Electroacupuncture induces c-Fos expression in the rostral ventrolateral medulla and periaqueductal gray in cats: relation to opioid containing neurons

Our previous studies have shown that electroacupuncture (EA) at the Neiguan-Jianshi (P5-P6) acupoints inhibits sympathetic outflow and attenuates excitatory visceral cardiovascular reflexes through enkephalin- or beta-endorphin-related opioid receptors in the rostral ventrolateral medulla (rVLM). It is not known whether EA at these acupoints activates neurons containing enkephalin or beta-endorphin in the rVLM as well as in the periaqueductal gray (PAG) that are involved in EA-mediated central neural regulation of sympathetic activity. The present study evaluated activated neurons in the rVLM and PAG by detecting c-Fos immunoreactivity, and identified the relationship between c-Fos nuclei and neuronal structures containing enkephalin or beta-endorphin in these regions. To enhance the detection of cell bodies containing enkephalin or beta-endorphin, colchicine (90-100 microg/kg) was injected into the subarachnoid space in anesthetized cats 28-30 h prior to EA or the sham-operated control for EA. Following bilateral barodenervation and cervical vagotomy, EA (1-4 mA, 2 Hz, 0.5 ms) was performed at the P5-P6 acupoints (overlying median nerve; n=7) for 30 min. Identical procedures, with the exception of electrical stimulation, were carried out in five control animals. EA decreased blood pressure (BP) in four of seven cats (5-15 mm Hg) while the sham procedure for EA produced no responses. Perikarya containing enkephalin were found in the rVLM and rarely in the PAG, while no cell bodies labeled with beta-endorphin were identified in either region. Compared to animals in the control group, more c-Fos immunoreactivity, located principally in close proximity to fibers containing enkephalin or beta-endorphin, was observed in the rVLM and ventrolateral PAG (vlPAG) in EA-treated cats. Moreover, neurons double-labeled with c-Fos and enkephalin in the rVLM were significantly increased in cats following EA stimulation (P<0.05). These data indicate that EA at the P5-P6 acupoints activates neurons in the rVLM and vlPAG. These activated neurons contain enkephalin in the rVLM, and most likely interact with nerve fibers containing enkephalin or beta-endorphin in both the rVLM and vlPAG. The results from this study provide the first anatomical evidence showing that EA at the P5-P6 acupoints has the potential to influence neuronal structures (perikarya, axons and/or dendrites) containing enkephalin or beta-endorphin in specific regions of the brain stem. These neurons likely form the substrate for EA's influence on sympathoexcitatory cardiovascular reflexes.     

Responses of T2-T4 spinal neurons to stimulation of the greater splanchnic nerves of the cat

Effects of electrical stimulation of the greater splanchnic nerves on T2-T4 spinal neurons were determined in 16 cats anesthetized with alpha-chloralose. Of 77 neurons responding to somatic stimuli, 65 (84%) were excited, inhibited, or both excited and inhibited by splanchnic input. Each of the splanchnic responsive cells also was responsive to electrical stimulation of cardiopulmonary sympathetic afferent fibers. All but one neuron with left splanchnic input also received input from the right splanchnic nerve. Short- and long-latency excitatory responses were observed after splanchnic stimulation. The cell response to splanchnic stimulation was greatly inhibited by a conditioning stimulus applied to the other splanchnic nerve. A similar, although weaker, interaction occurred between splanchnic and cardiopulmonary sympathetic afferent fibers. The activity of 17 cells was inhibited by repetitive stimuli applied to one or both splanchnic nerves. Cells were found in laminae I, IV, V, VII, and VIII. These data provide the first evidence for splanchnic modulation of upper thoracic dorsal horn neurons.     

Neuropeptide Y in the rostral ventrolateral medulla blocks somatosympathetic reflexes in anesthetized rats

The rostral ventrolateral medulla (RVLM) is a major integrative center of cardiovascular reflexes that modulate vasomotor tone. The functions of Neuropeptide Y (NPY) in the RVLM on cardiorespiratory responses remain unknown. Arterial blood pressure (AP), heart rate (HR), splanchnic sympathetic (sSNA) and phrenic nerve activities, and responsiveness to baro-, somatosympathetic, and chemoreflex stimulation were recorded before and after bilateral NPY injection (100 pmol, 200 nl/side) in the RVLM of vagotomized urethane-anesthetized rats (n=7). Responses were characterized by an initial increase in AP followed by prolonged hypotension (P<0.01). A similar biphasic effect was exerted on HR (P<0.01). NPY caused a large increase of sSNA (P<0.01), that gradually recovered towards baseline. Somatosympathetic responses evoked by tibial nerve stimulation were largely abolished following NPY microinjection (P<0.01), but sympathoexcitatory responses evoked by acute hypoxia or sympathoinhibition evoked by aortic depressor nerve stimulation were unchanged following NPY. There was no effect of NPY on phrenic nerve amplitude or frequency. We conclude that NPY exerts excitatory effects on sympathetic tone, but inhibits responses evoked by somatic inputs. We speculate that this apparent contradiction may be due to differential expression of NPY receptor subtypes on the soma of sympathetic premotor neurons in the RVLM and on the presynaptic terminals of neurons that comprise excitatory afferent pathways.     

Stimulation of the ventrolateral medulla inhibits the baroreceptor input to the nucleus tractus solitarius

Extracellular recording experiments were done in urethane- and chloralose-anaesthetized, paralyzed and artificially ventilated rabbits. Thirty-one baroreceptor-sensitive neurones were identified in the region of the nucleus tractus solitarius (NTS) by their excitatory responses to stimulation of the ipsilateral aortic nerve. A conditioning stimulus delivered to the ipsilateral or contralateral rostral ventrolateral medulla (rVLM) inhibited the excitatory responses of 80.0% (19 out of 24) neurones to an aortic test stimulus as early as 3 ms and extending as long as 400 ms after conditioning. The same inhibitory effect was also observed by application of excitant amino acid DL-homocysteic acid (0.2 M, 100 nl) into the ipsi- or contralateral rVLM area in 6 units. In 5 units inhibited by rVLM stimulation, evoked discharges were inhibited by prolonged electrical stimulation of the superficial peroneal nerve (SP) with low intensity and low frequency (0.1-0.3 mA and 5-10 Hz). These results provide the electrophysiological evidence for the suppressing effect of the rVLM on the excitatory responses of NTS neurones to baroreceptor afferent stimulation.     

Rostral ventrolateral medullary opioid receptor subtypes in the inhibitory effect of electroacupuncture on reflex autonomic response in cats

Electroacupuncture (EA) is used in traditional Chinese medicine to treat arrhythmias, hypertension and myocardial ischemia. Our previous work suggests that the inhibitory effect of EA on the pressor reflex induced by bradykinin (BK) applied to the gallbladder is due, in part, to the activation of opioid receptors, most likely located in the rostral ventrolateral medulla (rVLM). However, specific opioid receptor subtypes, and hence the neurotransmitters. responsible for this inhibition are unknown. Therefore, in anesthetized cats, BK (10 microg/ml) was applied to the gallbladder to induce transient reflex increases in arterial blood pressure (BP). EA (1-2 mA, 5 Hz, 0.5 ms pulses) was delivered through acupuncture needles inserted bilaterally into Neiguan and Jianshi acupoints on forelimbs, overlying the median nerves. EA attenuated the BK-induced pressor response by 39%. Opioid receptor subtype antagonists or agonists were microinjected unilaterally into the rVLM. The mu- and delta-receptor antagonists CTOP and ICI 174,864, respectively, significantly attenuated the EA-induced inhibition for at least 30 min. The K-receptor antagonist (nor-BNI) was less effective and was shorter acting. Like EA, microinjection of mu- and delta-opioid agonists, DAGO and DADLE, respectively, into the rVLM significantly decreased the pressor responses. In contrast, the kappa-opioid agonist, U50,488, failed to alter the BK-induced pressor response. We conclude that a significant portion of inhibition of the gallbladder pressor response by EA is related to activation of mu- and delta-opioid receptors in the rVLM. The endogenous neurotransmitters for mu- and delta-opioid receptors, beta-endorphins and enkephalins, in the rVLM, therefore appear to play a role in the EA-related modulation of cardiovascular reflex responses. Conversely, dynorphin is less likely to be involved in this response.     

Comparative study of sympathetic and parasympathetic discharges in the autonomic nerves during stimulation of different structures of the cat hypothalamus

Simultaneous recordings of hypothalamo-parasympathetic and hypothalamo-sympathetic evoked discharges in anesthetized cats demonstrated that both parasympathetic discharges in the pelvic nerve and sympathetic discharges in the splanchnic nerve of the lowest threshold and shortest latency could be obtained from stimulation of the posterior hypothalamus. The focus of the maximal neuronal activated elicited by stimulation of afferent fibres of visceral nerves and the focus evoking maximal efferent reactions of parasympathetic and sympathetic nerves were located in the same region of the postero-lateral hypothalamus. It is supposed (as a working hypothesis) that convergent polysensory neurons of the hypothalamus are also polyeffector divergent elements of the hypothalamo-visceral reflex system.     

Comparison of central versus peripheral nerve pathways to the guinea pig inferior mesenteric ganglion determined electrophysiologically after chronic nerve section

The contributions of central versus peripheral nerve pathways to neurons of the inferior mesenteric ganglion of guinea pigs were studied. Nerve trunks innervating neurons in the ganglion were surgically sectioned and intracellular electrical responses to nerve stimulation were measured 6-8 days after surgery. Guinea pigs were divided into two experimental groups: (1) those that had the lumbar sympathetic chain ganglia (LSG) L2 through L4 removed and (2) those that had the intermesenteric, lumbar colonic and hypogastric nerves sectioned leaving central connections intact. After 6-8 days fast excitatory postsynaptic potentials (EPSPs) and slow EPSPs were recorded intracellularly in randomly selected principal ganglionic neurons. The threshold stimulus voltage to elicit a fast EPSP, the amplitude of the slow EPSP and the number of neurons in which each type of synaptic potential occurred in response to stimulation of each of the nerve trunks was compared between surgically-sectioned animals and sham-operated controls. Neither section of preganglionic nerve trunks nor of postganglionic nerve trunks eliminated all synaptic input to neurons in the ganglion, indicating that neurons with cell bodies located central to the ganglion as well as in visceral target organs made synaptic connections in the ganglion. Both fast and slow synaptic potentials could be evoked by stimulation of postganglionic nerve trunks even after they were sectioned provided that preganglionic nerves were intact, indicating that axons of central origin which synapse in the ganglion may continue out into postganglionic nerve trunks. In like manner, evidence was obtained indicating that fibers from peripheral nerve trunks which initiate either fast or slow synaptic potentials in ganglionic neurons may continue out into the lumbar splanchnic nerves. These studies demonstrate that the synaptic potentials recorded in the inferior mesenteric ganglion arise not only from neurons with cell bodies central to the ganglion but also from neurons with cell bodies located in the visceral organs which this ganglion subserves.     

Units in tegmental nuclei responding to stimulation of gastric vagal and greater splanchnic fibers in the cat

The response of neurons in the ventral and dorsal tegmental nuclei during electrical stimulation of the gastric vagal fibers which serve the proximal stomach and the left greater splanchnic fibers were evaluated in chloralose-anesthetized cats. The mean latency of 181 gastric vagally evoked unitary responses recorded in the tegmental nuclei was 352.2 ms, whereas the latency of the left greater splanchnic-evoked tegmental response was significantly less (63.2 ms). The unitary responses to the gastric vagal and greater splanchnic fibers stimulation were bilaterally distributed in the ventral and dorsal tegmental nuclei. Convergence of the gastric vagal input from the proximal stomach and the left greater splanchnic input was observed in 151 units (83 percent). Stimulation of the greater splanchnic nerve usually resulted in a short latency excitation followed by an inhibitory effect on gastric vagally evoked responses. The results suggested that some convergent splanchnic inhibition of gastric vagally evoked responses was mediated via an interneuron. Projections from the nucleus tractus solitarius and the parabrachial nucleus to the tegmental nuclei were also identified electrophysiologically by direct microstimulation of the two former areas. The significant number of gastric vagal and splanchnic evoked unitary responses recorded in the ventral and dorsal tegmental nuclei suggested that they may serve as an important pontine site for processing of visceral information between the nucleus tractus solitarius and forebrain sites.     

Participation of interoceptive afferent excitations in sensory integration processes of cat visual cortex neurons]

In experiments on cats immobilized with diplacine it is shown that stimulation of the vagal and splanchnic nerves exerts a substantial influence on the neuronal background activity of field 17 in the brain cortex. Neuronal responses as prolonged tonic changes in impulse frequency were seen more often. Complex phasic neuronal responses with steady inhibitory periods and periods of activation appeared more rarely. The development of phasic reactions could begin both with activation and initial inhibition of the activity. Predominantly excitatory responses resulted from the splanchnic nerve stimulation. They could be also seen during somatic stimulations. The degree of neuronal involvement into the responses and their intensity increased by trains of stimuli. Visceral and somatic influences on the visual cortical neurons are shown to be mediated by the lateral hypothalamus and midbrain reticular formation.     

Parabrachial nucleus: neuronal evoked responses to gastric vagal and greater splanchnic nerve stimulation.

Unitary responses were recorded extracellularly in the parabrachial nucleus (PBN) in anesthetized cats during electrical stimulation of the 1) gastric branches of the ventral and dorsal vagal trunks which serve the proximal stomach, and 2) left greater splanchnic nerve. The gastric vagally evoked parabrachial responses consisted of phasic single and multiple spike orthodromic discharges, which were bilaterally distributed, with a mean latency of 349 ms (S.D. +/- 38.5). The parabrachial-evoked splanchnic unitary responses had a much shorter latency with a bimodal distribution (mean latencies, 53 and 128 ms, respectively). Convergence of gastric vagal input from the proximal stomach and the left greater splanchnic nerve upon single neurons in the PBN was electrophysiologically demonstrated in 132 units. Eighty-seven percent of the gastric vagally evoked parabrachial unitary responses were inhibited by simultaneous electrical stimulation of the splanchnic nerve. The condition-test paradigm was used to evaluate the time course of the splanchnic inhibition of the gastric vagally evoked parabrachial response. Reciprocal connections between neuronal populations in the nucleus tractus solitarius (NTS) which received gastric vagal input and the PBN were also identified electrophysiologically by direct microstimulation of the former structure. The density and characteristics of the gastric vagal and greater splanchnic input to the PBN suggested that this nucleus receives and processes a substantial amount of visceral afferent input. The PBN may serve as an important site for integrating visceral information governing the proximal stomach and ingestive processes.     

Effects of chemical and electrical stimulation of the midbrain on feline T2-T6 spinoreticular and spinal cell activity evoked by cardiopulmonary afferent input

Responses to electrical and chemical stimulation of the periaqueductal gray (PAG) and midbrain reticular formation (RF) were examined on extracellular activity of 28 spinoreticular tract (SRT) and 56 spinal neurons in T2-T6 segments of anesthetized cats. All cells received excitatory viscerosomatic convergent input from the left forelimb triceps region and from cardiopulmonary sympathetic afferents. Evoked activity that resulted from pinching the left triceps was reduced by PAG and midbrain RF stimulation (100 Hz, 100 microseconds, 50-500 microA). Visceral afferent input to the neurons was elicited during electrical stimulation of cardiopulmonary sympathetic afferent fibers and following injection of bradykinin into the left atrium of the heart. Electrical stimulation of the PAG and adjacent RF decreased A-delta and C-fiber activation of these neurons produced during electrical stimulation of cardiopulmonary afferents and decreased the activity of cells during excitatory responses to intracardiac bradykinin. Electrical stimulation of the PAG or midbrain RF similarly decreased spontaneous and evoked cell activity. Selective activation of cell bodies with microinjection of glutamate into the PAG reduced the activity of 6 of 8 cells whereas glutamate injections into midbrain RF reduced the activity of only 3 of 13 neurons. This difference in the effectiveness of chemically stimulating the PAG vs midbrain RF was significant (P less than 0.05). These data demonstrate that (1) neuronal activity evoked by visceral afferent input from the heart was decreased by electrical stimulation of the PAG and midbrain RF and (2) a portion of this descending inhibition may be mediated by cell bodies in the PAG.     

Reticulospinal neurons inactivated by warming of the preoptic area and anterior hypothalamus of rabbits

To identify the premotor neurons for vasoconstrictors of the skin, activities of reticulospinal neurons in the rostroventral medulla, the ear sympathetic nerve (ESNA) and the renal sympathetic nerve (RSNA) were recorded in anesthetized and immobilized Japanese White or New Zealand White rabbits. Two groups of neurons were identified according to their responses to thermal stimulation of the preoptic area and the anterior hypothalamus (POAH) and to electrical stimulation of baroreceptor afferents, the aortic nerve (AN). Neurons (Type I neurons, n = 21) whose activity was inhibited by warm stimulation of the POAH but not inhibited by the AN stimulation were located in sites medial to the rostral ventrolateral medulla (RVLM). The other neurons (Type II neurons, n = 20) whose activity was not inhibited by warm stimulation of the POAH but inhibited by the AN stimulation were located in the RVLM. Because the time course of the inhibitory response of Type I neurons to warm stimulation of the POAH was very similar to that of the inhibitory response of the ESNA and activities of these neurons and the ESNA were not inhibited by the stimulation of the AN, it was suggested the Type I neurons might participate in regulation of activity of the vasoconstrictors of the ear skin. The Type II neurons are considered to be the barosensitive RVLM neurons that regulate systemic arterial pressure by controlling the activity of visceral or muscular sympathetic vasoconstrictors or cardiac sympathetic fibers.     

Viscerosomatic convergence and responses to intestinal distension of neurons at the junction of midbrain and posterior thalamus in the cat

In cats anesthetized with sodium pentobarbital, microelectrode recordings were made from single neurons in the posterior thalamic region of termination of the spinothalamic tract (medial magnocellular division of the medial geniculate) and adjoining mesencephalic reticular formation, to determine if they receive input of visceral as well as of somatic origin. Of 309 units encountered, 136 (44%) demonstrated viscerosomatic convergency by responding to electrical stimulation of the greater splanchnic nerve as well as a somatic nerve (superficial radial and/or posterior tibial). Of 125 units tested, 42 (34%) had large somatic receptive fields spanning two or more limbs. Most of these responded best to intense skin stimuli (pressure, pinch, sometimes noxious heating). The remaining 83 had receptive fields restricted to part of one limb, and most of these responded to weak stimuli (e.g., hair movement, light tap) with no increment in responses to stronger stimuli. To determine of neurons in this region respond to intense visceral stimulation, the small intestine was distended by inflation of a balloon catheter which was inserted into a fistulated portion of the jejunum. Of 134 units with splanchnic nerve input, 28 responded to intestinal distension. Nine units responded only during the initial phase of distension. The other 19 units responded for all or part of the stimulus duration. Each of these 19 units tested had large somatic receptive fields, and most responded best to strong stimuli (e.g., pinch). In addition, they typically had thresholds for electrical stimulation of the splanchnic nerve which were well above threshold for the viscerointercostal reflex, suggesting that the input was mediated by Aδ-C fibers.     

Cardiopulmonary sympathetic afferent input to lower thoracic spinal neurons

Spinal neuronal responses to stimulation of cardiopulmonary sympathetic afferent (CPS) fibers were studied in 25 alpha-chloralose-anesthetized cats. Eighty-two neurons located in the T7-T9 segments were tested for responses to electrical stimulation of CPS fibers. Activity of 55 neurons was altered; 37 were excited, 10 were inhibited, and 8 were both excited and inhibited. All 55 cells with CPS input also responded to stimulation of somatic receptors and the left greater splanchnic nerve (SPL). Somatic receptive fields were primarily located on the upper portion of the abdomen and left lower rib cage. Short and long latency responses occurred following CPS and SPL stimulation. Latencies of responses to CPS stimulation were significantly longer than latencies of responses to SPL stimulation (P less than 0.05). Early responses to CPS stimulation were significantly less in magnitude compared to early responses to SPL stimulation (P less than 0.05). Cell responses to CPS stimulation were reduced in magnitude for as long as 300 ms when a conditioning stimulus was applied to SPL. Inhibitory responses of 10 cells to CPS fiber stimulation were best observed during repetitive stimulation. Eight of the cells were also inhibited by repetitive stimulation of SPL. Injection of bradykinin (4 micrograms/kg) into the left atrium increased activity of 16/30 cells from 8 +/- 2 to 22 +/- 5 spikes/s. The results demonstrate that CPS fiber stimulation alters activity of lower thoracic spinal neurons but not as intensely as SPL stimulation. These neurons may participate in cardiac-abdominal visceral reflexes or the pain of cardiac origin that is referred to the abdomen.     

Reflex control of the gastric motility by the vagus and splanchnic nerves in the guinea pig in vivo

Gastric responses to electrical stimulation of the vagus and splanchnic nerves and to esophageal distension were observed in studies of the reflex control of the gastric motility in anesthetized guinea pigs in vivo. The gastric motility was inhibited by both vago-vagal and splanchno-vagal reflexes through the activation of non-adrenergic inhibitory nerve fibers and by splanchno-splanchnic reflex through the inactivation of the intramural cholinergic excitatory neurons. Distension of the lower esophagus caused a relaxation of the stomach which was mediated by an intrinsic reflex via intramural non-adrenergic inhibitory neurons. Furthermore, stimulation of the splanchnic nerve innervating the stomach seems to activate adrenergic inhibitory, cholinergic excitatory and probably some non-adrenergic inhibitory fibers.     

Evidence for a tonic GABA-ergic inhibition of excitatory respiratory-related afferents to presympathetic neurons in the rostral ventrolateral medulla

The effect of blockade of ionotropic GABA and glutamate receptors in the rostral ventrolateral medulla (RVLM) on the relationship between phrenic nerve, splanchnic sympathetic nerve and lumbar sympathetic nerve activities was examined in urethane anesthetized, paralyzed and vagotomized Sprague-Dawley rats. Bilateral microinjection of the GABA-A receptor antagonist, bicuculline (4 mM, 100 nl), into the RVLM dramatically, and almost exclusively, increased the post-inspiratory related discharge in both splanchnic sympathetic nerve and lumbar sympathetic nerve activities and elicited hypertension with fluctuations of arterial pressure phase locked to the discharge of the phrenic nerve. Subsequent bilateral microinjection of kynurenate, a non-selective ionotropic excitatory amino acid receptor antagonist (50 mM, 100 nl), into the RVLM significantly attenuated the sympathoexcitation and hypertension evoked by injection of bicuculline. This was accompanied by an abolition of the post-inspiratory related burst discharge of splanchnic sympathetic nerve and lumbar sympathetic nerve activities. These data suggest that the GABAergic inputs to RVLM tonically inhibit glutamatergic inputs from central respiratory neurons that normally act to increase the firing of presympathetic neurons in the RVLM. Inputs from post-inspiratory neurons appear to be an especially potent excitatory synaptic drive to the presympathetic neurons in the absence of the GABAergic inhibition.     

Effects of electrical and chemical stimulation of nucleus raphe magnus on responses to renal nerve stimulation

Electrical stimulation of the nucleus raphe magnus (NRM) inhibits some somatic and visceral input at the spinal level. This study was designed to examine the effects of electrical and chemical stimulation of NRM on neuronal responses to afferent renal nerve (ARN) stimulation. In chloralose-anesthetized rats, electrical stimulation of ARN elicited predominantly excitatory responses in spinal gray neurons. In 10 neurons studied, electrical stimulation of the NRM elicited an inhibition of spontaneous activity of 8 neurons and inhibited evoked responses to ARN stimulation in 6 neurons. Microinjection of glutamate (5-10 nmol in 0.5-1 microliter) into the NRM elicited an inhibition of spontaneous activity in 9 neurons, a facilitation in 6 neurons and no response in 8 neurons receiving ARN input. Responses evoked by ARN stimulation were inhibited in 12 neurons, facilitated in 4 neurons and not affected in 8 neurons. We conclude that renal input can be modulated at the spinal level by activation of the NRM and adjacent tissue. Furthermore, the inhibition of spinal gray neuronal responses elicited by stimulation of the NRM appears to be due, at least in part, to activation of fibers of passage since non-selective electrical stimulation is more efficacious than selective chemical stimulation of neuronal soma and dendrites.     

Intersegmental connections and interactions of myelinated somatic and visceral afferents with sympathetic preganglionic neurons in the unanesthetized spinal cat

The aim of this study was to obtain a measure of the interactions through exclusively spinal circuits, of myelinated afferents with sympathetic preganglionic neurons. Experiments were performed on 16 unanesthetized cats rendered insensitive by bilateral vertebral and carotid occlusion, whose spinal cords had been transected at C1 6-12 h before recording. The evoked responses of 68 tonically active sympathetic preganglionic neurons were recorded from filaments dissected from the cervical sympathetic trunk. Excitation, inhibition and excitation-inhibition sequences were evoked by electrical stimulation of radial, femoral and pelvic nerve afferents. Inhibition was most often observed during pelvic nerve stimulation. Ninety percent of the sympathetic preganglionic neurons tested responded to radial, 77% to femoral and 85% to pelvic nerve stimulation. These differences in percentage of units responding to the three nerves were statistically insignificant. Thus, in the acute spinal cat, the fraction of tonically active sympathetic preganglionic neurons whose activity can be influenced by myelinated afferents is independent of the length of the intraspinal pathway which conveys the input. A main difference between the long pathway (mediating the responses to femoral or pelvic nerve) and the short pathway (mediating the responses to radial nerve) seems to be the efficacy of their connections. While single shocks reliably evoked responses from the radial nerve, trains (200 Hz, 20 msec) were usually necessary to elicit responses from femoral or pelvic nerve, indicating a requirement for summation in the long pathway.     

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.