Benzodiazepine receptors modulate 5-hydroxytryptamine neurotransmission in the rat hippocampus: in vivo electrophysiological evidence.

In the present electrophysiological studies the effects of the activation of putative presynaptic benzodiazepine receptors (BZR) in modulating serotonergic (5-HT) transmission in dorsal hippocampus were investigated in chloral hydrate-anesthetized rats. The effectiveness of the electrical stimulation of the ascending 5-HT pathway on the firing activity of CA3 hippocampus pyramidal neurons was modulated by the intravenous administration of BZR ligands. The BZR agonists, diazepam, lorazepam and CL 218,872, dose-dependently enhanced the effectiveness of the stimulation. In contrast, the BZR inverse agonist, FG 7142, dose-dependently decreased it. The effects of agonist and inverse agonist BZR ligands were blocked by the specific BZR antagonist flumazenil. Diazepam and FG 7142 did not modify the efficacy of microiontophoretic applications of gamma-aminobutyric acid and 5-HT onto hippocampal CA3 neurons. BMY 7378, a 5-HT1A receptor antagonist which blocks the effect of the endogenous 5-HT released by the electrical stimulation of 5-HT fibers in this paradigm, also blocked the enhancing action of diazepam on the effectiveness of the stimulation. These results indicate that the activation of BZR of type I modulates 5-HT neurotransmission, presumably through a presynaptic regulatory mechanism.