Contribution of the ascending cholinergic pathways in the production of ultrasonic vocalization in the rat.

It has been well documented that cholinergic stimulation of the mediobasal forebrain structures induces 20-30 kHz ultrasonic vocalization in adult rats. If the cholinergic system plays a triggering role for ultrasonic vocalization, the question arises as to where the source of the cholinergic fibres, which innervate the mediobasal forebrain and induce vocalization, is located. In the present study, the role of the ascending cholinergic projection from the ponto-mesencephalic cholinergic nuclei to the mediobasal hypothalamic-preoptic region in production of 22 kHz calls was investigated. Cholinergic neurons were stimulated by local injection of L-glutamate and eventual vocalization was recorded by a S200 bat detector and analyzed sonographically. Intracerebral injection of L-glutamate into the laterodorsal tegmental nucleus induced short latency, 20-30 kHz ultrasonic calls. Sound frequency (pitch) and single call duration of the L-glutamate-induced vocalization did not differ from those obtained by cholinergic stimulation of the mediobasal hypothalamic-preoptic region with carbachol. However, L-glutamate stimulation of the laterodorsal tegmental nucleus was ineffective or less effective in 70% of responses, when the terminal fields in the mediobasal hypothalamic-preoptic region were pretreated with scopolamine, a muscarinic antagonist. The results demonstrate that the ascending cholinergic projection from the laterodorsal tegmental nucleus plays a triggering role for 20-20 kHz vocalization in adult rats.