The effects of intraperitoneal administration of antagonists and development of morphine tolerance on the antinociception induced by stimulating the anterior pretectal nucleus of the rat.
1 The effects of intraperitoneal administration of antagonists to morphine, 5-hydroxytryptamine (5-HT), noradrenaline and dopamine have been studied on the antinociceptive effects of electrical stimulation of the anterior pretectal nucleus (APtN) of the rat. 2 A 15 s period of 35 microA sine wave stimulation of APtN significantly increased the latency of the tail flick reflex to noxious heat for periods up to 1 h. 3 Naloxone (0.25-1.0 mg kg-1) attenuated the effects of APtN stimulation in a dose-dependent manner. In rats made tolerant to morphine by daily administration of morphine, the antinociceptive effects of APtN stimulation were significantly reduced. 4 The 5-HT receptor antagonists methysergide (5 mg kg-1) and ketanserin (1 mg kg-1), the dopamine receptor antagonist haloperidol (1 mg kg-1) and the beta-adrenoceptor antagonist propranolol (1 mg kg-1) had little effect on the antinociceptive effects of stimulating the APtN. 5 alpha-Adrenoceptor antagonists caused a dose-dependent antagonism of the response. The order of potency was; idazoxan greater than prazosin greater than phenoxybenzamine, the respective ED50 for each drug being 0.08: 0.45: 1.5 mg kg-1. 6 It is concluded that antagonism at opioid receptors and alpha-adrenoceptors but not beta-adrenoceptors, dopamine or 5-HT receptors reduces the antinociceptive effects of APtN stimulation. This differs from the reported effects of these antagonists on the antinociception caused by stimulating other sites in the brain.
D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000319:Adrenergic beta-Antagonists / Q000494:pharmacology; D000700:Analgesics; D000818:Animals; D004305:Dose-Response Relationship, Drug; D004361:Drug Tolerance; D004558:Electric Stimulation; D006633:Histamine Antagonists / Q000494:pharmacology; D007274:Injections, Intraperitoneal; D008297:Male; D008636:Mesencephalon / Q000502:physiology*; D009020:Morphine / Q000494:pharmacology*; D009270:Naloxone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000008:administration & dosage / Q000494:pharmacology*; D051381:Rats; D011919:Rats, Inbred Strains; D011930:Reaction Time / Q000187:drug effects; D011954:Receptors, Dopamine / Q000494:pharmacology
D000317:Adrenergic alpha-Antagonists; D000319:Adrenergic beta-Antagonists; D000700:Analgesics; D006633:Histamine Antagonists; D009292:Narcotic Antagonists; D011954:Receptors, Dopamine; D009270:Naloxone; D009020:Morphine