alpha-MSH exhibits opioid antagonist-like effects in the brainstem of rats

Unilateral microinjections of alpha-MSH (0.3, 1.2 and 12 pmol) into the nucleus tractus solitarius (NTS) of urethane-anaesthetized rats did not modify blood pressure or heart rate (HR). Using a dual microinjection technique, it has been shown that prior injection of alpha-MSH (0.3 pmol) attenuated the pressor effect of a similar injection of dynorphin 1-9 (18 pmol) but did not modify the cardiovascular effects of [Met]enkephalin (14 pmol). Since alpha-MSH has been localized in the NTS, the results indicate that this peptide may play a role in central cardiovascular control, possibly acting in an antagonistic manner to the endogenous opioid peptides.