Fast excitatory postsynaptic potentials and the responses to excitant amino acids of sympathetic preganglionic neurons in the slice of the cat spinal cord

Effects of Postural Resonance on Skin Sympathetic Nerve Activity and Blood Pressure: A Pilot Study Evaluating Vascular Tone Baroreflex Stimulation Through Biofeedback

Heart rate and vascular tension baroreflex exhibit resonance characteristics at approximately 0.1 and 0.03 Hz. In this study, we aimed to induce postural resonance (PR) through rhythmic postural adjustments. To assess the viability of this technique, we investigated the acute impacts of postural resonance on blood pressure, sympathetic nerve activity, and mood. Fifteen healthy study participants, consisting of 8 males and 7 females, were selected for this self-controlled study. Skin sympathetic nerve activity was continuously monitored during both the intervention and stress test on the experimental day. After PR intervention, the diastolic blood pressure and mean arterial pressure in the PR group exhibited significant reductions compared to the CON group (P = 0.032, CON = 71.67 ± 2.348, PR = 64.08 ± 2.35; P = 0.041, CON = 75.00 ± 2.17, PR = 81.67 ± 2.17). After PR intervention both left brachial ankle pulse wave velocity and right brachial ankle pulse wave velocity exhibited a significant reduction compared to pre-intervention levels (from 1115.86 ± 150.08 to 1048.43 ± 127.40 cm/s, p < 0.001; 1103.86 ± 144.35 to 1060.43 ± 121.35 cm/s, p = 0.018). PR intervention also led to a significant decrease in burst frequency and duration (P = 0.049; CON = 8.96 ± 1.17, PR = 5.51 ± 1.17) and a noteworthy decrease in burst amplitude and burst threshold during the cold-pressor test (P = 0.002; P = 0.002). Additionally, VAS scores exhibited a substantial increase following PR (P = 0.035, CON = 28.4 ± 4.49, PR = 42.17 ± 4.10). PR can induce resonance effects within the cardiovascular system, resulting in the effective reduction of blood pressure, skin sympathetic nerve activity and pulse wave velocity, and decreased burst amplitude and burst threshold of the sympathetic nerve during the cold-pressor test.

Sympathetic preganglionic neurons: properties and inputs

The sympathetic nervous system comprises one half of the autonomic nervous system and participates in maintaining homeostasis and enabling organisms to respond in an appropriate manner to perturbations in their environment, either internal or external. The sympathetic preganglionic neurons (SPNs) lie within the spinal cord and their axons traverse the ventral horn to exit in ventral roots where they form synapses onto postganglionic neurons. Thus, these neurons are the last point at which the central nervous system can exert an effect to enable changes in sympathetic outflow. This review considers the degree of complexity of sympathetic control occurring at the level of the spinal cord. The morphology and targets of SPNs illustrate the diversity within this group, as do their diverse intrinsic properties which reveal some functional significance of these properties. SPNs show high degrees of coupled activity, mediated through gap junctions, that enables rapid and coordinated responses; these gap junctions contribute to the rhythmic activity so critical to sympathetic outflow. The main inputs onto SPNs are considered; these comprise afferent, descending, and interneuronal influences that themselves enable functionally appropriate changes in SPN activity. The complexity of inputs is further demonstrated by the plethora of receptors that mediate the different responses in SPNs; their origins and effects are plentiful and diverse. Together these different inputs and the intrinsic and coupled activity of SPNs result in the rhythmic nature of sympathetic outflow from the spinal cord, which has a variety of frequencies that can be altered in different conditions.

Effects of intrathecal kynurenate on arterial pressure during chronic osmotic stress in conscious rats

Increased plasma osmolality elevates mean arterial pressure (MAP) through activation of the sympathetic nervous system, but the neurotransmitters released in the spinal cord to regulate MAP during osmotic stress remain unresolved. Glutamatergic neurons of the rostral ventrolateral medulla project to sympathetic preganglionic neurons in the spinal cord and are likely activated during conditions of osmotic stress; however, this has not been examined in conscious rats. This study investigated whether increased MAP during chronic osmotic stress depends on activation of spinal glutamate receptors. Rats were chronically instrumented with an indwelling intrathecal (i.t.) catheter for antagonist delivery to the spinal cord and a radiotelemetry transmitter for continuous monitoring of MAP and heart rate. Osmotic stress induced by 48 h of water deprivation (WD) increased MAP by ~15 mmHg. Intrathecal kynurenic acid, a nonspecific antagonist of ionotropic glutamate receptors, decreased MAP significantly more after 48 h of WD compared with the water-replete state. Water-deprived rats also showed a greater fall in MAP in response to i.t. 2-amino-5-phosphonovalerate. Finally, i.t. kynurenic acid also decreased MAP more in an osmotically driven model of neurogenic hypertension, the DOCA-salt rat, compared with normotensive controls. Our results suggest that spinally released glutamate mediates increased MAP during 48-h WD and DOCA-salt hypertension.

Involvement of protein kinase C and Src tyrosine kinase in acute tolerance to ethanol inhibition of spinal NMDA-induced pressor responses in rats

BACKGROUND AND PURPOSE

The present study was carried out to examine the role of protein kinases in the development of acute tolerance to the effects of ethanol on spinal N-methyl-D-aspartate (NMDA) receptor-mediated pressor responses during prolonged ethanol exposure.

EXPERIMENTAL APPROACH

Blood pressure responses induced by intrathecal injection of NMDA were recorded. The levels of several phosphorylated residues on NMDA receptor NR1 (GluN1) (NR1) and NMDA receptor NR2B (GluN2B) (NR2B) subunits were determined by immunohistochemistry and Western blot analysis.

KEY RESULTS

Ethanol inhibited spinal NMDA-induced pressor responses at 10 min, but the inhibition was significantly reduced at 40 min following continuous infusion. This effect was dose-dependently blocked by chelerythrine [a protein kinase C (PKC) inhibitor, 1-1000 pmol] or PP2 (a Src family tyrosine kinase inhibitor, 1-100 pmol) administered intrathecally 10 min following ethanol infusion. A significant increase in the immunoreactivity of phosphoserine 896 of NR1 subunits (pNR1-Ser896) and phosphotyrosine 1336 of NR2B subunits (pNR2B-Tyr1336) was found in neurons of intermediolateral cell column during the development of tolerance. Levels of pNR1-Ser896 and pNR2B-Tyr1336 were also significantly increased in lateral horn regions of the spinal cord slices incubated with ethanol for 40 min in vitro. The increases in pNR1-Ser896 and pNR2B-Tyr1336 levels were inhibited by post-treatment with chelerythrine and PP2, respectively, both in the in vivo and in vitro studies.

CONCLUSIONS AND IMPLICATIONS

The results suggest that activation of PKC and Src tyrosine kinase during prolonged ethanol exposure leading to increases in the levels of pNR1-Ser896 and pNR2B-Tyr1336 may contribute to acute tolerance to inhibition by ethanol of NMDA receptor function.

Diversity of sympathetic vasoconstrictor pathways and their plasticity after spinal cord injury

Sympathetic vasoconstrictor pathways pass through paravertebral ganglia carrying ongoing and reflex activity arising within the central nervous system to their vascular targets. The pattern of reflex activity is selective for particular vascular beds and appropriate for the physiological outcome (vasoconstriction or vasodilation). The preganglionic signals are distributed to most postganglionic neurones in ganglia via synapses that are always suprathreshold for action potential initiation (like skeletal neuromuscular junctions).

Most postganglionic neurones receive only one of these “strong” inputs, other preganglionic connections being ineffective. Pre- and postganglionic neurones discharge normally at frequencies of 0.5–1 Hz and maximally in short bursts at <10 Hz. Animal experiments have revealed unexpected changes in these pathways following spinal cord injury. (1) After destruction of preganglionic neurones or axons, surviving terminals in ganglia sprout and rapidly re-establish strong connections, probably even to inappropriate postganglionic neurones. This could explain aberrant reflexes after spinal cord injury. (2) Cutaneous (tail) and splanchnic (mesenteric) arteries taken from below a spinal transection show dramatically enhanced responses in vitro to norepinephrine released from perivascular nerves. However the mechanisms that are modified differ between the two vessels, being mostly postjunctional in the tail artery and mostly prejunctional in the mesenteric artery. The changes are mimicked when postganglionic neurones are silenced by removal of their preganglionic input. Whether or not other arteries are also hyperresponsive to reflex activation, these observations suggest that the greatest contribution to raised peripheral resistance in autonomic dysreflexia follows the modifications of neurovascular transmission.

Effects of spinal cord injury on synaptic inputs to sympathetic preganglionic neurons

Spinal cord injuries often lead to disorders in the control of autonomic function, including problems with blood pressure regulation, voiding, defecation and reproduction. The root cause of all these problems is the destruction of brain pathways that control spinal autonomic neurons lying caudal to the lesion. Changes induced by spinal cord injuries have been most extensively studied in sympathetic preganglionic neurons, cholinergic autonomic neurons with cell bodies in the lateral horn of thoracic and upper lumbar spinal cord that are the sources of sympathetic outflow. After an injury, sympathetic preganglionic neurons in mid-thoracic cord show plastic changes in their morphology. There is also extensive loss of synaptic input from the brain, leaving these neurons profoundly denervated in the acute phase of injury. Our recent studies on sympathetic preganglionic neurons in lower thoracic and upper lumbar cord that regulate the pelvic viscera suggest that these neurons are not so severely affected by spinal cord injury. Spinal interneurons appear to contribute most of the synaptic input to these neurons so that injury does not result in extensive denervation. Since intraspinal circuitry remains intact after injury, drug treatments targeting these neurons should help to normalize sympathetically mediated pelvic visceral reflexes. Furthermore, sympathetic pelvic visceral control may be more easily restored after an injury because it is less dependent on the re-establishment of direct synaptic input from regrowing brain axons.

Cocaine- and amphetamine-regulated transcript peptide potentiates spinal glutamatergic sympathoexcitation in anesthetized rats

Cocaine- and amphetamine-regulated transcript (CART) is widely expressed in the rat central nervous system, notably in areas involved in control of autonomic and neuroendocrine functions. The aim of this study was to evaluate the effects of CART peptide fragment 55-102, referred to herein as CARTp, by intrathecal injection on blood pressure (BP) and heart rate (HR) before and after intrathecal glutamate in urethane-anesthetized male Sprague-Dawley rats. CARTp (0.1-10 nmol) administered intrathecally caused no or a small, statistically insignificant increase of blood pressure and heart rate, except at the concentration of 10 nmol, which caused a significant increase of blood pressure and heart rate. Intrathecal glutamate (0.1-10 nmol) produced a dose-dependent increase in arterial pressure and heart rate. Pretreatment with CARTp dose-dependently potentiated the pressor effects of glutamate (1 nmol), which by itself elicited a moderate increase of blood pressure and heart rate. Further, CARTp significantly potentiated the tachycardic effect of glutamate at 1 and 5 nmol, but attenuated the response at 10 nmol. The effect of CARTp was long-lasting, as it enhanced glutamatergic responses up to 90 min after administration. Prior injection of CARTp antiserum (1:500) but not normal rabbit serum nullified the potentiating effect of CARTp on glutamatergic responses. The result suggests that CARTp, whose immunoreactivity is detectable in sympathetic preganglionic neurons as well as in fibers projecting into the intermediolateral cell column, augments spinal sympathetic outflow elicited by glutamate at lower concentrations and may directly excite neurons in the intermediolateral cell column at higher concentrations.

Hydrogen peroxide increases the activity of rat sympathetic preganglionic neurons in vivo and in vitro

Reactive oxygen species (ROS) have been shown to modulate neuronal synaptic transmission and have also been implicated in cardiovascular diseases such as hypertension. The hypothesis that H(2)O(2) acting on sympathetic preganglionic neurons (SPNs) affects spinal sympathetic outflow was tested in the present study. H(2)O(2) was applied intrathecally via an implanted cannula to the T7-T9 segments of urethane-anesthetized rats. Blood pressure and heart rate were used as indices to evaluate the spinal sympathetic effects of H(2)O(2) in vivo. Intrathecal H(2)O(2) (100-1000 nmol) dose-dependently increased both the mean arterial pressure and heart rate. Reproducible pressor effects of H(2)O(2) (1000 nmol) applied consecutively at intervals of 30 min were observed. The pressor effects of intrathecal H(2)O(2) (1000 nmol) were attenuated by pretreatment with intrathecal administration of catalase (500 units), or N-acetyl-cysteine (1000 nmol). The pressor effects of intrathecal H(2)O(2) (1000 nmol) were also antagonized dose-dependently by prior intrathecal injection of AP-5 (DL-2-amino-5- phosphonovaleric acid, 10 and 30 nmol), or 6-cyano-7- nitroquinoxaline-2,3-dione, 10 and 30 nmol. In vitro electrophysiological study in spinal cord slices showed that superfusion of 1 mM H(2)O(2) for 3 min, which had no effect on membrane potential, caused an increase in amplitude of excitatory postsynaptic potentials in SPNs, but had little effect on that of inhibitory postsynaptic potentials. Taken together, these results demonstrated that oxidative stress in spinal cord may cause an increase in spinal sympathetic tone by acting on SPNs, which may contribute to ROS-induced cardiovascular dysfunction.

Inhibition by ethanol of NMDA-induced responses and acute tolerance to the inhibition in rat sympathetic preganglionic neurons in vitro and in vivo

N-methyl-d-aspartate (NMDA) receptors have been demonstrated to be a pivotal target for ethanol action. The present study examined the actions of acute ethanol exposure on NMDA-induced responses and the acute tolerance to ethanol actions in rat sympathetic preganglionic neurons (SPNs) in vitro and in vivo. NMDA (50 microM) applied every 5 min induced reproducible membrane depolarizations of SPNs in neonatal spinal cord slice preparations. Ethanol (50 - 100 mM) applied by superfusion for 15 min caused a sustained decrease in NMDA-induced depolarizations in a dose-dependent and reversible manner. When the superfusion time of ethanol (100 mm) was increased to 50 min, NMDA-induced depolarizations were attenuated initially but a gradual recovery was seen in approximately 40% of SPNs tested. Repeated injections of NMDA (2 nM) intrathecally at 30 min interval caused reproducible increases in mean arterial pressure (MAP) in urethane-anesthetized rats. Intravenous injections of ethanol (0.16 or 0.32 g, 1 ml) inhibited NMDA-induced pressor effects in a blood concentration-dependent manner. The inhibition by ethanol of NMDA-induced pressor effects was reduced over time during continuous infusion of ethanol or on the second injection 3.5 h after prior injection of a higher dose of ethanol. Ethanol, at concentrations significantly inhibited NMDA-induced responses, had no significant effects on alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-induced responses. The study demonstrated the selective inhibition by ethanol of NMDA-induced responses and the development of acute tolerance to the inhibitory effects in SPNs both in vitro and in vivo. These effects may play important roles in the ethanol regulation of cardiovascular function.

Dynamic balance of metabotropic inputs causes dorsal horn neurons to switch functional states

Sensory relay structures in the spinal cord dorsal horn are now thought to be active processing structures that function before supraspinal sensory integration. Dorsal horn neurons directly receive nociceptive (pain) signals from the periphery, express a high degree of functional plasticity and are involved in long-term sensitization and chronic pain. We show here that deep dorsal horn neurons (DHNs) in Wistar rats can switch their intrinsic firing properties from tonic to plateau or endogenous bursting patterns, depending upon the balance of control by metabotropic glutamate (mGlu) and GABA(B) receptors. We further show that this modulation acts on at least one common target, the inwardly rectifying potassium channel (Kir3). Finally, we found that these firing modes correspond to specific functional states of information transfer in which dorsal horn neurons can faithfully transmit, greatly enhance or block the transfer of nociceptive information.

Glutamate and GABA content of calbindin-immunoreactive nerve terminals in the rat intermediolateral cell column

Immunoreactivity for calbindin-D28K (calbindin) occurs in some bulbospinal vasopressor neurons in the rostral ventrolateral medulla and calbindin-immunoreactive terminals form synapses in the intermediolateral cell column (IML), where the cell bodies of sympathetic preganglionic neurons are located. In this study, we used post-embedding immunogold labelling to determine whether calbindin terminals in the IML contained the excitatory amino acid neurotransmitter glutamate. We also assessed GABA immunoreactivity in semi-serial sections through the same terminals since this inhibitory amino acid transmitter is present in the inputs to sympathetic preganglionic neurons that lack glutamate. Analysis of 42 calbindin-positive terminals whose postsynaptic targets were not identified revealed two major groups on the basis of amino acid content. One group was immunoreactive for glutamate; and the other, for GABA. In addition, about 20% of the calbindin terminals were positive for both glutamate and GABA. Our anatomical methods cannot differentiate whether this third group is a subset of the GABAergic terminals or a separate population capable of co-releasing the two amino acids.

GABA in the control of sympathetic preganglionic neurons

1. Amino acid neurotransmitters are critical for controlling the activity of most central neurons, including sympathetic preganglionic neurons (SPN), the spinal cord neurons involved in controlling blood pressure and other autonomic functions. 2. In studies reviewed here, SPN were identified either by retrograde tracing from a peripheral target (superior cervical ganglion or adrenal medulla) or by detection of immunoreactivity for choline acetyltransferase (ChAT), the acetylcholine-synthesizing enzyme that is a marker for all SPN, in intact or completely transected rat spinal cord. 3. Postembedding immunogold labelling on ultrathin sections was then used to detect GABA and sometimes glutamate in nerve terminals on SPN or near them in the neuropil of the lateral horn. 4. In some cases, the terminals were prelabelled to show an anterograde tracer or immunoreactivity for ChAT or neuropeptide Y. 5. This anatomical work has provided information that is helpful in understanding how SPN are influenced by their GABAergic innervation. 6. Immunogold studies showed that the proportion of input provided by GABAergic terminals varies between different groups of SPN. For some groups, this input may be preferentially targeted to cell bodies. 7. Anterograde tracing demonstrated that supraspinal as well as intraspinal GABAergic neurons innervate SPN and investigations on completely transected cord suggested that supraspinal neurons may provide a surprisingly large proportion of the GABAergic terminals that contact SPN. 8. The double-labelling studies in which other amino acids, ChAT or neuropeptide Y were localized along with GABA indicate that GABAergic terminals contain other neurochemicals that could modulate the actions of GABA, depending on the complement of receptors that are present pre- and post-synaptically. 9. Taken together, these data indicate that GABAergic transmission to SPN may be much more complicated than suggested by the currently available electrophysiological studies.

Dorsal column nuclei neurons recorded in a brain stem-spinal cord preparation: characteristics and their responses to dorsal root stimulation

Recordings were obtained from dorsal column nucleus (DCN) neurons in a neonatal rat brain stem-spinal cord preparation to study their basic electrophysiological properties and responses to stimulation of a dorsal root. Whole-cell patch-clamp recordings were made from 21 neurons that responded to dorsal root stimulation with a fast excitatory postsynaptic potential (EPSP). These neurons were located lateral to, but at the level of, the area postrema at depths of 100-268 microm below the dorsal surface of the brain. The neurons could be divided into groups according to the shape of their action potentials or voltage responses to hyperpolarizing current steps; however, the response profiles of the groups of neurons to dorsal root stimulation were not significantly different and all neurons were considered together. Dorsal root stimulation elicited excitatory postsynaptic potentials (EPSPs) in all neurons with a very low variability in onset latency and an ability to follow 100-Hz stimulation, indicating that they were mediated by activation of a monosynaptic pathway. The peak amplitude of the EPSP increased with membrane hyperpolarization, and applications of the non-NMDA receptor antagonists 6-nitro-7-sulfamoylbenzo[f]quinoxaline-2, 3-dione (NBQX) and 6,7-dinitroquinoxaline-2,3-dione (DNQX) decreased the amplitude of the EPSP to 14.2% of the control response (n = 6). The descending phase of the EPSP decreased with membrane hyperpolarization and was reduced by the N-methyl-D-aspartate (NMDA) receptor antagonist AP-5 (n = 2). The EPSPs were also reduced in amplitude by applications of the gamma-aminobutyric acid-B (GABA(B)) receptor agonist baclofen, which had no effect on membrane potential or input resistance. These results show that fast EPSPs in DCN neurons elicited by dorsal root stimulation are mediated by an excitatory amino acid acting at both non-NMDA and, to a lesser extent, NMDA receptors. In addition, GABA acting at presynaptic GABA(B) receptors can inhibit these responses.

Glutamate receptor subunits associated with rat sympathetic preganglionic neurons

The purpose of this study was to determine what subunits of the glutamate (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)) receptor are expressed by sympathetic preganglionic neurons in the spinal cord of adult rats. Preganglionic neurons were retrogradely labelled with Fluorogold, double-labelled with choline acetyltransferase immunofluorescence, and examined with confocal microscopy for evidence of immunoreactivity for GluR1, GluR2, GluR2/3 and GluR4 subunits. Quantitative analysis revealed that 92, 63 and 85% of preganglionic cells in the T8 segment express GluR1, GluR2 and GluR2/3 subunits, respectively. Cells were not immunoreactive for the GluR4 subunit. This evidence is consistent with the idea that most sympathetic preganglionic neurons form heteromeric AMPA receptors. Cells with GluR2 subunits will assemble receptors which are impermeable to calcium ions and may be resistant to excitotoxic cell death.

Metabotropic glutamate receptor-mediated excitation and inhibition of sympathetic preganglionic neurones

The effects of metabotropic glutamate receptor (mGluR) subtype selective compounds on the excitability of sympathetic preganglionic neurones (SPNs) were investigated. Non-selective mGluR agonists (1S,3R)-aminocyclopentane-1,3-dicarboxylic acid and (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine, induced dose-dependent depolarisations in 96 and 75% of SPNs, respectively and hyperpolarisations in 2 and 21% of SPNs. Both agonists could induce subthreshold membrane potential oscillations in previously non-oscillating SPNs and either increased or reduced the frequency of spontaneously occurring oscillations. A selective group I mGluR agonist, 3,5-dihydroxyphenylglycine, depolarised all SPNs tested, induced oscillations in membrane potential of otherwise non-oscillating SPNs and increased the frequency of spontaneous oscillations. Agonists with selectivity for group II mGluRs (1S,3S)-aminocyclopentane-1,3-dicarboxylic acid and (S)-4-carboxy-3-hydroxy-phenylglycine ((S)-4C3HPG) did not induce depolarising responses. However (S)-4C3HPG induced hyperpolarising responses associated with a reduction in the frequency of spontaneous oscillations in two of six SPNs tested. Depolarising and hyperpolarising responses were maintained in the presence of tetrodotoxin indicating a direct action of the agonists upon SPNs. In individual SPNs responses of opposite polarity could be induced from the same initial membrane potential using different agonists, indicating that the opposing responses involved different ionic mechanisms. The broad spectrum mGluR antagonist (S)-alpha-methyl-4-carboxyphenylglycine and the selective group I mGluR antagonist (S)-4-carboxyphenylglycine reversibly depressed mGluR agonist induced depolarisations. These results indicate that SPNs express two mGluR populations with opposing actions on neuronal excitability: group I mGluRs depolarise SPNs and can drive oscillatory membrane potential activity; a minority of SPNs express group II mGluRs which mediate membrane hyperpolarisations and reduce the frequency of membrane potential oscillations.

GABA- and glutamate-immunoreactive synapses on sympathetic preganglionic neurons projecting to the superior cervical ganglion

Our previous work suggests that virtually all of the synapses on sympathetic preganglionic neurons projecting to the rat adrenal medulla are immunoreactive for either the inhibitory amino acid, gamma-aminobutyric acid (GABA) or the excitatory amino acid, L-glutamate. To investigate whether or not this is true for other groups of sympathetic preganglionic neurons, and to determine whether or not the proportion of inputs containing each type of amino acid neurotransmitter is the same for different groups of sympathetic preganglionic neurons, we retrogradely labelled rat and rabbit sympathetic preganglionic neurons projecting to the superior cervical ganglion and used post-embedding immunogold on ultrathin sections to localise GABA- and glutamate-immunoreactivity. The cell bodies and dendrites of both rat and rabbit sympathetic preganglionic neurons projecting to the superior cervical ganglion received synapses and direct contacts from nerve fibres immunoreactive for GABA and from nerve fibres immunoreactive for glutamate. In the rat, GABA was present in 48.9% of the inputs to sympathetic preganglionic neurons projecting to the superior cervical ganglion, and glutamate was present in 51.7% of inputs. Double immunogold labelling for glutamate and GABA on the same section, as well as labelling of consecutive serial sections for the two antigens, indicated that GABA and glutamate occur in separate populations of nerve fibres that provide input to rat sympathetic preganglionic neurons projecting to the superior cervical ganglion. We now have shown that GABA or glutamate is present in virtually all of the inputs to sympathetic preganglionic neurons projecting to the superior cervical ganglion and in essentially all of the inputs to sympathetic preganglionic neurons supplying the adrenal medulla. These findings are consistent with the hypothesis that all fast synaptic transmission in central autonomic pathways may be mediated by either excitatory or inhibitory amino acids. Furthermore, we showed a statistically significant difference in the proportion of glutamate-immunoreactive inputs between sympathetic preganglionic neurons projecting to the superior cervical ganglion and sympathoadrenal neurons (data from Llewellyn-Smith et al. [Llewellyn-Smith, I.J., Phend, K.D., Minson, J.B., Pilowsky, P.M., Chalmers, J.P., 1992. Glutamate immunoreactive synapses on retrogradely labelled sympathetic neurons in rat thoracic spinal cord. Brain Res. 581, 67-80]), with preganglionics supplying the adrenal medulla receiving more excitatory inputs than those supplying the superior cervical ganglion. This increased excitatory input to sympathoadrenal neurons may explain the predominant activation of these neurons following baroreceptor unloading.

Bilaterally evoked monosynaptic EPSPs, NMDA receptors and potentiation in rat sympathetic preganglionic neurones in vitro

1. Whole-cell patch clamp and intracellular recordings were obtained from 190 sympathetic preganglionic neurones (SPNs) in spinal cord slices of neonatal rats. Fifty-two of these SPNs were identified histologically as innervating the superior cervical ganglion (SCG) by the presence of Lucifer Yellow introduced from the patch pipette and the appearance of retrograde labelling following the injection of rhodamine-dextran-lysine into the SCG. 2. Electrical stimulation of the ipsilateral (n = 71) or contralateral (n = 32) lateral funiculi (iLF and cLF, respectively), contralateral intermediolateral nucleus (cIML, n = 41) or ipsilateral dorsal horn (DH, n = 34) evoked EPSPs or EPSCs that showed a constant latency and rise time, graded response to increased stimulus intensity, and no failures, suggesting a monosynaptic origin. 3. In all neurones tested (n = 60), fast rising and decaying components of EPSPs or EPSCs evoked from the iLF, cLF, cIML and DH in response to low-frequency stimulation (0.03-0.1 Hz) were sensitive to non-NMDA receptor antagonists. 4. In approximately 50 % of neurones tested (n = 29 of 60), EPSPs and EPSCs evoked from the iLF, cLF, cIML and DH during low-frequency stimulation were reduced by NMDA receptor antagonists. In the remaining neurones, an NMDA receptor antagonist-sensitive EPSP or EPSC was revealed only in magnesium-free bathing medium, or following high-frequency stimulation. 5. EPSPs evoked by stimulation of the iLF exhibited a sustained potentiation of the peak amplitude (25.3 +/- 11.4 %) in six of fourteen SPNs tested following a brief high-frequency stimulus (10-20 Hz, 0.1-2 s). 6. These results indicate that SPNs, including SPNs innervating the SCG, receive monosynaptic connections from both sides of the spinal cord. The neurotransmitter mediating transmission in some of the pathways activated by stimulation of iLF, cLF, cIML and DH is glutamate acting via both NMDA and non-NMDA receptors. Synaptic plasticity is a feature of glutamatergic transmission in some SPNs where EPSPs are potentiated following a brief high-frequency stimulus. Our data also suggest a differential expression of NMDA receptors by these neurones.

Sympathetic burst activity: characteristics and significance

1. The activity recorded from mammalian sympathetic nerves comes in bursts, which result from large numbers of fibres firing synchronously. 2. Human sympathetic nerve activity behaves similarly to that in animals, although burst rates may be lower. 3. Vasomotor, cardiac and sudomotor nerve fibres all fire in bursts. Whether other sympathetic pathways do so is unknown. 4. Sympathetic activity is intrinsically 'bursty' but not intrinsically regular. 5. Bursting is a population phenomenon, not usually evident in the firing of individual neurons. 6. Bursts in post-ganglionic nerves are driven by synchronously firing preganglionic neurons. 7. The origin of bursts remains controversial. Preganglionic neuron properties are likely to be important in at least shaping bursts. 8. Burst amplitude, which reflects the number of fibres firing together, and burst probability are controlled independently. 9. Baroreceptors affect burst probability over a wide range, but have less effect on mean burst amplitude. How they affect burst timing within the cardiac cycle is discussed. 10. Burst probability is determined 'downstream' of the rostral ventrolateral medulla, implicating either the spinal cord or recurrent brainstem connections in burst generation. 11. Neuroeffector responses are too slow to follow individual bursts. However, bursting will promote spatial facilitation at both ganglionic and effector levels, which may increase the dynamic range of neural control.

Glutamate- and GABA-immunoreactive synapses on sympathetic preganglionic neurons caudal to a spinal cord transection in rats

Spinal cord injury destroys bulbospinal amino acid-containing pathways to sympathetic preganglionic neurons and severely disrupts blood pressure control, resulting in resting or postural hypotension and episodic hypertension. Almost all immunoreactivity for the excitatory amino acid L-glutamate has been reported to disappear from autonomic areas of the cord caudal to a transection, apparently depriving autonomic neurons of their major excitatory input. However, the magnitude of the neurogenic episodic hypertension after cord injury suggests that excitatory inputs to sympathetic preganglionic neurons must still be present. Moreover, the hypotension associated with high spinal injuries may reflect a enhanced role for inhibitory transmitters, such as GABA. This apparent contradiction regarding the presence of glutamate and lack of information about GABA prompted the present investigation. In rats seven days after spinal cord transection, we examined identified sympathetic preganglionic neurons caudal to the injury for the presence of synapses or direct contacts from varicosities that were immunoreactive for the amino acids, L-glutamate and GABA. Adrenal sympathetic preganglionic neurons were retrogradely labelled with cholera toxin B subunit and amino acid immunoreactivity was revealed with post-embedding immunogold labelling. In single ultrathin sections, 46% (98/212) of the synapses or direct contacts on adrenal sympathetic preganglionic neurons were immunoreactive for glutamate and 39% (83/214) were immunoreactive for GABA. Analysis of inputs with the physical disector yielded similar results for the two amino acids. The proportions of glutamatergic or GABAergic synapses on cell bodies and dendrites were similar. When alternate ultrathin sections were stained to reveal glutamate or GABA immunoreactivity, either one or the other amino acid occurred in 78.4% (116/148) of inputs; 4.1% (6/148) of inputs contained both amino acids and 17.5% (26/148) of inputs contained neither. These results demonstrate that nerve fibres immunoreactive for the neurotransmitter amino acids, glutamate and GABA, provide most of the input to sympathetic preganglionic neurons caudal to a spinal cord transection. Synapses containing glutamate and GABA could provide the anatomical substrate for the exaggerated sympathetic reflexes and the low sympathetic tone that result from spinal cord injury.

A novel slow excitatory postsynaptic current in substantia gelatinosa neurons of the rat spinal cord in vitro

Whole-cell patch-clamp recordings were made from neurons in the substantia gelatinosa of adult rat spinal cord slices with attached dorsal root to study a slow synaptic current evoked by focal or dorsal root stimulation. Repetitive focal stimulation with a monopolar electrode positioned within substantia gelatinosa elicited a slow excitatory postsynaptic current preceded by a fast excitatory postsynaptic current in 73 of 83 neurons. A similar slow excitatory postsynaptic current was also elicited by stimulation of A delta afferent fibres. The amplitude of slow excitatory postsynaptic currents was unchanged when the recording electrode contained guanosine-5'-O-(2-thiodiphosphate). The slow excitatory postsynaptic current and current evoked by aspartate revealed similar reversal potentials and showed a marked outward rectification at holding potentials more negative than -30 mV, while the glutamate-induced current exhibited a relatively linear voltage relationship. In addition, the slow excitatory postsynaptic currents were reversibly occluded during the aspartate-induced current but were not occluded during the glutamate-induced current. The slow excitatory postsynaptic currents evoked by focal stimulation were depressed but not abolished by 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM) or by 6-cyano-7-nitroquinoxaline-2,3-dione together with DL-2-amino-5-phosphonopentanoic acid (100 microM). Similarly, the aspartate- and glutamate-induced currents were also resistant to these antagonists. These observations suggest that a transmitter released from interneurons or descending fibres which are activated in part by A delta afferents, mediates a slow excitatory postsynaptic currents in substantia gelatinosa neurons and that an excitatory amino acid is implicated in the generation of the slow excitatory postsynaptic current, although the receptor appears to differ from the known ligand-gated channels. C afferents are unlikely to contribute to the slow excitatory postsynaptic current. This slow synaptic response may participate in the pain pathway and play an important role in the processing of nociceptive information in the spinal dorsal horn.

Voltage-dependent potassium currents of sympathetic preganglionic neurons in neonatal rat spinal cord thin slices

Voltage-dependent potassium currents were analyzed in the visually identified sympathetic preganglionic neurons (SPNs) of neonatal rat spinal cord thin slices by the whole-cell patch-clamp technique. Some of the SPNs were identified by the presence of retrogradely transported fluorescent dye, DiI, injected into the superior cervical ganglion several days prior to experimentation. In a tetrodotoxin (TTX)-containing solution, a step depolarization from the holding potential of -72 mV generated a slow outward current that was suppressed by tetraethylammonium (TEA) and by Ca(2+)-free/2.5 ImM Co2+ solution. Ca(2+)-dependent current consisted of a transient and a sustained components. In a Ca(2+)-free (substituted with Mg2+) solution with TTX and TEA, a step depolarization from a hyperpolarized potential evoked a transient outward current that was blocked by 4-aminopyridine (4-AP). A step hyperpolarization evoked a voltage-dependent inward current, the conductance of which was dependent not only on the membrane potential, but also on the extracellular K+ concentration. Tail current analyses revealed that all of these currents were carried by K+ ions. These results indicate that SPN possesses at least five types of voltage-dependent K+ current, including the delayed rectifier current (IK), Ca(2+)-dependent transient current (IC), Ca(2+)-dependent sustained current (IAHP), A-current (IA) and inward rectifying current (Iu), which may be targets of putative transmitters released from various descending and segmental inputs impinging upon the SPN.

Developmental changes in the effects of serotonin and N-methyl-D-aspartate on intrinsic membrane properties of embryonic chick motoneurons

A spinal cord slice preparation was developed in order to study developmental changes in intrinsic membrane properties and in responses to N-methyl-D-aspartate and serotonin in embryonic chick motoneurons. Transverse spinal cord slices were obtained from chick embryos over a series of developmental stages (embryonic days 12-18). Intracellular recordings were obtained from 87 antidromically identified motoneurons. During the stages examined, the average resting membrane potential did not vary significantly, the voltage threshold of current-evoked action potentials became significantly more negative, there was a non-significant trend towards a decrease in the recorded input resistance, but there were no significant changes observed in the membrane time constant. There were significant developmental changes in the waveform of the current-evoked action potentials. The average amplitude of the action potentials increased over the stages studied, while the action potential duration measured at half-amplitude decreased. All of the motoneurons examined were maximally depolarized by bath application of 50 microM N-methyl-D-aspartate. The depolarization persisted in the presence of tetrodotoxin but was blocked by 100 microM 2-amino-5-phosphonopentanoic acid and, therefore, was at least partially due to a direct action of N-methyl-D-aspartate on motoneuronal receptors. The average amplitude of the N-methyl-D-aspartate-induced depolarizations decreased significantly over the stages examined. In contrast, bath application of 50 microM serotonin produced either depolarizing or hyperpolarizing responses depending on the developmental age of the motoneuron. Serotonin induced a depolarization in about 50% of the motoneurons at embryonic day 12, 69% of the motoneurons at embryonic day 15 and 100% of the motoneurons recorded from at embryonic day 18. These findings reveal important developmental changes in intrinsic membrane responses and action potential properties of chick motoneurons recorded from a slice preparation. We have also documented changes in the motoneuronal responses to serotonin, a neurotransmitter used by a major descending projection, and N-methyl-D-aspartate, which activates glutamate receptors known to contribute to synaptic activity in segmental circuits.

Excitation of sympathetic preganglionic neurons via metabotropic excitatory amino acid receptors

The role of excitatory amino acid metabotropic receptors in the regulation of excitability of sympathetic preganglionic neurons was investigated. This study used both conventional intracellular and whole-cell patch clamp techniques to record from sympathetic preganglionic neurons in transverse spinal cord slices of the rat (9-21 days old). The metabotropic receptor agonists (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) (10-200 microM, superfused for 2-60 s) and quisqualate (1-50 microM, superfused for 2-60 s) induced concentration-dependent depolarizing responses which did not desensitize. These responses were unaffected by the glutamate ionotropic receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10-50 microM), 6,7-dinitroquinoxaline-2,3-dione (DNQX, 10 microM), dizocilpine (MK-801, 10-40 microM), 3-[(R)-2-carboxy-piperazin-4-yl]-propyl-1-phosphonic acid (D-CPP, 10-50 microM) and DL-2-amino-5-phosphonovaleric acid (DL-AP5, 20-100 microM). Depolarizing responses to 1S,3R-ACPD and quisqualate were unaffected by L-2-amino-3-phosphonopropionic acid (L-AP3, 30 microM-1mM) and L-2-amino-4-phosphonobutanoic acid (L-AP4, 100 microM-1 mM)). The responses to 1S,3R-ACPD and quisqualate were reduced by including the G-protein blocker GDP-beta-S (400 microM) in the patch pipette solution by 77 +/- 2% (mean +/- S.E) of control (n = 3), suggesting that these agonists activate a G-protein-coupled receptor. Metabotropic receptor-mediated responses were maintained in the presence of tetrodotoxin (500 nM), progressively reduced with increased membrane hyperpolarization to around -95 mV and associated with either an increase of 16.5 +/- 2.8% (data from four neurons) in the majority of neurons (n = 22 of 34) or no measurable change (n = 12) in neuronal input resistance. These data suggest that the agonists exert a direct action on 1S,3R-ACPD and quisqualate had several effects on sympathetic preganglionic neuron membrane properties including: inhibition of a slow apamin-insensitive component of the afterhyperpolarization; a reduction in spike frequency adaptation leading to increases in firing frequency from 6.4 +/- 2.8 Hz in control experiments up to 14.7 +/- 3.0 Hz (n = 6 neurons) in the presence of a metabotropic receptor agonist: a broadening of the action potential by 37.5 +/- 6.4% (n = 6 neurons) of control. These observations suggest that the metabotropic receptor-mediated depolarization is due, at least in part, to the reduction of potassium conductances involved in the spike afterhyperpolarisation potential.(ABSTRACT TRUNCATED AT 400 WORDS)

Medullary-evoked EPSPs in neonatal rat sympathetic preganglionic neurones in vitro

1. Whole-cell patch clamp recordings were made from twenty-three sympathetic preganglionic neurones (SPNs) in the upper thoracic segments of a neonatal rat brainstem-spinal cord preparation to study their synaptic responses to stimulation of the rostral ventrolateral medulla (RVLM) and the receptors involved. 2. SPNs were identified by their antidromic activation following stimulation of a ventral root, their morphology and their location in the spinal cord. 3. Electrical stimulation within the RVLM elicited EPSPs in all SPNs tested (n = 23). These EPSPs consisted of one or more components that had different time courses, voltage relationships and pharmacological sensitivities. 4. All SPNs responded to RVLM stimulation with a constant-latency fast EPSP that increased in size as the membrane was hyperpolarized. This EPSP was reduced in amplitude by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (10-20 microM). 5. In thirteen SPNs the response to RVLM stimulation was a complex EPSP consisting of a fast EPSP and a slow EPSP that either followed or summed with the fast EPSP. The amplitude of the slow EPSP was (i) either reduced in size or not affected as the membrane was hyperpolarized, and (ii) reduced by the NMDA receptor antagonist, D, L-2-amino-5-phosphonovaleric acid (50 microM). 6. Selective activation of neuronal cell bodies in the RVLM by chemical stimulation elicited slow depolarizations and increases in synaptic activity in SPNs. 7. These results provide evidence that an excitatory amino acid is involved in transmitting sympathoexcitatory drive from the RVLM, partly via a monosynaptic pathway. Both non-NMDA and NMDA receptors play a role in mediating this drive.

Membrane properties and synaptic potentials in rat sympathetic preganglionic neurons studied in horizontal spinal cord slices in vitro

Intracellular recordings were made from neurons in the intermediolateral column and adjacent white matter in horizontal slices of upper thoracic spinal cord from rats aged 21-28 days. Membrane properties were studied in the presence of picrotoxin (100 microM) to block ongoing inhibitory synaptic potentials. 37 neurons were identified as sympathetic preganglionic neurons (SPNs) by their electrical behaviour, anatomical location and/or morphology. SPNs had resting potentials of -57 +/- 2 mV and input resistances of 254 +/- 31 M omega (n = 14). Following a hyperpolarising voltage step, a transient outward current was activated which had a time constant of decay of approx. 400 ms. The inflection in the repolarising phase of the action potential and the following prolonged AHP were both abolished by Cd2+ (50 microM). The current underlying the AHP had two components with kinetic properties similar to the two calcium-activated potassium conductances, gKCa1, and gKCa2, characterized in other autonomic neurons. Noradrenaline (10-100 microM) caused a small depolarization and blocked the calcium component of the action potential suppressing the AHP. This revealed an afterdepolarization (ADP) with an underlying inward current with a decay time constant of approx. 150 ms. All effects of noradrenaline were blocked by phentolamine (10 microM). Graded stimulation of the lateral funiculus 0.5-1 mm rostral to the recording site evoked in all cells monosynaptic fast excitatory synaptic potentials (fEPSPs) which were graded in amplitude. fEPSPs decayed with a time constant identical to the cell input time constant and were reduced in amplitude by CNQX (10-20 microM). In 7 cells, higher stimulus voltages elicited slow EPSPs with a time to peak of 1.1 +/- 0.1 s and a half decay of 2.8 +/- 0.3 s (n = 7) which were not reduced by alpha-adrenoceptor antagonists. The AHP was not blocked when the action potential was initiated during the slow EPSP. We conclude that excitatory bulbospinal inputs to SPNs involve at least one fast transmitter which is likely to be glutamate and one slow transmitter which is not noradrenaline.

The isolated mammalian spinal cord

This review considers: spinal cord slices; isolated spinal cord sagitally or transversely hemisected; whole spinal cord; respiration control--[brain-stem spinal cord; brain-stem spinal cord with attached lungs]; nociception--[spinal cord with tail]; fictive locomotion--[spinal cord with one hind limb; spinal cord with two hind limbs]. Much of the functional circuitry of the CNS can be studied in the isolated spinal cord with the additional advantage that the isolated spinal cord can be perfused with known concentrations of ions, neurotransmitters, agonists, antagonists, and anaesthetics. These can be washed away, the circuitry allowed to recover and other drugs or different concentrations applied. Future preparations including the complete spinal cord, the two hind limbs, and a sagittal section of the complete brain will allow greater understanding of the multiple sensory and motor pathways and their interactions in the CNS.

Localization of glutamatergic neurons in the dorsolateral pontine tegmentum projecting to the spinal cord of the cat with a proposed role of glutamate on lumbar motoneuron activity

Glutamate is considered to be a major excitatory neurotransmitter in the central nervous system. The presence of glutamate-like immunoreactive neurons in the rodent locus coeruleus has been reported previously. In this study we used both immunohistochemical and electrophysiological techniques to answer two major questions: (1) Is there any glutamate-like immunoreactivity in the catecholaminergic coeruleospinal system of the cat? (2) What is the physiological role, if any, of glutamate in descending locus coeruleus control of spinal motoneurons? Following injections of rhodamine-labeled latex microspheres or Fast Blue into the seventh lumbar segment of the spinal cord of the cat, retrogradely labeled cells were found throughout the rostrocaudal extent of the dorsolateral pontine tegmentum. They were primarily observed in the nucleus locus coeruleus and the Kolliker-Fuse nucleus. Some labeled cells were also present in the nucleus subcoeruleus and, to a lesser extent, in the parabrachial nuclei. Data from immunohistochemical studies indicate that 86% of all dorsolateral pontine tegmentum neurons that project to the spinal cord contain glutamate-like immunoreactivity, and 77% co-contain both glutamate- and tyrosine hydroxylase-like immunoreactivity. Electrical stimulation (four pulses of 500 microseconds duration at 500 Hz; intensity = 50-200 microA) of the locus coeruleus, in decerebrate cats, consistently induced lumbar motoneuron discharges recordable ipsilaterally as ventral root responses. These motoneuronal responses were reversibly antagonized following chemical inactivation of noradrenergic locus coeruleus neurons by local infusion of the alpha 2-adrenergic agonist clonidine, suggesting the locus coeruleus neurons to be the main source of evoked ventral root responses. Additionally, the evoked ventral root responses were reversibly reduced by 34.20 +/- 4.45% (mean +/- S.E.M.) upon intraspinal injections of the non-N-methyl-D-aspartate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, into the ventral horn of seventh lumbar spinal cord segment (three to four injections, 20 nmol in 0.2 microliter of 0.1 M Tris-buffered saline for each injection). Similar volumes of vehicle injections had no significant effect on the locus coeruleus-evoked ventral root responses. These ventral root responses were also partially blocked (62.30 +/- 11.76%) by intravenous administration of the alpha 1-adrenergic receptor antagonist prazosin (20 micrograms/kg). In the light of several anatomical reports of noradrenergic and glutamatergic terminals in close contact with spinal motoneurons, our present findings suggest that the locus coeruleus-evoked ventral root response probably involves the synaptic release of both norepinephrine and glutamate onto lumbar motoneurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Properties of preganglionic and postganglionic neurones in vasoconstrictor pathways of rats and guinea pigs

The electrophysiological properties of pre- and postganglionic neurones and their synaptic inputs have been examined in both in vivo and in vitro preparations. Electrically, both neurone types have similar low resting conductance and compact dendritic trees. In preganglionic vasoconstrictor neurones, both slow and fast excitatory and fast inhibitory potentials are probably involved in baroreceptor reflexes, discharge being initiated after summation. In contrast, postganglionic vasoconstrictor neurones receive only one type of fast excitatory input. One of the converging preganglionic inputs has a very high safety factor and always fires the postganglionic neurone ensuring that the centrally-derived pattern of discharge reaches the neurovascular junctions. We do not know if the other subthreshold inputs summate during natural activity in vivo, as it is not known whether functionally distinct preganglionic inputs converge on vasoconstrictor neurones in ganglia.

Excitatory amino acid receptors in central cardiovascular regulation

Excitatory amino acid neurotransmitters and their receptors are the principal mediators of fast synaptic transmission within the central nervous system. Accumulating evidence suggests that synaptic activation of excitatory amino acid receptors in the nucleus of the tractus solitarius, rostral and caudal ventrolateral medulla, and in the spinal cord play a key role in neural transmission of cardiovascular information in the central nervous system. Pharmacological blockade of excitatory amino acid receptors at these sites eliminates a variety of centrally-mediated cardiovascular responses. These include baroreceptor reflexes and increases in arterial pressure produced by stimulation of various brain regions as well as peripheral afferent nerves. These observations indicate that synaptic activation of EAA receptors at specific sites within the brainstem and in the spinal cord play an important role in central cardiovascular regulation.

Spinal cord lamina V and lamina VII interneuronal projections to sympathetic preganglionic neurons

This light and electron microscopic study sought to localize spinal cord interneurons that contribute to the normal and abnormal physiological regulation of spinal sympathetic preganglionic function. Sympathetic preganglionic neurons in caudal C8 through T4 of rat spinal cord were retrogradely labeled with wheat germ agglutinin (WGA) and/or cholera beta subunit (CT beta) following injections into the superior cervical ganglion (SCG). With two exceptions, the observed locations of retrogradely WGA- and CT beta-labeled sympathetic preganglionic neurons were as expected from previous studies. The exceptions were restricted populations of cells in caudal C8 and rostral T1 spinal segments. These neurons were classified as ventrolateral (vlSPN) and ventromedial (vmSPN) sympathetic preganglionic neurons; their somata and dendrites encircled dorsolateral lamina IX motoneurons. Only WGA was transported transneuronally following the retrograde labeling of sympathetic preganglionic neurons. Transneuronally WGA-labeled spinal interneurons were located principally in the reticulated division of lamina V and dorsolateral lamina VII. A strict segmental organization was observed. All transneuronally labeled interneurons were ipsilateral to, and coextensive with, retrogradely WGA-labeled sympathetic preganglionic neurons. Electron microscopic observations suggested that retrograde transsynaptic passage of WGA occurred within the sympathetic preganglionic neuropil and showed further that similar classes of organelles were WGA immunoreactive in retrogradely labeled sympathetic preganglionic neurons and in transneuronally labeled lamina V and lamina VII neurons: 1) cisternae and vesicles at the trans face of the Golgi apparatus, 2) large endosomes/dense bodies, and 3) multivesicular bodies. The data are consistent with two hypotheses: 1) Somatic and visceral primary afferent inputs to thoracic spinal cord modify segmental sympathetic preganglionic function through activation of a disynaptic pathway involving lamina V and/or lamina VII interneurons, and 2) long-loop propriospinal pathways access sympathetic preganglionic neurons through symmetrical, segmental interneuronal circuitry.

Intracellular recording from sympathetic preganglionic neurons in cat lumbar spinal cord

Sympathetic preganglionic neurons (SPN) are responsible for the control of many autonomic targets including the heart and blood vessels. Previous intracellular studies have examined the morphology of SPN in the thoracic spinal cord, but there are no intracellular studies of SPN in the lumbar spinal cord. In this study we identified lumbar SPN using intracellular recording and dye-filling so that we could study their entire soma-dendritic tree, as well as their axons. At the same time, axonal conduction velocity was measured, and any evidence of an input in phase with phrenic nerve discharge was noted. Intracellular recordings were made from SPN in the L3 (n = 125) and T3 (n = 17) segments of the cat spinal cord. Axonal conduction velocities ranged from 0.6-8.4 m/s. In 85 lumbar SPN, the recordings lasted long enough to assess respiratory-related modulation. A respiratory-related modulation of the membrane potential was seen in 7 of these 85 neurons. All 7 respiratory-related neurons had a conduction velocity of 2.0 m/s or less, while none of the SPN with conduction velocities of more than 2.0 m/s had a respiratory rhythmicity. Histological analysis of 50 biocytin-filled SPN, including 3 with a respiratory-related modulation of their membrane potential, revealed that they occurred mostly in the principal part of the intermediolateral cell column and tended to be elongated in the rostro-caudal direction. Dendrites ramified in the intermediolateral cell column, the dorsolateral white matter and the ventral and medial gray matter. Axons arose either from cell bodies or from primary dendrites and did not bifurcate or have varicose intraspinal collaterals. This is the first report of the morphology of intracellularly filled SPN in the lumbar spinal cord.

Effects of MK-801 and CNQX, glutamate receptor antagonists, on bladder activity in neonatal rats

This study was undertaken to examine the role of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) glutamatergic receptors in the regulation of urinary bladder activity in the neonatal rat. Experiments were conducted using an in vitro spinal cord-bladder (SB) preparation from 1- to 5-day-old rats or awake neonatal rats 6 and 7 days old. SB preparations were isolated under hypothermic anesthesia. Isovolumetric bladder contractions occurred spontaneously, were induced by electrical stimulation (ES) of the bladder wall or were evoked reflexly by perineal tactile stimulation (PS). MK-801 (3-30 microM), an NMDA receptor antagonist, enhanced the amplitude of spontaneous, ES- and PS-evoked contractions. Removal of the spinal cord after MK-801 abolished PS-evoked reflex contractions but did not change the amplitude of spontaneous and ES-evoked contractions. Removal of the spinal cord in the absence of MK-801 increased the amplitude of spontaneous and ES-evoked contractions, indicating that the bladder is subject to a tonic inhibitory control originating in the spinal cord. 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 3-30 microM), an AMPA receptor antagonist, decreased the amplitude of PS-evoked contractions and the frequency of spontaneous contractions in the SB preparation. Removal of the spinal cord after CNQX enhanced the amplitude of spontaneous and ES-evoked contractions but abolished PS-evoked contractions. The frequency of spontaneous contractions which decreased after CNQX increased to near control levels after removal of the spinal cord. In awake neonatal rats, intraperitoneal injection of MK-801 (3 mg/kg) induced spontaneous micturition. A large dose of CNQX (30 mg/kg) decreased PS-evoked micturition volume. These results suggest that NMDA glutamatergic receptors are involved in a lumbosacral spinal inhibitory mechanism controlling bladder activity; whereas AMPA glutamatergic receptors are involved in the perineal-to-bladder reflex pathway in neonatal rats.

Phosphate-activated glutaminase immunoreactive synapses in the intermediolateral nucleus of rat thoracic spinal cord

A monoclonal antibody against phosphate-activated glutaminase was used to identify glutamatergic neuronal components in the intermediolateral nucleus of the thoracic spinal cord of the rat. Under electron microscopy of the intermediolateral nucleus, most glutaminase immunoreactivity was detected in the axoplasm surrounding spherical synaptic vesicles in the presynaptic axon varicosities which formed asymmetric synapses with small dendrites and occasionally with neuronal cell bodies. About 40% of axon varicosities within the intermediolateral nucleus and 49% of the axon varicosities forming asymmetric synaptic contacts showed glutaminase immunoreactivity. Glutaminase immunoreactivity was further seen in mitochondria of neuronal perikarya and dendrites in the intermediolateral nucleus, and occasionally in the cytoplasm of the dendrites and glial processes in the vicinity of glutaminase-immunoreactive axon varicosities. By the combined method of immunocytochemistry and retrograde axonal transport, glutaminase-immunoreactive axons were shown to make direct synaptic contacts with the preganglionic sympathetic neurons, which were retrogradely labeled by injection of horseradish peroxidase conjugated with choleratoxin B subunit into the superior cervical ganglion. The present results indicate that glutaminase-containing axons are the major synaptic inputs to intermediolateral nucleus neurons including preganglionic sympathetic ones, suggesting that glutamate is used as the neurotransmitter to control those neurons in the intermediolateral nucleus.

Enhancement of monosynaptic excitatory postsynaptic potentials by glutamate in frog spinal motoneurons

We analyzed glutamate-induced enhancement of the amplitude of monosynaptic excitatory postsynaptic potentials evoked by stimulation of the lateral column fibers (LC-EPSPs) on lumbar motoneurons in the frog spinal cord. Low concentrations (0.1-0.3 mM) of glutamate, which produced small depolarization, often enhanced EPSP associated with inhibition of a paired pulse facilitation and increased occurrence of spontaneous EPSPs. With 1 mM glutamate, transient enhancement of EPSP was seen in some cells during the early phase or prior to large depolarization, even when input conductance was increased. Transient or sustained enhancement of EPSP was occasionally seen with N-methyl-D-aspartate, kainate and quisqualate, but not with L-2-amino-4-phosphonobutyrate. The results suggest that glutamate enhanced release of excitatory transmitters at low concentrations that apparently did not affect the postsynaptic membrane.

An excitatory amino acid synapse in the thoracic spinal cord is involved in the pressor response to muscular contraction

The increase in arterial pressure and heart rate elicited during exercise are produced by descending central command and by feedback from contracting limb muscles. Previous studies from this laboratory have demonstrated that neurons in the ventrolateral medulla (VLM) that project to the intermediolateral (IML) columns of the thoracic spinal cord are involved in the mediation of the pressor response to contraction of hind limb muscles. This study determines if these VLM neurons utilize excitatory amino acids (EAA) as the neurotransmitter at the synapse on IML neurons. The arterial pressure and heart rate responses to static muscular contraction, elicited by stimulation of the L7 and S1 ventral roots, and to electrical stimulation in the caudal hypothalamus were examined in anesthetized cats. Both muscular contraction and hypothalamic stimulation elicited significant increases in arterial pressure and heart rate. Intrathecal administration of the broad spectrum, postsynaptic EAA antagonist, kynurenic acid, greatly reduced (-77%) the pressor response to muscular contraction. A smaller (-27%) decrease in the magnitude of pressor response elicited by muscular contraction was produced by intrathecal administration of 2-amino-4-phosphonobutyric acid which acts on a presynaptic EAA receptor. Neither antagonist affected the heart rate responses associated with muscular contraction or the cardiovascular responses to hypothalamic stimulation. These results indicate that the pressor response elicited by feedback from contracting hind limb muscles is mediated through an excitatory amino acid synapse in the spinal cord.

Sympathoexcitation from the rostral ventrolateral medulla is mediated by spinal NMDA receptors

These studies examined the role of spinal N-methyl-D-aspartic acid (NMDA) receptors in mediating sympathoexcitation evoked by stimulation of neurons in the rostral ventrolateral medulla (RVLM). In urethane-anesthetized rats, blood pressure, heart rate, and splanchnic sympathetic nerve activity (SNA) were recorded. The NMDA receptor antagonist D-2-amino-7-phosphonoheptanoic acid (D-AP7) was administered to the spinal cord via intrathecal (IT) catheter. Blockade of spinal NMDA receptors reduced arterial blood pressure, heart rate, and SNA. Spinal administration of D-AP7 markedly attenuated the pressor and sympathoexcitatory responses evoked by L-glutamate stimulation of the RVLM. The small increases in heart rate evoked by stimulation of the RVLM were not affected by IT administration of D-AP7. These results indicate that NMDA receptors in the spinal cord mediate the pressor and sympathoexcitatory responses evoked by activation of a bulbospinal pathway originating from the RVLM. Moreover, these data suggest that excitatory amino acid neurotransmitters and NMDA receptors in the spinal cord play an important role in the maintenance and regulation of SNA and cardiovascular function.

Characteristics of function-specific pathways in the sympathetic nervous system

The autonomic nervous system enables all of our body systems to operate in an external environment that is both physically and emotionally challenging. Despite voluntary and involuntary interventions, the composition of the internal environment is maintained. Autonomic dysfunction, particularly in aging people, reveals the importance of this efferent neural control for the wellbeing of our bodies and minds. Although the sympathetic component of this system has been widely thought to be concerned only with the body's response to stress, we discuss here how a range of neuroscientific techniques has started to reveal the specialized properties of functional pathways in the sympathetic system at molecular, cellular and integrative levels. The diversity observed is not compatible with a simple neuroendocrine role of this system.

Fast inhibitory postsynaptic potentials and responses to inhibitory amino acids of sympathetic preganglionic neurons in the adult cat

Intracellular recordings were obtained from sympathetic preganglionic neurons (SPNs) of the intermediolateral nucleus (IML) in slices of upper thoracic spinal cord of the anesthetized cat. A total of 44 neurons was studied. Single shock stimulation of an area of white matter dorsolateral to the IML, close to the recording electrode (< 0.5 mm), evoked fast IPSPs with rise time of 3.8 ms and 1/2 decay time of 14.7 ms (n = 12). In 17 other cells only fast EPSPs were recorded but, after suppression of the EPSPs by the excitatory amino acid receptor antagonists CNQX (20 microM) and APV (100-250 microM), fast IPSPs were unmasked. The IPSP reversed polarity at -63 mV (-67 mV in the presence of CNQX and APV). The reversal potential shifted to a less negative value when the extracellular chloride concentration was reduced. The IPSP was reversibly abolished by the GABAA receptor antagonist bicuculline in 32% of the cells, by the glycine receptor antagonist strychnine in 47% of the cells and by the combination of the two in 21% of the cells. The IPSP was abolished by TTX (0.5 microM), had constant latency and showed no failures during high frequency stimulation. The IPSP presumably resulted from the excitation of inhibitory axons and/or inhibitory neuron somata with monosynaptic connections to the SPN. Glycine and GABA (1-3 mM) produced hyperpolarization associated with decreased membrane resistance. Sixty-nine percent of cells responded to both agonists, 19% to glycine only and 12% to GABA only. The GABAB agonist baclofen (5 microM) had no effect.(ABSTRACT TRUNCATED AT 250 WORDS)