Prolactin-induced yawning behavior requires an intact nigro-striatal dopamine system

Herein, we evaluate the importance of the nigro-striatal dopamine system in prolactin-, apomorphine-, and physostigmine-induced yawning behavior. Bilateral 6-OH-dopamine lesions of the substantia nigra were performed on male rats (2-4 months old). The lesioned as well as control rats were injected with either physiological saline, physostigmine (200 micrograms/kg), apomorphine (50 micrograms/kg), or ovine prolactin (0.25 micrograms/kg) 72 hours after the surgical procedure. The results show that bilateral lesions of the substantia nigra did not affect physostigmine-induced yawning whereas both apomorphine- and prolactin-induced yawning were reduced by the lesion. Following the observation period the caudate nuclei were removed and analyzed for dopamine (DA) and dihydroxyphenylacetic acid (DOPAC) content. The lesions reduced DA and DOPAC content in all treatment groups compared to the respective intact groups. Also, both DA and DOPAC concentrations were lower in the intact apomorphine and prolactin treated groups compared to intact saline controls, at times that were temporally related to the display of yawning behavior suggesting a decrease in dopamine activity following apomorphine and prolactin treatment. Interestingly, DA and DOPAC concentrations were higher in the lesioned apomorphine group compared to lesioned saline controls; however, in the lesioned prolactin group only the DA concentrations were higher when compared to lesioned saline controls. These results indicate that prolactin- and apomorphine-induced yawning require an intact nigro-striatal dopamine system and that these substances induce yawning by different mechanisms.