Release of endogenous 5-hydroxytryptamine in rat ventral hippocampus evoked by electrical stimulation of the dorsal raphe nucleus as detected by microdialysis: Sensitivity to tetrodotoxin, calcium and calcium antagonists

Abstract

We have utilized the brain microdialysis technique in an attempt to measure excitation-secretion coupled release of endogenous 5-hydroxytryptamine in rat brain in vivo and investigated the pharmacology of the voltage-sensitive calcium channel involved in this process. All experiments were carried out using chloral hydrate anaesthetized rats. Ascending serotoninergic neurons were electrically stimulated using an electrode implanted into the dorsal raphe nucleus. A dialysis probe was implanted into the ventral hippocampus and continuously perfused with artificial cerebrospinal fluid containing the selective 5-hydroxytryptamine uptake inhibitor citalopram (1 μM). Twenty-minute perfusates were analysed for endogenous 5-hydroxytryptamine using high performance liquid chromatography with electrochemical detection. Electrical stimulation (cathodal monophasic 1 ms pulses, 300 μA, 2–10 Hz) of the dorsal raphe nucleus for 20 min induced an immediate release of 5-hydroxytryptamine which lasted for the duration of the stimulus and was frequency-dependent. The calculated amount of 5-hydroxy-tryptamine release per electrical impulse was constant over the frequency range used. Addition of tetrodotoxin (10 μM) to, or omission of calcium from, the perfusion medium reduced the spontaneous output of 5-hydroxytryptamine by 60–70% and caused a near complete inhibition of the effect of low frequency (3 Hz) electrical stimulation of the dorsal raphe nucleus. Local perfusion with cadmium (30 and 300 μM), which is reported to antagonize both N- and L-type voltage-sensitive calcium channels, also caused a pronounced decrease of basal output of 5-hydroxytryptamine and a marked, but not complete inhibition of the effect of nerve stimulation. In comparison nickel, a putative antagonist of the T-type voltage-sensitive calcium channel, caused a moderate reduction in both spontaneous and electrically evoked output of 5-hydroxytryptamine when locally perfused at a concentration of 300 μM, but had no effect at 30 μM. Addition to the perfusion medium of the selective L-type voltage-sensitive calcium channel antagonists nifedipine (10 μM) and diltiazem (10 μM) did not affect either basal or evoked output of 5-hydroxytryptamine. These findings are some of the first direct evidence that neuronal 5-hydroxytryptamine (in rat ventral hippocampus) can be released by an excitation-secretion coupled event in vivo. Furthermore, our findings fit best with the idea that depolarization-evoked release of neuronal 5-hydroxytryptamine in rat hippocampus is mediated by the opening of an N-type rather than an L- or T-type voltage-sensitive calcium channel.