AMPA and NMDA receptor regulation of firing activity in 5-HT neurons of the dorsal and median raphe nuclei

The glutamatergic regulation of 5-hydroxytryptamine (5-HT) neuronal activity has not been extensively studied. Here, we used extracellular single unit recording in midbrain slices to examine glutamate receptor mediated effects on 5-HT neuronal activity in the dorsal raphe nucleus (DRN) and the median raphe nucleus (MRN). Alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA; 1 and 3 mu M) concentration-dependently increased firing in 5-HT neurons in both the DRN and the MRN. The response to AMPA was blocked by the AMPA receptor antagonist, 6,7-dinitroquinoxaline-2,3(1H-4H)-dione (DNQX; 10 mu M) but not the N-methyl-D-aspartate (NMDA) receptor antagonist, 2-amino-5-phosphonopentanoic acid (AP-5; 50 mu M). NMDA (10-100 mu M) also increased 5-HT neuronal firing in a concentration-dependent manner in both the DRN and MRN; a response that was blocked by AP-5 (50 mu M). In some DRN neurons the NMDA response was partially antagonized by DNQX (10 mu M) suggesting that NMDA, as well as directly activating 5-HT neurons, evokes local release of glutamate, which indirectly activates AMPA receptors on 5-HT neurons. Responses of DRN 5-HT neurons to AMPA and NMDA were enhanced by the gamma-amino-butyric acid (GABA)(A) receptor antagonist, bicuculline (50 mu M), suggesting that both AMPA and NMDA increase local release of GABA. Finally in the DRN the 5-HT1A receptor antagonist, WAY100635 (100 nM), failed to enhance the response of 5-HT neurons to AMPA and caused only a small increase in the excitatory response to NMDA suggesting a low degree of tonic activation of 5-HT1A autoreceptors even when 5-HT neuronal firing rate is high.