Brainstem projections to the rat cuneate nucleus

Functional organization and connectivity of the dorsal column nuclei complex reveals a sensorimotor integration and distribution hub

The dorsal column nuclei complex (DCN-complex) includes the dorsal column nuclei (DCN, referring to the gracile and cuneate nuclei collectively), external cuneate, X, and Z nuclei, and the median accessory nucleus. The DCN are organized by both somatotopy and modality, and have a diverse range of afferent inputs and projection targets. The functional organization and connectivity of the DCN implicate them in a variety of sensorimotor functions, beyond their commonly accepted role in processing and transmitting somatosensory information to the thalamus, yet this is largely underappreciated in the literature. To consolidate insights into their sensorimotor functions, this review examines the morphology, organization, and connectivity of the DCN and their associated nuclei. First, we briefly discuss the receptors, afferent fibers, and pathways involved in conveying tactile and proprioceptive information to the DCN. Next, we review the modality and somatotopic arrangements of the remaining constituents of the DCN-complex. Finally, we examine and discuss the functional implications of the myriad of DCN-complex projection targets throughout the diencephalon, midbrain, and hindbrain, in addition to their modulatory inputs from the cortex. The organization and connectivity of the DCN-complex suggest that these nuclei should be considered a complex integration and distribution hub for sensorimotor information.

Target- and input-dependent organization of AMPA and NMDA receptors in synaptic connections of the cochlear nucleus

We examined the synaptic structure, quantity, and distribution of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)- and N-methyl-D-aspartate (NMDA)-type glutamate receptors (AMPARs and NMDARs, respectively) in rat cochlear nuclei by a highly sensitive freeze-fracture replica labeling technique. Four excitatory synapses formed by two distinct inputs, auditory nerve (AN) and parallel fibers (PF), on different cell types were analyzed. These excitatory synapse types included AN synapses on bushy cells (AN-BC synapses) and fusiform cells (AN-FC synapses) and PF synapses on FC (PF-FC synapses) and cartwheel cell spines (PF-CwC synapses). Immunogold labeling revealed differences in synaptic structure as well as AMPAR and NMDAR number and/or density in both AN and PF synapses, indicating a target-dependent organization. The immunogold receptor labeling also identified differences in the synaptic organization of FCs based on AN or PF connections, indicating an input-dependent organization in FCs. Among the four excitatory synapse types, the AN-BC synapses were the smallest and had the most densely packed intramembrane particles (IMPs), whereas the PF-CwC synapses were the largest and had sparsely packed IMPs. All four synapse types showed positive correlations between the IMP-cluster area and the AMPAR number, indicating a common intrasynapse-type relationship for glutamatergic synapses. Immunogold particles for AMPARs were distributed over the entire area of individual AN synapses; PF synapses often showed synaptic areas devoid of labeling. The gold-labeling for NMDARs occurred in a mosaic fashion, with less positive correlations between the IMP-cluster area and the NMDAR number. Our observations reveal target- and input-dependent features in the structure, number, and organization of AMPARs and NMDARs in AN and PF synapses.

Acoustic overexposure increases the expression of VGLUT-2 mediated projections from the lateral vestibular nucleus to the dorsal cochlear nucleus

The dorsal cochlear nucleus (DCN) is a first relay of the central auditory system as well as a site for integration of multimodal information. Vesicular glutamate transporters VGLUT-1 and VGLUT-2 selectively package glutamate into synaptic vesicles and are found to have different patterns of organization in the DCN. Whereas auditory nerve fibers predominantly co-label with VGLUT-1, somatosensory inputs predominantly co-label with VGLUT-2. Here, we used retrograde and anterograde transport of fluorescent conjugated dextran amine (DA) to demonstrate that the lateral vestibular nucleus (LVN) exhibits ipsilateral projections to both fusiform and deep layers of the rat DCN. Stimulating the LVN induced glutamatergic synaptic currents in fusiform cells and granule cell interneurones. We combined the dextran amine neuronal tracing method with immunohistochemistry and showed that labeled projections from the LVN are co-labeled with VGLUT-2 by contrast to VGLUT-1. Wistar rats were exposed to a loud single tone (15 kHz, 110 dB SPL) for 6 hours. Five days after acoustic overexposure, the level of expression of VGLUT-1 in the DCN was decreased whereas the level of expression of VGLUT-2 in the DCN was increased including terminals originating from the LVN. VGLUT-2 mediated projections from the LVN to the DCN are likely to play a role in the head position in response to sound. Amplification of VGLUT-2 expression after acoustic overexposure could be a compensatory mechanism from vestibular inputs in response to hearing loss and to a decrease of VGLUT-1 expression from auditory nerve fibers.

Cuneate and spinal trigeminal nucleus projections to the cochlear nucleus are differentially associated with vesicular glutamate transporter-2

There are distinct distributions and associations with vesicular glutamate transporters (VGLUTs) for auditory nerve and specific somatosensory projections in the cochlear nucleus (CN). Auditory nerve fibers project primarily to the magnocellular areas of the ventral cochlear nucleus and deepest layer of the dorsal cochlear nucleus and predominantly colabel with VGLUT1; whereas the spinal trigeminal nucleus (Sp5) projections terminate primarily in the granule cell domains (GCD) of CN and predominantly colabel with VGLUT2. Here, we demonstrate that the terminals of another somatosensory pathway, originating in the cuneate nucleus (Cu), also colabel with VGLUT2. Cu projections in cochlear nucleus exhibited a bilateral distribution pattern with ipsilateral dominance, with 30% of these classified as putative mossy fibers (MFs) and 70% as small boutons (SBs). Cu anterograde endings had a more prominent distribution in the GCD than Sp5, with a higher percentage of MF terminals throughout the CN and higher MF/SB ratio in GCD. 56% of Cu endings and only 25% of Sp5 endings colabeled with VGLUT2. In both cases these were mostly MFs with only 43% of Cu SBs and 18% of Sp5 SBs colabeled with VGLUT2. The few Cu and Sp5 terminals that colabeled with VGLUT1 (11% vs. 1%), were evenly distributed between MFs and SBs. The high number of VGLUT2-positive Cu MFs predominantly located in the GCD, may reflect a faster-acting pathway that activates primarily dorsal cochlear nucleus cells via granule cell axons. In contrast, the higher percentage of Sp5-labeled SB terminals and a greater number of projections outside the GCD suggest a slower-acting pathway that activates both dorsal and ventral cochlear nucleus principal cells. Both projections, with their associations to VGLUT2 likely play a role in the enhancement of VGLUT2 after unilateral deafness [Zeng C, Nannapaneni N, Zhou J, Hughes LF, Shore S (2009) J Neurosci 29:4210-4217] that may be associated with tinnitus.

Corticofugal modulation of the tactile response coherence of projecting neurons in the gracilis nucleus

Precise and reproducible spike timing is one of the alternatives of the sensory stimulus encoding. We test coherence (repeatability) of the response patterns elicited in projecting gracile neurons by tactile stimulation and its modulation provoked by electrical stimulation of the corticofugal feedback from the somatosensory (SI) cortex. To gain the temporal structure we adopt the wavelet-based approach for quantification of the functional stimulus-neural response coupling. We show that the spontaneous firing patterns (when they exist) are essentially random. Tactile stimulation of the neuron receptive field strongly increases the spectral power in the stimulus and 5- to 15-Hz frequency bands. However, the functional coupling (coherence) between the sensory stimulus and the neural response exhibits ultraslow oscillation (0.07 Hz). During this oscillation the stimulus coherence can temporarily fall below the statistically significant level, i.e., the functional stimulus-response coupling may be temporarily lost for a single neuron. We further demonstrate that electrical stimulation of the SI cortex increases the stimulus coherence for about 60% of cells. We find no significant correlation between the increment of the firing rate and the stimulus coherence, but we show that there is a positive correlation with the amplitude of the peristimulus time histogram. The latter argues that the observed facilitation of the neural response by the corticofugal pathway, at least in part, may be mediated through an appropriate ordering of the stimulus-evoked firing pattern, and the coherence enhancement is more relevant in gracilis nucleus than an increase of the number of spikes elicited by the tactile stimulus.

Anterior cingulate cortex contributes to the descending facilitatory modulation of pain via dorsal reticular nucleus

Supraspinal centres biphasically modulate spinal nociceptive transmission, including descending inhibition and facilitation. Recent studies have revealed that descending facilitatory modulation is a key mechanism underlying induction and maintenance of neuropathic and inflammatory pain. The anterior cingulate cortex (ACC) is not only involved in the transmission of pain sensation but also plays a role in processing pain-related emotion. The ACC also widely connects with relevant regions of the descending modulation system. Here we used electrophysiological and behavioural techniques to study the possible pathways behind the modulation of spinal nociceptive transmission from the ACC. C-fibre-evoked field potentials in the spinal dorsal horn were produced by electrical stimulation of the sciatic nerve at an intensity high enough to excite C fibres, and paw withdrawal latencies (PWLs) to noxious heating were recorded. The results showed that high-frequency tetanic electrical stimulation of the ACC both unilaterally enhanced the C-fibre-evoked field potentials in the spinal dorsal horn and bilaterally shortened PWLs, indicating a facilitation of spinal nociception. A similar effect was observed after microinjection of N-methyl-d-aspartic acid (NMDA; 10 nm, 1 microL) or homocysteic acid (HCA; 0.1 m, 1 microL) into the ACC. When the dorsal reticular nucleus (DRt) was electrolytically lesioned, ACC-induced facilitation of spinal nociception was blocked. These results imply that: (i) activation of the ACC may facilitate spinal nociception; (ii) NMDA receptors in the ACC may be involved in descending facilitation; and (iii) the DRt plays a crucial role in mediating ACC-induced facilitation of spinal nociception.

The chemo- and somatotopic architecture of the Galago cuneate and gracile nuclei

The pattern of peripheral nerve inputs into the dorsal column nuclei, cuneate and gracile, was investigated in the prosimian Galago garnetti. The major findings were, that there is a greater segregation of the inputs from the fingers/hand within the cuneate compared with input form the toes/foot within the gracile. In both nuclei, cell clusters can be identified as cytochrome oxidase dense blotches, reactive also for the activity-dependent enzyme nitric oxide synthase. In the cuneate, cell clusters were apparent as six main cytochrome oxidase/nitric oxide synthase-reactive ovals arranged in a medial to lateral sequence. In contrast in the gracile, a higher degree of parcellation was noted and several cytochrome oxidase/nitric oxide synthase blotches were distributed along the rostrocaudal axis of the nucleus. This different architecture parallels differences in the organization of the inputs from the hand and from the foot. In the cuneate, cholera toxin B subunit conjugated to horseradish peroxydase labeled terminals from the glabrous and hairy skin of digits d1 to d5 segregated in each of the five most lateral cytochrome oxidase/nitric oxide synthase blotches. Afferents from the thenar, palmar pads and hypothenar overlapped with those from digit 1, digit 2 to digit 4 and digit 5, respectively. Inputs from wrist arm and shoulder were segregated in the most medial blotch. In the gracile, multiple foci of cholera toxin B subunit conjugated to horseradish peroxydase labeled terminals were observed upon injections of single sites in the toes or plantar pads. Although in multiple foci, inputs from different toes segregated from one another as well. Terminals from the plantar pads appeared to converge on the same cytochrome oxidase/nitric oxide synthase blotches targeted by inputs from the toes. In both the cuneate and the gracile, cytochrome oxidase/nitric oxide synthase blotches also presented intense immunoreactivity for GABA, calbindin, parvalbumin, and brain derived neurotrophic factor. Finally, in the cuneate the cell cluster region presented similarities in prosimian galagos and four species of New World monkeys, whereas it appeared more differentiated and complex in the Old Word macaque monkeys. In conclusion, the different pattern of segregation of the inputs from the hand and from the foot can be related to the different metabolic organization of the cuneate and of the gracile, respectively.

Ascending and descending projections of the lateral vestibular nucleus in the frog Rana esculenta

The lectin Phaseolus vulgaris leucoagglutinin was injected into the frog lateral vestibular nucleus (LVN) to study its antero- and retrograde projections. The following new observations were made. 1) In the diencephalon, vestibular efferents innervate the thalamus in a manner similar to that of mammalian species. The projections show a preference for the anterior, central, and ventromedial thalamic nuclei. 2) In the mesencephalon, vestibular fibers terminate in the tegmental nuclei and the nucleus of medial longitudinal fascicle. 3) In the rhombencephalon, commissural and internuclear projections interconnect the vestibular nuclei. Some of the termination areas in the reticular formation can be homologized with the mammalian inferior olive and the nucleus prepositus hypoglossi. Another part of the vestibuloreticular projection may transmit vestibular impulses toward the vegetative centers of the brainstem. A relatively weak projection is detected in the spinal nucleus of the trigeminal nerve, dorsal column nuclei, and nucleus of the solitary tract. 4) In the spinal cord, vestibular terminals are most numerous in the ipsilateral ventral horn and in the triangular area of the dorsal horn. 5) The coincidence of retrogradely labeled cells with vestibular receptive areas suggests reciprocal interconnections between these structures and the LVN. 6) In seven places, the LVN projections overlap the receptive areas of proprioceptive fibers, suggesting a convergence of sensory modalities involved in the sense of balance.

Afferent synaptic contacts on glycine-immunoreactive neurons in the rat cuneate nucleus

This study was aimed to clarify whether the primary afferent terminals (PATs), GABAergic terminals, and glutamatergic terminals made direct synaptic contacts with glycine-IR neurons in the cuneate nucleus of rats. In this connection, injection of the anterograde tracer WGA-HRP into brachial plexus, antiglycine preembedding immunoperoxidase, and anti-GABA, along with antiglutamate postembedding immunogold labeling, were used to identify the PATs, glycine-IR neurons, GABA-IR terminals, and glutamate-IR terminals, respectively. The present results showed that HRP-labeled PATs, immunoperoxidase-labeled glycine-IR terminals, immunogold-labeled GABA-IR, and glutamate-IR terminals made axodendritic synaptic contacts with immunoperoxidase-labeled glycine-IR neurons. The latter three presynaptic elements also formed axosomatic synapses with glycine-IR neurons. Statistical analysis has shown that the minimum diameter of the glycine-IR dendrites postsynaptic to the above-mentioned four presynaptic elements did not differ significantly. In addition, the synaptic ratio of the glutamate-IR terminals on the glycine-IR dendrites was higher than that of GABA-IR terminals. The synaptic ratio of the GABA-IR terminals on glycine-IR dendrite was in turn higher than that of the PATs and glycine-IR terminals. It is suggested that the PATs and glutamate-IR terminals on the glycine-IR neurons may be involved in subsequent postsynaptic inhibition for spatial precision of lateral inhibition. On the other hand, the GABA-IR and glycine-IR terminals which make synaptic contacts with the dendrites of glycine-IR neurons may provide a putative means for disinhibition or facilitation to maintain the baseline neuronal activity in the rat cuneate nucleus. The results of quantitative analysis suggest that glutamate act as the primary excitatory neurotransmitter, while GABA, when compared with glycine, may serve as a more powerful inhibitory neurotransmitter on glycine-IR neurons in the rat cuneate nucleus.

Cuneothalamic relay neurons are postsynaptic to glycine-immunoreactive terminals in the rat cuneate nucleus

This study was aimed to clarify whether the cuneothalamic relay neurons (CTNs) in the rat cuneate nucleus contained glycine or whether the neurons were modulated directly by presynaptic glycine-IR terminals. For this purpose, retrograde transport of wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) and immunoperoxidase labelling for glycine have been used to ascertain if the CTNs in the rat are glycine-immunoreactive (glycine-IR). Our results have shown that the WGA-HRP-labelled CTNs (mean area = 318 +/- 6.5 microm(2)) were not reactive for glycine. Glycine immunoreactivity, however, was localized in some small-sized neurons (mean area = 210 +/- 6.2 microm(2)) and axon terminals associated with the CTNs. The synaptic organization between the glycine-IR terminals and CTNs was further analyzed using anti-glycine postembedding immunogold labelling. By electron microscopy, the immunogold-labelled glycine-IR terminals containing pleomorphic synaptic vesicles formed symmetrical synaptic contacts with the dendrites, dendritic spines, and somata of CTNs. Quantitative estimation showed that the mean ratios of glycine-IR terminals to total terminals associated with the soma, proximal dendrites and distal dendrites of the CTN were 49.5, 45.2, and 45.8%, respectively. The higher incidence of glycine-IR terminals on the soma, however, was not significantly different from that of the proximal and distal dendrites. Notwithstanding the above, this study has shown a large number of glycine-IR terminals making direct synaptic contacts with CTNs, suggesting that glycine is one of the important neurotransmitters involved in postsynaptic inhibition on the cuneothalamic relay neurons to modulate incoming somatosensory information from forelimb areas in the rat.

Control of size and excitability of mechanosensory receptive fields in dorsal column nuclei by homolateral dorsal horn neurons

Both accidental and experimental lesions of the spinal cord suggest that neuronal processes occurring in the spinal cord modify the relay of information through the dorsal column-lemniscal pathway. How such interactions might occur has not been adequately explained. To address this issue, the receptive fields of mechanosensory neurons of the dorsal column nuclei were studied before and after manipulation of the spinal dorsal horn. After either a cervical or lumbar laminectomy and exposure of the dorsal column nuclei in anesthetized cats, the representation of the hindlimb or of the forelimb was defined by multiunit recordings in both the dorsal column nuclei and in the ipsilateral spinal cord. Next, a single cell was isolated in the dorsal column nuclei, and its receptive field carefully defined. Each cell could be activated by light mechanical stimuli from a well-defined cutaneous receptive field. Generally the adequate stimulus was movement of a few hairs or rapid skin indentation. Subsequently a pipette containing either lidocaine or cobalt chloride was lowered into the ipsilateral dorsal horn at the site in the somatosensory representation in the spinal cord corresponding to the receptive field of the neuron isolated in the dorsal column nuclei. Injection of several hundred nanoliters of either lidocaine or cobalt chloride into the dorsal horn produced an enlargement of the receptive field of the neuron being studied in the dorsal column nuclei. The experiment was repeated 16 times, and receptive field enlargements of 147-563% were observed in 15 cases. These data suggest that the dorsal horn exerts a tonic inhibitory control on the mechanosensory signals relayed through the dorsal column-lemniscal pathway. Because published data from other laboratories have shown that receptive field size is controlled by signals arising from the skin, we infer that the control of neuronal excitability, receptive field size and location for lemniscal neurons is determined by tonic afferent activity that is relayed through a synapse in the dorsal horn. This influence of dorsal horn neurons on the relay of mechanosensory information through the lemniscal pathways must modify our traditional views concerning the relative independence of these two systems.

Primary motor cortex influences on the descending and ascending systems

The motor cortex plays a crucial role in the co-ordination of movement and posture. This is possible because the pyramidal tract fibres have access both directly and through collateral branches to structures governing eye, head, neck trunk and limb musculature. Pyramidal tract axons also directly reach the dorsal laminae of the spinal cord and the dorsal column nuclei, thus aiding in the selection of the sensory ascendant transmission. No other neurones in the brain besides pyramidal tract cells have such a wide access to different structures within the central nervous system. The majority of the pyramidal tract fibres that originate in the motor cortex and that send collateral branches to multiple supraspinal structures do not reach the spinal cord. Also, the great majority of the corticospinal neurones that emit multiple intracraneal collateral branches terminate at the cervical spinal cord level. The pyramidal tract fibres directed to the dorsal column nuclei that send collateral branches to supraspinal structures also show a clear tendency to terminate at supraspinal and cervical cord levels. These facts suggest that a substantial co-ordination between descending and ascending pathways might be produced by the same motor cortex axons at both supraspinal and cervical spinal cord sites. This may imply that the motor cortex co-ordination will be mostly directed to motor responses involving eye-neck-forelimb muscle synergies. The review makes special emphasis in the available evidence pointing to the role of the motor cortex in co-ordinating the activities of both descending and ascending pathways related to somatomotor integration and control. The motor cortex may function to co-operatively select a unique motor command by selectively filter sensory information and by co-ordinating the activities of the descending systems related to the control of distal and proximal muscles.

Multiple inputs of GABA-immunoreactive neurons in the cuneate nucleus of the rat

Using anterograde transport of WGA-HRP and the experimental degeneration method for identification of corticocuneate (CCT) and primary afferent (PAT) terminals in conjunction with gamma-amino butyric acid (GABA) and glutamate immunocytochemistry, this study has demonstrated that the GABA-immunoreactive (GABA-IR) neurons in the rat cuneate nucleus were post-synaptic to PATs (some of them being glutamate-IR), GABA-IR and GABA-negative terminals. The HRP-labelled CCTs did not make any synaptic contacts with GABA-IR neurons but with some GABA-negative dendrites. PATs labelled by HRP or showing degenerating features made direct synaptic contacts with the dendrites of GABA-IR neurons. Beside the above GABA-IR boutons also showed axosomatic and axodendritic synapses with the GABA-IR neurons. In 'triple labeling' method for GABA, PAT and glutamate, it was found that the PATs which were usually glutamate-positive were presynaptic to the dendrites of GABA-IR neurons. Furthermore, some glutamate-IR terminals which were of non-PAT's origin also synapsed with the dendrites and somata of GABA-IR neurons. It is concluded from this study that the major inputs of GABA-IR neurons were from glutamate immunopositive PATs and glutamate terminals of non-PATs origin; other GABA-IR terminals either intrinsic or extrinsic also contributed to the afferent sources of GABA-IR neurons. The CCTs contributed very little, if any, to this input. It is suggested that the PATs and glutamate-IR terminals on GABA-IR neurons may be involved in lateral inhibition for increase of spatial precision. The synaptic contacts between GABA-IR boutons and dendrites or somata of GABA-IR neurons may provide a possible means for disinhibition.

Anuran dorsal column nucleus: organization, immunohistochemical characterization, and fiber connections in Rana perezi and Xenopus laevis

As part of a research program on the evolution of somatosensory systems in vertebrates, the dorsal column nucleus (DCN) was studied with (immuno)histochemical and tract-tracing techniques in anurans (the large green frog, Rana perezi, and the clawed toad, Xenopus laevis). The anuran DCN contains some nicotinamide adenine dinucleotide phosphate diaphorase-positive neurons, very little calbindin D-28k, and a distinct parvalbumin-positive cell population. The anuran DCN is innervated by primary and non-primary spinal afferents, by primary afferents from cranial nerves V, VII, IX, and X, by serotonin-immunoreactive fibers, and by peptidergic fibers. Non-primary DCN afferents from the spinal cord appear to arise throughout the spinal cord, but particularly from the ipsilateral dorsal gray. The present study focused on the efferent connections of the DCN, in particular the targets of the medial lemniscus. The medial lemniscus could be traced throughout the brainstem and into the diencephalon. Along its course, the medial lemniscus gives off collaterals to various parts of the reticular formation, to the octavolateral area, and to the granular layer of the cerebellum. At mesencephalic levels, the medial lemniscus innervates the lateral part of the torus semicircularis as well as various tegmental nuclei. A striking difference between the two species studied is that while in R. perezi medial lemniscal fibers do not reach the tectum mesencephali, in X. laevis, intermediate and deep tectal layers are innervated. Beyond the midbrain, both dorsal and ventral thalamic areas are innervated by the medial lemniscus. The present study shows that the anuran "lemniscal pathway" is basically similar to that of amniotes.

Are there separate central nervous system pathways for touch and pain?

Information about bodily events is conveyed by primary sensory fibres to higher brain centres through neurons in the dorsal column nuclei (DCN) and spinal dorsal horn. The DCN route is commonly considered a 'touch pathway', separate from the spinal pain pathway', in part because DCN neurons respond to gentle tactile stimulation of small skin areas. Here we report that DCN neurons can additionally respond to gentle and noxious stimulation of viscera and widespread skin regions. These and other experimental and clinical data suggest that the DCN and spinal routes cooperate, rather than operate separately, to produce the many perceptions of touch and pain, an ensemble view that encourages novel approaches to health care and research.

Colocalization of neuronal nitric oxide synthase with GABA in rat cuneate nucleus

Pre- and post-embedding immunocytochemistry were employed in this electron microscopic investigation of cuneate neurons that are enriched in GABA and in nitric oxide synthase, the enzyme responsible for the synthesis of nitric oxide. GABAergic neurons are local circuit interneurons; 10-20% of them also contain nitric oxide synthase. These are among the smallest GABA-positive perikarya. We describe a network of processes in the rat cuneate nucleus that are immunopositive for nitric oxide synthase. Axon terminals positive for nitric oxide synthase are small and make synapses mainly onto dendrites; they make only occasional axo-axonic contacts. Double-labelling immunocytochemistry verified that the large majority of terminals positive for nitric oxide synthase also contained GABA. However, most GABA-positive profiles were negative for nitric oxide synthase and GABA-positive terminals that are negative for nitric oxide synthase frequently made axo-axonic contacts. These results suggest that nitric oxide synthase is within a specialized subpopulation of interneurons in the cuneate nucleus.

Bombesin-induced changes of sensory transmission in the dorsal column nuclei

Bombesin was intracisternally administered to determine the effects on the neuronal activities of the dorsal column nuclei (DCN) of anesthetized rats. Although averaged afferent somatosensory transmission through 22 DCN neurons did not appear to be altered by bombesin (-1.11 +/- 3.5%), 12 of them were actually augmented (+17.15 +/- 2.7%) and 10 of them were suppressed (-26.15 +/- 4.9%) during 40 min after bombesin (0.01 microgram) administration. Pretreatment of a bombesin antagonist ([Leu13,Psi(CH2NH)Leu14]bombesin, 0.1 microgram) blocked the effects of bombesin. Lower dose (0.001 microgram) of bombesin did not exert any influences. These results suggest that bombesin may heterogeneously influence afferent somatosensory information in the brain stem of the rat.

Differential patterns of local cerebral glucose utilisation associated with rilmenidine- or B-HT 933-induced hypotension

The anti-hypertensive drug, rilmenidine, has activity at both imidazoline-preferring receptors (IPRs) and alpha 2-adrenoceptors. However, available evidence suggests that its hypotensive effect is mediated via central IPRs. In the present study, the neuroanatomical regions involved in mediating the hypotensive response to rilmenidine were investigated using the [14C]2-deoxyglucose in vivo autoradiographic technique to map drug-induced changes in glucose utilisation within the CNS of conscious, spontaneously hypertensive rats (SHR). The cerebral metabolic effects of rilmenidine were compared with those of B-HT 933, a selective, alpha 2-adrenoceptor agonist with no selectivity for the IPR. Rilmenidine (1 mg/kg, s.c.) and B-HT 933 (2 mg/kg, s.c.) both elicited a moderate but significant hypotension (-24 +/- 2 and -18 +/- 5 mmHg, resp.) and bradycardia (-62 +/- 19.5 and -69 +/- 14 beats/min, resp.). [14C]2-deoxyglucose autoradiography, initiated after stabilisation of the drug-induced reduction in blood pressure, revealed significant reductions (P < 0.05) in local cerebral glucose utilisation (LCGU) in the intermediolateral cell column of the spinal cord, area postrema, ventrolateral medulla, nucleus tractus solitarius and cuneate nucleus of rilmenidine-treated rats. Rilmenidine did not significantly alter LCGU in a number of structures containing high densities of alpha 2-adrenoceptors such as nucleus accumbens, locus coeruleus, frontal cortex. No significant changes in glucose use were evident in any of the 26 CNS regions examined following B-HT 933 administration. These results provide evidence for the functional involvement of brainstem cardiovascular control centres in the central hypotensive effects of rilmenidine.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructural and immunocytochemical characterization of primary afferent terminals in the rat cuneate nucleus

The cuneate nucleus is a relay center for somatosensory information by receiving tactile and proprioceptive inputs from primary afferent fibers that ascend in the dorsal funiculus. The morphology, synaptic contacts, and neurochemical content of primary afferent terminals in the cuneate nucleus of rats were investigated by combining anterograde transport of horseradish peroxidase conjugated to wheat-germ agglutinin or to cholera toxin (injected in cervical dorsal root ganglia) with postembedding immunogold labeling for glutamate and GABA. Both tracers gave similar results. Two types of terminals were labeled: type I terminals were irregularly shaped, had a mean area of 4.0 microns 2, synapsed on several dendrites, and were contacted by other terminals, some of which were GABA positive. Type II terminals were dome-shaped, had a mean area of 2.18 microns 2, and made synaptic contact on a single dendrite. All the anterogradely labeled terminals (interpreted as endings of primary afferents) were enriched in glutamate but not in GABA. The finding that identified primary afferent terminals are enriched in glutamate with respect to other tissue profiles strongly suggests a neurotransmitter role for glutamate in this afferent pathway to the rat cuneate nucleus.

Neurotransmitters in subcortical somatosensory pathways

Investigations during recent years indicate that many different neuroactive substances are involved in the transmission and modulation of somesthetic information in the central nervous system. This review surveys recent developments within the field of somatosensory neurotransmission, emphasizing immunocytochemical findings. Increasing evidence indicates a widespread role for glutamate as a fast-acting excitatory neurotransmitter at different levels in somatosensory pathways. Several studies have substantiated a role for glutamate as a neurotransmitter in primary afferent neurons and in corticofugal projections, and also indicate a neurotransmitter role for glutamate in ascending somatosensory pathways. Other substances likely to be involved in somatosensory neurotransmission include the neuropeptides. Many different peptides have been detected in primary afferent neurons with unmyelinated or thinly myelinated axons, and are thus likely to be directly involved in primary afferent neurotransmission. Some neurons giving rise to ascending somatosensory pathways, primarily those with cell bodies in the dorsal horn, are also immunoreactive for peptides. Recent investigations have shown that the expression of neuropeptides, both in primary afferent and ascending tract neurons, may change as a result of various kinds of peripheral manipulation. The occurrence of neurotransmitters in intrinsic neurons and neurons providing modulating inputs to somatosensory relay nuclei (the dorsal horn, the lateral cervical nucleus, the dorsal column nuclei and the ventrobasal thalamus) is also reviewed. Neurotransmitters and modulators in such neurons include acetylcholine, monoamines, GABA, glycine, glutamate, and various neuropeptides.

Serotoninergic innervation of the dorsal column nuclei and its relation to cytoarchitectonic subdivisions: an immunohistochemical study in cats and monkeys (Aotus trivirgatus)

The serotoninergic innervation of the dorsal column nuclei (DCN) was investigated in cats and owl monkeys (Aotus trivirgatus) with immunohistochemical methods. A dense network of serotonin-immunoreactive fibers was present in the reticular regions of DCN in cats, and in the pars triangularis of the cuneate nucleus and the peripheral and caudal regions of the gracile nucleus in owl monkeys. The cat's cluster regions and the monkey's rotund regions were more sparsely innervated. Electron microscopic examination showed that the labeled fibers were thin and unmyelinated. Vesicle-containing, terminal-like structures were small. They were in contact with dendrites, other terminals and cell bodies, but synapses were rare. The results demonstrate that the serotoninergic projection to the DCN in both cats and owl monkeys is heterogeneously distributed in a pattern that is faithfully related to the cytoarchitectonic subdivisions of the DCN. The densely innervated reticular regions in the DCN of cats and the corresponding regions in monkeys are predominantly involved in the processing of sensory information to the cerebellum, either directly, or indirectly through projections to the inferior olive, pontine gray, tectum, pretectum, red nucleus, or zona incerta. Thus, the present findings suggest that the serotoninergic innervation of the DCN is primarily related to the DCN's involvement in motor functions.

Intrinsic and commissural connections of the rat inferior colliculus

This study analyzes the distribution of the intrinsic and commissural fiber plexuses originating in the central nucleus of the inferior colliculus in the rat. The anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) was injected iontophoretically at different places along the tonotopic axis of the central nucleus and visualized immunohistochemically. In coronal sections the terminal fields of axons originating at each injection site are seen to create four well-defined bands across the rostrocaudal extent of the inferior colliculus, two in the ipsilateral and two in the contralateral side. The "ipsilateral main band" extends dorsomedially and ventrolaterally from the injection site, in register with the known isofrequency contours of the central nucleus, spanning this nucleus and extending into the dorsal cortex of the inferior colliculus. The "ipsilateral external band" is located in the external cortex, where it is oriented dorsoventrally, slightly oblique to the pial surface. In caudal sections, the ventral portion of these two bands appear to join. The two bands in the contralateral inferior colliculus occupy a symmetric position to those of the ipsilateral side, forming a mirror-like image. The position of the four bands changes as the position of the injection site is varied along the frequency gradient axis of the central nucleus. After ventromedial (high frequency area) injections, the main band is ventral and medial, and the external band ventral and lateral. After more dorsolateral (lower frequency) injections, the main band is more dorsal and lateral, whereas the external band is more dorsal but more medial. Thus, the change in the position of the external band is separate and opposite to that of the main band. We suggest that the main bands represent isofrequency contours. Since the projection from the central nucleus to the external cortex of the inferior colliculus also appears to be tonotopic, we also propose a tonotopic organization for the external cortex. The main bands overlap the terminal field of the lemniscal fibers in the central nucleus; thus, it is concluded that the intracollicular fibers contribute to the formation of the known fibrodendritic laminae of the central nucleus. A possible role in preservation of frequency information and integration of other different acoustic parameters is proposed for the main bands. The external bands could participate in polysensory integration, and the commissural connections could be involved in hitherto unknown stages of binaural processing of sound. Based on our results, several modifications are proposed for delineating the subdivisions of the inferior colliculus.

Meso-diencephalic regions projecting to spinal cord and dorsal column nuclear complex in the hedgehog-tenrec, Echinops telfairi

The distribution of neurons projecting to the spinal cord and dorsal column nuclear complex was investigated in the mesodiencephalic regions of the lesser hedgehog-tenrec, Echinops telfairi (Insectivora) by using the retrograde flow technique. While only few neurons projected to the dorsal column nuclear complex, numerous cells were found to give rise to spinal projections. Rubro-spinal neurons of various sizes were distributed over the entire rostrocaudal extent of the contra-lateral nucleus; a few neurons were also located ipsilaterally, Unlike that of the opossum, the projection appeared to be somatotopically organised. Interstitio-spinal neurons were differentiated into several subpopulations according to their location and laterality of projection. In the ipsilateral periventricular grey, in addition, there was a distinct population of cells possibly corresponding to the nucleus of Darkschewitsch. The mesencephalic central grey contained relatively few labeled neurons, the great majority of them being mesencephalic trigeminal, ectopic cuneiform or midline cells. Labeled cuneiform and midline cells, on the other hand, were quite numerous, extending both from a level just caudal to the trochlear nucleus to levels far beyond the rostral tip of the somatic oculomotor nucleus. The discrepancy between the poorly differentiated oculomotor nuclei and the apparently well-developed Edinger-Westphal complex is discussed. Hypothalamo-spinal neurons were essentially restricted to dorsal regions: the hypothalamic paraventricular nucleus (PAV), the dorso-medial (DmHy) and dorso-intermediate cell groups as well as the lateral hypothalamic zone. The latter two cell groups were bilaterally labeled, while the labeled neurons in DmHy and PAV were located predominantly ipsilaterally. Labeled neurons in the amygdala, colliculus superior and mesencephalic trigeminal nucleus were only found following cervical injections; all other mentioned areas and the posterior commissure complex projected to, at least, midthoracic level.

Distribution and targets of the cartwheel cell axon in the dorsal cochlear nucleus of the guinea pig

This investigation attempted to determine the mode of distribution and synaptic targets of the cartwheel cell axon in the guinea pig dorsal cochlear nucleus (DCoN). Antiserum against PEP-19, a putative calcium-binding neuropeptide, was employed at the light and electron microscopic levels. We show that in the hind-brain of the guinea pig, cerebellar Purkinje cells and DCoN cartwheel cells are the most densely immunoreactive neurons. The PEP-19 immunoreaction product is localized to all neuronal compartments of these cells. Primary targets of cartwheel cell axons are the DCoN pyramidal cells, the large efferent neurons of layer 2. These neurons receive numerous immunoreactive synaptic boutons on their cell bodies and apical and basal dendritic arbors. A PEP-19-immunoreactive axonal plexus, largely formed by cartwheel cell axons, highlights layer 3, co-extensively with the basal arbors of pyramidal cells. This plexus is oriented predominantly in the transstrial plane of the DCoN, in parallel with the sheet-like basal dendritic arbor of pyramidal neurons and with the isofrequency bands of primary cochlear nerve fibers. PEP-19-positive boutons contain pleomorphic synaptic vesicles and form symmetric synaptic junctions, indicative of inhibitory innervation. In addition, immunoreactive boutons, similar to those synapsing on pyramidal neurons, were observed on the cell bodies and main dendritic trunks of cartwheel neurons, indicating a system of recurrent collaterals. Furthermore, a small number of PEP-19-positive axons of unknown origin reach the caudal rim of the posteroventral cochlear nucleus. Within the territory of distribution of the cartwheel cell axon are the dendrites of at least two other types of DCoN neuron, the vertical cells of Lorente de Nó and the giant cells. These neurons may represent additional targets of the cartwheel cell axon, but this remains to be ascertained with specific methods. Our data demonstrate that the cartwheel neurons modulate the activity of pyramidal neurons and, therefore, play a key role in shaping the output of the DCoN superficial layers.

Precortical filtering and selective attention: an evoked potential analysis

Attention-related changes in the human auditory brainstem response and the short-latency somatosensory evoked potential (SLSEP) were demonstrated in separate experiments. In Experiment 1, ABRs were recorded during conditions in which subjects awaited the presentation of either auditory or somatosensory target stimuli. In Experiment 2, median nerve SLSEPs were recorded in a similar task context. Analyses of ABR and SLSEP component amplitudes and latencies revealed that the auditory nerve (wave II) and rostral brainstem (wave V) components of the ABR, and the cervical component (N12) of the SLSEP, were facilitated when attention was directed toward stimuli that were presented in the same sensory modality and receptive field as the EP-eliciting stimuli. These results suggest that central mechanisms can modulate the transmission of auditory and somatosensory information at an early, precortical stage and that these mechanisms play a role in auditory and somatosensory selective attention.

Substance P-containing projections in the dorsal columns of rats and cats

Light and electron microscopic immunocytochemical methods were used to study the distribution and the morphology of substance P-positive fibers and axon terminals in the dorsal column nuclei of rats and cats, and to determine whether they are part of an ascending input to these nuclei. In rats, substance P-positive fibers and axon terminals are present throughout the rostrocaudal extent of the dorsal column nuclei. In cats, immunostained fibers and terminals are mostly confined to the ventral region of the caudal and middle portions of these nuclei but they are more homogeneously distributed at rostral levels. In both species, substance P-positive neurons are not present in the same nuclear complex. At the electron microscope level, substance P-positive terminals are small- to medium-sized and dome-shaped; they form asymmetric contacts on dendrites and contain many round, agranular vesicles and sparse dense core vesicles. In double-labeling experiments, visualization of substance P-immunoreactivity in the dorsal root ganglia and dorsal horn of the spinal cord was combined with the retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase or of colloidal gold-labeled wheat germ agglutinin conjugated to enzymatically inactive horseradish peroxidase. These experiments show that substance P-positive axon terminals may originate from both small dorsal root ganglion neurons and from spinodorsal column nuclei neurons in lamina IV. Although quantitative evaluation of the contribution of these two pathways to the substance P innervation of the dorsal column nuclei has not been performed and other sources cannot be discarded on the basis of the present evidence, it is proposed that non-primary afferents to the dorsal column nuclei account for most of the substance P-positive fibers and terminals in the dorsal column nuclei. The experiments support previous findings suggesting that nociceptive input may access the dorsal column nuclei and that this may be mediated, though to a very limited extent, directly by way of small dorsal root ganglion neurons.