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Abstract
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.