Contribution of alpha 2-adrenoceptors to the central cardiovascular effects of clonidine and S 8350 in anaesthetized rats.
1. The alpha 2-adrenoceptor agonist clonidine elicits centrally mediated effects through an interaction with both alpha 2-adrenoceptors and imidazoline binding sites. 2. We selected a new oxazoline derivative, S 8350, which competes with [3H]-yohimbine for binding to cerebral alpha 2-adrenoceptors (IC50, 67 +/= 17 nmol/L) and displays a higher affinity (35-fold) for alpha 2- than for alpha 1-adrenoceptors. 3. As observed for clonidine, intravenous (i.v.) administration of S 8350 resulted in a brief pressor effect followed by a prolonged hypotension. When S 8350 was administered i.v. to spinally pithed rats, only a rise in blood pressure was observed. 4. In order to discriminate the cardiovascular effects related to the central imidazoline receptor or alpha 2-adrenoceptor activation, the effects of intracisternal (i.c.) administration of clonidine and S 8350 were investigated in the rat. 5. In the anaesthetized rat, both clonidine and S 8350 displayed a profound central (i.c. route) hypotensive effect associated with a bradycardia. 6. The cardiovascular effects of S 8350 were abolished by the central administration of the selective alpha 2-adrenoceptor antagonist rauwolscine. Conversely, rauwolscine completely prevented bradycardia but it induced only a partial reversion of the hypotension elicited by clonidine. 7. These results suggest that central alpha 2-adrenoceptors are responsible for hypotension and bradycardia while imidazoline binding sites do not apparently contribute to heart rate control.
Deckert
V
V
Laboratoire de Pharmacologie, CNRS 161670, Facult è de Mèdecine Necker, Paris, France.
Lachaud
V
V
Parini
A
A
Elghozi
J L
JL
D000316:Adrenergic alpha-Agonists / Q000378:metabolism / Q000494:pharmacology; D000317:Adrenergic alpha-Antagonists / Q000378:metabolism / Q000494:pharmacology; D000818:Animals; D001667:Binding, Competitive; D001794:Blood Pressure / Q000187:drug effects; D002319:Cardiovascular System / Q000187:drug effects*; D002540:Cerebral Cortex / Q000378:metabolism / Q000648:ultrastructure; D003000:Clonidine / Q000494:pharmacology*; D006339:Heart Rate / Q000187:drug effects; D008297:Male; D010080:Oxazoles / Q000494:pharmacology*; D051381:Rats; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D015016:Yohimbine / Q000378:metabolism / Q000494:pharmacology
D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D010080:Oxazoles; D011942:Receptors, Adrenergic, alpha; D015016:Yohimbine; C071199:2-((1-cyclohexyl-2,2,2-trifluoro)ethylamino)oxazoline; D003000:Clonidine