Species difference in the modulatory effect of kappa agonist on 5-HT release from ground squirrel and rat hippocampus

Inclusion of the kappa agonist U50488 in the perifusion medium enhanced K(+)-stimulated 5-HT release from ground squirrel hippocampal slices, but reduced 5-HT outflow in both young and old rats' hippocampal slices. The stimulatory effect of U50488 on 5-HT release from ground squirrel hippocampus was not significantly attenuated by the non-specific opioid antagonist naloxone (10(-5) M), but was completely reversed by the specific kappa antagonist nor-BNI (10(-6)M), and the voltage-dependent sodium channel blocker TTX (10(-6)M). In contrast, the inhibitory effect of U50488 on 5-HT release from the young rat hippocampus was attenuated by naloxone (10(-6)M), but not by nor-BNI (10(-6)M) or TTX (10(-6)M). These results illustrate a significant species difference in the modulatory effect of opioids on neurotransmitter release in the hippocampus.