GABA- and glutamate-mediated synaptic potentials in rat dorsal raphe neurons in vitro

Synaptic potentials were recorded with intracellular electrodes from rat dorsal raphe neurons in a slice preparation. Synaptic potentials were evoked by applying electrical pulses to bipolar stimulating electrodes positioned immediately dorsal to the raphe nucleus; these arose after a latency of 0.5-5 ms and had a duration of 20-200 ms. The synaptic potential was biphasic (at the resting potential) when the recording electrodes contained potassium citrate; a depolarization was followed by a hyperpolarization. The hyperpolarization reversed in polarity at -70 mV and was blocked by bicuculline. The depolarizing synaptic potential was reduced to 50-90% of control by kynurenate (1-2 mM) or 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline (CNQX) (10 μM) and increased in amplitude and duration by magnesium-free solution. In magnesium-free solutions (with CNQX), the depolarizing synaptic potential was blocked by DL-2-amino-5-phosphonovaleric acid (APV, 50 μM). APV also blocked depolarization caused by adding N-methyl-D-aspartate (NMDA) to the superfusion solution. The results indicate that raphe neurons display two synaptic potentials having a duration of 150-200 ms: one that is mediated by GABA and a second that is due to an excitatory amino acid. The component mediated by an excitatory acid involves, in part, a receptor of the NMDA type.