An electrophysiological study of convergence of entopeduncular and lateral preoptic inputs on lateral habenular neurons projecting to the midbrain.

Extracellular recordings were made from single neurons in the lateral habenular nucleus of urethane-anesthetized rats. Single pulse stimulation of the entopeduncular nucleus influenced the spontaneous activity of 200 out of 293 (68%) lateral habenular neurons tested, with the most frequent response being suppression of activity and initial activation followed by suppression. Single pulse stimulation of the lateral preoptic area influenced the activity of 140 out of 165 (85%) lateral habenular neurons tested. Response were similar to those for entopeduncular stimulation. In a series of 137 lateral habenular units, 85 (62%) were influenced by stimulation of both sites indicating convergence of inputs from these two forebrain areas. Approximately half of the lateral habenular neurons which received converging inputs from entopeduncular nucleus and lateral preoptic area were activated antidromically by stimulation of the fasciculus retroflexus, indicating that they were output neurons. These electrophysiological findings support the suggestion from recent neuroanatomical studies that the lateral habenular nucleus is a site of integration for entopeduncular and limbic inputs and in turn sends signals to the midbrain.