screening models for central and peripheral analgesics
TRANSCRIPT
SCREENING MODELS FOR CENTRAL AND PERIPHERAL ANALGESICS
by Bajgire Krushna bALIRAMDepartment of Pharmacology and Toxicology
National Institute of Pharmaceutical Education and ResearchMohali, 160062
FLOW OF PRESENTATION :
1. INTRODUCTION2. CLASSIFICATION3. MECHANISM OF ACTION4. SCREENING MODELS
CENTRAL ANALGESIC ACTIVITY
PERIPHERAL ANALGESIC ACTIVITY
INTRODUCTION
Pain is a symptom of many diseases which requires
analgesic treatment
Analgesics is defined as the are agents which selectively
relieve pain by acting in the CNS or by peripheral pain mechanisms
CLASSIFICATION OF ANALGESICS
Analgesics
Narcotics
Ex. Morphine, Codeine,
Pethidine, Dihydromorphine
, Diacetylmorphine
Non-narcotics
Ex. Aspirin Paracetamol,
Ibuprofen, Ketoprofen, Diclofenac,
Piroxicam
MECHANISM OF ACTION
Analgesic drugs act in various ways on the peripheral and central nervous system Opioids produce analgesia by binding to specific G – protein coupled receptors in brain and spinal cord NSAIDs inhibit the activity of both cyclooxygenase-1 (COX- 1) and cyclooxygenase-2 (COX-2) and thereby the synthesis of prostaglandins and thromboxane's Inhibition of COX1 and COX-2 leads to the anti- inflammatory, analgesic and antipyretic effects
SCREENING
MODELS
CENTRAL ANALGESIC ACTIVITY
1. IN VITRO MODELS: * 3H-Naloxone binding assay * 3H-Dihydromorphine binding to opiate𝜇 receptors in rat brain * Receptor binding of nociceptin * Bioassays for nociception * Receptor binding of cannabinoids * Vanilloid receptor binding
2.IN VIVO MODELS:
Haffner’s Tail Clip Method Hot - plate test Electrical stimulation of the tail Grid - shock test Formalin test in rats Chemotherapy-Induced Pain Spinal Cord Injury Radiant heat method Tail immersion test
Purpose and rationale: * The paws of mice and rats are sensitive to heat at temperatures which are not damaging to skin * The responses are jumping, withdrawal of the paws and licking of the paws * The responses is prolonged after administration of centrally acting analgesics
Hot plate method :
Procedure :
The latency is recorded before & after 20, 60 and 90 min after the administration of standard or test compound
Groups of 10 mice (18-22g) are selected and divided into standard, test & control group respectively
The temperature of the hot plate is maintained at 55° to 56°C.
The animals are placed on the hot plate & time until either licking or jumping occurs is recorded.
Evaluation :
The prolongation of latency time between the test, standard and control animals are compared
Using various doses ED50 values can be calculated
Grid-shock test :
Purpose and rationale : * The electric grid shock test in mice has been described by Blake et al
* The analgesic properties of drugs like Morphine, Acetylsalicylic acid can be measured by the Flinch – jump response of rats
Procedure :Male mice (18-20g) are selected and placed
individually in plastic chamber.
The floor of the box is wired with stainless steel wire.
The stimulus is given in the form of square wave pulses ( 30 cycles per second)
The output of stimulator is connected to alternate wires of grid.
The fixed resistance is placed with the grid & parallel to an oscilloscope to allow calibration in milliamperes.
With increase in shock intensities the mice flinch, exhibit startling reaction & increase locomotion or attempt to jump
The behavior is accurately reflected on the oscilloscope by marked fluctuations of the displayed pulse
Pain thresholds are determined in each individual mouse twice before & after the administration of the test drug.
Evaluation :
The current measured in milliamperes is recorded for
each animal before and after administration of the drug
The average pain threshold values for each group at
each time interval are calculated and statistically
compared with the control values.
Tail immersion test :
PURPOSE AND RATIONAL :
* The method has been developed to be selective for morphine-like compounds
* The procedure is based on the observation that morphine- like drugs are capable of prolonging the reaction time of the typical tail withdrawal reflex in rats induced by immersing end of tail in warm water at 55°C
Procedure :
Young female wistar rats ( 170-210 g) are placed into individual restraining cages leaving the tail hanging out freely
The animals are allowed to adapt to the cages 30 minutes prior to testing
The lower 5 cm portion of tail is marked and this portion is then immersed in a cup of freshly filled water of exactly 55°C
Within a few seconds the rat reacts by withdrawing the tail. The reaction time is recorded
The reaction time is determined before and periodically after either oral or subcutaneous administration of the test substance
Evaluation :
ED50 values can be calculated for each compound and time response curves (onset, peak and duration of the effect) be measured
All the morphine-like analgesics have been shown to be active at doses which do not produce gross behavioral changes
PERIPHERAL ANALGESIC ACTIVITY
Writhing test
Pain in Inflamed Tissue (RANDALL-SELITTO-Test)
Mechanical Visceral Pain Model in the Rat
Antagonism to Nerve Growth Factor
Effect of Analgesics on Spinal Neurons
Antagonism Against Local Effects of Bradykinin
Writhing test : PURPOSE AND RATIONALE : Pain is induced by injecting irritants like acetic
acid into peritoneal cavity of mice
The animals react with characteristic stretching
behavior which is writhing
The test is suitable to detect analgesic activity of
peripherally acting drugs
Procedure:
Mice (20-25g) are selected and divided into standard, test & control group respectively
Appropriate volume of acetic acid solution is administered to the mice (control group) and
placed individually in the glass jar.
The onset of writhing, abdominal contractions & trunk twist response are recorded for 10 min.
The test and standard drug is administered 15 min prior to the acetic acid administration.
Evaluation :
The writhing period is recorded and
compared with the control group
Writhing response in the drug treated must
be less when compared to the acetic acid
treated control
Pain in Inflamed Tissue (RANDALL- SELITTO-Test) :
This method for measuring analgesic activity is based on the principle that inflammation increases the sensitivity to pain and that this sensitivity is susceptible to modification by analgesics
Inflammation decreases the pain reaction threshold and this low pain reaction threshold is readily elevated by non-narcotic analgesics of the salicylate-amidopyrine type as well as by the narcotic analgesics
Purpose and rationale :
Procedure :
For a time response, groups of at least 7 animals are used, four groups for the agent to be tested and one for the vehicle control
Groups of male Wistar rats (130 to 175 g) are used
0.1ml of a 20% suspension of Brewer’s yeast in distilled water is injected subcutaneously into the plantar surface of the left hind paw of the rat to induce
inflammation
Three hours later, pressure is applied through a tip to the plantar surface of the rat’s foot at a constant rate by a special apparatus to the point at which the
animal struggles, squeals or attempts to bite
The tests are done at 15 min interval after subcutaneous administration and at 30 min intervals after oral administration for any change in
pain threshold
Evaluation :
The mean applied force is determined for each time interval tested
The percentage increase in pain threshold is calculated by subtracting the applied force of the vehicle control from the applied force of the drug group which is divided by the applied force of the vehicle control in order to give the percentage of increase in pain threshold of the drug group
The interval of time which indicates the greatest increase in pain threshold is regarded as the peak time