Dexmedetomidine, the highly selective alpha 2-adrenergic agonist, produces a dose-dependent hypnotic response in rats through a central mechanism. Because the locus coeruleus (LC) contains pathways involved in the maintenance of vigilance and a high prevalence of alpha 2 adrenoceptors, we investigated the role of this brainstem nucleus in the hypnotic response to dexmedetomidine. The experimental model consisted of chronic, stereotactically cannulated rats (n = 157) in which the hypnotic response to dexmedetomidine was assessed by the duration of the loss of their righting reflex. Correct placement of the cannula was confirmed histologically at necropsy. The hypnotic response to dexmedetomidine 0.3-333.3 micrograms administered into the LC increased in a dose-dependent fashion. Dexmedetomidine 6.6 micrograms injected 2 mm lateral to the LC did not cause the animals to lose their righting response. Atipamezole 0.07 micrograms-12 micrograms, a selective alpha 2-adrenergic antagonist, blocked the hypnotic response to dexmedetomidine 6.6 micrograms when both were administered into the LC. Also, atipamezole 0.7-30 micrograms, administered into the LC, blocked in a dose-dependent manner the hypnotic response to intraperitoneal (ip) dexmedetomidine 50 micrograms.kg-1. Atipamezole injected into the LC did not block the hypnotic response to pentobarbital 40 mg.kg-1 ip. Prazosin, an alpha 1-adrenergic antagonist, 4.2 micrograms into the LC or 1.0 mg.kg-1 ip, did not alter the hypnotic response to dexmedetomidine 6.6 micrograms into the LC. The present data suggest that alpha 2-adrenergic receptors in the LC appear to be a major site for the hypnotic action of dexmedetomidine.(ABSTRACT TRUNCATED AT 250 WORDS)