Evidence that hypophagia induced bymCPP and TFMPP requires 5-HT1C and 5-HT1B receptors; hypophagia induced by RU 24969 only requires 5-HT1B receptors

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Abstract
Male Sprague-Dawley rats deprived of food for 18 h were injected with the 5-HT agonists RU 24969, 1-(3-chlorophenyl)piperazine (mCPP) or 1-[3-(trifluoromethyl)phenyl)]piperazine (TFMPP) and 20 min later presented with their normal diet. Food intake was determined 1, 2 and 4 h later. All three drugs reduced intake over 1 and 2 h. Three out of four drugs with high affinity for 5-HT1C receptors (metergoline, mianserin, and mesulergine but not cyproheptadine) opposed hypophagia caused bymCPP. Another drug reported to have high affinity for the 5-HT1C site, 1-naphthyl-piperazine (1-NP), also blocked the hypophagic response tomCPP at doses which attenuatedmCPP-induced hypolocomotion. Only one of the above drugs (metergoline) which also has high affinity for other 5-HT sites opposed hypophagia caused by RU 24969. Two out of three 5-HT1B receptor antagonists [(±) cyanopindolol, (−) propranolol, but not (−) pindolol)] which oppose hypophagia caused by RU 24969 (Kennett et al. 1987) also opposed hypophagia caused bymCPP. The 5-HT2 antagonists ketanserin and ritanserin, the 5-HT3 antagonist ICS 205-930 and the α2 adrenoceptor antagonist idazoxan did not oppose the hypophagic effect ofmCPP. In agreement with results formCPP, hypophagia caused by TFMPP was opposed by both, mianserin and (±) cyanopindolol. Given alone, mianserin 1-NP and cyproheptadine but not ICS 205-930 increased food consumption of normally fed rats. The results suggest that RU 24969-induced hypophagia depends on 5-HT1B receptors but not on 5-HT1C receptors, whilemCPP (and TFMPP)-induced hypophagia may depend on both receptors. Thus, 5-HT1C and 5-HT1B receptors may evoke hypophagia via a common pathway but the effect of antagonists implies that at the doses usedmCPP and TFMPP act predominantly at 5-HT1C receptors. Since only the hypophagic response tomCPP is blocked by cyanopindolol and (−) propranolol (Kennett and Curzon 1988) it is unlikely to be secondary to hypoactivity induced by the drug.