MODULATION OF INTRINSIC AND SYNAPTIC PROPERTIES OF TWO SUBICULAR NEURON TYPES BY 5-HT2 AGONIST, AND ITS ROLE IN STDP

The subiculum represents one of the two primary outputs of the hippocampal formation, transferring neuronal information to cortical areas involved in spatial navigation and cognitive processing. In this region, at least three subtypes of pyramidal cells have been identified and classified according to their firing patterns: regular spiking (RS), weak bursting (WB), and strong bursting (SB). In this context, the strong serotonergic (5-HT) innervation of subicular neurons has been shown to modulate their output discharge by selectively activating postsynaptic 5-HT receptors. Because modulation of bursting behavior is critical for the induction of spike timing-dependent plasticity (STDP), this study explores the specific role of the 5-HT2_C receptor in firing discharge and STDP processes of WB and SB neurons of the proximal subiculum in the dorsal hippocampus, using acute hippocampal slices. Bath perfusion of the 5-HT_2C agonist Ro60-0175 fumarate (5µM) increased the somatic input resistance and decreased the rheobase current required to elicit an action potential. Consistent with these findings, Ro60-0175 increased firing discharge of WB and SB neurons, accompanied by a reduction in the I_h-mediated sag conductance. Ro60-0175 perfusion did not alter the RMP in either neuronal group. While fast, medium, and slow afterhyperpolarization (AHP) conductances were decreased in WB, they all increased in SB. Notably, although Ro60-0175 modified the firing discharge of subicular neurons, it did not interfere with burst firing in either WB or SB neurons. Lastly, EPSPs elicited by CA1-Subiculum synapsis stimulation exhibited systematic synaptic depression in the presence of Ro60-0175, in both neuron subtypes.