Investigations of the cholinergic modulation of GABA release in rat thalamus slices

The thalamus receives a dense cholinergic projection from the pedunculopontine tegmentum. A number of physiological studies have demonstrated that this projection causes a dramatic change in thalamic activity during the transition from sleep to wakefulness. Previous anatomical investigations have found that muscarinic type 2 receptors are densely distributed on the dendritic terminals of GABAergic interneurons, as well as the somata and proximal dendrites of GABAergic cells in the thalamic reticular nucleus. Since these structures are the synaptic targets of cholinergic terminals in the thalamus, it appears likely that thalamic pedunculopontine tegmentum terminals can activate muscarinic type 2 receptors on GABAergic cells. To test whether activation of muscarinic type 2 receptors affects the release of GABA in the thalamus, we have begun pharmacological studies using slices prepared from the rat thalamus. We have found that the application of the nonspecific muscarinic agonist, methacholine, and the muscarinic type 2-selective agonist, oxotremorine.sesquifumarate, diminished both the baseline, and K(+) triggered release of [(3)H]GABA from thalamic slices. This effect was calcium dependent, and blocked by the nonselective muscarinic antagonist atropine, the muscarinic type 2-selective antagonist, methoctramine, but not the muscarinic type 1 antagonist, pirenzepine. Thus, it appears that one function of the pedunculopontine tegmentum projection is to decrease the release of GABA through activation of muscarinic type 2 receptors. This decrease in inhibition may play an important role in regulating thalamic activity during changes in states of arousal.