A comparison of different sensory stimuli in producing conditioned apomorphine effects

In previous studies, it was shown that apomorphine-induced stereotyped behaviour could be conditioned when apomorphine was repeatedly paired with sensory stimuli (CS). Since in these experiments, the sum of various sensory stimuli were applied, it seemed of interest to use each sensory stimulus separately in order to evaluate the relevance of each of the stimuli for the development of the conditioned responses (CRs). Therefore, apomorphine (0.5mg/kg s.c.) was repeatedly (six times) paired either with an auditory, an olfactory, a tactile or a compound (auditory + olfactory + tactile) stimulus. On the seventh (drug-free test) day, the rats were injected with solvent in the presence of the CS previously applied. It was shown that the olfactory stimulus alone when used as CS produced similar CRs (in particular, stereotyped sniffing and licking) as the compound stimulus, whereas the other stimuli applied did not noticeably contribute to the development of the CRs. Furthermore, similarly to the compound stimulus, the olfactory stimulus, but not the auditory or tactile one, enhanced the apomorphine-induced stereotypies in the presence of the CS. This result suggests that the application of olfactory stimuli might be of particular relevance for the development of conditioned dopaminergic responses.