Cells of origin, thalamic relay and termination of the external cuneothalamocortical tract in the gerbil

Dorsal column nuclei projection to the cerebellar caudal vermis in the rabbit revealed by the fluorescent double-labeling method

The organization of the projection from the dorsal column nuclei (DCN) to the lobules of the cerebellar caudal vermis was studied in the rabbit. Following unilateral injections of the retrograde fluorescent tracers fast blue (FB) and diamidino yellow (DY) into the pyramis (Pr) and uvula (Uv), respectively, a great number of single FB- (40%) and DY-labeled (60%) neurons were observed in the ipsilateral (79%) and contralateral (21%) DCN subdivisions. These neurons, as parents for the DCN-Pr and DCN-Uv projections, were numerous in the lateral cuneate nucleus (CuL; 84 and 74%, respectively) and in the complex of the gracile (Gr) and medial cuneate nuclei (CuM; Gr+CuM; 14 and 25%, respectively). A small percentage of the Pr projecting neurons was found in the CuM and Gr nuclei (2% in total). As regards the Uv, a rare and only ipsilateral projection arose from the CuM (1%), and no connection originated from the Gr. The distribution pattern of labeled neurons within individual subnuclei indicates that there are both separate regions and, to a great extent, common regions of the DCN-Pr and DCN-Uv projections. In these common regions, a small population of double FB+DY-labeled neurons (1.2%) was identified. Such neurons, present exclusively in the ipsilateral CuL and Gr+CuM, were the source of projection by way of axonal collaterals to the Pr and Uv simultaneously. It is suggested that the described connections may play a role in coordination of the axial and proximal forelimb muscles.

Ultrastructural localization of acetylcholinesterase and choline acetyltransferase in oligodendrocytes, glioblasts and vascular endothelial cells in the external cuneate nucleus of the gerbil

This study reports the reactivities of acetylcholinesterase (AChE) and choline acetyltransferase (ChAT) in some of the nonneuronal elements in the external cuneate nucleus (ECN) of gerbils. AChE reaction products were localized in some oligodendrocytes in their cisternae of rough endoplasmic reticulum, nuclear envelope and Golgi saccules. The basal lamina lining the capillary endothelia also displayed AChE reactivity. In ChAT immunocytochemistry, the reaction products were found to be associated with the vascular basal lamina as well as the endothelial plasma membrane facing the lumen. The most remarkable finding was the localization of ChAT immunoreactivity in some oligodendrocytes and occasional glioblasts (small glial precursor cells containing a thin rim of cytoplasm surrounding an irregular nucleus with homogeneous chromatin materials). The ChAT-positive oligodendrocytes consisted of two types, medium-dense and dark cells, either associated with blood vessels or ChAT-stained neuronal elements. It is suggested from these new findings that at least some of the oligodendrocytes and glioblasts in the ECN of gerbils may be involved in the synthesis, storage, release and degradation of acetylcholine.

Ultrastructural study of external cuneothalamic neurons and their synaptic relationships with primary afferents in the gerbil

The present study examined the synaptic organization of external cuneothalamic neurons and their relationships with primary afferents in the gerbil external cuneate nucleus (ECN) following an injection of horseradish peroxidase (HRP) into the anterodorsal cap of the ventrobasal thalamus in conjunction with a simultaneous injection of HRP into the contralateral brachial and cervical nerve plexuses. The thalamus-projecting neurons have been shown to be confined to the intermediate portion of the caudal half of the ECN at the light microscopic level (Lan et al., 1994c). In this study, HRP-labelled external cuneothalamic neurons were ultrastructurally characterized by their relatively small-sized soma bearing a variable number of somal spines. Their nucleus had a slightly indented contour with an eccentric nucleolus. The HRP-labelled somata were postsynaptic to many axon terminals, which were classified into round (Rs type; 53.0%), pleomorphic (Ps type; 32.7%), and flattened (Fs type; 14.3%) vesicle-containing boutons. The HRP-labelled dendritic elements were postsynaptic to a greater number of axon terminals, which were also classified into the round (Rd; 64.7%), pleomorphic (Pd; 25.2%), and flattened (Fd; 10.1%) type boutons. These presynaptic axonal boutons tended to synapse on distal and secondary dendrites of external cuneothalamic neurons. In the present simultaneous HRP labelling study, some of the primary afferent terminals made direct synaptic contacts with the dendrites of the external cuneothalamic neurons. In view of the multiple inputs onto the external cuneothalamic neurons, impinging particularly on their somata and secondary dendrites, it is suggested that the proprioceptive information reaching these neurons is intensively modulated and integrated before transmission ultimately to the cerebral sensorimotor cortex.

Ultrastructural study of phenylethanolamine-N-methyltransferase, corticotropin-releasing factor and neurotensin immunoreactive neurons in the external cuneate nucleus of the gerbil

The present study examined the existence of catecholamine-, corticotropin-releasing factor (CRF)- and neurotensin (NT)-containing neurons in the external cuneate nucleus (ECN) of the gerbil using single label pre-embedding immunocytochemistry in an attempt to shed light on the increasing evidence for autonomic involvement of the ECN. Peroxidase immunoreactivity of phenylethanolamine-N-methyl-transferase (PNMT), CRF or NT was identified in the heterogeneous population of the ECN neurons characterized by a deeply infolded nucleus. The label was localized in their somata, dendrites, myelinated axons and axon terminals. The immunolabelled dendrites were contacted by spherical (S) and flattened (F) types of presynaptic boutons containing spherical and flattened synaptic vesicles, respectively. The PNMT-labelled dendrites, however, were postsynaptic to an additional type of axon terminals containing pleomorphic (P) synaptic vesicles. Among the immunoreactive axon terminals, the PNMT-labelled boutons consisted of two types: S and F; in the CRF- and NT-labelled axon terminals, only the S type was observed. The catecholamine-containing ECN neurons differed from the CRF- and NT-immunoreactive neurons in their synaptic organization. The latter two were considered to be of the same cell population because of their similarities in ultrastructural features and synaptic relations. In view of a high frequency (48% for PNMT, 50% for CRF and 46% for NT) of the F-typed boutons associated with the three categories of immunolabelled neurons in the ECN, it is possible that they are under considerable inhibitory control. The presence of catecholamine, CRF and NT in the ECN suggests that the nucleus may be involved in the integration of proprioception-, exercise- or stress-evoked autonomic responses.

Ultrastructural identification of cholinergic neurons in the external cuneate nucleus of the gerbil: acetylcholinesterase histochemistry and choline acetyltransferase immunocytochemistry

Using acetylcholinesterase histochemical and choline acetyltransferase immunocytochemical localization methods, this study has provided conclusive evidence for the existence of cholinergic neurons in the external cuneate nucleus of gerbils. By light microscopy, both acetylcholinesterase and choline acetyltransferase labelling was confined to the rostral portion of the external cuneate nucleus. Ultrastructurally, acetylcholinesterase reaction products were found in the nuclear envelope, cisternae of rough endoplasmic reticulum and Golgi saccules of some somata and large dendrites as well as in the membranes of small dendrites, myelinated axons and axon terminals. These neuronal elements were also stained for choline acetyltransferase; immunoreactivity was associated with nuclear pores, nuclear envelope, perikaryal membrane and all the membranous structures within the cytoplasm. Of the total choline acetyltransferase-labelled neuronal profiles analysed, 79% were myelinated axons, 15% dendrites, 4% somata and 2% axon terminals. The immunostained axon terminals consisted of two types containing either round (Rd type; 62.5%) or pleomorphic (Pd type; 37.5%) vesicles. Both were associated directly with choline acetyltransferase-positive dendrites. In contrast to the paucity of choline acetyltransferase-labelled axon terminals, numerous choline acetyltransferase-positive myelinated axons were present. It may thus be hypothesized that most, if not all, of the external cuneate nucleus cholinergic neurons are projection cells; such cells may give rise to axonal collaterals which synapse onto their own dendrites for possible feedback control. Choline acetyltransferase-positive dendrites were contacted by numerous unlabelled presynaptic boutons, 60% of which contained round or spherical synaptic vesicles (Rd boutons) and 40% flattened vesicles (Fd boutons), suggesting that these neurons are under strong inhibitory control. The preferential concentration of cholinergic components in the rostral external cuneate nucleus may be significant in the light of the highly organized somatotopy in the external cuneate nucleus and its extensive efferent projections to medullary autonomic-related nuclei. Our results suggest that the cholinergic neurons may be involved in somatoautonomic integration.

An ultrastructural study of cuneocerebellar neurons and primary afferent terminals in the external cuneate nucleus of gerbils as revealed by retrograde and transganglionic transport of horseradish peroxidase

The present study examined the synaptic organization of cuneocerebellar neurons and their relationships with the primary afferents in the gerbil external cuneate nucleus following multiple injections of horseradish peroxidase over a widespread area in the cerebellum in conjunction with a simultaneous injection of horseradish peroxidase into the cervical or brachial nerve plexus. The external cuneate nucleus is topographically organized: the rostral portion receiving the primary afferents from the cervical plexus and the caudal portion primary afferents from the brachial plexus. This study attempted to correlate the synaptology with the topography and different cytoarchitecture in these two specific regions in the external cuneate nucleus. Ultrastructurally, the profiles of horseradish peroxidase-labelled cuneocerebellar neurons could be divided into three types, namely, small, medium and large on the basis of their cross-sectional areas. Axon terminals which formed axosomatic synapses could be classified into: round (Rs type; 22.2%), pleomorphic (Ps type; 55.6%) and flattened (Fs type; 22.2%) vesicle boutons. The horseradish peroxidase-labelled dendritic elements of the cuneocerebellar neurons were postsynaptic to a greater number of axon terminals which were also classified into Rd (77.5%), Pd (18.8%) and Fd (3.7%) type boutons. Some of the Rd boutons making direct synaptic contacts with the cuneocerebellar neurons originated from primary afferents since they were simultaneously labelled by transganglionic transport of horseradish peroxidase. In the rostral external cuneate nucleus, synapses on cuneocerebellar neurons were more frequent on their primary dendrites as compared with those on the primary dendrites of the caudal cuneocerebellar neurons. The latter, on the other hand, showed more synapses on their distal dendrites. This may have functional implications with regard to the afferent inputs to cuneocerebellar neurons in the rostral and caudal external cuneate nucleus.

Efferent connections from the external cuneate nucleus to the medulla oblongata in the gerbil

The present study revealed the efferent projections from the external cuneate nucleus (ECN) to various medullary nuclei in the gerbil as demonstrated in fresh living brainstem slices by using in vitro anterogradely tracing with the dextran-tetramethyl-rhodamine-biotin. The tracer-labelled ECN axon terminals were observed (1) in most of the vital autonomic-related nuclei: the nucleus solitary tractus, nucleus ambiguus, rostroventrolateral reticular nucleus and C2 adrenergic area, (2) in the reticular formation: the medullary, parvocellular, intermediate, gigantocellular, dorsal paragigantocellular and lateral paragigantocellular reticular nuclei and medullary linear nucleus, and (3) in sensory nuclei: the cuneate nucleus, spinal trigeminal nuclei caudalis and interpolaris, paratrigeminal nucleus, medial and spinal vestibular nuclei, inferior olive and prepositus hypoglossal nucleus. These new findings are discussed in relation to possible roles of the ECN in cardiovascular, respiratory and sensorimotor controls.