Effects on hamster vocalization and aggression of carbachol injections into the MPOA/AH

Recent work has shown that microinjections of the cholinergic agonist carbachol into the preoptic area (MPOA) increase 20–30 kHz ultrasonic vocalization in rats. This response could be interpreted as a quite specific and direct effect on a central mechanism for vocalization or ultrasound production. Alternatively, it could reflect a more general drug-induced increase in defensiveness or defensive aggression. Two studies were conducted to distinguish these possibilities. In both, MPOA carbachol treatments very similar to those in previous work (unilateral injections into the MPOA/AH of 1 µg of carbachol in 0.2 µl of saline) were administered to female hamsters. The first study focused on the rate of ultrasonic courtship vocalization, finding it to decrease after treatment. This result is inconsistent with the view that cholinergic agonists directly stimulate vocalization. In contrast, although it does not completely exclude an effect on a mechanism for vocalization, it provides at least suggestive support for an interpretation in terms of defensiveness: Because hamster ultrasounds convey sexual readiness rather than alarm or aggressiveness, they would be expected to decrease in the event of a drug-induced increase in an incompatible motivational or emotional state. The second study exploited the aggressiveness of hamsters to even more directly test carbachol's ability to increase defensiveness or defensive aggression. Its results showed that minimally-estrous female subjects bit their male pursuers more quickly if recently treated with carbachol rather than placebo. Taken together, both results support the existence of a cholinergic system that extends through the MPOA and regulates defensiveness or a related motivational or emotional state.