An autoradiographic study of the efferent connections of the preoptic region in the rat

Abstract The normal morphology of the rat preoptic region has been briefly described on the basis of Nissl‐ and silver‐stained preparations and its efferent connections have been studied autoradiographically in over 50 rat brains with single small injections of 3 H‐proline, or various mixtures of 3 H‐proline, 3 H‐leucine, and 3 H‐lysine. Injections in the anteroventral part of the lateral preoptic area labeled fibers projecting through, and perhaps to, the anterior and lateral hypothalamic areas and ending in the supramammillary region and ventral fiber lamina of the mammillary complex; other labeled fibers ended in the periventricular hypothalamic gray and the internal lamina of the median eminence. The posteromedial lateral preoptic area projects to the same regions, as well as to the medial septal‐diagonal band complex, and to the lateral habenula through the stria medullaris. Injections of the posterolateral lateral preoptic area labeled each of the above fiber systems as well as fibers to the main olfactory bulb, anterior olfactory nucleus and taenia tecta. Other fibers coursed over the genu of the corpus callosum, through the stria terminalis and ansa peduncularis to the medial, cortical and basal amygdaloid nuclei and the anterior amygdaloid area, and through the medial forebrain bundle to the substantia nigra. The transition region between the lateral preoptic and lateral hypothalamic areas at the level of the supraoptic nucleus has widespread connections as a whole (a) with the medial septal‐diagonal band complex, lateral septum and bed nucleus of the stria terminalis, (b) through or to most of the hypothalamus, the substantia nigra, central tegmental field, central gray, superior central nucleus, and the locus coeruleus, (c) through the stria medullaris to the lateral habenula (bilaterally), parataenial, paraventricular, and mediodorsal nuclei of the thalamus, (d) through the stria terminalis and ansa peduncularis to the central, medial and cortical nuclei of the amygdala, and (e) to the main olfactory bulb, anterior olfactory nucleus, cingulate bundle, olfactory tubercle, medial septaldiagonal band complex and the lateral septum. Injections of the magnocellular preoptic nucleus label fibers (a) anteriorly to the main olfactory bulb, anterior olfactory nucleus, taenia tecta, piriform cortex and olfactory tubercle, and to the medial septal‐diagonal band complex, (b) posteriorly through the medial forebrain bundle to the lateral preoptic and lateral hypothalamic areas, internal lamina of the median eminence, supramammillary region, ventral fiber lamina of the mammillary complex, posterior hypothalamic area, central gray, locus coeruleus, and ventral tegmental area, (c) to the medial, cortical and basal amygdaloid nuclei and the anterior amygdaloid area, and (d) through the stria medullaris to the mediodorsal nucleus, lateral habenula, and contralateral magnocellular preoptic nucleus. The preoptic continuation of the bed nucleus of the stria terminalis projects to the medial, central, and basomedial amygdaloid nuclei, to the paraventricular nucleus of the thalamus and lateral habenular nucleus: through the stria medullaris, and to the substantia innominata, anterior and lateral hypothalamic areas, hypothalamic periventricular gray, fiber capsule of the ventromedial nucleus, internal lamina of the median eminence, supramammillary region, ventral fiber lamina of the mammillary complex, posterior hypothalamic area, central gray, locus coeruleus and ventral tegmental area. The medial preoptic area appears to project to the hypophysiotrophic zone of the hypothalamus including the periventricular gray, internal lamina of the median eminence, lateral hypothalamic area, ventromedial nucleus, and ventral fiber lamina of the mammillary complex. Neurons in the dorsal part of the periventricular preoptic nucleus project to the laterally adjacent lateral preoptic and lateral hypothalamic areas while those in an intermediate location send additional fibers to the supraoptic nucleus. The ventralmost cells in the preoptic periventricular nucleus project to the hypophysiotrophic zone of the hypothalamus in much the same way as the anteriorly adjacent suprachiasmatic nucleus (Swanson and Cowan, '75c).