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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
Author(s) -
Rees H.,
Prado W.A.,
Rawlings S.,
Roberts M.H.T.
Publication year - 1987
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1987.tb11380.x
Subject(s) - idazoxan , methysergide , pharmacology , yohimbine , phenoxybenzamine , ketanserin , (+) naloxone , medicine , stimulation , endocrinology , chemistry , morphine , opioid , prazosin , nociception , antagonist , 5 ht receptor , receptor , serotonin
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 μA 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 β‐adrenoceptor antagonist propranolol (1 mg kg −1 ) had little effect on the antinociceptive effects of stimulating the APtN. 5 α‐Adrenoceptor antagonists caused a dose‐dependent antagonism of the response. The order of potency was; idazoxan > prazosin > phenoxybenzamine, the respective ED 50 for each drug being 0.08: 0.45: 1.5 mg kg −1 . 6 It is concluded that antagonism at opioid receptors and α‐adrenoceptors but not β‐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.