Afferent connections of the caudal raphe pallidus nucleus in rats: a study using the fluorescent retrograde tracers fluorogold and true-blue

Hypothalamic Regulation of Cardiorespiratory Functions: Insights into the Dorsomedial and Perifornical Pathways

The dorsomedial hypothalamus nucleus (DMH) plays a pivotal role in the orchestration of sympathetic nervous system activities. Through its projections to the brainstem and pontomedullary nuclei, it controls heart rate, contractility, blood pressure, and respiratory activity, such as timing and volumes. The DMH integrates inputs from higher brain centers and processes these signals in order to modulate autonomic outflow accordingly. It has been demonstrated to be of particular significance in the context of stress responses, where it orchestrates the physiological adaptations that are necessary for all adaptative responses. The perifornical region (PeF), which is closely associated with the DMH, also makes a contribution to autonomic regulation. The involvement of the PeF region in autonomic control is evidenced by its function in coordinating the autonomic and endocrine responses to stress, frequently in conjunction with the DMH. The DMH and the PeF do not function in an isolated manner; rather, they are components of a comprehensive hypothalamic network that integrates several autonomic responses. This neural network could serve as a target for developing therapeutic strategies in cardiovascular diseases.

Systemic serotonin inhibits brown adipose tissue sympathetic nerve activity via a GABA input to the dorsomedial hypothalamus, not via 5HT1A receptor activation in raphe pallidus

AIM

Serotonin (5-hydroxytryptamine, 5-HT), an important neurotransmitter and hormone, modulates many physiological functions including body temperature. We investigated neural mechanisms involved in the inhibition of brown adipose tissue (BAT) sympathetic nerve activity (SNA) and BAT thermogenesis evoked by 5-HT.

METHODS

Electrophysiological recordings, intravenous (iv) injections and nanoinjections in the brains of anaesthetized rats.

RESULTS

Cooling-evoked increases in BAT SNA were inhibited by the intra-rostral raphé pallidus (rRPa) and the iv administration of the 5-HT_1A receptor agonist, 8-OH-DPAT or 5-HT. The intra-rRPa 5-HT, the intra-rRPa and the iv 8-OH-DPAT, but not the iv 5-HT-induced inhibition of BAT SNA were prevented by nanoinjection of a 5-HT_1A receptor antagonist in the rRPa. The increase in BAT SNA evoked by nanoinjection of NMDA in the rRPa was not inhibited by iv 5-HT, indicating that iv 5-HT does not inhibit BAT SNA by acting in the rRPa or in the sympathetic pathway distal to the rRPa. In contrast, under a warm condition, blockade of 5HT_1A receptors in the rRPa increased BAT SNA and BAT thermogenesis, suggesting that endogenous 5-HT in the rRPa contributes to the suppression of BAT SNA and BAT thermogenesis. The increases in BAT SNA and BAT thermogenesis evoked by nanoinjection of NMDA in the dorsomedial hypothalamus (DMH) were inhibited by iv 5-HT, but those following bicuculline nanoinjection in the DMH were not inhibited.

CONCLUSIONS

The systemic 5-HT-induced inhibition of BAT SNA requires a GABAergic inhibition of BAT sympathoexcitatory neurones in the DMH. In addition, during warming, 5-HT released endogenously in rRPa inhibits BAT SNA.

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.

Paraventricular nucleus modulates autonomic and neuroendocrine responses to acute restraint stress in rats

The paraventricular nucleus of the hypothalamus (PVN) has been implicated in several aspects of neuroendocrine and cardiovascular control. The PVN contains parvocellular neurons that release the corticotrophin release hormone (CRH) under stress situations. In addition, this brain area is connected to several limbic structures implicated in defensive behavioral control, as well to forebrain and brainstem structures involved in cardiovascular control. Acute restraint is an unavoidable stress situation that evokes corticosterone release as well as marked autonomic changes, the latter characterized by elevated mean arterial pressure (MAP), intense heart rate (HR) increases and decrease in the tail temperature. We report the effect of PVN inhibition on MAP and HR responses, corticosterone plasma levels and tail temperature response during acute restraint in rats. Bilateral microinjection of the nonspecific synaptic blocker CoCl(2) (1 mM/100 nL) into the PVN reduced the pressor response; it inhibited the increase in plasma corticosterone concentration as well as the fall in tail temperature associated with acute restraint stress. Moreover, bilateral microinjection of CoCl(2) into areas surrounding the PVN did not affect the blood pressure, hormonal and tail vasoconstriction responses to restraint stress. The present results show that a local PVN neurotransmission is involved in the neural pathway that controls autonomic and neuroendocrine responses, which are associated with the exposure to acute restraint stress.

Stress- and lipopolysaccharide-induced c-fos expression and nNOS in hypothalamic neurons projecting to medullary raphe in rats: a triple immunofluorescent labeling study

Neurons in the rostral raphe pallidus (rRP) have been proposed to mediate experimental stress-induced tachycardia and fever in rats, and projections from the dorsomedial hypothalamus (DMH) may signal their activation in these settings. Thus, we examined c-fos expression evoked by air jet/restraint stress and restraint stress or by systemic administration of lipopolysaccharide (10 microg/kg and 100 microg/kg) as well as the distribution of the neuronal nitric oxide synthase (nNOS) in neurons retrogradely labeled from the raphe with cholera toxin B in key hypothalamic regions. Many neurons in the medial preoptic area and the dorsal area of the DMH were retrogradely labeled, and approximately half of those in the medial preoptic area and moderate numbers in the dorsal DMH were also positive for nNOS. Either stress paradigm or dose of lipopolysaccharide increased the number of c-fos-positive neurons and nNOS/c-fos double-labeled neurons in all regions examined. However, retrogradely labeled neurons positive for c-fos were increased only in the dorsal DMH and adjoining region in both stressed and lipopolysaccharide-treated groups, and triple-labeled neurons were found only in this area in rats subjected to either stress paradigm. Thus, hypothalamic neurons that project to the rRP and express c-fos in response to either experimental stress or systemic inflammation are found only in the dorsal DMH, and many of those activated by stress contain nNOS, suggesting that nitric oxide may play a role in signaling in this pathway.

Collateralization of the tectonigral projection with other major output pathways of superior colliculus in the rat

Dopaminergic (DA) neurons exhibit a short-latency, phasic response to unexpected, biologically salient stimuli. The superior colliculus (SC) is also sensitive to such stimuli and sends a projection directly to DA-containing regions of the ventral midbrain. Recent evidence suggests that the SC is a critical relay for transmitting short-latency visual information to DA neurons. An important question is whether the ventral midbrain is an exclusive target of tectonigral neurons, or whether the tectonigral projection is a collateral branch of other tectofugal pathways. Double-label retrograde anatomical tracing techniques were used to address this issue. Injections of either Diamidino Yellow or Fluorogold into substantia nigra pars compacta (SNc) were combined with larger injections of True Blue into one of the following efferent projections of the SC: 1) target regions of the ipsilateral ascending projection to the thalamus; 2) the crossed descending tecto-reticulo-spinal pathway; 3) target structures of the ipsilateral descending projection; and 4) the contralateral superior colliculus. Moderate numbers of double-labeled neurons were observed following combined injections into substantia nigra and individual nuclei in the thalamus (ventromedial nucleus, 21.3%; central lateral, 18.4%; parafasicular nucleus 6.0%). Much less double-labeling was associated with injections into either of the descending projections (crossed, 1.0-3.2%; uncrossed, 0.2-2.7%) or the contralateral SC (0.7-1.9%). These results suggest: i) the SC may provide a coordinated input concerning the occurrence of unpredicted sensory events to both the substantia nigra and striatum (via the thalamus); and ii) few gaze-related motor signals are simultaneously relayed to DA-containing regions of the ventral midbrain.

The dorsomedial hypothalamus: a new player in thermoregulation

Neurons in the dorsomedial hypothalamus (DMH) play key roles in physiological responses to exteroceptive (“emotional”) stress in rats, including tachycardia. Tachycardia evoked from the DMH or seen in experimental stress in rats is blocked by microinjection of the GABAA receptor agonist muscimol into the rostral raphe pallidus (rRP), an important thermoregulatory site in the brain stem, where disinhibition elicits sympathetically mediated activation of brown adipose tissue (BAT) and cutaneous vasoconstriction in the tail. Disinhibition of neurons in the DMH also elevates core temperature in conscious rats and sympathetic activity to least significant difference interscapular BAT (IBAT) and IBAT temperature in anesthetized preparations. The latter effects are blocked by microinjection of muscimol into the rRP, while microinjection of muscimol into either the rRP or DMH suppresses increases in sympathetic nerve activity to IBAT, IBAT temperature, and core body temperature elicited either by microinjection of PGE2 into the preoptic area (an experimental model for fever), or central administration of fentanyl. Neurons concentrated in the dorsal region of the DMH project directly to the rRP, a location corresponding to that of neurons transsynaptically labeled from IBAT. Thus these neurons control nonshivering thermogenesis in rats, and their activation signals its recruitment in diverse experimental paradigms. Evidence also points to a role for neurons in the DMH in thermoregulatory cutaneous vasoconstriction, shivering, and endocrine adjustments. These directions provide intriguing avenues for future exploration that may expand our understanding of the DMH as an important hypothalamic site for the integration of autonomic, endocrine, and behavioral responses to diverse challenges.

Stress-induced cardiac stimulation and fever: Common hypothalamic origins and brainstem mechanisms

Our past results provide considerable evidence that activation of neurons somewhere in the region of the dorsomedial hypothalamus (DMH) plays a key role in the generation of many of the effects typically seen in "emotional" stress in rats, including activation of the hypothalamic-pituitary-adrenal (HPA) axis, the neuroendocrine hallmark of the generalized response to stress, and sympathetically mediated tachycardia. More recently, we demonstrated that (1) the tachycardia resulting either from chemical stimulation of the DMH or from experimental stress is markedly attenuated by microinjection of the GABAA receptor agonist muscimol, a neuronal inhibitor, into the medullary raphe pallidus (RP); and (2) the specific subregion of the DMH mediating stimulation-induced tachycardia corresponds to the dorsal hypothalamic area (DHA), a site where neurons projecting to the RP are densely concentrated. Thus, the pathway from neurons in the DHA to sympathetic premotor neurons in the RP may constitute a key relay mediating the increases in heart rate seen in emotional stress--a role that had never been proposed previously for either of these regions. Instead, sympathetic premotor neurons were known to exist in the RP but had been most closely associated with sympathetic thermoregulatory mechanisms, including activation of brown fat, the principal means for nonshivering thermogenesis in rats, and cutaneous vasoconstriction in the tail, an important method of conserving body heat in this species. These sympathetic effects serve to maintain body temperature in a cold environment or to increase it in fever--and are typically accompanied by tachycardia. Interestingly, we and others have now shown that (1) disinhibition of neurons in the DMH also increases body temperature, at least in part through activation of brown fat, (2) microinjection of the neuronal inhibitor muscimol into the DMH reduces experimental fever and the associated tachycardia in rats. We hypothesize that activation of neurons in the DMH mediates both the increased body temperature and cardiac stimulation produced in rats by experimental "emotional" stress and fever, and that these effects are mediated in large part through direct projections to sympathetic premotor neurons in the RP. Thus, this pathway may constitute a common effector circuit upon which a variety of forebrain inputs converge in response to diverse environmental challenges.

Projections from bed nuclei of the stria terminalis, dorsomedial nucleus: implications for cerebral hemisphere integration of neuroendocrine, autonomic, and drinking responses

The overall projection pattern of a tiny bed nuclei of the stria terminalis anteromedial group differentiation, the dorsomedial nucleus (BSTdm), was analyzed with the Phaseolus vulgaris-leucoagglutinin anterograde pathway tracing method in rats. Many brain regions receive a relatively moderate to strong input from the BSTdm. They fall into eight general categories: humeral sensory-related (subfornical organ and median preoptic nucleus, involved in initiating drinking behavior and salt appetite), neuroendocrine system (magnocellular: oxytocin, vasopressin; parvicellular: gonadotropin-releasing hormone, somatostatin, thyrotropin-releasing hormone, corticotropin-releasing hormone), central autonomic control network (central amygdalar nucleus, BST anterolateral group, descending paraventricular hypothalamic nucleus, retrochiasmatic area, ventrolateral periaqueductal gray, Barrington's nucleus), hypothalamic visceromotor pattern-generator network (five of six known components), behavior control column (ingestive: descending paraventricular nucleus; reproductive: lateral medial preoptic nucleus; defensive: anterior hypothalamic nucleus; foraging: ventral tegmental area, along with interconnected nucleus accumbens and substantia innominata), orofacial motor control (retrorubral area), thalamocortical feedback loops (paraventricular, central medial, intermediodorsal, and medial mediodorsal nuclei; nucleus reuniens), and behavioral state control (subparaventricular zone, ventrolateral preoptic nucleus, tuberomammillary nucleus, supramammillary nucleus, lateral habenula, and raphé nuclei). This pattern of axonal projections, and what little is known of its inputs suggest that the BSTdm is part of a striatopallidal differentiation involved in coordinating the homeostatic and behavioral responses associated thirst and salt appetite, although clearly it may relate them to other functions as well. The BSTdm generates the densest known inputs directly to the neuroendocrine system from any part of the cerebral hemispheres.

Projections from bed nuclei of the stria terminalis, magnocellular nucleus: implications for cerebral hemisphere regulation of micturition, defecation, and penile erection

The basic structural organization of axonal projections from the small but distinct magnocellular and ventral nuclei (of the bed nuclei of the stria terminalis) was analyzed with the Phaseolus vulgaris leucoagglutinin anterograde tract tracing method in adult male rats. The former's overall projection pattern is complex, with over 80 distinct terminal fields ipsilateral to injection sites. Innervated regions in the cerebral hemisphere and brainstem fall into nine general functional categories: cerebral nuclei, behavior control column, orofacial motor-related, humorosensory/thirst-related, brainstem autonomic control network, neuroendocrine, hypothalamic visceromotor pattern-generator network, thalamocortical feedback loops, and behavioral state control. The most novel findings indicate that the magnocellular nucleus projects to virtually all known major parts of the brain network that controls pelvic functions, including micturition, defecation, and penile erection, as well as to brain networks controlling nutrient and body water homeostasis. This and other evidence suggests that the magnocellular nucleus is part of a corticostriatopallidal differentiation modulating and coordinating pelvic functions with the maintenance of nutrient and body water homeostasis. Projections of the ventral nucleus are a subset of those generated by the magnocellular nucleus, with the obvious difference that the ventral nucleus does not project detectably to Barrington's nucleus, the subfornical organ, the median preoptic and parastrial nuclei, the neuroendocrine system, and midbrain orofacial motor-related regions.

Regulation of thermogenesis by the central melanocortin system

Adaptive thermogenesis represents one of the important homeostatic mechanisms by which the body maintains appropriate levels of stored energy and its core temperature. Dysregulation of adaptive thermogenesis promotes obesity. The central melanocortin system, in particular the melanocortin 4 receptor (MC4R) signaling pathway, influences the regulation of every aspect of energy balance, including thermogenesis, and plays a critical role in energy homeostasis in both rodent and man. This review will outline our current understanding of adaptive thermogenesis, focusing on the role of the central melanocortin pathway in the regulation of thermogenesis.

Dorsomedial hypothalamic sites where disinhibition evokes tachycardia correlate with location of raphe-projecting neurons

Disinhibition of neurons in the region of the dorsomedial hypothalamus (DMH) elicits sympathetically mediated tachycardia in rats through activation of the brain stem raphe pallidus (RP), and this same mechanism appears to be largely responsible for the increases in heart rate (HR) seen in air jet stress in this species. Neurons projecting to the RP from the DMH are said to be concentrated in a specific subregion, the dorsal hypothalamic area (DA). Here, we examined the hypothesis that the location of RP-projecting neurons in the DA correspond to the sites at which microinjection of bicuculline methiodide (BMI) evokes the greatest increases in HR. To determine the distribution of RP-projecting neurons in the DA, cholera toxin B was injected in the RP in four rats. A consistent pattern of retrograde labeling was seen in every rat. In the hypothalamus, RP-projecting neurons were most heavily concentrated midway between the mammillothalamic tract and the dorsal tip of the third ventricle dorsal to the dorsomedial hypothalamic nucleus approximately 3.30 mm caudal to bregma. In a second series of experiments, the HR response to microinjections of BMI (2 pmol/5 nl; n = 76) was mapped at sites in the DA and surrounding areas in 22 urethane-anesthetized rats. All injection sites were located from 2.56 to 4.16 mm posterior to bregma, and the microinjections that evoked the largest increase in HR (i.e., >100 beats/min in some instances) were located in a region where RP-projecting neurons were most densely concentrated. Thus RP-projecting neurons in the DA may mediate DMH-induced tachycardia and thus play a role in stress-induced cardiac stimulation.

Excitatory amino acid receptor activation in the raphe pallidus area mediates prostaglandin-evoked thermogenesis

To investigate the role of excitatory amino acid neurotransmission within the rostral raphe pallidus area (RPa) in thermogenic and cardiovascular responses, changes in sympathetic nerve activity to brown adipose tissue (BAT), BAT temperature, expired CO(2), arterial pressure, and heart rate were recorded after microinjection of excitatory amino acid (EAA) receptor agonists into the RPa in urethan-chloralose-anesthetized, ventilated rats. To determine whether EAA neurotransmission within the RPa is necessary for the responses evoked by disinhibition of the RPa or by prostaglandin E(2) acting within the medial preoptic area, BAT sympathetic nerve activity, BAT temperature, expired CO(2), arterial pressure, and heart rate were measured during these treatments both before and after blockade of EAA receptors within the RPa. Microinjection of EAA receptor agonists into the RPa resulted in significant increases in all measured variables; these increases were attenuated by prior microinjection of the respective EAA receptor antagonists into the RPa. Microinjection of prostaglandin E(2) into the medial preoptic area or microinjection of bicuculline into the RPa resulted in respective significant increases in BAT sympathetic nerve activity (+approximately 190% and +approximately 235% of resting levels), in BAT temperature (approximately 1.8 degrees C and approximately 2 degrees C), in expired CO(2) (approximately 1.1% and approximately 1.1%), and in heart rate (approximately 97 beats per minute (bpm) and approximately 100 bpm). Blockade of ionotropic EAA receptors within the RPa by microinjection of kynurenate completely reversed the prostaglandin E(2) or bicuculline-evoked increases in all of the measured variables. Blockade of either N-methyl-D-aspartate (NMDA) receptors or non-NMDA receptors alone resulted in marked attenuations of the prostaglandin E(2)-evoked effects on all of the measured variables. These data demonstrate that activation of an EAA input to the RPa is necessary for the BAT thermogenic and the cardiovascular effects resulting from the actions of prostaglandin E(2) within the medial preoptic area or from the disinhibition of local neurons in the RPa.

Excitatory amino acid receptors in the dorsomedial hypothalamus mediate prostaglandin-evoked thermogenesis in brown adipose tissue

We determined whether the dorsomedial hypothalamus (DMH) plays a role in the thermogenic, metabolic, and cardiovascular effects evoked by centrally administered PGE2. Microinjection of PGE2 (170 pmol/60 nl) into the medial preoptic area of the hypothalamus in urethane-chloralose-anesthetized, artificially ventilated rats increased brown adipose tissue (BAT) sympathetic nerve activity (SNA; +207 +/- 18% of control), BAT temperature (1.5 +/- 0.2 degrees C), expired CO2 (0.9 +/- 0.1%), heart rate (HR; 106 +/- 12 beats/min), and mean arterial pressure (22 +/- 4 mmHg). Within 5 min of subsequent bilateral microinjections of the GABAA receptor agonist muscimol (120 pmol.60 nl-1.side-1) or the ionotropic excitatory amino acid antagonist kynurenate (6 nmol.60 nl-1.side-1) into the DMH, the PGE2-evoked increases were, respectively, attenuated by 91 +/- 3% and 108 +/- 7% for BAT SNA, by 73 +/- 12% and 102 +/- 28% for BAT temperature, by 100 +/- 4% and 125 +/- 21% for expired CO2, by 72 +/- 11% and 70 +/- 16% for HR, and by 84 +/- 19% and 113 +/- 16% for mean arterial pressure. Microinjections outside the DMH within the dorsal hypothalamic area adjacent to the mamillothalamic tracts or within the ventromedial hypothalamus were less effective for attenuating the PGE2-evoked thermogenic, metabolic, and cardiovascular responses. These results demonstrate that activation of excitatory amino acid receptors within the DMH is necessary for the thermogenic, metabolic, and cardiovascular responses evoked by microinjection of PGE2 into the medial preoptic area.

Somatic and visceral afferents to the 'vasodepressor region' of the caudal midline medulla in the rat

Previous research has found that the integrity of a restricted region of the caudal midline medulla (including caudal portions of nucleus raphé obscurus and nucleus raphé pallidus) was critical for vasodepression (hypotension, bradycardia, decreased cardiac contractility) evoked either by haemorrhage or deep pain. In this anatomical tracing study we found that the vasodepressor part of the caudal midline medulla (CMM) receives inputs arising from spinal cord, spinal trigeminal nucleus (SpV) and nucleus of the solitary tract (NTS). Specifically: (i) a spinal-CMM projection arises from neurons of the deep dorsal horn, medial ventral horn and lamina X at all spinal segmental levels, with approximately 60% of the projection originating from the upper cervical spinal cord (C1-C4); (ii) a SpV-CMM projection arises primarily from neurons at the transition between subnucleus caudalis and subnucleus interpolaris; (iii) a NTS-CMM projection arises primarily from neurons in ventrolateral and medial subnuclei. In combination, the specific spinal, SpV and NTS regions which project to the CMM receive the complete range of somatic and visceral afferents known to trigger vasodepression. The role(s) of each specific projection is discussed.

Microinjection of muscimol into raphe pallidus suppresses tachycardia associated with air stress in conscious rats

Sympathetically mediated tachycardia is a characteristic feature of the physiological response to emotional or psychological stress in mammals. Activation of neurons in the region of the dorsomedial hypothalamus appears to play a key role in the integration of this response. Tachycardia evoked by chemical stimulation of the dorsomedial hypothalamus can be suppressed by microinjection of the GABA(A) receptor agonist and neuronal inhibitor muscimol into the raphe pallidus (RP). Therefore, we tested the hypothesis that neuronal excitation in the RP mediates tachycardia seen in experimental air stress in rats. Microinjection of the GABA(A) receptor antagonist bicuculline methiodide (BMI) into the RP evoked increases in heart rate. At the same sites, microinjection of muscimol (80 pmol (100 nl)(-1)) had no effect on heart rate under baseline conditions but virtually abolished air stress-induced tachycardia, while microinjection of lower doses (10 or 20 pmol) produced transient but clear suppression. Microinjection of muscimol at sites outside the RP had no effect on stress-induced tachycardia, although modest suppression was apparent after injection at two sites within 500 microm of the RP. In another series of experiments, microinjection of muscimol (80 pmol (100 nl)(-1)) into the RP failed to influence the changes in heart rate produced by baroreceptor loading or unloading. These findings indicate that activity of neurons in the RP plays a previously unrecognized role in the generation of stress-induced tachycardia.

Haemorrhage-evoked compensation and decompensation are mediated by distinct caudal midline medullary regions in the urethane-anaesthetised rat

Previous research using microinjections of excitatory amino acids suggested that the caudal midline medulla (including nucleus raphe obscurus and nucleus raphe pallidus) contained a mixed population of sympathoexcitatory and sympathoinhibitory neurones. The results of this study indicate that different anaesthetic regimes (urethane versus halothane) determine whether sympathoexcitatory (urethane only) or sympathoinhibitory (halothane only) responses are evoked by stimulation within distinct caudal midline medullary regions. In addition, anaesthetic regimes also affect the caudal midline medullary-mediated response to haemorrhage. Specifically, under conditions of urethane anaesthesia, inactivation (lignocaine) of the midline medullary region immediately caudal to the obex, prematurely triggered and dramatically potentiated the hypotension and bradycardia evoked by 15% haemorrhage; whereas under halothane anaesthesia, inactivation of the same region had no effect. In contrast, under urethane anaesthesia, inactivation of the midline medullary region immediately rostral to the obex, delayed the onset of the hypotension and bradycardia to 15% haemorrhage; inactivation of the same region under halothane anaesthesia blocked haemorrhage-evoked hypotension and bradycardia. Our findings indicate that topographically distinct parts of the caudal midline medulla contain neurones (i) that differentially regulate the timing and magnitude of the compensatory (normotensive) versus decompensatory (hypotensive) phases of the response to haemorrhage; and (ii) whose activity is altered by urethane versus halothane anaesthesia.

Delta- and kappa-opioid receptors in the caudal midline medulla mediate haemorrhage-evoked hypotension

In mammals blood loss can trigger, shock, an abrupt, life-threatening hypotension and bradycardia. In the halothane-anaesthetised rat this response is blocked by inactivation of a discrete, vasodepressor area in the caudal midline medulla (CMM). Haemorrhagic shock is blocked also by systemic or ventricular injections of the opioid antagonist, naloxone. This study investigated, in the halothane anaesthetised rat, the contribution of delta-, kappa- and mu-opioid receptors in the CMM vasodepressor region to haemorrhage-evoked shock (i.e. hypotension and bradycardia) and its recovery. It was found that microinjections into the CMM of the delta-opioid receptor antagonist, naltrindole delayed and attenuated the hypotension and bradycardia evoked by haemorrhage, but did not promote recompensation. In contrast, CMM microinjections of the kappa-opioid receptor antagonist, nor-binaltorphamine, although it did not alter haemorrhage-evoked hypotension and bradycardia, did lead to a rapid restoration of AP, but not HR. CMM microinjections of the mu-opioid receptor antagonist, CTAP had no effect on haemorrhage-evoked shock or recompensation. These data indicate that delta- and kappa- (but not mu-) opioid receptor-mediated events within the CMM contribute to the hypotension and bradycardia evoked by haemorrhage and the effectiveness of naloxone in reversing shock.

Nitric oxide synthase in the hypothalamic paraventricular nucleus of the female rat; organization of spinal projections and coexistence with oxytocin or vasopressin

We investigated the distributions and interrelations of neuronal nitric oxide (NO) synthase- (nNOS), oxytocin- (OT), and 8-arginine vasopressin- (AVP) immunoreactive (IR) neurons in the paraventricular nucleus (PVN), and the occurrence and distribution of nNOS spinally projecting neurons in the PVN of the female rat. Using double labelling immunohistochemistry, we mapped the distribution of nNOS-, OT- and AVP-immunoreactive (IR) neuronal cell bodies in the different parts of the PVN. About 80% of nNOS-IR cell bodies were magnocellular. About 30% of the nNOS-IR cell bodies were OT-IR, colocalization being most frequent in the rostral parts. In comparison, only approximately 3% of all nNOS-IR cell bodies were AVP-IR, evenly distributed throughout the PVN. True Blue (TB), administered unilaterally into the spinal cord, disclosed that most spinally projecting cell bodies in the PVN were localized in caudal parts. Combined TB tracing and nNOS immunohistochemistry showed that approximately 30% of spinally projecting neurons in the PVN were nNOS-IR, and that approximately 40% of these were magnocellular. Ipsilateral nNOS spinal projections were about eight times more frequent than the contralateral nNOS projections. The study describes the detailed neuroanatomical organization of nNOS neurons coexpressing OT or AVP, and of nNOS spinally projecting neurons within defined parts of the PVN. In contrast to the paraventriculo-spinal system in general, we show that the nNOS paraventriculo-spinal pathway to a large extent originates in magnocellular cell bodies. The results suggest that NO is an important messenger in the paraventriculo-spinal pathway that may in part act in concert with OT.