Morphological and electrophysiological characteristics of neurons within identified subnuclei of the lateral habenula in rat brain slices.

Based on the specificity of its inputs and targets, the lateral habenular complex (LHb) constitutes a pivotal motor-limbic interface implicated in various cerebral functions particularly in regulating monoamine transmission. Despite its functional significance, cellular characteristics underlying LHb functionality have not been examined systematically. The present study aimed to correlate morphological and electrophysiological properties of neurons within the different subnuclei of the LHb using whole-cell recording and neurobiotin labeling in rat slice preparations. Morphological analysis revealed a heterogeneous population of projection neurons randomly distributed throughout the LHb. According to somatodendritic characteristics four main categories were classified including spherical, fusiform, polymorphic and vertical cells. Electrophysiological characterization of neurons within the different categories demonstrated homologous profiles and no significant differences between groups. Typically, LHb neurons possessed high input resistances and long membrane time constants. They also displayed time-dependent inward rectification and distinct afterhyperpolarization. A salient electrophysiological feature of LHb neurons was their ability to generate rebound bursts of action potentials in response to membrane hyperpolarization. Based on the pattern of spontaneous activity, neurons were classified as silent, tonic or bursting. The occurrence of distinctive firing modes was not related to topographic allocation. The patterns of spontaneous firing and evoked discharge were highly sensitive to alterations in membrane potential and merged upon de- and hyperpolarizing current injection and synaptic stimulation. Besides projection neurons, recordings revealed the existence of a subpopulation of cells possessing morphological and physiological properties of neocortical neurogliaform cells. They were considered to be interneurons. Our data suggest that neurons within the different LHb subnuclei behave electrophysiologically more similar than expected, considering their morphological heterogeneity. We conclude that the formation of functional neuronal entities within the LHb may be achieved through defined synaptic inputs to particular neurons, rather than by individual neuronal morphologies and intrinsic membrane properties.