Nipecotic acid: preferential accumulation in the cochlea by GABA uptake systems and selective retrograde transport to brainstem

[³H]Nipecotic acid was shown to be preferentially accumulated by the same cochlear structures which selectively accumulate [³H]γ- aminobutyric acid ([³H]-GABA), including the terminals of a subset of olivocochlear neurons. With both amino acids, olivocochlear fibers selectively transported label in a retrograde direction, from cochlea to brainstem. However, only [³H]nipecotic acid produced dense labeling, and labeling of cell bodies in the superior olive, presumably because it is metabolized very slowly. Nipecotic acid appears to provide a selective retrograde tracer, specific to neurons whose terminals exhibit preferential GABA uptake.     

Crossed divergent axon collaterals from cerebellar nuclei to thalamus and lateral medulla oblongata in the rat

The divergent collateralization of the ipsilateral descending limb of the brachium conjunctivum was here studied in the rat by means of fluorescent retrograde double-labeling. Tracer injections in the lateral part of the medulla oblongata were combined with injections of another tracer in the contralateral thalamus. Retrogradely single-labeled cells, as well as a relatively high number of double-labeled ones, were found in the lateral part of the interpositus and in the dorsolateral hump in the cerebellar nuclei ipsilateral to the medullary injections. The present results demonstrate that the same cerebellar cells interconnect by means of axon collaterals the ipsilateral medulla oblongata and the contralateral thalamus.     

GABA-like immunoreactivity in the squirrel monkey organ of corti

The distribution of gamma-aminobutyric acid (GABA)-like immunoreactivity in the squirrel monkey organ of Corti was determined using an antiserum against GABA conjugated to bovine serum albumin. Immunoreactive labeling was seen in the region of the inner spiral bundle, the synaptic region below inner hair cells, in terminals contacting the basal part of outer hair cells, and in tunnel spiral fibers. Examples of each of these immunoreactive components could be observed in all cochlear turns. In the region of inner hair cells, immunoreactive labeling took the form of numerous small puncta randomly distributed below the base of the cells. In the region of outer hair cells, large globular immunoreactive structures reminiscent of terminal endings at the subnuclear level were observed. Since similar structures were seen at the base of outer hair cells in other cochleas processed for AChE, we conclude that GABA-like immunoreactivity was contained in efferent terminals which synapse on outer hair cells. These results strengthen previous evidence for the presence of GABA in the olivocochlear system of the mammalian cochlea.     

Distribution of lumbar dorsal root fibers in the lower thoracic and lumbosacral spinal cord of the rat studied with choleragenoid horseradish peroxidase conjugate

Spinal cord projections from lumbar dorsal root ganglia (DRGs) were investigated in adult rats following injections of choleragenoid horseradish peroxidase (B-HRP) into each of the six lumbar DRGs. This method is known to label primarily thick fibers. Labeling was found in all laminae except in the outer part of lamina II. Labeled fibers and terminal-like structures were found 8-13 segments rostral and 1-5 segments caudal to the injected DRGs. A somatotopic organization was revealed in laminae III, where the labeling seemed to be organized in mediolateral zones. Some of these protruded as fingerlike processes through segments rostral and caudal to the root entry level. An interdigitating pattern for these fingerlike processes was seen between some DRGs, while an extensive overlap was found between other DRGs. Many zones were found to correspond to the known central projections of peripheral sensory nerves supplied by the injected ganglion. This suggests that the central projection of a DRG is highly related to the projections of the peripheral nerves included in the DRG. The projections to lamina IV were organized in a similar manner as in lamina III, even though the projections showed a higher degree of overlap than in lamina III. No clear somatotopic organization was found in laminae V-IX. Provided that the topographical relationship between central projections of single peripheral nerves and of DRGs correspond to their peripheral projections, the results of this study, together with results of earlier studies suggest that the outlines of dermatomes are highly related to the territories of peripheral nerves included in the dermatomes.     

Neurophysiology of limbic system pathways in the rat: Projections from the subicular complex and hippocampus to the entorhinal cortex

We studied the responses of rat entorhinal neurons to electrical stimulation of the dentate gyrus, hippocampus and subicular complex. Three main results were obtained. Excitatory postsynaptic potentials were recorded in entorhinal neurons in response to electrical stimulation. Cell in layers II, III and V of the entorhinal cortex were responsive. Frequency potentiation of excitatory responses was observed when 10/s stimulation was used. Excitatory responses were followed by inhibitory postsynaptic potentials. The results provide evidence for an excitatory projection from the hippocampus and subiculum to the entorhinal cortex, and are consistent with the existence of feed-forward inhibition of entorhinal principal neurons.     

Distribution and hypothalamic projection of tyrosine-hydroxylase containing neurons of the nucleus of the solitary tract in the pigeon.

The avian nucleus of the solitary tract has an extensive subnuclear organization. Several subnuclear cell groups can be distinguished on the basis of cytoarchitectonic criteria. In general, the subnuclei of the medial division of the nucleus of the solitary tract receive gastrointestinal afferents, whereas the subnuclei of the lateral division of the nucleus of the solitary tract receive cardiopulmonary afferents. Forebrain afferents to the nucleus of the solitary tract are segregated to medial and lateral subnuclei, which are located at the periphery of the nucleus. These peripheral subnuclei of the nucleus of the solitary tract are also the source of ascending axonal projections to the forebrain. In this study, the tyrosine hydroxylase (initial enzyme for catecholamine synthesis) content of the anteromedial hypothalamic projecting neurons of the nucleus of the solitary tract is determined by use of a combined retrograde fluorescent dye-immunofluorescence method. Fast Blue implanted into the anteromedial hypothalamus (in the region of the nucleus periventricularis magnocellularis) resulted in the retrograde labeling of neurons in the caudal two-thirds of the nucleus of the solitary tract. At levels rostral to the obex, dye-labeled cells were mostly observed in the dorsally located subnuclei medialis superficialis pars posterior and lateralis dorsalis pars posterior and in the ventrally located subnucleus medialis ventralis pars posterior. More centrally located subnuclei contained few labeled cells, if any. For example, subnucleus medialis intermedius pars posterior only had a few retrogradely labeled cells, whereas the centrally located subnucleus medialis dorsalis pars posterior was almost devoid of labeled cells. At levels caudal to the obex, many retrogradely labeled neurons of the nucleus of the solitary tract were observed. Neurons immunoreactively labeled for tyrosine hydroxylase were mostly found within subnuclei, which contain anteromedial hypothalamic projection neurons. In subnuclei medialis superficialis pars posterior and lateralis dorsalis pars posterior, 87% of the retrogradely dye-labeled cells were also immunoreactively labeled, whereas in the caudal nucleus of the solitary tract (at levels caudal to the obex), 68% of the retrogradely labeled cells were immunoreactively labeled. Not all tyrosine hydroxylase containing cells had projections to the implantation site in the anteromedial hypothalamus since only 40% of the immunoreactive cells in the caudal nucleus of the solitary tract and 59% of the immunoreactive cells in the subnucleus medialis superficialis pars posterior were retrogradely labeled with Fast Blue.(ABSTRACT TRUNCATED AT 400 WORDS)     

Vestibular and cochlear efferent neurons in the monkey identified by immunocytochemical methods

Attempts were made to identify vestibular (VEN) and cochlear (CEN) efferent neurons in the squirrel monkey using retrograde transport of horseradish peroxidase (HRP) and immunocytochemical methods. HRP implants in the ampulla of the lateral semicircular duct retrogradely labeled cells of VEN bilaterally and some cells of CEN. VEN located lateral to the rostral part of the abducens nucleus formed a compact collection of cells, all of which were immunoreactive only to antisera for choline acetyltransferase (ChAT). CEN, identified by immunoreactivity to ChAT were located at the hilus of the lateral superior olive (LSO), along the lateral border of the LSO and sparsely near lateral parts of the ventral trapezoid nucleus (VTN). A small number of cells and fibers near the border of the VTN and lateral to the LSO were immunoreactive for leucine enkephalin (L-ENK). Fibers immunoreactive for L-ENK also were identified in the hilus of the LSO. No cells of the superior olivary complex were immunoreactive for antisera to ChAT, L-ENK, substance P, gamma-aminobutyric acid or glutamic acid decarboxylase. Cells of VEN and CEN can be identified by their immunoreactivity to ChAT, and some cells and fibers of CEN also contain L-ENK.     

Retinal ganglion cells projecting to the accessory optic system in the rat

The present data identify the distribution and morphological features of a homogeneous group of rat retinal ganglion cells. These cells were labelled after injection of either horseradish peroxidase or a fluorescent tracer, Fast Blue, into the medial terminal nucleus (MTN) of the accessory optic system. After retrograde fluorescent labelling, MTN-projecting retinal ganglion cells were intracellularly injected with Lucifer Yellow to reveal their complete dendritic morphology. There were on average 1,750 MTN-projecting cells fairly evenly distributed over the entire retinal ganglion cell layer. Their density ranged from 40-49 cells/mm2 in superior retina to 10-19 cells/mm2 towards the peripheral regions of both inferior and superior retina. The area of highest density formed a nasal-temporal band suggestive of a visual streak. Soma diameters ranged from 8.7 to 14.5 micron centrally and from 9.9 to 17.1 microns peripherally. Maximal dendritic field diameter ranged from 431 to 644 micron and averaged 516 micron with no obvious eccentricity dependence. The majority of MTN-projecting cells were bistratified. Dendrites stratified predominantly in the inner sublamina of the inner plexiform layer (IPL) with a varying number of branches from the remaining dendrites contained within the outer IPL, both strata presumably corresponding to the electrophysiologically determined on-off dichotomy. Cells projecting to the MTN were characterised by higher-order dendritic branching patterns that resulted in a dense dendritic tree with minor dendritic overlap. The slender dendrites had a beaded appearance and displayed spiny protrusions. The dendritic coverage of 5-6, stratification pattern, and overall morphological appearance of rat MTN-projecting cells renders them suitable candidates for on-direction--selective cells shown electrophysiologically to be linked with the MTN of the accessory optic system.     

The serotoninergic innervation of cerebral cortex: different classes of axon terminals arise from dorsal and median raphe nuclei

The objective of the present study was to characterize the morphology of serotoninergic axons in cerebral cortex of the rat and to determine whether dissimilar axon terminals arise from the dorsal vs. the median raphe nuclei. The anterograde tracer PHA-L was administered by iontophoresis into the dorsal (DR) and median (MR) raphe nuclei, and the morphologic features of the respective axonal projections from raphe to forebrain were analyzed. We have observed consistent structural differences between the axons from these two nuclei. Anterogradely labeled axons which arise from cells in the MR are characterized by large, spherical varicosities (type M axons) and by variations in axonal diameter. In contrast, DR fibers are very fine and typically have small, pleomorphic varicosities that are granular or fusiform in shape (type D axons). Similar features of serotonin (5-HT) axon morphology are also evident in 5-HT immunocytochemical preparations. In addition to structural differences, there is a differential topographic distribution of MR vs. DR fibers, with MR axons concentrated in particular areas of limbic cortex such as dentate gyrus, posterior cingulate, and entorhinal areas as well as in parietal cortex. Immunofluorescence with dual labels shows that over two-thirds of the raphe-cortical axons are serotoninergic. The dissimilarities in axon morphology indicate that individual raphe nuclei may form different patterns of synaptic organization. Based on the evidence that the dorsal and median raphe nuclei give rise to morphologically different axon terminals, we conclude that 5-HT axons in cortex may be subdivided into two distinct projections. This proposal is in accord with other, recent data showing that the two axon types have different pharmacologic properties and are likely to be functionally different.     

Cytology of brain stem neurons projecting to the caudal neurosecretory complex: An HRP-electron microscopic study

Brain stem projections to the neurons in the caudal neurosecretory complex (CNC) of Poecilia sphenops (molly) have been studied with HRP retrograde tracing. Using light microscopic procedures, HRP-labelled neurons were located in the reticular nucleus of the medulla (RMN) and in the vicinity of the nucleus of the medial longitudinal fascicle (NMLF). The present study was undertaken to examine the cytology of the brain stem neurons that project to the caudal neurosecretory complex using combined HRP-electron microscopic methods. Cells in the midbrain NMLF containing HRP-filled profiles were located bilaterally close to the midline and just beneath the ependyma. HRP reaction product was found additionally in the neurosecretory cells of the midbrain dorsal tegmental magnocellular nucleus (DTMN) located dorsal to the NMLF. Cells containing HRP-labelled profiles were also seen in the RMN and in neurons dorsal-lateral to the RMN. This latter group of neurons contained small dense core vesicles in addition to HRP labelled dense bodies.