Dissimilar interaction between dopaminergic and cholinergic systems in the initiation of emission of 50-kHz and 22-kHz vocalizations

Abstract

Rats emit 22-kHz or 50-kHz ultrasonic vocalizations (USVs) to signal their emotional state to other conspecifics. The 22-kHz USVs signal a negative emotional state while 50-kHz USVs reflect a positive affective state. The initiation of 22-kHz USVs is dependent on the activity of cholinergic neurons within the laterodorsal tegmental nucleus that release acetylcholine along the medial cholinoceptive vocalization strip. Emission of 50-kHz USVs is dependent upon the activation of dopaminergic neurons located within the ventral tegmental area that release dopamine into the medial shell of the nucleus accumbens. There have been reports that showed an antagonistic interaction between acetylcholine and dopamine during the expression of emotional states, and dopamine agonists decreased carbachol-induced emission of 22-kHz USVs. The current study tests the hypothesis that initial antagonism of dopamine receptors by systemic haloperidol or intraacumbens raclopride should increase the subsequent emission of 22 kHz USVs induced by carbachol from the lateral septum. Our findings showed that antagonism of dopaminergic signaling either via systemic haloperidol or via intracerebral raclopride did not alter the number of emitted 22-kHz USVs. Thus, inhibition of the mesolimbic dopamine system did not increase the magnitude of a negative emotional state. It was found, however, that prolonged emission of 22-kHz USVs initiated by carbachol caused a delayed rebound emission (R) of 50-kHz USVs appearing after 300 s of emission of 22-kHz USVs, i.e., when the response was subsiding. The R-50-kHz USVs were predominantly frequency modulated (FM) USVs and their number was directly proportional to the number of recorded 22-kHz USVs. The emission of R-50-kHz USVs was reversed by systemic pretreatment with haloperidol or intraacumbens injection of raclopride. It is argued that the R-50-kHz USVs represent a rebound emotional state that is opposite in valence and arousal induced by carbachol. Importantly, prolonged emission of amphetamine-induced 50 kHz USVs failed to show any vocalization rebound effect.