Axonal projections from the lateral and medial superior olive to the inferior colliculus of the cat: A study using electron microscopic autoradiography

The superior olivary complex is the first site in the central auditory system where binaural interactions occur. The output of these nuclei is direct to the central nucleus of the inferior colliculus, where binaural inputs synapse with monaural afferents such as those from the cochlear nuclei. Despite the importance of the olivary pathways for binaural information processing, little is known about their synaptic organization ir the colliculus. The present study investigates the structure of the projections from the lateral and medial superior olivary nuclei to the inferior colliculus at the electron microscopic level. Stereotaxic placement and electrophysi ological responses to binaural sounds were used to locate the superior olive. Anterograde axonal transport of 3H-leucine was combined with light and electron microscopic autoradiography to reveal the location and morphology of the olivary axonal endings. The results show that the superior olivary complex contributes different patterns of synaptic input to the central nucleus of the inferior colliculus. Each projection from the superior olivary complex to the colliculus differs in the number and combinations of endings. Axonal endings from the ipsilateral medial superior olive were exclusively the round (R) type that contain round synaptic vesicles and make asymmetrical synaptic junctions. This morpholo is usually associated with excitatory synapses and neurotransmitters such as glutamate. Endings from medial superior olive terminate densely in the central nucleus. The projection from the contralateral lateral superior olive also terminates primarily as R endings. This projection also includes small numbers of pleomorphic (PL) endings that contain pleomorphic synaptic vesicles and usually make symmetrical synaptic junctions. The PL morpholo is associated with inhibitory synapses and transmitters such as gamma-aminobutyric acid and glycine. All endings from the contralateral lateral superior olive terminate much less densely than endings from the medial olive. In contrast, the projection from the ipsilateral lateral superior olive contributes both R and PL endings in roughly equal proportions. These ipsilateral afferents are heterogeneous in density and can terminate in lower or higher concentrations than endings from the contralateral side. These data show that the superior olive is a major contributor to the synaptic organization of the centr nucleus of the inferior colliculus. The ipsilateral projections of the medial and lateral superior olive may produce higher concentrations of R endings than other inputs to the central nucleus. Such endings may participate in excitatory synapses. The highest concentra tions of PL endings come from the ipsilateral lateral superior olive. In combination with inputs from the contralateral dorsal nucleus of the lateral lemniscus, PL endings from the superior olive may participate in many inhibitory synapses found in the central nucleus. These different patterns of synaptic input from the superior olivary complex will influence how binaural information is transmitted to the inferior colliculus. © 1995 Wiley-Liss, Inc.     

Organization of the efferent projections of the medial superior olivary nucleus in the cat as revealed by HRP and autoradiographic tracing methods

Features of the organization of the efferent axonal projections from the medial superior olivary nucleus (MSO) in the cat were studied. In order to determine the origin and distribution of projections from MSO, the retrograde horseradish peroxidase (HRP) and autoradiographic tracing methods were used. The results showed that (1) in both HRP and autoradiographic studies the projection to the inferior colliculus was largely ipsilateral, although a contralateral component was present; (2) the projection field of MSO was confined to the ventral division of the central nucleus of the inferior colliculus, and within this field the labeling was heavier in the rostral and dorsolateral parts of the ventral division; (3) the projection to the inferior colliculus was topographic with ventral parts of MSO projecting ventrally and dorsal parts of MSO projecting dorsolaterally; (4) the projection field in the central nucleus formed successive laminae oriented from ventrolateral to dorsomedial; (5) the axonal course was via the medial or internal segment of the lateral lemniscus; and (6) some fibers in this course ended additionally within the dorsal nucleus of the lateral lemniscus. This latter projection was also topographically organized. These observations supported previously described features of lamination and tonotopic order for afferents of the inferior colliculus, as well as recent suggestions that functional segregation of afferent connections exists within the laminated portion of the central nucleus of the inferior colliculus.     

Glycine-immunoreactive projection of the cat lateral superior olive: possible role in midbrain ear dominance

Neurons in the lateral superior olive are optimally excited by stimulation of the ipsilateral ear, as are those in the inferior colliculus by stimulation of the contralateral ear. This reversal of ear dominance may result, in part, from distinct crossed excitatory and uncrossed inhibitory pathways ascending from the lateral superior olive. To explore this possibility, immunoreactivity for two putative inhibitory neurotransmitters, glycine and GABA, was examined in projection neurons that retrogradely transported horseradish peroxidase from the cat inferior colliculus. The results suggest that the projection from the lateral superior olive can be segregated, immunocytochemically, into three components: 1) a crossed, glycine-negative (-) projection; 2) an uncrossed, glycine-positive (+) projection; and 3) an uncrossed, glycine(-) projection. Additional evidence suggests that the terminal fields of the two uncrossed projections may distribute differently within the inferior colliculus. Glycine(+) or glycine(-) projection neurons, crossed or uncrossed, do not differ in the size, shape, or location of their somata. However, most glycine(-) neurons are heavily encrusted with glycine(+) endings; glycine(+) neurons have 40-60% fewer of these endings. Glycine(-) neurons located in the lateral limb have fewer glycine (+) perisomatic endings than those in the medial limb. Few projection neurons are GABA(+), and GABA(+) perisomatic endings are rare in the lateral superior olive. Thus, there is a heavy uncrossed projection from the cat lateral superior olive to the inferior colliculus that may be glycinergic and inhibitory. Furthermore, there is a bilateral projection that is not glycinergic or GABAergic, which may be excitatory. The potential contribution of these pathways to contralateral ear dominance in the inferior colliculus is discussed.     

Organization of binaural excitatory and inhibitory inputs to the inferior colliculus from the superior olive

The major excitatory, binaural inputs to the central nucleus of the inferior colliculus (ICC) are from two groups of neurons with different functions-the ipsilateral medial superior olive (MSO) and the contralateral lateral superior olive (LSO). A major inhibitory, binaural input emerges from glycinergic neurons in the ipsilateral LSO. To determine whether these inputs converge on the same postsynaptic targets in the ICC, two different anterograde tracers were injected in tonotopically matched areas of the MSO and the LSO on the opposite side in the same animal. The main findings were that the boutons from MSO axons terminated primarily in the central and caudal parts of the ICC laminae but that contralateral LSO terminals were concentrated more rostrally and on the ventral margins of the MSO inputs. In contrast, the ipsilateral LSO projection converged with the MSO inputs and was denser than the contralateral LSO projection. Consistent with this finding, retrograde transport experiments showed that the very low-frequency areas of the ICC with dense MSO inputs also received inputs from greater numbers of ipsilateral LSO neurons than from contralateral LSO neurons. The results suggest that different binaural pathways through the low-frequency ICC may be formed by the segregation of excitatory inputs to ICC from the MSO and the contralateral LSO. At the same time, the ipsilateral LSO is a major inhibitory influence in the target region of the MSO. These data support the concept that each frequency-band lamina in the ICC may comprise several functional modules with different combinations of inputs.     

Neuronal death and synapse elimination in the olivocerebellar system. I. Cell counts in the inferior olive of developing rats

A transient multiple innervation of cerebellar Purkinje cells by climbing fibers has been described during postnatal development of the rat. The aim of the present study was to determine if the regression of redundant synapses is related to the loss of presynaptic cells in the inferior olivary nucleus (ION), which is the sole source of climbing fibers in rodents. To this end, the population size of the ION was evaluated by counting healthy cells of the four main subnuclei in rats from birth to adulthood. The cell population at birth was found to be very similar to that of the adult animal (27,655 versus 28,385), but a loss of 25% of the cells occurred in the first five days, presumably through their death since degenerating cells were observed over the same period. Although cell loss was found throughout the whole nucleus, it was more pronounced in the medial accessory olive. A subsequent apparent increase of the cell population was observed so that the adult value was again reached at 15 days. The evolution of the ION population is then characterized by a period of moderate cell death which takes place before the peak of polyneuronal innervation of Purkinje cells by olivary axons is attained. This strongly suggests that the removal of the redundant synaptic contacts established by climbing fibers onto Purkinje cells during development is caused by a progressive reduction of the branching of olivary axons rather than by degeneration of the presynaptic cells.     

Neuron types in the rat lateral superior olive and developmental changes in the complexity of their dendritic arbors

The lateral superior olive (LSO), a conspicuous mammalian brainstem nucleus that is involved in sound localization, has become a model system for investigating the formation of topographically organized inhibitory and excitatory connections. In experiments employing intracellular injections of Lucifer yellow or neurobiotin into lightly fixed brain slices, we have examined the soma-dendritic morphology of 483 LSO neurons of rats between postnatal day (P) 4 and P36. A detailed analysis of the shape and complexity of dendritic arbors was performed in 238 neurons in order to identify different cell classes and to determine whether age-related changes occur that may relate to a topographical refinement. Regardless of age, seven classes of LSO neurons were identified, more than had been delineated previously with the Golgi technique. Bipolar neurons and multipolar neurons comprised the two major cell types, whereas small multipolar cells, banana-like cells, bushy cells, unipolar cells, and marginal cells were found less frequently. Age-related changes were analyzed in bipolar and multipolar neurons, and several modifications of their dendritic arbors were observed that are in accordance with a refinement of topography. For example, at P4, bipolar and multipolar cells had relatively broad dendritic arbors, with an average of 140 and 138 dendritic end branches, respectively. During further development, their numbers became drastically reduced by about 80%, such that an average of less than 30 endpoints remained by P36. As the dendritic arbors became smaller specifically along the transverse axis of the LSO, they became confined to a smaller frequency area. We conclude from our results that considerable remodeling takes place in the LSO and that the selective loss of dendritic branches may be a morphological correlate for the formation of exquisite tonotopy. J. Comp. Neurol. 390:20–40, 1998. © 1998 Wiley-Liss, Inc.     

Vestibular afferents of the inferior olive and the vestibulo-olivo-cerebellar climbing fiber pathway to the flocculus in the cat

The projection of the vestibular nuclei to the inferior olive was investigated by means of anterograde transport of tritiated leucine. Following injections in the medial and descending vestibular nuclei, terminal labeling was found ipsilaterally in the dorsomedial cell column, subnucleus beta and the caudal medial accessory olive, while the latter also received afferents from the nucleus prepositus hypoglossi. At the contralateral side termination in the dorsomedial cell column and the medial accessory olive was found after injections in the nucleus vestibularis superior and group Y. The ventrolateral outgrowth and different parts of the principal olive also received afferents from these two nuclei and also from ventral parts of the lateral cerebellar nucleus. The dorsal cap was labeled exclusively from the contralateral nucleus prepositus hypoglossi. The termination in the inferior olive of the vestibular afferents is compared with the projection from a number of pretectal nuclei. Furthermore the consequences of the divergence and convergence of both types of projections at the level of the inferior olive is discussed in relation to the subsequent climbing fiber projection to the flocculus.     

Electrophysiological identification of neurons in ventrolateral medulla sending collateral axons to paraventricular and supraoptic nuclei in the cat

In recent studies, it has been reported that high-frequency stimulation restricted to A alpha beta fibers in man can be perceived as painful and evoke a nociceptive flexion reflex. These results would indicate that some patterns of activity in low-threshold mechanoreceptors can lead to painful sensations. Because of the theoretical importance of this question, the above studies were extended by recording the evoked neural activity with the technique of percutaneous microneurography. Painful sensations and the nociceptive reflex did not appear unless the evoked nerve response contained activity in A delta fibers. The results support the theory that painful sensations occur in normal man only when nociceptor afferents are activated.     

Organization and synaptic connections of cholinergic fibers in the cat superior colliculus

The cat superior colliculus (SC) receives a dense cholinergic input from three brainstem nuclei, the pedunculopontine tegmental nucleus, the lateral dorsal tegmental nucleus, and the parabigeminal nucleus (PBG). The tegmental inputs project densely to the intermediate gray layer (IGL) and sparsely to the superficial layers. The PBG input probably projects only to the superficial layers. In the present study, the morphology of choline acetyltransferase (ChAT)-immunoreactive axons and synaptic endings in the superficial and deep layers of the SC was examined by light and electron microscopy to determine whether these cholinergic afferents form different types of synapses in the superifical and deep layers. Two types of fibers were found within the zonal (ZL) and upper superficial gray layers (SGL): small diameter fibers with few varicosities and larger diameter fibers with numerous varicosities. Quantitative analysis demonstrated a bimodal distribution of axon diameters, with one peak at approximately 0.3–0.5 μm and the other at 0.9–1.0 μm. On the other hand, ChAT-immunoreactive fibers in the IGL were almost all small and formed discrete patches within the IGL. Two types of ChAT-immunoreactive synaptic profiles were observed within the ZL and upper SGL using the electron microscope. The first type consisted of small terminals containing predominantly round synaptic vesicles and forming asymmetric synaptic contacts, mostly on dendrites. The second type was comprised of varicose profiles that also contained round synaptic vesicles. Their synaptic contacts were always symmetric in profile. ChAT-immunoreactive terminals in the IGL patches contained round or pleomorphic synaptic vescles, and the postsynaptic densities varied from symmetric to asymmetric, including intermediate forms. However, no large varicose profiles were observed. This study suggests that cholinergic fibers include at least two differnt synaptic morphologies: small terminals with asymmetric thickenings and large varicose profiles with symmetric terminals. The large varicose profile in the superficial layers is absent in the IGL. This result suggests that the cholinergic inputs that innervate the superficial layers and the patches in the IGL of the cat SC differ in their synaptic organization and possibly also in their physiological actions. © 1993 Wiley-Liss, Inc.     

Neuronal death and synapse elimination in the olivocerebellar system: III. Cell counts in the inferior olive of developing rats X-irradiated from birth

The change with age of cell number in the developing inferior olivary nucleus (ION) of the normal rat, compared to the time course of the regression of the polyneuronal innervation of Purkinje cells by olivary axons (i.e., the climbing fibers), suggests that the involution of the redundant olivocerebellar contacts is caused by a reduction of axonal branching rather than by degeneration of the parent cells, this being also suggested by the normal size of the olivary population in adult rodents whose Purkinje cells retain polyneuronal innervation. However, the similar size of the adult ION population does not necessarily imply that the developmental history is the same in normal and multiply innervated adult rodents. Therefore, cell counts were performed in developing rats which had been repeatedly X-irradiated from birth until postnatal day 14 and which retained polyneuronal innervation. The results show that, although less marked than during normal development, the evolution of the ION population is also characterized by a phase of cell loss followed by a slow increase. However, the number of cells in X-irradiated rats is higher than in their controls from birth to postnatal day 15 but becomes identical at 20 days and later. These data confirm that cell death in the ION does not play a major role in the shaping of olivocerebellar connections.     

Development of glycinergic cells and puncta in nuclei of the superior olivary complex of the postnatal ferret

The distribution of glycine-immunopositive cells and axonal endings was studied in the adult and early postnatal ferret superior olive. As in other species, the most prominent glycine-immunopositive cell group in the adult ferret superior olive was the medial nucleus of the trapezoid body. Other darkly immunostained cells were present, although more scattered, in most periolivary regions, including the lateral and ventral trapezoid body nuclei. In the lateral superior olivary nuclei, glycine-immunopositive cells were intermingled with immunonegative cells. A comparable population of cells in the ipsilateral lateral superior olivary nucleus was retrogradely labeled in cases with unilateral injections of tritiated glycine in the inferior colliculus. Glycine-immunopositive puncta were widely distributed in the neuropil in most periolivary regions, including dense accumulations in the dorsomedial periolivary region and ventral and lateral nuclei of the trapezoid body. In the lateral and medial superior olivary nuclei, immunopositive puncta were distributed around the principal cells in characteristic perisomatic halos. In postnatal ferrets, immunopositive cell bodies were first observed by postnatal day 7 and were distributed in regions comparable to regions in the adult, with the exception that immunopositive cells in the lateral superior olivary nucleus did not appear until about postnatal day 28. There was diffuse staining in the neuropil in principal and periolivary nuclei by postnatal day 7. During the third postnatal week, the immunostaining in the neuropil began to take on a more granular appearance and immunopositive puncta could be seen by postnatal day 35. In the lateral and medial superior olivary nuclei, the earliest distribution of immunostaining in the neuropil was nonuniform, being greater in the high-frequency, medial, and ventral regions, respectively. The density gradient in these areas was gradually eliminated over the next 2 postnatal weeks as immunostained processes and endings appeared over greater portions of the nuclei. © 1995 Wiley-Liss, Inc.     

The corticotrigeminal projection in the cat. A study of the organization of cortical projections to the spinal trigeminal nucleus

The projection from the cerebral cortex to the spinal trigeminal nucleus has been studied light microscopically in adult cats. Both orthograde degeneration and orthograde intra-axonal labeling techniques have been applied. Our results indicate that the projection from the coronal gyrus (face area of primary somatosensory cortex) to the spinal trigeminal complex is somatotopically organized. In subnucleus caudalis this somatotopy is organized dorsoventrally and appears to match the somatotopic distribution of the divisional trigeminal afferents. Hence cortical fibers originating from the posterior coronal gyrus (upper representation) project ventrolaterally into caudalis where division I trigeminal afferents terminate. Likewise cortical fibers from the anterior coronal gyrus (jaw and tongue representation) terminate dorsomedially in caudalis to overlap with division III trigeminal afferents. In contrast, the distribution of corticofugal afferents to the rostral spinal trigeminal subnuclei (pars interpolaris and oralis) is organized mediolaterally. Therefore in these subnuclei the cortical projection does not appear to overlap the dorsoventral lamination of the divisional trigeminal afferents. In addition, our results suggest that the cortical projection to subnucleus caudalis includes fibers which terminate in the marginal zone (lamina I) and its extensions into the spinal trigeminal tract (the interstitial cells of Cajal). We have been unable to document a projection from the proreate gyrus to the spinal trigeminal complex.     

Neuronal death and synapse elimination in the olivocerebellar system. II. Cell counts in the inferior olive of adult x-irradiated rats and weaver and reeler mutant mice

Cell death in the developing rat inferior olive precedes the regression of the polyneuronal innervation of Purkinje cells by olivary axons (i.e., climbing fibers), suggesting that the involution of the redundant olivocerebellar contacts is caused by a withdrawal of supernumerary axonal collaterals rather than by degeneration of the parent cell. However, a subsequent apparent increase of the olivary population occurs, which could eventually mask a residual presynaptic cell death taking place at the same time. Therefore, cell counts were performed in the inferior olive of adult rodents in which the multiple innervation of Purkinje cells by olivary axons is maintained, with the idea that if cell death plays a role in the regression of supernumerary climbing fibers, the number of olivary cells should be higher in these animals than in their controls. The results show that the size of the cell population in the inferior olive of weaver and reeler mutant mice and rats degranulated by early postnatal x-irradiation does not differ significantly from that of their controls. Similarly, the distribution of the cells in the four main olivary subnuclei is not modified in weaver mice and x-irradiated rats. The present data further support the assumption that the regression of the polyneuronal innervation of Purkinje cells occurs independently of cell death in the presynaptic population.     

Acoustic chiasm. III: Nature, distribution, and sources of afferents to the lateral superior olive in the cat.

The outcomes of seven experiments are reported, each directed to the nature and sources of the excitation and inhibition impinging on the lateral superior olive (LSO) in cats. In the first experiment, we used conventional 14C 2-DG methods to determine the specificity, precision, and extent of symmetry in the stimulation reaching LSO from the ipsilateral and contralateral ears. In Experiment 2, we sought the presence of GABA and glycine receptors in LSO using conventional, in vitro receptor-binding methods. On the basis of these results, we used in vitro high-affinity uptake methods in Experiment 3 to seek evidence that some of the terminals as well as the receptors in LSO are glycinergic. In Experiment 4, we used immunocytochemical methods to show that the somata known to supply the contralateral projections to LSO, and their terminals in LSO, are each immunoreactive with an antibody directed to a glycine-protein conjugate. In Experiment 5, we made use of a glycinergic neuron's avidity for transporting glycine retrogradely to label the likely sources of the glycinergic terminals in LSO. In Experiment 6, we used immunocytochemical methods to show that the spherical and globular cells of the ventral cochlear nucleus and terminals in LSO and in MTB are glutamatergic and/or aspartergic. In Experiment 7, we used receptor binding methods to determine whether the glutamate/aspartate receptors in LSO are probably of the kainate or of the quisqualate type. The results of the several experiments suggest that probably glutamate-quisqualate synapses mediate LSO's ipsilaterally driven excitatory responses and glycinergic synapses mediate its contralaterally driven inhibitory responses. The two types of input appear to be well matched in LSO's medial and middle limbs with glycinergic terminals mostly perisomatic and glutamatergic terminals mostly peridendritic. However, LSO's low frequency lateral limb appears to be somewhat different; it receives less stimulation from the contralateral ear. Instead, LSO's lateral limb may receive some of its glycinergic input directly from the ipsilateral ventral cochlear nucleus and/or indirectly via the juxtaposed lateral nucleus of the trapezoid body.     

Brain stem influences on the parasympathetic supply to the urinary bladder of the cat

(1)The brain stem of anaesthetized cats has been mapped between Horsley-Clark planes APO and P8.5 with electrical stimuli of low intensity in order to determine the areas which can produce excitatory of inhibitory influences on the spontaneously contracting, sympathetically denervated, urinary bladder. (2) Two inhibitory areas were found. The first extended from P3.0 to P8.5 and at all levels was coincident with the midline raphe nuclei. The second area occurred largely 2-3 mm lateral to the midline, in the area of the nucleus reticularis pontis caudalis rostrally, and the nuclei reticularis gigantocellularis and parvocellularis caudally. Both of these areas were found to inhibit bladder contractions with threshold stimulus parameters of 20-60 microamperemeter, 400 microseconds, 20 Hz. (3)One excitatory area was found, largely 3-4 mm lateral to the midline. This area appeared large and diffuse in the lateral reticular formation. It is possible that it originated in the pontine micturition centre in the rostral pontine tegmentum. Caudally, it shifted to occupy a ventrolateral position. This excitatory area was in close approximation to, and was probably interspersed with, the lateral inhibitory area. (4) In decerebrate preparations the areas that produced excitation or inhibition had the same distribution as those found in anaesthetized animals. (5) Single shock stimulation (100 microamperemeter, 400 microseconds, 0.5 Hz repetition frequency in the excitatory area could produce firing in pelvic nerve efferents to the bladder at latencies of 60-110 ms. The amplitude of such responses was dependent on the level of intravesical pressure. (6) Stimulation of the inhibitory areas produced no evoked responses in the pelvic nerve efferents, but could inhibit reflexly evoked responses in this nerve. The similarity in the time courses of the inhibitory effects from the two areas raises the possibility that one acts via the other.     

Differential recruitment of hypothalamic neuroendocrine and ventrolateral medulla catecholamine cells by non-hypotensive and hypotensive hemorrhages

We performed c-fos expression experiments in conscious rats to quantify the threshold and extent of activation of hypothalamic neuroendocrine cells in response to non-hypotensive and hypotensive hemorrhages allowing us to assess whether their pattern of recruitment corresponded to known oxytocin, vasopressin and ACTH release patterns. Also, because previous studies have implicated ventrolateral medulla catecholamine cells in the generation of certain hypothalamic neuroendocrine cell responses, we examined the response of ventrolateral medulla catecholamine cells to non-hypotensive and hypotensive hemorrhages and directly tested their role in regulating neuroendocrine cell responses to hypotensive hemorrhage. Animals were subjected to hemorrhages of 0, 4, 8, 12 or 16 ml/kg BW, the latter two levels being hypotensive. We found that only supraoptic nucleus vasopressin cells were significantly activated by the smallest non-hypotensive hemorrhage (4 ml/kg), which corresponds to reports that only vasopressin is released into the plasma after a small hemorrhage. Hypotensive hemorrhages resulted in significant recruitment of paraventricular and supraoptic oxytocin and vasopressin cells and parvocellular cells of the medial division of the paraventricular nucleus. Vasopressin cells were recruited in much greater numbers than oxytocin cells, which is in agreement with previous findings that there is a greater release of vasopressin than oxytocin into the plasma after hypotensive hemorrhage. In addition, medial parvocellular cells of the paraventricular nucleus, most likely to be tuberoinfundibular-projecting corticotropin-releasing factor cells, were activated by hypotensive hemorrhage only when arterial pressure dropped below 60 mmHg which also corresponds well with the plasma release response of ACTH. Ventrolateral medulla catecholamine cells were only recruited by hypotensive hemorrhages. While caution must be exercised in interpreting an absence of response, this certainly suggests that catecholamine cells are unlikely to have a role in the activation of supraoptic neurosecretory cells in response to non-hypotensive hemorrhages. Unilateral lesions of the ventrolateral medulla catecholamine cell column, corresponding primarily to the location of A1 noradrenergic cells, significantly reduced the hypotensive hemorrhage-induced activation of hypothalamic vasopressin, oxytocin and medial parvocellular paraventricular nucleus cells. This suggests that A1 noradrenergic cells contribute to the activation of these neuroendocrine cell populations, including oxytocin cells, which is an unexpected finding. More significantly, however, because the reduction in responsiveness after A1 lesions was similar for all cell categories, it seems likely that other factors must determine the differential recruitment of hypothalamic neuroendocrine cells in response to a hypotensive hemorrhage.     

Ultrastructural study of the GABAergic, cerebellar, and mesodiencephalic innervation of the cat medial accessory olive: anterograde tracing combined with immunocytochemistry

The rostral medial accessory olive (MAO) of the cat was studied by using an ultrastructural technique combining wheat germ agglutinin-coupled horseradish peroxidase (WGA-HRP) anterograde tracing and postembedding GABA immunocytochemistry. One group of cats received a WGA-HRP injection in the posterior interposed nucleus of the cerebellum and another group received an injection in the nucleus of Darkschewitsch. Based on differences in their morphology three types of GABAergic and three types of nonGABAergic terminals were observed. One type of GABAergic terminal was often GABA/WGA-HRP double-labeled in the cerebellar experiments, and one type of nonGABAergic terminal was often WGA-HRP-labeled in the mesodiencephalic experiments. Following injections of WGA-HRP in the cerebellar nuclei virtually all WGA-HRP-labeled terminals were GABA positive. Quantification of these GABA/WGA-HRP-double-labeled terminals showed that 1) 30% of the GABAergic terminals randomly selected from the entire neuropil were double-labeled, 2) 13% of the GABAergic terminals adjacent to perikarya were double-labeled, and 3) 34% of the GABAergic terminals strategically located next to both of the dendritic elements linked by a gap junction were double-labeled. Statistical analysis of the above data showed that significantly fewer GABAergic terminals adjacent to perikarya were double-labeled (P less than .001) than would be expected from the double-labeled proportion of the randomly selected GABAergic terminals. Following injection of WGA-HRP in the nucleus of Darkschewitsch, all WGA-HRP-labeled terminals were GABA-negative. Quantification of these terminals showed that 1) 26% of the randomly selected nonGABAergic terminals were WGA-HRP labeled, 2) 20% of the nonGABAergic terminals adjacent to perikarya were WGA-HRP labeled, and 3) 23% of the nonGABAergic terminals strategically located next to a gap junction were WGA-HRP labeled. No significant differences were found among these populations. Quantification of terminals of both groups of experiments mentioned above showed that GABAergic terminals composed 1) 38% of the randomly selected terminals, 2) 64% of the terminals apposed to perikarya, and 3) 53% of the terminals strategically located next to gap junctions. Statistical analysis of the above data showed that significantly more GABAergic terminals were located adjacent to perikarya (P less than .001) and strategically next to a gap junction (P less than .05) than would be expected from the random GABAergic innervation. The above findings of the GABAergic, cerebellar, and mesodiencephalic input are discussed with regard to their functional role in the neuronal circuitry of the ros     

Ultrastructure of neurons and large synaptic terminals in the lateral nucleus of the trapezoid body of the cat

Neurons of the lateral nucleus of the trapezoid body (LNTB), the most prominent periolivary nucleus of the cat superior olivary complex, form an important component of the descending auditory pathways and also innervate the medial superior olive. Cells forming the posteroventral subnucleus (pvLNTB), when investigated by light microscopy, exhibit morphological similarities with globular bushy cells of the cochlear nucleus and principal cells of the medial nucleus of the trapezoid body. These latter two cell types are integral components of brainstem circuitry mediating the early stages of sound localization. In this report, ultrastructural features of LNTB neurons are described. pvLNTB cell bodies are characterized by a round to oval shape, smooth nuclear membrane, and the relative paucity of stacks of rough endoplasmic reticulum. In addition, pvLNTB cell bodies and proximal dendrites are contacted by large synaptic terminals which contain round synaptic vesicles and form multiple asymmetric synaptic junctions. These ultrastructural characteristics are similar to those previously described for globular and principal cells and distinguish pvLNTB cells from cells of the main subnucleus. Large terminals contacting pvLNTB cells contain a specialized organelle assembly, including an adherens plaque associated by filamentous strands with a mitochondrion. We name this organelle assembly the mitochondria-associated adherens complex (MAC) and note its proximity to synaptic junctions. Because high activity rates are characteristic of large terminals in the lower auditory system, the MAC may play a specialized role in membrane stabilization at synapses which generate high rates of vesicle membrane turnover. J. Comp. Neurol. 398:257–272, 1998. © 1998 Wiley-Liss, Inc.     

Central distribution of afferent and efferent components of the chorda tympani in the cat as revealed by the horseradish peroxidase method

Central distribution of afferent and efferent components of the chorda tympani (CT) in the cat was examined by using the anterograde and retrograde tracing techniques of horseradish peroxidase (HRP). HRP was applied to the CT in the tympanic cavity. HRP-labeled CT fibers were traced to the brain stem along the ventral surface of the vestibular nerve. The afferent CT fibers were divided into ascending and descending components. The rostrally directed ascending fibers ended within and around the dorsomedial portions of the principal sensory trigeminal nucleus. The descending fibers entered the solitary tract to run caudally as far as the levels slightly rostral to the obex, giving terminals to the solitary nucleus. A cluster of HRP-labeled neurons were seen ipsilaterally in the lateral reticular formation medial to the spinal trigeminal nucleus; it was observed from the caudalmost levels of the exiting root of the facial nerve to the caudal levels of the facial nucleus. HRP-labeled axons arising from the HRP-labeled neurons firstly ran dorsomedially and then medially under the genu of the facial nerve to form a small genu at the region medial to the genu of the facial nerve. Subsequently the labeled axons ran laterally and ventrolaterally to join other CT fibers at the dorsomedial aspect of the spinal trigeminal tract.     

Distinct X- and Y-streams in the cat visual cortex revealed by bicuculline application

The projections from the central nucleus of the amygdala to the dorsal vagal complex were examined in the rat by means of anterograde and retrograde axonal transport of wheat germ agglutinin-horseradish peroxidase and anterograde degeneration. Light microscopic findings confirmed that the amygdala projects to the dorsal motor nucleus (DMV) and the nucleus of the solitary tract. Electron microscopic experiments demonstrated degenerating axosomatic and axodendritic terminals in the DMV following electrolytic lesions in the central nucleus of the amygdala.