Behavioral effects of centrally administered LH-RH agonist in rats.

The neuropharmacological actions of the agonist analog D-Trp-6-LH-RH were investigated in several tests after intracerebroventricular (ICV) administrations to male rats. The doses applied were 10, 100 and 1000 ng/animal. In the open field the 1000 ng ICV dose of the peptide D-Trp-6-LH-RH suppressed the ambulation, rearing and grooming. In a combined catalepsy test, the 10 ng and 1000 ng dose of D-Trp-6-LH-RH increased the total duration of immobility. The LH-RH agonist inhibited stereotyped behavior induced by both apomorphine and amphetamine, and the effects of 100 and 1000 ng D-Trp-6-LH-RH were significant. Naloxone in a dose of 0.5 mg/kg IP totally abolished the inhibition of apomorphine-induced stereotypy by 1000 ng D-Trp-6-LH-RH, but the opiate antagonist did not influence amphetamine-induced stereotypy but significantly potentiated the inhibitory effect of 100 ng D-Trp-6-LH-RH. In the tail-flick test the latencies were significantly increased after D-Trp-6-LH-RH ICV, both 20 or 40 min after the injections. The peptide-induced analgesia was totally naloxone reversible. The results indicate that the agonist analog of LH-RH exert potent actions on the central nervous system, and the mechanism of effects may involve dopaminergic transmission and/or endogenous opiates.