Control of cardiovascular function by electrical stimulation within the medullary raphe region of the cat

"On-" and "off-" cells in the rostral ventromedial medulla of rats held in thermoneutral conditions: are they involved in thermoregulation?

In thermal neutral condition, rats display cyclic variations of the vasomotion of the tail and paws, synchronized with fluctuations of blood pressure, heart rate, and core body temperature. "On-" and "off-" cells located in the rostral ventromedial medulla, a cerebral structure implicated in somatic sympathetic drive, 1) exhibit similar spontaneous cyclic activities in antiphase and 2) are activated and inhibited by thermal nociceptive stimuli, respectively. We aimed at evaluating the implication of such neurons in autonomic regulation by establishing correlations between their firing and blood pressure, heart rate, and skin and core body temperature variations. When, during a cycle, a relative high core body temperature was reached, the on-cells were activated and within half a minute, the off-cells and blood pressure were depressed, followed by heart rate depression within a further minute; vasodilatation of the tail followed invariably within ∼3 min, often completed with vasodilatation of hind paws. The outcome was an increased heat loss that lessened the core body temperature. When the decrease of core body temperature achieved a few tenths of degrees, sympathetic activation switches off and converse variations occurred, providing cycles of three to seven periods/h. On- and off-cell activities were correlated with inhibition and activation of the sympathetic system, respectively. The temporal sequence of events was as follows: core body temperature → on-cell → off-cell ∼ blood pressure → heart rate → skin temperature → core body temperature. The function of on- and off-cells in nociception should be reexamined, taking into account their correlation with autonomic regulations.

Modulation of sympathetic and somatomotor function by the ventromedial medulla

The ventromedial medulla is implicated in a variety of functions including nociceptive and cardiovascular modulation and the control of thermoregulation. To determine whether single microinjections into the ventromedial medulla elicit changes in one or multiple functional systems, the GABA(A) receptor antagonist bicuculline was microinjected (70 nl, 5-50 ng) into the ventromedial medulla of lightly anesthetized rats, and cardiovascular, respiratory, and nociceptive measures were recorded. Bicuculline microinjection into either the midline raphe or the laterally adjacent reticular nucleus simultaneously increased interscapular brown adipose tissue temperature, heart rate, blood pressure, expired [CO(2)], and respiration rate and elicited shivering. Bicuculline microinjection also decreased the noxious stimulus-evoked changes in heart rate and blood pressure, decreased the frequency of heat-evoked sighs, and suppressed the cortical desynchronization evoked by noxious stimulation. Although bicuculline suppressed the motor withdrawal evoked by noxious tail heat, it enhanced the motor withdrawal evoked by noxious paw heat, evidence for specifically patterned nociceptive modulation. Saline microinjections into midline or lateral sites had no effect on any measured variable. All bicuculline microinjections, midline or lateral, evoked the same set of physiological effects, consistent with the lack of a topographical organization within the ventromedial medulla. Furthermore, as predicted by the isodendritic morphology of cells in the ventromedial medulla, midline bicuculline microinjection increased the number of c-fos immunoreactive cells in both midline raphe and lateral reticular nuclei. In summary, 70-nl microinjections into ventromedial medulla activate cells in multiple nuclei and elicit increases in sympathetic and somatomotor tone and a novel pattern of nociceptive modulation.

Midline medullary depressor responses are mediated by inhibition of RVLM sympathoexcitatory neurons in rats

Mechanisms underlying the depressor and sympathoinhibitory responses evoked from the caudal medullary raphe (MR) region were investigated in pentobarbital sodium-anesthetized, paralyzed rats. Intermittent electrical stimulation (0.5 Hz, 0.5-ms pulses, 200 microA) of the MR elicited a mixed sympathetic response that consisted of a long-latency sympathoexcitatory (SE) peak (onset = 146 +/- 7 ms) superimposed on an inhibitory phase (onset = 59 +/- 10 ms). Chemical stimulation of the MR (glutamate; Glu) most frequently elicited depressor responses accompanied by inhibition of sympathetic nerve discharge. Occasionally, these responses were preceded by transient pressor and SE responses. We examined the influence of intermittent electrical stimulation (0.5 Hz, 0.5-ms pulses, 25-200 microA) and Glu stimulation of the MR on the discharge of rostral ventrolateral medulla (RVLM) premotor SE neurons. Peristimulus-time histograms of RVLM unit discharge featured a prominent inhibitory phase in response to MR stimulation (onset = 20 +/- 2 ms; duration = 42 +/- 4 ms; n = 12 units). Glu stimulation of the MR reduced blood pressure (-37 +/- 2 mmHg, n = 19) and inhibited the discharge of RVLM SE neurons (15 of 19 neurons). Depressor and sympathoinhibitory responses elicited by chemical and electrical stimulation of the MR region are mediated by inhibition of RVLM premotor SE neurons and withdrawal of sympathetic vasomotor discharge.

The ventrolateral periaqueductal gray projects to caudal brainstem depressor regions: a functional-anatomical and physiological study

The reaction of shock, a precipitous, life-threatening fall in arterial pressure and heart rate, is evoked often by the combination of deep pain and blood loss following traumatic injury. A similar "shock-like" pattern of response can be evoked by excitation of the ventrolateral midbrain periaqueductal gray. Further, ventrolateral periaqueductal gray neurons are selectively activated by deep somatic or visceral pain and haemorrhage. The pathways mediating ventrolateral periaqueductal gray evoked hypotension and bradycardia are not known. In this study, the projections from the ventrolateral periaqueductal gray to "cardiovascular" regions in the caudal medulla of the rat were examined. Injections of the anterograde tracer, biotinylated dextran amine at physiologically-defined, ventrolateral periaqueductal gray depressor sites, revealed strong projections to the caudal midline medulla and to the depressor region of the caudal ventrolateral medulla. Injections of excitatory amino acids established that substantial falls in arterial pressure could be evoked from the ventrolateral periaqueductal gray-recipient parts of the caudal midline medulla. Injections of the retrograde tracer, cholera toxin subunit B at physiologically-defined, depressor sites in the caudal midline medulla and the caudal ventrolateral medulla confirmed the existence of substantial projections from the ventrolateral periaqueductal gray. Although previous studies have emphasized the importance of projections from the ventrolateral periaqueductal gray to the pressor region of the rostral ventrolateral medulla, this study has revealed the existence of strong ventrolateral periaqueductal gray projections to depressor regions within the caudal medulla (caudal midline medulla and caudal ventrolateral medulla) which likely contribute to ventrolateral periaqueductal gray-mediated hypotension and bradycardia.

Raphe nuclei in three cartilaginous fishes, Hydrolagus colliei, Heterodontus francisci, and Squalus acanthias

The vertebrate reticular formation, containing over 30 nuclei in mammals, is a core brainstem area with a long evolutionary history. However, not all reticular nuclei are equally old. Nuclei that are widespread among the vertebrate classes are probably ones that evolved early. We describe raphe nuclei in the reticular formation of three cartilaginous fishes that diverged from a common ancestor over 350 million years ago. These fishes are Hydrolagus colliei, a holocephalan, Squalus acanthias, a small-brained shark, and Heterodontus francisci, a large-brained shark. Nuclear identification was based on immunohistochemical localization of serotonin and leu-enkephalin, on brainstem location, and on cytoarchitectonics. Raphe nuclei are clustered in inferior and superior cell groups, but within these groups individual nuclei can be identified: raphe pallidus, raphe obscurus, and raphe magnus in the inferior group and raphe pontis, raphe dorsalis, raphe centralis superior, and raphe linearis in the superior group. Hydrolagus lacked a dorsal raphe nucleus, but the nucleus was present in the sharks. The majority of immunoreactive cells are found in the superior group, especially in raphe centralis superior, but immunoreactive cells are present from spinal cord to caudal mesencephalon. The distribution and cytoarchitectonics of serotoninergic and enkephalinergic cells are similar to each other, but raphe nuclei contain fewer enkephalinergic than serotoninergic cells. The cytoarchitectonics of immunoreactive raphe cells in cartilaginous fishes are remarkably similar to those described for raphe nuclei in mammals; however, the lack of a raphe dorsalis in Hydrolagus indicates that either it evolved later than the other raphe nuclei or it was lost in holocephalan fishes.

Map of spinal neurons activated by chemical stimulation in the nucleus raphe magnus of the unanesthetized rat

The expression of the proto-oncogene c-fos was used as a cellular marker of spinal cord neurons activated by microinjection of kainic acid into the medullary nucleus raphe magnus of awake and drug-free Sprague-Dawley rats. The c-FOS protein was detected by immunocytochemistry. We found increased immunoreactivity bilaterally in laminae I-VI of the dorsal horn. The strongest c-FOS expression was observed within the inner layer of lamina II near its border with lamina III. In the ventral horn no c-FOS immunoreactivity was observed. Thus, the present results provide evidence for a descending excitation of neurons predominantly in inner lamina II, possibly mediating nucleus raphe magnus-induced inhibition of neurons in other laminae.

Fronto-parietal control of electrodermal activity in the cat

The aim of this work was to investigate the direct involvement of the fronto-parietal cortex in the control of spinal autonomic centers eliciting electrodermal activity (EDA). This autonomic response, linked with the activity of sweat glands, was recorded as skin potential responses (SPRs) from forepaws in the cat. Animals were paralyzed by gallamine and SPRs were obtained under halothane anaesthesia. For each animal, a transection of the medulla sparing only pyramidal tracts was carried out. SPRs were elicited by direct electrical stimulation of pericruciate and posterior parietal cortical areas before and after such a transection. Results showed that in intact preparations, stimulation of the pericruciate cortex evoked SPRs at lower thresholds than the posterior parietal cortex. After the bulbar transection, only the stimulation of pericruciate areas still elicited SPRs at low intensities. Results are interpreted as indicating that fronto-parietal control of EDA is probably mediated by a double descending system: one involving corticoreticulospinal pathways and a direct corticospinal one. We hypothesized that the somatic motor cortex initiates descending programs to autonomic centers at bulbar and spinal levels, and that these centers are involved in autonomic adjustments to somatomotor movements.

Internal connections in the rostral ventromedial medulla of the rat

Physiological and pharmacological data suggest that the rostral ventromedial medulla (RVM) is an important site where integration between somatic and visceral functions might occur. The aim of the present study was to describe the interconnections between various nuclei of the rostral ventromedial medulla and thus reveal the possible anatomical basis for such functional interactions. The topography of anterogradely labelled internal projections was examined following iontophoretic microinjections of Phaseolus vulgaris leucoagglutinin (PHA-L). The results revealed that the nuclei of the rostral ventromedial medulla have strong interconnections and, to varying degrees, they also have bilateral projections into the rostral ventrolateral medulla. A particularly dense projection to widespread regions of the ventral medulla was traced from the raphe obscurus. Terminals, originating from the raphe pallidus were similarly dispersed but very low density in comparison. The focus of the projections of the gigantocellular nucleus pars ventralis and pars alpha shifted from the lateral paragigantocellular nucleus towards the RVM in rostral direction. Connections from the raphe magnus were altogether restricted to the RVM and the medial aspects of the lateral paragigantocellular nucleus. The diffuse and dense intramedullary connections of the raphe obscurus suggest that it might have an important role in coordinating the activity of rostral ventral medullary cells. The raphe pallidus and the ventral gigantocellular nuclei, areas that were innervated from widespread regions of the rostral ventral medulla but gave only limited projections there, are more likely to be involved in the direct descending control of spinal activities.

Facilitation of the arterial baroreflex by the ventrolateral part of the midbrain periaqueductal grey matter in rats

1. The effects of stimulation of the ventrolateral part of the midbrain periaqueductal grey matter (PAG) on the arterial baroreflex were investigated in urethane-chloralose anaesthetized and artificially ventilated rats. 2. Both electrical and chemical stimulation of the ventrolateral PAG provoked hypotension, vagal bradycardia and marked facilitation of baroreflex vagal bradycardia (BVB), which was induced by stimulation of the aortic depressor nerve. The magnitude of ventrolateral PAG facilitation of BVB was 328 +/- 193% (n = 34) for electrical stimulation and 243 +/- 224% (n = 13) for chemical stimulation. Baroreflex hypotension was slightly augmented during either electrical or chemical stimulation of the ventrolateral PAG in vagotomized rats. 3. Stimulation of the nucleus raphe magnus (NRM) also provoked hypotension, vagal bradycardia and facilitation of BVB. The magnitude of BVB facilitation was 234 +/- 132% (n = 8) for electrical stimulation and 328 +/- 170% (n = 7) for chemical stimulation. After microinjection of kainic acid into the NRM region, baroreflex facilitation, as well as hypotension and vagal bradycardia, produced by ventrolateral PAG stimulation, was almost abolished. 4. In conclusion, the ventrolateral PAG, besides producing hypotension and bradycardia, facilitates arterial baroreflexes. These effects are exerted via the NRM, sharply contrasting with effects of the dorsal PAG.

Responses of feline raphespinal neurons to urinary bladder distension

Effects of distending the urinary bladder were studied on extracellular activity of 77 raphespinal neurons in 19 alpha-chloralose anesthetized cats. Neurons were activated antidromically from thoracic spinal cord; recording sites were located in nucleus raphe magnus (NRM). Mean conduction velocity was 48 +/- 2 m/s. Urinary bladder distension (UBD) increased activity in 12 cells and decreased activity in 17 cells. Spontaneous bladder contractions also affected activity in raphespinal neurons responsive to UBD. Noxious pinch stimulus applied to proximal hindlimbs or forelimbs either increased or decreased activity in 28 raphespinal neurons. No cells were excited both by UBD and pinching of skin and deep tissues of the limbs. Thus, excitatory viscerosomatic convergence was not observed with the stimuli tested in raphespinal neurons examined in this study. Urinary bladder input to descending projection neurons in NRM might participate in descending modulation of dorsal horn neurons. In addition, micturition reflexes might be affected by urinary bladder input to these neurons.

Innervation of serotonergic medullary raphe neurons from cells of the rostral ventrolateral medulla in rats

The rostral ventral medulla has been shown to consist of three distinct subregions: the midline or raphé region, the lateral paragigantocellular-gigantocellular region and the rostro-ventrolateral reticular nucleus. All three regions have been shown to contribute to central vaso-regulation and to project towards sympathetic preganglionic neurons of the thoracic spinal cord. Therefore it is of particular interest to describe the interconnections between the three regions and to see if local afferents reach cells which have been implicated in the regulation of descending inputs. Following injections of the anterograde tract tracer Phaseolus vulgaris leucoagglutinin into the lateral paragigantocellular nucleus or the rostroventrolateral reticular nucleus, labelled axons were traced into the medullary raphé nuclei and the contralateral rostral ventrolateral medulla. Efferents originating from both regions innervated the raphé pallidus, raphé obscurus and raphé magnus. However the distribution of terminals originating from the two regions was different in the contralateral ventrolateral medulla oblongata. The data indicate that the connection between the ipsi- and contralateral equivalents of both the lateral paragigantocellular-gigantocellular region and the rostroventrolateral reticular nucleus are stronger than the cross-connection between the ipsi- and contralateral parts of the two different regions. In the second part of the study, the existence of direct projections from the rostroventrolateral reticular nucleus and the lateral paragigantocellular-gigantocellular region onto serotonin-immunogold-labelled cells of the ventromedial medulla were investigated. The correlated light and electron microscopic analysis revealed direct synaptic contacts between axons originating from both the lateral paragigantocellular-gigantocellular region and the rostroventrolateral reticular nucleus, and serotonin-immunoreactive cells of the raphé obscurus and raphé pallidus. The results of the present light microscopic tract-tracing study revealed a different pattern of the intramedullary projection of the lateral paragigantocellular-gigantocellular region and the rostroventrolateral reticular nucleus. These data are in support of the proposed parcellation of the two cytoarchitectonically different areas of the rostral ventrolateral medulla into two functionally distinct subdivisions. Furthermore, the direct anatomical connection revealed in the present study between cells of the rostral ventrolateral and ventromedial medulla oblongata indicates the possibility that vasoregulatory effects of some cells of the rostral ventrolateral medulla oblongata might be executed via direct projections onto serotonin-immunoreactive cells of the medullary raphé nuclei.

Responses of neurons in the caudal medullary raphe nuclei of the cat to stimulation of the vestibular nerve

In the decerebrate cat, recordings were made from neurons in the caudal medullary raphe nuclei to determine if they responded to electrical stimulation of the vestibular nerve and thus might participate in vestibulosympathetic reflexes. Many of these cells projected to the upper thoracic spinal cord. The majority (20/28) of raphespinal neurons with conduction velocities between 1 and 4 m/s received vestibular inputs; 13 of the 20 were inhibited, and 7 were excited. Since many raphespinal neurons with similar slow conduction velocities are involved in the control of sympathetic outflow, as well as in other functions, these cells could potentially relay vestibular signals to sympathetic preganglionic neurons. The onset latency of the vestibular effects was long (median of 15 ms), indicating the inputs were polysynaptic. In addition, 34 of 42 raphespinal neurons with more rapid conduction velocities (6-78 m/s) also received long-latency (median of 10 ms) labyrinthine inputs; 26 were excited and 8 were inhibited. Although little is known about these rapidly-conducting cells, they do not appear to be involved in autonomic control, suggesting that the function of vestibular inputs to raphe neurons is not limited to production of vestibulosympathetic reflexes. One hypothesis is that raphe neurons are also involved in modulating the gain of vestibulocollic and vestibulospinal reflexes; this possibility remains to be tested.

Differential actions of the medial region of caudal medulla on autonomic nerve activities

1. The inhibitory effects produced by activation of the medial region of caudal medulla on activities of the left and right cardiac sympathetic, vagus and greater splanchnic nerves were studied in chloralose-urethane anaesthetized cats. 2. Electrical stimulation of the medial region produced an 80-92% inhibition of the sympathetic nerve activities, and a 45% and 58% inhibition of the left and right cardiac vagal nerve activities, respectively. There were no significant differences between effects elicited in the left and right autonomic nerves. Similar but smaller inhibitory effects were produced by micro-injection of sodium glutamate (0.5 mol/L) or DL-homocysteic acid (50 mmol/L) to the same medullary sites. 3. These data suggest that neurons residing in the medial medullary region exert strong inhibitory effects on autonomic nerve activities. Since the vasculature is principally innervated by sympathetic nerves, inhibition of sympathetic nerve activities might be the principal factor responsible for the depressor effects caused by activation of the medial region of caudal medulla. The heart is innervated both by sympathetic and parasympathetic nerves. Thus, their simultaneous inhibition during activation of the medial region elicits only a weak and variable inhibition of the heart.

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

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

Comparison of the influence of rostral and caudal raphe neurons on the adrenal secretion of catecholamines and on the release of adrenocorticotropin in the cat

Neuroendocrine and autonomic responses were assessed in chloralose-anesthetized cats after chemical stimulation of medial brain-stem regions, including those that influence nociceptive input to the medullary or spinal dorsal horn. Microinjections of L-glutamate (0.5 M, 160 nl) were directed at the following rostral and caudal raphe nuclei: the periaqueductal gray (PAG), the dorsal raphe nucleus (DR), the raphe magnus (RM), and the raphe obscurus/raphe pallidus (Ro/Rpa). Activation of DR neurons evoked a significant increase in the adrenal secretion of epinephrine (+2.6 +/- 1.1 ng/min, P less than 0.01) that returned towards prestimulus values by 6 min, whereas microinjections into other raphe nuclei had no consistent effect. Activation of Ro/Rpa neurons evoked an increase in the plasma concentration of adrenocorticotropin (ACTH, +47.9 +/- 12.3 pg/ml, P less than 0.01), whereas microinjections into other raphe nuclei did not affect ACTH. Arterial pressure increased significantly after activation of PAG (+7.5 +/- 2.1 mm Hg, P less than 0.01) or of DR (+4.8 +/- 2.0 mm Hg, P less than 0.05) neurons, whereas heart rate increased significantly (P less than 0.05) after stimulation of cells within the Ro/Rpa. Glutamate microinjections within the RM, a raphe nucleus that exerts a significant descending influence on nociceptive input to the medullary and to the spinal dorsal horns, had no consistent effect on any measured variable. No evidence was seen to suggest that chemical activation of neurons within raphe nuclei inhibited the adrenal secretion of catecholamines or inhibited the release of ACTH. The results indicated that glutamate activation of neurons within different raphe nuclei evoked non-uniform effects on neuroendocrine and autonomic function. Further, these data suggested that the neural substrate underlying the control of the adrenal secretion of catecholamines and of the release of ACTH in response to activation of raphe neurons is likely distinct from that which contributes to the descending influence on nociceptive input to the medullary and spinal dorsal horn.

Involvement of the raphe in the respiratory effects of gigantocellular area activation

Previous reports indicate that the nucleus reticularis gigantocellularis (NGC) of the brainstem reticular formation is involved in inhibitory respiratory and cardiovascular reflexes. Stimulation of portions of the nearby bulbar raphe complex, specifically the raphe magnus (RM), have also been shown to suppress phrenic activity and to decrease blood pressure and heart rate. Since synaptic connectivity between the NGC and the RM has been demonstrated, we hypothesized that the RM may be involved in the cardiopulmonary effects of NGC stimulation. This study found that electrolytic lesions in the raphe magnus attenuated the inhibitory respiratory effects but not the cardiovascular suppression due to NGC stimulation. Lesions in the raphe magnus also lowered resting blood pressure and resting breath frequency. We conclude that the RM may mediate part of the NGC-mediated respiratory effects.

Evidence for colocalization of substance P and 5-hydroxytryptamine in spinally projecting neurons from the cat medulla oblongata

Substance P (SP)- and 5-hydroxytryptamine (5-HT)-like immunoreactivities were localized in bulbospinal neurons of the raphe nuclei and ventrolateral medulla (VLM). In raphe pallidus and raphe obscurus virtually all of the spinally projecting neurons contained SP and/or 5-HT. Furthermore, SP and 5-HT were colocalized in half of these spinal-raphe neurons. In raphe magnus few spinally projecting neurons contained either SP or 5-HT. Half of the bulbospinal neurons in the caudal VLM contained SP and/or 5-HT and in 50% of these SP and 5-HT were colocalized. However, no SP-containing neurons in the rostral VLM projected to the spinal cord.

Cardiovascular responses elicited by electrical and chemical stimulation of the rostral medullary raphe of the rabbit

Electrical stimulation of the rostral medullary raphe (RMR) of the rabbit elicited pressor responses that were accompanied by tachycardia or bradycardia. Stimulation of dorsal sites (the dorsal raphe obscurus) evoked a pressor/tachycardia response and stimulation of ventral sites (the ventral raphe obscurus, raphe magnus and raphe pallidus) produced a pressor/bradycardia response. Electrical stimulation of the RMR after sinoaortic denervation led to an increase in the magnitude of the pressor response elicited from all stimulation sites, a decrease in the magnitude of the bradycardia produced by stimulation at the ventral sites, but had no effect upon the magnitude of the tachycardia observed from stimulation of the dorsal sites. These findings suggest that electrical stimulation of the dorsal sites leads to inhibition of the cardiomotor component of the baroreceptor reflex. The results of vagal blockade experiments demonstrated that baroreceptor attenuation of the pressor responses at ventral sites was mediated primarily by parasympathetic input to the heart. Chemical stimulation of the RMR with L-glutamate also led to a pressor/tachycardia response at the dorsal sites and a pressor/brachycardia response at the ventral sites. This finding provides evidence that neuronal cell bodies, not axon of passage, mediated the responses elicited by electrical stimulation.

Medullary substrates mediating antinociception produced by electrical stimulation of the vagus

Electrical stimulation of afferents of the right cervical vagus inhibited the tail-flick reflex elicited by noxious heat in barbiturate-anesthetized rats. This inhibitory effect was eliminated in rats receiving local anesthetic blockade of either the nucleus tractus solitarii (NTS), the lateral reticular nuclei, the nucleus raphe magnus-medullary reticular formation, or nucleus raphe obscurus regions of the medulla. Similarly, the vasodepressor and bradycardic effects of vagal stimulation were either attenuated or eliminated by local anesthetic blockade of these regions. Microinjection of the non-specific glutamate antagonist gamma-D-glutamylglycine (DGG) into the NTS region also eliminated vagally evoked inhibition of the tail-flick reflex, hypotension, and bradycardia. Conversely, microinjection of glutamate into the NTS region resulted in inhibition of the tail-flick reflex, hypotension, and bradycardia. These findings with DGG and glutamate are consistent with the view that glutamate serves as a neurotransmitter of the primary vagal afferents mediating these antinociceptive and cardiovascular responses. These results are discussed in terms of vagal afferent influences on somatosensory, somatomotor, and cardiovascular function.

Serotonin- and substance P-containing projections to the nucleus tractus solitarii of the rat

The objective of the present study was to determine the location of the neurons that give rise to serotonin- and substance P-containing terminals in the nucleus tractus solitarii. This was done by injecting rhodamine-filled latex microspheres into the nucleus tractus solitarii of rats to retrogradely label neuronal cell bodies and by processing sections from the brains of these animals to determine whether the labelled neurons contained serotonin or substance P immunoreactivity. Serotonin-immunoreactive neurons that projected to the nucleus tractus solitarii were found in the nucleus raphe magnus, nucleus raphe obscurus, nucleus raphe pallidus, and in the ventral medulla, lateral to the pyramidal tract. Substance P-immunoreactive neurons that projected to the nucleus tractus solitarii were found in similar areas but were proportionately less numerous in the nucleus raphe magnus and proportionately more numerous in the nucleus raphe pallidus. It is concluded that neurons in the medullary raphe nuclei, some of which presumably utilize serotonin or substance P as a neurotransmitter, could regulate autonomic function via direct projections to the nucleus tractus solitarii.

Responses of phrenic motoneurones of the cat to stimulation of medullary raphe nuclei

Responses of phrenic motoneurones to stimulation of the three medullary raphe nuclei (raphe magnus (r. magnus), raphe obscurus (r. obscurus) and raphe pallidus (r. pallidus] were recorded in anaesthetized and decerebrated cats. Stimulation of r. magnus or r. obscurus depressed phrenic motoneurones. Stimulation at 100 Hz reduced action potential frequency within each inspiratory burst, without appreciable changes in inspiratory duration, or number of inspiratory bursts per unit time. The depression was proportional to the stimulus intensity (40-160 microA) and frequency (12-100 Hz) and lasted throughout the period of stimulation. Intracellular recording revealed concomitant depression of central respiratory drive potentials (c.r.d.p.s) and increased membrane input resistance during r. obscurus or r. magnus stimulation. In motoneurones which discharged action potentials during expiratory as well as inspiratory phases following intracellular chloride injection, stimulation of r. magnus or r. obscurus depressed cell firing during both phases. Both c.r.d.p.s and reversed inhibitory post-synaptic potentials (i.p.s.p.s) were depressed. These findings indicate that the depression is not related to post-synaptic inhibition of phrenic motoneurones. Stimulation (100 Hz) of r. pallidus produced discharges of action potentials in phrenic motoneurones. Stimulation lengthened the duration of each inspiratory discharge in proportion to stimulus intensity. Continuous firing occurred throughout the period of stimulation with maximal intensities. Intracellular recordings revealed sustained depolarization and reduction in membrane input resistance during the discharge. Responses were recorded extracellularly from medullary inspiratory neurones of the dorsal respiratory group (d.r.g.) and ventral respiratory group (v.r.g.) and from vagal axons which fired in phase with phrenic nerve activity. Responses to raphe stimulation were similar to those recorded from phrenic motoneurones. Evidence is presented that the responses are not related to stimulation of decussating bulbo-spinal axons from d.r.g. or v.r.g. neurones. It is suggested that medullary respiratory neurones receive inhibitory and excitatory synaptic inputs from medullary raphe neurones. Hypercapnia (5% CO2 in O2) or hypoxia (15% O2 in N2) reduced markedly the inhibition produced during stimulation of r. obscurus or r. magnus, and restored expiratory-linked silent periods during stimulation of r. pallidus. Activation of Hering-Breuer or baroreceptor reflexes did not alter responses to r. pallidus stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Vagal cardioinhibitory area in and around the caudal inferior olivary nucleus of cats. I. Localization and connection

Bradycardia was produced by electrical stimulation and injection of glutamate (200 nl, 1 M) to the regions in and around the extremely caudal parts of the inferior olivary nucleus (ION) in chloralose-urethane anesthetized cats. Either medullary transection at 4 mm rostral to the obex, spinal transection at C2-3 or both transections together did not abolish the ION-induced bradycardia. Bradycardia induced by stimulating one ION was slightly reduced and completely abolished by unilateral and bilateral vagotomies, respectively. The ION-evoked bradycardia was also slightly reduced by a raphe lesion. This lesion together with unilateral vagotomy abolished only the bradycardia induced from the ION on the vagotomized side. In 28 left vagotomized cats, the left ION-induced bradycardia was subjected to the effects of destroying the right ION, dorsal motor/solitary nucleus (DM/SN) and ambiguous nucleus (AN). The bradycardia was completely abolished by ION lesion (7/7), greatly reduced (7/13) or completely abolished (6/13) by DM/SN lesion, and slightly increased (5/8) or decreased (3/8) by AN lesion. We concluded that cells and/or fibers in and around the ION may project fibers to the ipsilateral DM/SN and through the raphe nucleus to the contralateral ION which may send fibers to the other DM/SN.

Interaction between naloxone and serotonin in the control of the cardiovascular system in hemorrhaged cats

We investigated the mechanism for the pressor effect of intravenous administration of naloxone in pentobarbital-anesthetized cats. Comparisons were made between groups of hemorrhaged animals that received either naloxone or an equivalent volume of saline after 1 h of hemorrhage. Two other groups of hemorrhaged animals were depleted of serotonin by pretreatment with para-chlorophenylalanine 40-48 h before the experiment. One group of serotonin-depleted animals received naloxone after 60 min of hemorrhage and the other group received saline. Animals with normal brain serotonin content showed a significant pressor effect following naloxone when compared with animals given saline. Animals with reduced brain serotonin content also had a pressor response following naloxone administration. Serotonin-depleted animals showed an increase in maximum left ventricular dP/dt following naloxone administration when compared to serotonin-depleted animals given saline. Our data are consistent with the hypothesis that naloxone can exert a pressor effect in hemorrhaged cats by actions at central and at peripheral sites. In cats with normal serotonin values, the peripheral action of naloxone is predominant. In serotonin-depleted animals given naloxone, central and peripheral sites contribute to the pressor effect.

Inhibition of feline spinal cord dorsal horn neurons following electrical stimulation of nucleus paragigantocellularis lateralis. A comparison with nucleus raphe magnus

The effect of electrical stimulation applied to the nucleus paragigantocellularis lateralis (PGL) was assessed on the somatosensory responses of functionally identified spinal cord dorsal horn neurons in the cat. Neurons were classified as low threshold mechanoreceptive, wide dynamic range or nociceptive specific. The responses of over 95% of all neurons tested were inhibited by a conditioning stimulus to the PGL. For each cell the threshold current intensity necessary to produce inhibition from the PGL (inhibitory threshold) was determined. Analysis of the incidence of inhibition and the inhibitory thresholds showed that the PGL-induced inhibition was not selective for a particular class of neuron. Due to the many similarities between the PGL and the nucleus raphe magnus (NRM), a comparison was made between each region's potency in inhibiting the responses of spinal cord neurons. Based on an analysis of inhibitory thresholds, the PGL was found to be significantly more potent than the NRM. These results indicate the PGL to be an important site from which descending modulation of spinal cord somesthetic information emanates.

Evidence for a serotonergically mediated sympathoexcitatory response to stimulation of medullary raphe nuclei

The cardiovascular role of serotonin (5-HT) containing neurons in the midline medullary raphe nuclei was studied in anesthetized cats. High frequency electrical stimulation of nucleus (n.) raphe (r.) pallidus, n.r. obscurus and n.r. magnus produced both pressor and depressor responses. Single shock stimulation of pressor sites produced an excitatory evoked potential of sympathetic nervous discharge (SND) recorded from the inferior cardiac nerve. Conversely, single shock stimulation of vasodepressor sites resulted in a computer-summed inhibition of SND. The mean conduction velocity in the sympathoexcitatory medullo-spinal pathway to sympathetic preganglionic neurons was calculated to be 1.24 m/s. The 5-HT antagonists methysergide and metergoline blocked the excitation of sympathetic activity evoked from medullary raphe nuclei. In contrast, these agents failed to alter the sympathoexcitatory response to electrical stimulation of lateral medulla pressor sites or the sympathoinhibitory response elicited by raphe stimulation. The 5-HT uptake inhibitor chlorimipramine increased the duration of the sympathoexcitatory response evoked from the raphe but not from the lateral medulla. Finally, mid-collicular transection did not effect the excitation of sympathetic activity elicited by stimulation of medullary raphe nuclei. These data suggest that serotonergic neurons in the midline medullary raphe nuclei provide an excitatory input to sympathetic neurons in the spinal cord.

Interactions between cardiovascular and pain regulatory systems

A review of pharmacological, neuroanatomical, electrophysiological, and behavioral data indicates that systems controlling cardiovascular function are closely coupled to systems modulating the perception of pain. This view is directly supported by experiments from our laboratory showing that activation of either the cardiopulmonary baroreceptor reflex arc or the sinoaortic baroreceptor reflex arc induces antinociception. The outcomes of studies using pharmacological treatments, peripheral nerve stimulation, peripheral nerve resection, and CNS lesions are also presented as a preliminary means of characterizing cardiovascular input to pain regulatory systems. The network formed by these systems is proposed to participate in the elaboration of adaptive responses to physical and psychological stressors at various levels of the neuroaxis, and possibly to participate in "diseases of adaptation." In particular, the present analysis suggests that the inhibition of pain brought about by elevations in either arterial or venous blood pressure may provide a form of psychophysiological relief under situations of stress and contribute to the development of essential hypertension in humans.