Analgesics Lab # 3

Types of Pain - Stimulation of skin & mucous membranes Superficial: - Stimulation of skin & mucous membranes - Fast response Deep: - Arises from muscles, joints, tendons, heart ..etc. - Slow response

Analgesics Definition Classification Drugs that selectively inhibit the perception (sensation) of the pain Classification 1- Peripheral (miscellaneous): - Causal - Non-causal 2- Central: - Narcotic - Non-narcotic

Peripheral Analgesics Causal -Treat the cause Example: Atropine (antispasmodic) Non-causal - Not treat the cause Examples: 1- Local anaesthetics (for superficial tumor) 2- Counter-irritant (apply pain that counteract or mask the original one e.g. acupuncture)

Central Analgesics Narcotics Non-narcotic The class - Opioids (morphine & morphine like drugs) Examples 1- Natural (as codeine) 2- Semi synthetic e.g. di-hydromorphine& diacetylmorphine (heroin) 3- Synthetic e.g. pethidine 4- Endogenous opiates as endorphins & encephalins Non-narcotic - NSAID 1- Aspirin 2- Paracetamol 3- Diclofenac 4- Piroxicam 5- Ibuprofin 6- Ketoprofin

Central Analgesics Narcotics Non-narcotic Site of Cortex & thalamus action Antagonist Naloxone, nalorphine & levallorphan Uses Sever & deep pain e.g. cancer, MI & anginal pian Potency High Side effect Addiction MOA ‡ of opiates receptors (m, k ,s, d) and relief the pain through the release of endorphines & encephalins Non-narcotic Subcortical “thalamus” No antagonist Dull pain e.g. headache, toothache & backache Low No addiction. ↑in bleeding tendency & ulcer Inhibits prostaglandin synthesis by inhibition of cycloxygenase enzyme

Screening Methods For Analgesics Principle Pain is induced to a suitable animal and the response of the animal to the painful stimuli is recorded before and after administration

Screening methods Narcotics: 1- Thermal method a- Hot plate b- Tail flick 2- Mechanical method

Screening methods Non-narcotic: 1- Electrical method 2- Chemical ( Writhing method)

Hot – plate Material Animal: Mouse Instrument: Hot plate analgesiometer Painful stimulus: Heat (55°C) Drug used: Morphine

Tail-flick Material Animal: Mouse Instrument: Tail-flick analgesiometer Painful stimulus: Heat (by apply a beam of light 130°C) Drug used: Morphine

Writhing Method Principle: The painful stimulus is induced by IP injection of an irritant substance (e.g. acetic acid) Writhing: Stretching of the body, withdrawing of the limb, retraction of the abdomen & the stomach touches the ground

Writhing Method Material Animal: Mouse Painful stimulus: Chemicals e.g. acetic acid Drug used: NSAID e.g. Na salicylate Drug Onset of writhing No. of writhing Acetic acid (control) Na salicylate (100mg/kg) Na salicylate (200mg/kg)