5-HYDROXYTRYPTAMINE(2) RECEPTOR FACILITATES GABAERGIC NEUROTRANSMISSION IN RAT HIPPOCAMPUS

5-Hydroxytryptamine (5-HT; serotonin) administration enhances GABAergi c synaptic activity recorded in pyramidal neurons of the CA1 region of hippocampus. Previous studies have attributed this effect to the acti vation of HT-5(3) receptors located on GABAergic interneurons. During unrelated experiments, we noticed that under our recording conditions, 5-HT can still increase GABAergic synaptic activity after the complet e blockade of 5-HT3 receptors. This indicated the involvement of an ad ditional 5-HT receptor subtype. Therefore, we reinvestigated the effec ts of 5-HT on GABAergic synaptic activity recorded in pyramidal cells of the CA1 region. The ability of 5-HT to increase GABAergic synaptic activity in the presence of 5-HT3 receptor blockade was mimicked by th e selective 5-HT2 agonist (+)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopr opane and blocked by the selective 5-HT2 antagonist ketanserin. This i ndicated that the additional 5-HT receptor belongs to 5-HT2 receptor f amily. 5-HT2 receptor activation resulted in an increase in the freque ncy of spontaneous inhibitory postsynaptic currents as well as a shift in their amplitude distribution toward larger sizes. These effects we re absent in the presence of tetrodotoxin. We interpret these results to indicate that 5-HT2 receptors activate GABAergic interneurons in th e slice, leading to an increase in GABAergic synaptic activity onto py ramidal cells of the CA1 region.