Lesion of subthalamic or motor thalamic nucleus in 6-hydroxydopamine-treated rats: effects on striatal glutamate and apomorphine-induced contralateral rotations

A unilateral lesion of the rat nigrostriatal pathway with 6-hydroxydopamine (6-OHDA) results in a decrease in the basal extracellular level of striatal glutamate, a nearly complete loss of tyrosine hydroxylase (TH) immunolabeling, an increase in the density of glutamate immunogold labeling within nerve terminals making an asymmetrical synaptic contact, and an increase in the number of apomorphine-induced contralateral rotations. [Meshul et al. (1999) Neuroscience 88:1-16; Meshul and Allen (2000) Synapse 36:129-142]. In Parkinson's disease, a lesion of either the subthalamic nucleus (STN) or the motor thalamic nucleus relieves the patient of some of the motor difficulties associated with this disorder. In this rodent model, either the STN or motor thalamic nucleus was electrolytically destroyed 2 months following a unilateral 6-OHDA lesions. Following a lesion of either the STN or motor thalamic nucleus in 6-OHDA-treated rats, there was a significant decrease (40-60%) in the number of apomorphine-induced contralateral rotations compared to the 6-OHDA group. There was a significant decrease (<30%) in the basal extracellular level of striatal glutamate in all of the experimental groups compared to the sham group. Following an STN and/or 6-OHDA lesion, the decrease in striatal extracellular levels was inversely associated with an increase in the density of nerve terminal glutamate immunolabeling. There was no change in nerve terminal glutamate immunogold labeling in either the motor thalamic or motor thalamic plus 6-OHDA lesion groups compared to the sham group. The decrease in the number of apomorphine-induced rotations was not due to an increase in TH immunolabeling (i.e., sprouting) within the denervated striatum. This suggests that alterations in striatal glutamate appear not to be directly involved in the STN or motor thalamic lesion-induced reduction in contralateral rotations.     

The effects of nanoliter ejections of lidocaine into the pontomedullary reticular formation on cortically induced rhythmical jaw movements in the guinea pig.

In the ketamine/urethane anesthetized guinea pig, electromyographic (EMG) responses of the anterior digastric muscle were studied when loci within the lower brainstem were microejected with lidocaine (2%) during rhythmical jaw movements (RJMs) evoked by repetitive electrical stimulation of the masticatory area of the cortex. The area investigated was between the trigeminal motor nucleus (Mot V) and the rostral pole of the inferior olive. Microejections of lidocaine, contralateral to the cortical stimulus site, into the ventral-medial portion of Mot V where digastric motoneurons are known to be located, resulted in reduction or complete abolishment of the digastric EMG activity ipsilateral to the ejection with no effective change in mean cycle duration (CD) or mean percent normalized integrated amplitude of the contralateral digastric EMG. Microejections of lidocaine, contralateral to the cortical stimulus site, into the ponto-medullary reticular formation in areas that included portions of the caudal nucleus pontis caudalis (PnC), nucleus gigantocellularis (GC), medial nucleus parvocellularis (PCRt), and dorsal paragigantocellularis (dPGC), in most cases produced a bilateral reduction in the mean normalized integrated amplitude and a bilateral increase in the mean cycle duration. In these sites, the bilateral increase in mean cycle duration of digastric EMG bursts was also associated with a significant increase of coefficient of variation in CD. In many cases, microejection of lidocaine completely abolished rhythmical digastric activity, bilaterally. HRP injections into Mot V were performed to determine the locations of trigeminal premotoneurons and their relationship to effective lidocaine sites for rhythmical jaw movement suppression. Retrogradely labeled cells were found mainly in the mesencephalic nucleus of V; trigeminal principal and spinal V sensory nuclei, bilaterally; and within the intermediate and lateral regions of reticular formation, bilaterally. No labeling was found in the medial reticular formation, including the nucleus gigantocellularis and dorsal paragigantocellularis.(ABSTRACT TRUNCATED AT 400 WORDS)     

Corticothalamic projections from layer V cells in rat are collaterals of long-range corticofugal axons

The vast majority of corticothalamic (CT) axons projecting to sensory-specific thalamic nuclei arise from layer VI cells but intralaminar and associative thalamic nuclei also receive, to various degrees, a cortical input from layer V pyramidal cells. It is also well established that all long-range corticofugal projections reaching the brainstem and spinal cord arise exclusively from layer V neurons. These observations raise the possibility that the CT input from layer V cells may be collaterals of those long-range axons projecting below thalamic level. The thalamic projections of layer V cells were mapped at a single cell level following small microiontophoretic injections of biocytin performed in the motor, somatosensory and visual cortices in rats. Camera lucida reconstruction of these CT axons revealed that they are all collaterals of long-range corticofugal axons. These collaterals do not give off axonal branches within the thalamic reticular nucleus and they arborize exclusively within intralaminar and associative thalamic nuclei where they form small clusters of varicose endings. As layer V cells are involved in motor commands everywhere in the neocortex, these CT projections and their thalamic targets should be directly involved in the central organization of motor programs.     

A golgi study of the class V cell in the visual thalamus of the cat

Golgi methods were used to study class V cells within the cat visual thalamus. Counterstaining was combined with Golgi staining to assess the distribution of dendrites relative to cytoarchitectural boundaries. Class V cells were encountered within all laminae of the lateral geniculate nucleus, the medial interlaminar nucleus, and the lateral posterior complex. The cells possess medium-sized perikarya and smooth and varicose or moniliform dendrites. Dendritic appendages are sparse and occur as single or serial swellings on thin processes. Many class V cells exhibit large, sparse dendritic arbors which span laminar or nuclear borders; dendrites were seen to lie within and to cross the interlaminar zones of the lateral geniculate nucleus, and extend beyond this nucleus into the perigeniculate nucleus and medial interlaminar nucleus. Class V cells of the lateral posterior complex send dendrites into the external medullary lamina. Indirect evidence favors the interpretation that the class V cells are thalamo-cortical relay cells.     

Functional implications of the anatomical organization of the callosal projections of visual areas V1 and V2 in the macaque monkey

The efferent and afferent connections of the V1/V2 border with the contralateral hemisphere have been examined using anatomical tracers. The V1/V2 border was found to exchange connections with the contralateral V2 area as well as a restricted strip of V1 lying adjacent to the V1/V2 border. Besides these homotopic projections, two heterotopic projections were found to V3/V3A and V5. Anterograde tracing of callosal connections showed that terminals in these heterotopic sites were focused in layer 4, the recipient layer of projections originating from the ipsilateral V1/V2 border. Bilateral injections of fluorescent dyes showed that these heterotopic targets of the V1/V2 border are connected to the homologous ipsilateral V1/V2 border region. The laminar location of callosal projecting neurons as well as their terminals were characteristic for each cortical region. The laminar pattern of callosal connectivity was found to differ markedly from that of associational visual pathways. Two principal hypotheses are suggested by these results. First, the fact that V1 in part is reciprocally callosally connected in all mammals supports the notion that this interhemispheric pathway completes long-range intrinsic cortical connections. Second, the convergence of inter- and intrahemispheric pathways could provide the anatomical basis for the modulation of the sensory processing within one hemisphere by ongoing activity in the contralateral hemisphere.     

Central projections of the mesencephalic nucleus of the fifth nerve: an autoradiographic study

Projections of cells of the mesencephalic nucleus of the fifth nerve (Mes V) to brainstem structures in the cat were studied by labelling Mes V cells with tritiated leucine. Before the leucine injections were made, however, large kainic acid lesions were produced in the vicinity of Mes V cells because these neurons are resistant to being killed or injured by this neurotoxin. Thus Mes V cells were selectively labelled by the leucine even though they are scattered among many other neurons. Leucine injections near Mes V cells located in the mesencephalon, which are primarily the somata of jaw muscle spindle afferent fibers, produced essentially the same pattern of terminal labelling as injections near a caudally located group of Mes V cells that includes the somata of many tooth mechanoreceptive afferents. Labelling was dense above the trigeminal motor nucleus in the nucleus supratrigeminalis and in the most medial portion of the principal trigeminal sensory nucleus. A scattering of labelled axons and diffuse label was seen along the length of the tract of Probst, which follows the medial border of the descending trigeminal sensory nucleus as far caudally as the dorsal motor nucleus of the vagus. Labelling within most of the trigeminal motor nucleus, which is known to receive direct synaptic input from Mes V cells, was very light. The only reasonably dense region of label was confined to a small dorsolateral portion of the motor nucleus. Although Mes V has generally been supposed to be involved with jaw control in a direct, reflexive manner, the extensive projections to nucleus supratrigeminalis and parts of the trigeminal sensory system draw attention to the potential proprioceptive sensory contribution of Mes V.     

Input-output relations of the red nucleus in the cat.

In unanesthetized cats, microstimulation within the red nucleus produces contraction of single muscles of the contralateral limbs and face. Separate zones may activate different muscles. Forelimb muscles were primarily activated from areas in the dorsomedial quadrants of the red nucleus whereas hindlimb muscles were predominantly activated from the ventorlateral quadrants. With stimulus currents of 10 muA there was considerable overlap in the effective zones activating different muscles. In the majority of cases the minimal threshold was under 10 muA when stimilating with a 50-msec pulse train. Current thresholds for electromypgraphic changes in the muscles varied inversely with pulse frequency and train duration. When long stimulus trains were applied to the red nucleus, the resulting muscle contraction was sustained for the duration of the stimulus. These motor effects did not depend upon the motor cortex or pyramidal tract but were mediated by a tract in contralateral dorsal quadrants of the spinal cord which was likely to be the rubrospinal tract. Units within the red nucleus typically had wide cutaneous receptive fields and responded to deep pressure and joint rotation in one or more limbs. Usually the focus driving the cell most briskly was located in one of the contralateral limbs and corresponded to the limb where muscle contraction was elicited by microstimulation with the same electrode. It is concluded that the red nucleus includes overlapping efferent neuronal colonies controlling individual muscles irrespective of their functional class. This property is shared by the motor cortex and suggests that these two structures may complement each other in the control of movement. The more diffuse activation of rubral than cortical neurons by natural stimuli suggests that rubral activity may not be as tightly linked as that of the motor cortex to specific peripheral input.     

The rubro-spinal tract of the opposum (Didelphis virginiana)

The course, caudal extent and termination of the opossum rubro-spinal tract were determined by employing the Nauta-Gygax and Fink-Heimer techniques on the spinal cords of animals with lesions either within the red nucleus or within the descending fibers emanating from that nucleus. Prior to placement of the lesion, the area at the tip of the electrode was stimulated and the concomitant movements of skeletal muscles were palpated. The rubro-spinal tract traversed the entire length of the spinal cord and was located within the dorsolateral portion of the lateral funiculus. The tract was partially separated from the surface throughout cervical and thoracic levels by fibers of the dorsal spino-cerebellar tract. More caudally the rubro-spinal tract coursed immediately beneath the cord surface. At cervical levels, rubro-spinal fibers ended extensively within the lateral portion of laminae V and VI (Rexed, '52) and minimally within the medial portions of the same laminae. Fascicles also ended within lamina VII. At thoracic levels, lamina VI disappeared and rubro-spinal fibers terminated within the lateral and, to a lesser extent, the medial portions of laminae V and VII. Lamina VI was present at lumbar and rostral sacral levels and the majority of rubro-spinal fibers ended within the lateral part of that lamina. However, a few fibers terminated within laminae V and VII at such levels. At caudal sacral and coccygeal levels lamina VI disappeared and rubro-spinal bundles ended within laminae V and VII. Stimulation of the caudal red nucleus initiated contraction of the flexor musculature of the contralateral forelimb, and to a lesser degree, the contralateral hindlimb. Contraction of the contralateral paraxial musculature was also noted at cervical and thoracic levels during such experiments. Stimulation of the rubro-spinal tract and adjacent tegmentum immediately caudal to the red nucleus resulted in similar movements, but they were essentially limited to the side of the stimulation.     

Afferent projections to the central nucleus of the inferior colliculus in the rat

The afferent projections to the inferior colliculus of the rat were studied using the method of retrograde transport of horseradish peroxidase (HRP).Following large injections of HRP into the central nucleus, cells within the cochlear nuclei, superior olivary complex and auditory cortex were stained. Within the contralateral dorsal cochlear nucleus, fusiform cells were heavily labeled. Giant cells were also labeled in deeper layers. In the contralateral ventral cochlear nucleus, virtually all major cell types were labeled, with some types being labeled in greater numbers than others. Octopus cells of posteroventral division of ventral cochlear nucleus (PVCN) were never labeled. HRP-positive cells were found in ipsilateral and contralateral lateral superior olivary nucleus (LSO), ipsilateral medial superior olivary nucleus (MSO), ipsilateral and contralateral lateral nucleus of the trapezoid body (LTB), ipsilateral ventral nucleus of the trapezoid body (VTB), and ipsilateral superior paraolivary nucleus (SPN). Pyramidal cells of layer V of auditory cortex were heavily labeled.Small injections of HRP into the central nucleus resulted in labeled cells within restricted regions of the cochlear nuclei, superior olivary complex and auditory cortex. Injections into dorsal regions of the central nucleus resulted in cells labeled in ventral regions of the dorsal and ventral cochlear nuclei, and in lateral regions of LSO. These regions contain neurons which are considered to have low best frequencies. Injections placed in more ventral regions of the central nucleus led to labeling of cells in more dorsal regions of the cochlear nuclei and more medial regions of LSO in agreement with the tonotopical progressions within these structures.     

Direct projections from the masseteric nerve to the mesencephalic nucleus

The caliber spectrum of the nerve to the masseter muscle of the cat was found to be unimodal, ranging from 1 to 16 μ, with the majority of the fibers in the 4-8 μ range. The upper 25%, which includes the afferent propriococoptive fibers, averaged 10.4 μ. The nerve to the masseter was stimulated and evoked potentials were recorded from the ipsi- and contralateral mesencephalic nuclei of the trigeminus and from the contralateral nerve. The average latencies were 0.846, 0.935 and 1.46 msec respectively. The mesencephalic nucleus was stimulated and evoked potentials were recorded from the contrlateral nucleus with an average latency of 0.385 msec. Analysis of the latencies of the responses recorded from the ipsilateral nucleus suggested that about half of the fibers of the nerve to the masster projected directly and the other half projected by way of an intervening synapse. A similar distribution of the fibers to the contralateral nucleus was deduced from the recorded latencies. Potentials recorded in the nuclei as a result of stimulation of the contralateral nucleus demonstrated that there were direct internuclear pathways without intervening synapses. The latencies of potentials recorded in the nerve as a result of stimulation of the contralateral nerve indicated that there was always at least one synapse intervening. It was also shown that there were direct projections from the nerve of the masseter to the contralateral motor nucleus of V. The work confirms our earlier report of bilateral projections from a muscle of mastication to the mesencephalic nuclei and provides a pathway for bilteral simultaneous activation of the muscles of mastication resulting from stimulation of unilateral tension receptors.     

The role of the nucleus accumbens in sensitization to drugs of abuse.

1. Male rats received cannula implants above the nucleus accumbens for monitoring extracellular concentrations of dopamine via in vivo microdialysis. 2. Daily injections with cocaine led to an augmentation in both the behavioral response and the neurochemical response (i.e. cocaine-induced increase in extracellular dopamine within the nucleus accumbens) to this drug. 3. Pertussis toxin injections into the A10 region led to sensitized behavioral and neurochemical responses to an acute injection of cocaine. 4. Prior exposure to footshock stress augmented the cocaine-induced increase of motor activity and of extracellular dopamine within the nucleus accumbens. 5. These data suggest that treatments which lead to behavioral sensitization also lead to sensitization within the mesolimbic dopamine system as measured by an augmented dopamine release in the nucleus accumbens.     

Peripheral and central oculomotor organization in the goldfish, Carassius auratus

Peripheral and central oculomotor organization was studied in the goldfish. The sizes of the extraocular muscles were quantified by counting the fibers contained in a given muscle and by area measurements of the cross-sectional surfaces. All the muscles were of approximately similar size. Kinematics were determined by electrical stimulation of a given muscle. The macroscopic appearance and kinematics of the muscles had the characteristics of other lateral-eyed animals (e.g., rabbit). Locations of extraocular motor neurons were found by retrograde transport of horseradish peroxidase (HRP) following injections into individual extraocular muscles. The eye muscles were innervated by four ipsilateral (lateral rectus, medial rectus, inferior oblique, inferior rectus) and two contralateral (superior rectus, superior oblique) motor neuron pools. The oculomotor nucleus was found in the midbrain, at the level of the caudal zone of the inferior lobe of the hypothalamus. Inferior rectus motor neurons were located rostrally in the oculomotor nucleus, whereas medial rectus, superior rectus, and inferior oblique motor neurons were intermingled in its more caudal portions. All labelled cells were located dorsally and medially to the medial longitudinal fasciculus (MLF) in close proximity to either the floor of the ventricle or the midline region. Occasionally, motor neurons were interspersed within the fiber bundles of the MLF or the exiting fibers of the oculomotor nerve. The trochlear nucleus, containing superior oblique motor neurons, was found in the immediate lateral and caudal neighborhood of the oculomotor nucleus, where its rostral border overlapped with the caudal border of the latter. The abducens nucleus, containing lateral rectus motor neurons, was located in the posterior brainstem in the neighborhood of the vestibular nuclear complex. This nucleus was divided into a rostral and a caudal portion. The axons of ipsilaterally projecting motor neurons headed toward their respective nerve roots via the shortest possible route, as did the axons of superior rectus motor neurons, which crossed the midline without detour to enter the contralateral oculomotor nerve. In contrast, trochlear motor neuron axons arched around the dorsal aspect of the ventricle through the cerebellar commissure to reach the contralateral trochlear nerve. The morphology of individual motor neurons was visualized by intrasomatic injection of HRP. Cell somata had oblong shapes, and their large dendrites were oriented laterally and ventrally. The axons did not collateralize within the midbrain region or the oculomotor nerve as far as they could be traced.(ABSTRACT TRUNCATED AT 400 WORDS)     

A pretectofacial projection in the cat: a possible link in the visually-triggered blink reflex pathways

A direct projection from the pretectum to the facial motor nucleus was shown to exist in the cat by the anterograde and retrograde horseradish peroxidase (HRP) methods. Pretectofacial fibers arise from the olivary pretectal nucleus and end mainly in the dorsal division of the facial motor nucleus, bilaterally, with a contralateral predominance. It is known that the olivary pretectal nucleus receives retinal fibers, and that the dorsal division of the facial motor nucleus contains orbicularis oculi motoneurons. Thus, the pretectofacial fibers are assumed to cause protective lid closure with certain visual stimuli.     

A comparative topographical analysis of dorsal column nuclear and cerebral cortical projections to the basilar pontine gray in rats

The somatotopic distribution of dorsal column nuclear projections within the basilar pontine gray was examined in relation to the massive corticopontine projection system that emanates most heavily from motor and somatosensory cortex. The distribution patterns of these two systems were compared by combining autoradiographic and degeneration axonal tracing methods within individual animals. Stereotaxic injections of tritiated leucine (50 microCi/microliter) and lesions by aspiration were made in animals under ketamine hydrochloride anesthesia. The forelimb cortical injections (0.1-0.3 microliter) were centered in either sensory or motor cortical regions as determined by intracortical microstimulation and multiunit recording techniques. Because sensory and motor hindlimb cortical areas overlap extensively in the rat, hindlimb cortical injections (0.1-0.3 microliter) were limited to a single hindlimb sensorimotor cortical region. The corresponding contralateral dorsal column nucleus, cuneatus or gracilis, was then aspirated. A somatotopic distribution of fore- and hindlimb corticopontine fibers were found in discrete regions of the ipsilateral pontine gray. Hindlimb sensorimotor corticopontine fibers distributed caudal to forelimb projections. Similarly, pontine afferents from the dorsal column nuclei terminated somatotopically in the caudal half of the contralateral pontine gray in that gracilopontine fibers distributed caudal to cuneopontine fibers. Within individual animals, partially overlapping terminations were seen from nucleus cuneatus and the forelimb sensory cortical area as well as from nucleus gracilis and the hindlimb sensorimotor cortical area. No overlap existed in the pontine terminations from nucleus cuneatus and the forelimb motor cortical area.     

Corticopulvinar connections of areas V5, V4, and V3 in the macaque monkey: a dual model of retinal and cortical topographies

The connection zones of cortical areas V3, V4, and V5 (MT) with the thalamic pulvinar nucleus in the macaque monkey were identified. A combination of single- and dual-tracer techniques was used to study their topography and to establish whether these zones occupy separate or overlapping pulvinar territories. In each case, the retinotopic distribution of tracer in the pulvinar was charted by reference to its parallel distribution within the maps of cortical areas V1 and V2. Each of the areas V3, V4, and V5 were found to connect with both the 1 degrees and the 2 degrees maps located within the inferior and lateral pulvinar nuclei and to respect the previously identified topographies of these maps. However, V5 connects to a narrow zone lining the rostrolateral margin of the lateral and inferior pulvinar and V4 to a broader zone within the body of these two nuclei, which is adjacent to but separate from the V5 zone; the V3 zone overlaps both. Focal injections into cortex produce columns of pulvinar label whose trajectory defines a line of isorepresentation. The lines of isorepresentation in the 1 degrees and 2 degrees maps are approximately linear and parallel and adopt a rostrolateral to caudomedial axis; in the 1 degrees map, this axis is roughly perpendicular to the facet of the inferior pulvinar that lies adjacent to the lateral geniculate nucleus. The connections of V5 and V4 can be modelled as successive zones along the axis of isorepresentation, with registered visual topographies. The scheme is extended by existing reports that inferotemporal cortex connects to the caudomedial pole of this axis-reflecting an occipitotemporal cortical gradient, in that V1 and other prestriate areas, e.g., V3, connect to the opposite pole. Thus a simple model of the mapped volume in the pulvinar arises, in which a unidimensional cortical topography is represented orthogonally to retinal topography. Adjoining this volume medially, within the inferior and medial pulvinar, is a second, heavier zone of V5 connectivity, which is poorly topographic. Both the medial and the rostrolateral zones of V5 connectivity may overlap with previously identified regions of tectal input to the pulvinar.     

Corticoreticulospinal control of the tonic vibration reflex in the cat

The cortico-reticulospinal system has been investigated using vibration to produce a tonic reflex in an intact cat. Stimulation of the motor cortex potentiated the effects of medial medullary inhibition but not the facilitation evoked from the lateral medulla.The cortico-bulboreticular pathways were found to project bilaterally to the medulla. Medullary stimulation produced bilateral facilitation or inhibition of the tonic vibration reflex (TVR). Although the cortico-bulboreticular pathway projected bilaterally the effect evoked from the motor cortex was clearly contralateral in its end result. The cortico-reticulospinal system usually inhibits the tonic vibration reflex of both triceps surae and tibialis anterior, but on occasions inhibition of triceps surae was accompanied by a phasic contraction of the tibialis anterior. Stimulation of the contralateral caudate nucleus failed to influence the TVR directly or the response evoked by medullary stimulation. Stimulation of the contralateral red nucleus evoked a facilitation of the TVR which continued several seconds after red nucleus stimulation ceased.It was concluded that those medullary neurones which inhibit tone in flexor and extensor hind limb muscles of the cat are controlled directly by the motor cortex and are uninfluenced by caudate nucleus or red nucleus. The facilitatory neurones in the lateral medulla appear to be independent of cortical control.     

Three bulbospinal pathways from the rostral medulla of the cat: an autoradiographic study of pain modulating systems

Small amounts of ³H-leucine were injected into discrete regions in the rostral medulla of the cat. Descending projections from these sites were studied with autoradiographic methods. On the basis of differential projections to the medulla and spinal cord, three distinct regions were delineated. Nucleus reticularis gigantocellularis (Rgc), located dorsally in the medullary reticular formation, projects primarily to “motor” related sites, including cranial motor nuclei VI, VII, XII, nucleus intercalatus, and a part of the ipsilateral medial accessory olive. The projection to the spinal cord is primarily via the ipsilateral ventrolateral and contralateral ventral funiculi. The Rgc terminal field is in lamina VII and VIII ipsilateral and lamina VIII contralateral to the injection site. In contrast, nucleus raphe magnus, (NRM) located ventrally, in the midline of the rostral medulla projects primarily to structures with known nociceptive and/or visceral afferent input. These sites include the solitary nucleus, the dorsal motor nucleus (X) and the marginal and gelatinous layers of the spinal trigeminal nucleus caudalis. The projection to the spinal cord is bilateral, via the dorsolateral funiculus. Terminal fields are found in the marginal zone and the substantia gelatinosa of the dorsal horn, and more deeply in lamina V, medial VI and VII. Nucleus reticularis magnocellularis (Rmc), located lateral to NRM and ventral to Rgc, has an overlapping projection with NRM, but the projection is ipsilateral. This difference between Rmc and Rgc is correlated with cytoarchitectural features of the two regions. The possibility that the raphe-spinal pathway in the DLF mediates opiate and brain stimulation-produced analgesia is discussed.     

Afferent and efferent projections of the VIIIth cranial nerve in the lamprey Lampetra japonica

Anterograde and retrograde transport of horseradish peroxidase was used to examine the afferent and efferent projections of the VIIIth cranial nerve in the lamprey Lampetra japonica. Ganglion cells of the VIIIth nerve are classified into three types on the basis of their morphology. The central processes of these ganglion cells enter the medulla in two groups: the anterior group (mostly thick fibers) and the posterior group (mostly thin fibers). Afferent fibers mainly terminate within the ipsilateral ventral and octavomotor nuclei of the octavolateralis area and within the granular and molecular layer of the cerebellum. Some fibers terminate in the contralateral cerebellum, the medial and dorsal nuclei of the octavolateralis area, the descending nucleus of the trigeminal nerve, some cranial motor nuclei, and the lateral octavus nucleus, which has not been described previously. This small nucleus is located beneath the descending nucleus of the trigeminal nerve near the obex. Within the ventral nucleus, thin fibers occupy the dorsal part and thick fibers occupy the ventral part. The basic projection pattern of the primary afferents of the VIIIth nerve in the lampreys was similar to that of gnathostome fishes that have been studied to date. Cell bodies of the efferent vestibular neurons are located between the ipsilateral trigeminal motor nucleus and the facial motor nucleus. The lateral location of these cell bodies differs from that of all other fish species that have been studied.