| The mechanisms underlying a clinically relevant synergistic behavioral impairment caused by clonidine and ethanol were investigated. A model was developed that permitted dose-related evaluation of rotorod performance and loss of righting reflex (LORR) in male, Sprague-Dawley rats. The following contributing factors were determined: (i) prior surgical procedures significantly enhanced clonidine-ethanol evoked behavioral impairment, (ii) injection interval was inversely related to behavioral impairment, and (iii) clonidine did not affect blood ethanol levels. Investigation of mechanisms underlying clonidine-ethanol behavioral synergy revealed that: (i) increasing levels of alpha2-adrenergic receptor (AR) activation (30, 60, and 90 mug/kg clonidine, i.v.) caused proportionate increases in behavioral impairment elicited by ethanol (1 g/kg, i.v.), (ii) the synergistic behavioral interaction was only evident when clonidine was administered prior to ethanol, (iii) involvement of the imidazoline (I1) receptor, also activated by clonidine, was ruled out, and (iv) pharmacological blockade of the central alpha 2A-AR with RX821002 abolished the clonidine, but not the ethanol; component of the behavioral response, while blockade of the central alpha2B-AR with ARC-239 independently evoked a strong sedative effect. Together, these results indicate that prior activation of the central alpha2B-AR, but not the I1-imidazoline or alpha2B-AR, is required to elicit synergistic behavioral impairment by clonidine-ethanol combination. Analysis of neurochemical effects (c-Fos expression in the locus ceruleus) of clonidine, ethanol, or their combination suggests that the alpha 2A-AR is involved in diverse signaling pathways to modulate ethanol-induced c-Fos expression. The contribution of p-ERK1/2 to the synergistic interaction was briefly studied. Finally, the involvement of NOS-NO signaling in the behavioral interaction between ethanol and clonidine was investigated using non-selective NOS (No-nitro-L-arginine methyl ester. L-NAME), selective nNOS (N-propyl-L-arginine, NPLA) and selective eNOS (N(5)-(1-iminoethyl)-L-ornithine, L-NIO) inhibitors. NOS-derived NO is differentially involved in the underlying mechanisms for clonidine-ethanol induced behavioral impairment, such that a decrease in nNOS-derived NO contributes to impaired rotorod performance, while an increase in eNOS-derived NO contributes to LORR duration elicited by clonidine-ethanol combination. The present findings yielded new mechanistic insight into the role of central alpha 2A-AR-NOS signaling in the synergistic behavioral interaction between clonidine and ethanol. |
