Figure 5. 5-HT neurons inhibit BA PNs via 5-HT1A receptors and enhanced GABA release. A, Representative biocytin-filled PN showing a sparse axon with long-projecting branches (left) and spiny dendrites (right). B, Example of a voltage-clamp (−50 mV) trace from a BA PN showing a slow IPSC evoked by 30 s optical stimulation at 20 Hz of DRN axons, blocked by the 5-HT1A receptor antagonist WAY 100635 (1 μm). Inset, Stereotypical firing pattern of a BA PN exhibiting the light-evoked slow IPSC. C, Slow IPSC evoked in BA PNs is blocked by the 5-HT1A receptor antagonist WAY 100635 (1 μm; Wilcoxon matched-pairs signed-rank test, p = 0.0313, n = 7), but not by the GABAA and GABAB receptor antagonists SR 95531 (50 μm) and CGP54626 (5 μm; Wilcoxon matched-pairs signed-rank test, p = 0.6250, n = 5). D, Relative distribution (as a percentage) of response types in BA PNs evoked by optical stimulation of 5-HT axons (n = 13). E, Example of a voltage-clamp (−50 mV) trace from a BA PN showing an increase in sIPSC frequency during 20 Hz optical stimulation of DRN axons (top). Mean PSTH showing an increase in sIPSC frequency in BA PNs during 30 s optical stimulation at 20 Hz of DRN axons (n = 14; bottom) is shown. F, Increase in sIPSC frequency in BA PNs is blocked by the GABAA and GABAB receptor antagonists SR 95531 (50 μm) and CGP54626 (5 μm), but not the 5-HT1A antagonist WAY100635 (1 μm; control, Wilcoxon matched-pairs signed-rank test, p = 0.0254, n = 13; WAY 100635, paired t test, p = 0.0609, n = 7; SR 95531 and CGP54626, Wilcoxon matched-pairs signed-rank test, p > 0.9999, n = 5). G, Top, Example of a current-clamp trace from a BA PN. Rheobase-positive current was constantly injected to evoke baseline firing. Thirty seconds of optical stimulation at 20 Hz of 5-HT axons reduced the firing rate. Bottom, Firing rate of BA PNs is significantly reduced by optical stimulation at 20 Hz of 5-HT axons (repeated-measures one-way ANOVA, main effect of stimulation time p = 0.0371; Dunnett's multiple-comparisons test, baseline vs 5 s p = 0.0261, baseline vs 10 s p = 0.0456, baseline vs 15 s p = 0.0410, n = 8). Data are expressed as means ± SEM. *p < 0.05.