Coexistence of varying combinations of neuropeptides with 5-hydroxytryptamine in neurons of the raphe pallidus et obscurus projecting to the spinal cord

Embracing diversity in the 5-HT neuronal system

Neurons that synthesize and release 5-hydroxytryptamine (5-HT; serotonin) express a core set of genes that establish and maintain this neurotransmitter phenotype and distinguish these neurons from other brain cells. Beyond a shared 5-HTergic phenotype, these neurons display divergent cellular properties in relation to anatomy, morphology, hodology, electrophysiology and gene expression, including differential expression of molecules supporting co-transmission of additional neurotransmitters. This diversity suggests that functionally heterogeneous subtypes of 5-HT neurons exist, but linking subsets of these neurons to particular functions has been technically challenging. We discuss recent data from molecular genetic, genomic and functional methods that, when coupled with classical findings, yield a reframing of the 5-HT neuronal system as a conglomeration of diverse subsystems with potential to inspire novel, more targeted therapies for clinically distinct 5-HT-related disorders.

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.

Origins and neurochemical complexity of preganglionic neurons supplying the superior cervical ganglion in the domestic pig

The superior cervical ganglion (SCG) is a center of sympathetic innervation of all head and neck organs. SCG sympathetic preganglionic neurons (SPN) were found in the nucleus intermediolateralis pars principalis (IMLpp), the nucleus intermediolateralis pars funicularis (IMLpf), the nucleus intercalatus spinalis (IC), and the nucleus intercalatus spinalis pars paraependymalis (ICpe). Despite its importance, little is known of SCG innervation and chemical coding in the laboratory pig, a model that is physiologically and anatomically representative of humans. Here in our study, we established the distribution and chemical coding of Fast Blue (FB) retrogradely labelled SPN innervating porcine SCG. After unilateral injection of FB retrograde tracer into the left SCG, labeled neurons were found solely on the ipsilateral side with approximately 98 % located in Th₁–Th₃ segments and predominantly distributed in the IMLpp and IMLpf. Neurochemical analysis revealed that approximately 80 % of SPN were positive both to choline acetyltransferase (ChAT) and nitric oxide synthase (NOS) and were surrounded by a plethora of opioidergic and peptiergic nerve terminals. The results of our study provide a detailed description of the porcine preganglionic neuroarchitecture of neurons controlling the SCG, setting the stage for further studies concerning SPN plasticity under experimental/pathological conditions.

Neurodegenerative changes are prevented by Erythropoietin in the pmn model of motoneuron degeneration

Motoneuron diseases are fatal neurodegenerative disorders characterized by a progressive loss of motoneurons, muscle weakness and premature death. The progressive motor neuronopathy (pmn) mutant mouse has been considered a good model for the autosomal recessive childhood form of spinal muscular atrophy (SMA). Here, we investigated the therapeutic potential of Erythropoietin (Epo) on this mutant mouse. Symptomatic or pre-symptomatic treatment with Epo significantly prolongs lifespan by 84.6% or 87.2% respectively. Epo preserves muscle strength and significantly attenuates behavioural motor deficits of mutant pmn mice. Histological and metabolic changes in the spinal cord evaluated by immunohistochemistry, western blot, and high-resolution (1)H-NMR spectroscopy were also greatly prevented by Epo-treatment. Our results illustrate the efficacy of Epo in improving quality of life of mutant pmn mice and open novel therapeutic pathways for motoneuron diseases.

Orexin inputs to caudal raphé neurons involved in thermal, cardiovascular, and gastrointestinal regulation

Orexin-expressing neurons in the lateral hypothalamus with their wide projections throughout the brain are important for the regulation of sleep and wakefulness, ingestive behavior, and the coordination of these behaviors in the environmental context. To further identify downstream effector targets of the orexin system, we examined in detail orexin-A innervation of the caudal raphe nuclei in the medulla, known to harbor sympathetic preganglionic motor neurons involved in thermal, cardiovascular, and gastrointestinal regulation. All three components of the caudal raphe nuclei, raphe pallidus, raphe obscurus, and parapyramidal nucleus, are innervated by orexin-A-immunoreactive fibers. Using confocal microscopy, we demonstrate close anatomical appositions between varicose orexin-A immunoreactive axon profiles and sympathetic premotor neurons identified with either a transneuronal retrograde pseudorabies virus tracer injected into the interscapular brown fat pads, or with in situ hybridization of pro-TRH mRNA. Furthermore, orexin-A injected into the fourth ventricle induced c-Fos expression in the raphe pallidus and parapyramidal nucleus. These findings suggest that orexin neurons in the hypothalamus can modulate brown fat thermogenesis, cardiovascular, and gastrointestinal functions by acting directly on neurons in the caudal raphe nuclei, and support the idea that orexin's simultaneous stimulation of food intake and sympathetic activity might have evolved as a mechanism to stay alert while foraging.

Peptidergic neuromodulation of the lumbar locomotor network in the neonatal rat spinal cord

It is now well established that a dynamic balance of neurotransmitters and neuromodulators finely influence the output of neuronal networks and subsequent behaviors. In the present study, to further understand the modulatory processes that control locomotor behavior, we investigated the action of 11 neuropeptides, chosen among the various peptide subfamilies, on the lumbar neuronal network in the in vitro neonatal rat spinal cord preparation. Peptides were bath-applied alone, in combination with N-methyl-D,L-aspartate (NMA) or with the classical 'locomotor cocktail' of NMA and serotonin. Using these different experimental paradigms, we show that each peptide can neuromodulate the lumbar locomotor network and that peptides exhibit different neuromodulatory profiles and potencies even within the same family. Only vasopressin, oxytocin, bombesin and thyrotropin releasing hormone triggered tonic or non-organized rhythmic activities when bath-applied alone. All the neuropeptides modulated NMA induced activity and/ or ongoing sequences of fictive locomotion to varying degrees. These results suggest that neuropeptides play an important role in the control of the neural network for locomotion in the neonatal rat. Their various profiles of action may account in part for the great flexibility of motor behaviors.

Somatostatin inhibition of fictive respiration is modulated by pH

We studied the respiratory effects of the tetradecapeptide somatostatin (SST) upon fictive respiration using the in vitro brain stem spinal cord preparation from new-born mouse. We found that SST inhibits respiration, an effect that was potentiated when the chemical drive to respiration was increased. SST inhibited fictive respiration decreasing both the frequency and amplitude in a dose-dependent way. SST inhibition was not antagonized by cyclosomatostatin (cyclo [7-aminoheptanoyl-Phe-D-Trp-Lys-Thr(Bzl)]), a putative SST antagonist, which in contrast behaved as a partial agonist. When the chemical drive to respiration was increased, by lowering the pH of the brain stem superfusion medium from 7.4 to 7.3, the inhibitory effect of SST on respiratory frequency was potentiated. These results suggest an interaction between SST and respiratory central chemoreception in new-born mouse.

A mapping study of cardiorespiratory responses to chemical stimulation of the midline medulla oblongata in ventilated and freely breathing rats

The aim of this study was to examine the cardiorespiratory effects of chemically stimulating neurons in the midline medulla oblongata (MM) of artificially ventilated and freely breathing anesthetized rats. Earlier studies reported that stimulation of the MM elicits increases or decreases in mean arterial pressure (MAP) and phrenic nerve activity, depending on the mode and site of stimulation, anesthetic, and species. In the first series of experiments, rats were anesthetized with urethane, artificially ventilated, paralyzed, and bilaterally vagotomized. The rostrocaudal extent of the MM was mapped by microinjections of DL-homocysteic acid or L-glutamate (both 100 mM, 100 nl), and, in line with previous studies, most injections produced only small responses in MAP, heart rate, and splanchnic sympathetic nerve activity. Increases in respiratory parameters were evoked in caudal regions. However, activation of a discrete region of the MM at the level of the caudal pole of the facial nucleus (CP7) consistently caused a dramatic reduction in phrenic nerve amplitude and/or frequency and, in six rats, produced a prolonged apnea. The second series of experiments was carried out on freely breathing pentobarbitone sodium-anesthetized rats, with a diaphragmatic electromyogram used to monitor respiratory activity. Respiratory activity could again be abolished at CP7 after microinjections of glutamate (100 mM, 50 nl); however, these responses were accompanied by large decreases in MAP and moderate reductions in heart rate. This depression of respiratory activity may be due to activation of propriobulbar inhibitory neurons that project to known respiratory centers in the brain stem.

Growth and neurotrophic factors regulating development and maintenance of sympathetic preganglionic neurons

The functional anatomy of sympathetic preganglionic neurons is described at molecular, cellular, and system levels. Preganglionic sympathetic neurons located in the intermediolateral column of the spinal cord connect the central nervous system with peripheral sympathetic ganglia and chromaffin cells inside and outside the adrenal gland. Current knowledge is reviewed of the development of these neurons, which share their origin with progenitor cells, giving rise to somatic motoneurons in the ventral horn. Their connectivities, transmitters involved, and growth factor receptors are described. Finally, we review the distribution and functions of trophic molecules that may have relevance for development and maintenance of preganglionic sympathetic neurons.

Effects of electroconvulsive stimuli and MK-801 on neuropeptide Y, neurokinin A, and calcitonin gene-related peptide in rat brain

Rats were pretreated with 0.9% NaCl, or 0.1 or 1.0 mg/kg MK-801, an anticonvulsant and a psychotomimetic drug, and 60 minutes later given ECS or sham ECS. After six sessions the animals were sacrificed and neuropeptide Y (NPY-), neurokinin A (NKA-), and calcitonin gene-related peptide (CGRP-) like immunoreactivity (-LI) measured with radioimmunoassays. ECS increased NPY-LI in frontal cortex, striatum, occipital cortex and hippocampus, and NKA-LI in occipital cortex and hippocampus. MK-801 increased CGRP in a dose-response manner in frontal cortex, and NKA-LI in occipital cortex. Although the higher MK-801 dose reduced seizure duration by 50%, the ECS induced NPY-LI increase in striatum, occipital cortex and hippocampus, and NKA-LI in occipital cortex was not diminished. In contrast, there was a parallel decrease in seizures and NPY-LI and NKA-LI changes in frontal cortex and hippocampus, respectively. Investigation of neuropeptides in brain may contribute to understanding of the mechanisms of action of antidepressive and antipsychotic treatments and of psychotomimetic drugs.

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.

Modulation by 5-hydroxytryptamine of the somatostatin receptor-effector system and somatostatin levels in rat brain

The role of 5-hydroxytryptamine (5-HT) in the acute regulation of the rat brain somatostatin (SS) receptor-effector system and somatostatin-like immunoreactivity (SSLI) content was examined. 5-HT administered i.c.v. in a volume of 10 microliters at a dose of 0.5 microgram (pH 3.4) increased the SSLI concentration at 60 min in the Wistar rat frontoparietal cortex and hippocampus (60%, P < 0.05; 72%, P < 0.01; respectively). These changes were associated with a significant increase in the total number of specific SS receptors in the frontoparietal cortex (24%, P < 0.05) and hippocampus (20%, P < 0.05), without changes in the affinity constant as compared with the control group. No significant differences were seen in the basal and forskolin (FK)-stimulated adenylate cyclase (AC) activities in both brain areas of 5-HT-treated rats when compared to the control group. The capacity of SS to inhibit the FK-stimulated AC activity in the frontoparietal cortex and hippocampus of 5-HT-treated rats was lower than in the control groups. The ability of the stable GTP analogue 5'-guanylylimidodiphosphate (Gpp(NH)p) to inhibit FK-stimulated AC activity in frontoparietal cortical and hippocampal membranes was markedly decreased in 5-HT-treated rats. To determine if the above-mentioned changes were related to the 5-HT activation of central 5-HT1 and 5-HT2 receptors, a non-selective 5-HT1 and 5-HT2 receptor antagonist, methysergide, was administered 60 min before the 5-HT injection. Pretreatment with methysergide (5 mg/kg i.p. in a volume of 400 microliters) prevented the 5-HT-induced changes in the SS receptor-effector system and in SSLI levels in both brain areas. Methysergide alone had no observable effect on the somatostatinergic system. These results suggest that the frontoparietal cortical and hippocampal somatostatinergic system can be regulated by 5-HT receptors.

Developmental changes in the brain-stem serotonergic nuclei of teleost fish and neural plasticity

1. During early ontogeny, the serotonergic neurons in the brain stem of the three-spined stickleback shows a temporal and spatial developmental pattern that closely resembles that of amniotes. 2. However, in the adult fish, only the midline nuclei of the rostral group (dorsal and median raphe nuclei) and the dorsal lateral tegmental nucleus are consistently serotonin-immunoreactive (5-HTir), whereas the groups of the upper and lower rhombencephalon (raphe pontis, raphe magnus, and raphe pallidus/obscurus nuclei) are variable and, when present, contain relatively small numbers of 5-HTir neurons. 3. Using specific antisera against tryptophan 5-hydroxylase and aromatic L-amino acid decarboxylase, we have shown that the lateral B9 group and the groups of the upper and lower rhombencephalon are consistently present in adult sticklebacks. The results are discussed in relation to other known instances of neurotransmitter plasticity or transient neurotransmitter expression in teleost fish. 4. While there are several instances of transient expression of neurotransmitter markers by discrete neuronal populations, there is so far no evidence of changes from one neurotransmitter phenotype to another in the brain of teleost fish. However, there are indications of plasticity of expression of catecholamines and indoleamines, and their respective synthesizing enzymes, as reflected in age-dependent changes and variation between individuals of different physiological status. 5. As the brain grows continuously in teleost fish, and new neurons are added from proliferative regions, synaptic connections may be expected to undergo remodeling in all brain regions throughout life. Thus, the teleostean brain may be considered a suitable model for experimental studies of different aspects of neural plasticity.

Light and electron microscopic studies of the effects of p-chloroamphetamine on the monoaminergic innervation of the rat spinal cord

A previous report has shown that small diameter serotoninergic (5-HT) axons innervating the forebrain are selectively eliminated by treatment with an amphetamine derivative, (+/-)p-chloroamphetamine (PCA; Mamounas et al., [1991] J. Comp. Neurol. 314:558-586). It is well known that the spinal cord is the target of numerous monoaminergic fibers of different types. We have previously shown that the dorsal and ventral horns and the intermediolateral cell column are innervated by numerous serotonin-, noradrenaline- and dopamine-containing axons, including both thin axons with small varicosities and beaded axons with large varicosities. In all these regions, the large majority of fine indolaminergic fibers do not establish synaptic contacts, contrasting with the beaded axons which mostly exhibit synapses. The present work was conducted to study the effect of PCA on the monoaminergic innervation of the adult rat spinal cord. Animals received two subcutaneous doses of PCA 24 hours apart and were perfused 3 weeks later. Immunocytochemistry was performed to detect 5-HT, noradrenergic and dopaminergic fibers by using light and electron microscopy. Light microscopy revealed that PCA treatment caused a marked and selective elimination of the fine 5-HT-immunoreactive fibers, mainly found in the dorsal horn, but spared all other larger axons. This selective effect on the dorsal horn innervating thin 5-HT fibers was confirmed with the electron microscope by calculating the synaptic incidence(s) of monoaminergic innervation. These results suggest that fine and beaded 5-HT axons correspond to two anatomically, biochemically and pharmacologically different types of fibers, which could arise from two subpopulations of brainstem neurons. In addition, this drug could be used to provide an experimental animal, devoid of 5-HT nonsynaptic fibers, thereby facilitating a study on the role of dorsal horn nonsynaptic system in pain modulation.

Distribution of glutamate- and GABA-immunoreactive neurons projecting to the cardioacceleratory center of the intermediolateral nucleus of the thoracic cord of SHR and WKY rats: a double-labeling study

We aimed at (1) determining the distribution of glutamate (Glu)- and gamma-aminobutyric acid (GABA)-containing neurons in the brainstem with projections to the cardioacceleratory sympathetic preganglionic neurons in the intermediolateral nucleus (IML) of the upper thoracic cord and (2) determining whether such afferent projections in spontaneously hypertensive rats (SHR) differ from those of control Wistar-Kyoto (WKY) rats. We used a combination of electrophysiological methods to determine the site of HRP injection in the spinal cord and double-labeling methods for plotting the distribution of Glu- and GABA-immunoreactive neurons with projections to this site. HRP/Glu-labeled neurons (possibly glutamatergic) and HRP/GABA-labeled neurons (possibly GABAergic) were detected in 27% and 7% of the total HRP-labeled neurons of the central autonomic nuclei of 3 SHR rats and 3 WKY rats. HRP/Glu-labeled neurons were distributed predominantly ipsilaterally in 20 nuclei of the medulla oblongata, pons and hypothalamus, while HRP/GABA-labeled neurons were distributed in 7 nuclei of the medulla oblongata. No significant differences were found between the average percentages of HRP/Glu-labeled and HRP/GABA-labeled neurons in SHR and WKY rats. These findings indicate that: (1) the Glu-containing neurons represent a greater proportion than the GABA-containing neurons, (2) the proportions of these neurons appear to be similar in WKY and SHR rats and (3) generation of inbred tachycardia and hypertension in SHR rats can not be attributed to the topological and quantitative differences in the distribution of the glutamatergic and GABAergic neurons in the central autonomic nuclei.

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.

Muscle denervation increases thyrotropin-releasing hormone (TRH) biosynthesis in the rat medullary raphe

To determine whether thyrotropin-releasing hormone (TRH) could exert a trophic role in ventral horn motor neurons, we examined the effect of muscle denervation with botulinum toxin A on TRH mRNA in the rat medullary raphe by in situ hybridization histochemistry. Compared to controls, denervated rats showed a significant increase in the number and silver grain density of hybridized medullary raphe neurons. Increased proTRH gene expression in the medullary raphe in response to motor unit perturbation indicates that TRH may be trophic to lower motor neurons.

Immuno-electronmicroscopic studies on the gamma-aminobutyric acid and glycine receptor in the intermediolateral nucleus of the thoracic spinal cord of rats and guinea pigs

Gamma-aminobutyric acid (GABA) and glycine are known as major inhibitory neurotransmitters in the intermediolateral nucleus of the spinal cord. Distribution and density of GABA immunoreactive axon varicosities and glycine receptor immunoreactive dendrites and somata in the intermediolateral nucleus were examined by immuno-electronmicroscopy. GABA immunoreaction was observed in the axon varicosities of axo-dendritic and axo-somatic synapses. Glycine receptor immunoreaction was seen in association with the postsynaptic membrane of dendrites and soma. GABA immunoreactive axon varicosities were larger (1.01 +/- 0.49 x 1.20 +/- 0.38 microns) than axon varicosities presynaptic to glycine receptors (0.72 +/- 0.22 x 0.98 +/- 0.33 microns). The density of GABA immunoreactive axon varicosities was 3.65/100 microns 2 and that of glycine receptor immunoreactive synapses was 4.78/100 microns 2. A subpopulation of GABA immunoreactive axons (42%) made synaptic contact with glycine receptor immunoreactive dendrites or soma, indicating the coexistence of GABA and glycine.

Immunohistochemical evidence for the coexistence of substance P, thyrotropin-releasing hormone, GABA, methionine-enkephalin, and leucin-enkephalin in the serotonergic neurons of the caudal raphe nuclei: a dual labeling in the rat

By means of dual immunohistochemical labeling on the same brain section examined with a light microscope, the present study reports the presence with serotonin (5-hydroxytryptamine; 5-HT) of gamma-aminobutyric acid (GABA), substance P (SP), thyrotropin-releasing hormone (TRH), leucin-enkephalin (LEU-enk), or methionine-enkephalin (MET-enk), within the same neuron in the nuclei raphe magnus, raphe obscurus, and raphe pallidus of the rat. On the one hand, peptides or GABA are detected with specific rabbit antibodies by indirect peroxidase labeling using peroxidase-conjugated Fab fragments, and on the other, 5-HT is detected with a rabbit antibody against the BSA-serotonin conjugate by radio-immunocytochemistry using [125I]-labeled protein A. The possible coexistence of TRH and SP in these neurons is also investigated by using peroxidase labeling and radio-immunocytochemical detection, respectively. In the whole caudal raphe nuclei the proportion of each coexisting peptide with 5-HT appears in decreasing order as: TRH greater than SP greater than MET-enk # LEU-enk greater than GABA. In all instances the level of coexistence differs considerably in B1-B2 vs. B3 cell groups. No SP/TRH dually labeled cells have ever been found in any of the serotonergic nuclei of the caudal raphe. Given the evidence that these raphe nuclei project possibly to the spinal cord, these data constitute an anatomical substrate for the several distinct physiological functions presumably subserved by 5-HT in the cord, namely the modulation of nociception, motor, and autonomic functions.

Direct synaptic contacts of 5-hydroxytryptamine-, neuropeptide Y-, and somatostatin-immunoreactive nerve terminals on the preganglionic sympathetic neurons of the guinea pig

Direct synapses from 5-hydroxytryptamine-, neuropeptide Y-, and somatostatin-immunoreactive axon varicosities to the preganglionic sympathetic neurons retrogradely labeled with horseradish peroxidase were observed in the thoracic intermediolateral nucleus of the guinea pig. Symmetric synapses were predominant and both axo-dendritic and axo-somatic contacts were observed.