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

Dopaminergic Dependency of Cholinergic Pallidal Neurons

We employed the whole-cell patch-clamp method and ChAT-Cre mice to study the electrophysiological attributes of cholinergic neurons in the external globus pallidus. Most neurons were inactive, although approximately 20% displayed spontaneous firing, including burst firing. The resting membrane potential, the whole neuron input resistance, the membrane time constant and the total neuron membrane capacitance were also characterized. The current-voltage relationship showed time-independent inward rectification without a "sag". Firing induced by current injections had a brief initial fast adaptation followed by tonic firing with minimal accommodation. Intensity-frequency plots exhibited maximal average firing rates of about 10 Hz. These traits are similar to those of some cholinergic neurons in the basal forebrain. Also, we examined their dopamine sensitivity by acutely blocking dopamine receptors. This action demonstrated that the membrane potential, excitability, and firing pattern of pallidal cholinergic neurons rely on the constitutive activity of dopamine receptors, primarily D2-class receptors. The blockade of these receptors induced a resting membrane potential hyperpolarization, a decrease in firing for the same stimulus, the disappearance of fast adaptation, and the emergence of a depolarization block. This shift in physiological characteristics was evident even when the hyperpolarization was corrected with D.C. current. Neither the currents that generate the action potentials nor those from synaptic inputs were responsible. Instead, our findings suggest, that subthreshold slow ion currents, that require further investigation, are the target of this novel dopaminergic signaling.

Catecholaminergic C3 neurons are sympathoexcitatory and involved in glucose homeostasis

Brainstem catecholaminergic neurons play key roles in the autonomic, neuroendocrine, and behavioral responses to glucoprivation, yet the functions of the individual groups are not fully understood. Adrenergic C3 neurons project widely throughout the brain, including densely to sympathetic preganglionic neurons in the spinal cord, yet their function is completely unknown. Here we demonstrate in rats that optogenetic stimulation of C3 neurons induces sympathoexcitatory, cardiovasomotor functions. These neurons are activated by glucoprivation, but unlike the C1 cell group, not by hypotension. The cardiovascular activation induced by C3 neurons is less than that induced by optogenetic stimulation of C1 neurons; however, combined stimulation produces additive sympathoexcitatory and cardiovascular effects. The varicose axons of C3 neurons largely overlap with those of C1 neurons in the region of sympathetic preganglionic neurons in the spinal cord; however, regional differences point to effects on different sympathetic outflows. These studies definitively demonstrate the first known function of C3 neurons as unique cardiovasomotor stimulatory cells, embedded in the brainstem networks regulating cardiorespiratory activity and the response to glucoprivation.

Increased intrinsic excitability of muscle vasoconstrictor preganglionic neurons may contribute to the elevated sympathetic activity in hypertensive rats

Hypertension is associated with pathologically increased sympathetic drive to the vasculature. This has been attributed to increased excitatory drive to sympathetic preganglionic neurons (SPN) from brainstem cardiovascular control centers. However, there is also evidence supporting increased intrinsic excitability of SPN. To test this hypothesis, we made whole cell recordings of muscle vasoconstrictor-like (MVClike) SPN in the working-heart brainstem preparation of spontaneously hypertensive (SH) and normotensive Wistar-Kyoto (WKY) rats. The MVClike SPN have a higher spontaneous firing frequency in the SH rat (3.85 ± 0.4 vs. 2.44 ± 0.4 Hz in WKY; P = 0.011) with greater respiratory modulation of their activity. The action potentials of SH SPN had smaller, shorter afterhyperpolarizations (AHPs) and showed diminished transient rectification indicating suppression of an A-type potassium conductance (IA). We developed mathematical models of the SPN to establish if changes in their intrinsic properties in SH rats could account for their altered firing. Reduction of the maximal conductance density of IA by 15-30% changed the excitability and output of the model from the WKY to a SH profile, with increased firing frequency, amplified respiratory modulation, and smaller AHPs. This change in output is predominantly a consequence of altered synaptic integration. Consistent with these in silico predictions, we found that intrathecal 4-aminopyridine (4-AP) increased sympathetic nerve activity, elevated perfusion pressure, and augmented Traube-Hering waves. Our findings indicate that IA acts as a powerful filter on incoming synaptic drive to SPN and that its diminution in the SH rat is potentially sufficient to account for the increased sympathetic output underlying hypertension.

Mapping the cellular electrophysiology of rat sympathetic preganglionic neurones to their roles in cardiorespiratory reflex integration: a whole cell recording study in situ

Sympathetic preganglionic neurones (SPNs) convey sympathetic activity flowing from the CNS to the periphery to reach the target organs. Although previous in vivo and in vitro cell recording studies have explored their electrophysiological characteristics, it has not been possible to relate these characteristics to their roles in cardiorespiratory reflex integration. We used the working heart–brainstem preparation to make whole cell patch clamp recordings from T3–4 SPNs (n = 98). These SPNs were classified by their distinct responses to activation of the peripheral chemoreflex, diving response and arterial baroreflex, allowing the discrimination of muscle vasoconstrictor-like (MVC_like, 39%) from cutaneous vasoconstrictor-like (CVC_like, 28%) SPNs. The MVC_like SPNs have higher baseline firing frequencies (2.52 ± 0.33 Hz vs. CVC_like 1.34 ± 0.17 Hz, P = 0.007). The CVC_like have longer after-hyperpolarisations (314 ± 36 ms vs. MVC_like 191 ± 13 ms, P < 0.001) and lower input resistance (346 ± 49  MΩ vs. MVC_like 496 ± 41 MΩ, P < 0.05). MVC_like firing was respiratory-modulated with peak discharge in the late inspiratory/early expiratory phase and this activity was generated by both a tonic and respiratory-modulated barrage of synaptic events that were blocked by intrathecal kynurenate. In contrast, the activity of CVC_like SPNs was underpinned by rhythmical membrane potential oscillations suggestive of gap junctional coupling. Thus, we have related the intrinsic electrophysiological properties of two classes of SPNs in situ to their roles in cardiorespiratory reflex integration and have shown that they deploy different cellular mechanisms that are likely to influence how they integrate and shape the distinctive sympathetic outputs.

Monoaminergic modulation of spinal viscero-sympathetic function in the neonatal mouse thoracic spinal cord

Descending serotonergic, noradrenergic, and dopaminergic systems project diffusely to sensory, motor and autonomic spinal cord regions. Using neonatal mice, this study examined monoaminergic modulation of visceral sensory input and sympathetic preganglionic output. Whole-cell recordings from sympathetic preganglionic neurons (SPNs) in spinal cord slice demonstrated that serotonin, noradrenaline, and dopamine modulated SPN excitability. Serotonin depolarized all, while noradrenaline and dopamine depolarized most SPNs. Serotonin and noradrenaline also increased SPN current-evoked firing frequency, while both increases and decreases were seen with dopamine. In an in vitro thoracolumbar spinal cord/sympathetic chain preparation, stimulation of splanchnic nerve visceral afferents evoked reflexes and subthreshold population synaptic potentials in thoracic ventral roots that were dose-dependently depressed by the monoamines. Visceral afferent stimulation also evoked bicuculline-sensitive dorsal root potentials thought to reflect presynaptic inhibition via primary afferent depolarization. These dorsal root potentials were likewise dose-dependently depressed by the monoamines. Concomitant monoaminergic depression of population afferent synaptic transmission recorded as dorsal horn field potentials was also seen. Collectively, serotonin, norepinephrine and dopamine were shown to exert broad and comparable modulatory regulation of viscero-sympathetic function. The general facilitation of SPN efferent excitability with simultaneous depression of visceral afferent-evoked motor output suggests that descending monoaminergic systems reconfigure spinal cord autonomic function away from visceral sensory influence. Coincident monoaminergic reductions in dorsal horn responses support a multifaceted modulatory shift in the encoding of spinal visceral afferent activity. Similar monoamine-induced changes have been observed for somatic sensorimotor function, suggesting an integrative modulatory response on spinal autonomic and somatic function.

Electrophysiological, pharmacological and molecular profile of the transient outward rectifying conductance in rat sympathetic preganglionic neurons in vitro

Transient outward rectifying conductances or A-like conductances in sympathetic preganglionic neurons (SPN) are prolonged, lasting for hundreds of milliseconds to seconds and are thought to play a key role in the regulation of SPN firing frequency. Here, a multidisciplinary electrophysiological, pharmacological and molecular single-cell rt-PCR approach was used to investigate the kinetics, pharmacological profile and putative K+ channel subunits underlying the transient outward rectifying conductance expressed in SPN. SPN expressed a 4-aminopyridine (4-AP) sensitive transient outward rectification with significantly longer decay kinetics than reported for many other central neurons. The conductance and corresponding current in voltage-clamp conditions was also sensitive to the Kv4.2 and Kv4.3 blocker phrixotoxin-2 (1-10 μM) and the blocker of rapidly inactivating Kv channels, pandinotoxin-Kα (50 nM). The conductance and corresponding current was only weakly sensitive to the Kv1 channel blocker tityustoxin-Kα and insensitive to dendrotoxin I (200 nM) and the Kv3.4 channel blocker BDS-II (1 μM). Single-cell RT-PCR revealed mRNA expression for the α-subunits Kv4.1 and Kv4.3 in the majority and Kv1.5 in less than half of SPN. mRNA for accessory β-subunits was detected for Kvβ2 in all SPN with differential expression of mRNA for KChIP1, Kvβ1 and Kvβ3 and the peptidase homologue DPP6. These data together suggest that the transient outwardly rectifying conductance in SPN is mediated by members of the Kv4 subfamily (Kv4.1 and Kv4.3) in association with the β-subunit Kvβ2. Differential expression of the accessory β subunits, which may act to modulate channel density and kinetics in SPN, may underlie the prolonged and variable time-course of this conductance in these neurons.

Regulation of visceral sympathetic tone by A5 noradrenergic neurons in rodents

The ventrolateral pons contains the A5 group of noradrenergic neurons which regulate the circulation and probably breathing. The present experiments were designed to identify these neurons definitively in vivo, to examine their response to chemoreceptor stimuli (carotid body stimulation and changes in brain pH) and to determine their effects on sympathetic outflow. Bulbospinal A5 neurons, identified by juxtacellular labelling in anaesthetized rats, had a slow regular discharge, were vigorously activated by peripheral chemoreceptor stimulation with cyanide, but only mildly activated by hyperoxic hypercapnia (central chemoreceptor stimulation). The caudal end of the A5 region also contained neurons with properties reminiscent of retrotrapezoid neurons. These cells lacked a spinal axon and were characterized by a robust response to CO2. The pH sensitivity of A5 neurons, examined in brain slices from neonatal (postnatal days 6–10) tyrosine hydroxylase (TH)-GFP transgenic mice, was about 10 times smaller than that of similarly recorded retrotrapezoid neurons. Selective stimulation of the A5 neurons in rats using channelrhodopsin optogenetics (A5 TH neurons represented 66% of transfected cells) produced fivefold greater activation of the renal nerve than the lumbar sympathetic chain. In summary, adult A5 noradrenergic neurons are vigorously activated by carotid body stimulation. This effect presumably contributes to the increase in visceral sympathetic nerve activity elicited by acute hypoxia. A5 neurons respond weakly to hypercapnia in vivo or to changes in pH in slices suggesting that their ability to sense local variations in brain pH or Pco₂ is limited.

Electrophysiological characteristics of identified kidney-related neurons in adult rat spinal cord slices

Whole-cell patch-clamp recordings were made from kidney-related neurons in the intermediolateral cell column (IML) in horizontal slices of thoracolumbar spinal cord from adult rats. Kidney-related neurons were identified in vitro subsequent to inoculation of the kidney with a fluorescent, retrograde, transynaptic pseudorabies viral label (i.e., PRV-152). Kidney-related neurons detected in the IML expressed choline acetyltransferase, characteristic of spinal preganglionic motor neurons. Their mean resting potential was -51+/-4 mV and input resistance was 448+/-39 MOmega. Both spontaneous inhibitory and excitatory post-synaptic currents (i.e., sIPSCs and sEPSCs) were observed in all neurons. The mean frequency for sEPSCs (3.1+/-1 Hz) was approximately 2.5 times that for sIPSCs (1.4+/-0.3 Hz). Application of the glycine and GABA(A) receptor-linked Cl(-) channel blocker, picrotoxin (100 microM) blocked sIPSCs, while the ionotropic glutamate receptor antagonist, kynurenic acid (1 mM) blocked all sEPSCs, indicating they were mediated by GABA/glycine and glutamate receptors, respectively. Thus, using PRV-152 labeling allowed whole-cell patch-clamp recording of neurons in the adult spinal cord, which were kidney-related. Excitatory glutamatergic input dominated synaptic responses in these cells, the membrane characteristics of which resembled those of immature IML neurons. Combined PRV-152 pre-labeling and whole-cell patch-clamp recordings may allow more effective analysis of synaptic plasticity seen in adult models of injury or chronic disease.

Heterogeneity of membrane properties in sympathetic preganglionic neurons of neonatal mice: evidence of four subpopulations in the intermediolateral nucleus

Spinal cord sympathetic preganglionic neurons (SPNs) integrate activity from descending and sensory systems to determine the final central output of the sympathetic nervous system. The intermediolateral column (IML) has the highest number and density of SPNs and, within this region, SPN somas are found in distinct clusters within thoracic and upper lumbar spinal segments. Whereas SPNs exhibit a rostrocaudal gradient of end-target projections, individual clusters contain SPNs with diverse functional roles. Here we explored diversity in the electrophysiological properties observed in Hb9-eGFP-identified SPNs in the IML of neonatal mice. Overall, mouse SPN intrinsic membrane properties were comparable with those seen in other species. A wide range of values was obtained for all measured properties (up to a 10-fold difference), suggesting that IML neurons are highly differentiated. Using linear regression we found strong correlations between many cellular properties, including input resistance, rheobase, time constant, action potential shape, and degree of spike accommodation. The best predictor of cell function was rheobase, which correlated well with firing frequency-injected current (f-I) slopes as well as other passive and active membrane properties. The range in rheobase suggests that IML neurons have a recruitment order with stronger synaptic drives required for maximal recruitment. Using cluster analysis, we identified at least four subpopulations of SPNs, including one with a long time constant, low rheobase, and high f-I gain. We thus propose that the IML contains populations of neurons that are differentiable by their membrane properties and hypothesize they represent diverse functional classes.

Biophysical and histological determinants underlying natural firing behaviors of splanchnic sympathetic preganglionic neurons in neonatal rats

Isolated thoracic spinal cords of neonatal rats spontaneously generate splanchnic sympathetic nerve discharge (SND) with a quasiperiodic rhythm approximately 1-Hz. Using in vitro nerve-cord preparations that retained T6-T12 spinal segments, we investigated whether the natural firing behavior of sympathetic preganglionic neurons (SPNs) encoded the SND rhythm and what were the main biophysical and histological determinants of SPN firing. Under extracellular recording conditions, electrical stimulation of splanchnic nerves elicited antidromic responses in 212 SPNs. Among them, 92 SPNs were quiescent; 120 active SPNs had an average firing rate of 0.72+/-0.04 Hz, which was close to the quasiperiodic rhythm of SND. SPNs with rhythmic burst firing were rare. Probability plots of interspike intervals were constructed to extract mathematical features underlying SPN firing. Most active SPNs (88%) had a firing well described by unimodal Gaussian, suggesting a predominantly tonic pattern with normal variations. Biophysical properties of 112 SPNs were measured under whole-cell recording conditions. The charging time constant, tau, is positively correlated with the average firing rate. Histological properties were examined in 45 SPNs with intracellular diffusion of Lucifer Yellow or biocytin. SPNs with pyramidal somata and multipolar dendrites tend to be spontaneously active. In contrast, those with bipolar somata and fewer dendritic branches were quiescent in firing. These observations suggest that activity levels of SPNs are correlated with their capacity for temporal and spatial summation of synaptic inputs. How the seemingly tonic firing of individual SPNs is integrated into whole-nerve SND with quasiperiodic rhythms is discussed.

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.

Historical reflections on the afterhyperpolarization--firing rate relation of vertebrate spinal neurons

In mammalian spinal motoneurons (MNs), the slow component of the afterhyperpolarization (AHP) that follows the spike of each action potential is a major but not the sole determinant of the cells' firing rate. In this brief historical review, we emphasize four points about the AHP-firing rate relation. (1) There is a relatively sparse literature across vertebrates that directly addresses this topic. (2) After the advent of intracellular recording in the early 1950s, there was evidence from mammals to the contrary of an idea that subsequently became prevalent: that the high-firing rates attainable by spinal interneurons (INs) and low-threshold MNs was attributable to their small AHP at rheobase. (3) Further work is needed to determine whether our present findings on the AHP-firing rate relation of turtle cells generalize to the spinal neurons of other vertebrate species. (4) Relevant to point 3, substantial in vivo and in vitro work is potentially available in raw data used in reports on several mammalian and non-mammalian vertebrates. In summary, the factors in addition to the slow AHP that help determine spinal INs and MN firing rate deserve further evaluation across vertebrates, with relevant data already potentially available in several laboratories.

Transient expression of A-type K channel alpha subunits Kv4.2 and Kv4.3 in rat spinal neurons during development

A-type K(+) currents (I(A)s) have been detected from the ventral horn neurons in rat spinal cord during embryonic day (E) 14 to postnatal day (P) 8 but not in adulthood. It is not known which types of neurons and which A-type K(+) channel alpha subunits express the I(A)s and what the possible function might be. Here, we examined the expression of two A-type K(+) channel alpha subunits, Kv4.2 and Kv4.3, in rat spinal cord at various developmental stages by immunohistochemistry. We found a transient expression of Kv4.2 in somatic motoneurons during E13.5-P8 with a peak around E17.5, which coincides temporally with the natural selection of motoneurons. Transient expression of Kv4.2 and Kv4.3 was also observed in the intermediate gray (IG) interneurons. During E19.5-P14, some IG interneurons express Kv4.2, some express Kv4.3 and a subset co-express Kv4.2 and Kv4.3. Peak expression of Kv4.2 and Kv4.3 in the IG interneurons was detected around P1, which coincides temporally with the developmental selection of IG interneurons. In contrast to the I(A)-expressing subunits Kv4.2 and Kv4.3, a delayed-rectifier K(+) channel alpha subunit Kv1.6 is persistently expressed in somatic motoneurons and IG interneurons. Together, these data support the hypothesis that expression of I(A)s may protect I(A)-expressing somatic motoneurons, and possibly also IG interneurons, from naturally occurring cell death during developmental selection.

Association of potassium channel Kv3.4 subunits with pre- and post-synaptic structures in brainstem and spinal cord

Voltage-gated K+ channels (Kv) are divided into eight subfamilies (Kv1-8) and play a major role in determining the excitability of neurones. Members of the Kv3 subfamily are highly abundant in the CNS, with each Kv3 gene (Kv3.1-Kv3.4) exhibiting a unique pattern of expression, although single neurones can express more than one subtype. Of the Kv3 subunits relatively little is known of the Kv3.4 subunit distribution in the nervous system, particularly in the brainstem and spinal cord of the rat. We performed immunohistochemistry to determine both the cellular and sub-cellular distribution of the Kv3.4 subunit in these areas. Kv3.4 subunit immunoreactivity (Kv3.4-IR) was widespread, with dense, punctate staining in many regions including the intermediolateral cell column (IML) and the dorsal vagal nucleus (DVN), nucleus ambiguus (NA) and nucleus tractus solitarius (NTS). In the ventral horn a presynaptic location was confirmed by co-localization of Kv3.4-IR with the synaptic vesicle protein, SV2 and also with the glutamate vesicle markers vesicular glutamate transporter (VGluT) 1, VGluT2 or the glycine transporter GlyT2, suggesting a role for the channel in both excitatory and inhibitory neurotransmission. Electron microscopy confirmed a presynaptic terminal location of Kv3.4-IR in the VH, IML, DVN, NA and NTS. Interestingly however, patches of Kv3.4-IR were also revealed postsynaptically in dendritic and somatic structures throughout these areas. This staining was striking due to its localization at synaptic junctions at terminals with morphological features consistent with excitatory functions, suggesting an association with the postsynaptic density. Therefore the pre and postsynaptic localization of Kv3.4-IR suggests a role both in the control of transmitter release and in regulating neuronal excitability.

Transmission of signals through sympathetic ganglia--modulation, integration or simply distribution?

AIM

On structural grounds, synaptic transmission in sympathetic ganglia is potentially complex with extensive divergence and convergence between preganglionic and postganglionic neurones. In this review, the focus is on what constitutes a functional synapse in sympathetic ganglia and how intracellular recordings have enabled us to identify how the transmission process operates in vivo.

RESULTS

Only one or two suprathreshold or 'strong' inputs are involved in activating each postganglionic neurone. The functional significance of the subthreshold or 'weak' inputs remains obscure. The strong inputs, and sometimes the weak ones as well, respond in the same way during reflexes. The expansion of ineffective weak connections enables the rapid restoration of functional control after lesions that damage preganglionic neurones. These novel connections may generate erroneous reflex responses after spinal injury. Postganglionic discharge in vivo consists of the summed firing of the strong preganglionic inputs limited, at high preganglionic discharge rates, by the properties of the afterhyperpolarization.

CONCLUSION

Preganglionic signals are distributed widely through paravertebral ganglia with little modification.

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.

Properties of interneurones in the intermediolateral cell column of the rat spinal cord: role of the potassium channel subunit Kv3.1

Sympathetic preganglionic neurones located in the intermediolateral cell column (IML) are subject to inputs descending from higher brain regions, as well as strong influences from local interneurones. Since interneurones in the IML have been rarely studied directly we examined their electrophysiological and anatomical properties. Whole cell patch clamp recordings were made from neurones in the IML of 250 microM slices of the thoracic spinal cord of the rat at room temperature. Action potential durations of interneurones (4.2+/-0.1 ms) were strikingly shorter than those of sympathetic preganglionic neurones (9.4+/-0.2 ms) due to a more rapid repolarisation phase. Low concentrations of tetraethylammonium chloride (TEA) (0.5 mM) or 4-aminopyridine (4-AP) (30 microM) affected interneurones but not sympathetic preganglionic neurones by prolonging the action potential repolarisation as well as decreasing both the afterhypolarisation amplitude and firing frequency. Following recordings, neurones sensitive to TEA and 4-AP were confirmed histologically as interneurones with axons that ramified extensively in the spinal cord, including the IML and other autonomic regions. In contrast, all cells that were insensitive to TEA and 4-AP were confirmed as sympathetic preganglionic neurones. Both electrophysiological and morphological data are therefore consistent with the presence of the voltage-gated potassium channel subunit Kv3.1 in interneurones, but not sympathetic preganglionic neurones. Testing this proposal immunohistochemically revealed that Kv3.1b was localised in low numbers of neurones within the IML but in higher numbers of neurones on the periphery of the IML. Kv3.1b-expressing neurones were not sympathetic preganglionic neurones since they were not retrogradely labelled following intraperitoneal injections of Fluorogold. Since Kv3.2 plays a similar role to Kv3.1 we also tested for the presence of Kv3.2 using immunohistochemistry, but failed to detect it in neuronal somata in the spinal cord. These studies provide electrophysiological and morphological data on interneurones in the IML and indicate that the channels containing the Kv3.1 subunit are important in setting the firing pattern of these neurones.

Correlation between electrophysiology and morphology of three groups of neuron in the dorsal commissural nucleus of lumbosacral spinal cord of mature rats studied in vitro

The dorsal commissural nucleus (DCN) in the lumbosacral spinal cord receives afferent inputs from the pelvic organs via pudendal and pelvic nerves. Electrophysiological and morphological properties of neurons in the DCN of L6-S1 were examined using whole-cell recordings with biocytin-filled electrodes in transverse slices of mature rat spinal cord. Neurons were categorized into three groups according to their discharge in response to suprathreshold depolarizing pulses; neurons with tonic (19/42) and phasic (13/42) firing patterns, and neurons (10/42) that fired in bursts arising from a Ca(2+)-dependent hump. The predominantly fusiform somata of neurons labeled during recording (n = 31) had on average 3.1 primary dendrites, 7.5 terminating dendritic branches, 3.1 axon collaterals, and 14.2 axon terminations per neuron. The groups were morphologically distinct on the basis of their dendritic branching patterns. Phasic neurons (n = 10) had the most elaborate dendritic branching and the largest numbers of axon collaterals. All tonic neurons (n = 11) had axons/collaterals projecting to the intermediolateral area but none to the funiculi, suggesting that they function as interneurons in local autonomic reflexes. Many axons/collaterals of all phasic neurons lay within the DCN, suggesting that they integrate segmental and descending inputs. Seven of 10 neurons with Ca(2+)-dependent humps had axons/collaterals extending into one of the funiculi, suggesting that they project intersegmentally or to the brain. Ca(2+) hump neurons also had more axons/collaterals within the DCN and fewer in the intermediolateral area than tonic neurons. This correlation between firing pattern and morphology is an important step toward defining the cellular pathways regulating pelvic function.

Diversity of channels involved in Ca(2+) activation of K(+) channels during the prolonged AHP in guinea-pig sympathetic neurons

The types of Ca(2+)-dependent K(+) channel involved in the prolonged afterhyperpolarization (AHP) in a subgroup of sympathetic neurons have been investigated in guinea pig celiac ganglia in vitro. The conductance underlying the prolonged AHP (gKCa2) was reduced to a variable extent in 100 nM apamin, an antagonist of SK-type Ca(2+)-dependent K(+) channels, and by about 55% in 20 nM iberiotoxin, an antagonist of BK-type Ca(2+)-dependent K(+) channels. The reductions in gKCa2 amplitude by apamin and iberiotoxin were not additive, and a resistant component with an amplitude of nearly 50% of control remained. These data imply that, as well as apamin- and iberiotoxin-sensitive channels, other unknown Ca(2+)-dependent K(+) channels participate in gKCa2. The resistant component of gKCa2 was not abolished by 0.5-10 mM tetraethylammonium, 1 mM 4-aminopyridine, or 5 mM glibenclamide. We also investigated which voltage-gated channels admitted Ca(2+) for the generation of gKCa2. Blockade of Ca(2+) entry through L-type Ca(2+) channels has previously been shown to reduce gKCa2 by about 40%. Blockade of N-type Ca(2+) channels (with 100 nM omega-conotoxin GVIA) and P-type Ca(2+) channels (with 40 nM omega-agatoxin IVA) each reduced the amplitude of gKCa2 by about 35%. Thus Ca(2+) influx through multiple types of voltage-gated Ca(2+) channel can activate the intracellular mechanisms that generate gKCa2. The slow time course of gKCa2 may be explained if activation of multiple K(+) channels results from Ca(2+) influx triggering a kinetically invariant release of Ca(2+) from intracellular stores located close to the membrane.

Intracellular recording of lamina X neurons in a horizontal slice preparation of rat lumbar spinal cord

A horizontal slice preparation of postnatal rat lumbar spinal cord has been developed which allows correlative observations of the morphology, electrophysiology, and receptor pharmacology of lamina X neurons. These slices better maintain afferent input and somatodendritic morphology and are amenable to subsequent immunohistochemical processing. Stable intracellular recordings obtained from postnatal day 14-45 animals reveal that a number of different intrinsic membrane conductances contribute to the regulation of excitability in lamina X neurons. In addition, lamina X neurons possess inhibitory GABAergic as well as excitatory glutamate and cholecystokinin receptors. This preparation will be useful in future studies designed to characterize developmental changes in the intrinsic membrane properties, synaptic profiles and neuropeptide responsiveness of lamina X neurons in the rat. Such a characterization is important given that lamina X represents a unique sexually dimorphic region that is a convergence site for somatic and visceral afferent inputs, which includes nociceptive information.

Dynamics of intrinsic electrophysiological properties in spinal cord neurones

The spinal cord is engaged in a wide variety of functions including generation of motor acts, coding of sensory information and autonomic control. The intrinsic electrophysiological properties of spinal neurones represent a fundamental building block of the spinal circuits executing these tasks. The intrinsic response properties of spinal neurones--determined by the particular set and distribution of voltage sensitive channels and their dynamic non-linear interactions--show a high degree of functional specialisation as reflected by the differences of intrinsic response patterns in different cell types. Specialised, cell specific electrophysiological phenotypes gradually differentiate during development and are continuously adjusted in the adult animal by metabotropic synaptic interactions and activity-dependent plasticity to meet a broad range of functional demands.

Fast excitatory post synaptic potentials and their response to catecholaminergic antagonists in rat sympathetic preganglionic neurones in vitro

In an in vitro slice preparation from neonatal rats intracellular recordings were made from electrophysiologically identified sympathetic preganglionic neurones. Electrical stimulation in the lateral funiculus (>500 microm) from the recording site elicited a mono- or polysynaptic excitatory post synaptic potential. The latter potential was blocked with the dopamine D2 antagonist haloperidol but not with the dopamine D1 antagonist SCH 23390. We therefore report the first showing of a functional descending pathway in an in vitro slice preparation describing both the transmitter and the receptor subtype involved and physiologically show that dopamine may exert an indirect excitatory influence on sympathetic preganglionic neurones possibly via interneurones present in the spinal cord.

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.

Presynaptic inhibition by noradrenaline of the EPSC evoked in neonatal rat sympathetic preganglionic neurons

Visually identified and electrophysiologically characterized sympathetic preganglionic neurons (SPNs) were recorded using the whole-cell voltage clamp technique in slices of neonatal rat spinal cord. Monosynaptic excitatory postsynaptic currents (EPSCs) were evoked by electrical stimulation of the nucleus intercalatus in the presence of strychnine (5 microM) and bicuculline (10 microM). These EPSCs were abolished by the antagonist of AMPA-type glutamate receptors, 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX; 10 microM). Bath applied noradrenaline (NA; 0.5-50 microM) dose-dependently and reversibly decreased by up to around 60% the amplitude of the EPSC, without affecting the holding current. The EPSC depression by NA was not accompanied by a change in EPSC reversal potential (around +5 mV), nor were inward currents generated by pressure application of glutamate affected by NA application. A comparable degree of EPSC depression was also seen with the alpha2-adrenoceptor agonist clonidine (5 microM), and the alpha2A-agonist oxymetazoline (5 microM), while the alpha1-agonist phenylephrine (100 microM) caused only a 22% depression. The EPSC depression caused by NA (10 microM) was completely antagonized by either the alpha-antagonist phentolamine (10 microM) or the alpha2-antagonist idazoxan (2 microM). Conversely, the beta-adrenoceptor antagonist popranolol (5 microM), and the alpha1-, alpha2B- and alpha2C-antagonist prazosin (2 microM) were without effect. These results indicate that activation of presynaptic alpha2A-adrenoceptors on inputs to SPNs decreases glutamate release.

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.

Actions of the anaesthetic Saffan on rat sympathetic preganglionic neurones in vitro

1. Whole-cell patch-clamp recordings were used to investigate the effects of the anaesthetic Saffan on the electrophysiological properties of sympathetic preganglionic neurones (SPNs) in rat spinal cord slices. 2. Saffan (1-54 microM) abolished or reduced the frequency of spontaneous action potential firing and abolished spontaneous, sub-threshold membrane potential oscillations. Saffan caused dose-dependent decreases in input resistance and depending upon the initial resting membrane potential, either a depolarization, a hyperpolarization or no change in membrane potential. 3. Responses to Saffan were blocked by the GABAA receptor antagonists bicuculline (5-20 microM) and picrotoxin (20 microM), but not by the glycine receptor antagonist strychnine (20 microM) indicating that they were mediated by GABAA receptors. 4. Changes in the properties of SPN action potentials were also observed. In the presence of Saffan the amplitude and duration of the action potential after-hyperpolarization were reduced and larger depolarizations were required in order to evoke trains of action potentials. 5. To examine the effects of Saffan on electrotonic coupling between SPNs, experiments were performed with the Na+ channel blocker QX-314 in the intracellular solution and antidromic oscillations were evoked by ventral root stimulation. Saffan failed to abolish antidromic oscillations, but reduced their amplitude and duration. This indicates that the abolition of spontaneous membrane potential oscillations was not a direct effect on the coupling between SPNs, but was a result of the abolition of spontaneous activity by Saffan. 6. The responses to Saffan occurred within the plasma concentration range of Saffan during anaesthesia, suggesting that the electrophysiological properties of SPNs may be altered during anaesthesia with Saffan. This would be expected to lead to changes in sympathetic tone and in the integration of sympathetic output.

Electrotonic coupling between rat sympathetic preganglionic neurones in vitro

1. Using the whole-cell recording technique in rat spinal cord slices we have shown that 26% of sympathetic preganglionic neurones (SPNs) show spontaneous membrane potential oscillations. These oscillations consist of trains of biphasic waves, which we have termed spikelets because of their similarity to truncated action potentials. 2. The spikelets were inhibited by TTX and anaesthetics such as alpha-chloralose but not by the intracellular application of lidocaine N-ethyl bromide (QX-314). 3. By stimulating the ventral roots we have demonstrated the presence of short-latency depolarizations (SLDs) in oscillating neurones. These SLDs have a similar waveform to the spontaneous spikelets, and also show the ability to override the frequency of occurrence of the spontaneous spikelets. These observations suggest that the spikelets result from electrotonic coupling between the oscillating SPNs. 4. SLDs were also observed in a population of non-oscillating, electrotonically coupled, quiescent SPNs. It was possible to induce oscillations in these neurones by the injection of depolarizing current (in the presence of QX-314), suggesting that these neurones are also gap-junction coupled. 5. Simultaneous whole-cell recordings were obtained from twenty-three pairs of SPNs. Two pairs displayed both spontaneous, synchronized oscillations and action potentials. Electrotonic coupling was confirmed by the detection of membrane polarization in both neurones in response to current injected into one neurone. In a further two pairs of quiescent SPNs, injection of depolarizing current pulses into one neurone induced action potential discharge in that neurone and a depolarization and oscillations in the other neurone. 6. The ability of groups of electrotonically coupled SPNs to generate spontaneous discharges within the spinal cord provides a novel mechanism for the integration and synchronization of information within the sympathetic nervous system.

Three electrophysiological classes of guinea pig sympathetic postganglionic neurone have distinct morphologies

Sympathetic postganglionic neurones can be differentiated electrophysiologically into three classes (phasic, Ph; tonic, T; and long-afterhyperpolarising, LAH) based on their potassium channel expression and consequent differences in excitability. We tested whether neuronal morphology differs between these classes. Neurones in coeliac, inferior mesenteric, and lower lumbar paravertebral ganglia of guinea pigs were filled with biocytin during in vitro experiments in which electrical properties were recorded. The dimensions of somata and dendrites were measured in approximately equal numbers of stained neurones of each class. The three electrophysiological classes were distinct in terms of soma shape, soma size (Ph < T = LAH), total dendritic length (LAH < Ph < T) and average length of dendrites (LAH < Ph < T) (P < 0.0001, multivariate analysis of variance). The mean number of primary dendrites also differed (LAH 13, Ph 16, T 20). The majority of dendrites did not branch, the ratios of terminations to primary dendrites being 1.36 (LAH), 1.63 (Ph) and 1.81 (T). Overall, LAH neurones, with medium-sized somata but the smallest dendritic trees, were more distinct morphologically than Ph and T neurones. The morphological differences between classes were not dependent on differences in location. Further, there was no apparent relation between morphology and the pattern of synaptic input each class receives. The results indicate that three distinct groups of sympathetic postganglionic neurone exist in adult guinea pigs, although more than three functions are subserved by these neurones.