Supraspinal administration of [D-Met2]FMRFamide produces a naloxone-sensitive increase in heart rate in unrestrained spontaneously hypertensive rats.

Intracerebroventricular (i.c.v.) administration of either Phe-Met-Arg-Phe-NH2 (FMRFamide; molluscan cardioexcitatory neuropeptide; 3-30 micrograms) or the FMRFamide analog Phe-D-Met-Arg-Phe-NH2 ([D-Met2]FMRFamide; 15 micrograms) to conscious unrestrained spontaneously hypertensive rats (SHR) produced a relatively long lasting (greater than 1 h) increase in heart rate. The increase in heart rate produced by [D-Met2]FMRFamide was attenuated by i.c.v. injection of the opiate antagonist naloxone (2 micrograms). These results extend to a second endpoint an apparent opioid agonist-like (naloxone-reversible) action of [D-Met2]FMRFamide.