Alpha 2A-adrenergic receptor signaling underlies synergistic enhancement of ethanol-induced behavioral impairment by clonidine.
BACKGROUND
We tested the hypothesis that central alpha(2A)-adrenergic receptor (alpha(2A)AR) signaling plays a key role in clonidine-ethanol evoked synergistic behavioral impairment.


METHODS
Male Sprague-Dawley rats, with intracisternal and jugular vein cannulae implanted 6 days earlier, were tested for drug-induced behavioral impairment. The latter was assessed as the duration of loss of righting reflex (LORR) and rotorod performance every 15 minutes until the rat recovered to the baseline walk criterion (180 seconds). In a separate cohort, c-Fos expression in locus coeruleus (LC) and cerebellum was determined as a marker of neuronal activity following drug treatment.


RESULTS
Rats that received clonidine (60 microg/kg, i.v.) followed by ethanol (1 g/kg, i.v.) exhibited synergistic impairment of rotorod performance and LORR. The mixed alpha(2A)AR and I(1)-imidazoline receptor agonist clonidine (30, 60, and 90 microg/kg) synergistically and dose-dependently enhanced behavioral impairment elicited by ethanol (1 g/kg). Possible involvement of I(1)-imidazoline receptors was ruled out because selective I(1)-agonist rilmenidine (300 microg/kg, i.v.) did not cause behavioral impairment alone or enhance ethanol-evoked behavioral impairment. Pharmacological blockade of central alpha(2A)AR (RX821002, 0.3 mg i.c.) abolished the synergy between clonidine and ethanol; the behavioral response caused by the drug combination was similar to that caused by ethanol alone. Conversely, involvement of central alpha(2B)AR in the interaction was ruled out because blockade of central alpha(2B)AR (ARC-239) independently evoked a strong sedative effect. Clonidine (60 microg/kg) or ethanol (1 g/kg) alone increased, but their combination decreased, c-Fos levels in LC, while inconsistent c-Fos responses were observed in cerebellum.


CONCLUSIONS
Central alpha(2A)AR, but not I(1)-imidazoline or alpha(2B)AR, signaling is implicated in the synergistic enhancement of ethanol-evoked behavioral impairment by clonidine. Although the mechanism of c-Fos response remains to be investigated, this neurochemical response highlights the LC as a neuroanatomical target for clonidine-ethanol behavioral interaction.
Bender
Tara Summer
TS
Department of Pharmacology and Toxicology, Brody School of Medicine, East Carolina University, Greenville, North Carolina 27834, USA.
Abdel-Rahman
Abdel A
AA
D058647:Adrenergic alpha-2 Receptor Agonists; D058669:Adrenergic alpha-2 Receptor Antagonists; D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects*; D002492:Central Nervous System Depressants / Q000009:adverse effects*; D002531:Cerebellum / Q000187:drug effects / Q000378:metabolism; D003000:Clonidine / Q000009:adverse effects*; D004357:Drug Synergism; D000431:Ethanol / Q000009:adverse effects*; D054755:Imidazoline Receptors / Q000819:agonists / Q000378:metabolism; D008125:Locus Coeruleus / Q000187:drug effects* / Q000378:metabolism; D008297:Male; D009043:Motor Activity / Q000187:drug effects; D010080:Oxazoles / Q000494:pharmacology; D016760:Proto-Oncogene Proteins c-fos / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D018341:Receptors, Adrenergic, alpha-2 / Q000378:metabolism*; D012018:Reflex / Q000187:drug effects; D000077769:Rilmenidine; D045442:Rotarod Performance Test
C547770:Adra2a protein, rat; D058647:Adrenergic alpha-2 Receptor Agonists; D058669:Adrenergic alpha-2 Receptor Antagonists; D002492:Central Nervous System Depressants; D054755:Imidazoline Receptors; D010080:Oxazoles; D016760:Proto-Oncogene Proteins c-fos; D018341:Receptors, Adrenergic, alpha-2; C410887:imidazoline I1 receptors; D000431:Ethanol; D003000:Clonidine; D000077769:Rilmenidine