Membrane resistance increase induced in thalamic neurons by stimulation of brainstem cholinergic afferents

The synaptic response induced in thalamic relay cells by stimulation of peribrachial cholinergic afferents was studied in cats maintained under urethane anesthesia. Recordings were performed in the ventral lateral nucleus after destruction of the reticular thalamic complex and cortical lesion. Some animals were also pretreated with reserpine (1.2 mg/kg) 24 h before recording sessions. Repetitive stimulation (25-60 Hz for 200-600 ms) of the peribrachial area produced a slow depolarization and tonic firing in thalamic cells. The depolarization was associated with an increase in membrane resistance and its amplitude decreased during membrane hyperpolarization. The reactivity of thalamic relay cells to a corticothalamic volley was also increased during the slow depolarization. Similar results were obtained in reserpinized animals. On the basis of previous data obtained in vitro, it is concluded that the slow depolarization resulted from the activation of muscarinic receptors by brainstem cholinergic afferents.