Organisation of lateral line and auditory areas in the midbrain of Xenopus laevis

Lateral line areas in the midbrain of Xenopus laevis were identified by recording evoked potentials and neural activity elicited by stimulating anterior and posterior lateral line nerves. Spike activity was found in the lateral half of the optic tectum, ventrolateral tectum, and torus semicircularis. Contra- and ipsilateral lateral line pathways to these regions were identified. Spike discharge was associated with an evoked potential (EP) consisting of a large negative-positive wave sometimes preceded by a small positive-negative deflection. EP depth profiles varied according to electrode position within the lateral line midbrain projection field. In the middle of the field a dramatic increase in EP growth occurred as the electrode passed through the torus semicircularis, with peak amplitudes being achieved 900-1,100 micron from the surface within nucleus principalis and magnocellularis. Tracks at the lateral edge of the field showed a steady growth of EP, with peak amplitudes around 600 micron as the electrode passed through ventrolateral tectum. Auditory responses to tone pips were found in the nucleus laminaris and principalis in caudomedial regions of the torus semicircularis, in areas lying medial to the main centers of lateral line evoked activity; this is a similar organisation to that found in teleost fish. The results indicate the torus semicircularis and deep layers of the lateral tectum to be involved in lateral line processing Some topographic separation of the representation of anterior and posterior lateral line systems is indicated. The possible involvement of these areas in lateral line stimulus localisation is discussed.     

Auditory and lateral line inputs to the midbrain of an aquatic anuran: neuroanatomic studies in Xenopus laevis

Computation of rate in auditory signals is essential to call recognition in anurans. This task is ascribed to a group of central nervous system nuclei in the dorsal midbrain or torus semicircularis, homologous to the inferior colliculus of mammals. We have mapped the connections of the subnuclei of the torus semicircularis in Xenopus laevis to determine which receive auditory and which receive lateral line information. Relative to terrestrial anurans, the torus of X. laevis is hypertrophied and occupies the entire caudal, dorsal midbrain. Auditory input to the torus, that arising directly from the dorsal medullary nucleus, is present only in the laminar nucleus. The principal and magnocellular nuclei receive their input from the lateral line nucleus of the medulla. All three nuclei of the torus also have reciprocal connections with the superior olive and the nucleus of the lateral lemniscus. Ascending efferents from all three nuclei of the torus innervate central and lateral thalamic nuclei, and all have a weak reciprocal connection with the posterior thalamus. The laminar and magnocellular nuclei have reciprocal connections with the ventral thalamus, and all three nuclei of the torus receive descending input from the anterior entopeduncular nucleus. The laminar and magnocellular nuclei also receive descending input from the preoptic area. Based on our identification of toral nuclei and these results we assign a major function for the detection of water-borne sounds to the laminar nucleus and a major function for the detection of near field disturbances in water pressure to the principal and magnocellular nuclei.     

Spinal ascending pathways in amphibians: Cells of origin and main targets

As part of a research program on the evolution of somatosensory systems in vertebrates, the various components of ascending spinal projections were studied with in vivo and in vitro tract-tracing techniques in representative species of amphibians (the large green frog, Rana perezi, the clawed toad, Xenopus laevis and the ribbed newt, Pleurodeles waltl). Three main ascending sensory channels, each with largely separate targets, were demonstrated: 1. Ascending projections via the dorsal funiculus include primary and nonprimary projections that ascend to terminate mainly in the dorsal column nucleus at obex levels. A small component ascends farther rostralwards to terminate in the reticular formation, the octavolateral area, the trigeminal nuclear complex, and in the granular layer of the cerebellum. 2. Projections ascending via the dorsolateral funiculus reach other spinal and supraspinal targets than the dorsal funicular fibers, mainly ipsilaterally. At upper cervical cord and obex levels, many fibers innervate a region considered the amphibian homologue of the lateral cervical nucleus of mammals. In the medulla, these fibers ascend ventral to the descending trigeminal tract to terminate in the dorsal column and the solitary tract nuclei, and more rostrally, in the reticular formation, the descending trigeminal nucleus and the medial aspect of the ventral octaval nucleus. Major projections reach the area between the facial motor nucleus and the ventral octaval nucleus, and a mediolateral subcerebellar band. These projections arise in neurons located mainly in the ipsilateral deep dorsal and lateral fields throughout the spinal cord. 3. Ascending spinal projections via the ventral quadrant of the spinal cord (the ventral and ventrolateral funiculi) ascend throughout the brainstem up to the diencephalon. Along its course, this component innervates various parts of the reticular formation, the octavolateral area, the granular layer of the cerebellum, the region ventromedial and ventrolateral to the isthmic nucleus, and the subcerebellar region. In the mesencephalon, the torus semicircularis, the midbrain tegmentum and, sparsely, the tectum mesencephali are innervated. Beyond the midbrain, various dorsal and particularly ventral thalamic nuclei and the posterior tubercle are innervated by this ascending sensory channel. The cells of origin of some of these projections were observed in the dorsal, and to a lesser extent, in the lateral and ventral spinal fields of the spinal cord. Evidence for the presence of these three main ascending sensory channels throughout vertebrates will be discussed. The presence of such channels appears to be a shared character in the brain of both amniotes and anamniotes. J. Comp. Neurol. 378:205–228, 1997. © 1997 Wiley-Liss, Inc.     

Role of nerve growth factor in the adult dorsal root ganglia neuron and its response to injury

The response of dorsal root ganglia (DRG) neurons to NGF deprivation and to axotomy was examined in adult guinea pigs. The success of NGF deprivation by means of an autoimmune approach was monitored by the measurement of serum antibody titer levels against guinea pig NGF with the standard bioassay for NGF activity. That the antibody produced NGF deprivation was confirmed by histologic evidence of neuronal atrophy and apparent cell loss in sections of the superior cervical ganglia (SCG) and by marked decreases (65-80%) of SCG neurotransmitter-synthesizing enzyme activity levels. By using the autoimmune approach a new source of guinea pigs was found which consistently produced high titers of cross-reacting anti-NGF antibodies. Experiments were designed to examine the response of the sensory neuron to injury while chronically deprived of NGF. Total neuronal counts in the sixth lumbar DRG 98 days after sciatic nerve crush showed no difference between NGF-deprived and control ganglia. Measurement of the size spectrum of DRG neurons showed evidence of atrophy of the NGF-deprived neurons in both the uninjured and axotomized side compared to respective controls. The mean volume of uninjured sensory neurons measured in the NGF-deprived guinea pigs was decreased 27.7% (P less than .05) compared with that of control guinea pigs. The degree of regeneration 6 days following a nerve crush was the same in NGF-deprived sensory neurons and in controls when measured by the "pinch test" and by isotope-labeled axonal transport studies.(ABSTRACT TRUNCATED AT 250 WORDS)     

Xenopus laevis CB1 cannabinoid receptor: molecular cloning and mRNA distribution in the central nervous system

In the present research we isolated and characterized Xenopus laevis CB1 cannabinoid receptor mRNA. The CB1 coding sequence shows a high degree of identity with those of other vertebrates, mammals included, confirming that CB1 receptor is conserved over the course of vertebrate evolution. Notably, the similarity between the X. laevis CB1 sequence and that of the urodele amphibian Taricha granulosa is not higher than the similarity existing between Xenopus and mammals, thus supporting phylogenetic distance between anurans and urodeles. By means of in situ hybridization histochemistry, CB1 mRNA expression and distribution was investigated in the X. laevis central nervous system. As revealed, CB1 mRNA-containing neurons are numerous in the prosencephalon, especially in the olfactory bulbs, telencephalic pallium, and hypothalamus. In the midbrain and hindbrain, labeled cells were observed in the mesencephalic tegmentum and dorsolateral romboencephalon. Abundant CB1 mRNA positive neurons are localized throughout the gray matter of the spinal cord, in particular in the dorsal and ventral fields, where labeled motor neurons are also observed. The distribution of CB1 mRNA in the Xenopus CNS is generally consistent with the CB1-like-immunohistochemistry results we have previously obtained, showing in amphibians a well developed cannabinergic system almost comparable to that described in mammals. However, some differences, such as the abundance of CB1 mRNA-containing neurons in the olfactory system and the rich CB1 spinal innervation, are found.     

Tachykinin Actions on Deep Dorsal Horn Neurons In Wm: An Electrophysiological and Morphological Study in the Immature Rat

To assess whether functional neurokinin receptors exist in the deep dorsal horn of the rat, the actions of the selective neurokinin-1 receptor (NK1R) agonist [Sar⁹,Met(O₂)¹¹]substance P [Sar⁹,Met(O₂)¹¹]]SP), the neurokinin-2 receptor (NK2R) agonists [β-Ala⁸]NKA_4-10 and GR64349 and the neurokinin-3 receptor (NK3R) agonist senktide were examined intracellularly in vitro. [Sar⁹,Met(O₂)¹¹]]SP (1–4 μM) and senktide (1-2 μM) elicited slow depolarizations (40 mV) associated with increased synaptic activity and cell firing. [β-Ala⁸]NKA_4-10 (10-20 μM) and GR64349 (0.25-10 μM) caused small depolarizations (<2.0 mV) and no firing. Neurons were categorized as either ‘tonic’ or ‘phasic’ depending on their firing response to direct current step depolarizations. Tonic neurons, which, unlike phasic neurons, display no spike firing accommodation, generated a significantly larger depolarization to the NK1R and NK3R agonists. The putative contribution of these receptors to primary afferent-mediated synaptic transmission was assessed by testing the NKIR antagonist GR82334 (1 μM), the NK2R antagonist MEN10,376 (1 μM) and the NK3R antagonist [Trp⁷,β-Ala⁸]NKA4_-10 (1 μM) against the dorsal root-evoked excitatory postsynaptic potential (DR-EPSP). GR82334 and [Trp⁷,β-Ala⁸]NKA_4-10 significantly reduced (P ≤ 0.05) the duration but not the amplitude of the DR-EPSP. MEN10,376 (1 μM) had no effect on DR-EPSP amplitude or duration. Morphological detail was obtained for seven biocytin-filled deep dorsal horn neurons tested with [Sar⁹,Met(O₂)¹¹]SP. Five neurons responded to the NKIR agonist, and two of these had dorsally directed dendrites into the substantia gelatinosa. The other three [Sar⁹,Met(O₂)¹¹]SP-sensitive neurons had dendrites within deeper laminae. These data support the existence of functional NK1Rs and NK3Rs in the deep dorsal horn which may be involved in mediating sensory afferent inputs from nociceptors.     

An experimental study of the connections of the telencephalon in the rainbow trout (Oncorhynchus mykiss). I: Olfactory bulb and ventral area

The fluorescent carbocyanine dye 1,1'-dioctadecyl 3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) was used in fixed tissue to comprehensively analyze the connections of the olfactory bulbs and the different regions of the ventral (V) area of the telencephalic lobes (subpallium) of the rainbow trout. With this goal, DiI was applied to the different telencephalic nuclei and zones, as well as to the olfactory nerve, the olfactory bulb, the retina, and to several structures in the diencephalon and brainstem of juvenile trout. The olfactory bulbs maintain reciprocal connections with several regions of the telencephalon [ventral nucleus of V (Vv), supracommissural nucleus (Vs), posterior zone of D (Dp), preoptic nucleus], and also project to the diencephalon (posterior tuberal nucleus, posterior hypothalamic lobe). Vv receives afferents from Vs, the dorsal nucleus of V (Vd), the preoptic nucleus, and from several nuclei in the diencephalon and brainstem (suprachiasmatic nucleus, anterior and lateral tuberal nuclei, preglomerular complex, tertiary gustatory nucleus, posterior tubercle, inferior hypothalamic lobes, thalamus, torus semicircularis, secondary gustatory nucleus, locus coeruleus, superior raphe nucleus, central gray, and reticular formation), and projects to dorsal (pallial) regions and most of the nuclei afferent to Vv. The dorsal nucleus of V (Vd) and Vs mainly project to the dorsal area. In an accompanying article (Folgueira et al., 2004), we present the results of application of DiI to dorsal (pallial) telencephalic regions, as well as of several experiments of tracer application to extratelencephalic regions. The results presented here, together with those of the accompanying article, reveal a complex connectional pattern of the rainbow trout ventral telencephalon, most of these connections having not been described previously in salmonids.     

Segmental and laminar organization of the spinothalamic neurons in cat: evidence for at least five separate clusters

The spinothalamic tract (STT), well known for its role in the relay of information about noxe, temperature, and crude touch, is usually associated with projections from lamina I, but spinothalamic neurons in other laminae have also been reported. In cat, no complete overview exists of the precise location and number of spinal cells that project to the thalamus. In the present study the laminar distribution of retrogradely labeled cells in all spinal segments (C1-Coc2) was investigated after large WGA-HRP injections in the thalamus. The results show that this distribution of STT cells differed greatly between the different spinal segments. Quantitative analysis showed that there exist at least five separate clusters of spinothalamic neurons. Lamina I neurons in cluster A and lamina V neurons in cluster B are mainly found contralaterally throughout the length of the spinal cord. Cluster C neurons are located bilaterally in the ventrolateral part of laminae VI-VII and lamina VIII of the C1-C3 spinal cord. Cluster D neurons were found contralaterally in lamina VI in the C1-C2 segments, and cluster E neurons were located mainly contralaterally in the medial part of laminae VI-VII and lamina VIII of the lumbosacral cord. Most spinothalamic neurons are not located in the enlargements and most spinothalamic neurons are not located in lamina I, as suggested by several other authors. The location of the spinothalamic neurons shows remarkable similarities, but also differences, with the location of spino-periaqueductal gray neurons.     

Membrane Properties of Physiologically Classified Rat Dorsal Horn Neurons In Vitro: Correlation with Cutaneous Sensory Afferent Input

Dorsal horn neurons in the young rat spinal cord-hindlimb preparation were physiologically classified as wide dynamic range (WDR), nociceptive specific (NS) or low threshold (LT) according to their excitatory responses to low and high intensity mechanical stimuli applied to the hindlimb skin. Two additional types were classified: neurons displaying only sub-threshold excitations (SUB) and neurons displaying inhibitory events (INH), such as inhibitory post-synaptic potentials or interruption of spontaneous spiking following cutaneous stimulation. Direct intracellular current injection revealed four different patterns of spiking behaviour: group A neurons were characterized by tonic firing in response to depolarizing current pulses; group B neurons were strongly phasic, producing only one spike at the beginning of the pulse; group A-B neurons generated an early unsustained (< 300 ms) burst of spikes; and group C neurons exhibited anomalous rectification in response to hyperpolarizing current which was followed by a voltage-dependent rebound excitation. A statistically significant ( P ≤ 0.01) association existed between a neuron's physiological classification and its electrophysiological profile. The majority of WDR neurons responded with tonic firing and were assigned to group A, while NS neurons were strongly represented in group A-B. All INH neurons were assigned to group C. LT neurons were distributed between groups A and A-B, and SUB neurons were distributed between groups A and B. These data indicate, firstly, that dorsal horn neurons possess heterogeneous membrane properties and, secondly, that a relationship exists between a neuron's biophysical profile and its excitatory or inhibitory response to peripheral cutaneous afferent stimulation. The implications for dorsal horn somatosensory processing are discussed.     

Central projections of thoracic splanchnic and somatic nerves and the location of sympathetic preganglionic neurons in Xenopus laevis

The central and peripheral organization of thoracic visceral and somatic nervous elements was studied by applying dextran amines to the proximal cut ends of the thoracic splanchnic and somatic nerves in Xenopus laevis. Many labeled dorsal root ganglion cells of visceral afferents, and all somatic afferents, were located in a single ganglion of one spinal segment, and the two types of cells were distributed topographically within the ganglion. The labeled sympathetic preganglionic neurons were located predominantly in the same area of the thoracic spinal gray as in other frogs and in mammals. The labeled visceral afferents projected to Lissauer's tract and the dorsal funiculus. The visceral fibers of the tract ascended to the level of the subcerebellar area, supplying collateral branches to the lateral one-third of the dorsal horn and to the area of brainstem nuclei, including lateral cervical and descending trigeminal nucleus, and descended to the filum terminale. The visceral fibers of the dorsal funiculus were distributed to the dorsal column nucleus and the solitary tract. A similar longitudinal projection was also seen in the somatic afferents. The dual central pathway of thoracic primary afferents in the anuran spinal cord is a property held in common with mammals, but the widespread rostrocaudal projection through Lissauer's tract may be a characteristic of the anuran central nervous system. In frogs, the direct transmission of primary afferent information to an extremely wide area of the central nervous system may be important for prompt assessment of environmental factors and control of body functions.     

Cytoarchitectonic organization of the spinal cord in the rat: II. The cervical and upper thoracic cord

A laminar cytoarchitectonic scheme of the cervical and upper thoracic segments of the rat spinal cord is presented in which Rexed's principles for the cat are applied. The material examined in the current investigation consists of 50-80 microns-thick unstained or Nissl-stained sections, and 2 microns-thick plastic-embedded sections stained with paraphenylenediamine. The cytoarchitectonic organization was found to be basically similar to that of the cat. As in our previous study of the cytoarchitectonic organization of the lower thoracic and lumbosacral spinal cord (Molander et al.; J. Comp. Neurol. 230:133-141, '84), the borderlines between the laminae were often found to be ambiguous, suggestive of zones of transition rather than sharp borders. In addition to the laminar scheme, the distribution of certain important cell groups, including the column of Clarke, the central cervical nucleus, the lateral cervical nucleus, the lateral spinal nucleus, and the internal basilar nucleus, is described.     

An autoradiographic study of the spinofacial projection in the monkey

The spinofacial projection was revealed using anterograde transport of radioactively labeled protein in the monkey. The projection arises from cells in the lateral part of the spinal dorsal horn (i.e. the lateral part of lamina V of Rexed) at the upper cervical cord, mainly C1 segment, ascends in the medial or ventromedial part of the anterior funiculus after crossing the anterior white commissure, then courses through the dorsolateral part of the inferior olivary complex. Finally, it terminates within the medial parts of the facial nuclei bilaterally, with the cells from the side ipsilateral to the injection contributing more heavily. Some fibers of this projection cross initially at spinal levels and recross again at levels through the rostral medulla and caudal pons.     

Pattern of calbindin‐D28k and calretinin immunoreactivity in the brain of Xenopus laevis during embryonic and larval development

The present study represents a detailed spatiotemporal analysis of the localization of calbindin‐D28k (CB) and calretinin (CR) immunoreactive structures in the brain of Xenopus laevis throughout development, conducted with the aim to correlate the onset of the immunoreactivity with the development of compartmentalization of distinct subdivisions recently identified in the brain of adult amphibians and primarily highlighted when analyzed within a segmental paradigm. CR and CB are expressed early in the brain and showed a progressively increasing expression throughout development, although transient expression in some neuronal subpopulations was also noted. Common and distinct characteristics in Xenopus, as compared with reported features during development in the brain of mammals, were observed. The development of specific regions in the forebrain such as the olfactory bulbs, the components of the basal ganglia and the amygdaloid complex, the alar and basal hypothalamic regions, and the distinct diencephalic neuromeres could be analyzed on the basis of the distinct expression of CB and CR in subregions. Similarly, the compartments of the mesencephalon and the main rhombencephalic regions, including the cerebellum, were differently highlighted by their specific content in CB and CR throughout development. Our results show the usefulness of the analysis of the distribution of these proteins as a tool in neuroanatomy to interpret developmental aspects of many brain regions. J. Comp. Neurol. 521:79–108, 2013. © 2012 Wiley Periodicals, Inc.     

Mechanisms underlying the noradrenergic modulation of longitudinal coordination during swimming in Xenopus laevis tadpoles

Noradrenaline (NA) is a potent modulator of locomotion in many vertebrate nervous systems. When Xenopus tadpoles swim, waves of motor neuron activity alternate across the body and propagate along it with a brief rostro-caudal delay (RC-delay) between segments. We have now investigated the mechanisms underlying the reduction of RC-delay s by NA. When recording from motor neurons caudal to the twelfth postotic cleft, the mid-cycle inhibition was weak and sometimes absent, compared to more rostral locations. NA enhanced and even unmasked inhibition in these caudal neurons and enhanced inhibition in rostral neurons, but to a lesser extent. Consequently, the relative increase in the amplitude of the inhibition was greater in caudal neurons, thus reducing the RC-inhibitory gradient. We next investigated whether NA might affect the electrical properties of neurons, such that enhanced inhibition under NA might promote postinhibitory rebound firing. The synaptic inputs during swimming were simulated using a sustained positive current, superimposed upon which were brief negative currents. When these conditions were held constant NA enhanced the probability of rebound firing--indicating a direct effect on membrane properties--in addition to any indirect effect of enhanced inhibition. We propose that NA preferentially enhances weak caudal inhibition, reducing the inhibitory gradient along the cord. This effect on inhibitory synaptic transmission, comprising parallel pre- and postsynaptic components, will preferentially facilitate rebound firing in caudal neurons, advancing their firing relative to more rostral neurons, whilst additionally increasing the networks ability to sustain the longer cycle periods under NA.     

Pre- and Post-synaptic Actions of 5-Hydroxytryptamine in the Rat Lumbar Dorsal Horn In Vitro: Implications for Somatosensory Transmission

Since the relative contribution of pre- versus post-synaptic actions of 5-hydroxytryptamine (5-HT) to modulation of somatosensory processing in the dorsal horn is not known, recordings fro m primary afferents and dorsal horn neurons from in vitro rat spinal cord were used to address this issue. 5-HT produced a depression of spontaneous dorsal root potentials and a slow primary afferent depolarization (PAD): the PAD versus 5-HT concentration-response curve was bell shaped (maximum at 5 μM; 250±C 41.5 μV). In 28/40 dorsal horn neurons, 5-HT elicited a slow depolarization not clearly associated with a specific input resistance change. Excitatory synaptic transmission from primary afferents to dorsal horn neurons was depressed by 5-HT in 40/45 neurons. 5-HT ≥ 5 μM significantly ( P ≤ 0.05) decreased the amplitude, shortened the total duration and half-decay time of the excitatory post-synaptic potential (EPSP). A dominant effect of 5-HT on longer latency EPSP components was evident. There was no direct relationship between the magnitude of PAD and the reduction of the EPSP by 5-HT. 5-Carboxamidotryptamine, an agonist for 5-HT¹ receptors, mimicked the depression of neurotransmission in the dorsal horn without producing PAD. A sample of dorsal horn neurons ( n = 8) was injected with biocytin and their morphology described. All had somata within laminae III-VI. In five of these neurons 5-HT depressed the EPSP but in one interneuron-like and one unclassed neuron the EPSP was potentiated. These data suggest that whilst depression of synaptic transmission is the predominant effect of 5-HT in the deep dorsal horn, this is not easily related to PAD or cellular actions of 5-HT on dorsal horn neurons.     

Localization of the spinal accessory motoneurons in the cervical cord in connection with the phrenic nucleus: an HRP study in cats

The localization of the spinal accessory motoneurons (SAMNs) that innervate the accessory respiratory muscles, the sternocleidomastoid (SCM) and trapezius (TP) muscles, was identified in the cat using the horseradish peroxidase (HRP) method. In the cases of HRP bathing of the transected spinal accessory nerve (SAN), HRP-labeled motoneurons were observed ipsilaterally from the C1 to the rostral C6 segments of the spinal cord. Labeled neurons were located principally in the medial and central regions of the dorsomedial cell column of the ventral horn in the C1 segment, in the lateral region of the ventrolateral cell column in the C2-C4 segments, between the ventrolateral and ventromedial cell columns in the C5 segment and in the lateral region of the ventromedial cell column in the C6 segment. In the cases of HRP injection into either SCM or TP muscles, labeled SCM motoneurons were found in the C1-C3 segments of the spinal cord and labeled TP motoneurons were chiefly localized more caudally within the spinal accessory nucleus. The present study revealed that, in the C5 and C6 segments, the SAMNs have a very similar topographic localization to the phrenic nucleus in the ventral horn. This finding implicated the functional linkage of the SAMNs with the phrenic motoneurons in particular types of respiration.     

Organization of subcortical pathways for sensory projections to the limbic cortex. I. Subcortical projections to the medial limbic cortex in the rat

Subcortical afferent projections to the medial limbic cortex were examined in the rat by the use of retrograde axonal transport of horseradish peroxidase. Small iontophoretic injections of horseradish peroxidase were placed at various locations within the dorsal and ventral cingulate areas, the dorsal agranular and ventral granular divisions of the retrosplenial cortex and the presubiculum. Somata of afferent neurons in the thalamus and basal forebrain were identified by retrograde labeling. Each of the anterior thalamic nuclei was found to project to several limbic cortical areas, although not with equal density. The anterior dorsal nucleus projects primarily to the presubiculum and ventral retrosplenial cortex; the anterior ventral nucleus projects to the retrosplenial cortex and the presubiculum with apparently similar densities; and the anterior medial nucleus projects primarily to the cingulate areas. The projections from the lateral dorsal nucleus to these limbic cortical areas are organized in a loose topographic fashion. The projection to the presubiculum originates in the most dorsal portion of the lateral dorsal nucleus. The projection to the ventral retrosplenial cortex originates in rostral and medial portions of the nucleus, whereas afferents to the dorsal retrosplenial cortex originate in caudal portions of the lateral dorsal nucleus. The projection to the cingulate originates in the ventral portion of the lateral dorsal nucleus. Other projections from the thalamus originate in the intralaminar and midline nuclei, including the central lateral, central dorsal, central medial, paracentral, reuniens, and paraventricular nuclei, and the ventral medial and ventral anterior nuclei. In addition, projections to the medial limbic cortex from the basal forebrain originate in cells of the nucleus of the diagonal band. Projections to the presubiculum also originate in the medial septum. These results are discussed in regard to convergence of sensory and nonsensory information projecting to the limbic cortex and the types of visual and other sensory information that may be relayed to the limbic cortex by these projections.     

Central projections of C4-C8 dorsal root ganglia in the rat studied by anterograde transport of WGA-HRP.

Injections of WGA-HRP were made in the rat C4-C8 dorsal root ganglia (DRGs) individually to study the central projections and their relations to each other. The main dorsal horn projections from these DRGs to the dorsal horn lamina II extended for about two segments rostrally and caudally to the injected DRG, whereas the projections to laminae I, III, and IV were less restricted rostrocaudally. Comparisons of the dorsal horn projections from the DRGs investigated indicated a tendency for a somatotopic organization, which was most prominent in lamina II. Labeled central branches from the C4-8 DRGs could be traced in the dorsal column as far caudally as 12-17 segments caudal to the level of entrance. Most of these fibers appeared to end in the medial dorsal horn base, including the column of Clarke. Labeling of primary afferents in the ventral horn generally extended for at least 3-4 segments rostral and caudal to the level of the injected DRG. Projections to the central cervical nucleus were most prominent from the C4 DRG and gradually became less prominent from the more caudal DRGs. Heavy projections to the cuneate nucleus (Cun) originated from the C7 and C8 DRG, whereas those from the C4-C6 DRGs were less extensive. The Cun projections from the different DRGs appeared to overlap, and the same was true for the projections to the external cuneate nucleus. Projections to the gracile nucleus, the vestibular nuclear complex, including nucleus X, and to trigeminal sensory nuclei were seen from all DRGs investigated.     

Evidence for new growth and regeneration of cut axons in developmental plasticity of the rubrospinal tract in the North American opossum.

We have shown previously that rubral axons can grow around a lesion of their spinal pathway in the developing opossum and that a critical period exists for that plasticity (Martin and Xu, Dev Brain Res 39:303, 1988). Since most rubrospinal neurons degenerate after axotomy during the critical period, we have proposed that plasticity results primarily from growth of late arriving axons around the lesion rather than regeneration of cut axons (Xu and Martin, J Comp Neurol 279:368, 1989). In the present study, we used a double-labeling paradigm to test that hypothesis. Four groups of pouch young opossums received bilateral or unilateral injections of Fast Blue (FB) into the caudal thoracic or rostral lumbar cord (T12-L2) at different ages in order to label rubrospinal neurons. Three or 4 days later, the rubrospinal tract was transected unilaterally, four to five segments rostral to the injection(s). If the injection was unilateral, the lesion was made ipsilateral to it. The animals were maintained for about 1 month before a second marker, Diamidino Yellow (DY), was injected, usually bilaterally, between the FB injection(s) and the lesion. The animals were maintained for about 5 days before sacrifice and sections through the red nucleus and spinal cord were examined with a fluorescence microscope. During the critical period for plasticity, only a few rubral neurons contralateral to the lesion were labeled by FB alone, supporting our previous contention that most axotomized neurons degenerate. In contrast, many neurons were labeled by DY alone, indicating that their axons were not present in the caudal cord at the time of the FB injection and that they grew around the lesion during the 1 month survival to incorporate DY. A few double-labeled neurons were also found. One interpretation of such neurons is that they survived axotomy, as evidenced by the presence of FB, and supported axons which grew around the lesion to take up DY. Another interpretation is that they supported late growing axons which incorporated residual FB as well as DY. In order to choose between these alternatives, a similar double-labeling paradigm was carried out, but with removal of FB at the time of the lesion. Since a few neurons were still double labeled, we conclude that regeneration of cut axons also contributed to rubrospinal plasticity. Our results support our previous suggestion that developmental plasticity of the rubrospinal tract results primarily from growth of late arriving axons around the lesion, but they also suggest that regeneration of cut axons occurs.     

The differentiation of the olfactory placode in Xenopus laevis: a light and electron microscope study

Corticothalamic projections from postcruciate area 4, located on the rostral part of the posterior sigmoid gyms, were traced with the autora-diographic technique in the dog. Injections of tritiated amino acids were made into the lateral and medial parts of area 4 in regions corresponding to the forelimb and hindlimb areas of the primary motor cortex, respectively. In cases with injections placed in the lateral part of areas, dense accu-mulations of label were present in the lateral part of the ventral anterior nucleus (VA), the central part of the ventral lateral nucleus (VL), the ventral half of the ventral posterior inferior nucleus (VPI), the caudal part of the central lateral nucleus (CL), and the centrum medianum (CM). Lighter label was also present in the lateral part of the cytoarchitectonically distinct VL region bordering the ventrobasal complex (VB), as well as in the ventro-lateral part of the mediodorsal nucleus (MD), and in the lateral posterior nucleus (LP). In one case in which the injection site involved an adjacent part of area 3a, label was also seen ventrally in the medial division of the posterior nuclear group (POm). However, no detectable differences in VL, MD, or intralaminar labeling patterns were noted between this case and the four other cases with injections confined to the lateral part of area 4. In two cases with injections restricted to the medial part of area 4, dense label was present in the lateralmost part of VL, the ventral part of VPI, the caudal part of CL, and CM. Lighter label was also present in the VL region bordering the dorsolateral edge of VB and in LP. An additional case in which the injection also involved the rostral border of area 3a showed a similar pattern cf thalamic labeling. Projections from both the lateral and medial parts of area 4 were also noted in the subthalamic nucleus, zona incerta, and nucleus of Darkschewitsch. These results suggest that Corticothalamic projections from postcruciate area 4 to VL are organized topographically such that projections from the lateral part of area 4 project centrally within VL while those from the medial part of area 4 project more laterally. Both parts of area 4 also project top-ographically to a cytoarchitectonically distinct region of VL located im-mediately adjacent to VB, In contrast, the projections to the intralaminar nuclei do not appear to be topographically organized. The data from cases involving spread of the injection into area 3a suggest that projection pat-terns from area 3a to ventral, intralaminar, and medial thalamic nuclei are similar to those from area 4. However, it appears that at least the lateral part of area 3a also projects to POm.