Serotonin-receptor-mediated modulation of Ca²⁺-dependent 5-hydroxytryptamine release from neurones of the rat brain cortex

Summary

Rat brain cortex slices were incubated with ³H-5-hydroxytryptamine and subsequently superfused. Tritium overflow from slices superfused with physiological salt solution was stimulated by rectangular electrical pulses (normal frequency 3 Hz), and overflow from slices superfused with Ca²⁺-free solution containing 25 mM K⁺ was stimulated by introduction of (usually) 1.3 mM CaCl₂. In most of the experiments the neuronal uptake of 5-hydroxytryptamine (5-HT) was blocked by paroxetine.

• 1. Electrically evoked ³H overflow was abolished by tetrodotoxin or omission of Ca²⁺ from the superfusion fluid, whereas Ca²⁺-evoked overflow was not affected by tetrodotoxin, but inhibited by Mg²⁺ ions.

• 2. Both electrically and Ca²⁺-evoked ³H overflow were reduced by unlabelled 5-HT and increased by methiothepin; the potencies of the drugs in modifying evoked overflow were similar with both methods of stimulation. Methiothepin caused a substantial shift to the right of the concentration-response curve of unlabelled 5-HT for its inhibitory effect. By contrast, metergoline produced only a very slight shift of this curve.

• 3. The inhibitory effect of unlabelled 5-HT on electrically evoked ³H overflow increased with decreasing Ca²⁺ concentration (1.3–0.65 mM; 3 Hz) or with decreasing frequency of stimulation (10–0.3 Hz; 1.3 mM Ca²⁺).

• 4. When ³H overflow was stimulated by introduction of various Ca²⁺ concentrations (0.65–4.55 mM) into the Ca²⁺-free, K⁺-rich solution, the data obtained yielded a straight line in the double reciprocal plot (K _m =2.5 mM Ca²⁺). Straight lines were also obtained, when the effects of unlabelled 5-HT and methiothepin on ³H overflow evoked by different Ca²⁺ concentrations were analyzed; these lines and that calculated for the controls had a common intercept on the ordinate, whereas the K _m values were considerably different from each other (in the presence of unlabelled 5-HT and methiothepin, 5.4 and 1.2 mM Ca²⁺, respectively).

It is concluded that the modification of ³H-5-HT release (caused by interaction of methiothepin and 5-HT with presynaptic 5-HT autoreceptors on serotoninergic neurones) is mediated by decreasing the availability of Ca²⁺ ions for stimulus-release coupling, probably by decreasing the affinity of the voltagesensitive permeability channel of the cell membrane for Ca²⁺ ions.