Postsynaptic dorsal column pathway of the rat. I. Anatomical studies

As one of a series of studies of the ascending spinal cord pathways that might be involved in nociception in the rat, we have examined the projection to the dorsal column nuclei that originates in the spinal cord dorsal horn using the retrograde transport of horseradish peroxidase (HRP). This projection in other animals has been called the postsynaptic dorsal column (PSDC) pathway. Small iontophoretic injections of HRP into the cuneate nucleus (CN) labeled more than 350 neurons in alternate sections within the ipsilateral gray matter of segments C6-8. Fewer than 25 neurons were labeled in L4-6 by injections into CN. Injections of HRP confined to the gracile nucleus (GN) labeled more than 200 neurons within a narrow band extending across the ipsilateral dorsal horn subjacent to substantia gelatinosa of L4-6. Fewer than 10 cells were labeled in C6-8 by such injections. Labeling in lumbar neurons following injections into GN was prevented by transection of the dorsal columns at T10, T8, or C2. Thus, neurons labeled by such injections ascend entirely within the dorsal columns. Lesions of the dorsal columns in C2 reduced the number of labeled neurons in the cervical cord following CN injections by approximately 90%. Combined lesions of the dorsal columns and ipsilateral dorsal lateral funiculus (DLF) reduced the number of cells labeled in C6-8 by approximately 98%. Thus, the majority of labeled neurons in the cervical enlargement project to CN via the dorsal columns; a small secondary component of the cervical projection to CN appears to ascend within the DLF. To compare the relative sizes of the projections to the dorsal column nuclei from PSDC neurons and dorsal root ganglion cells (DRG), labeled neurons were counted in the gray matter of the cervical and lumbar enlargements and the corresponding DRG. In the four animals so examined, PSDC neurons constituted over 38% of the neurons that projected to CN and approximately 30% of the cells that projected to GN. These findings indicate that the PSDC projection of the rat is capable of providing a large somatotopically organized input to the dorsal column nuclei.     

Early postnatal development of the rat corticostriatal pathway: an anterograde axonal tracing study using biocytin pellets.

The ontogenetic development of the neocortical projections to the striatum was studied in postnatal rats by sensitive anterograde tracing with biocytin. The developmental status of this mainly glutamatergic pathway is important as it plays a major role in regulation of striatal maturation and induction of excitotoxic striatal neurodegeneration resembling the type found in Huntington's disease. Biocytin pelletts were injected into the motorcortex caudal forelimb area of newborn rats and of rats aged 3, 6, 13, 27 and 61 days followed by sacrifice and visualisation of the tracer 24 h later, at P1, P7, P14, P28 and P62, respectively. Biocytin pellets were also injected into the motorcortex jaw-lip-tongue area and into the cingulate cortex of newborn and 6-day-old rats. The biocytin pellets produced intense anterograde labelling of corticofugal projection fibres from the injection sites, as well as a local Golgi-like labelling of neurons. From postnatal day 1 into adult age efferent fibres from the motorcortex caudal forelimb area displayed a progressively denser innervation of the ipsilateral and contralateral, dorsolateral striatum. The terminating fibres were most dense in the ipsilateral striatum. In the dorsolateral striatum the corticostriatal fibres displayed a patch/matrix-like arrangement from postnatal day 14 onwards. Injections into the motorcortex jaw-lip-tongue area at postnatal days 0 and 6 displayed a progressive, denser innervation of the ipsilateral and contralateral ventrolateral striatum. The cingulate corticostriatal fibre projection, was more developed at birth than the projection from the motorcortex and increased its fibre density in the ipsilateral dorsomedial striatum up to at least day 7. In conclusion: the ipsilateral corticostriatal projections from the rat motorcortex and cingulate areas are present in newborn rats. During postnatal life the pathway develops further and specialisation of the terminating fibres into a patch/matrix like arrangement can be identified by postnatal day 14.     

Cellular mechanisms of motor control in the vibrissal system

In this article we discuss the experimental advantages that the vibrissal motor system offers for analysis of motor control and the specializations of this system related to the unique characteristics of whisker movements. Whisker movements are often rhythmic, fast, and bilateral. Movements of individual whiskers have simple characteristics, whereas, movements of the entire vibrissae array are complex and sophisticated. In the last few years, powerful methods for high precision tracking of whisker movements have become available. The whisker musculature is arranged to permit forward movements of individual whiskers and consists—depending on the species—mainly or exclusively of fast contracting, fast fatigable muscle fibers. Whisker motor neurons are located in the lateral facial nucleus and their cellular properties might contribute to the rhythmicity of whisking. Numerous structures provide input to the lateral facial nucleus, the most mysterious and important one being the putative central pattern generator (CPG). Although recent studies identified candidate structures for the CPG, the precise identity and the functional organization of this structure remains uncertain. The vibrissa motor cortex (VMC) is the largest motor representation in the rodent brain, and recent work has clarified its localization, subdivisions, cytoarchitectonics, and connectivity. Single-cell stimulation experiments in VMC allow determining the cellular basis of cortical motor control with unprecedented precision. The functional significance of whisker movements remains to be determined.     

Oral motor deficits following lesions of the central nervous system in the rat.

The effects of lesions of the zona incerta (ZI), globus pallidus (GP), midlateral and far lateral hypothalamus (MLH and FLH), and the central amygdaloid complex (CAC) on oral motor deficits were investigated. Lesions of the ZI, GP, and CAC resulted in a significant reduction in tongue extension and lap volume. MLH lesions significantly reduced tongue extension whereas FLH lesions significantly reduced both tongue extension and lap volume. Injections of 6-hydroxydopamine into the GP, MLH, and CAC also significantly reduced tongue extension and lap volume. Water and/or food intakes were reduced in some of the lesioned groups but the oral motor deficits were observed both in the presence and in the absence of reduced water and food intakes. The possibility that oral motor deficits are associated with damage to striatal and non-striatal dopamine neurons needs further investigation.     

Convergence in a thermal afferent pathway in the rat.

1. In anaesthetized rats, unit activity was recorded in the afferent somatosensory pathway leading from the scrotum. Recording sites were in the dorsal horn near the entry zone of the scrotal nerve, in the ventrobasal complex of the thalamus and in the somatosensory (SI) cortex. During recording, the temperatures of the left and right sides of the scrotum were varied independently. 2. Almost all (64/67) the units in dorsal horn, thalamus and cortex responding specifically to scrotal temperature were equally affected by temperature changes on either side of the scrotum. The receptive fields of these units were bilateral and large, implying a massive convergence of fibres from thermoreceptors on to each central unit. In contrast, mechanosensitive units responded only to unilateral stimulation. 3. As a consequence of the convergence in the thermal pathway, the firing rate of each central unit was a function of an additive combination, often simply the sum, of the temperatures of the two sides of the scrotum. 4. The relationship between firing rate and the temperature of one side of the scrotum was sigmoid, the position, but not the shape, of the curve depending on the temperature at which the opposite side was maintained. An increase in the maintained temperature shifted the sigmoid response curve towards lower temperatures and vice versa. 5. The convergence which this pathway exhibits would be well suited to integration of the temperature of the scrotal skin, but not to spatial discrimination.     

Selective rostral transection of the fornix spares the hippocampal commissural pathway in the rat: a Phaseolus vulgaris leucoagglutinin tracing study

This study describes an approach for disconnecting the septal region from the hippocampus by fimbria-fornix lesions while sparing the commissural projections. After a frontal cut through the rostral fornix, commissural fibres were labelled with the anterograde tracer Phaseolus vulgaris leucoagglutinin. The commissural fibre bundle located in the posterior-basal fornix (ventral hippocampal commissure) remained unaffected by the rostral fornix transection, whereas the absence of septal fibres in the hippocampus could be verified using AChE histochemistry. Thus, using this approach, selective studies of the septo-hippocampal projection can be performed while leaving the overwhelming portion of the commissural fibre system intact.     

Reticulo-collicular projections: a neuronal tracing study in the rat

Neuroanatomical tract-tracing methods were used to study the topography of the reticulocollicular projections. Injections of gold-HRP or BDA tracers into the medial and/or central portions of the superior colliculus resulted in labelled neurones mainly in the medial reticular formation, whereas injections into the lateral portion of the superior colliculus showed labelling in the medial and lateral reticular formation. When tracer was injected into the lateral portion of the caudal superior colliculus, extensive lateral labelling was observed in the contralateral parvocellular reticular nucleus and the contralateral dorsal medullary reticular nucleus, two areas involved in reflex blinking. The present study shows that these reticular areas project to the lateral superior colliculus, which is known to be involved in the coordination of eye and eyelid movements.     

The pathway from Ia forelimb afferents to the motor cortex: a new hypothesis.

Forelimb target-reaching and food-taking in cats depend on different interneuronal circuitry in the spinal cord. On the basis of previous findings regarding the effect of transection of the corticospinal tract in the spinal cord, of high dorsal column (DC) transection, of low pyramidotomy and of pyramidotomy after previous DC transection, it is proposed that the food-taking movement is temporally linked to target-reaching as follows: During target-reaching, the position of the paw is signalled by the pathway from forelimb proprioceptors (mainly Ia) to the motor cortex with a relay in the main cuneate nucleus. The command for food-taking is issued by the motor cortex only when the pathway from the forelimb signals that the paw approaches the target correctly, as may be determined by a comparison of the information from the forelimb with an efference copy of the motor program for target-reaching. The hypothesis is based on previous results regarding the organization of the pathway from the forelimb to area 3a and the motor cortex, and regarding the cortico-cuneate pathway with selective projection from area 3a and motor cortex to the basal caudal part of the cuneate nucleus, where the proprioceptive information from the forelimb is relayed. Results relevant to the present hypothesis regarding responses of precentral neurones during active and passive movements in awake animals are briefly discussed.     

Neurons with access to the general circulation in the central nervous system of the rat: a retrograde tracing study with fluoro-gold.

Central nervous system neurons which have access to the general circulation were identified by injecting the retrograde tracer Fluoro-Gold peripherally. Fluoro-Gold does not penetrate the blood-brain barrier but is taken up by nerve terminals which project to areas supplied by fenestrated capillaries or to the periphery. Fluoro-Gold-accumulating neurons were present in the following regions or cell groups of the central nervous system: diagonal band of Broca; medial preoptic area; organum vasculosum of the lamina terminalis; subfornical organ; anterior periventricular area; paraventricular nucleus; arcuate nucleus; accessory magnocellular nuclei of the hypothalamus; motor neurons of cranial nerves III-VII, and IX-XII in the brainstem and spinal cord; autonomic ganglionic cells of cranial nerve III (Westphal-Edinger nucleus) in the mesencephalon and the intermediolateral column of the spinal cord; sensory ganglia of the cranial nerve V (mesencephalic trigeminal nucleus); and the C1-C2 and A2 adrenergic cell groups in the medulla. In addition, Fluoro-Gold-accumulating neurons were seen in the sensory ganglia of cranial and spinal nerves. Retrograde labeling with Fluoro-Gold can be combined with immunocytochemistry to identify the chemical messengers within Fluoro-Gold-labeled perikarya. Although a large number of neurons are labeled in the central nervous system with Fluoro-Gold when it is administered peripherally, this technique in combination with immunocytochemistry can be a powerful tool to identify selected neuronal systems in the central nervous system.     

The development of the corticotectal pathway in the albino rat: transient projections from the visual and motor cortices

In rats ranging in age from the second postnatal day (23rd postconceptional day 23 PCD) to adulthood, we have studied the distribution of corticotectal terminals labelled anterogradely by unilateral injections of horseradish peroxidase (conjugated with wheat germ agglutinin) into the visual or motor cortices. No projection to the contralateral superior colliculus (SC) was observed. The earliest age at which the labelled axons and/or terminals from the visual cortex were observed in the ipsilateral SC was 25 PCD. At this stage the projection only involves the optic layer. From 28 to 34 PCD, the projection involves the optic layer, the intermediate layers and the deep part of superficial gray layer. Between 34 and 40 PCD the projection becomes restricted to the superficial laminae (i.e. adultlike). On the 23 PCD (the earliest age examined) we observed a projection from the motor cortex to the intermediate laminae and to a lesser extent the optic layer of the ipsilateral SC. By 34 PCD only the adult-like projection extending from the brachium to the periaqueductal gray (PAG) is apparent. The disappearance of the transient projections to the intermediate collicular laminae may be the result of withdrawal of ‘misprojecting’ axonal collaterals.     

The influence of the autonomic nervous system in the control of erythropoietin secretion in the hypoxic rat.

1. Erythropoietin has been measured in the serum of hypoxic rats that have been exposed to 6 hr of hypoxia, by use of a biological assay. 2. The effects of section of splanchnic, renal and sinus nerves on the concentration of Erythropoietin present in the serum ofthe hypoxic animals has been investigated. 3. Serum levels of Erythropoietin were reduced in the hypoxic animals immediately after cutting the splanchnic nerves, but unaltered one week after the operation. 4. The concentration of Erythropoietin in the serum of hypoxic rats was increased by the secretion of the renal nerves. Section of the sinus nerves abolished the effect. 5. Elevated levels of Erythropoietin were present in the sera of hypoxic rats immediately after section of the sinus nerves. The effect was still obtained when the animals were exposed to hypoxia 1 week after cutting the nerves, though in attenuated form. 6. The rise in the concentration of the hormone after chronic section of the sinus nerves was estimated as as 261% (fiducial limits 167-513, P = 0-05), by parallel line bio-assay. 7. The concentration of Erythropoietin in the serum of hypoxic rats was progressively elevated as the arterial PO2 of the animals was reduced. Section of the sinus nerves did not alter the secretion of Erythropoietin in these experiments.     

The visual cortico-striato-nigral pathway in the rat

The organization of the visual cortico-striato-nigral pathway in the rat has been investigated in two sets of experiments using anterograde autoradiographic tracing techniques. First, in one group of animals, injections of tritiated amino acids were placed throughout the visual cortex to demonstrate the visual corticostriatal pathway. The results indicate that visual corticostriatal fibres terminate in a distinctive clustered pattern throughout the entire length of the ipsilateral dorsomedial striatum. The projection shows a longitudinal topographic organization with cortical loci projecting onto narrow longitudinal regions of the dorsomedial striatum. In addition to the ipsilateral projection, a substantially smaller contralateral visual corticostriatal projection was also demonstrated. In the second set of experiments, the visual corticorecipient region of the striatum demonstrated in the first set of experiments was injected with tritiated amino acids to demonstrate the "visual" striatonigral projection. The results indicate that striatonigral fibres from the dorsomedial striatum project throughout the rostrocaudal extent of the ventral region of the pars reticulata. As with the visual corticostriatal pathway, the projection shows a longitudinal topographic organization with striatal loci projecting onto longitudinal regions of the ventral pars reticulata; the most rostral regions of the dorsal striatum project to the most medial regions of the ventral pars reticulata and, likewise, successively more caudal regions of the dorsal striatum project to successively more lateral regions of the ventral pars reticulata. In addition to the main projection to the ventral pars reticulata, a second smaller component of the striatonigral pathway was traced to adjacent regions of the dorsal pars reticulata and ventral pars compacta. These findings provide evidence of a visual cortico-striato-nigral pathway to both the pars reticulata and pars compacta in the rat. It is suggested that the major projection onto the ventral pars reticulata may provide an input onto nigrotectal neurons and thereby complete an indirect visual corticotectal connection mediated via links in the basal ganglia.     

Anatomical analysis of ventrolateral thalamic input to primate motor cortex.

1. The origin and topographical organization of input to the arm area of the primate motor cortex from the ventrolateral thalamus were examined using the method of retrograde transport of horseradish peroxidase (HRP). 2. A thin, continuous slab of labeled neurons was found in the ventrolateral thalamus followingmultiple injections of HRP into the arm area of the motor cortex. The slab of labeled neurons was flanked, medially and laterally, by groups of unlabeled neurons. 3. The origin of ventrolateral thalamic input was more extensive than previously thought. Labeled neurons were found from A10.0 to A6.0 and occurred in three ventolateral thalamic subdivisions: ventralis lateralis pars oralis (VLo), ventralis lateralis pars caudalis (VLc), and ventralis posterior lateralis pars oralis (VPLo). For simplicity this region containing labeled neurons has been termed the ventrolateral thalamic (VL) arm area. 4. Injections of HRP into the somatic sensory cortex indicated that the thalamic regions which project to the somatic sensory cortex are separate from the VL arm area. 5. The distribution of labeled neurons following single injections of HRP into different regions of the motor cortex arm area indicated that the VL arm area is topographically organized, particularly its caudal part. Ventral regions of the VL arm area were labeled following HRP injections into motor cortex regions adjacent to the central sulcus where the representation of largely distal musculature is localized. Dorsal regions of the VL arm area were labeled following HRP injections into motor cortex regions more rostral to the central sulcus where the representation of more proximal musculature is localized. 6. A larger region of the VL arm area was labeled following HRP injections adjacent to the central sulcus than following the more rostral motor cortex injections. This suggests that, like the arm area of the motor cortex, more of the VL arm area is allotted to the representation of distal than proximal musculature. 7. Following very small cortical HRP injections, isolated labeled thalamic neurons were diffusely scattered throughout a 3-mm rostrocaudal extent of the VL arm area. In addition, a small focal cluster of labeled thalamic neurons was also seen. The labeled cluster was limited to 0.5 mm rostrocaudally and 300 mum in width. The focal distribution of labeled thalamic neurons suggests that aspects of a point to point organization may exist in the connection between VL and the motor cortex arm area.     

The vascular system of the upper eyelid. Anatomical study and clinical interest

Thorough knowledge of the vascular supply is indispensable for repair and oncologic surgery of the eyelids, and has a significant impact on the management of complex defects of this region. This anatomic study was performed with five fresh cadavers after arterial injection of coloured neoprene latex. The distribution of the vascular system of the upper eyelid was examined after dissection and photographic study. It is made up of three arcades: the preseptal arcade, the supratarsal arcade, and the marginal arcade, under the orbicularis oculi muscle. These arcades are supplied by branches of the ophthalmic artery (supraorbital artery, supratrochlear artery and medial palpebral artery) and branches of the facial artery and temporal artery. Small vertical branches arising out of these arcades provide an anastomotic network. This anatomical study aimed to describe the vascular system of the upper eyelid in order to search for constant features and to map the blood supply of the principal upper lid flaps.     

Early postnatal development of the rat corticostriatal pathway: an anterograde axonal tracing study using biocytin pellets

 The ontogenetic development of the neocortical projections to the striatum was studied in postnatal rats by sensitive anterograde tracing with biocytin. The developmental status of this mainly glutamatergic pathway is important as it plays a major role in regulation of striatal maturation and induction of excitotoxic striatal neurodegeneration resembling the type found in Huntington’s disease. Biocytin pelletts were injected into the motorcortex caudal forelimb area of newborn rats and of rats aged 3, 6, 13, 27 and 61 days followed by sacrifice and visualisation of the tracer 24 h later, at P1, P7, P14, P28 and P62, respectively. Biocytin pellets were also injected into the motorcortex jaw-lip-tongue area and into the cingulate cortex of newborn and 6-day-old rats. The biocytin pellets produced intense anterograde labelling of corticofugal projection fibres from the injection sites, as well as a local Golgi-like labelling of neurons. From postnatal day 1 into adult age efferent fibres from the motorcortex caudal forelimb area displayed a progressively denser innervation of the ipsilateral and contralateral, dorsolateral striatum. The terminating fibres were most dense in the ipsilateral striatum. In the dorsolateral striatum the corticostriatal fibres displayed a patch/matrix-like arrangement from postnatal day 14 onwards. Injections into the motorcortex jaw-lip-tongue area at postnatal days 0 and 6 displayed a progressive, denser innervation of the ipsilateral and contralateral ventrolateral striatum. The cingulate corticostriatal fibre projection, was more developed at birth than the projection from the motorcortex and increased its fibre density in the ipsilateral dorsomedial striatum up to at least day 7. In conclusion: the ipsilateral corticostriatal projections from the rat motorcortex and cingulate areas are present in newborn rats. During postnatal life the pathway develops further and specialisation of the terminating fibres into a patch/matrix like arrangement can be identified by postnatal day 14. Accepted: 5 January 1999     

The organization of the crossed geniculogeniculate pathway of the rat: a Phaseolus vulgaris-leucoagglutinin study.

The intergeniculate leaflet of the thalamus is known to give rise to neuronal projections to the suprachiasmatic nuclei and the rostral part of the pineal gland. Via these projections the intergeniculate leaflet is considered to play a role in regulation of circadian rhythms. Iontophoretic injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin were placed in various subnuclei of the lateral geniculate nucleus in order to study the topographical organization of the crossed geniculogeniculate pathway in the rat. Injections involving neurons in the intergeniculate leaflet or the medial subpart of the ventral nucleus (which presumably is part of the intergeniculate leaflet of the thalamus too) gave rise to labeled nerve fibers in the opposite lateral geniculate nucleus. The axons contained in this pathway were followed either medially via the posterior commissure, or via the optic tracts and optic chiasm, to the contralateral hemisphere. In the contralateral lateral geniculate nucleus, the intergeniculate leaflet was most densely innervated, but a substantial innervation of the ventral lateral geniculate nucleus was observed as well. Only a few labeled fibers were observed in the dorsal subnucleus. However, the dense innervation of the contralateral intergeniculate leaflet not only covered the small zone between the dorsal and ventral nuclei, but also a dorsomedial part of the ventral nucleus that merged caudally with the lateral part of the zona incerta. In the remaining part of the ventral nucleus, single Phaseolus vulgaris-leucoagglutinin-labeled fibers surrounded specific cells. The demonstration of a divergent projection between the intergeniculate leaflet and specific subparts of the contralateral geniculate nuclei indicates that the two lateral geniculate nuclei are regulating each other. The function of this pathway is suggested to be related to the regulation of circadian rhythmicity, but experimental evidence for this hypothesis is still lacking.     

Immunohistochemical evidence for a crossed cholecystokinin corticostriatal pathway in the rat.

Using the indirect immunofluorescence technique, the effects of decortication and callosotomy on the pattern of cholecystokinin (CCK)-like immunoreactivity were studied in the striatum of the rat. Decortication plus callosotomy, but not decortication alone, caused a strong decrease in the immunoreactivity on the side ipsilateral to the lesion. An almost complete disappearance of CCK immunoreactive patches in the medial-dorsal aspects of the striatum was observed. These results indicate that part of the striatal CCK immunoreactive fibres are of cortical origin, to a considerable extent from the contralateral side.     

Histochemical tracing of zinc ions in the rat testis

To detect free zinc ions in the rat testes four rats were transcardially perfused with Na2S, and the seminiferous tubules from two other rats were incubated in Na2S. Sections from the two sources were autometallographically (AMG) developed, whereby zinc sulphide crystal lattices created in the tissue by the sulphide treatment were silver enhanced. Light microscopical analysis showed zinc ions in primary spermatogonia until the zygotene primary spermatocytes (stage I), in late pachytene spermatocytes (stages XII and XIII), and in late spermatids from step 15 to step 19 (stages I-VIII). The highest intensity of AMG grains was detected in the residual bodies and tails of step 19 spermatids. Grains were occasionally found in the cytoplasm of Leydig cells. Sections from animals treated with the chelator diethyldithiocarbamate prior to sulphide treatment showed a complete lack of AMG staining. At ultrastructural levels the AMG grains were found in smooth-surfaced endoplasmic reticulum of all spermatogonial stages, and in the acrosome, midpiece, and tail of late spermatids. The presence of zinc ions in preleptotene spermatocytes and cytoplasmic lobes of late spermatids suggests a specific role of free zinc at the onset of meiosis and at spermiation.