VPT 321: Veterinary Neuropharmacology (2+1) copy 1

OPIOIDS-INTRODUCTION AND CHEMISTRY

• Narcotic is a Greek word used for stupor and this word narcotic is most appropriately used for any drug that induces sleep. Opioid drugs are commonly used as narcotic analgesics.
• Opium has been used in medicine since the dawn of history. Opium is the air-dried milky exudates obtained from the incised unripe seed capsules of the poppy plant Papaver somniferum.
• Opium contains about 24 alkaloids but only a few like morphine and codeine are of importance. The alkaloids may be phenantherenes like morphine, codeine and thebaine or benzylisoquinolines like papaverine and noscapine.
• The term opioid is applied to any substance, whether endogenous or synthetic, that produces morphine-like effects that are blocked by antagonists such as naloxone.
• The older term, opiate, is restricted to synthetic morphine-like drugs with non-peptidic structures.
• Tincture of opium is known as laudanum and camphorated tincture of opium is known as paregoric. Paregoric is used in the treatment of diarrhoea.

Chemistry

• Morphine  was the first plant alkaloids that was isolated. Morphine molecule consists of a partially hydrogenated phenantherene nucleus.
• In addition two hydroxy groups (alcoholic and phenolic) are important in maintaining the pharmacological integrity of the morphine molecule.
  • Alteration in the phenolic hydroxy group reduces analgesic potency, respiratory depression and likelihood of constipation. A stimulant activity upon the CNS is noted when substitution is made in this position.
  • If substitution is made in the alcoholic hydroxy position, narcotic and respiratory depressions are enhanced. Eg: Hydromorphone
  • Substitution in either of the hydroxy positions lessens the emetic activity of the parent molecule. Codeine and hydromorphone are less potent than morphine in producing emesis.
  • Semisynthetic derivatives of morphine are apomorphine hydrochloride, a potent emetic agent and naloxone hydrochloride, an antagonist of opiate drugs that have important clinical applications.