LAUSR

&NA; Rats can produce ultrasonic vocalizations (USVs) in a variety of different contexts that signal their emotional state to conspecifics. Under distress, rats can emit 22‐kHz USVs, while during positive pro‐social interactions rats can emit frequency‐modulated (FM) 50‐kHz USVs. It has been previously reported that rats with increasing emission of FM 50‐kHz USVs in anticipation of rewarding electrical stimulation or positive pro‐social interaction decrease the number of emitted 22‐kHz USVs. The purpose of the present investigation was to determine, in a pharmacological‐behavioural experiment, if the positive emotional arousal of the rat indexed by the number of emitted FM 50‐kHz USVs can decrease the magnitude of a subsequent negative emotional state indexed by the emission of 22‐kHz USVs. To induce a positive emotional state, an intracerebral injection of a known D1/D2 agonist R‐(‐)‐apomorphine (3.0 &mgr;g/0.3 &mgr;l) into the medial nucleus accumbens shell was used, while a negative emotional state was induced by intracerebral injection of carbachol (1.0 &mgr;g/0.3 &mgr;l), a known broad‐spectrum muscarinic agonist, into the anterior hypothalamic‐medial preoptic area. Our results demonstrated that initiation of a positive emotional state was able to significantly decrease the magnitude of subsequently expressed negative emotional state measured by the number of emitted 22‐kHz USVs. The results suggest the neurobiological substrates that initiate positive emotional state indirectly antagonize the brain regions that initiate negative emotional states.