Excitatory effects of serotonin on rat striatal cholinergic interneurones

Abstract

We investigated the effects of 5-hydroxytryptamine (5-HT, serotonin) in striatal cholinergic interneurones with gramicidin-perforated whole-cell patch recordings. Bath-application of serotonin (30 microm) significantly and reversibly increased the spontaneous firing rate of 37/45 cholinergic interneurones tested. On average, in the presence of serotonin, firing rate was 273 +/- 193% of control. Selective agonists of 5-HT(1A), 5-HT3, 5-HT4 and 5-HT7 receptors did not affect cholinergic interneurone firing, while the 5-HT2 receptor agonist alpha-methyl-5-HT (30 microm) mimicked the excitatory effects of serotonin. Consistently, the 5-HT2 receptor antagonist ketanserin (10 microm) fully blocked the excitatory effects of serotonin. Two prominent after-hyperpolarizations (AHPs), one of medium duration that was apamin-sensitive and followed individual spikes, and one that was slower and followed trains of spikes, were both strongly and reversibly reduced by serotonin; these effects were fully blocked by ketanserin. Conversely, the depolarizing sags observed during negative current injections and mediated by hyperpolarization-activated cationic currents were not affected. In the presence of apamin and tetrodotoxin, the slow AHP was strongly reduced by 5-HT, and fully abolished by the calcium channel blocker nickel. These results show that 5-HT exerts a powerful excitatory control on cholinergic interneurones via 5-HT2 receptors, by suppressing the AHPs associated with two distinct calcium-activated potassium currents.

Figure 1. Serotonin increases the spontaneous firing rate of cholinergic interneurones

A and B, bath-application of 30 μm serotonin strongly and reversibly increased spike frequency in two cells which displayed high (A) or low (B) spontaneous firing rates in control solution (note different time scale). Typical responses of a cholinergic interneurone to current injections are shown in the inset below B. C, distribution of 30 μm serotonin effects on firing rate of responsive cells as a function of the spontaneous firing rate in control solution. D, average effects of different serotonin concentrations on spike frequency of cholinergic interneurones.

Figure 2. The 5-HT₂ receptor antagonist ketanserin reverses the effects of serotonin on the firing rate of cholinergic interneurones

A, bath-application of the 5-HT₂ receptor agonist α-methyl-5-HT (30 μm) strongly increased the spontaneous firing rate of a cholinergic interneurone; subsequent application of ketanserin (10 μm) reduced the firing rate to a level lower than control. B, similarly, bath-application of 30 μm serotonin increased the firing rate of another cholinergic interneurone; subsequent application of ketanserin (10 μm) reduced the firing rate to a level lower than control. C, average effects of serotonin and serotonin plus ketanserin on spike frequency. Data from 5 cells were normalized to average control solution value (100%).

Figure 3. Serotonin strongly reduces the medium AHP (mAHP) of cholinergic interneurones

A, serotonin application reversibly reduced the mAHP amplitude in a cholinergic interneurone. Traces in A and B (same calibrations) represent the average of 5 events (grey traces) ±s.d. (black traces). B, in a different neurone, serotonin reduced the mAHP amplitude, while subsequent application of ketanserin increased this amplitude to more than control level. C, average effects of serotonin and serotonin plus ketanserin on the mAHP amplitude (data from 10 cells were normalized to average control solution value (100%)).

Figure 4. Serotonin strongly reduces the slow AHP (sAHP) of cholinergic interneurones

A, in control solution, serotonin strongly reduced the sAHPs (arrows) induced by positive current steps, and this effect was reversed by subsequent addition of ketanserin. B, in a different cell, in the presence of TTX (1 μm) and apamin (100 nm), a current injection caused depolarizations followed by negative sags (black arrows); after the end of the step, sAHPs (grey arrows) were present. Serotonin (30 μm) strongly and reversibly reduced both the sag and the sAHP. Calibrations as in C. C, in the same cell as in B, NiCl (1 mm) abolished the sag and the sAHP; in the presence of NiCl serotonin failed to affect the current-induced depolarization. D and E, average serotonin effects (in the presence of TTX and apamin) on the negative sag amplitude (D) and on the sAHP amplitude (E). Data pooled from 11 cells and normalized to control solution average values.