Sleeping times evoked by alpha adrenoceptor agonists in two-day-old chicks: an experimental model to evaluate full and partial agonists at central alpha-2 adrenoceptors.

The ability of a series of alpha adrenoceptor agonists to induce a sleep-like state (as measured by the time interval between the loss and regaining of the righting reflex) has been assessed in 2-day-old chicks in order to understand their pharmacological profile better. Guanabenz, guanoxabenz, UK-14,304, guanfacine and xylazine produced dose-related increases in sleeping time, the highest dose of these agonists causing the chicks to sleep for over 120 min. In contrast, the dose-response curves to tiamenidine and clonidine were flatter and bell-shaped with maxima of 30 and 60 min, respectively. The effects of all these compounds were antagonized by idazoxan (RX781094) and yohimbine (two selective alpha-2 adrenoceptor antagonists) but were moderately enhanced or unaffected by prazosin (a selective alpha-1 adrenoceptor antagonist) confirming that the state of arousal in chicks can be depressed by stimulation of alpha-2 adrenoceptors. In particular, idazoxan displaced significantly the guanoxabenz dose-response curve to the right without affecting its slope and apparent maximum and blocked the sleep induced by clonidine. However, idazoxan failed to affect the sleep evoked by ethanol, etorphine or pentobarbital. Naloxone antagonized the effects of etorphine but not those of guanoxabenz, ethanol or pentobarbital. The relatively selective alpha-1 adrenoceptor agonist, cirazoline, given in doses up to 20 mg/kg i.m., produced in chicks behavioral manifestations suggestive of enhanced arousal.(ABSTRACT TRUNCATED AT 250 WORDS)