Apomorphine enhances conditioned responses induced by aversive stimulation of the inferior colliculus

Consistent evidence has shown that learning may be produced in paradigms using electrical stimulation of the inferior colliculus (IC) as unconditioned stimulus (UCS). Recent reports have also demonstrated that aversive stimulation of the IC, at the escape threshold, enhances dopamine (DA) release in the prefrontal cortex. The purpose of the present study was to determine whether dopaminergic mechanisms are involved in the Pavlovian conditioning and latent inhibition using IC stimulation as UCS and light as conditioned stimulus (CS). Rats were placed inside a shuttle box and subjected to a two-way avoidance paradigm. IC aversive electrical stimulation was used as UCS and shuttle box illumination as CS. The rats quickly learned to avoid or interrupt the IC stimulation. Apomorphine injections produced a dose-dependent increase in the number of avoidance responses. On the other hand, chlorpromazine administration promoted a dose-dependent reduction of the avoidance responses. Previous injections of chlorpromazine inhibited the effects of apomorphine. Also, previous exposure to unreinforced light weakened the strength of the conditioning. Apomorphine blocked this latent inhibition effect, which was antagonized by previous injections of chlorpromazine. These findings bring evidence for the involvement of DA in the setting up of adaptive responses to aversive states generated at the IC level, which may underlie stressful situations present in anxiety.