NEURAL CONTROL OF PULPAL BLOOD FLOW

L OlgartDepartment of Physiology and Pharmacology, Division of Pharmacology, Karolinska Institute!, S171 77 Stockholm, Sweden

ABSTRACT: Blood flow of mammalian dental pulp is under both remote and local control. There is evidence for the existenceof parasympathetic nerves in the pulp, but functionally the cholinergic influence is weak, and the physiological significance ofthis autonomic system seems to be low. The evidence for sympathetic vasoconstrictor nerves in the pulp is robust, and thereis convincing support for the contention that these nerves play a physiological role, operating via release of noradrenaline andneuropeptide Y. However, there is no significant functional evidence in support of sympathetic beta-adrenoceptor-mediatedvasodilation in the pulp. The local control of blood flow involves a subset of intradental sensory nerves. By virtue of their neu-ropeptide content, these afferent fibers cause vasodilation and inhibit sympathetic vasoconstriction in response to painfulstimulation of the tooth. Such locally governed control may serve to meet immediate demands of the pulp tissue. A locallytriggered reflex activation of sympathetic nerves in the pulp may modulate this control and limit its magnitude. Thus, there arecompetitive interactions between local and remote vascular controls which may be put out of balance in the injured andinflamed dental pulp.

Key words. Blood flow, autonomic nerves, sensory nerves, dental pulp.

Introduction

H emodynamic regulation in the dental pulp has sev-eral important functions. It serves to provide opti-mal nutrition to pulpal cells, supports the removal ofmetabolites and waste products from the tissue, andacts to maintain a blood pressure within the vasculartree of the pulp in harmony with the pulpal tissue pres-sure. The remote neural control of pre- and post-capil-lary muscle sphincters in the pulp may not be the mostimportant regulatory system. There are also local factors,including certain nerves, which serve to balance theexchange between blood and tissue during resting con-ditions or to alter an inappropriate remote signal. Localfactors may also serve to satisfy specific demandsbrought about by external stimuli. The physiology ofblood-tissue interactions has previously been extensive-ly reviewed (Heyeraas, 1985, 1990). Therefore, the aim ofthis article is critically to evaluate hypotheses aboutmechanisms that regulate pulpal blood flow.

Parasympathetic SystemVasodilation as mediated by parasympathetic nerves ispart of a reflexogenic reaction in many organs, e.g., nasalmucosa, salivary glands, skeletal muscles, and skin. Forexample, the reflex-evoked cholinergic transmissionleading to salivary secretion also triggers vasodilation in

the gland to support the secretory process.In the salivary glands, as well as in the nasal mucosa

of several species, secretion is mediated mainly byacetylcholine (Ach), whereas blood flow, to a greatextent, is mediated by vasoactive intestinal peptide (VIP)(Lundberg et al, 1981b,c). In these tissues, both media-tors are found in parasympathetic fibers associated withblood vessels (Lundberg et al, 1981a). The possible exis-tence of parasympathetic vasodilation in the dental pulphas been debated for several decades (Weiss et al, 1972;Edwall et al, 1973; Tonder, 1976; Cauvin and Kirkendol,1980; Okabe et al, 1989), and there is still some contro-versy in this matter. Previous histochemical studiesreported the presence of acetylcholine esterase, respon-sible for the degradation of Ach in mammalian pulps(Pohto and Antila, 1968a, 1972; cf. Avery and Chiego,1990). When applied locally on dog pulps, Ach triggers arise in pulpal blood flow (Okabe et al, 1989; Liu et al,1990). Ach receptors have also been encountered inporcine dental pulp (Sano et al, 1989). The existence ofspecific receptors and agonist effects is not unequivocalproof of a parasympathetic nervous influence on thepulp. Nor is the presence of choline esterase evidencefor the presence of cholinergic nerves (Lehman andFibiger, 1979). The presence of the Ach-synthesizingenzyme choline acetyl transferase (ChAT) is a more reli-

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CONTROL ATROPINE CHLORISONDAMINE

Figure 1. Blood flow changes in lower canine teeth evoked bynerve stimulation in three groups of cats. Lingual nerve stim-ulation (10 V, 2 ms, 30 Hz) in inferior alveolar nerve-denervat-ed animals (10 days before) (n = 6), 3 inferior alveolar nervestimulation (10 V, 2 ms, 5 impulses, 2 Hz) (n = 4), bipolarelectrical stimulation (100 IJLA, 5 ms, 5 impulses, 2 Hz) of thecanine tooth (n = 5). Duplicate stimulations were performed ineach animal of all groups before (control) and after administra-tion of atropine (0.5 mg/kg) and after chlorisondamine (30m9,/kg). Blood flow was monitored by laser-Doppler flowmetry(LDF). The anticholinergic drugs influenced only vasodilationevoked by lingual nerve stimulation.

able indicator of cholinergic nerves, but studies to local-ize this enzyme have not been carried out on the pulp.The other mediator (VIP), which co-exists with Ach inpost-ganglionic neurons, is released upon parasympa-thetic stimulation and is a candidate for mediation of thenon-cholinergic (atropine-resistant) vasodilationobserved in, e.g., the cat nasal mucosa and salivaryglands (Lundberg et al, 1981b,c). In oro-facial tissues,VIP-immunoreactivity (IR) can thus be used as a markerfor parasympathetic post-ganglionic neurons. VlP-con-taining nerve fibers have been found in the dental pulpof several species, including man (Uddman et al, 1980;Akai and Wakisaka, 1990; Casasco et al, 1990; Luthman etal, 1992), and intra-arterial injection of synthetic VIP inthe picomolar range causes vasodilation in the cat pulp(Olgart et al, 1988). An interesting finding is that theadjacent gingiva is sensitive to even lower doses of VIP.Denervation experiments clearly demonstrate that theVIP in the pulp of cats does not originate from sensory orsympathetic nerves (Akai and Wakisaka, 1990; Olgart,1990). However, the functional evidence for parasympa-thetic regulation of blood flow in the pulp is still the sub-ject of controversy. Different results and opinions may bepartly due to differences in experimental design, tech-niques, and species differences.

In a preliminary study, long-lasting electrical stimu-lation of the lingual nerve in cats led to depletion of the

stores of VIP-IR in tooth pulps of ipsilateral lower canineteeth (Gazelius and Olgart, 1989). This would suggest aparasympathetic influence on pulpal blood flow. The lin-gual nerve carries parasympathetic fibers to the sub-mandibular and lingual glands and to the tongue. Anadditional finding was that a brief (30 Hz) electrical stim-ulation of this nerve occasionally induced a weak vasodi-lation in the ipsilateral lower canine pulp, provided thatnerve branches supplying adjacent tissues were cut.Interestingly, in cats subjected to unilateral inferior alve-olar nerve (IAN) neurotomy one week prior to the experi-ment, predictable and enhanced vasodilator responseswere obtained. The Ach antagonist atropine reduced thisresponse by 46%, thus leaving a remaining atropine-resistant response. Chlorisondamine (an autonomicganglion blocker) completely blocked the remainingresponse (Fig. 1). Since these experiments were carriedout under non-physiological conditions {i.e., in theabsence of sensory and sympathetic supply), completeinterpretation must await further experiments. In con-trast, Sasano and collaborators recently reported experi-ments done on the cat which suggest that there is noparasympathetic control of pulpal blood flow, althoughsuch a control was shown to exist in the adjacent gingi-va (Sasano et al,. 1995a, b). Electrical stimulation of facialand glossopharyngeal nerve roots, known to carryparasympathetic fibers, elicited hexamethonium-sensi-tive blood flow increases in the ipsilateral lip, but not inthe adjacent canine pulp. These results corroborate ear-lier histochemical studies showing the existence ofmarkers for parasympathetic nerves in the gingiva and lipof the cat (Izumi and Karita, 1991, 1993; Kaji etal, 1988,1991). The weakness or absence of signs of parasympa-thetic pulpal vasodilation obtained in animal experi-ments could possibly be due to vigorous parasympathet-ic vasodilation in neighboring innervated tissues whichwould then "steal" perfusion pressure from the pulp(Tonder, 1976). Experiments designed to trigger a physio-logical reflex activation of the autonomic nerves wouldperhaps circumvent this experimental problem. Suchstudies were recently carried out in conscious humans(Aars et al, 1992, 1993; Kemppainen et al, 1994). In the1993 study by Aars et al, the isometric hand-grip test(used as a stress stimulus) induced a moderate rise inpulpal blood flow which could be blocked by atropine(Fig. 2). It is noteworthy that, after cholinergic blockade,the identical stress stimulus resulted in a reversed effect,i.e., vaso-constriction, suggesting that the two antago-nistic autonomic systems were simultaneously activated.It is thus possible that a concomitant vasoconstrictorinfluence in adjacent tissues prevented "stealing ofblood" from the pulp, thus allowing a cholinergic vasodi-lation to occur in these human experiments.

Interesting information suggesting possible interac-tion between cholinergic and adrenergic nerves in the

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human pulp was recently reported (Parker et al, 1995).Adrenergic terminals in the pulp appear to be equippedwith receptors of both the muscarinic and the nicotinictype. Agonist activation of these receptors was shown toreduce the release of noradrenaline from activated sym-pathetic nerves in human dental pulp in vitro. It thereforeseems possible that cholinergic neurons in the pulpexert a modulatory influence on sympathetic functions.These intriguing findings call for further investigation.

In summary, all prerequisites (receptors, chemicalmediators, and nerves) for a parasympathetic control ofpulpal blood flow are present. This remote regulationappears to have only a weak influence on pulpal bloodflow, which may explain the divergent experimentalresults. The physiological significance of an existingparasympathetic vascular control in the pulp is probablylow.

Sympathetic SystemThe sympathetic system is involved in many vital func-tions, not all related to hemodynamic regulation. Amajor role of the sympathetic system is the maintenanceof blood pressure. An appropriate stimulus triggers anincrease in sympathetic activity, which results in anincrease in blood pressure. This effect is generated byvasoconstriction of arterioles in most tissues, and bydirect effects on the heart. Sympathetic vaso-constric-tion in the pulps and jaws is insignificant for circulatoryhomeostasis, since these tissues represent a relativelysmall blood volume. The sympathetic vasomotor controlof the pulp may have local importance.

There is overwhelming evidence that the pulpalmicrocirculation is under the control of sympatheticnerves. Taylor (1950), using vital microscopy, showedthat stimulation of the transsected cervical sympathetictrunk caused a reduction of blood flow in rat incisor pulp.This finding was later confirmed by Pohto and Scheinin(1962), who also observed that an adrenaline solutionapplied at the apical foramen of the rat incisor caused areduction of blood flow in the coronal pulp. Thus, con-striction of vessels in and in the vicinity of the pulp causedsimilar effects. The evidence that a-adrenoceptors existin pulpal vessels was based, in part, on the finding thatlocal application of noradrenaline (NA) on the exposedpulp produced vaso-constriction (Ogilvie et al, 1966;Ogilvie, 1967). Using histochemical and biochemicaltechniques, many researchers have provided conclusiveevidence for adrenergic vasomotor innervation in thedental pulp of several species, including man (Annerothand Norberg, 1968; Pohto and Antila, 1968b; Kukletova etal, 1968; Larsson and Linde, 1971; Parker et al, 1986;Kerezoudis et al, 1992; Luthman et al, 1992). The post-ganglionic fibers originate from the cervical sympatheticganglion, and after joining the trigeminal nerve at itsganglion, most of them follow the course of the sensory

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Figure 2. Experiment in human upper incisors showing the influ-ence of atropine on changes in pulpal blood flow (upper panel)and mean arterial pressure (lower panel) evoked by isometrichand grip (IHG) followed by arterial occlusion of the upper arm(OCCL). Results are expressed as percentage of baseline andmedian values with 25-75 percentiles. Solid line is controlresponses without atropine, and dashed line is after administra-tion of atropine (0.015 mg/kg). * P < 0.05 compared withbaseline; ^r P < 0.05 compared with control (n = 7). Blood flowwas measured with laser-Doppler flowmetry (LDF). (Modifiedwith permission from Aars et al., 1993.)

nerves to the teeth and adjacent tissues in the cat and rat(Matthews and Robinson, 1980; Marfurt et al, 1986;Kerezoudis et al, 1995). In the rat, a small but significantproportion of the sympathetic nerves follow anotherroute to the teeth, possibly traveling via the vessels(Kerezoudis etal, 1995).

Detailed functional studies have shown that sympa-thetic vasoconstriction in the cat and dog pulp is medi-ated mainly by vascular receptors of the a,-type (Edwalland Kindlova, 1971). These receptors are located post-junctionally, e.g., on the vascular smooth muscle cells.Adrenoceptors of the a2-type are located both pre-and post-junctionally. Activation of pre-junctionala2-adrenoceptors by NA results in a reduced aj-evokedvasoconstriction, due to an auto-inhibitory control of NArelease from the sympathetic nerve endings. Post-junc-tional activation of a2-receptors usually results in a weak

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fFigure 3. Effects of a2-, a^adrenergic blockers on pulpal bloodflow (LDF) responses in rat incisor pulp upon sympathetic nervestimulation (4 V, 1 ms, 4 and 16 Hz, 1 min). Responses beforeand ^ after administration of the a2-adrenoceptor antagonistidazoxan (0.5 mg/kg) and i the apadrenoceptor antagonistprazosin (50 ixg/kg). Numbers in columns are numbers of ani-mals; values are means SEM; * * * P < 0.001 as comparedwith control responses.

vasoconstriction. Pulpal vessels in the dog, like peri-odontal vessels in the cat, are equipped with both a,-and a2-adrenoceptors (Edwall et al, and Gazelius, 1988;Kim et al, 1989; Ibricevic et al, 1991). The influence ofvasoconstrictor a2-receptors on blood flow in the dogand the cat was shown to be smaller than that ofa,-receptors. Evidently, there are species differences,since in the rat incisor the a2-adrenergic blocker idazox-an was shown not to influence sympathetic vasocon-striction (Fig. 3) (Kerezoudis etal, 1993a). The a-adreno-ceptors seem to be distributed at both the arterioles andthe venules in the rat dental pulp (Kim et al, 1989). Thismay mean that the remote sympathetic control canselectively regulate the pre- and post-capillary sphinc-ters in the pulp in order to adjust pressure in the inter-mediate capillary sections, as required by the tissue.However, such selective regulation of regional flow hasnot been demonstrated. There is no detailed descriptionabout the distribution of adrenergic receptors in thehuman teeth.

The influence of sympathetic activity on pulpal cir-culation in resting conditions seems to be low in bothanimals and man. In anesthetized animals in a supineposition, cutting the sympathetic nerve or administeringan a-adrenoceptor antagonist does not alter blood flow

(Edwall, 1971; Heyeraas Tonder and Naess, 1978). Inseated conscious humans, mandibular block anaesthe-sia (mepivacain) does not change resting pulpal bloodflow, whereas it blocks stress-induced vasoconstriction(Aars et al, 1992). The low resting level of sympatheticactivity and a low degree of autoregulation may, at leastto some extent, explain why pulpal blood flow in experi-mental animals under general anesthesia is very depen-dent on alterations in systemic perfusion pressure (Fig.4) (see also Sasano et al, 1989). The same explanationmay be applied to the observation that, in conscioushumans resting in a comfortable chair, pulpal blood flowdecreases if the subject falls asleep. When these subjectsare abruptly awakened, pulpal blood flow instantlyincreases as blood pressure and heart rate increase(Gazelius and Olgart, unpublished). Changes in bloodpressure may also explain the increase in blood flowobserved in adolescent human pulps after physical exer-cise and the successive decrease as systemic circulatoryparameters normalize (Olgart, 1995). In these examplesfrom human recordings, the apparent absence of pres-sure autoregulation may, to some extent, be misleading.It cannot be ruled out that autonomic nerves were acti-vated, resulting in blood flow alterations overrulingautoregulation. The delicate question about the exis-tence of autoregulation in pulpal circulation in con-scious humans needs further research. At any rate, mostevidence points to the fact that the level of sympatheticcontrol of blood flow to the pulp in resting conditions islow. However, in certain threatening situations, includingphysical and mental stress, which cause a general activa-tion of the sympathetic system, the neural vasoconstric-tor control is also activated in the pulp (see Aars et al,1992, 1993). This has been demonstrated in anesthetizeddogs (Heyeraas Tonder, 1975; Kim et al, 1980). In the lat-ter study, a reflex activation of the sympathetic systemcould be induced by experimental hypotension (hemor-rhage and nitro-prusside infusion) or by a decrease inoxygen transport (by hemodilution). The concomitantreduction in pulpal blood flow was related to sympathet-ic vasoconstriction, since the effect was partly blocked byan a-adrenoceptor antagonist. However, part of theeffect was also due to a fall in systemic blood pressure.

The involvement of a p-adrenoceptor-mediatedvasodilator component in the sympathetic control ofpulpal blood flow has been a matter of great controversy(Tonder, 1976; Heyeraas Tonder and Naess, 1978;Gazelius and Olgart, 1980; Kim et al, 1980). Althoughlocal application of p-adrenoceptor agonists on the pulpcauses vasodilation, thereby implying the presence ofthese receptors in the pulp (Okabe et al, 1989; Liu et al,1990), there is little evidence that they have any signifi-cance in neural control. The only functional evidence fora sympathetic nerve-induced vasodilation mediated byP-adrenoceptors in the pulp was obtained by direct

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tooth stimulation of the incisor in the rat (Kerezoudis etal, 1992). The specific (3-adrenoceptor blocker, timolol,abolished a small vasodilator response which was visibleafter a-adrenergic blockade in these experiments.However, in the same series of experiments, the authorswere unable to evoke vasodilation by stimulation of thesympathetic nerve trunk. These results imply that theexisting (3-adrenoceptors in the pulp may not be of phys-iological/functional importance in the neural control ofblood flow. The possibility that the (3-receptors react tocirculating adrenaline has not found support in animalexperiments (Olgart, unpublished).

Dopamine, which is a precursor of NA, is present insympathetic nerves in rat incisor pulp and may be usedas a marker for these nerves in the pulp (Kerezoudis et al,1995). It has agonistic actions on a- and p-adrenocep-tors, as well as on dopamine receptors. As shown in arecent study (Kerezoudis et al, 1995), dopamine does notseem to contribute to neural control of pulpal vessels,since appropriate dopamine antagonists do not influ-ence vasoconstriction induced by sympathetic nervestimulation.

The fact that some vasoconstriction remains afterappropriate a-adrenoceptor blockade, particularly dur-ing high-frequency stimulation of the sympathetic sup-ply (Heyeraas Tonder and Naess, 1978; Kerezoudis et al,1993a) (see Fig. 3), can be explained by the presence ofneuropeptide Y (NPY) in sympathetic terminals in thepulp of several species, including man (Uddman et al,1984; Edwall et al, 1985; Wakisaka, 1990; Casasco et al,1990; Luthmanetal., 1992; Heyeraas et al, 1993). The dis-tribution of axons with NPY-IR is very similar to the dis-tribution of axons with dopamine- p-hydroxylase, anenzyme involved in catecholamine synthesis (seeWakisaka, 1990). In addition, following ablation of thesuperior cervical ganglion, NPY-containing axons disap-pear from the pulp. Intra-arterial injection of NPY in catscauses a sustained reduction in pulpal blood flow whichis resistant to a-adrenoceptor blockade (Edwall et al,1985). Results obtained from various other tissues in dif-ferent species suggest that the release of NPY occurspredominantly at higher frequencies of sympatheticnerve stimulation (Lundberg et al, 1986). Since a physio-logically evoked activity in post-ganglionic sympatheticfibers shows an irregular bursting pattern (Wallin, 1981),conditions for a complementary role of the classic (NA)and novel (NPY) transmitters in pulpal vasomotor con-trol are at hand. Since NA exerts pre-synaptic inhibitionon NPY release via a2-adrenoceptors, the use ofnon-specific a-adrenoceptor blockers may enhance theNPY-evoked component of the vasoconstriction, thusoffering an additional explanation for the resistance tosuch an a-blockade.

It may be concluded that there is firm evidence for asympathetic vascular control in the dental pulp in many

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minFigure 4. Simultaneous recording of blood pressure (upperpanel) and pulpal blood flow in the ferret lower canine tooth(lower panel). Blood flow (LDF perfusion units) measured withlaser-Doppler flowmetry. Alteration in blood pressure wasinduced by intravenous injection with pentobarbital (arrows, 2+ 2 mg/kg).

species, including man. However, a reduction of bloodflow to the pulp due to sympathetic activation may also,to various degrees, depend on the constriction of feedingarterioles upstream, outside the pulp proper. The media-tors presently known are noradrenaline and neuropep-tide Y. The system does not seem to be tonically active inoral tissues, but physical and mental stress may trigger asympathetic vasoconstriction in the oral tissues, inclu-ding the pulp.

SYMPATHETIC MODULATION OFLOCAL PAIN TRANSMISSION?

Sympathetic vasoconstriction in the cat dental pulp hasbeen shown strongly to modulate the excitability ofintradental sensory nerves (Edwall and Scott, 1971).Thus, sensory terminals of the A-type readily lose theirnormal sensitivity during pulpal ischemia (Olgart and

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ARTERIALPRESSUREmm Hg

2001

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DISAPPEARANCE 6 .

RATEk 102min'

TIME (40sec)SIGNAL 4 hours

Figure 5. Influence of deep cavity preparation on sympathetic vaso-constrictor response in the pulp of the cat; initial response (1) andafter 4 hours (2). 1, 2 sympathetic stimulation with 6 Hz. Pulpalblood flow was measured with the iodide disappearance technique.(From Forssell-Ahlberg and Edwall, 1977, with permission.)

Gazelius, 1977). A heat stimulus, which excites thesenerves under normal conditions, was shown to be with-out such an effect during a period of sympathetic activa-tion. In animals, a reduction in pain sensation as a partof the "fight and flight" reaction may appear valuable, butfor the human dental pulp, such an effect would not gen-erally be considered useful. Extrapolated to humanteeth, fluctuations in pulpal pain intensity may beexplained, for example, by stress-induced sympatheticvasoconstriction. Perhaps this is one reason why somepatients with pulpal symptoms find that the pain disap-pears on the way to the dentist. On the other hand,increased activity in sympathetic nerves and release ofNA may result in the opposite effect, namely, direct exci-tation of certain sensory nerve endings. As recentlyshown in experimentally induced chronic inflammationin rat cutaneous tissue, a2-adrenoceptor-mediated sym-pathetic activity excites certain nociceptors (Sato et al,1993). Therefore, a-adrenoceptors present on sensorynerve endings may stimulate pain transmission whenactivated by NA and may hypothetically contribute tofluctuations in pain symptoms from inflamed pulps.

One critically important aspect of autonomic vaso-motor regulation, and other possible functions of auto-nomic nerves, is that both the sympathetic and parasym-pathetic systems operate at the general or segmentallevels and tend to ignore the needs of an individual tis-sue such as the pulp. However, as we shall see, reflexactivation of sympathetic nerves to the pulp may play arole in modulating locally triggered vascular reactions.Furthermore, local mechanisms in the pulp may modu-late the effects of the remote control.

Local Modulation

INHIBITION OF SYMPATHETIC VASCULAR CONTROL

Sympathetic vasoconstriction in the pulp is susceptibleto inhibition following local insults directed to the tis-sue. In the cat, particularly in mature teeth, deep cavitypreparation and heating or cooling of the tooth mayabolish the vasoconstrictor response to sympatheticactivation for several hours after application of the localstimulus (Fig. 5) (Edwall, 1971; Forssell-Ahlberg andEdwall, 1977). Nearby intra-arterial infusions of acetyl-choline, histamine, bradykinin, and substance P alsocounteract an ongoing sympathetic vasoconstriction(Edwall eta!., 1973; Gazelius et al, 1977). This effect of thevasodilator substances may somehow be related to alocal inhibition of sympathetic influence, since some ofthese agents may be released under physiological condi-tions and in conjunction with early signs of pulp inflam-mation. This view is supported by observations showingthat pre-capillary sphincters become refractory to vaso-constrictor stimuli in the initial stages of inflammation(Zweifach, 1971, 1973). It is therefore possible thatvasoactive mediators released locally in the pulp over-ride the effects of an existing sympathetic influence. Theremoval of the flow- and pressure-limiting control mayexplain why the pulsating pain from pulpitis becomesworse when a patient bends over. In such a case, pulp tis-sue pressure may increase uncontrolled by sympatheticneural influence, thus activating sensitized pain recep-tors in the pulp.

An active sympathetic control is not necessarily aprerequisite for local blood flow regulation. Also, in theabsence of neural vasoconstriction, basal myogenic tonein the healthy pulp permits a pronounced vasodilation totake place in the pulp via an increased metabolism andlocal release of vasoactive substances (Edwall etal, 1973;Heyeraas Tonder, 1980).

NON-ADRENERGIC, NON-CHOLINERGICVASODILATOR NERVES

A certain population of afferent (sensory) nerves in thepulp of animals and man seems to exert vasodilatoreffects in the pulp. The fibers belong to a morphological-ly and functionally diverse subset of sensory nerves shar-ing the trait of being susceptible to the stimulatory andsensory blocking actions of capsaicin, the pungent ingre-dient in hot peppers (Szolcsanyi, 1984; Holzer, 1991; seeOlgart, 1996). These nerves are excited by a variety ofnoxious stimuli, and they are peptidergic, small- andmedium-sized neurons associated with unmyelinated(C) or thin-myelinated (A-delta) fibers (Janig and Lisney,1989; Matthews and Vongsavan, 1994; Olgart, 1996).These nerves contain vasoactive neurokinins (substanceP [SP] and neurokinin A [NKA]) and calcitonin

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gene-related peptide (CGRP) (Olgart et al, 1977a; Akaiand Wakisaka, 1990; Luthman et al, 1992). Both SP andCGRP are potent vasodilators in the pulp, whereas NKAhas a much smaller effect on pulpal blood flow (Gazeliuset al, 1987; Olgart, 1990). Upon activation of such nervesin the pulp, SP is released (Olgart et al, 1977b; Brodin etal, 1981a). After exerting its effects, SP is rapidly degrad-ed by enzymes in the pulp tissue (Gazelius et al, 1981a).Similar observations of release and degradation havebeen made for the other neuropeptides in a number oftissues.

Activation of sensory nerves in the pulp, either bybrief antidromic electrical stimulation of the inferioralveolar nerve or by direct stimulation on the toothcrown, induces a long-lasting blood flow increase in thepulp (Heyeraas Tonder and Naess, 1978; Gazelius andOlgart, 1980; Kerezoudis et al, 1993b). The mechanism ofthis reaction in rat incisor pulp was recently shown toinvolve both SP and CGRP (Kerezoudis et al, 1994a,b).Specific antagonists were used to show that the initialcomponent of the response was mediated by SP, where-as the continued long-lasting rise in blood flow wasdependent on CGRP. Stimulation of the supplying nervesor direct stimulation of the tooth crown, for several min-utes, also increased vascular permeability in the pulp, asshown by the Evans blue extravasation technique(Kerezoudis et al, 1993c). This delayed reaction wasshown to be mediated by SP and prostaglandins. In theneighboring gingiva, histamine is also involved.Interestingly, only a few or single pulses are necessary toevoke a large vasodilator response in the pulp, and it isnoteworthy that stimulation of adjacent tissues, inclu-ding the lip, gingiva, and neighboring teeth, also resultsin pulpal vasodilation in the canine tooth on the ipsilat-eral side (Olgart, 1996; Sasano et al, 1995a). An addition-al intriguing finding is that vasodilation is also evoked inadjacent soft tissues by painful stimulation of humanteeth (Kemppainen et al, 1994; Kuriwada et al, 1995).Taken together, these results indicate branching of sen-sory axons and suggest that there is an axonreflex-mediated spread of vascular reactions occurringbeyond the site or tissue stimulated. This arrangementimplies that a painful stimulus directed at the tooth mayresult in counteraction of an increased vascular tone,regardless of whether the vasoconstriction is takingplace inside the pulp or in the feeding arterioles outsidethe pulp.

Other stimuli of clinical relevance, like drilling andprobing of exposed dentin, application of ultrasound,and percussion of teeth, also cause vasodilation in thepulp, which is mediated by intradental sensory nerves(Fig. 6) (Olgart et al, 1991; Matthews and Vongsavan,1994). In addition, brief extensive load, causing elasticdeformation of dentin in cat teeth, evokes bursts ofimpulses in intradental nerves and vasodilation in the

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Figure 6. Influence of grinding ( 3 x 1 s) of dentin on pulpalblood flow in control and inferior alveolar nerve denervatedlower canine teeth of the cat. (A) At enamel-dentin junction; (B)in inner half of dentin. Deep preparation in denervated teeth (B,lower panel) probably caused a direct inhibition of the myo-genic tone of pulpal vessels.

pulp (Olgart et al, 1988). Thus, stimuli known to causepain in human teeth by hydrodynamic mechanisms initi-ate neurogenic vascular responses in the pulp. Such anaxon reflex-mediated vasodilation is regarded as anappropriate defense reaction which enhances the trans-port of nutrients and metabolites in the tissue. A furtherconsequence related to the sudden increase in bloodflow and volume in the encapsulated pulp tissue is anenhanced outward dentinal fluid flow, presumably dueto the concomitant increase in local tissue pressure(Matthews and Vongsavan, 1994). The outward dentinalfluid flow may also play a part in local defense by pro-tecting the pulp from invasive threats. In this context, itis noteworthy that sympathetic vasoconstriction altersthe movement of dentinal fluid flow to an inward direc-tion under exposed dentin surfaces of the cat (Matthewsand Vongsavan, 1994). Such a condition would be lessfavorable, since it may promote transport of external irri-tants to the pulp. The finding that sympathetic vasocon-striction can be counteracted and reversed to a vasodila-tion by the local nerve-induced vasodilator mechanisms(Kerezoudis et al, 1993a) draws attention to a valuableinteraction which may support pulpal health in certainsituations.

Indirect evidence suggests that the sensory neu-ropeptides can also be released without obvious stimu-

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Activation

SympatheticNerve

Figure 7. Schematic drawing showing proposed bidirectionalinteractions between remote (sympathetic) control and local(sensory) control of pulpal blood flow. Sympathetic vasocon-striction (a) is counteracted by local activation of sensory nervesand release of vasodilator neuropeptides (CGRP and SP) (b).Reflex activation (c) of sympathetic nerves and local release ofNA attenuates the release of CGRP and SP (d).

lation of the tooth (Olgart and Gazelius, 1988). Thisassumption was based on findings in the cat, showingthat neurotomy-induced depletion of the peptide storesin the pulp parallels enhanced vasodilator responses toinjections with SP (Olgart et al, 1993). Interpreted as aphenomenon of receptor supersensitivity, this findingimplies that the tachykinins, by a slow release, exert acontinuous influence on their receptors on pulpal ves-sels in the normal unstimulated pulp. Such a "spill-over"of the neuropeptides, in all probability, is not associatedwith any impulse propagation centrally. The chemosensi-tive nature of the peptidergic nerves may lead to endoge-nous activation by inflammatory mediators released inthe pulp. This aspect became obvious when it was foundthat pulpal vasodilation in response to local applicationof bradykinin occurred, to a great extent, via activation ofsensory nerves (Olgart et al, 1991). Thus, afferent nervesin the pulp may, to various degrees, participate inhemoregulation depending on the status of the pulp.This view is interesting in relation to the phenomenon ofsprouting of sensory nerve terminals, and up-regulationof the neuropeptides, observed in acute stages of pulpinflammation (Byers and Taylor, 1990). Under such con-ditions, it would be expected that afferent nerves partici-pate in the inflammatory process by an increased releaseof the neuropeptides.

Sympathetic modulation oflocal vascular reactions

Rat incisor teeth provide a special feature in that bothsympathetic vasoconstriction and sensory nerve-induced vasodilation may be studied in response to

direct electrical stimulation of the tooth crown(Kerezoudis et al, 1992). In this model, interactionsbetween local and remote neurogenic control of bloodflow can be analyzed in some detail. After short trains ofelectrical pulses on the tooth, a transient vasoconstric-tion followed by a long-lasting vasodilation is seen. Aftera-adrenoceptor blockade and following acute cutting ofthe sympathetic supply, the vasodilator component ofthe response is significantly enhanced (Kerezoudis et al,1993d). This implies that, when triggered, the local neu-rogenic control is under a sympathetic influence. Thefindings also suggest that an inhibitory sensory-sympa-thetic reflex may be evoked by a nociceptive tooth stim-ulation, thus limiting the locally induced vasodilation.Since previous in vitro experiments have shown that NAinhibits stimulus-evoked CGRP release in bovine pulptissue (Engelstad et al, 1992), it appears that NA releasedfrom sympathetic terminals exerts an inhibitory actionon sensory neuropeptide release in the pulp. The mech-anism of this action of NA is probably via activation ofpre-synaptic a-adrenoceptors on the sensory nerve ter-minals, thus attenuating the release of the vasodilatoragents. The delayed increase in vascular permeability ini-tiated by SP is similarly controlled by NA released fromsympathetic terminals (Kerezoudis et al, 1993a).

In summary, there is evidence to suggest the exis-tence of mutual and competitive interactions betweenthe remote and local neural mechanisms controlling pul-pal blood flow. A locally triggered neurogenic vasodila-tion may override sympathetic vasoconstriction to sup-port local demands, and a stimulus-induced sympathet-ic nerve activity may limit such local control (Fig. 7).Under physiological conditions, these counteractingmechanisms may keep necessary control of blood flow inbalance. However, in a state of acute inflammation, whenthe sympathetic function is impaired (see above), andwhen there is sprouting of afferent CGRP-containingfibers in the pulp tissue (Byers and Taylor, 1990), thelocal mechanisms will apparently dominate. In suchcompromised pulps, other flow-limiting factors, such asendothelium-derived endothelins (see below) and anincreased tissue pressure, may be of importance forrecovery (Heyeraas Tonder, 1980).

OTHER MECHANISMS MODULATINGPULPAL BLOOD FLOW

There are several less-well-investigated options forblood flow regulation in the pulp. One example is theobservation that endothelins (ET) are present in thehuman dental pulp (Casasco et al, 1991). Particularly,one subtype (ET-1) in this group of endothelium-derivedpeptides exerts powerful and prolonged vasoconstrictiveeffects when injected into peripheral vessels, including

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those of the dental pulp (Gilbert et al, 1992). Recently, ithas been suggested that ET-1 contributes to basal vas-cular tone, as shown in porcine pulmonary vessels(Weitzberg et al, 1994) and in the brachial artery of con-scious humans (Haynes and Webb, 1994). Local infusionof novel, specific ET-selective receptor antagonistscaused vasodilation, implying that endogenous genera-tion of ET-1 maintains vascular tone through activationof ET receptors. These intriguing findings will extend ourunderstanding of regional blood flow regulation.

Another example of a possible mechanism modulat-ing pulpal blood flow is the observation that somato-statin-positive nerves, probably of trigeminal origin, arepresent in the pulp (Luthman et al, 1992). Injection ofpharmacologically relevant doses of this peptide in thecat reduces the stimulus-induced release of SP (Gazeliuset al, 1981b). Parallel to this finding, somatostatin atten-uates afferent nerve-induced vasodilation in the cat pulp(Brodin et al, 1981b). Thus, if released, this neuropeptidecould play a modulatory role on local vasodilator mech-anisms. However, there is as yet only limited evidence fora functional role of somatostatin in the pulp (Olgart,1996).

Enkephalins, present in certain pulpal cells of thedog (Kudo et al, 1981), have been shown to reduce bothSP release and vasodilation induced by stimulation ofpulpal afferent nerves in the cat (Brodin et al, 1981b,1983). However, such peripheral actions of these opioidsappear to be of significance primarily in tissue inflam-mation (Stein, 1993).

Nitric oxide (NO), which is a molecule with powerfulvasodilator capacity, is, in all probability, enzymaticallyproduced in certain cells in the odontoblast layer and invascular endothelium (Kerezoudis et al, 1993e). Underbasal blood flow conditions, NO appears to play a role inmodulating the local myogenic tone of pulpal vessels(Kerezoudis et al, 1993b). Thus, a basal formation of NOappears to be maintained as a consequence of the con-tinuous enzyme activation of the endothelium, e.g., byshear stress. Support for a role of NO in regulation ofvascular tone was obtained in studies on rat incisors andcat canine teeth which showed that treatment with aninhibitor of NO-synthase caused vasoconstriction(Kerezoudis et al, 1993b; Lohinai et al, 1995). Since thevascular endothelium continously generates both avasoconstrictor substance (ET) and a vasodilator agent(NO), there are prerequisites for modulation of pulp cir-culation without influence of nerves. An interesting pre-liminary finding that we obtained in experimentallyinflamed rat pulps is that NO-synthase activity is dra-matically increased in pulpal cells and endothelium ascompared with normal pulps. This is yet another exam-ple demonstrating that, in developing pulpitis, condi-tions for regulation of pulpal microcirculation are con-tinuously altered and therefore are extremely difficult to

predict. Thus, early pulpitis is an important target forcontinued studies, because, at an early stage of inflam-mation, there are unexplored treatment possibilities.Therefore, we urgently need more research on how vas-cular reactions can be controlled and perhaps manipu-lated in the compromised dental pulp.

ConclusionThe remote (autonomic) neural control of pulpal bloodflow is not tonically active but is typically activated bystress stimuli and by painful stimuli directed at almostany part of the body. Thus, the documented effects ofsympathetic and parasympathetic nerves on pulpalblood flow are mostly incidental and reflect a more wide-ly spread of centrally mediated reflexes that affect muchof the body. There is no evidence for pulpal blood flowbeing selectively adjusted by sympathetic or parasympa-thetic nerve activity to meet specific requirements of thetissue.

Local neural control, on the other hand, operates ona local scale. When triggered by a local stimulus (usual-ly a painful one), a subset of intradental sensory fibersmediates relaxation of pulpal vessels by counteracting amyogenic or sympathetic vasoconstrictor tone. Manyother non-neural mechanisms, including endothelium-derived vasoactive principles and local tissue pressure,also contribute to maintaining an optimal blood circula-tion in the normal pulp.

In the compromised pulp, the remote vasoconstric-tor control is attenuated, and the delicate interplaybetween local mechanisms may be put out of balance.Blood circulation in such a pulp becomes very depen-dent on alterations in systemic perfusion and tissuepressures, and this may contribute to further progress ofpulp inflammation.

Future DirectionsThe present knowledge about regulation of pulpal bloodflow is based mainly on results obtained in anesthetizedanimals. We need critically to confirm and extend thisknowledge by studying the pulp in conscious humans.Methods are available, such as non-invasive laser-Doppler flowmetry for blood flow recording, and electro-physiological techniques for recording of nerve func-tions. Novel biochemical techniques should also beapplied on pulps of extracted human teeth and teeth insitu for the tracing of alterations in the expression ofmediators of neurovascular reactions. Perhaps some ofthese functional and biochemical parameters obtainedin humans will show the existence of even more complexinteractions among the many regulatory systems in thepulp than we thought.

Circulatory control should also be further studied inpulps with early signs of inflammation in both animalsand man. Such studies have been hampered by difficul-

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ties in attempts to induce inflammation in a predictablemanner. Indeed, the ultimate goal of functional studiesof the pulp is to improve diagnostic and clinical proce-dures in order to improve our understanding of how toavoid insults and how to cure pulpal inflammation.

A further track to follow is continuous cooperation,world-wide, between and among groups of scientistsusing different approaches and technology in theirresearch, but sharing the same goal.

AcknowledgmentsThis review is based upon valuable collaboration with many colleaguesand students, to whom I am greatly indebted. This work was supportedby Swedish MRC Grant 00816 and Karolinska Institutet.

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