Thyrotropin-releasing hormone-immunoreactive neurons project from the ventral medulla to the intermediolateral cell column: partial coexistence with serotonin

Leptin amplifies the action of thyrotropin-releasing hormone in the solitary nucleus: an in vitro calcium imaging study

Leptin exerts a powerful permissive influence on neurogenic thermogenesis. During starvation and an absence of leptin, animals cannot produce thermogenic reactions to cold stress. However, thermogenesis is rescued by restoring leptin. We have previously observed a highly cooperative interaction between leptin and thyrotropin-releasing hormone [TRH] to activate hindbrain-generated thermogenic responses (Hermann et al., 2006). In vivo physiological studies (Rogers et al., 2009) suggested that the thermogenic impact of TRH in the hindbrain is amplified by the action of leptin through a leptin receptor-mediated production of phosphoinositol-trisphosphate [PIP3]. In turn, PIP3 can activate a tyrosine kinase whose target is the Src-SH2 regulatory site on the phospholipase C [PLC] complex. The TRH receptor signals through the PLC complex. Our immunohistochemical studies (Barnes et al., 2010) suggest that this transduction interaction between leptin and TRH occurs within neurons of the solitary nucleus [NST], though this interaction had not been verified. The present in vitro live cell calcium imaging study shows that while medial NST neurons are rarely activated by leptin alone, leptin pre-treatment significantly augments NST neurons' responsiveness to TRH. This leptin-mediated priming of NST neurons was uncoupled by pre-treatment with the phosphoinositide 3-kinase [PI3K] inhibitor [wortmannin], the phospholipase C inhibitor [U73122] and the Src-SH2 antagonist [PP2]. TTX did not eliminate the synergistic response of the agonists, thus the sensitization cannot be attributed to pre-synaptic mechanisms. It seems likely that NST neurons are involved in the leptin-mediated increase in BAT temperature by sensitizing the TRH-PLC-IP3-calcium release mechanism.

Co-localization of TRHR1 and LepRb receptors on neurons in the hindbrain of the rat

We have reported a highly cooperative interaction between leptin and thyrotropin releasing hormone (TRH) in the hindbrain to generate thermogenic responses (Hermann et al., 2006) (Rogers et al., 2009). Identifying the locus in the hindbrain where leptin and TRH act synergistically to increase thermogenesis will be necessary before we can determine the mechanism(s) by which this interaction occurs. Here, we performed heat-induced epitope recovery techniques and in situ hybridization to determine if neurons or afferent fibers in the hindbrain possess both TRH type 1 receptor and long-form leptin receptor [TRHR1; LepRb, respectively]. LepRb receptors were highly expressed in the solitary nucleus [NST], dorsal motor nucleus of the vagus [DMN] and catecholaminergic neurons of the ventrolateral medulla [VLM]. All neurons that contained LepRb also contained TRHR1. Fibers in the NST and the raphe pallidus [RP] and obscurrus [RO] that possess LepRb receptors were phenotypically identified as glutamatergic type 2 fibers (vglut2). Fibers in the NST and RP that possess TRHR1 receptors were phenotypically identified as serotonergic [i.e., immunopositive for the serotonin transporter; SERT]. Co-localization of LepRb and TRHR1 was not observed on individual fibers in the hindbrain but these two fiber types co-mingle in these nuclei. These anatomical arrangements may provide a basis for the synergy between leptin and TRH to increase thermogenesis.

Leptin "gates" thermogenic action of thyrotropin-releasing hormone in the hindbrain

Leptin, acting as a measure of metabolic fuel availability, exerts a powerful permissive influence on neurogenic thermogenesis. During starvation and an absence of leptin, animals cannot produce thermogenic reactions to cold stress. However, thermogenesis is rescued by restoring leptin. We have previously observed (Hermann, G.E., Barnes, M.J., Rogers, R.C., 2006. Leptin and thyrotropin-releasing hormone: cooperative action in the hindbrain to activate brown adipose thermogenesis. Brain Res. 1117, 118-124.) a highly cooperative interaction between leptin and thyrotropin-releasing hormone [TRH] to activate hindbrain generated thermogenic responses. Specifically, exposure to both leptin and TRH elicited a 3.5 degrees C increase in brown adipose tissue [BAT] thermogenesis, while leptin alone did not evoke any change, and TRH alone caused only approximately 1 degrees C increase. The present study shows that the leptin-TRH synergy in controlling brown adipose [BAT] thermogenesis is order-specific and dependent on the feeding status of the animal. That is, fourth ventricular [4V] application of leptin to the food-deprived animal, before TRH injection, yields a substantial increase in BAT; while the reverse order yields a significantly smaller effect. If the animal were fed within minutes of anesthesia, then exogenous leptin was not necessary for TRH to yield a large increase in BAT temperature. The leptin-TRH synergy was uncoupled by pretreatment with the phosphoinositol-tris phosphate kinase [PI3K] inhibitor, wortmannin and the Src-SH2 antagonist, PP2. The TRH transduction mechanism utilizes phospholipase C [PLC] potently regulated by the SH2 site. Previous work in culture systems suggests that the product of PI3K activity [PIP3] potently upregulates PLC by activating the SH2 domain of the PLC complex. Perhaps leptin "gates" the thermogenic action of TRH in the hindbrain by invoking this same mechanism.

Functional mapping of the cardiorespiratory effects of dorsal and median raphe nuclei in the rat

The dorsal (DRN) and median (MRN) raphe nuclei are important sources of serotonergic innervation to the forebrain, projecting to sites involved in cardiovascular regulation. These nuclei have been mapped using electrical stimulation, which has the limitation of stimulating fibers of passage. The present study maps these areas with chemical stimulation, investigating their influence on cardiorespiratory parameters. Urethane-anesthetized (1.2 g/kg, iv) male Wistar rats (280-300 g) were instrumented for pulsatile and mean blood pressure (MBP), heart rate, renal nerve activity, and respiratory frequency recordings. Microinjections of L-glutamate (0.18 M, 50-100 nl with 1% Pontamine Sky Blue) were performed within the DRN or the MRN with glass micropipettes. At the end of the experiments the sites of microinjection were identified. The majority of sites within the MRN (86.1%) and DRN (85.4%) evoked pressor responses when stimulated (DRN: DeltaMBP = +14.7 +/- 1.2; MRN: DeltaMBP = +13.6 +/- 1.3 mmHg). The changes in renal nerve activity and respiratory rate caused by L-glutamate were +45 +/- 11 and +42 +/- 9% (DRN; P < 0.05%), +40 +/- 10 and +29 +/- 7% (MRN, P < 0.05), respectively. No significant changes were observed in saline-microinjected animals. This study shows that: a) the blood pressure increases previously observed by electrical stimulation within the raphe are due to activation of local neurons, b) this pressor effect is due to sympathoexcitation because the stimulation increased renal sympathetic activity but did not produce tachycardia, and c) the stimulation of cell bodies in these nuclei also increases the respiratory rate.

Projections from the nucleus reticularis magnocellularis to the rat cervical cord using electrical stimulation and iontophoretic injection methods

The aim of this study is to clarify the fiber distribution of the nucleus reticularis magnocellularis (NRMC) and adjacent areas in the rat spinal cord. Biotinylated dextran amine was injected iontophoretically through a glass capillary into the areas, in which a single cell responded to noxious electrical stimulation of the sciatic nerve and to a pinch of the thigh skin with multiple spikes. Labeled fibers descended bilaterally through the ventral funiculi of the medulla oblongata and then through the ventral and lateral funiculi of the cervical cord with an ipsilateral predominance, and terminated in the spinal gray (laminae I-X). A single fiber sometimes ran through several laminae while bifurcating many short branches with axon varicosities and terminal buttons in one transverse section, that is, through laminae V, VII and X, through laminae V, IIl-IV and I-II, and through laminae VII to I-II. The present study showed that the wide distribution of a single fiber and a mass of fibers descending from the NRMC and adjacent areas might modulate not only somatic sensory and motor functions but also autonomic functions in the spinal cord.

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.

Contributions of the medullary raphe and ventromedial reticular region to pain modulation and other homeostatic functions

The raphe magnus is part of an interrelated region of medullary raphe and ventromedial reticular nuclei that project to all areas of the spinal gray. Activation of raphe and reticular neurons evokes modulatory effects in sensory, autonomic, and motor spinal processes. Two physiological types of nonserotonergic cells are observed in the medullary raphe and are thought to modulate spinal pain processing in opposing directions. Recent evidence suggests that these cells may modulate stimulus-evoked arousal or alerting rather than pain-evoked withdrawals. Nonserotonergic cells are also likely to modulate spinal autonomic and motor circuits involved in thermoregulation and sexual function. Medullary serotonergic cells have state-dependent discharge and are likely to contribute to the modulation of pain processing, thermoregulation, and sexual function in the spinal cord. The medullary raphe and ventromedial reticular region may set sensory, autonomic, and motor spinal circuits into configurations that are appropriate to the current behavioral state.

Inhibition of pudendal reflexes in spinal rats. Reassessing the role of serotonin

The effects of serotonin (5-HT) and thyrotropin-releasing hormone (TRH) on penile reflexes were investigated in intact and spinally transected male rats. Doses of intrathecal 5-HT (0.0, 1.13, 2.26, 11.3, 22.6, and 113.0 nmol), in a range previously shown to inhibit pudendal reflexes in anesthetized spinal preparations, prolonged the latency to the first penile erection in awake intact rats. However, these doses also provoked hyperreactivity and vocalization. Doses of intrathecal TRH (100 and 500 pmol) that effectively inhibited penile erection in intact animals were less effective in spinalized animals. Finally, a combination of subthreshold doses of TRH (100 pmol) and 5-HT (4.0 nmol) at a ratio known to affect other TRH/5-HT-mediated circuits significantly extended erection latency in animals with spinal transections. These data suggest that 5-HT and TRH are both involved in the inhibitory circuits regulating penile erection, either through corelease onto the same population of cells or through independent release onto different populations of neurons.

The discharge of a subset of serotonergic raphe magnus cells is influenced by baroreceptor input

In order to determine whether serotonergic cells in the medullary raphe magnus (RM) receive baroreceptor input, cells were tested for their responses to descending aortic occlusion, aortic nerve stimulation, or systemic phenylephrine administration in the lightly anesthetized rat. Serotonergic cells were identified physiologically by a quantitative analysis of their slow and steady discharge. Greater than 40% of the serotonergic RM cells tested responded to brief occlusion of the descending aorta at the level of the coeliac arteries, a stimulus that elevated blood pressure by about 30 mmHg. Similarly, about 40% of the serotonergic RM cells responded to stimulation of the aortic nerve, a nerve that contains primarily baroreceptor afferents from the aortic arch. Greater than 70% of RM serotonergic cells responded to phenylephrine administration which elevated blood pressure by an average of 50 mmHg. Serotonergic cell responses to all methods of baroreceptor activation were small in magnitude and were largely restricted in time to the stimulus duration. The results indicate that a subset of serotonergic cells in RM are influenced by baroreceptor activity.

Central neural regulation of penile erection

Penile erection is caused by a change of the activity of efferent autonomic pathways to the erectile tissues and of somatic pathways to the perineal striated muscles. The spinal cord contains the cell bodies of autonomic and somatic motoneurons that innervate the peripheral targets. The sympathetic outflow is mainly antierectile, the sacral parasympathetic outflow is proerectile, and the pudendal outflow, through contraction of the perineal striated muscles, enhances an erection already present. The shift from flaccidity to erection suggests relations among these neuronal populations in response to a variety of informations. Spinal neurons controlling erection are activated by information from peripheral and supraspinal origin. Both peripheral and supraspinal information is capable of eliciting erection, or modulating or inhibiting an erection already present. One can hypothesize a spinal network consisting of primary afferents from the genitals, spinal interneurons and sympathetic, parasympathetic and somatic nuclei. This system is capable of integrating information from the periphery and eliciting reflexive erections. The same spinal network, eventually including different populations of spinal interneurons, would be the recipient of supraspinal information. Premotor neurons that project directly onto spinal sympathetic, parasympathetic or somatic motoneurons, are present in the medulla, pons and diencephalon. Several of these premotor neurons may in turn be activated by sensory information from the genitals. Aminergic and peptidergic descending pathways in the vicinity of spinal neurons, exert complex effects on the spinal network that control penile erection. This is caused by the potential interaction of a great variety of receptors and receptor subtypes present in the spinal cord. Brainstem and hypothalamic nuclei (among the latter, the paraventricular nucleus and the medial preoptic area) may not necessarily reach spinal neurons directly. However they are prone to regulate penile erection in more integrated and coordinated responses of the body, such as those occurring during sexual behavior. Finally, the central and spinal role of regulatory peptides (oxytocin, melanocortins, endorphins) has only recently been elucidated.

Synaptic control of motoneuronal excitability

Movement, the fundamental component of behavior and the principal extrinsic action of the brain, is produced when skeletal muscles contract and relax in response to patterns of action potentials generated by motoneurons. The processes that determine the firing behavior of motoneurons are therefore important in understanding the transformation of neural activity to motor behavior. Here, we review recent studies on the control of motoneuronal excitability, focusing on synaptic and cellular properties. We first present a background description of motoneurons: their development, anatomical organization, and membrane properties, both passive and active. We then describe the general anatomical organization of synaptic input to motoneurons, followed by a description of the major transmitter systems that affect motoneuronal excitability, including ligands, receptor distribution, pre- and postsynaptic actions, signal transduction, and functional role. Glutamate is the main excitatory, and GABA and glycine are the main inhibitory transmitters acting through ionotropic receptors. These amino acids signal the principal motor commands from peripheral, spinal, and supraspinal structures. Amines, such as serotonin and norepinephrine, and neuropeptides, as well as the glutamate and GABA acting at metabotropic receptors, modulate motoneuronal excitability through pre- and postsynaptic actions. Acting principally via second messenger systems, their actions converge on common effectors, e.g., leak K(+) current, cationic inward current, hyperpolarization-activated inward current, Ca(2+) channels, or presynaptic release processes. Together, these numerous inputs mediate and modify incoming motor commands, ultimately generating the coordinated firing patterns that underlie muscle contractions during motor behavior.

Dorsal raphe nucleus serotonergic neurons innervate the rostral ventrolateral medulla in rat

Stimulation of the dorsal raphe nucleus (DRN) alters arterial pressure, heart rate and cerebral blood flow, yet projections from the DRN to medullary autonomic nuclei have not been described. We examined whether serotonergic (5-HT) projections from the DRN terminate in the rostral ventrolateral medulla (RVL) and if so, whether the projection mediates cardiovascular responses to DRN stimulation. Studies were performed in adult male Sprague-Dawley rats. Horseradish peroxidase or choleratoxin B was injected unilaterally or bilaterally into the RVL. Levels of 5-HT, its precursors L-tryptophan and 5-hydroxytryptophan and the metabolite 5-hydroxyindole acetic acid were measured in the ventral medulla by HPLC three weeks following placement of electrolytic lesions in DRN. Serotonin transporter (3H-cyanoimipramine binding) was quantified by autoradiography in DRN-lesioned animals. Horseradish peroxidase or choleratoxin B injections into the medulla at the level of the RVL resulted in retrogradely labeled neurons bilaterally, with ipsilateral predominance, in the DRN. Labeled cells were preponderant in rostral ventrolateral portions of the DRN, but were also observed in the dorsal, lateral and interfascicular DRN subnuclei; fewer neurons were observed in caudal portions of the DRN. Three weeks following placement of electrolytic lesions in the DRN, the concentrations of 5-HT and 5-hydroxyindole acetic acid, but not L-tryptophan or 5-hydroxytryptophan, were reduced in the medulla by 45 and 48%, respectively, compared to sham-operated or unoperated controls. DRN lesions reduced binding to the 5-HT transporter in the RVL by approximately 30% compared to unlesioned controls. Unilateral lesions of the RVL reduced the evoked blood pressure response by 53+/-15%; bilateral RVL lesions reduced the response by 86+/-9%. The increase in cortical blood flow elicited by DRN stimulation was unchanged after unilateral or bilateral RVL lesions. These studies demonstrate that there is a descending serotonergic projection from the DRN to the RVL. This projection may mediate autonomic changes elicited by DRN stimulation.

Nucleus raphe obscurus (nRO) regulation of anorectal motility in rats

Previous research has demonstrated that anorectal contractions in the rat are modulated by activation of spinal autonomic circuits. In the present study, anterograde tracing of descending pathways originating from the caudal nucleus raphe obscurus (nRO) revealed that this nucleus projects to cells within the intermediolateral (IML) cell column of the thoracic cord and the sacral parasympathetic nucleus (SPN). These anatomical studies suggested that the nRO may influence the regulation of spinal reflexes of the pelvic floor. In a second set of experiments, acute rat preparations were used to investigate changes in anorectal motility during electrical stimulation of the nRO. Anorectal contractions were measured by a fluid-filled manometer. Electrical stimulation of the nRO significantly reduced spontaneous anorectal activity when compared to baseline contractions recorded for 1 min prior to stimulation. Stimulation sites outside the nRO did not affect anorectal contractions when compared to either (a) the 1-min pre-stimulation baseline for that site or (b) the 1-min stimulation period for sites within the nRO. Stimulation of caudal portions of the nRO were more likely than the rostral nRO to reduce anorectal contractions. Given that the SPN contains preganglionic neurons which may be involved in control of anorectal contractions (mediated via the pelvic nerve), the studies presented here suggest a functional role for nRO regulation of preganglionic motoneurons innervating the distal gut of the rat.

Spinal control of penile erection

Smooth muscle relaxation of penile arteries, the corpus cavernosum, and the corpus spongiosum, leading to penile erection, results from parasympathetic neural pathway activation and, likely, simultaneous inhibition of sympathetic outflow. Proerectile parasympathetic outflow is reflexively activated by sensory information of peripheral origin, conveyed by the dorsal penile nerve, and reflexive erections are supported by an intraspinal circuitry. Supraspinal influences modulate the reflex. Information integrated at or originating from supraspinal structures may also elicit penile erection. Several neurotransmitters are involved in either the modulation of the spinal reflex or the mediation of supraspinal influences. Spinal cord injury differently alters reflexive penile erection or erection from a central origin, depending on the neurologic level of injury.

Differential effects of intrathecal thyrotropin-releasing hormone (TRH) on perineal reflexes in male rats

The effects of thyrotropin-releasing hormone (TRH) on the sexual and defecatory reflexes regulated by pudendal motoneurons were investigated. Intrathecal TRH (10 microliters volume; 0.0, 0.01, 1.0 or 100 microM concentration) at lumbosacral spinal segments (L4-S1) in acute preparations produced a dose-dependent increase in external anal sphincter (EAS), but not bulbospongiosus (BS), electromyographic (EMG) activity. Intraspinal (L6) injection of 100 microM TRH (1 microliter/micropipette), significantly increased EAS EMG activity in acute preparations. Electromyographic activity of the BS muscle was unchanged. All doses of intrathecal TRH (10 microliters volume; 0, 10, 50, 100, or 500 microM concentration) in awake animals significantly reduced the proportion of responders to a penile reflex test. Subsequently, all measures of penile reflexes were significantly reduced. Glans tumescence and defecation bouts before or during penile reflex testing were unaffected by intrathecal TRH as were indices of behavioral and motor hyper-reactivity analogous to that produced by serotonin. These data indicate that pudendal motoneurons, in the dorsomedial nucleus, are differentially regulated by neuropeptides present in the lumbosacral spinal cord.

Peptidergic transmission: from morphological correlates to functional implications

Like non-peptidergic transmitters, neuropeptides and their receptors display a wide distribution in specific cell types of the nervous system. The peptides are synthesized, typically as part of a larger precursor molecule, on the rough endoplasmic reticulum in the cell body. In the trans-Golgi network, they are sorted to the regulated secretory pathway, packaged into so-called large dense-core vesicles, and concentrated. Large dense-core vesicles are preferentially located at sites distant from active zones of synapses. Exocytosis may occur not only at synaptic specializations in axonal terminals but frequently also at nonsynaptic release sites throughout the neuron. Large dense-core vesicles are distinguished from small, clear synaptic vesicles, which contain "classical' transmitters, by their morphological appearance and, partially, their biochemical composition, the mode of stimulation required for release, the type of calcium channels involved in the exocytotic process, and the time course of recovery after stimulation. The frequently observed "diffuse' release of neuropeptides and their occurrence also in areas distant to release sites is paralleled by the existence of pronounced peptide-peptide receptor mismatches found at the light microscopic and ultrastructural level. Coexistence of neuropeptides with other peptidergic and non-peptidergic substances within the same neuron or even within the same vesicle has been established for numerous neuronal systems. In addition to exerting excitatory and inhibitory transmitter-like effects and modulating the release of other neuroactive substances in the nervous system, several neuropeptides are involved in the regulation of neuronal development.

Thyrotropin-releasing hormone (TRH) and CNS regulation of anorectal motility in the rat

The effect of thyrotropin-releasing hormone (TRH) upon anorectal motility was investigated in acute male rat preparations. Micromolar doses of TRH were intrathecally (i.t.) infused at the L6 spinal level at a rate of 1 microliter/min over 8 min. TRH infusions in 1.0-1000 microM concentrations elicited biphasic, dose-dependent anorectal contractions as measured by a rectal manometer. The 100 microM dose yielded the most significant increase in contractions over the greatest period of time. Atropine, administered as a pretreatment (100 micrograms s.c.), blocked contractions normally produced by i.t. infusion of TRH (1000 microM). Intravenous infusions of atropine (10 micrograms) through a jugular catheter immediately blocked anorectal contractions produced by i.t. infusion of 100 microM TRH. Sectioning of the hypogastric nerve, which supplies sympathetic innervation to the colon and internal anal sphincter, did not significantly affect contractions induced by 100 microM TRH applied intrathecally. Disruption of the major pelvic ganglion fibers, however, completely abolished the contractions induced by 100 microM TRH, either through the interruption of preganglionic parasympathetic fibers in the pelvic nerve, or by disrupting postganglionic fibers. These findings extend the role of TRH in the regulation of defecatory behaviors.

Antisense to thyrotropin releasing hormone receptor reduces arterial blood pressure in spontaneously hypertensive rats

We report in the present study the effect of intrathecal treatment with antisense oligonucleotides complementary to thyrotropin releasing hormone (TRH) receptor mRNA on the pressor response to intrathecal administration of TRH and on resting arterial blood pressure in Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). In 16-week-old male WKY rats, 18-base phosphodiester antisense or mismatch oligonucleotides to TRH receptor mRNA (100 micrograms per day) were injected intrathecally for 3 days. Twenty-four hours after the last injection, the magnitude of the pressor response to intrathecal TRH (10 micrograms) was significantly smaller in the antisense-treated group (n = 7) compared with mismatch-treated controls (n = 7) (change in mean arterial pressure, +20.3 +/- 3.0 versus +32.6 +/- 2.5 mm Hg, P < .01). No differences were observed in the pressor responses to injection of N-methyl-D-aspartic acid. Resting arterial blood pressure was unaffected by antisense treatment in WKY rats. In separate experiments, 16-week-old male SHR were treated with antisense (n = 7) or mismatch (n = 6) oligonucleotides for 3 days. Mean resting arterial blood pressure was significantly reduced by treatment with antisense oligonucleotides (from 157 +/- 4.8 to 119 +/- 8.8 mm Hg, P < .01), but no significant changes were observed in mismatch-treated animals. Our results suggest that the expression of TRH receptors in spinal sympathetic preganglionic neurons can be selectively reduced by intrathecal treatment with antisense oligonucleotides and that TRH projections to sympathetic preganglionic neurons play an important role in the elevation of arterial blood pressure in SHR.

Transneuronal labeling of CNS neuropeptide and monoamine neurons after pseudorabies virus injections into the stellate ganglion

The viral transneuronal labeling method was used in combination with immunohistochemical procedures to identify CNS neuropeptide and monoamine neurons that innervate the sympathetic preganglionic neurons (SPNs) which project to the stellate ganglion--the principal source of the sympathetic supply to the heart. Transneuronal labeling was found at three CNS levels: spinal cord, brainstem, and hypothalamus. In the thoracic spinal cord, apart from the pseudorabies virus (PRV)-labeled stellate SPNs, PRV-labeled neurons were localized in laminae I/II, IV, V, VII, and X as well as in the lateral spinal nucleus and lateral funiculus. In the C1-C4 spinal segments, labeled neurons were found in the lateral funiculus as well as laminae V and VII of the spinal gray matter. PRV-labeled cells were identified in lamina V and the dorsolateral funiculus of the lumbar spinal cord. Three medullary areas were consistently labeled: rostral ventromedial medulla (RVMM), rostral ventrolateral medulla (RVLM), and caudal raphe nuclei. The greatest concentration of labeling was found in the RVMM. This projection arose from adrenergic, serotonergic (5-HT), thyrotropin releasing hormone (TRH), substance P, somatostatin, enkephalin, and vasoactive intestinal peptide (VIP) immunoreactive neurons. The RVLM projection originated mainly from C1 adrenergic neurons, some of which contained immunoreactive neuropeptide Y (NPY). C3 adrenergic-NPY neurons lying near the floor of the 4th ventricle were also labeled. Enkephalin-, somatostatin- and VIP-immunoreactive RVLM neurons also contributed to this projection. 5-HT neurons of the caudal raphe nuclei (raphe pallidus, raphe obscurus, and raphe magnus) were labeled; some of these contained substance P or TRH-immunoreactivity with an occasional neuron staining for all three putative neurotransmitters. In the pons, catecholamine neurons in the A5 cell group, subcoeruleus and Kolliker-Fuse nuclei were labeled. The midbrain contained relatively few infected cells, but some were present in the Edinger-Westphal and precommissural nuclei. Forebrain labeling was concentrated in the paraventricular hypothalamic nucleus (PVN) with lesser amounts in the lateral hypothalamic area (LHA) and the perifornical region. In the PVN, oxytocin-immunoreactive neurons accounted for the greatest chemically-defined projection while corticotrophin releasing factor (CRF), vasopressin-, and angiotensin II-immunoreactive neurons provided successively lesser inputs. In the LHA, angiotensin II-immunoreactive neurons were labeled. In summary, this study provides the first detailed map of the chemically-coded CNS neurons involved in the control of the cardiosympathetic outflow.

Serotonin inputs to rabbit sympathetic preganglionic neurons projecting to the superior cervical ganglion or adrenal medulla

The input from serotonin-containing nerve fibres to rabbit sympathetic preganglionic neurons projecting to either the superior cervical ganglion or the adrenal medulla was investigated by combining retrograde tracing with the B subunit of cholera toxin and immunocytochemistry for serotonin. There were pronounced rostrocaudal variations in the density of serotonin fibres in the rabbit intermediolateral cell column from T1 to L4; maximum numbers of fibres were found in T3-6 and L3-4 and minimum numbers in T1 and T10-12. By light microscopy, retrogradely labelled sympathetic preganglionic neurons projecting to the superior cervical ganglion or the adrenal medulla received variable densities of close appositions from serotonin-immunoreactive fibres. Some neurons from each population received many close appositions, whereas others received moderate numbers or few appositions. Appositions occurred on the cell bodies, dendrites, and occasionally axons of sympathetic preganglionic neurons. Rare neurons in both groups of retrogradely labelled cells received no appositions from serotonin-containing nerve fibres. At the ultrastructural level, synapses were found between serotonin-positive boutons and sympathetic preganglionic neurons projecting either to the superior cervical ganglion or to the adrenal medulla. These results indicate that, through direct synaptic contacts, serotonin-immunoreactive, presumably bulbospinal, nerve fibres affect the activity of the vast majority of sympathetic preganglionic neurons that send axons either to the superior cervical ganglion or to the adrenal medulla. This serotonin input may be sympathoexcitatory and could mediate increases in sympathetic nerve activity and in the release of catecholamines from the adrenal medulla.

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.

Response of neurons in the dorsal motor nucleus of the vagus to thyrotropin-releasing hormone

Autonomic motoneurons in the dorsal motor nucleus of the vagus (DMX) were recorded intracellularly in an in vitro slice preparation of the guinea pig brainstem. Bath-applied thyrotropin releasing hormone (TRH) (1-10 microM) induced a reversible depolarization of neurons that was typically accompanied by an increase in the spontaneous firing of the cells. In some cells, TRH induced rhythmic bursting activity. The TRH-induced depolarization occurred also in the presence of reduced Ca2+ and TTX. The response was dose-dependent over TRH concentrations of 0.1-10 microM. The TRH-induced depolarization was accompanied by an increase in input resistance. The reversal potential of this effect corresponded to that of K+. Our results indicate that TRH increases the excitability of DMX neurons by reducing a resting K+ conductance.

Opioid action on spinal cord reflexes due to dorsolateral pontine tegmentum stimulation

Electrical stimulation of the dorsolateral pontine tegmentum (DLPT) produces phasic facilitatory and inhibitory actions on the lumbar spinal monosynaptic reflexes (MSRs) of both flexor and extensor muscle nerves in the decerebrate cat. Naloxone, an opioid receptor antagonist, given intravenously or intraspinally enhanced the DLPT-induced potentiation of MSRs in most of the reflexes studied. However, systemic naloxone had no significant effect on the unconditioned MSR of the spinal cord. Intraspinal microinjections of naloxone significantly attenuated the DLPT-induced inhibition of MSRs of both flexors and extensors, similar to the action of systemic injection of naloxone, indicating a direct opioid action at the spinal ventral horn level upon DLPT stimulation. Results of the present experiment further support the anatomical finding that there are pontospinal enkephalinergic pathways in the cat, and indicate that these descending pathways modulate spinal motor outflow.

Monosynaptic projections from the rostral ventrolateral medulla oblongata to identified sympathetic preganglionic neurons

The rostral ventrolateral medulla oblongata plays an important role in the control of arterial blood pressure and it has strong descending projections into the intermediolateral nucleus of the thoracic spinal cord, where the majority of sympathetic preganglionic neurons are located. The purpose of this study was to see whether these projections form synaptic contacts with sympathetic preganglionic neurons in the rat. Projections from both the lateral part of the rostral ventrolateral medulla (rostroventrolateral reticular nucleus) and from the more medial region (lateral paragigantocellular nucleus) were investigated separately in view of their different functional roles in sympatho-regulation and their different chemical composition. Using anterograde tract-tracing of descending medullary pathways with Phaseolus vulgaris leucoagglutinin and retrograde labelling of sympatho-adrenal preganglionic neurons with cholera B chain conjugated to horseradish peroxidase, the existence of monosynaptic connections was sought by electron microscopy. Synaptic inputs from both the lateral and medial aspects of the rostral ventrolateral medulla oblongata were found on identified sympathetic preganglionic neurons. Synaptic specializations were of both the symmetrical and asymmetrical type. The targets of boutons forming asymmetrical synaptic contacts differed according to their origin: boutons originating from neurons in the rostroventrolateral reticular nucleus were mainly in contact with dendrites of sympathetic preganglionic neurons, while those originating from the lateral paragigantocellular nucleus mainly innervated the cell bodies. Our observations provide anatomical support for the view that there are two distinct classes of sympatho-regulatory cells in the rostral ventrolateral medulla, each of which can directly influence the activity of sympathetic preganglionic neurons; they also emphasize the importance of detailed investigation of the subregions of the ventrolateral medulla with respect to their sympatho-regulatory functions.

Distribution of thyrotropin-releasing hormone receptor messenger RNA in the rat brain: an in situ hybridization study

Based on the recent cloning of the mouse thyrotropin-releasing hormone receptor, oligonucleotide probes complementary to the DNA sequence were constructed and used for in situ hybridization studies on the rat brain. Thyrotropin-releasing hormone receptor messenger RNA was found in many areas of the brain, mostly showing high degree of overlap with the distribution thyrotropin-releasing hormone binding sites as previously revealed in autoradiographic studies. Thus, a strong signal was observed in the accessory olfactory bulb, the perirhinal sulcus, the ventral aspects of the hippocampal formation, some amygdaloid nuclei, the diagonal band nucleus, parts of nucleus accumbens, the bed nucleus of the stria terminalis, dorsomedial, lateral and perifornical hypothalamic regions, the septohippocampal nucleus, parts of the vestibular complex, as well as many bulbar motoneurons including the facial, dorsal vagal, ambiguus and hypoglossal nuclei, the superficial layer of the spinal trigeminal nucleus, and motoneurons and dorsal horn neurons in the spinal cord. Cells within one and the same nucleus expressed varying levels of thyrotropin releasing hormone receptor messenger RNA suggesting marked differences in rate of receptor synthesis. Most of these areas receive an input by thyrotropin-releasing hormone-positive nerve endings. Taken together these results suggest that thyrotropin-releasing hormone receptors are mostly localized in the vicinity of the cell bodies which express thyrotropin-releasing hormone receptor messenger RNA and mediate the wide range of actions that have been recorded after administration of exogenous thyrotropin-releasing hormone.

Thyrotropin-releasing hormone (TRH)-like immunoreactivity in the grey monkey (Macaca fascicularis) spinal cord and medulla oblongata with special emphasis on the bulbospinal tract

The distribution of thyrotropin-releasing hormone (TRH)-like immunoreactivity (LI) has been studied in the grey monkey (Macaca fascicularis) spinal cord and medulla oblongata by the use of indirect immunofluorescence and the peroxidase-antiperoxidase (PAP) technique. Furthermore, double-labeling experiments were performed in order to study colocalization of 5-hydroxytryptamine (5-HT)- and substance P-LI. A dense innervation of TRH-immunoreactive (IR) varicose fibers was found in the ventral horn motor nuclei, in the region surrounding the central canal, in the intermediolateral cell column, and in the dorsal horn laminae II and III. In addition, cell bodies harboring TRH-LI were found in the dorsal horn laminae II-IV. In the ventral horn, many of the large cell bodies and their proximal dendrites were totally encapsulated by TRH-IR fibers. From double-labeled sections a high degree of coexistence could be established between TRH-/5-HT-LI, TRH-/substance P-LI, and 5-HT-/substance P-LI in fibers in the motor nuclei; as a consequence, a large proportion of these fibers should harbor TRH-/5-HT-/substance P-LI. A coexistence between TRH-/5-HT-LI could also be demonstrated in the intermediolateral cell column. However, no unequivocal coexistence could be found between TRH-/substance P-LI and 5-HT-/substance P-LI in this region. In the dorsal horn, no clear coexistence could be encountered for any of the above indicated combinations. Electron microscopic analysis of material from the lumbar lateral motor nucleus demonstrated TRH-IR terminals making synapses with large cell bodies and dendrites. In addition, contacts lacking synaptic specializations could also be verified. In the medulla oblongata, with the use of the PAP technique, a large number of cell bodies containing TRH-LI were encountered in the midline raphe nuclei and in nucleus reticularis lateralis. A similar distribution pattern could be found for 5-HT-LI, but no cell bodies containing substance P-LI could be seen in these regions. Chemical analysis of specimens from cervical, thoracic, and lumbar spinal cord revealed higher concentrations of TRH- and 5-HT-LI in the ventral quadrants, whereas substance P-LI dominated in the dorsal quadrants. Thus, the concentrations of TRH-, 5-HT-, and substance P-LI was in accordance with the observed regional variation in density of IR-fibers and varicosities found in the spinal cord. We have shown that TRH-LI has a distribution in the monkey spinal cord and medulla oblongata similar to that previously demonstrated in other species.(ABSTRACT TRUNCATED AT 400 WORDS)

Some regional anatomical relationships of TRH to 5-HT in rat limbic forebrain

It is now a recognized principle that various neuropeptides are neuronally co-localized with biogenic amine or aminoacid neurotransmitters. In the rat CNS it has previously been shown that TRH is co-localized with 5-HT (and also with substance P) in cell bodies of the posterior raphe that project to the spinal cord. Although TRH cell bodies are known to be widely distributed throughout the forebrain there is no other known co-localization with 5-HT. In this study we further specify the forebrain there is no other known co-localization with 5-HT. In this study we further specify the anatomical relationship of TRH with 5-HT by use of surgical and neurotoxic lesioning with reference to limbic forebrain regions wherein TRH is greatly increased following seizures. In groups of rats, the fimbria-fornix was lesioned alone, or combined with a lesion of the dorsal perforant path or the ventral perforant path. There was a sham lesioned control group. Additional groups were lesioned with 5,7 dihydroxytryptamine, 100 micrograms i.v.t., 45 min. after i.p. desipramine, 25 mg/kg. All rats were sacrificed three weeks after lesions. Indoleamines were determined by HPLC in left anterior cortex, left pyriform/olfactory cortex, left dorsal hippocampus and left ventral hippocampus. TRH was determined by specific RIA in the corresponding right brain regions. The modal n was 7 rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Rostrocaudal differences in morphology and neurotransmitter content of cells in the subretrofacial vasomotor nucleus

The rostral ventrolateral medulla (RVLM) contains sympathoexcitatory neurons that exert a powerful control over the sympathetic outflow to the cardiovascular system. In the cat there is a concentration of such neurons (but not neurons subserving other functions) within a narrow longitudinal column in the RVLM termed the subretrofacial (SRF) nucleus. Furthermore, it has been suggested that there are subgroups of cells, located at different rostrocaudal levels of the SRF nucleus, that preferentially or exclusively control different vascular beds (e.g. in the kidney and hindlimb). The aim of this study was to map quantitatively the rostrocaudal distribution within the nucleus of different cell types, defined according to morphological and/or chemical criteria, and to correlate this with the regional vasomotor effects (in hindlimb and kidney) evoked by stimulation of SRF cells at the corresponding rostrocaudal levels. SRF cells were highly heterogeneous with respect to both their morphology and chemical properties. They varied greatly in size (equivalent diameter ranging from 10-40 microns) as well as in shape and orientation. An immunohistochemical examination using the avidin-biotin procedure revealed that many SRF cells (estimated 57% of all SRF cells) were immunoreactive for tyrosine hydroxylase (TH, a marker of catecholamine cells). In addition, there were SRF cells immunoreactive for neuropeptide Y (NPY, 11% of total), enkephalin (ENK, 16% of total), and serotonin (5HT, 10% of total), but not for substance P, galanin or somatostatin. Different cell types, defined according to their morphology and/or chemical properties, were unevenly distributed throughout the nucleus. In the most caudal part of the SRF nucleus, virtually all cells were TH-positive, and the large majority (estimated 80%) were NPY-positive, suggesting that many cells at this level contained both TH and NPY. In contrast, in the most rostral part of the SRF nucleus, only 30% of cells were TH-positive, and no NPY-positive cells were observed. Both 5HT- and ENK-positive cells were found throughout the rostrocaudal extent of the nucleus, but predominantly within its rostral part. Furthermore, TH-positive cells in the rostral SRF nucleus were on average significantly larger (mean equivalent diameter 18-43% greater) than TH/NPY-positive cells in the caudal part of the nucleus, but smaller than 5HT- or ENK-positive cells at the same level. Overall, rostral cells (regardless of their chemical type) were larger than caudal cells within the SRF nucleus (mean equivalent diameter 13-28% greater).(ABSTRACT TRUNCATED AT 400 WORDS)

Distribution of thyrotropin-releasing hormone in the rat spinal cord with special reference to sympathetic nuclei: a light- and electron-microscopic immunocytochemical study

This paper deals with the distribution of thyrotropin-releasing hormone-like immunoreactivity in the spinal cord of the rat, and particularly in the sympathetic nuclei, at light and electron microscopic levels. In the dorsal horn, the inner part of laminae II and III displayed thin thyrotropin-releasing hormone immunoreactive profiles. Electron microscopy revealed small immunoreactive varicosities which made synaptic contact with small dendrites or dendritic spines. Dense thyrotropin-releasing hormone-like immunoreactivity was observed in all sympathetic nuclei (nucleus intermediolateralis pars fascicularis and principalis, nucleus intercalatus and dorsal commissural nucleus) except the nucleus intercalatus pars ependymalis. Electron microscopy showed many immunoreactive varicosities which were often in synaptic contact with dendrites (proximal or distal), rarely with perikarya and never with axons. Sometimes, the same immunoreactive varicosity made axodendritic contacts with two dendrites and, conversely one dendrite was sometimes synaptically contacted by two or more immunoreactive varicosities. The ventral horn displayed a diffuse thyrotropin-releasing hormone-like immunoreactivity except for the cremaster nucleus (at lumbar level) which was densely outlined by immunoreactive profiles. Occasionally a large cell body in lamina IX (a putative motoneuron) was outlined by immunoreactive profiles but ultrastructural studies revealed very few immunoreactive axosomatic synapses, while immunoreactive symmetrical or asymmetrical axodendritic synapses were observed. The present study clearly confirms the existence of thyrotropin-releasing hormone immunoreactive synapses, thus substantiating the physiological role of this hormone in the spinal cord.

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.

Spinal cord substance P mediates the inhibition of gastric acid secretion induced by electrical stimulation of the preoptic area

A possible role of spinal substance P (SP) in the mediation of signals to inhibit gastric acid secretion by central activation of the sympatho-adrenomedullary system was examined in urethane-anesthetized rats. Intrathecal (i.t.) administration of SP (1-10 nmol) inhibited vagally induced acid output. I.t. administration of spantide, a SP receptor antagonist, reduced the inhibitory effect of 3 nmol SP. I.t. administration of spantide (0.1-1 nmol) blocked the inhibition of vagally induced gastric acid output evoked by electrical stimulation of the preoptic area. Atropine, hexamethonium, phentolamine, propranolol, DL-para-chlorophenylalanine (PCPA) and 5,7-dihydroxytryptamine (5,7-DHT) were without effect. Repeated i.t. administration of 10 nmol SP produced desensitization to the SP-induced inhibitory response on gastric acid output. In these animals, electrical stimulation of the preoptic area did not inhibit vagally induced gastric acid output. These results suggest that electrical stimulation of the preoptic area excites SP-containing neurons in the spinal cord, and a resultant sympatho-adrenomedullary system-mediated inhibition of gastric acid secretion occurs.

Serotoninergic denervation suppresses the sympathetic outflow induced by thyrotropin-releasing hormone in conscious rats

Intrathecal administration of thyrotropin-releasing hormone (TRH) resulted in an increase in plasma epinephrine (E) and glucose levels in conscious rats. To elucidate the mechanisms by which TRH stimulates the release of E, the animals were pretreated with 5,7-dihydroxytryptamine, parachlorophenylalanine, methysergide or ketanserin. Serotoninergic denervation suppressed the TRH-induced increase in plasma E and glucose levels, whereas inhibition of serotonin (5-HT) synthesis or blockade of 5-HT receptors did not suppress the responses. These findings suggest that the serotoninergic neurons, but not 5-HT itself, are involved in stimulating the sympathetic outflow by TRH at the spinal level.

Effects of intrathecal administration of thyrotropin releasing hormone and its analogue, DN1417, on plasma glucose and catecholamine levels in conscious rats

Effects of intrathecal (i.t.) administration of TRH and the TRH analogue, DN1417, on plasma catecholamines and glucose levels were studied in conscious male rats. The i.t. administration of TRH (0.6 and 3 nmol) at the T8-10 vertebral level resulted in a dose-related increase in epinephrine (E), norepinephrine (NE), and glucose levels, which was suppressed by prior administration of the ganglionic blocker, hexamethonium (1.5 mg/100 g b. wt.). I.t. administration of TRH (3 nmol) caused small increases in plasma E and glucose at the C7-T1 vertebral level, but it did not change plasma E, NE and glucose levels at the sacral level. DN1417 (3 nmol) administered i.t. at the T8-10 or C7-T1 vertebral level had a more potent and long-lasting effect in stimulating the release of E than TRH. These findings suggest that TRH may act on sympathetic preganglionic neurons at the T7-10 spinal levels and stimulate the release of catecholamines from the adrenal medulla.

Bulbospinal and intraspinal thyrotropin releasing hormone systems: modulation of spinal cord pain transmission

Recent research indicates that pain processing by spinal nociceptive neurons is modulated by: 1) descending bulbospinal pathways originating in the rostral ventral medulla (RMV) and 2) short intraspinal peptide systems located in the dorsal horn. Immunohistochemical studies have identified both bulbospinal and intraspinal thyrotropin releasing hormone (TRH) systems, however, the role of these two TRH systems in pain modulation has not been defined. The purpose of the present study was to explore the role of the bulbospinal and intraspinal TRH systems in pain modulation. Three TRH mediated neural network were examined in electrophysiologic experiments; each model predicting specific testable outcomes. Model A represented a direct TRH projection from RVM to spinal cord. This model predicted that the effect of RVM stimulation and TRH micropressure administration on dorsal horn unit activity would be in the same direction, either both inhibitory effects or both excitatory. Of 44 dorsal horn units inhibited by RVM stimulation, 82% were excited by TRH, therefore, Model A was rejected. Model B interposed an intraspinal neuron between the bulbospinal TRH projection and the dorsal horn nociceptive unit. This model predicted that desensitization of spinal TRH receptor systems should block the effect of RVM stimulation on dorsal horn neuron. In TRH desensitization experiments, no attenuation or blockade of RVM stimulation was observed, therefore, the effect of RVM stimulation did not appear mediated by TRH and Model B was rejected.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of neurotransmitters in the central regulation of the cardiovascular system

The last decade has seen tremendous progress in determining the nature of the neurotransmitters which regulate central nervous system pathways involved in the regulation of blood pressure. Investigations are now pursuing the identity and functional importance of neurotransmitters contained within pathways shown to be important in cardiovascular regulation. In addition, several key components of the brain stem networks involved in the control of sympathetic activity have been identified. For example, numerous studies indicate the importance of neurons located in the rostral ventrolateral medulla in the regulation of SPN. Indeed, this area contains medullospinal sympathoexcitatory neurons which represent the final site of integration of many brain stem and reflex pathways involved in the regulation of sympathetic nerve activity. The neurotransmitter which is utilized by this medullospinal pathway remains unknown. Epinephrine, substance P and glutamate have all been hypothesized as primary chemical mediators in the descending pathway from the brain stem to SPN. Interestingly, lesions of, or antagonists to, epinephrine, substance P, glutamate and 5-HT neurons all abolish sympathetic activity and reduce blood pressure to a level similar to that in a spinal animal. Clearly, not all these transmitters are primary mediators of sympathetic information carried from the brain stem to the spinal cord. It is likely that monoamines and neuropeptides act in the IML, as in other area of the central nervous system, as neuromodulators to set the level of excitability of SPN rather than relaying sympathetic information over a functionally specific medullospinal pathway. This conclusion is supported by the observation that midline medullary 5-HT neurons provide a tonic excitatory input to SPN, but receive no afferent inputs from other central sympathetic or baroreceptor pathways. However, the firing of 5-HT neurons appears to relate to the state of vigilance of the animal. This suggests that 5-HT neurons may lower the threshold of SPN to sympathetic inputs during states of wakefulness. In addition, the time course of the norepinephrine-mediated slow EPSPs and IPSPs in SPN is consistent with a gain-setting function. By analogy, epinephrine is likely to act as a neuromodulator in the IML rather than to serve as the primary mediator of sympathetic information descending from the rostral ventrolateral medulla.(ABSTRACT TRUNCATED AT 400 WORDS)

5-Hydroxytryptamine, substance P, and thyrotropin-releasing hormone in the adult cat spinal cord segment L7: immunohistochemical and chemical studies

The terminal projections of the descending 5-hydroxytryptamine (5-HT) bulbospinal pathway and the coexistence among 5-HT-, substance P (SP)-, and thyrotropin-releasing hormone (TRH)-like immunoreactivities (LI) in fibers innervating the L7 segment in the cat spinal cord were studied quantitatively and semiquantitatively by use of the indirect double-staining immunofluorescence technique. The content of 5-HT, SP, and TRH in different parts of the spinal cord was determined by use of radioimmunoassay (RIA) (SP and TRH) and high-performance liquid chromatography with electrochemical detection (HPLC-ECD) (5-HT). For all three substances studied, immunoreactive (IR) axon terminals were found in all parts of the gray matter, but with clear regional variation in the density of innervation. Thus, all three substances showed a dense innervation in the motor nucleus, particularly in the ventral part of the nucleus, while the superficial dorsal horn was very densely innervated by SP-IR fibers (laminae I and II) and TRH-IR fibers (laminae II and III). In the motor nucleus, the studied substances coexisted to a very high degree, but some 5-HT-IR fibers (about 10%) lacked peptide-LI and some SP-IR fibers (about 10%) lacked 5-HT-LI while virtually all TRH-IR fibers also contained 5-HT-LI. In the superficial dorsal horn (laminae I-III), no coexistence was detected, while other parts of the gray matter displayed various degrees of coexistence in between those found in the motor nucleus and laminae I-III. The quantitative analysis of IR varicosities in the motor nucleus suggested that the unilateral L7 motor nucleus is innervated by about 55-110 x 10(6) 5-HT-IR nerve terminals, which may indicate as many as 4,000 boutons per descending 5-HT cell body in the brain stem only with this restricted projection. When combing these results with the biochemical data, it could be calculated that the concentration of 5-HT in IR varicosities is about 3-6 x 10(-3) M, while the corresponding figures for SP and TRH was 0.3-0.5 x 10(-3) M and 0.1-0.2 x 10(-3) M, respectively. In cats subjected to spinal cord transection at the lower thoracic level, all 5-HT-IR fibers in the L7 segment had disappeared 44 days after the lesion, indicating a strict suprasegmental origin of 5-HT-IR fibers in this segment.(ABSTRACT TRUNCATED AT 400 WORDS)

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

The coexistence of varying combinations of substance P (SP), somatostatin (SOM), thyrotropin-releasing hormone (TRH) and met-enkephalin-Arg-Gly-Leu (ENK) with 5-hydroxytryptamine (5-HT) as semiquantitatively revealed by immunocytochemistry in neuronal perikarya of the raphe pallidus et obscurus in the guinea-pig was analyzed. SOM coexisted most frequently with 5-HT, followed by SP, ENK and TRH. Many 5-HT neurons were immunoreactive to 2 or more peptides such as SP/SOM, SOM/ENK, SP/ENK, SOM/TRH, SP/TRH or SOM/SP/ENK. Most of these neurons were shown to project to the spinal cord by retrograde HRP labeling combined with immunocytochemistry. After hemisection of the cervical spinal cord at the C5 level, ENK and 5-HT immunoreactive nerve terminals in the ipsilateral intermediolateral nucleus of the thoracic spinal cord were decreased in number. The results indicate that neurons in the raphe pallidus et obscurus projecting to the spinal cord can be classified into subpopulations according to which peptides coexist with 5-HT, and may have different functions.

Enkephalin-immunoreactive neuronal projections from the medulla oblongata to the intermediolateral cell column: relationship to substance P-immunoreactive neurons

The present study investigated the ventral medullary distribution of enkephalin-immunoreactive neurons that project to the intermediolateral cell column and the relationship of these neurons to substance P-immunoreactive neurons. Neurons that projected to the intermediolateral cell column were identified by the presence of rhodamine-labeled microspheres within the neuronal cell body after an injection of the microspheres into the intermediolateral cell column of the third thoracic spinal cord segment. Enkephalin- and substance P-immunoreactivities were identified by dual-color immunohisto-chemistry. Enkephalin-immunoreactive neurons that projected to the intermediolateral cell column were present in the raphe magnus, the nucleus reticularis magnocellularis pars alpha, the paragigantocellular reticular nucleus, and the parapyramidal region. These neurons were present throughout the rostrocaudal extent of each of these nuclei. However, in the raphe magnus the greatest number was present at more rostral levels of the nucleus. The morphology and distribution of enkephalin-immunoreactive neurons that projected to the intermediolateral cell column were similar to those of enkephalin-immunoreactive neurons that were not observed to contain rhodamine-labeled microspheres. Substance P- and enkephalin-immunoreactive neurons that projected to the intermediolateral cell column were present in similar distributions in each of the nuclei studied, except the raphe magnus. The raphe magnus contained more enkephalin- than substance P-immunoreactive neurons at rostral levels and more substance P-immunoreactive neurons than enkephalin-immunoreactive neurons at caudal levels. Coexistence of substance P- and enkephalin-immunoreactivities in ventral medullary neurons that projected to the intermediolateral cell column was rarely seen. These studies support the hypothesis that ventral medullary enkephalinergic neurons project to the intermediolateral cell column where they could act to modulate preganglionic sympathetic activity.

CNS cell groups regulating the sympathetic outflow to adrenal gland as revealed by transneuronal cell body labeling with pseudorabies virus

The CNS cell groups that innervate the sympathoadrenal preganglionic neurons of rats were identified by a transneuronal viral cell body labeling technique combined with neurotransmitter immunohistochemistry. Pseudorabies virus was injected into the adrenal gland. This resulted in retrograde viral infections of the ipsilateral sympathetic preganglionic neurons (T4-T13) and caused retrograde transneuronal cell body infections in 5 areas of the brain: the caudal raphe nuclei, ventromedial medulla, rostral ventrolateral medulla, A5 cell group, and paraventricular hypothalamic nucleus (PVH). In the spinal cord, the segmental distribution of virally infected neurons was the same as the retrograde cell body labeling observed following Fluoro-gold injections in the adrenal gland except there was almost a 300% increase in the number of cells labeled and a shift in cell group distribution. These results imply there are local interneurons that regulate the sympathoadrenal preganglionic neurons. In the medulla oblongata, serotonin (5-HT)-, substance P (SP)-, thyrotropin-releasing hormone-, Met-enkephalin-, and somatostatin-immunoreactive neurons of the raphe pallidus and raphe obscurus nuclei and the ventromedial medulla were infected. In the ventromedial and rostral ventrolateral medulla, immunoreactive phenylethanolamine-N-methyltransferase, SP, neuropeptide Y, somatostatin, and enkephalin neurons were infected. The A5 noradrenergic cells were labeled, as were some somatostatin-immunoreactive neurons in this area. In the were infected. The A5 noradrenergic cells were labeled, as were some somatostatin-immunoreactive neurons in this area. In the hypothalamus, tyrosine hydroxylase- and SP-immunoreactive neurons of the dorsal parvocellular PVH were infected. Only a few immunoreactive vasopressin, oxytocin, Met-enkephalin, neurotensin, and somatostatin PVH neurons were labeled.