Nitric oxide-containing neurons in long-term grafts in a rat model of Parkinson's disease.

The role that nitric oxide may play in modulating graft function in long-term fetal ventral mesencephalic grafts in an animal model of Parkinson's disease was investigated. Mature grafts harvested from the entire fetal ventral mesencephalon possessed a large number of neuronal nitric oxide synthase (nNOS)/NADPH-diaphorase-containing neurons throughout the graft intermingled with dopaminergic neurons. The morphological and neurochemical characteristics of these NADPH-diaphorase neurons resembled those in centers adjacent to the substantia nigra of adult brain but not that of the striatum. Pretreatment with the nNOS blocker, 7-nitroindazole, resulted in contralateral rotations following methamphetamine challenge in long-term grafted animals that previously showed normalized rotational behavior. In contrast, mature grafts derived from fetal ventral mesencephalon without the midline areas possessed only a few nNOS-containing neurons within the grafts, and a similar methamphetamine challenge following 7-nitroindazole pretreatment in long-term grafted rats that previously showed normalized rotational behavior resulted in random movements. Our results indicate that nitric oxide-containing neurons inadvertently included during grafting may affect graft function, and excluding the midline areas of the ventral mesencephalon during tissue harvesting may minimize this effect.

Time course of mossy fiber degeneration following pontine ablation in the rat.

The normal ultrastructure of mossy fiber terminals within the ansiform lobule of the rat was investigated in parasagittal sections of the cerebellar cortex. Four types of mossy fiber varicosities (simple and complex dispersed, simple and complex clustered) were distinguished on the basis of the presence or absence of clublike excrescences and the concentration of synaptic vesicles within the cortical zone of the terminal. Removal of the pontocerebellar input to the contralateral cerebellar hemisphere was performed electrolytically. Such lesions, when placed in the basilar pontine gray using a dorsal approach, resulted in the degeneration of nearly the entire population of mossy fiber varicosities within the contralateral ansiform lobule. As early as 3 days following pontine ablation, degenerative changes were observed within the synaptic portions of the mossy fibers. Two distinct axonal reactions were apparent within this population of degenerating varicosities. A small population of mossy fiber varicosities (12%) of the simple clustered variety underwent a rapid course of electron-dense degeneration, which was complete by the fifth day. These mossy fiber varicosities were very susceptible to phagocytosis by reactive glial elements. The second group which consisted of simple (60%) and complex (26%) varieties of dispersed terminals, underwent a very slow course of electron-dense degeneration. Reactive glial cells were rarely found in association with this second group of degenerating varicosities. As a result, most glomeruli were found to contain debris from degenerating mossy fiber varicosities throughout the first 57 days following pontine lesions. The majority of the cerebellar glomeruli, however, were denervated by 80 days.

Long-term effect of mossy fiber degeneration in the rat.

Mossy fiber-deafferentated rats (20) were permitted to survive from 34 to to 120 days and subsequently examined following Golgi-Cox preparation or after processing for electron microscopy. The primary response to mossy fiber deafferentation was transneuronal degeneration of the granule cell system. Morphological evidence is provided that suggests that the mossy fiber varicosity plays an important role in the fragmentation and removal of the granule cell digitiform dendrite. Computer-assisted image analysis of Golgi-impregnated Purkinje cells indicated significant losses in both smooth branch and spiny branchlet numbers following loss of the mossy fiber input. Ultrastructural examination revealed that a secondary transneuronal degeneration occurred within the dendritic arborization of both Purkinje cells and molecular layer interneurons. Although an overall reduction in the number of dendritic spines occurred along the terminal branchlets following mossy fiber deafferentation, several of the existing spines underwent marked changes in length, with some elongating to more than twice their size. By increasing the length of their spines, denervated Purkinje cells may acquire new synaptic contacts.

Cytology and synaptology of the lateral reticular nucleus of the cat.

The organization of lateral reticular nucleus (LRN) of the cat was investigated using electron microscopy and Golgi techniques. Golgi-Cox preparations revealed that the LRN consists of allodendritic and, especially, isodendritic neurons. The latter have been associated with neural centres that have important roles integrating signals from distant sources. Several forms of spines were identified with the Golgi method, and their ultrastructural correlates were determined. Somatic spines resembled stubby protrusions, while dendritic spines, where were usually observed on distal dendrites, appeared as pedunculated spines, racemose appendages and spine-crowned appendages. Ultrastructural examination of this nuclease revealed various synaptic relationships. The majority of the synaptic terminals were small (1.5--2.5 micrometer in diameter), contained round vesicles and usually contacted dendrites and spines. Other small terminals contained pleomorphic vesicles and contacted distal dendrites and spines. Large terminals (greater than 2.5 micrometer in diameter) with round or pleomorphic vesicles contacted the somata or proximal dendrites. Three types of "synaptic configurations," which consisted of discrete aggregations of neuronal processes invested by astrocytic lamellae, were also identified. These structural arrangements likely provide a basis for the integration of inputs to the LRN from spinal and supraspinal centres.

A light microscopic investigation of the afferent connections of the lateral reticular nucleus in the cat.

The topographical organization of the projections to the lateral reticular nucleus (LRN) of the cat was investigated using the horseradish peroxidase (HRP), silver-impregnation and autoradiographic tracing methods. Following injection of HRP into the LRN, labelled cells were found mainly within Rexed's laminae VII and VIII of the spinal cord, the contralateral red nucleus, the ventro-rostral aspect of the contralateral fastigial nucleus and the contralateral anterior sigmoid and coronal gyri of the cerebral cortex. Animals with injections of tritiated amino acids placed within the pericruciate cortex, red nucleus or fastigial nucleus were processed for autoradiography. In a corresponding series of animals, electrolytic lesions were placed selectively into the above sources of reticular afferents, and terminal degeneration within the LRN was studied by light microscopy. An extensive input from the spinal cord was found to terminate predominantly on the ipsilateral side throughout the rostrocaudal extent of the LRN, except for a small ventromedial area of the rostral parvocellular division and a small rostromedial area of the magnocellular division. The cortical projection terminated diffusely within the middle one-half of the contralateral magnocellular division, while the rubral projection terminated extensively within the contralateral subtrigeminal division and the dorsolateral region of the rostral magnocellular and neighbouring parvocellular divisions. The rubral projection did not overlap the cortical projection. The fastigial nucleus projected sparsely to the contralateral LRN, mainly to the medial aspect of the rostral two-thirds of the magnocellular division, with less to the parvocellular and subtrigeminal divisions. The LRN therefore receives spinal and supraspinal projections within at least its rostral one-half, and these terminate within specific areas in a partially overlapping fashion, whereas the caudal one-half is primarily a spinal receiving region. No convergence of the rubral and sensorimotor cortical projections was evident.

A light and electron microscopic study of the effects of 3-acetylpyridine intoxication on the inferior olivary complex and cerebellar cortex.

The effects of 3-acetylpyridine (3-AP) intoxication on the inferior olivary complex and cerebellar cortex of the rat were examined at both the light and electron microscopic level. Following intraperitoneal injection of 65 mg of 3-AP per kg body weight, the inferior olivary neurons were observed to undergo a rapid form of electron dense degeneration. A complete bilateral involvement of the nuclear complex was well advanced as early as 12 hours following injection. Marked astrocytic proliferation also occurred by 12 hours and appeared essential for neuronal fragmentation and disintegration. Microglial activity was prominent in the later stages, from 60 hours onwards, and participated in the phagocytic removal of degenerating neuronal fragments. By the end of the second week, all cytoplasmic and nuclear debri was removed. Concurrently, degenerative changes in the cerebellar cortex were evident from 12 hours onwards. All climbing fiber varicosities were observed to be degenerative as early as 24 hours following treatment. Electron microscopic observations revealed that these electron dense fragments were largely phagocytized and cleared by Bergmann glial cells around 7 days. The sensitivity of the olivocerebellar system to 3-AP thus provides a convenient and selective means of eliminating all of the inferior olivary neurons and their axons, the climbing fibers of the cerebellar cortex. In contrast to the more conventionally used electrolytic methods, 3-AP causes a complete bilateral ablation of all olivary neurons while avoiding the problems inherent to electrolytic procedures, such as incomplete destruction of the nucleus and involvement of fibers of passage.

Afferent connections of the interpeduncular nucleus and the topographic organization of the habenulo-interpeduncular pathway: an HRP study in the rat.

The HRP tracing method was employed to investigate the organization and afferent connections of the interpeduncular nucleus (IPN) in the rat. To study the topographical features of the different projections, a method was devised for obtaining HRP placements of limited size in different areas of the IPN. The main afferent connection of the IPN is a topographically organized projection from the medial habenula (Hb). This projection follows a reversed caudorostral pattern, terminating throughout all but the caudalmost part of the IPN. The dorsal part of the IPN receives a sparse innervation arising mainly from a narrow lateral and ventrolateral area of the medial Hb. The ventral two thirds of the IPN receives a much heavier projection, as follows: A large ventrolateral area of the medial Hb projects to the lateral part of the IPN in a completely bilateral way. An additional projection, which is predominantly ipsilateral, arises from the rostral half of the dorsolateral part of the medial Hb and terminates in the caudal IPN. The medial part of the medial Hb projects preferentially to central areas of the IPN. The projection from the lateral Hb is quantitatively much smaller but appears to be distributed to the entire length of the IPN, following a nonreversed caudorostral arrangement, with the ipsilateral projection predominating. The projections from the medial and lateral Hb to the IPN were confirmed by tracing anterogradely transported HRP as well. No reciprocal connection from the IPN to the Hb could be demonstrated. A sparse projection to the IPN with a strong ipsilateral predominance arises from the horizontal limbs of the nucleus of the diagonal band of Broca. This was the only projection observed from the septal region. Sparse projections from the premammillary and supramammillary nuclei were also demonstrated. Confirmatory data and some details of organization were also obtained for projections to the IPN from other areas, including the medial and dorsal raphe nuclei, the dorsal tegmental nucleus of Gudden, and the adjacent dorsolateral tegmental nucleus. Very small projections from the ventral tegmental nucleus and the locus coeruleus were also found.

An electron microscopic study of the afferent connections of the lateral reticular nucleus of the cat.

The mode and pattern of termination of the afferents to the lateral reticular nucleus (LRN) of the cat were examined at the cellular level through the ultrastructural localization of induced degeneration. Examination of the LRN following hemicordotomy at the fifth and sixth cervical levels revealed that most of the degenerating terminals were in contact with intermediate and distal dendrites, and that most of these degenerating terminals were small and contained round vesicles. Fewer degenerating terminals were observed on the somata and proximal dendrites after spinal hemisection, and most of these terminals were large and contained round vesicles. Following lesions of the pericruciate cortex, small degenerating terminals were occasionally observed making contact onto intermediate and distal dendrites. Degenerating rubral terminals were observed synapsing on somata, somatic and dendritic spines, proximal dendrites and most commonly on intermediate and distal dendrites following lesioning of the red nucleus. The degenerating axosomatic rubro-LRN terminals belonged to the large, round-vesicle terminal population, while those degenerating terminals contacting intermediate and distal dendrites belonged to the small, round-vesicle population. Small, degenerating terminals were occasionally seen following lesions of the fastigial nucleus, and they made synaptic contact mainly onto intermediate and distal dendrites and dendritic spines. The present ultrastructural observations taken together with the convergence pattern of LRN afferents and the available electrophysiological data on inputs to the LRN suggest an extensive integration of converging impulses from two or more afferent sources to the rostral LRN neurons. The results of this study therefore support the veiw that the rostral LRN functions as a comparator of command signals from the motor cortex and red nucleus and feedback signals from the spinal cord and cerebellum during ongoing movement.

Compartmental origin of the striato-entopeduncular projection in the rat.

The mammalian neostriatum is divisible into neurochemically and cytoarchitectonically distinct striosome and matrix compartments. This compartmentalization is respected by many afferent and efferent projections of the striatum. The distribution of distinct types of neuroactive substances and receptors and the unique connections of the striosome and matrix suggest a functional segregation between these compartments. The present study examines the organization of efferent projections from each of the striatal compartments to the entopeduncular nucleus (EPN), a major output center of the basal ganglia. The fluorescent retrograde tracer fluorogold, or rhodamine-conjugated dextran, was injected into the lateral habenula or the ventrolateral nucleus of the thalamus of adult Wistar rats to identify the topographical organization of EPN-habenular and EPN-thalamic neurons. Fluorogold was then placed into the rostral or caudal parts of the EPN, identified from the previous experiment as areas containing predominantly EPN-habenular or EPN-thalamic neurons, respectively. Sections containing retrogradely labeled neurons in the neostriatum were simultaneously immunolabeled for calbindin-D28kDa, a calcium-binding protein found exclusively in the projection neurons of the matrix. The results indicate that the striatal projection to the EPN-habenular and EPN-thalamic parts of the EPN originates from striosome and matrix neurons, respectively. The duality of striatal outflow involving the EPN suggests a mechanism whereby the striosome is integrated into subcortical pathways that modulate the activity of the basal ganglia via the ascending serotoninergic projection from the dorsal raphe nucleus, whereas the matrix is involved in a loop that includes the thalamus and the cerebral cortex.

Axonal branching of the olivocerebellar projection in the rat: a double-labeling study.

Collateralization of olivocerebellar (climbing) fibers was studied in the rat by means of the fluorochrome double-labeling technique. Most of the olivocerebellar projection is crossed except for a minimal ipsilateral component which arises from the most rostal part of the inferior olivary nucleus (ION). ION neurons in the caudolateral part of the medial accessory olive (MAO) and the dorsal accessory olive (DAO) give off axons that branch to supply both hindlimb areas of the contralateral cerebellar cortex, i.e., the rostral anterior lobe and the caudal paramedian lobule. In addition, neurons in the middle one-third of the contralateral MAO and DAO send axons that divide to terminate in both the caudal part of the anterior lobe and the rostral part of the paramedian lobule (forelimb receiving areas). Neurons within the caudal part of the MAO, the lateral part of the DAO, the ventral lamella of the principal olive (PO), and the dorsomedial cell column (DMCC) send axonal branches that terminate within at least two different areas of the same sagittal zones throughout the contralateral cerebellar cortex. Thus, the ION contains specialized cells that provide a divergence of integrated information from the ION to at least two cerebellar regions.

Double-labeling study of axonal branching within the lateral reticulocerebellar projection in the rat.

Collateral axonal branching to the cerebellum from the lateral reticular nucleus (LRN) was studied in the rat by using the fluorescent double-labeling technique. Following injection of Fast Blue (FB) into the cerebellar cortex, followed 3 days later by injection of Nuclear Yellow (NY) into a different region of the cortex, single- and double-labeled cells were found within the LRN. Most LRN-cerebellar projections were bilateral with ipsilateral preponderance, except for the projection to the paramedian lobule, which was completely ipsilateral. The dorsolateral area of the magnocellular division of the LRN contained cells whose axons branch to terminate in the rostral anterior lobe and the caudal part of the ipsilateral paramedian lobule (hindlimb areas of the cerebellar cortex), while the medial area of the LRN contained cells that supply, via collateral axonal branching, the caudal area of the contralateral anterior lobe and the rostral part of the ipsilateral paramedian lobule (forelimb areas of the cerebellum). Branched LRN-cerebellar axons projected to both hemispheres and to both sides of the caudal anterior lobe. No axonal branching was evident in the LRN-cerebellar projection to the rostral anterior lobe. The projection to the anterior and posterior lobe vermis also contained collateral axonal branching.

Colocalization of somatostatin, neuropeptide Y, and NADPH-diaphorase in the caudate-putamen of the rat.

Somatostatin, neuropeptide Y, and nicotinamide adenine dinucleotide phosphate-diaphorase are colocalized within a small population of medium aspiny neurons in the caudate-putamen of the rat. The extent of colocalization, however, appears to be in dispute. In order to examine the question of colocalization between these three neuroactive substances, a series of double-labelling experiments was performed. This was accomplished by combining immunocytochemistry for somatostatin or neuropeptide Y or enzyme histochemistry for nicotinamide adenine dinucleotide phosphate-diaphorase with in situ hybridization for somatostatin and/or neuropeptide Y mRNA. The results of such analysis indicate that nicotinamide adenine dinucleotide phosphate-diaphorase and somatostatin mRNA are 100% colocalized throughout the caudate-putamen, except for the area bordering the globus pallidus. All neurons that contain neuropeptide Y contain somatostatin message. Only 84% of the neurons that contain somatostatin mRNA, however, also contain neuropeptide Y. Neurons that contain somatostatin 28 but not neuropeptide Y are found throughout the caudate-putamen. These results indicate that the somatostatin neuron population in the rat caudate-putamen is not homogeneous. Instead, the medium aspiny neuron population is actually composed of several subpopulations based on the content of neuroactive substances.

Colocalization of prosomatostatin-derived peptides in the caudate-putamen of the rat.

In the striatum of rat, somatostatin 14, somatostatin 28, and somatostatin 28(1-12) have previously been localized within a small population of medium aspiny local circuit neurons. Because all three peptide fragments are generated through the cleavage of prosomatostatin by different converting enzymes, the possibility for differential expression of these peptides exists. In order to investigate this possibility, frozen sections were collected from the brains of adult female Wistar rats fixed with 4% paraformaldehyde and double labelled using immunocytochemistry and in situ hybridization. Sections were first processed for somatostatin 14, somatostatin 28, or somatostatin 28(1-12) by using the avidin-biotin complex immunocytochemical technique followed by in situ hybridization using 35S-labelled antisense riboprobes to somatostatin mRNA. The results of such analysis revealed that somatostatin 28 and somatostatin mRNA are 100% colocalized. Somatostatin 14 and somatostatin 28(1-12), in contrast, are only present within 66% of the neurons that express somatostatin mRNA. Examination of the anatomical distribution of neurons that express both somatostatin mRNA and somatostatin 14 or somatostatin 28(1-12) protein reveals that these neurons are present throughout the caudate-putamen of rat but are more prevalent in the ventromedial regions. Neurons that express somatostatin mRNA but not somatostatin 14 or somatostatin 28(1-12) are also present throughout the caudate-putamen but are most numerous within a dorsolateral strip just beneath the corpus callosum. These results suggest that the somatostatin neuron population within the rat caudate-putamen is actually composed of two smaller subpopulations based on neuropeptide content. The first subpopulation contains somatostatin 28 and constitutes one-third of the total somatostatin population, whereas the other contains somatostatin 28, somatostatin 14, and somatostatin 28(1-12) and represents the remaining two-thirds of the cells that express somatostatin mRNA.

Parvalbumin-containing GABAergic neurons in the basal ganglia output system of the rat.

The output of the basal ganglia is directed through the entopeduncular nucleus (EPN) and the substantia nigra pars reticulata (SNR) and pars lateralis (SNL), which provide a gamma-aminobutyric acidergic (GABAergic) projection to various nuclei of the thalamus and brainstem. Although many neurons within the SNR and EPN have been described as modality specific, the morphological and neurochemical similarities preclude their precise identification. In the present study, the immunocytochemical localization of parvalbumin, a calcium-binding protein, is used in combination with axonal tracing to verify neuronal heterogeneity within the SNR, SNL, and EPN. The results reveal that the majority of neurons in all three centers contain parvalbumin. The parvalbumin-containing neurons are distributed in the caudal two-thirds of the EPN, the rostral part of the SNL, and the lateral two-thirds of the entire rostrocaudal extent of the SNR, the areas involved in sensorimotor function of the basal ganglia. Moreover, the nigrothalamic, nigrocollicular, and EPN-thalamic neurons possess parvalbumin immunoreactivity, whereas the EPN-habenular neurons are devoid of parvalbumin immunoreactivity. The results indicate a neurochemical heterogeneity within the GABAergic output neurons of the basal ganglia and suggest that the parvalbumin-containing neurons of the SNR, SNL, and EPN are the tonically active output neurons that form a major link in the disinhibitory neuronal circuit of the basal ganglia, especially that concerned with sensorimotor function.

Somatostatin and the patch/matrix compartments of the rat caudate-putamen.

In the caudate-putamen of the rat, two subpopulations of medium aspiny neurons exist that contain somatostatin. The first subpopulation contains somatostatin 14, somatostatin 28, and somatostatin 28(1-12). The other subpopulation contains only somatostatin 28. To examine the relationship between somatostatin-containing neurons and the patch/matrix compartments, a series of double-labelling experiments using antisera directed against different somatostatin peptides and calbindin were used. Sections stained in this manner were examined with the aid of a confocal microscope. The results of these experiments indicate that somatostatin 28(1-12)-containing neurons may play a role in matrix integration, with some input directed from the patch compartment. In addition, somatostatin 28-containing neurons are more numerous in the patch compartment than somatostatin 28(1-12)-containing neurons, suggesting a possible role for these neurons in patch integration.

An HRP-TMB ultrastructural study of rubral afferents in the rat.

The projections from the deep cerebellar nuclei and the sensorimotor cortex to the red nucleus were studied in the rat using anterograde transport of horseradish peroxidase conjugated with wheat germ agglutinin (HRP-WGA). The anterogradely transported HRP-WGA was visualized ultrastructurally by using a modification of the tetramethylbenzidine (TMB) histochemical technique of Carson and Mesulam ('82). Following injection of HRP-WGA into the sensorimotor cortex, ultrastructural examination of anterograde labeling in the ipsilateral red nucleus revealed labeled synaptic terminals located on small-diameter dendrites of the parvocellular region. These terminals made asymmetrical contacts and contained round vesicles. HRP-WGA placement in the nucleus lateralis resulted in anterograde labeling of synaptic terminals which made asymmetrical contacts with small- to medium-sized dendrites of the parvocellular red nucleus. Similar placements in the nucleus interpositus gave rise to anterograde labeling of synaptic terminals which made asymmetrical contacts with somata and proximal dendrites of magnocellular neurons. In addition, retrograde labeling of magnocellular neurons was also observed following HRP-WGA placements in the nucleus interpositus. Anterogradely labeled interpositorubral synaptic terminals were located on retrogradely labeled rubrocerebellar neurons. The rat red nucleus thus receives topographically organized afferents which are characterized by their specificity in location at the cellular level.

Ultrastructural study of remodeled rubral afferents following neonatal lesions in the rat.

Following neonatal hemicerebellectomy, an aberrant ipsilateral cerebellorubral projection develops that maintains the topographic specificity of the normal contralateral projection. Similarly, neonatal lesions of the sensorimotor cortex lead to the appearance of an aberrant contralateral corticorubral projection that mirrors the topographic specificity of the normal ipsilateral input. The specificity of synaptic localization in these aberrant projections was studied by use of ultrastructural visualization of anterogradely transported HRP-WGA. Following neonatal ablations, adults received HRP-WGA injections in the unablated deep cerebellar nuclei or sensorimotor cortex. After 48 hours, animals were sacrificed and processed for ultrastructural localization of anterogradely transported HRP-WGA. In hemicerebellectomized animals, both the contralateral and ipsilateral interpositorubral projections terminated on the somatic and proximal dendritic membrane of magnocellular neurons. Some of these labeled synaptic terminals were located on somatic and dendritic spines. Following HRP-WGA injection in the unablated nucleus lateralis, anterogradely labeled synaptic terminals were located bilaterally on small- to medium-sized dendrites of parvicellular neurons. Injection of HRP-WGA in the remaining sensorimotor cortex of animals that had undergone neonatal unilateral ablation of the sensorimotor cortex resulted in labeled corticorubral synaptic terminals that contacted distal dendrites of ipsilateral and contralateral parvicellular neurons. These results demonstrate that, following neonatal deafferentation of the rat red nucleus, the topographic specificity of the aberrant rubral afferents is accompanied by a specificity of synaptic localization on discrete membrane areas of rubral neurons.

Topographical and ultrastructural investigation of the habenulo-interpeduncular pathway in the rat: a wheat germ agglutinin-horseradish peroxidase anterograde study.

The topographical and ultrastructural organization of the habenular projection to the interpeduncular nucleus (IPN) of the rat was examined employing the anterogradely transported tracer wheat germ agglutinin-horseradish peroxidase (WGA-HRP) and the chromogen tetramethylbenzidine (TMB). Unilateral placements of WGA-HRP in the habenular complex resulted in heavy terminal labelling in the rostral, central, and intermediate subnuclei bilaterally, and in the lateral subnuclei ipsilaterally. The apical subnucleus possessed only a sparse amount of label. Placements confined to the medial habenula (mH) produced similar results to those observed when the entire habenula was filled, suggesting that the afferent contribution made by the lateral habenula (lH) to the IPN is small. Unilateral placements of WGA-HRP in the dorsal portion of the mH resulted in heavy, predominantly ipsilateral labelling in the lateral subnucleus and the dorsal cap of the rostral subnucleus. In the lateral subnucleus labelled habenular terminals consistently contacted single dendritic processes shared by one or more other boutons, possibly of nonhabenular origin. Labelled habenular terminals in the rostral subnucleus normally contacted one or two dendrites. Labelled terminals in both subnuclei possessed clear, spherical vesicles and a variable number of dense-core vesicles. Unilateral placements of WGA-HRP in the ventral portion of the mH resulted in heavy labelling in the rostral half of the rostral subnucleus with a slight ipsilateral predominance, and in the central and intermediate subnuclei bilaterally. Terminal labelling was observed in crest and S synapses in the intermediate and central subnuclei respectively. Crest synapses, which consist of two parallel habenular terminals contacting an attenuated dendritic process, normally possessed label in only one of the two boutons. In the central subnucleus labelled horizontal axons formed several en passant S synapses with dendritic processes of small and medium diameter. These synaptic specializations of habenular axons contained numerous clear, spherical vesicles. This study demonstrates that a major topographically organized projection to the IPN originates from two distinct subpopulations of habenular neurons which comprise a dorsal sector and a ventral sector of the mH. Ultrastructural examination demonstrated that axons originating from neurons in the ventral and dorsal mH form characteristic contacts in the various IPN subnuclei.

Spinal projections to the lateral reticular nucleus in the rat: a retrograde labelling study using horseradish peroxidase.

The organization of the spinal projection to the lateral reticular nucleus (LRN) in the rat was investigated by means of a retrograde pathway tracing method in which horseradish peroxidase (HRP) served as an enzyme marker. Discrete placements of HRP into the LRN were achieved by a combination of dorsal (stereotaxic) and ventral (microsurgical) approaches. The extent and distribution of retrogradely labelled neurons in the spinal cord indicated a substantial and highly ordered projection to the LRN. All segments of the spinal cord contributed to the projection to the LRN. The limb enlargements were the riches sources of projection. The projection from the cervical spinal cord was bilateral with ipsilateral preponderance. The cervical neurons projecting to the ipsilateral LRN were located mainly in lamina VII of the spinal cord, while those projecting to the contralateral LRN were located mainly in lamina VIII. The lumbar spinal projection terminated in the contralateral LRN, and the neurons of the origin were located mainly in lamina VIII. Most if not all parts of the LRN received afferents from the spinal cord. The projections terminated most abundantly in the caudomedial portions of the magnocellular LRN. The medial aspect of the LRN was the site of preferential termination of the cervical spinal projection. Lumbar spinal projections terminated preferentially in the rostrolateral region of the nucleus. These two imputs overlap in the central region of the nucleus.

A horseradish peroxidase study of the rubral and cortical afferents to the lateral reticular nucleus in the rat.

The origin and organization of supraspinal afferents to the lateral reticular nucleus (LRN) in the rat were studied by means of the retrograde axonal transport of horseradish peroxidase (HRP). HRP was deposited into the LRN via both dorsal (stereotaxic) and ventral (microsurgical) routes. The entire cerebrum, brainstem, and cerebellum were surveyed for retrogradely labelled neurons. Significant projections arose from the contralateral red nucleus and the contralateral frontoparietal cortex. The rubral projection arose from neurons in the caudal two-thirds of the red nucleus. Ventrally and ventrolaterally located neurons projected to rostrolateral LRN, while dorsal and dorsomedial neurons projected to rostromedial LRN. The projection from the cerebral cortex arose from neurons located in layer V of the frontoparietal region. Rubral and cerebrocortical projections overlap in the rostral LRN, making this region of the nucleus a site of integration of descending inputs with ascending spinal signals.