The interpeduncular nucleus (ITP). Ultrastructure data indicative of a possible neurosecretory activity

The development of somatostatin immunoreactivity in the interpeduncular nucleus of the cat

Somatostatin (SS) immunoreactivity was localized in a population of neurons in the interpeduncular nucleus (IPN) of the developing and adult cat. SS-immunoreactive perikarya and fibers were found in several subdivisions of the IPN, but were most heavily concentrated in the central nucleus (IPC). Some immunoreactive fibers appeared to be associated with the ventral surface of the IPN where they may come in contact with the interpeduncular cistern. Bundles of immunoreactive fibers also appeared to leave the rostral pole of the IPN bilaterally. Still other SS-positive perikarya and fibers were associated with the tegmentopeduncular tract and the medial division of the dorsal tegmental nucleus. SS was present in both perikarya and fibers of the IPN at birth. The concentration of immunoreactivity increased dramatically during the first 100 days postnatal, but by 180 days the overall intensity of immunoreactivity had decreased to adult levels. These data indicate that SS may be present in the cat IPN in a specific population of neurons that is most active during early postnatal development. The data are consistent with previous suggestions that SS plays a special role in the development of certain neuronal systems and may be particularly important in the integration of behaviors during the neonatal period.

Cobalt injected into the right and left fasciculi retroflexes clarifies the organization of this pathway

A cobaltic lysine complex was injected separately into the right and left fasciculi retroflexes of the frog. This tracing technique labeled, in a Golgi-like manner, the neurons which initiate the fasciculi retroflexes and revealed details of their morphological pattern. The fasciculi retroflexes originate from various neurons distributed in the diencephalon and mesencephalon, but their main source is the habenular nuclei. In the frog there are dorsal and ventral habenulae which are homologous to the medial and lateral habenulae, respectively, of mammals. In the frog the dorsal habenulae are strikingly asymmetric. Our study shows that the fasciculus retroflexus is composed of several compact separated bundles of fibers and that the fibers originating from the dorsal habenular nuclei project to the interpeduncular nucleus, while those originating from the ventral habenular nuclei project beyond the interpeduncular nucleus, but so far we have not been able to show the exact site of their termination. The labeling of cells within the interpeduncular nucleus raises the possibility that the habenulo-interpeduncular tract is reciprocal in function. The finds support our previously reported hypothesis on the theoretical interpretation of the functional circuitry of the frog habenulo-interpeduncular system.

The ultrastructure and organization of methionine-enkephalin immunoreactive profiles in the interpeduncular nucleus of the cat

An ultrastructural analysis of the dorso-intermediate (CDIZ) and dorso-median zones (CDMZ) at the caudal end of the interpeduncular nucleus was undertaken. Serial vibratome sections were stained for met-enkephalin-like immunoreactivity by using the indirect antibody peroxidase-antiperoxidase method of Sternberger, and ultrathin sections were examined electron microscopically. Met-enkephalin-like immunoreactivity was observed in two different types of somata in the two regions. The dendritic architecture of the dorso-intermediate zone was characterized by the presence of many dendritic crests. A large number of these dendritic crests were immunostained for met-enkephalin. A variety of terminals was also seen in the CDIZ and CDMZ and they are categorized according to their vesicular morphology into three classes. One class contained mainly clear round vesicles. The met-enkephalin-like immunoreactive (MELI) terminal profiles in both regions were of this class and contained small round vesicles which were less than 35 nm in diameter. These MELI terminals appeared in significant numbers, sometimes in close proximity to unlabeled somata and dendrites. The dorso-median zone had, in addition, two more classes of terminals. One class was characterized by a large number of dense-cored vesicles--class II, and the other group had a large proportion of flat clear vesicles--class III. The possible neurochemical identities of some of the unlabeled terminals in relation to the MELI dendritic structures are discussed in the light of known neurochemically established afferent projections to the CDIZ and CDMZ of the interpeduncular nucleus.

Quantitative electron microscopic evidence for reinnervation in the adult rat interpeduncular nucleus after lesions of the fasciculus retroflexus

The method of electron dense degeneration has been used to make a quantitative study of the projection from the habenula through the fasciculus retroflexus (FR) to the interpeduncular nucleus (IPN) in the rat. The IPN si a midline structure onto which the right and left fasciculi converge. In the rostral part of the IPN the fascicular axons from each side form synapses throughout the mediolateral extent of the ventral two-thirs of the nucleus. In the caudal part of the IPN the fascicular axons from each side terminate to an equal extent in two discrete, parasagittal zones, situated one on side in the mid-mediolateral extent of the IPN. These zones contain clusters of neurons located along the course of a characteristic row of arterioles and venules penetrating the IPN from its ventral surface. In both rostral and caudal parts of the IPN the fascicular axons form single synaptic contaerpeduncular neurons, but caudally, in the two parasagittal zones they also form crest synapses. Crest synapses are only found in this part of the IPN. In crest synapses two presynaptic terminals form markedly asymmetrical contacts with the parallel opposing sides of an attenuated dendritic appendage (the crest). After unilateral fascicular lesions only one member of a pair of axon terminals contacting a crest degenerates. After bilateral fascicular lesions, however, there are many instances in which both members of a crest pair degenerate. This indicates that the axon terminals from the right and left fasciculi are segregated at the level of the crests, in such a way that one terminal comes from the right fasciculus and one from the left. At longer survivals after unilateral or bilateral fasciculus lesions the degeneration is completely removed, but crest synapses are still present, indicating that the presence of fascicular axons is not necessary for the maintenan ce of crests in the IPN. To investigate the effects of chronic deafferentation, the left fasciculus was destroyed and, after a survival of at least six weeks (sufficient for all degeneration to be removed) the right fasciculus was destroyed one day before killing. Under these conditions there are many crests in which both axon terminals show degeneration. The proportion of such doubly degenerating crest synapses is similar to that found after acute (1 day) bilateral lesions, indicating that axons from the right fasciculus have reinnervated sites formerly occupied by the left fasciculus. We conclude that during normal development there is some constraint which prevents both sides of a crest being innervated by axons from the fasciculus of the same side of the brain, but that this constraint is not effective after unilateral fascicular lesions in the adult.

An electron microscope observation of the right and the two left portions of the habenular nuclei of the frog

The right dorsal habenula (RDH), the left dorsal habenula, lateral portion (LDH-LP) and the left dorsal habenula, medial portion (LDH-MP) of the adult frog Rana esculenta have been studied by the electron microscope with the use of three types of fixatives: osmium tetroxide, aldehydes and potassium permangarate. The study has demonstrated in the habenulae of both sides a variety of syraptic vesicles, special synaptic patterns (en passant, serial, axo-somatic) containing small spherical transparent vesicles, never mixed with other types of vesicles, and large pale processes containing big granules. However, a particular type of synaptic vesicles characterized by small clear spherical vesicles with a tiny dark granule revealed by potassium permanganate fixition has been demonstrated only in the neuropil of the LDH and not in the RDH. Further, a peculiar type of intracytoplasmic inclusion appears in the form of myelin-like and crystal-like formations only in the LDH-MP. On the basis of these consistent ultrastructural differences, the dorsal habenular nuclei of the frog can be considered as composed of three individual nuclei, each with its own morphological characterization, dislocated, for some unknown reason, one on the right and two on the left side of the epithalamus. The results suggest that the LDH-MP is a nucleus metabolically different from the others and that the projections to the dorsal habenular nuclei are different on the right and left sides of the brain.