An autoradiographic and electron microscopic study of retino-hypothalamic connections

Olfactory relationships of the telencephalon and diencephalon in the rabbit. II. An autoradiographic and horseradish peroxidase study of the efferent connections of the anterior olfactory nucleus

The efferent connections of the anterior olfactory nucleus in the female albino rabbit have been studied using the autoradiographic and horseradish peroxidase methods for tracing axonal pathways. Following a unilateral injection of 3H-leucine into the olfactory peduncle, radioactively labeled efferent projections from the anterior olfactory nucleus were traced into all layers of the ipsilateral main olfactory bulb beneath the olfactory nerve layer and through the ipsilateral anterior limb of the anterior commissure and plexiform layer of the medial side of the cerebral hemisphere to the deep half of the plexiform (IB) and pyramidal cell (II) layers of the prepyriform cortex, the tenia tecta, and the entire surface of the olfactory tubercle. Labeled projections crossing the midline within the anterior commissure were followed to the layers IB and II of the contralateral anterior prepyriform cortex and pars externa, pars lateralis, and pars dorsalis of the anterior olfactory nucleus, and through the periventricular layer of the olfactory peduncle to all layers of the main olfactory bulb beneath the olfactory nerve layer. No well-defined labeled projection was traced to the contralateral accessory olfactory bulb. Evidence for possible anterior olfactory nucleus and/or prepyriform cortical projections to the ipsilateral paleocortical half of the claustrum, horizontal limb of the nucleus of the diagonal band, the posterior lateral hypothalamus at the level of the mammillary complex, and to the bed nucleus of the stria terminalis is discussed. Intra-axonal retrograde transport of horseradish peroxidase from axon terminals to parent cell bodies after unilateral injection of the protein into the main olfactory bulb or anterior olfactory nucleus revealed that anterior olfactory nucleus projections to the olfactory bulbs and the contralateral anterior olfactory nucleus arise predominately from the pars externa. The autoradiographic data indicate that the anterior olfactory nucleus projects to olfactory cortical structures which also receive afferent input from the olfactory bulb and that the termination of these projections is complementary to those from the olfactory bulb.

An autoradiographic study of the projections from the lateral geniculate body of the rat

The projections from the lateral geniculate body of the rat were followed using the technique of autoradiography after injections of [3H] proline into the dorsal and/or ventral nuclei of this diencephalic structure. Autoradiographs were prepared from either frozen or paraffin coronal sections through the rat brain. The dorsal nucleus of the lateral geniculate projected via the optic radiation to area 17 of the cerebral cortex. There was also a slight extension of label into the zones of transition between areas 17, 18 and 18a. The distribution of silver grains in the various layers of the cerebral cortex was analyzed quantitatively and showed a major peak of labeling in layer IV with minor peaks in outer layer I and the upper half and lowest part of layer VI. The significance of these peaks is discussed in respect to the distribution of geniculocortical terminals in other mammalian species. The ventral nucleus of the lateral geniculate body had 5 major projections to brain stem structures both ipsilateral and contralateral to the injected nucleus. There were two dorsomedial projections: (1) a projection to the superior colliculus which terminated mainly in the medial third of the stratum opticum, and (2) a large projection via the superior thalamic radiation which terminated in the ipsilateral pretectal area; a continuation of this projection passed through the posterior commissure to attain the contralateral pretectal area. The three ventromedial projections involved: (1) a geniculopontine tract which coursed through the basis pedunculi and the lateral lemniscus to terminate in the dorsomedial and dorsolateral parts of the pons after giving terminals to the lateral terminal nucleus of the accessory optic tract, (2) a projection via Meynert's commissure to the suprachiasmatic nuclei of both sides of the brain stem as well as to the contralateral ventral lateral geniculate nucleus and lateral terminal nucleus of the accessory optic tract, and (3) a medial projection to the ipsilateral zona incerta. The results obtained in these experiments are contrasted with other data on the rat's central visual connections to illustrate the importance of these connections in many subcortical visual functions.

Lamination and differential distribution of thalamic afferents within the sensory-motor cortex of the squirrel monkey

The structure of the first somatic sensory area (areas 3, 1 and 2), of the motor area (area 4) and the intervening transitional field (area 3a) is described in the squirrel monkey (Saimiri sciureus) using Nissl, Bodian, Weil and Golgi preparations. The laminar arrangement of both cells and axons is briefly described and this correlated with the distribution of thalamic afferents as identified in experiments conducted with the Nauta and autoradiographic techniques. The latter method was used particularly in order to assess quantitative differences in the density of thalamic projections to the five cytoarchitectonic fields. In the somatic sensory areas thalamic afferents terminate not only in layer IV but a large extent also in a recognizable part of layer III (layer IIIb). In area 4 thalamic terminals fill much of layer III, reaching almost to layer II. In area 3a the extent is intermediate between that seen in areas 3 and 4. It is thought that the extensive spread of thalamic terminals is related to the elongated form of a particular class of spine-bearing cell whose somata are situated in layer IV (Jones, '75). In all areas a small proportion of thalamic afferents end also in layer I. Evidence is presented to indicate that specific afferent fibers emanating from the ventrobasal and ventrolateral complexes of the thalamus terminate in both the deep and superficial parts of layer I while "non-specific" afferents from other thalamic sources end in the superficial part. The autoradiographic studies indicate that there are considerable differences between the number of thalamic afferents ending in area 3 on the one hand and in areas 1 and 2 on the other. Given this and the nature of the degenerating thalamic afferents observed in Nauta preparations, it is possible to identify thalamic afferents in normal Golgi preparations and significant differences are detectable in areas 4, 3 and 1 and 2. It it is as yet uncertain whether the slightly thinner, more sparsely distributed thalamic afferents ending in areas 1 and 2 are branches of those directed primarily to area 3.

The efferent connections of the suprachiasmatic nucleus of the hypothalamus

The efferent connections of the suprachiasmatic nucleus of the hypothalamus have been studied in the rat by the injection of 3H-proline into the nucleus and the surrounding regions of the rostral hypothalamus, and by the injection of the enzyme marker, horseradish peroxidase, into the region of the ventromedial hypothalamic nucleus. After an injection of 3H-proline confined to the ventral portion of the suprachiasmatic nucleus, transported label can be followed, in the autoradiographs, dorsally and caudally in the periventricular area as far as the caudal end of the ventromedial nucleus, into the triangular area between this nucleus and the arcuate nucleus, and along the ventral aspect of the tuberal region, just lateral to the ventromedial nucleus. A small number of silver grains are also seen over the internal lamina of the median eminence. No label can be followed rostrally or immediately lateral to the nucleus. Comparable injections into adjoining regions of the hypothalamus (especially the anterior hypothalamic area, the medial preoptic area, and the retrochiasmatic region) show transported label over the same regions, but with a somewhat different pattern of grain distribution; in addition, the anterior hypothalamic area shows an extensive projection through the medial forebrain bundle to the mammillary and supramammillary nuclei, the midbrain tegmentum, and certain of the midline thalamic nuclei. Although it is difficult in our autoradiographs to distinguish between the course of the efferent fibers from the suprachiasmatic nucleus and the zones in which they terminate, our evidence favors a termination among the cells of the periventricular area, and upon dendrites of the cells in the ventromedial, dorsomedial and arcuate nuclei, which extend beyond the limits of the nuclei into the periventricular area and to the area beneath the ventromedial nucleus.