Modulation of the activity of pyramidal neurons in rat prefrontal cortex by raphe stimulation in vivo: Involvement of serotonin and GABA

Serotonin is involved in psychiatric disorders exhibiting abnormal prefrontal cortex (PFC) function (e.g. major depression, schizophrenia). We examined the effect of the stimulation of the dorsal and median raphe nuclei (DR and MnR, respectively) on the activity of PFC neurons. Electrical stimulation of DR/MnR inhibited 66% (115/173) of pyramidal neurons in the medial PFC (mPFC). The rest of the cases exhibited orthodromic excitations, either pure (13%) or preceded by short-latency inhibitions (20%). Excited neurons had a lower prestimulus firing rate than those inhibited. Excitations evoked by MnR stimulation had a shorter latency than those evoked by DR stimulation. WAY-100635 [a 5-hydroxytryptamine(1A) (5-HT1A) antagonist] and the selective gamma aminobutyric acid(A) (GABA(A)) antagonist picrotoxinin partially antagonized DR/MnR-evoked inhibitions, suggesting the involvement of 5-HT1A- and GABA(A)-mediated components. The presence of a direct DR/MnR-mPFC GABAergic component is suggested by the short latency of evoked inhibitions (9+/-1 ms), faster than those evoked in the secondary motor area (20+/-3 ms), and that of antidromic spikes evoked by DR/MnR stimulation in mPFC pyramidal neurons (15+/-1 ms). Stimulation of the DR/MnR with paired pulses enhanced the duration of inhibitions and turned some excitations into inhibitions. Thus, the DR/MnR control the activity of mPFC pyramidal neurons in vivo in a complex manner, involving 5-HT-mediated excitations and GABA- and 5-HT-mediated inhibitions.