Septal complex of the telencephalon of the lizard Podarcis hispanica. II. afferent connections

Abstract

The afferent connections to the septal complex were studied in the lizard Podarcis hispanica (Lacertidae) by means of a combination of retrograde and anterograde tracing. The results of these experiments allow us to classify the septal nuclei into three main divisions. The central septal division (anterior, lateral, dorsolateral, ventrolateral, and medial septal nuclei plus the nucleus of the posterior pallial commissure) receives a massive, topographically organized, cortical projection (medial, dorsal, and ventral areas) and widespread afferents from the tuberomammillary hypothalamus and the basal telencephalon. Moreover, it receives discrete projections from the dorsomedial anterior thalamus, the ventral tegmentum, the midbrain raphe, and the locus coeruleus. The ventromedial septal division (ventromedial septal nucleus) receives a massive projection from the anterior hypothalamus, dense serotonergic innervation, and a faint amygdalohypothalamic projection, but it is devoid of direct cortical input. The midline septal division (nucleus septalis impar and dorsal septal nucleus) receives a nontopographic cortical projection (dorsomedial and dorsal cortices) and afferents from the preoptic hypothalamus, the dorsomedial anterior thalamus, the midbrain central gray, and the reptilian A8 nucleus/substantia nigra.

Our results indicate that the cortex provides a physiologically complex, massive input to the septum that terminates over the whole dendritic tree of septal cells. In contrast, most of the ascending afferents make axosomatic contacts by means of pericellular nests. The chemical nature of the main septal afferents and the comparative implications of the available hodological data on the organization of the septal complex of tetrapod vertebrates are discussed.