Neurons of the ventral medulla oblongata that contain both somatostatin and enkephalin immunoreactivities project to nucleus tractus solitarii and spinal cord

Somatostatin 1.1 contributes to the innate exploration of zebrafish larva

Pharmacological experiments indicate that neuropeptides can effectively tune neuronal activity and modulate locomotor output patterns. However, their functions in shaping innate locomotion often remain elusive. For example, somatostatin has been previously shown to induce locomotion when injected in the brain ventricles but to inhibit fictive locomotion when bath-applied in the spinal cord in vitro. Here, we investigated the role of somatostatin in innate locomotion through a genetic approach by knocking out somatostatin 1.1 (sst1.1) in zebrafish. We automated and carefully analyzed the kinematics of locomotion over a hundred of thousand bouts from hundreds of mutant and control sibling larvae. We found that the deletion of sst1.1 did not impact acousto-vestibular escape responses but led to abnormal exploration. sst1.1 mutant larvae swam over larger distance, at higher speed and performed larger tail bends, indicating that Somatostatin 1.1 inhibits spontaneous locomotion. Altogether our study demonstrates that Somatostatin 1.1 innately contributes to slowing down spontaneous locomotion.

Somatostatin 2 Receptors in the Spinal Cord Tonically Restrain Thermogenic, Cardiac and Other Sympathetic Outflows

The anatomical and functional characterization of somatostatin (SST) and somatostatin receptors (SSTRs) within the spinal cord have been focused in the dorsal horn, specifically in relation to sensory afferent processing. However, SST is also present within the intermediolateral cell column (IML), which contains sympathetic preganglionic neurons (SPN). We investigated the distribution of SSTR2 within the thoracic spinal cord and show that SSTR2A and SSTR2B are expressed in the dorsal horn and on SPN and non-SPN in or near the IML. The effects of activating spinal SSTR and SSTR2 were sympathoinhibition, hypotension, bradycardia, as well as decreases in interscapular brown adipose tissue temperature and expired CO₂, in keeping with the well-described inhibitory effects of activating SSTR receptors. These data indicate that spinal SST can decrease sympathetic, cardiovascular and thermogenic activities. Unexpectedly blockade of SSTR2 revealed that SST tonically mantains sympathetic, cardiovascular and thermogenic functions, as activity in all measured parameters increased. In addition, high doses of two antagonists evoked biphasic responses in sympathetic and cardiovascular outflows where the initial excitatory effects were followed by profound but transient falls in sympathetic nerve activity, heart rate and blood pressure. These latter effects, together with our findings that SSTR2A are expressed on GABAergic, presumed interneurons, are consistent with the idea that SST2R tonically influence a diffuse spinal GABAergic network that regulates the sympathetic cardiovascular outflow. As described here and elsewhere the source of tonically released spinal SST may be of intra- and/or supra-spinal origin.

Hindbrain noradrenergic A2 neurons: diverse roles in autonomic, endocrine, cognitive, and behavioral functions

Central noradrenergic (NA) signaling is broadly implicated in behavioral and physiological processes related to attention, arousal, motivation, learning and memory, and homeostasis. This review focuses on the A2 cell group of NA neurons, located within the hindbrain dorsal vagal complex (DVC). The intra-DVC location of A2 neurons supports their role in vagal sensory-motor reflex arcs and visceral motor outflow. A2 neurons also are reciprocally connected with multiple brain stem, hypothalamic, and limbic forebrain regions. The extra-DVC connections of A2 neurons provide a route through which emotional and cognitive events can modulate visceral motor outflow and also a route through which interoceptive feedback from the body can impact hypothalamic functions as well as emotional and cognitive processing. This review considers some of the hallmark anatomical and chemical features of A2 neurons, followed by presentation of evidence supporting a role for A2 neurons in modulating food intake, affective behavior, behavioral and physiological stress responses, emotional learning, and drug dependence. Increased knowledge about the organization and function of the A2 cell group and the neural circuits in which A2 neurons participate should contribute to a better understanding of how the brain orchestrates adaptive responses to the various threats and opportunities of life and should further reveal the central underpinnings of stress-related physiological and emotional dysregulation.

Effects of somatostatin on the responses of rostrally projecting spinal dorsal horn neurons to noxious stimuli in cats

Somatostatin (SOM) is a widely distributed peptide in the central nervous system and exerts a variety of hormonal and neural actions. Although SOM is assumed to play an important role in spinal nociceptive processing, its exact function remains unclear. In fact, earlier pharmacological studies have provided results that support either a facilitatory or inhibitory role for SOM in nociception. In the current study, the effects of SOM were investigated using anesthetized cats. Specifically, the responses of rostrally projecting spinal dorsal horn neurons (RPSDH neurons) to different kinds of noxious stimuli (i.e., heat, mechanical and cold stimuli) and to the Adelta-and C-fiber activation of the sciatic nerve were studied. Iontophoretically applied SOM suppressed the responses of RPSDH neurons to noxious heat and mechanical stimuli as well as to C-fiber activation. Conversely, it enhanced these responses to noxious cold stimulus and Adelta-fiber activation. In addition, SOM suppressed glutamate-evoked activities of RPSDH neurons. The effects of SOM were blocked by the SOM receptor antagonist cyclo-SOM. These findings suggest that SOM has a dual effect on the activities of RPSDH neurons; that is, facilitation and inhibition, depending on the modality of pain signaled through them and its action site.

Collateral axonal projections from rostral ventromedial medullary nitric oxide synthase containing neurons to brainstem autonomic sites

The magnocellular reticular nucleus and adjacent lateral paragigantocellular nucleus have been shown to contain a large population of nitric oxide synthase (NOS) immunoreactive neurons. However, little is known about the projections of these neurons within the central nervous system. Retrograde tract-tracing techniques combined with immunohistochemistry were used in this study to investigate whether NOS neurons in this rostral ventromedial medullary (RVMM) region send collateral axonal projections to autonomic sites in the nucleus of the solitary tract (NTS) and in the nucleus ambiguus (Amb). Fluorogold and/or rhodamine labeled latex microspheres were microinjected into the NTS and Amb at sites that elicited bardycardia and/or depressor responses (l-glutamate; 0.25 M; 10 nl). After a survival period of 10-14 days, the rats were sacrificed and tissue sections of the brainstem were processed immunohistochemically for the identification of NOS containing neuronal perikarya. After unilateral injection of the tract-tracers into the NTS and Amb, retrogradely labeled neurons were observed bilaterally throughout the RVMM region. Of the number of RVMM neurons retrogradely labeled from the NTS (684+/-143), 9% were found to be immunoreactive to NOS. Similarly, of those RVMM neurons retrogradely labeled from the Amb (963+/-207), 7% also contained NOS immunoreactivity. Neurons with collateral axonal projections to NTS and Amb (14% and 10%, respectively) were observed predominantly within a region of RVMM that extended co-extensively with approximately the rostrocaudal extent of the facial nucleus. Of these double labeled neurons, 36.4+/-20 (39%) were also found to be immunoreactive to NOS. These data indicate that the RVMM contains at least three population of NOS neurons that send axons to innervate functionally similar cardiovascular responsive sites in the NTS and Amb. Although the function of these NOS containing medullary pathways in cardiovascular control is not known, it is likely that those with collateral axonal projections represent the anatomical substrate by which the RVMM may simultaneously coordinate cardiovascular responses during physiological changes associated with respiration and/or motor movements.

Expression and localisation of somatostatin receptor subtypes sst1-sst5 in areas of the rat medulla oblongata involved in autonomic regulation

Somatostatin is known to modulate the activity of neurones of the medulla oblongata involved in autonomic regulation, mediated through five subtypes of G protein-coupled receptors, sst1-sst5. This study utilises reverse transcription polymerase chain reaction and immunohistochemistry to investigate the expression of sst1-sst5, including the sst2(A)/sst2(B) isoforms, in the main autonomic centres of the rat medulla oblongata: nucleus of the solitary tract (NTS), dorsal motor vagal nucleus (DVN) and ventrolateral medulla (VLM). In tissue from the cerebral cortex, hippocampus and cerebellum all subtype mRNAs were detected, but sst5 signals were weak, and the distribution of sst1-sst5 immunoreactivities was consistent with previous reports. In the medulla, all sst mRNAs gave clear amplicons and subtype-specific antibodies produced characteristic patterns of immunolabelling, frequently in areas of somatostatinergic innervation. Anti-sst1 labelled beaded fibres, sst2(A), sst2(B), sst4 and sst5 gave somatodendritic labelling and sst3 labelled presumptive neuronal cilia. In NTS tissue, sst1, sst2(A), sst4 and sst5 mRNAs were strongly expressed, while in VLM tissue sst1, sst2(A), sst2(B) and sst4 predominated. In both areas of the medulla, neurones with intense somatodendritic sst2(A) immunoreactivity were principally catecholaminergic in phenotype, being double labelled for tyrosine hydroxylase (TH) and phenylethanolamine-N-methyl-transferase (PNMT). Some TH/PNMT positive neurones were also sst2(B) and sst4 immunoreactive. Cholinergic parasympathetic neurones in the DVN were immunoreactive for the sst2(A), sst2(B), sst4 and sst5 subtypes. These observations are consistent with the proposal that multiple somatostatin receptor subtypes, possibly combining as heterodimers, are involved in mediating the modulatory effects of somatostatin on autonomic function, including cardiovascular, respiratory and gastrointestinal reflex activity.

GABAB receptors on central terminals of C-afferents mediate intersegmental Adelta-afferent evoked hypoalgesia

The current study tested the hypothesis that repetitive activation of sciatic Adelta-afferents evokes a saphenous C-afferent hypoalgesia mediated by pre-synaptic GABA(B) receptors. Tonic activation of sciatic Adelta-afferents was produced by cutaneous application of dimethyl sulfoxide (DMSO) followed by repetitive thermal activation of Adelta-afferents on the dorsolateral hind paw. The tonic activation of sciatic Adelta-afferents produced hypoalgesia in saphenous C-afferents. Intrathecal administration of the GABA(B) receptor antagonist, saclofen, attenuated saphenous hypoalgesia demonstrating at least partial mediation by central GABA(B) receptors. To determine if this central GABA(B) receptor activation occurs at pre-synaptic primary afferent terminals or postsynaptic spinal cord neurons, the dorsal hind paws of mice were infected with a recombinant herpes simplex virus type 1 (HSV-1) designed to selectively knock down expression of the GABA(B1a) receptor subunit (PAGB1a) in primary afferents or a control virus encoding the E. coli lacZ gene (PZ). Four weeks after infection, GABA(B) receptor immunoreactivity in the superficial dorsal horns ipsilateral to PAGB1a application was reduced and hypoalgesia in saphenous C-afferents was attenuated when compared to PZ-infected mice. These findings indicate an intersegmental, sciatic Adelta-afferent-evoked hypoalgesic effect on saphenous C-afferent responses that is mediated by pre-synaptic GABA(B) receptors on the terminals of those C-afferents.

Substance P (NK1) and somatostatin (sst2A) receptor immunoreactivity in NTS-projecting rat dorsal horn neurones activated by nociceptive afferent input

Spinal neurones that receive inputs from primary afferent fibres and have axons projecting supraspinally to the medulla oblongata may represent a pathway through which nociceptive and non-nociceptive peripheral stimuli are able to modulate cardiorespiratory reflexes. Expression of the neurokinin-1 (NK1) receptor is believed to be an indicator of lamina I cells that receive nociceptive inputs from substance P releasing afferents, and similarly, sst2A receptor expression may be a marker for neurones receiving somatostatinergic inputs. In this study, immunoreactivity for these two receptors was investigated in rat spinal neurones retrogradely labelled by injections of cholera toxin B or Fluorogold into the nucleus of the solitary tract (NTS). In addition, nociceptive activation of these labelled cells was studied by immunodetection of Fos protein in response to cutaneous and visceral noxious chemical stimuli. NK1 and sst2A receptors in lamina I were localised to mainly separate populations of retrogradely labelled cells with fusiform, flattened and pyramidal morphologies. Examples of projection neurones expressing both receptors were, however observed. With visceral stimulation, many retrogradely labelled cells expressing c-fos were immunoreactive for the NK1 receptor, and a smaller population was sst2A positive. In contrast, with cutaneous stimulation, only NK1 positive retrogradely labelled cells showed c-fos expression. These data provide evidence that lamina I neurones receiving noxious cutaneous and visceral stimuli via NK1 receptor activation project to NTS and so may be involved in coordinating nociceptive and cardiorespiratory responses. Moreover, a subpopulation of projection neurones that respond to visceral stimuli may receive somatostatinergic inputs of peripheral, local or supraspinal origins.

Medullary and spinal cord projections from cardiovascular responsive sites in the rostral ventromedial medulla

The rostral ventromedial medulla (RVMM) is a sympathoexcitatory area. However, little is known about its efferent projections. In this study, biotinylated dextran amine (BDA) or Phaseolus vulgaris leucoagglutinin (PHA-L) were used to investigate the medullary and spinal cord projections from pressor sites in RVMM. Initially, RVMM was systematically explored in urethane-anesthetized rats using microinjection of L-glutamate for sites that elicited increases in arterial pressure. A pressor area was identified that included the rostral magnocellular reticular and rostral lateral paragigantocellular reticular nuclei. In the second series of experiments, BDA or PHA-L was iontophoretically injected into RVMM pressor sites. Anterograde labeling was observed throughout the brainstem and spinal cord, bilaterally, but with an ipsilateral predominance. Dense labeling was observed within the nucleus of the solitary tract (NTS); the greatest density of labeling was observed in the caudal dorsolateral, medial, and ventrolateral subnuclei. Additionally, light to moderately dense labeling was found within the nucleus substantia gelatinosus and commissural nucleus. In the nucleus ambiguus/ventrolateral medullary (Amb/VLM) region, the density of labeling was greatest in caudal regions. Within Amb, most of the labeling was localized to its external formation. Anterograde labeling was also found throughout the spinal cord. In the thoracolumbar segments, dense axonal labeling was observed within the dorsolateral funiculus. These labeled axons innervated the intermediolateral nucleus and the central autonomic area. Taken together, these data suggest that RVMM neurons elicit increases in sympathetic activity by likely providing a direct excitatory input to spinal sympathetic preganglionic neurons, and by a direct inhibitory input to medullary cardioinhibitory and depressor areas.

Opioid circuits originating from the nucleus paragigantocellularis and their potential role in opiate withdrawal

Neurons in the rat nucleus paragigantocellularis (PGi), located in the ventrolateral medulla, send collateral projections to the locus coeruleus (LC) and to the nucleus of the solitary tract (NTS). The present study examined whether neurons in the PGi that project to both the LC and NTS contain leucine(5)-enkephalin (ENK), and also whether opioid-containing neurons in the PGi are activated following withdrawal from opiates. Retrograde transport of Fluoro-Gold (FG) from the LC and transport of a protein-gold tracer from the NTS was combined with detection of an antibody directed against ENK in the PGi. Using fluorescence and brightfield microscopy, it was established that more than half of the neurons containing both FG and the protein-gold tracer, also exhibited immunolabeling for ENK. The most frequent location of triply labeled neurons was the retrofacial portion of the PGi. In a separate series, rats were chronically implanted with morphine or placebo pellets and, on the fifth day, were subjected to an intraperitoneal injection of naltrexone. Two hours following initiation of withdrawal, rat brains were obtained and processed for detection of c-fos and in situ hybridization labeling of preproenkephalin (PPE) mRNA. Naltrexone injections into morphine-dependent rats caused a dramatic increase in c-fos as compared to control rats. Approximately 66% of the c-fos-labeled neurons exhibited labeling for PPE mRNA. These were also enriched in the retrofacial portion of the PGi. Taken together, the present data indicate that withdrawal from opiates engages opioid neurons in the PGi, some of which may coordinate activity of neurons in both the NTS and the LC.

Projection sites of superficial and deep spinal dorsal horn cells in the nucleus tractus solitarii of the rat

By using anterograde transport of biotin dextran amine injected into the cervical spinal dorsal horn, we have shown that fibres from superficial and deep dorsal horn project to the nucleus tractus solitarii via two distinct pathways. Afferent fibres from the superficial lamina (I-III) were found to course in the dorsal funiculus and terminate bilaterally in the caudal zone of the nucleus tractus solitarii (NTS), mainly within the commissural subnucleus. In contrast, afferents from the deeper dorsal horn laminae (IV-V) were found to course in the dorsolateral fasciculus and terminate ipsilaterally, mostly in the lateral areas of the caudal nucleus tractus solitarii. Similar, but more extensive patterns of labelled fibres were produced by injections into the white matter of the dorsal funiculus and dorsolateral fasciculus, respectively. These observations suggest that the caudal NTS not only serves as a location of visceral afferent convergence and integration, but may also be a receptive area for monosynaptic projections from dorsal horn neurons receiving sensory afferent inputs. Such projections may represent pathways through which NTS neurons are influenced by nociceptive and non-nociceptive information from the dorsal horn and thereby can co-ordinate the appropriate autonomic response, including adjustments in cardiorespiratory reflex output.

Interrelations of opioids with monoamines in descending inhibition of nociceptive transmission at the spinal level: an immunocytochemical study

This study was designed to reexamine a previous proposal of whether the opioid-like substances (OLS) being acting mainly as an intrinsic spinal mediator in the descending inhibition of nociception of the bulbospinally projecting NE-ergic, and/or 5-HT-ergic terminals in the dorsal horn by using an immunocytochemical method. The effects of intrathecal (i.t.) phentolamine (Ph), cyproheptadine (Cyp), and naloxone (Nal), administered separately or coadministered by two of them, on the expression of Fos-like-immunoreactive (FLI) neurons were observed on both sides of the lumbar dorsal horn of rats, in which equal volumes of formalin were injected into two hindpaws and the ipsilateral dorsolateral funiculus (DLF) was transected at the thoracic level antecedently. The results showed: (1) when rats were pretreated with i.t. saline, the number of nociceptive FLI neurons was significantly lowered 44% (p<0.01) on the side of the lumbar dorsal horn with intact DLF compared to the opposite side with sectioned DLF; (2) when rats were separately pretreated with i.t. Ph, Cyp and Nal, the reduction of FLI neurons on the DLF-intact side were decreased by 27% (p<0.01), 21% (p<0.01), and 25% (p<0.01), respectively; (3) when rats were pretreated with combined i.t. Ph+Cyp, the reduction on the intact side was eliminated almost completely (4%); (4) when rats were pretreated with combined i.t. Ph+Nal, the reduction on the intact side was 21% (p<0.01); and (5) when rats were pretreated with i.t. Cyp+Nal, the reduction on the intact side was 9.1%. These results suggest that: (1) nearly all the suppressive action exerted by the DLF-descending fibers are produced by the release of either NE or 5-HT as neurotransmitters at the spinal level; (2) most of the opioid-like substances act as an intrinsic spinal mediator mainly for the descending NE-ergic, but in a lesser extent for the 5-HT-ergic terminals in the dorsal horn circuitry; and (3) some OLS-ergic interneurons may only be activated by local nociceptive input.

A quantitative study of neurons which express neurokinin-1 or somatostatin sst2a receptor in rat spinal dorsal horn

The neurokinin-1 and somatostatin sst2a receptors have both been identified on spinal cord neurons. In this study we have estimated the proportions of neurons in different parts of the spinal cord which express these receptors, by using a monoclonal antibody against a neuronal nuclear protein named NeuN and combining the optical disector method with confocal microscopy. The NeuN antibody was initially tested on over 3200 neurons identified with antisera against a variety of compounds, including neuropeptides, enzymes and receptors, and also on astrocytes and oligodendrocytes. All of the neurons, but none of the glial cells that were examined possessed NeuN-immunoreactivity, which suggests that NeuN is a reliable marker for all spinal cord neurons. We found that approximately 45% of neurons in lamina I, 23-29% of those in laminae IV-VI and 18% in lamina X possessed the neurokinin-1 receptor, while the receptor was present on a smaller proportion of neurons in laminae II and III (6% and 11%, respectively). Thirteen percent of lamina I neurons and 15% of those in lamina II expressed the sst2a receptor. To provide further information about the types of neuron which possess the sst2a receptor, we searched for possible co-existence with the neurokinin-1 receptor as well as with GABA and glycine. sst2a and neurokinin-1 receptors were not co-localized on neurons in laminae I and II. All of the sst2a-immunoreactive neurons examined were also GABA-immunoreactive, and 83.5% were glycine-immunoreactive, indicating that the receptor is located on inhibitory neurons in the superficial dorsal horn. These results demonstrate the proportions of neurons in each region of the spinal cord which can be directly activated by substance P or somatostatin acting through these receptors. Levels of receptors can change in pathological states, and this method could be used to determine whether or not these changes involve alterations in the number of neurons which express receptors. In addition, the method can be used to estimate the sizes of neurochemically-defined populations of spinal cord neurons.

Evidence for the existence of the beta-endorphin-sensitive "epsilon-opioid receptor" in the brain: the mechanisms of epsilon-mediated antinociception

Recently, mu-, delta- and kappa-opioid receptors have been cloned and relatively well-characterized. In addition to three major opioid receptor types, more extensive studies have suggested the possible existence of other opioid receptor types that can be classified as non-mu, non-delta and non-kappa. Based upon anatomical and binding studies in the brain, the sensitive site for an endogenous opioid peptide, beta-endorphin, has been postulated to account for the unique characteristics of the opioid receptor defined as a putative epsilon-opioid receptor. Many epsilon-opioid receptors are functionally coupled to G-proteins. The functional epsilon-opioid receptors in the brain are stimulated by bremazocine and etorphine as well as beta-endorphin, but not by selective mu-, delta- or kappa-opioid receptor agonists. Epsilon-opioid receptor agonists injected into the brain produce profound antinociception. The brain sites most sensitive to epsilon-agonist-induced antinociception are located in the caudal medial medulla such as the nucleus raphe obscures, nucleus raphe pallidus and the adjacent midline reticular formation. The stimulation of epsilon-opioid receptors in the brain facilitates the descending enkephalinergic pathway, which probably originates from the brainstem terminating at the spinal cord. The endogenous opioid Met-enkephalin, released in the spinal cord by activation of supraspinal epsilon-opioid receptors, stimulates spinal delta2-opioid receptors for the production of antinociception. It is noteworthy that the epsilon-opioid receptor-mediated pain control system is different from that of other opioid systems. Although there appears to be no epsilon-selective ligand currently available, these findings provide strong evidence for the existence of the putative epsilon-opioid receptor and its unique function in the brain.

Transient somatostatin-immunoreactive ganglion cells in the developing rat retina

The peptide somatostatin (SRIF) is likely to play important roles in neuronal differentiation and maturation. In the mammalian retina, it is reported to be expressed by populations of amacrine and/or displaced amacrine cells and, in some species, by some ganglion cells. Previous studies have shown that in the rat retina the maturation of somatostatinergic systems encompasses late prenatal and early postnatal periods, suggesting a role of SRIF in maturative events of the retina. SRIF-expressing ganglion cells have not been reported in the rat retina at any developmental age. In the present study, we re-evaluated the postnatal development of SRIF-containing neurons of the rat retina focusing on the analysis of SRIF-containing cells in the ganglion cell layer (GCL), to test the possibility that SRIF is expressed by some ganglion cells during development. To this aim we combined immunocytochemical staining of SRIF-positive neurons with retrograde tracing of ganglion cell bodies through Fluoro-Gold injections into the superior colliculus. Double-labelling experiments revealed the presence of SRIF-containing ganglion cells at postnatal day (PND) 10. They accounted for 14% of the total SRIF-containing cells in the GCL. Such double-labelled cells were not observed either before (PND 7) or after (PND 15 and PND 45) this period. This transient expression of SRIF in retinal ganglion cells suggests that SRIF may be a factor regulating the maturation of retinocollicular projections in a restricted period of postnatal development.

Co-localization of c-Fos and neurotransmitter immunoreactivities in the cat brain stem after carotid sinus nerve stimulation

To reveal neurones in the cat medulla oblongata involved in carotid baroreceptor/chemoreceptor reflexes, the distribution of c-Fos oncoprotein immunoreactivity was studied following electrical stimulation of the right carotid sinus nerve. The neurochemistry of the activated neurones was investigated using antisera to tyrosine hydroxylase, neuropeptide Y, somatostatin, and glutamate. Nitric oxide containing neurones were identified using antiserum to nitric oxide synthase (NOS) and by the histochemical localization of nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase. Following sinus nerve stimulation numerous c-Fos-IR cells were detected both ipsilaterally and contralaterally in the nucleus tractus solitarii, the area postrema and throughout the ventrolateral medulla. Dual labelling studies revealed that 3.3% of c-Fos-immunoreactive cells in the nucleus tractus solitarii were also immunoreactive for tyrosine hydroxylase. The double labelled cells were scattered within the medial and ventrolateral subnuclei, predominantly rostral to obex. A higher proportion (10.3%) of c-Fos-IR cells in the ventrolateral medulla also showed tyrosine hydroxylase immunoreactivity. Caudal to obex, these were scattered in the reticular formation between the spinal trigeminal nucleus and the lateral reticular nucleus, while more rostrally they were found within the lateral reticular nucleus, the nucleus ambiguus and the lateral tegmental field. Cells expressing c-fos and reactive for glutamate, neuropeptide Y or NADPH-diaphorase (or NOS) were only rarely seen, and co-localization of c-Fos and somatostatin immunoreactivities was not seen. These results suggest that of the neurones forming pathways within the medulla activated on carotid sinus nerve stimulation, presumably mediating baro- and chemoreceptor reflexes, relatively few utilize catecholamines, glutamate, neuropeptide Y or nitric oxide as their transmitter substance.

A confocal microscopic survey of serotoninergic axons in the lumbar spinal cord of the rat: co-localization with glutamate decarboxylase and neuropeptides

Patterns of co-localization of serotonin with glutamate decarboxylase (the synthetic enzyme for GABA) or each one of eight neuropeptides (calcitonin gene-related peptide, dynorphin, enkephalin, galanin, neuropeptide Y, neurotensin, substance P and somatostatin) were investigated with dual-colour confocal laser scanning microscopy in the lumbar spinal cords of three adult rats. Four regions of the gray matter were studied (laminae I-II, V, IX and X). The extent of co-localization was estimated by direct assessment of merged pairs of optical sections and by automated image analysis. Co-localization of serotonin and glutamate decarboxylase was found only in a few axons of laminae I-II but was not detected in other laminae. Peptides were not co-localized with serotonin in the superficial dorsal horn but considerable co-localization was found in motor nuclei and sparse co-localization was found in laminae V and X. Galanin and substance P frequently co-existed with serotonin in lamina IX but some co-localization with dynorphin, somatostatin, [Met]enkephalin and neuropeptide Y was also detected. Galanin, substance P and dynorphin were also co-localized with serotonin in a few axons of the deep dorsal horn and in the gray matter around the central canal. Neurotensin and calcitonin gene-related compound did not co-exist with serotonin in any of the laminae investigated. This evidence suggests that different populations of serotoninergic axons project to different regions of the spinal gray matter. Those containing glutamate decarboxylase terminate in the superficial dorsal horn and are likely to be involved in antinociception, whereas those containing peptides terminate principally in motor nuclei and are likely to modulate motor activity.

Cardiovascular effects of Leu-enkephalin in the nucleus tractus solitarius of the rat

Microinjections of Leu-enkephalin into the dorsal vagal complex induced hypotension and bradycardia. Both naloxone, given at a dose conferring selectivity for mu receptors, and the delta antagonist ICI 154,129 prevented the cardiovascular effects of Leu-enkephalin. Naloxone was also found to decrease the gain of the baroreflex. These results suggest that Leu-enkephalin is involved in cardiovascular regulation through activation of delta-, and possibly mu-, opioid receptors in the dorsal vagal complex.

Monosynaptic projections from the medullary gigantocellular reticular formation to sympathetic preganglionic neurons in the thoracic spinal cord

Microinjection of L-glutamate into a restricted area of the medullary gigantocellular reticular formation, the gigantocellular depressor area (GiDA), lowers arterial pressure. Unlike the nuclei tractus solitarii and the caudal ventrolateral medulla, the two principle medullary vasodepressor areas, the GiDA projects directly to the spinal cord and not to the rostral ventrolateral medulla (Aicher et al. [1994] Neuroscience 60:761-779). We investigated whether neurons within GiDA directly innervate autonomic areas of the thoracic spinal cord. Fluoro-Gold injected into the thoracic spinal cord labeled neurons within functionally defined vasodepressor sites in the GiDA in the same animal. To examine the morphology of GiDA efferents to the spinal cord, the anterograde tracer Phaseolus vulgaris-leucoagglutinin was iontophoresed into the GiDA, and efferent processes in the intermediolateral cell column and nucleus intercalatus spinalis were examined by electron microscopy. Labeling was confined to axons and axon terminals (n = 144) that usually contained primarily small clear vesicles, contacted large and small dendrites, and formed symmetric (inhibitory) synapses. To determine whether some of the postsynaptic targets of GiDA efferent terminals in the thoracic spinal cord were sympathoadrenal preganglionic neurons, these neurons were retrogradely labeled from the adrenal gland with Fluoro-Gold in rats that had deposits of the anterograde tracer, biotinylated dextran amine (BDA), in the GiDA. Some BDA-containing terminals formed symmetric synapses with dendrites containing Fluoro-Gold. We conclude that a population of neurons in the GiDA monosynaptically innervates some sympathetic preganglionic neurons. The findings suggest the presence of a novel reticulospinal sympathoinhibitory projection originating in the GiDA.

SMS 201-995-induced stimulation of gastric acid secretion via the dorsal vagal complex and inhibition via the hypothalamus in anaesthetized rats

1. SMS 201-995, a somatostatin analogue which interacts with highest affinities at somatostatin receptor subtypes 5 > 2 > or = 3, was microinjected into selective brain sites and its influence on pentagastrin (10 micrograms kg-1 h-1, i.v.)-stimulated gastric acid secretion was investigated in rats anaesthetized with urethane. Gastric acid secretion was measured by flushing the stomach with saline through a gastric cannula every 10 min. 2. SMS 201-995 microinjected into the dorsal vagal complex (DVC, 7, 15, 30 and 60 ng) dose-dependently increased pentagastrin-stimulated gastric acid secretion. The peak acid response was reached within 20 min and returned to basal level 50 min post-injection. SMA 201-995 (30 ng) microinjected into the surrounding area or the central amygdala did not modify pentagastrin-stimulated acid secretion. 3. SMS 201-995 injected into the lateral ventricle (i.c.v., 100, 200, or 300 ng), paraventricular nucleus (PVN) or lateral hypothalamus (LH) (7.5, 15, or 30 ng) dose-dependently inhibited pentagastrin-stimulated gastric acid secretion. SMS 201-995 (30 ng) microinjected into the area surrounding the PVN or LH did not modify the acid secretion response to pentagastrin. 4. Vagotomy prevented the effects of SMS 201-995 (30 ng) microinjected into the DVC and LH. 5. Spinal cord transection abolished the inhibitory action of SMS 201-995 (30 ng) microinjected into the PVN but not the LH. 6. These results demonstrate that SMS 201-995 acts in the DVC to enhance and in the LH and PVN to inhibit pentagastrin-stimulated gastric acid secretion. The action is mediated through vagal (DVC, LH)or spinal (PVN) pathways. The site specific pattern of acid responses to SMS 201-995 may be linked to the distribution of receptor subtypes at these sites that convey the different biological actions of somatostatin.

Co-localization of neurotransmitter immunoreactivities in putative nitric oxide synthesizing neurones of the cat brain stem

The distribution of nitric oxide producing neurones in the medulla oblongata of the cat was investigated using nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase histochemistry, and nitric oxide synthase (NOS) immunohistochemistry. The pattern of staining obtained with both methods was found to be similar. Strongly diaphorase and NOS reactive neurones were present in the paramedian and lateral tegmental fields, including the regions occupied by the A1/C1 catecholamine cell groups, the nucleus ambiguus and lateral reticular nucleus, and in a number of sensory nuclei including the nucleus of the tractus solitarius and the dorsal column nuclei. The extent of co-localization of NADPH-diaphorase with a number of neuropeptides and neurotransmitters was investigated by combining NADPH-diaphorase histochemistry with immunocytochemistry for neuropeptide Y, somatostatin, glutamate, cholecystokinin and tyrosine hydroxylase. NADPH-diaphorase reaction product was observed in neurones immunoreactive for glutamate and somatostatin. These double-labelled cells were found in the paramedian region, lateral reticular field, the nucleus prepositus hypoglossi and in the rostral nucleus of the tractus solitarius. In the rostral ventrolateral medulla NADPH-diaphorase/somatostatin immunoreactive cells were found in the paragigantocellular nucleus. NADPH-diaphorase/glutamate immunoreactive cells overlapped the nucleus ambiguus, the lateral reticular nucleus and the A1/C1 catecholaminergic cell groups. In addition, a few NADPH-diaphorase/glutamate immunoreactive cells were found in the paraolivary area and gigantocellular tegmental field, in the external cuneate and infratrigeminal nuclei. The functional implications of the co-localization of nitric oxide with these neurotransmitters in areas of the medulla concerned with cardiovascular regulation is discussed.

Influence of intracisternal injection of somatostatin analog receptor subtypes 2, 3 and 5 on gastric acid secretion in conscious rats

Somatostatin (SST) and its analogs injected into the CSF induce different gastric acid response patterns. Five SST receptors have been characterized (SSTR-1 to -5). We studied the influence of selective SSTR-2, -3 and -5 ligands on basal gastric acid secretion after intracisternal (i.c.) injection in conscious rats equipped with chronic gastric and i.c. cannulae. Compared with pre-injection level, the SSTR-3 agonist, BIM-23056 (1 microgram) increased acid secretion by 274 +/- 43% while the SSTR-2 agonist, DC 32-87 (1 microgram) inhibited acid secretion by 50.7 +/- 13.3%. SST-14 (1 microgram), the SSTR-5 agonist, BIM-23052, (0.5-1 microgram), SSTR-3 (0.5 micrograms) and -2 (0.5 microgram) or vehicle injected i.c. did not modify basal acid secretion. These results show that the activation of brainstem SSTR-3 receptors stimulate and SSTR-2 inhibit basal gastric acid secretion in conscious rats with chronic gastric fistulae.

Medullary visceral reflex circuits: local afferents to nucleus tractus solitarii synthesize catecholamines and project to thoracic spinal cord

Visceral feedback circuits in lower brainstem were elucidated with retrograde tracers by mapping neurons that issue local projections to the general visceral afferent division of the nucleus tractus solitarii (NTS) and dorsomotor vagal nucleus (DMX) in adult male rats. In study 1, spinal and intramedullary afferents to the visceral-sensorimotor complex (NTS-X) were traced to contiguous populations of cell bodies arranged in cylindrical segmental organization. NTS-X afferents derive from curvilinear arrays of neurons that parallel the efferent radiations of the solitariotegmental tract. Newly discovered afferents arise from circumscribed cell groups in the dorsal reticular formation and periventricular zone. Another source was traced to a paraambigual cell column in the apex of the rostral ventrolateral reticular nucleus (n.RVL). In study 2, catecholaminergic afferents were initially defined with combined retrograde transport-immunocytochemical methods. Deposits of retrograde tracers into NTS-X transported to neurons containing tyrosine hydroxylase (TH) in the A1, C1, and C3 areas or phenylethanolamine N-methyltransferase (PNMT) in the C1 area of the n.RVL and C3 area. In study 3, it was revealed that NTS-X afferents arise, in part, as collaterals of thoracic reticulospinal neurons. Deposits of the retrograde fluorescent tracer Fluorogold into the upper thoracic cord and rhodamine-labeled microbeads into NTS-X transported to the same neurons within a subambigual locus in n.RVL and parts of nucleus raphe magnus. In study 4, dual retrograde tracer-immunocytochemical analysis demonstrated that catecholamines are synthesized by a subset of neurons in the n.RVL that issue collaterals to the NTS-X and thoracic cord. Double retrogradely labeled TH- or PNMT-immunoreactive cell bodies were restricted to the C1 area within a 450-microns column bordered rostrally by the facial nucleus and ventrally by the medullary subpial surface. We conclude that visceral reflex arcs are reciprocally organized. Targets of NTS projection are also sources of local NTS-X afferent innervation. Catecholaminergic and other local afferents from reticular formation, periventricular, and spinal gray may, via collaterals, simultaneously modulate visceral reflex excitability at the level of NTS and the outflow of autonomic and respiratory motoneurons.

Immunolocalization of putative neurotransmitters innervating autonomic regulating neurons (correction of neurones) of cat ventral medulla

This study investigated possible sites of contact of nerve fibers containing a range of putative neurotransmitter substances onto neurons in the cat ventral medulla oblongata concerned with autonomic, particularly cardiovascular, regulation. The parasympathetic preganglionic neurons of the nucleus ambiguous (correction of ambiguus) were identified by retrograde horseradish peroxidase tracing from the vagus nerve, and the groups of neurons in the A1 and C1 cell areas and the raphe nucleus by catecholamine enzyme or 5-hydroxytryptamine (5-HT) immunohistochemistry, respectively. Immunoreactive (-ir)nerve fibers and terminals in the vicinity if these neurons were visualized by subjecting the sections to a dual-staining technique using a brown peroxidase-diaminobenzidine reaction product and a blue alkaline phosphatase-Fast blue reaction product. By employing monochrome photography with combinations of blue and orange-red filters, it was possible to discriminate neural elements displaying one or the other reaction product, or colocalization of reaction products. The results revealed the presence of calcitonin gene-related peptide (CGRP) and galanin (GAL)-ir in some motoneurons of the nucleus ambiguus, but not in those innervating the heart via the cardiac vagus nerve. The latter group of parasympathetic efferent neurons were found to be densely innervated by fibers immunoreactive for dopamine beta-hydroxylase (DBH, indicating noradrenaline), glycine (GLY), gamma-aminobutyric acid (GABA), 5-HT, enkephalin (ENK), neuropeptide Y (NPY), substance P (SP), and thyrotropin-releasing hormone (TRH), and, to a lesser extent, by other neuropeptide-ir fibers. The catecholamine cells of the rostral C1 and caudal A1 groups showed a broadly similar pattern of innervation, most noticeably by fibers immunoreactive for DBH, GABA, 5-HT, cholecystokinin (CCK), CGRP, ENK, GAL, NPY, and SP. The 5-HT-ir neurons of the raphe nucleus, some also containing SP, TRH, ENK, or corticotropin-releasing factor (CRF)-ir, were most prominently innervated by terminals containing DBH, GABA, CCK, ENK, NPY, TRH, somatostatin (SRIF), and vasoactive intestinal polypeptide (VIP)-ir. Although the proof that these groups of neurons receive functional synaptic contacts from the immunoreactive fibers awaits further ultrastructural studies, the results do suggest that a wide range of putative transmitters may influence the activity of efferent neurons in the cat medulla controlling autonomic functions.

Anatomical characterization of a novel reticulospinal vasodepressor area in the rat medulla oblongata

Microinjection of L-glutamate into a subregion of the gigantocellular nucleus of the rat medulla oblongata significantly lowers arterial pressure. This vasodepressor area, the gigantocellular depressor area, is topographically distinct from other vasoactive areas of the medulla. We sought to determine the efferent projections of the gigantocellular depressor area and compare these to the efferent projections of sympathoexcitatory neurons within the rostral ventrolateral medulla. The anterograde tracer Phaseolus vulgaris-leucoagglutinin was deposited into sites in the gigantocellular depressor area or rostral ventrolateral medulla (pressor area) functionally defined as vasodepressor or vasopressor by microinjections of L-glutamate. Following Phaseolus vulgaris-leucoagglutinin injections into the gigantocellular depressor area, labeled punctuate fibers were seen bilaterally within distinct areas of a number of autonomic regions including the nuclei of the solitary tract, subcoeruleus area, parabrachial complex, the medial medullary reticular formation of the medulla and pons, and laminae 7 and 10 of the thoracic spinal cord. Following deposits into the rostral ventrolateral medulla (pressor area), labeled fibers were seen in many of these same autonomic nuclei; however, efferents from the gigantocellular depressor area to the nucleus of the solitary tract, the parabrachial complex and the reticular formation were medial to rostral ventrolateral medulla (pressor area) efferents to these same areas. These data indicate that neurons within the gigantocellular depressor area and the rostral ventrolateral medulla (pressor area) project to autonomic nuclei throughout the central nervous system and further suggest a heterogeneity of function with regard to autonomic control both within the reticular formation and its efferent targets. In addition, these data support the view that the gigantocellular depressor area may be a novel reticulospinal sympathoinhibitory area.

Neurokinin A coexists with substance P and serotonin in ventral medullary spinally projecting neurons of the rat

The coexistence of neurokinin A (NKA) with substance P (SP) and serotonin (5-HT) in ventral medullary neurons of the parapyramidal region and nucleus raphe pallidus of the rat was studied using multiple immunofluorescence labeling. Nearly all of the NKA-immunoreactive (IR) cells in the parapyramidal region and raphe pallidus were SP-IR nd 5-HT-IR, whereas about 70% of the SP-IR neurons and about 60% of the 5-HT-IR neurons contained NKA-IR. There were no apparent differences in the patterns of coexistence between parapyramidal and raphe pallidus neurons. NKA-IR neurons, which colocalized SP-IR and 5-HT-IR, were studied for projections to the lumbar and thoracic spinal cord by use of retrograde transport of fluorescent tracer. Whereas about 50% of the retrogradely labeled neurons of the parapyramidal region and raphe pallidus contained NKA-IR, nearly all of the NKA-IR neurons projected to the thoracic and lumbar spinal cord. In addition, some NKA-IR neurons in the ventral medulla were retrogradely labeled with tracer from localized injections into the thoracic intermediolateral cell column. In summary, this study demonstrated that NKA-IR is colocalized with SP-IR in bulbospinal serotonergic neurons of the parapyramidal region and raphe pallidus, which are known to regulate sensory, motor, and autonomic activities of the spinal cord.

Nucleus of the solitary tract and dorsal motor nucleus of the vagus nerve of the pigeon: localization of peptide and 5-hydroxytryptamine immunoreactive fibers

The distribution of peptide and serotonin fibers in the nucleus of the solitary tract (NTS) and the dorsal motor nucleus of the vagus nerve (DMNX) in the pigeon (Columba livia) was investigated immunocytochemically. This information was correlated with the viscerotopic organization of the nuclei and with central NTS circuitry to suggest the role of the neurochemical containing fibers in the regulation of organ function. The distribution of fibers containing cholecystokinin (CCK), calcitonin gene-related peptide (CGRP), enkephalin (ENK), neuropeptide Y (NPY), neurotensin (NT), substance P (SP), somatostatin (SS), vasoactive intestinal peptide (VIP), and 5-hydroxytryptamine (5-HT) was determined. Each substance had a distinct distribution within the subnuclei of NTS-DMNX, but certain generalities can be deduced. In the DMNX, fibers immunoreactive for ENK, NT, and SP were found in greatest concentration, while CGRP and 5-HT immunoreactive fibers were the least dense. This suggests that ENK, NT, and SP may have a significant modulatory effect on gastrointestinal functions. In the NTS overall, ENK, NT, SP, and VIP fibers were found in high density, CCK, NPY, SS, and 5-HT fibers were found in moderate density, and CGRP fibers were found in low density. However, some individual NTS subnuclei were found to contain moderate to high concentrations of each of the substances, including CGRP. Fibers containing CCK, ENK, NT, SP, SS, and VIP in the medial dorsal NTS subnuclei may regulate gastroesophageal functions. The caudal part of subnucleus lateralis parasolitarius did not contain most of the substances, which suggests that pulmonary function is not modulated by these neurochemicals. The boundaries of a subnucleus could sometimes be demarcated by a change in density of immunoreactive fibers between adjacent subnuclei. This was particularly evident in NTS subnuclei medialis dorsalis anterior centralis and lateralis parasolitarius, and in DMNX subnucleus posterior dorsalis magnocellularis. The selective distribution of peptide and serotonin immunoreactive fibers in various subnuclei of NTS-DMNX suggests that these substances may be differentially involved in neural circuits that mediate cardiovascular and gastrointestinal functions.

Origin of serotoninergic afferents to the hypoglossal nucleus in the rat

The hypoglossal nucleus contains serotonin and several different serotonin receptors, and serotonin is present in fibers and terminals contacting hypoglossal motoneurons. Serotonin alters the excitability of hypoglossal motoneurons, and may influence hypoglossal motoneuron activity in a variety of physiological processes. Since the hypoglossal nucleus contains no serotoninergic somata, the present study sought to identify the sources of serotoninergic afferents to the hypoglossal nucleus. Fluorogold was injected into the hypoglossal nucleus and serotoninergic immunofluorescence was utilized in a dual-fluorescence technique to identify the sources of serotoninergic afferents to the hypoglossal nucleus. The results demonstrate that most serotoninergic afferents to the hypoglossal nucleus originate from the nuclei raphe pallidus and obscurus, while fewer originate from the nucleus raphe magnus and the parapyramidal region. Other regions of the medial tegmental field and the pons that contain both serotoninergic neurons and neuronal afferents to the hypoglossal nucleus contain no double-labeled neurons.

Intrathecal coadministration of serotonin and morphine differentially modulates the tail-flick reflex of intact and spinal rats

In a previous study, we found that the antinociceptive effect of IT-administered morphine on the tail-flick (TF) reflex of rats was potentiated within 1 day after spinal transection. This suggested that the analgesic effect of spinal morphine in the intact animal was tonically suppressed, presumably by the release of a transmitter(s) from descending supraspinal pathway(s), and that the potency of IT morphine was increased because these inputs were removed by spinalization. Because spinally projecting serotonin [5-hydroxytryptamine (5-HT)] fibers are known to be involved in modulating nociception at this site, the present studies examined the possibility that 5-HT might be the proposed "antiopiate" at the spinal cord. Separate groups of intact and spinal rats were pretested on the TF and then injected IT with either morphine (intact: 0.25-5.0 micrograms, spinal: 0.0312-0.5 microgram) or 5-HT (1-200 micrograms), or combinations of these two agents, in a single solution. All rats were then retested 15 min later and the difference in latency was used to compare the effect of these treatments. The results confirmed that the antinociceptive effect of IT morphine was significantly increased by spinalization, whereas the antinociceptive effect of 5-HT was essentially abolished. In intact rats, morphine-induced analgesia was potentiated by a low (10 micrograms) dose of 5-HT but not by higher doses. However, in the spinal rat morphine-induced antinociception was antagonized by the same (10 micrograms) dose. The data suggest that IT 5-HT promotes antinociception in intact rats but acts pro-nociceptively in spinal rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Organization of presumptive catecholamine-synthesizing neurons in the canine medulla oblongata

Immunocytochemical methods were used to identify cells and processes containing two major catecholamine (CA)-biosynthetic enzymes in areas of the canine medulla implicated in autonomic control. Antisera were employed against tyrosine hydroxylase (TH) and phenylethanolamine N-methyltransferase (PNMT). These enzymes respectively catalyze the conversions of tyrosine to L-DOPA and noradrenaline to adrenaline. Immunocytochemical studies laid the groundwork for subsequent investigations in conscious dog in which we characterized an area of cardiovascular control in the rostral ventrolateral medulla (RVLM). In the anatomical studies, previously unidentified neuronal somata and processes were demonstrated in the canine medulla. Presumptive adrenergic (CI) neurons in the canine RVLM were subjacent to the nucleus ambiguous (NA) and most numerous at a level where the compact and semicompact divisions of NA merged. In contrast to their distribution in rodents, C1 neurons were skewed caudally and did not extend rostrally to the caudal pole of the facial nucleus. C1 neurons were also relatively less concentrated in the RVLM. A large number of C1 neurons extended dorsally into the lateral tegmental field (LTF). Most C1 neurons in the LTF (like those in the A1 area) were aligned with catecholaminergic (TH- and PNMT-ir) processes traversing the intermediate reticular zone. Since the numbers and locations of TH- and PNMT-ir neurons in the C1 area of the RVLM and rostral LTF were virtually identical on adjacent sections, it can be implicitly inferred that the enzymes are co-localized to the same somata and that these neurons are capable of biosynthesizing adrenaline. The C1 and A5 areas were clearly separated by a transitional zone, sparsely populated by TH-ir somata (1-2 cells per section), where the facial nucleus and rostral pole of the NA pars compacta (NAc) occupied the same level. A5 neurons were more abundant and complexly organized than suggested by previous CA-histofluorescence data. In addition, a new parvicellular subgroup was identified and composed of neurons containing TH but not PNMT. In contrast to other species, the A1 cell group was not confined to the VLM. A large number of A1 neurons extended into the caudal LTF and were situated between the nucleus tractus solitarii-motor vagal complex (NTS-X) and caudal VLM (CVLM). In contrast to previous reports, presumptive adrenergic (TH- and PNMT-ir) cell groups were more densely represented in the C2-3 areas of the canine NTS and dorsomedial reticular formation.(ABSTRACT TRUNCATED AT 400 WORDS)

Emerging relationships between cytochemical properties and sensory modality transmission in primary sensory neurons

Primary sensory neurons have been categorized according to a variety of characteristics, including modality responsiveness, somal size, cytology, cytochemistry, and the organization of their central axon collateral arborizations. A major aim in the study of primary afferents has been to determine the relationships between dorsal root ganglia neuronal physiology, anatomy, and chemistry that could provide a basis for a classification scheme more directly relevant to function. Here we briefly review these relationships and examine the utility of specific histochemical and immunohistochemical markers representative of distinct populations of neurons that may transmit particular sensory modalities. In addition, we discuss some of our observations suggesting that one population of dorsal root ganglia neurons contains high levels of cytochrome oxidase, carbonic anhydrase, parvalbumin, and calbindin D28k, while a separate population contains fluoride-resistant acid phosphatase, calcitonin gene-related peptide, and displays immunoreactivity with an antibody that labels the central arborization of a specific class of unmyelinated afferents in the dorsal horn. This may have implications for the combinations of substances contained within neurons with distinct sensory functions.

An atlas of the rat subpostremal nucleus tractus solitarius

The nucleus tractus solitarius (NTS) in the dorsal medulla is the principal visceral sensory relay nucleus in the brain. In the rat, numerous lines of evidence indicate that the caudal NTS at the level of the area postrema serves as a major integrating site for coordinating cardiorespiratory reflexes and viscerobehavioral responses. This region of the caudal NTS not only exhibits high densities of binding sites for an impressive array of transmitters and modulators but microinjections of many of these same neuroactive substances into the rat subpostremal NTS elicit pronounced cardiorespiratory and visceral response patterns. This report provides an abbreviated atlas of the rat subpostremal NTS consisting of a series of transverse, sagittal, and horizontal plates. Photomicrographs, together with their corresponding schematic drawings, are provided for the serial sections generated from each reference plane.

Quantitative analysis of bulbospinal projections from the rostral ventrolateral medulla: contribution of C1-adrenergic and nonadrenergic neurons

The contribution of C1-adrenergic and nonadrenergic neurons to the spinal projection from the rostral ventrolateral medulla (RVLM) and their relative innervation density throughout thoracic spinal segments were examined by combining the Fluorogold (FG) retrograde tracing technique with immunofluorescent labeling for the epinephrine-synthesis enzyme phenylethanolamine N-methyltransferase (PNMT). The results indicate that the RVLM-spinal projection is comprised of both PNMT-positive and PNMT-negative neurons located in the subretrofacial area of the RVLM, approximately 1 to 1.7 mm rostral to obex. The bulbospinal projection from the RVLM is predominantly ipsilateral, and bulbospinal neurons do not appear to be organized within the RVLM in a manner indicating their segmental termination site. Eighty-one percent (+/- 4%, n = 2) of the PNMT-positive cells in the ipsilateral subretrofacial RVLM were retrogradely labeled after unilateral FG injections into multiple thoracic levels of the intermediolateral cell column (IML). Following single level FG injections, the number of retrogradely labeled PNMT-positive neurons in the subretrofacial RVLM decreased with injections in more caudal thoracic segments, indicating a heavier innervation of the upper thoracic IML by C1 neurons. PNMT-negative neurons were the main component of the RVLM-spinal population with 63 +/- 8% (n = 7) of the non-PNMT-containing neurons within the ipsilateral subretrofacial RVLM innervating all thoracic levels of the IML. The results indicate that both C1-adrenergic and nonadrenergic neurons in the RVLM make a substantial contribution to the innervation of the IML.

Multiple putative neuromessenger inputs to the phrenic nucleus in rat

Immunohistochemical reactions for 12 putative neuromessengers combined with retrograde labeling of phrenic motoneurons identified seven neuromessengers (5-hydroxytryptamine, substance P, thyrotropin-releasing hormone, methionine enkephalin, cholecystokinin, galanin, neuropeptide Y) located within terminal varicosities in the phrenic nucleus. The degree of terminal labeling in the phrenic nucleus varied depending on the peptide. Substance P, thyrotropin-releasing hormone and methionine enkephalin were each tested for colocalization with 5-hydroxytryptamine within terminal varicosities in the phrenic nucleus, and the coincidence of double-labeling varied for each peptide. These results indicate that phrenic motoneurons are subject to modulation by many peptide neuromessengers that may alter their responsiveness to primary excitatory and inhibitory inputs.

Co-localization of Met-enkephalin and somatostatin in the spinal cord of the rat

A double-labelling immunofluorescence study of rat spinal dorsal horn was carried out with antisera to Met-enkephalin and somatostatin. Varicosities in laminae I and II were frequently immunoreactive with both antisera, and in addition some neuronal cell bodies in lamina II possessed both types of immunoreactivity. These findings suggest that enkephalin and somatostatin coexist in some axons within the rat superficial dorsal horn and that at least some of these axons are derived from local neurones.

Morphology of identified preganglionic neurons in the dorsal motor nucleus of the vagus

To determine the degree of variation of neuronal morphology both within and between the subnuclei of the dorsal motor nucleus of the vagus (dmnX), structural features of the preganglionic neurons of each of the five primary subnuclei in the rat dmnX were characterized quantitatively. Each of the columnar subnuclei was separately labeled by application of the retrograde tracer fast blue to its corresponding subdiaphragmatic vagal branch. Fixed brain slices of 100 microns thickness were then prepared in coronal, sagittal, and horizontal orientations. Next, randomly selected fast blue labeled neurons (n = 1,256) were injected with Lucifer yellow, drawn with camera lucida, and digitized. For each cell, three features of the perikaryon and twelve of the dendritic tree were measured. Dorsal motor nucleus neurons with up to eight primary dendrites, 30 dendritic segments, and seventh order dendritic branches were observed. Throughout the dmnX, the dendrites of preganglionic neurons were preferentially oriented in the horizontal plane. Consistent with an organizing role for the columnar subnuclei, most dendrites remained within their column of origin. However, between 5 and 30% of the neurons in each of the columns projected dendrites into adjacent dmnX subnuclei or other brainstem nuclei, including the nucleus of the solitary tract (NTS). The cyto- and dendroarchitectural analyses revealed systematic gradations in morphology, although they did not support the idea that the dmnX was composed of multiple distinct preganglionic types. The most parsimonious interpretation of the data is that dmnX motorneurons are variants of a single prototype, with dendrites varying widely in length and degree of ramification. The extent of an individual preganglionic neuron's dendritic field was predicted by three factors: the cell's rostrocaudal position within the dmnX, its location within a transverse plane (i.e., its coronal position within or ectopic to the dmnX), and its subnucleus of origin. Neurons at rostral and midlongitudinal levels of each column had more extensive dendritic arbors than those at caudal levels. Ectopic neurons had more extensive dendritic fields than similar cells in the corresponding columns; in fact, of all vagal preganglionic neurons, ectopics had the most extensive dendritic fields. Somata and dendrites of celiac column neurons were more extensive than those of hepatic and gastric column cells. These differential regional distributions of vagal preganglionics suggest that their structure and function are correlated.

Effects of intrathecal monoamine antagonists and naloxone on the descending inhibition of the spinal transmission of noxious input in rats: study with a new experimental model

An electrophysiological model has been developed to explore the transmitters and their relationships in the descending control of spinal transmission of noxious inputs. Nociceptive discharges were recorded extracellularly in parafascicular (Pf) neurons, and the caudal stump of longitudinally isolated dorsal half of the lower thoracic spinal cord was stimulated to simulate the descending volleys coming from the supraspinal structures. Nociceptive discharges in 34 Pf cells were markedly suppressed (83.2 +/- 13.9%) by the preceding spinal stimulation. Phentolamine, methysergide and naloxone were separately administered with random sequence in each of 25 cells by an intrathecal route to observe if the descending inhibition could be blocked by these drugs. The results suggested that in the dorsal spinal cord there exist at least two neurochemically different descending inhibitory fiber systems which comprise either the long descending adrenergic or serotonergic fibers and, most of the adrenergic fibers are succeeded by a propriospinal opioidergic neuron while a few of the serotonergic fibers have such a succession.

Central effects of tachykinin peptide on tracheal secretion

Tachykinin peptides acting on structures located on the ventral surface of the medulla can increase cholinergic outflow to the tracheal smooth muscles and augment respiratory motor output. In the experiments reported here (performed in anesthetized, paralyzed and artificially ventilated dogs), we examined the effects of tachykinin peptides substance P on secretion from submucosal glands. Changes in secretion were measured in an exposed section of tantalum-coated tracheal epithelium. Substances P was administered intracisternally or applied topically on the intermediate area of the ventral surface of medulla (VMS). Intracisternal infusion and the local medullary administration of tachykinin peptide caused a significant increase in tracheal submucosal gland secretion. Atropine given intravenously prevented the secretory changes induced by central action of tachykinins. In addition, prior application of 2% lidocaine to the medullary surface blocked the responses caused by substance P locally applied on the VMS. These findings suggest that substances P acting centrally can tracheal fluid secretion mainly via cholinergic mechanisms, and that the ventral surface of the medulla is one of the site of these action.

Distribution and hypothalamic projection of tyrosine-hydroxylase containing neurons of the nucleus of the solitary tract in the pigeon

The avian nucleus of the solitary tract has an extensive subnuclear organization. Several subnuclear cell groups can be distinguished on the basis of cytoarchitectonic criteria. In general, the subnuclei of the medial division of the nucleus of the solitary tract receive gastrointestinal afferents, whereas the subnuclei of the lateral division of the nucleus of the solitary tract receive cardiopulmonary afferents. Forebrain afferents to the nucleus of the solitary tract are segregated to medial and lateral subnuclei, which are located at the periphery of the nucleus. These peripheral subnuclei of the nucleus of the solitary tract are also the source of ascending axonal projections to the forebrain. In this study, the tyrosine hydroxylase (initial enzyme for catecholamine synthesis) content of the anteromedial hypothalamic projecting neurons of the nucleus of the solitary tract is determined by use of a combined retrograde fluorescent dye-immunofluorescence method. Fast Blue implanted into the anteromedial hypothalamus (in the region of the nucleus periventricularis magnocellularis) resulted in the retrograde labeling of neurons in the caudal two-thirds of the nucleus of the solitary tract. At levels rostral to the obex, dye-labeled cells were mostly observed in the dorsally located subnuclei medialis superficialis pars posterior and lateralis dorsalis pars posterior and in the ventrally located subnucleus medialis ventralis pars posterior. More centrally located subnuclei contained few labeled cells, if any. For example, subnucleus medialis intermedius pars posterior only had a few retrogradely labeled cells, whereas the centrally located subnucleus medialis dorsalis pars posterior was almost devoid of labeled cells. At levels caudal to the obex, many retrogradely labeled neurons of the nucleus of the solitary tract were observed. Neurons immunoreactively labeled for tyrosine hydroxylase were mostly found within subnuclei, which contain anteromedial hypothalamic projection neurons. In subnuclei medialis superficialis pars posterior and lateralis dorsalis pars posterior, 87% of the retrogradely dye-labeled cells were also immunoreactively labeled, whereas in the caudal nucleus of the solitary tract (at levels caudal to the obex), 68% of the retrogradely labeled cells were immunoreactively labeled. Not all tyrosine hydroxylase containing cells had projections to the implantation site in the anteromedial hypothalamus since only 40% of the immunoreactive cells in the caudal nucleus of the solitary tract and 59% of the immunoreactive cells in the subnucleus medialis superficialis pars posterior were retrogradely labeled with Fast Blue.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence of a Monosynaptic Pathway Between Cells of the Ventromedial Medulla and the Motoneuron Pool of the Thoracic Spinal Cord in Rat: Electron Microscopic Analysis of Synaptic Contacts

Previous electrophysiological and anatomical data have suggested the existence of a descending pathway from the ventromedial medulla into the thoracic motoneuron pool. However, systematic light and electron microscopic analysis have not yet been done to reveal such a projection. In the present study, the anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHA-L) was injected into several discrete regions of the medioventral medulla and descending PHA-L-labelled axons were investigated in the thoracic ventral horn using both light and electron microscopy. Light microscopic analysis of descending projections from 20 distinct areas of the medioventral medulla showed that neurons that project predominantly to the intermediate and ventral regions of the thoracic spinal grey matter are located caudal to the facial nucleus. Monosynaptic contacts were found between axons originating from five distinct regions of the medioventral medulla (containing raphé and/or gigantocellular reticular neurons) and cells in the thoracic motoneuron pool. PHA-L-labelled boutons formed synaptic contacts with large calibre dendrites and with somata. Seventy-two per cent of the investigated 32 boutons appeared to have symmetrical synaptic membrane specializations. The majority of the boutons contained only small, pleomorphic vesicles. Our findings show the existence of a direct monosynaptic pathway between the neurons of the ventromedial medulla and thoracic motor nuclei, providing anatomical support for previous physiological data.

Thyrotropin-releasing hormone-immunoreactive projections to the dorsal motor nucleus and the nucleus of the solitary tract of the rat

Thyrotropin-releasing hormone-immunoreactive nerve terminals heavily innervate the dorsal motor nucleus and nucleus of the solitary tract, whereas cell bodies containing thyrotropin-releasing hormone residue most densely in the hypothalamus and raphe nuclei. By using double-labeling techniques accomplished by retrograde transport of Fluoro-Gold following microinjection into the dorsal motor nucleus/nucleus of the solitary tract combined with immunohistochemistry for thyrotropin-releasing hormone, it was demonstrated that thyrotropin-releasing hormone-immunoreactive neurons projecting to the dorsal motor nucleus/nucleus of the solitary tract reside in the nucleus raphe pallidus, nucleus raphe obscurus, and the parapyramidal region of the ventral medulla, but not in the paraventricular nucleus of the hypothalamus. The parapyramidal region includes an area along the ventral surface of the caudal medulla, lateral to the pyramidal tract and inferior olivary nucleus and ventromedial to the lateral reticular nucleus. Varying the position of the Fluoro-Gold injection site revealed a rostral to caudal topographic organization of these raphe and parapyramidal projections.

[The nucleus tractus solitarius: neuroanatomic, neurochemical and functional aspects]

The nucleus tractus solitarii (NTS) has long been considered as the first central relay for gustatory and visceral afferent informations only. However, data obtained during the past ten years, with neuroanatomical, biochemical and electrophysiological techniques, clearly demonstrate that the NTS is a structure with a high degree of complexity, which plays, at the medullary level, a key role in several integrative processes. The NTS, located in the dorsomedial medulla, is a structure of small size containing a limited number of neurons scattered in a more or less dense fibrillar plexus. The distribution and the organization of both the cells and the fibrillar network are not homogeneous within the nucleus and the NTS has been divided cytoarchitectonically into various subnuclei, which are partly correlated with the areas of projection of peripheral afferent endings. At the ultrastructural level, the NTS shows several complex synaptic arrangements in form of glomeruli. These arrangements provide morphological substrates for complex mechanisms of intercellular communication within the NTS. The NTS is not only the site of vagal and glossopharyngeal afferent projections, it receives also endings from facial and trigeminal nerves as well as from some renal afferents. Gustatory and somatic afferents from the oropharyngeal region project with a crude somatotopy within the rostral part of the NTS and visceral afferents from cardiovascular, digestive, respiratory and renal systems terminate viscero-topically within its caudal part. Moreover the NTS is extensively connected with several central structures. It projects directly to multiple brain regions by means of short connections to bulbo-ponto-mesencephalic structures (parabrachial nucleus, motor nuclei of several cranial nerves, ventro-lateral reticular formation, raphe nuclei...) and long connections to the spinal cord and diencephalic and telencephalic structures, in particular the hypothalamus and some limbic structures. The NTS is also the recipient of several central afferent inputs. It is worth to note that most of the structures that receive a direct projection from the NTS project back to the nucleus. Direct projections from the cerebral cortex to the NTS have also been identified. These extensive connections indicate that the NTS is a key structure for autonomic and neuroendocrine functions as well as for integration of somatic and autonomic responses in certain behaviors. The NTS contains a great diversity of neuroactive substances. Indeed, most of the substances identified within the central nervous system have also been detected in the NTS and may act, at this level, as classical transmitters and/or neuromodulators.(ABSTRACT TRUNCATED AT 400 WORDS)

Transient expression of somatostatin messenger RNA and peptide in the hypoglossal nucleus of the neonatal rat

The postnatal developmental expression of somatostatin mRNA and peptide in the rat hypoglossal nucleus was analyzed using immunocytochemical and in situ hybridization techniques. Both the neuropeptide and its cognate mRNA were found to be transiently present within a subpopulation of hypoglossal motoneurons during the neonatal period. At the day of birth, a large population of perikarya situated in caudal, ventral regions of the hypoglossal nucleus expressed somatostatin. By postnatal day 7, the number of hypoglossal somata which expressed somatostatin had diminished considerably, and by 2 weeks postnatal, only few such cell bodies were found. By 3-4 weeks postnatal, somatostatin peptide- and mRNA-containing hypoglossal motoneurons were rarely observed, and in the adult, they were never detected, despite the use of colchicine. A double-labeling co-localization technique was used to demonstrate that somatostatin, when present perinatally, always coexisted with calcitonin gene-related peptide in hypoglossal motoneurons. The latter peptide, in contrast to somatostatin, was expressed in large numbers of somata throughout the entire hypoglossal nucleus and persisted within the motoneurons throughout development into adulthood. These results demonstrate that somatostatin is transiently expressed in motoneurons of the caudal, ventral tier of the hypoglossal nucleus in the neonatal rat. The developmental disappearance of somatostatin is most likely not due to cell death; hypoglossal somata continue to express calcitonin gene-related peptide, with which somatostatin coexisted perinatally, a high levels throughout development. Thus, it appears that the regulation of somatostatin expression in hypoglossal neurons occurs at the level of gene transcription or mRNA stability/degradation.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship of Met-enkephalin-like immunoreactivity to vagal afferents and motor dendrites in the nucleus of the solitary tract: a light and electron microscopic dual labeling study

Methionine (Met5)-enkephalin has been implicated in autonomic functions involving vagal reflexes within the nucleus of the solitary tract (NTS). We examined the light and electron microscopic relationships between neurons containing methionine (Met5)-enkephalin-like immunoreactivity (MELI) and vagal afferents and motor dendrites in the rat NTS. A polyclonal antibody raised against Met5-enkephalin and showing maximal cross-reactivity with this peptide was localized by immunoautoradiography. In the same sections, vagal afferents and motor neurons were identified by histochemical detection of anterogradely and retrogradely transported horseradish peroxidase (HRP). By light microscopy, the MELI was detected in perikarya distributed principally in the dorsomedial, intermediate and parasolitary subdivisions of the NTS. These subnuclei as well as medial and commissural divisions of the NTS also showed: (1) aggregates of silver grains thought to overlie terminals containing MELI, and (2) anterogradely transported HRP in varicose processes. Electron microscopic analysis of the dorsomedial NTS at the level of the area postrema established that MELI was detectable in perikarya, dendrites, and axon terminals. Most of the MELI was associated with large dense core vesicles (dcvs). These opioid terminals formed primarily symmetric synapses on proximal and asymmetric synapses on distal dendrites. Analysis of the dendritic targets of terminals containing MELI revealed that 13/222 were in synaptic contact with dendrites also containing MELI. The remainder of the terminals containing MELI either lacked recognized junctions or formed synapses with unlabeled dendrites. In comparison to the terminals containing MELI in the same series of sections, anterogradely labeled vagal terminals extensively formed asymmetric junctions with distal dendrites and spines. Of the observed anterogradely labeled terminals 6/84 formed synapses with dendrites containing MELI and 3/84 with dendrites containing retrogradely transported HRP. The remainder of the junctions were with dendrites lacking detectable immunoautoradiographic or HRP-labeling. The majority of the recognized synapses on labeled dendrites were at more proximal sites possibly reflecting more limited detection of both MELI and retrogradely transported HRP in smaller dendrites. However, the presence of even a few junctions at proximal sites on dendrites where synaptic transmission is known to be more effective suggests a potentially strong modulation of both opioid and vagal motor neurons by visceral afferents in the NTS. In addition to forming synapses on dendrites, both vagal afferents and terminals containing MELI showed frequent synaptic associations with unlabeled terminals, but not with each other. This finding suggests that the previously demonstrated opiate binding sites on vagal afferents is most likely attributed to other endogenous opiates.