Presynaptic inhibition of glutamate transmission by a2 receptors in the VTA

The ventral tegmental area (VTA) forms part of the mesocorticolimbic system and plays a pivotal role in reward and reinforcing actions of drugs of abuse. Glutamate transmission within the VTA controls important aspects of goal-directed behavior and motivation. Noradrenergic receptors also present in the VTA have important functions in the modulation of neuronal activity. Here we studied the effects of a2 noradrenergic receptor activation in the alteration of glutamate neurotransmission in VTA dopaminergic neurons from male SpragueDawley rats. We used whole-cell patch-clamp recordings from putative VTA dopaminergic neurons and measured excitatory postsynaptic currents. Clonidine (40 mu m) and UK 14,304 (40 mu m), both a2 receptor agonists, reduced (approximately 40%) the amplitude of glutamate-induced excitatory postsynaptic currents. After clonidine administration, there was a dose-dependent reduction over the concentration range of 15-40 mu m. Using yohimbine (20 mu m) and two other a2 adrenergic receptor antagonists, idaxozan (40 mu m) and atipemazole (20 mu m), we demonstrated that the inhibitory action is specifically mediated by a2 receptors. Moreover, by inhibiting protein kinases with H-7 (75 mu m), Rp-adenosine 3',5'-cyclic (11 mu m) and chelerythrine (1 mu m) it was shown that the clonidine-induced inhibition seems to involve a selective activation of the protein kinase C intracellular pathway. Increased paired-pulse ratios and changes in spontaneous and miniature excitatory postsynaptic current frequencies but not amplitudes indicated that the effect of the a2 agonist was presynaptically mediated. It is suggested that the suppression of glutamate excitatory inputs onto VTA dopaminergic neurons might be relevant in the regulation of reward and drug-seeking behaviors.