LATERAL SEPTAL PROJECTIONS ONTO TUBEROINFUNDIBULAR NEURONS IN THE HYPOTHALAMUS OF THE GUINEA-PIG

Efferent projections of the lateral septal nucleus (LS) to the preopti c area and the hypothalamus were identified in 20 female guinea pigs a fter iontophoretic injection of the anterograde axonal tracer Fluoro-R uby. Tubero-infundibular (TI) neurons of the preoptic area and the hyp othalamus were retrogradely labeled after intracardiac injection of Gr anular Blue or Fluoro-Gold. Magnocellular neurons of the supraoptic an d paraventricular nuclei were also labeled. The double labeling proced ure allowed an estimation of the extent of the direct relationship bet ween LS efferents and TI neurons. Contacts between lateral septal fibe rs and TI cell bodies were mainly observed at the light-microscopical level in the preoptic area. A group of labeled fibers coursing along t he third ventricle established sparse connections with hypothalamic pe riventricular TI neurons. A few appositions was observed in the infund ibular (arcuate) nucleus, suggestive of a monosynaptic regulation of T I neurons by a septo-arcuate tract. Close association with labeled mag nocellular neurons was also noted at the edge of the supraoptic and pa raventricular nuclei. The sparse but direct connections between LS and TI neurons may be involved in the neuroendocrine functions of the LS.