Caudal Raphe Nuclei Research Articles

Feedback Regulation of Thyrotropin-Releasing Hormone Gene Expression by Thyroid Hormone in the Caudal Raphe Nuclei in Rats**This work was supported by NIH Grants DK-50255 (H.Y.), DK-30110 (Y.T.), and DK-41301, Pilot and Feasibility Award (to H.Y.).

Medullary TRH regulates autonomic activity, and altered thyroid status is associated with autonomic disorders. We investigated whether thyroid hormone exerts a negative feedback regulation on TRH gene expression in the medullary caudal raphe nuclei. Medullary pro-TRH messenger RNAs (mRNAs) were mainly located in the raphe pallidus and raphe obscurus neurons as shown by in situ hybridization and were significantly increased by 70% and 160-230% by Northern blot analyses in 24 h fasted rats at 1 and 3-5 weeks after thyroidectomy, respectively, when serum T4 levels were reduced by 75-87%. The increased pro-TRH mRNA on the 30th day after thyroidectomy was reversed to euthyroid levels by T4 replacement (2 or 4 microg/100 g x day). T4 injections (10 or 100 microg/100 g x day for 30 days) did not significantly influence medullary pro-TRH mRNA levels in sham-operated rats. Thyroidectomized rats fed normally showed a 500% increase in pro-TRH mRNA levels 30 days after the surgery, while those fasted for 24 h showed only a 180% increase. These data indicate that medullary TRH gene expression is enhanced during hypothyroidism due to the lack of negative feedback regulation by thyroid hormone, and this response is modulated by feeding state. These findings may have important relevance to understanding autonomic-related visceral alterations induced by hypothyroidism.

Serotonin regulation of aggressive behavior in male golden hamsters (Mesocricetus auratus).

These studies examined the neurochemistry and neuroanatomy of the serotonin (5-HT) system innervating the anterior hypothalamus (AH) and the interaction of 5-HT receptor agonists with arginine vasopressin (AVP) in the regulation of offensive aggression in golden hamsters. Because specific 5-HT1A, 5-HT1B, and AVP V1A binding sites were observed within the AH by in vitro autoradiography, the hamsters were tested for offensive aggression after microinjections of AVP in combination with either the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino) tetraline (DPAT) or the 5-HT1B agonist CGS-12066A (CGS) directly within the AH. Though treatment with DPAT resulted in a dose-dependent inhibition of AVP-facilitated offensive aggression, CGS was ineffective. In addition, a retrograde tracer was injected within the AH to localize the distribution of 5-HT neurons projecting to the area. Retrogradely labeled 5-HT neurons were found within the dorsal, median, and caudal linear raphe nuclei and are suspected to inhibit AVP-facilitated offensive aggression by an activation of 5-HT1A receptors in the AH.

Expression of trkB and trkC mRNAs by adult midbrain dopamine neurons: a double-label in situ hybridization study.

The documented trophic actions of the neurotrophins brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4/5 (NT-4/5) upon ventral mesencephalic dopamine neurons in vitro and in vivo are presumed to be mediated through interactions with their high-affinity receptors TrkB (for BDNF and NT-4/5) and TrkC (for NT-3). Although both neurotrophin receptor mRNAs have been detected within the rat ventral midbrain, their specific association with mesencephalic dopaminergic cell bodies remains to be elucidated. The present study was performed to determine the precise organization of trkB and trkC mRNAs within rat ventral midbrain and to discern whether the neurotrophin receptor mRNAs are expressed specifically by dopaminergic neurons. In situ hybridization with isotopically labeled cRNA probes showed that trkB and trkC mRNAs were expressed in all mesencephalic dopamine cell groups, including all subdivisions of the substantia nigra and ventral tegmental area, and in the retrorubral field, rostral and caudal linear raphe nuclei, interfascicular nucleus, and supramammillary region. Combined isotopic/nonisotopic double-labeling in situ hybridization demonstrated that virtually all of the tyrosine hydroxylase (the catecholamine biosynthetic enzyme) mRNA-containing neurons in the ventral midbrain also expressed trkB or trkC mRNAs. Additional perikarya within these regions expressed the neurotrophin receptor mRNAs but were not dopaminergic. The present results demonstrate that essentially all mesencephalic dopaminergic neurons synthesize the neurotrophin receptors TrkB and TrkC and thus exhibit the capacity to respond directly to BDNF and NT-3 in the adult midbrain in vivo. Moreover, because BDNF and NT-3 are produced locally by subpopulations of the dopaminergic cells, the present data support the notion that the neurotrophins can influence the dopaminergic neurons through autocrine or paracrine mechanisms.

Neuronal Labeling in the Rat Brain and Spinal Cord from the Ovary Using Viral Transneuronal Tracing Technique

In the present investigations the viral transneuronal labeling method, which is able to reveal hierarchial chains of central nervous system (CNS) neurons, was applied to identify sites in the CNS connected with the ovary and presumably involved in the control of ovarian functions. Pseudorabies virus was injected into the ovaries of rats and a few days later (at various times after the injection) the spinal cord and brain were examined for virus-infected neurons from the ovary. The virus-labeled nerve cells were identified by immunocytochemistry using polyclonal antiviral antibody. Virus-labeled neurons were detected both in the spinal cord and the brain. In the spinal cord such elements were observed in the intermediolateral cell column, in the dorsal horn close to the marginal zone and in the central autonomic nucleus. In the medulla oblongata and pons, neurons of several nuclei and cell groups (area postrema, nucleus of the solitary tract, dorsal vagal complex, nucleus ambiguus, paragigantocellular nucleus, parapyramidal nucleus, A1, A5 and A7 cell groups, caudal raphe nuclei, locus ceruleus, subceruleus nucleus, Barrington’s nucleus, Kölliker-Fuse nucleus) were found to be transneuronally labeled. In the mesencephalon, the ventrolateral part of the periaqueductal gray matter contained virus-labeled neurons. In the diencephalon, a very intensive cell body labeling was observed in the hypothalamic paraventricular nucleus and a few virus-infected neurons could be detected in the lateral and dorsal hypothalamus, in the arcuate nucleus, zona incerta, perifornical area and in the anterior hypothalamus. Concerning the telencephalic structures, virus-labeled cells were found in the bed nucleus of the stria terminalis and in the central amygdala nucleus. These findings provide the first neuromorphological evidence for the existence of a multisynaptic neuronal pathway between the ovary and the CNS, and give a detailed account of the structures involved in this pathway.

Central nervous system origins of the sympathetic nervous system outflow to white adipose tissue.

White adipose tissue (WAT) is innervated by postganglionic sympathetic nervous system (SNS) neurons, suggesting that lipid mobilization could be regulated by the SNS [T. G. Youngstrom and T. J. Bartness. Am. J. Physiol. 268 (Regulatory Integrative Comp. Physiol. 37): R744-R751, 1995]. A viral transsynaptic retrograde tract tracer, the pseudorabies virus (PRV), was used to identify the origins of the SNS outflow from the brain to WAT neuroanatomically. PRV was injected into epididymal or inguinal WAT (EWAT and IWAT, respectively) of Siberian hamsters and IWAT of rats. PRV-infected neurons were visualized by immunocytochemistry and found in the spinal cord, brain stem (medulla, nucleus of the solitary tract, caudal raphe nucleus, C1 and A5 regions), midbrain (central gray), and several areas within the forebrain. The general pattern of infection of WAT in both species was more similar than different and resembled that seen after PRV injections into the adrenal medulla in rats (A. M. Strack, W. B. Sawyer, J. H. Hughes, K. B. Platt, and A. D. Loewy. Brain Res. 491: 156-162, 1989). EWAT versus IWAT injected hamsters had relatively less labeling in the suprachiasmatic, dorsomedial, and arcuate nuclei. Overall, it appeared that the SNS innervation of WAT originates from the general SNS outflow of the central nervous system and therefore may play a significant role in lipid mobilization.

Cellular localization of neuropeptide Y mRNA and peptide in the brain of the Japanese quail and domestic chicken

Neuropeptide Y (NPY) has been implicated in the control of a number of physiological functions in birds including food intake and reproduction. In the present study, sites of NPY synthesis were localized in the brains of Japanese quail and domestic chickens by in situ hybridization histochemistry using a digoxigenin-labelled riboprobe. NPY mRNA was detected in three main cell groups in both species. The most prominent group was associated with structures in the lateral thalamus including the anterior lateral thalamic nucleus, lateral forebrain bundle, rotund nucleus, pretectal nucleus and occipitomesencephalic tract. Other major cell groups were detected in the hippocampus, and in the caudal linear nucleus and raphe nucleus of the brainstem. NPY mRNA was also present in the piriform cortex and taenial nucleus. Double-labelling of NPY mRNA and peptide was demonstrated in individual cells of the hippocampal, thalamic and brainstem cell groups, suggesting that NPY is synthesized and stored in these areas. However, the identity of other cell groups, notably in the hyperstriatal, archistriatal and neostriatal regions of the telencephalon, which exhibit NPY-immunoreactive cell bodies but no NPY mRNA, remains to be determined.

CNS cell groups projecting to sympathetic outflow of tail artery: neural circuits involved in heat loss in the rat

In the rat, ≈20% of total body heat-loss occurs by sympathetically mediated increases in blood flow through an elaborate system of arteriovenous anastomoses in the skin of its tail. In this study, the CNS cell groups that regulate this sympathetic outflow were identified by the viral transneuronal labeling method. Pseudorabies virus was injected into the wall of the ventral tail artery in rats that had their cauda equina transected to eliminate the somatic innervation of the tail. After 4–7 days survival, the pattern of CNS transneuronal labeling was studied. Sympathetic preganglionic neurons in the T11–L2 (mainly L1) levels of the intermediolateral cell column (IML) were labeled by 4 days. After 5 days, sympathetic pre-motor neurons (i.e., supraspinal neurons that project to the IML) were identified near the ventral medullary surface; some of these contained serotonin immunoreactivity. Additional groups of the sympathetic premotor areas were labeled by 6 days post-injection, including the rostral ventrolateral medulla (C1 adrenergic neurons), rostral ventromedial medulla, caudal raphe nuclei (serotonin neurons in the raphe pallidus and magnus nuclei), A5 noradrenergic cell group, lateral hypothalamic area and paraventricular hypothalamic area (oxytocin-immunoreactive neurons). Seven days after the PRV injections, additional cell groups in the telencephalon (viz., bed nucleus of the stria terminalis, medial and lateral preoptic areas and medial preoptic nucleus), diencephalon (viz., subincertal nucleus, zona incerta as well as dorsal, dorsomedial, parafascicular, posterior and ventromedial hypothalamic nuclei) and midbrain (viz., periaqueductal gray matter, precommissural nucleus, Edinger–Westphal nucleus and ventral tegmental area) were labeled. The discussion is focused on the CNS cell groups involved in the control of body temperature and fever.

Colocalization of substance P or enkephalin in serotonergic neuronal afferents to the hypoglossal nucleus in the rat

The serotonergic innervation of the hypoglossal nucleus originates from the caudal raphe nuclei. Non-serotonergic neurons in the caudal raphe nuclei also project to the hypoglossal nucleus. We employed a triple-fluorescence technique to determine whether the substance P- or the enkephalin-containing neurons in the caudal raphe nuclei that projected to the hypoglossal nucleus also contained serotonin. Rhodamine latex microspheres were injected into the hypoglossal nucleus, and then serotonin and peptide dual-immunofluorescence was performed to colocalize perikarya containing serotonin, substance P, and rhodamine microspheres; or perikarya containing serotonin, enkephalin, and rhodamine microspheres. Our results demonstrate that most substance P-containing neuronal afferents to the hypoglossal nucleus colocalize serotonin. In contrast, few enkephalin-containing neuronal afferents to the hypoglossal nucleus also contain serotonin. These data suggest that substance P projections to the hypoglossal nucleus are a subset of serotonergic projections and that limited overlap exists between the populations of enkephalinergic and serotonergic neuronal afferents to the hypoglossal nucleus. Either substance P- or enkephalin-containing somata account for a very small proportion of non-serotonergic caudal raphe projections to the hypoglossal nucleus. Finally, these data demonstrate the medial tegmental field origins of the substance P projections and the enkephalin projections to the hypoglossal nucleus.

Ouabain-like immunoreactivity in the medulla oblongata of rats.

An isomer of ouabain, the ouabain-like compound (OI,C), may participate in the regulation of body fluid volume and vascular tone. Forebrain regions, especially the hypothalamus, are reported to be sites of OLC action in the central nervous system. The medulla oblongata is another critical area involved in central cardiovascular regulation. We reported that the microinjection of either monoclonal antibody to ouabain T8B11 or Fab fragment of digoxin-specific antibody into the rostral ventrolateral medulla significantly decreased mean arterial pressure and renal sympathetic nerve activity in anesthetized normotensive rats (TERUYA et al.: J. Clin. Invest. 99: 2791-2798, 1997). Using T8B11, we examined the ouabain-like immunoreactivity in the medulla oblongata of normotensive rats. In periodate-lysine-paraformaldehyde fixed tissues, ouabain-like immunoreactive neurons were detected in the nuclei and regions in the medulla oblongata including the ventrolateral medulla, ventromedial medulla, nucleus ambiguus, caudal raphe nuclei, nucleus of solitary tract, and dorsal motor nucleus of the vagus. When an Fab fragment of digoxin-specific antibody was used as a first antibody, the digoxin-like immunoreactive neurons were distributed in almost the same pattern as those observed with the use of T8B11. In the brain fixed with the "three-step" procedure developed by YAMADA et al. (1987), which was used in a previous ouabain immunohistochemical study of the hypothalamus, ouabain-like immunoreactivity in the medulla oblongata was much weaker in intensity and less restricted in distribution than that in the hypothalamus. These findings suggest that ouabain-like immunoreactivities are present in the medulla oblongata with a manner of distribution different from that seen in the hypothalamus. Some ouabain-immunopositive nuclei and regions in the medulla oblongata, especially the rostral ventrolateral medulla, may be other OLC action sites.

Decreased frontal activation of schizophrenics during stimulation with the continuous performance test—A functional magnetic resonance imaging study

Mesopontine tegmental nuclei such as the cuneiform, pedunculotegmental, oral pontine reticular, paramedian raphe and caudal linear raphe nuclei, are deep brain structures involved in arousal and motor function. Dysfunction of these nuclei is implicated in the pathogenesis of disorders of consciousness and sleep, as well as in neurodegenerative diseases. However, their localization in conventional neuroimages of living humans is difficult due to limited image sensitivity and contrast, and a stereotaxic probabilistic neuroimaging template of these nuclei in humans does not exist. We used semi-automatic segmentation of single-subject 1.1 mm-isotropic 7 T diffusion-fractional-anisotropy and T2-weighted images in healthy adults to generate an in vivo probabilistic neuroimaging structural template of these nuclei in standard stereotaxic (Montreal Neurological Institute, MNI) space. The template was validated through independent manual delineation, as well as leave-one-out validation and evaluation of nuclei volumes. This template can enable localization of five mesopontine tegmental nuclei in conventional images (e.g. 1.5 T, 3 T) in future studies of arousal and motor physiology (e.g. sleep, anesthesia, locomotion) and pathology (e.g. disorders of consciousness, sleep disorders, Parkinson's disease). The 7 T magnetic resonance imaging procedure for single-subject delineation of these nuclei may also prove useful for future 7 T studies of arousal and motor mechanisms.

Postsynaptic 5-HT 1A receptors mediate 5-hydroxytryptamine release in the amygdala through a feedback to the caudal linear raphe

Using brain microdialysis, it was demonstrated that the release of 5-hydroxytryptamine (5-HT) in the central nucleus of the amygdala is under inhibitory control of somatodendritic and postsynaptic 5-HT 1A receptors. Systemic administration of flesinoxan, a selective 5-HT 1A receptor agonist, significantly reduced the extracellular levels of 5-HT in the central nucleus of the amygdala. This effect could be completely antagonized by the 5-HT 1A receptor antagonist N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)- N-(2-pyridyl)cyclohexane carboxamine trihydrochloride (WAY 100635). Local administration of these compounds by reversed microdialysis into the raphe nuclei revealed that extracellular 5-HT levels in the central nucleus of the amygdala can be regulated through 5-HT 1A receptors in the caudal linear raphe nucleus, but not in the dorsal and median raphe nuclei. Interestingly, administration of flesinoxan into the central nucleus of the amygdala also decreased dialysate 5-HT levels both locally and in the caudal linear raphe nucleus. The former effect could be blocked by pretreatment with WAY 100635 when applied into the central nucleus of the amygdala, but not when applied into the caudal linear raphe nucleus. These data provide circumstantial evidence for the existence of a 5-HT 1A receptor mediated feedback loop from the central nucleus of the amygdala to the caudal linear raphe nucleus.

Responses of neurones in the medullary raphe nuclei to inputs from visceral nociceptors and the ventrolateral periaqueductal grey in the rat.

The ventrolateral periaqueductal grey matter (PAG) is believed to have a role in mediating cardiovascular responses to noxious visceral stimuli. The present study was carried out as a first stage in establishing whether the ventrolateral PAG may exert these influences after a relay in the caudal medullary raphe nuclei (nucleus raphe obscurus and nucleus raphe pallidus). Single unit extracellular recordings were made from neurones in the caudal raphe nuclei and, for comparison, in the more rostral nucleus raphe magnus in Saffan-anaesthetized and paralysed rats. Neurones in the mid-line medulla were tested for their responses to electrical stimulation at chemically identified depressor sites in the ventrolateral PAG and to noxious visceral stimuli (distensions of the urinary bladder and electrical stimulation of the greater splanchnic nerve). Fifty-two per cent of caudal and 74% of rostral mid-line neurones gave short latency excitatory responses to stimulation of depressor sites in the ventrolateral PAG. Of the neurones that were also tested with noxious visceral stimuli, 5% of the caudal and 47% of the rostral neurones responded to bladder distension, while 33 and 35%, respectively, of caudal and rostral neurones responded to splanchnic nerve stimulation. These results indicate that many mid-line medullary neurones receive inputs from both the ventrolateral PAG and visceral nociceptors and may, therefore, be part of the output pathway by which the ventrolateral PAG produces integrated physiological responses to noxious visceral stimuli.

Expression of the homeobox-containing genes EN1 and EN2 in human fetal midgestational medulla and cerebellum.

Homeobox-containing genes En-1 and En-2 have been implicated in the control of pattern formation during development of the central nervous system in experimental animals. In order to determine whether the expression of homologous human EN genes can be used as a developmental genetic marker of the arcuate nucleus of the medulla (a putative precerebellar nucleus that shows developmental deficiency in a subset of sudden infant death syndrome [SIDS]), we performed in situ hybridization with human EN1 and EN2 RNA probes in human fetal midgestational medulla and cerebellum (18-21 weeks gestational age, n=4). Expression of EN genes was demonstrated in all neuronal groups of the medulla and throughout the cerebellum. The RNA signal for both EN1 and EN2 was strongest in the cerebellar granule cell layers, white matter of the vermis and flocculus, inferior olive, arcuate nucleus, caudal raphe nuclei, corpus pontobulbare and nucleus ambiguus. Most of the structures that showed the strongest EN signal originate in the rhombic lip. Some of these structures are functionally interconnected, and show pathologic changes in the syndrome of infantile olivopontocerebellar hypoplasia/atrophy. Strong expression of EN signal in the arcuate nucleus could be used as a genetic marker of this nucleus in further developmental studies of the arcuate nucleus in SIDS. Although EN expression is not specific to the arcuate nucleus or to the rhombic lip derivatives, our results suggest that rhombic lip derivatives have the highest levels of EN RNA message among the medullary structures at midgestation.

Serotonergic neurons in the caudal raphe nuclei discharge in association with activity of masticatory muscles

There is a dense serotonergic projection from nucleus raphe pallidus and nucleus raphe obscurus to the trigeminal motors nucleus and serotonin exerts a strong facilitatory action on the trigeminal motoneurons. Some serotonergic neurons in these caudal raphe nuclei increase their discharged during feeding. The objective of the present study was to investigate the possibility that the activity of these serotonergic neurons is related to activity of masticatory muscles. Cats were implanted with microelectrodes and gross electrodes. Caudal raphe single neuron activity, electrocorticographic activity, and splenius, digastric and masseter electromyographic activities were recorded during active behaviors (feeding and grooming), during quiet waking and during sleep. Seven presumed serotonergic neurons were identified. These neurons showed a long duration action potential (> 2.0 msec) and discharged slowly (2-7 Hz) and very regularly (interspike interval coefficient of variation < 0.3) during quiet waking. The activity of these neurons decreased remarkably during fast wave sleep (78-100%). Six of these neurons showed tonic changes in their activity positively related to digastric and/or masseter muscles activity but not to splenius muscle activity during waking. These data are consistent with the hypothesis that serotonergic neurons in the caudal raphe nuclei play an important role in the control of jaw movements.

The brainstem network involved in coordination of inspiratory activity and cholinergic outflow to the airways

The respiratory rhythm modulates cholinergic outflow to the tracheal smooth muscle through the parasympathetic nerves. To determine the basis of this modulation, we combined the retrograde tracer technique to identify bulbospinal cells projecting to phrenic motoneurons, and the transneuronal labeling method to visualize medullary neurons that innervate airway-related vagal preganglionic cells. Following injections of fluorogold into the ventral horns of the cervical spinal cord and injections of pseudorabies virus (PRV) into the wall of the extrathoracic trachea of superior cervical ganglioctomized Sprague-Dawley rats. A large number of the double-labeled cells were identified along the ventral aspect of the medulla oblongata. Most frequently, double-labeled neurons were seen in the medial tegmental field, particularly in the parapyramidal region, within the gigantocellular nuclei, and the caudal raphe nuclei. Less frequently, double-labeled neurons were found in the ventrolateral medulla. No double-labeled cell was observed in the dorsal aspect of the medulla oblongata. This study indicates that a subset of medullary neurons that project to phrenic motoneurons also innervate the airway-related vagal preganglionic cells, allowing the coupling of inspiratory activity and parasympathetic outflow to the airways.

Caudal Brainstem Raphe Nuclei: Neural Substrate for their Involvement in the Expression of Some Biological Rhythms

Nucleus raphe pallidus (RPa) lies ventrally in the caudal brainstem, where it is coextensive rostrally with the nucleus raphe magnus (RMg) and caudally with the nucleus raphe obscurus (ROb). Retrograde neuronal tracing studies of our laboratory, carried out in rats and presented elsewhere, with fluorogold, true-blue or fast-blue, iontophoretically injected or by crystalline deposit, along the RPa extent, displayed many labeled pericaria at the preoptic area (POA), as well as lateral (LH) and dorsomedial (DMH) hypothalamus; paraventricular nucleus (PVN) and dorsal (DR) and median (MnR) raphe nuclei among others structures. In addition, RPa, which projects to the intermediolateral column, has been demonstrated to bear relation to many of the somatic-visceral functions also reported for POA. Iontophoretic injections of PHA-L, an anterograde tracer, in the POA subnuclei, presented terminal and varicose labeled fibers in RPa, as well as in the RMg, ROb, paraventricular thalamic (PVA), PVN and supraoptic nucleu...

The effects of serotonin antagonists in an animal model of sleep-disordered breathing.

Recent studies have shown excitatory effects of serotonin on upper airway motoneurons. This excitatory effect is normally present and arises from cells in the caudal raphe nuclei. The firing of these serotonergic neurons is reduced during sleep. To determine the importance of serotonin in the maintenance of patient airways and normal respiration in waking in obstructive sleep apnea, we studied the effects of two serotonin antagonists on upper airway dilator muscle activity, diaphragm activity, Sao2, and upper airway cross-sectional area in an animal model of sleep-disordered breathing, the English bulldog. Systemic administration of both antagonists resulted in significant reductions in the peak amplitudes of upper airway muscle respiratory bursts (range, 39 to 62% suppression; p < 0.05). Lesser reductions in diaphragm activity were noted (range, 10 to 33% suppression; p < 0.05). Oxyhemoglobin saturations also fell (p < 0.05), coinciding with suppressions in upper airway muscle activity. With reductions in dilator muscle activity, upper airway cross-sectional areas, as measured with cine CT, showed significant inspiratory collapse. These results support the hypothesis that serotonin is important in the maintenance of patent upper airways in obstructive sleep apnea.

Presence of vasomotor and respiratory rhythms in the discharge of single medullary neurons involved in the regulation of cardiovascular system

We analyzed the discharges of 77 single neurons located in the rostral ventrolateral medulla (RVLM, n = 25), caudal ventrolateral medulla (CVLM, n = 18), lateral tegmental field (LTF, n = 190) and caudal raphe nuclei ( n = 15). These recordings were made from 36 vagotomized and sinoaortic denervated cats that were either decerebrate ( n = 27) or anesthetized with urethane ( n = 9) and from 3 decerebrate cats with intact sinoartic and vagal nerves. These neurons were classified as sympathetic-related ( n = 61) if spike triggered averaging showed that their naturally occurring discharges were correlated to either the cardiac related (2–6 Hz) or a faster (10 Hz) oscillation in inferior cardiac sympathetic nerve discharge. Neurons were classified as sympathetic-unrelated ( n = 16) if they lacked these characteristics. We used autoregressive spectral techniques to detect additional slower oscillations hidden in the variability of neuronal discharge and possibly correlated to the oscillations of systolic arterial pressure (SAP). This analysis revealed the existence of a low frequency (LF) oscillation (0.12 ± 0.02 Hz) in the discharges of 36 sympathetic-related and 9 sympathetic-unrelated neurons. In relation to 35 neurons in 21 animals there was also an LF component in SAP variability. In 29 instances the LF neuronal discharges and SAP variabilities were significantly correlated. In addition, there was a high frequency (HF) oscillation (0.34 ± 0.06 Hz) in the discharges of 59 medullary neurons. In 56 cases the HF in neuronal discharge variability cohered to that in SAP variability. These data are the first to demonstrate the existence of an LF component in the discharges of individual medullary neurons, at least some of which were likely to be involved in the regulation of the cardiovascular system. Since these oscillations were evident in cats with section of sinoaortic and vagal nerves, they likely reflect central rhythmogenic properties.

Neuroendocrine and autonomous mechanisms underlying thermoregulation in cold environment.

This review focuses on the central regulation of thermoregulatory responses with special attention to the participation of thyrotropin-releasing hormone (TRH) in both autonomous and endocrine responses to a cold environment. Besides a direct projection of TRH neurons from paraventricular nuclei (PVN) to the median eminence, and the subsequent activation of the thyroid axis, there are direct projections from the PVN to the autonomic preganglionic neurons controlling autonomous responses. There projections convey information to peripheral targets involved in thermogenesis through the dorsal vagal complex and the spinal cord, for parasympathetic and sympathetic neurotransmissions respectively. Furthermore, cold exposure increases TRH mRNA levels in the PVN but also in dorsal motor and caudal raphe nuclei, thus providing strong evidence for a functional link between autonomous and neuroendocrine systems involved in thermoregulation. The review also focuses on neuroendocrine regulation of cold-induced TRH/TSH release associated with modifications in somatostatin release, with special reference to the participation of several central neurotransmitters (catecholamines, serotonin or GABA) or the influence of sex steroids.

Bulbospinal sympatho-excitatory neurons in the rat caudal raphe

To explore the rat caudal raphe nuclei for neurons that respond to activation of baroreceptor nerves and that have a spinal axon, and to compare the behavioural properties of barosensitive bulbospinal neurons in the rat caudal raphe with the properties of barosensitive bulbospinal neurons in the rostral ventrolateral medulla. Extracellular unit recordings were obtained from an area extending up to 1.0 mm caudally from the caudal edge of the facial nucleus. Two sites were explored: the rostral ventrolateral medulla and the midline. Single-unit recordings were made in anaesthetized (75 mg/kg chloral hydrate and 30 mg/kg sodium pentobarbitone then 3-6 mg intravenously as required), immobilized (2 mg pancuronium as required) Sprague-Dawley rats. Central respiratory drive was recorded from phrenic nerve discharge. The barosensitivity of single units was assessed by R-wave triggered histograms and by histograms of their responses to aortic nerve stimulation or to intravenous injection of phenylephrine. Nociceptors were activated by a brief pinch of the tail. Eleven spontaneously active units in the midline that were inhibited by baroreceptor stimulation and had a spinal axon were studied. Respiratory modulation was present and was predominantly inspiratory. Barosensitive neurons in the rostral ventrolateral medulla were activated by nociceptive inputs; midline barosensitive neurons were not. The behavioural characteristics of midline neurons differ from those of the bulbospinal barosensitive neurons in the rostral ventrolateral medulla, indicating that raphe spinal neurons have different sets of afferent inputs and may subserve to a distinct physiological role. The present paper is the first report of bulbospinal neurons in the rat caudal raphe that are inhibited by activation of arterial baroreceptors.