Three types of sympathetic preganglionic neurones with different electrophysiological properties are identified by intracellular recordings in the cat

Advancing respiratory-cardiovascular physiology with the working heart-brainstem preparation over 25 years

Twenty‐five years ago, a new physiological preparation called the working heart–brainstem preparation (WHBP) was introduced with the claim it would provide a new platform allowing studies not possible before in cardiovascular, neuroendocrine, autonomic and respiratory research. Herein, we review some of the progress made with the WHBP, some advantages and disadvantages along with potential future applications, and provide photographs and technical drawings of all the customised equipment used for the preparation. Using mice or rats, the WHBP is an in situ experimental model that is perfused via an extracorporeal circuit benefitting from unprecedented surgical access, mechanical stability of the brain for whole cell recording and an uncompromised use of pharmacological agents akin to in vitro approaches. The preparation has revealed novel mechanistic insights into, for example, the generation of distinct respiratory rhythms, the neurogenesis of sympathetic activity, coupling between respiration and the heart and circulation, hypothalamic and spinal control mechanisms, and peripheral and central chemoreceptor mechanisms. Insights have been gleaned into diseases such as hypertension, heart failure and sleep apnoea. Findings from the in situ preparation have been ratified in conscious in vivo animals and when tested have translated to humans. We conclude by discussing potential future applications of the WHBP including two‐photon imaging of peripheral and central nervous systems and adoption of pharmacogenetic tools that will improve our understanding of physiological mechanisms and reveal novel mechanisms that may guide new treatment strategies for cardiorespiratory diseases.

The sympathetic innervation of the heart: Important new insights

Autonomic control of the heart has a significant influence over development of life threatening arrhythmias that can lead to sudden cardiac death. Sympathetic activity is known to be upregulated during these conditions and hence the sympathetic nerves present a target for treatment. However, a better understanding of the anatomy and physiology of cardiac sympathetic nerves is required for the progression of clinical interventions. This review explores the organization of the cardiac sympathetic nerves, from the preganglionic origin to the postganglionic innervations, and provides an overview of literature surrounding anti-arrhythmic therapies including thoracic sympathectomy and dorsal spinal cord stimulation. Several features of the innervation are clear. The cardiac nerves differentially supply the nodal and myocardial tissue of the heart and are dependent on activity generated in spinal neurones in the upper thoracic cord which project to synapse with ganglion cells in the stellate complex on each side. Networks of spinal interneurones determine the pattern of activity. Groups of spinal neurones selectively target specific regions of the heart but whether they exhibit a functional selectivity has still to be elucidated. Electrical or ischemic signals can lead to remodeling of nerves in the heart or ganglia. Surgical and electrical methods are proving to be clinically beneficial in reducing atrial and ventricular arrhythmias, heart failure and severe cardiac pain. This is a rapidly developing area and we need more basic understanding of how these methods work to ensure safety and reduction of side effects.

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.

Uric acid, indoxyl sulfate, and methylguanidine activate bulbospinal neurons in the RVLM via their specific transporters and by producing oxidative stress

Patients with chronic renal failure often have hypertension, but the cause of hypertension, other than an excess of body fluid, is not well known. We hypothesized that the bulbospinal neurons in the rostral ventrolateral medulla (RVLM) are stimulated by uremic toxins in patients with chronic renal failure. To investigate whether RVLM neurons are sensitive to uremic toxins, such as uric acid, indoxyl sulfate, or methylguanidine, we examined changes in the membrane potentials (MPs) of bulbospinal RVLM neurons of Wister rats using the whole-cell patch-clamp technique during superfusion with these toxins. A brainstem-spinal cord preparation that preserved the sympathetic nervous system was used for the experiments. During uric acid, indoxyl sulfate, or methylguanidine superfusion, almost all the RVLM neurons were depolarized. To examine the transporters for these toxins on RVLM neurons, histological examinations were performed. The uric acid-, indoxyl sulfate-, and methylguanidine-depolarized RVLM neurons showed the presence of urate transporter 1 (URAT 1), organic anion transporter (OAT)1 or OAT3, and organic cation transporter (OCT)3, respectively. Furthermore, the toxin-induced activities of the RVLM neurons were suppressed by the addition of an anti-oxidation drug (VAS2870, an NAD(P)H oxidase inhibitor), and a histological examination revealed the presence of NAD(P)H oxidase (nox)2 and nox4 in these RVLM neurons. The present results show that uric acid, indoxyl sulfate, and methylguanidine directly stimulate bulbospinal RVLM neurons via specific transporters on these neurons and by producing oxidative stress. These uremic toxins may cause hypertension by activating RVLM neurons.

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 (MVC_like) SPN in the working-heart brainstem preparation of spontaneously hypertensive (SH) and normotensive Wistar-Kyoto (WKY) rats. The MVC_like 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 (I_A). 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 I_A 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 I_A 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.

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.

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.

The rat spinal cord slice: Its use in generating pharmacological evidence for cholinergic transmission using the α7 subtype of nicotinic receptors in the central autonomic nucleus

Lamina X surrounds the central canal of the spinal cord and is an important site for the convergence of somatic and visceral afferent inputs relaying nociceptive information. Lamina X contains sympathetic preganglionic neurons (SPN) in the so-called central autonomic nucleus which may participate to viscero-autonomic reflexes. Here, we describe a transversal slice preparation of postnatal rat thoracolumbar spinal cord which allows the detailed characterization of the morphology, electrophysiological properties, synaptic activities and receptor pharmacology of neurons surrounding the central canal. By means of the patch clamp technique, in its whole cell configuration, and by the use of various pharmacological tools, we show here that lamina X neurons of the central autonomic nucleus express functional alpha7 nicotinic receptors which are located postsynaptically on SPNs where they are involved in a fast cholinergic transmission. Thus, this in vitro preparation is useful to study the mechanisms and the pharmacology of viscero-autonomic reflexes.

Active and passive membrane properties of rat sympathetic preganglionic neurones innervating the adrenal medulla

The intravascular release of adrenal catecholamines is a fundamental homeostatic process mediated via thoracolumbar spinal sympathetic preganglionic neurones (AD-SPN). To understand mechanisms regulating their excitability, whole-cell patch-clamp recordings were obtained from 54 retrogradely labelled neonatal rat AD-SPN. Passive membrane properties included a mean resting membrane potential, input resistance and time constant of -62 +/- 6 mV, 410 +/- 241 MOmega and 104 +/- 53 ms, respectively. AD-SPN were homogeneous with respect to their active membrane properties. These active conductances included transient outward rectification, observed as a delayed return to rest at the offset of the membrane response to hyperpolarising current pulses, with two components: a fast 4-AP-sensitive component (A-type conductance), contributing to the after-hyperpolarisation (AHP) and spike repolarisation; a slower prolonged Ba(2+)-sensitive component (D-like conductance). All AD-SPN expressed a Ba(2+)-sensitive instantaneous inwardly rectifying conductance activated at membrane potentials more negative than around -80 mV. A potassium-mediated, voltage-dependent sustained outward rectification activated at membrane potentials between -35 and -15 mV featured an atypical pharmacology with a component blocked by quinine, reduced by low extracellular pH and arachidonic acid, but lacking sensitivity to Ba(2+), TEA and intracellular Cs(+). This quinine-sensitive outward rectification contributes to spike repolarisation. Following block of potassium conductances by Cs(+) loading, AD-SPN revealed the capability for autorhythmicity and burst firing, mediated by a T-type Ca(2+) conductance. These data suggest the output capability is dynamic and diverse, and that the range of intrinsic membrane conductances expressed endow AD-SPN with the ability to generate differential and complex patterns of activity. The diversity of intrinsic membrane properties expressed by AD-SPN may be key determinants of neurotransmitter release from SPN innervating the adrenal medulla. However, factors other than active membrane conductances of AD-SPN must ultimately regulate the differential ratio of noradrenaline (NA) versus adrenaline (A) release secreted in response to various physiological and environmental demands.

T (tail)-rhythm oscillators: principles of nervous control of the vasculature revealed?

This review focuses on the nervous control of the caudal ventral artery of the rat tail, and aims to convince the reader that sympathetic control of the vasculature can be mediated via neural oscillators intrinsic to the sympathetic nervous system. The definitive functional significance of these oscillators is unknown at present. However, it is expected that through dynamic relationships with modulating and driving inputs, such oscillators would permit graded vascular responses.

Reflex patterns in preganglionic sympathetic neurons projecting to the superior cervical ganglion in the rat

Reflex patterns in preganglionic neurons projecting in the cervical sympathetic trunk (CST) were analyzed in response to stimulation of various afferent systems. We focused on the question whether these preganglionic neurons can be classified into functionally distinct subpopulations. Reflex responses were elicited by stimulation of trigeminal and spinal nociceptive, thermoreceptive as well as baroreceptor and chemoreceptor afferents. Multi- and single fiber preparations were studied in baroreceptor intact and sino-aortically denervated animals. Spontaneous activity of 36 preganglionic single neurons ranged from 0.2 to 3.5 imp/s (median= 1.11 imp/s). The degree of cardiac rhythmicity (CR) in the activity of sympathetic neurons was 69.5+/-13% (mean+/-S.D.; N=52; range=39-95%). Noxious stimulation of acral skin activated the majority (67%) of sympathetic preparations by 37+/-25% (N=35) above pre-stimulus activity; 15% were inhibited. In these neurons the response to noxious stimulation of acral skin was significantly correlated with the degree of CR (P<0.001, N=52) in that neurons showing the strongest excitation to noxious stimulation displayed the strongest CR. Noxious mechanical stimulation of body trunk skin (N=60) inhibited the majority (80%) of fiber preparations tested (by 34+/-18% of pre-stimulus activity, N=48); an activation was not observed. Cold stimulation of acral (N=9) and body trunk skin (N=42) activated most fiber preparations. Trigeminal stimulation evoked a uniform reflex activation of preganglionic neurons (+79+/-73% of pre-stimulus activity, N=32). Chemoreceptor stimulation by systemic hypercapnia elicited inhibitory (-31+/-19%, N=8) as well as excitatory (+59+/-5%, N=4) responses. These results show that preganglionic sympathetic neurons projecting to target organs in the head exhibit distinct reflex patterns to stimulation of various afferent systems; however, a clear classification into different functional subgroups did not emerge. Furthermore, reflex patterns showed a segmental organization to noxious cutaneous stimulation of acral parts and body trunk reflecting a differential central integration of spinal afferent input. Compared with the cat the reflex organization of sympathetic neurons projecting to the head seems to be less differentiated in the anesthetized rat.

Fast (3 Hz and 10 Hz) and slow (respiratory) rhythms in cervical sympathetic nerve and unit discharges of the cat

1. In seven decerebrate cats, recordings were taken from the preganglionic cervical sympathetic (CSy) nerves and from 74 individual CSy fibres. Correlation and spectral analyses showed that nerve and fibre discharges had several types of rhythm that were coherent (correlated) between population and unit activity: respiratory, '3 Hz' (2-6 Hz, usually cardiac related), and '10 Hz' (7-13 Hz). 2. Almost all units (73/74) had respiratory modulation of their discharge, either phasic (firing during only one phase) or tonic (firing during both the inspiratory (I) and expiratory (E) phases). The most common pattern consisted of tonic I-modulated firing. When the vagi were intact, lung afferent input during I greatly reduced CSy unit and nerve discharge, as evaluated by the no-inflation test. 3. The incidence of unit-nerve coherent fast rhythms (3 Hz or 10 Hz ranges) depended on unit discharge pattern: they were present in an appreciable fraction (30/58 or 52 %) of tonic units, but in only a small fraction (2/15 or 13 %) of phasic units. 4. When baroreceptor innervation (aortic depressor amd carotid sinus nerves) was intact, rhythms correlated to the cardiac cycle frequency were found in 20/34 (59 %) of units. The cardiac origin of these rhythms was confirmed by residual autospectral and partial coherence analysis and by their absence after baroreceptor denervation. 4. The 10 Hz coherent rhythm was found in 7/34 units when baroreceptor innervation was intact, where it co-existed with the cardiac-locked rhythm; after barodenervation it was found in 9/50 neurones. Where both rhythms were present, the 10 Hz component was sometimes synchronized in a 3:1 ratio to the 3 Hz (cardiac-related) frequency component. 5. The tonic and phasic CSy units seem to form distinct populations, as indicated by the differential responses to cardiac-related afferent inputs when baroreceptor innervation is intact. The high incidence of cardiac-related correlation found among tonic units suggests that they are involved in vasomotor regulation. The high incidence of respiratory modulation of discharge suggests that the CSy units may be involved in regulation of the nasal vasculature and consequent ventilation-related control of nasal airway resistance.

Coherent rhythmic discharges in sympathetic nerves supplying thermoregulatory circulations in the rat

1. In anaesthetised rats, activity recorded from sympathetic postganglionic neurones innervating the tail circulation has characteristic rhythmicity (0.4-1.2 Hz). At the population level this rhythmicity can be seen as a peak (T-peak) in autospectra of sympathetic activity recorded from ventral collector nerves (VCNs). 2. Here we investigated whether nerves supplying thermoregulatory circulations share common rhythmic discharges at T-peak frequency. Activity was recorded from nerve pairs consisting of left ventral collector nerve (LVCN) and one of the following: right ventral collector nerve (RVCN), left dorsal collector nerve (DCN), left saphenous nerve (SN) or left renal nerve (RN). 3. During central apnoea, T-peak frequencies in RVCN autospectra were similar to those of simultaneously recorded LVCN and these activities were coherent. Similar observations were made for nerve pairs involving LVCN-DCN and LVCN-SN. In contrast, autospectra of RN activity did not contain T-peaks. 4. In comparison to the peaks in autospectra of RN activity, when the frequency of rhythmic phrenic nerve activity was manipulated T-peaks in VCN, DCN and SN autospectra did not show obligatory 1:1 locking. 5. We conclude that T-peaks are a robust feature of autospectra of sympathetic discharges supplying thermoregulatory circulation but not those influencing the kidney. The high coherence demonstrated between the T-peak discharges is consistent with the view that common/coupled oscillators located within the CNS influence cutaneous vasoconstrictor sympathetic activity.

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.

Spinal segments communicating resting sympathetic activity to postganglionic nerves of the stellate ganglion

It has been shown earlier using sympathetic reflexes and anatomic techniques that preganglionic neurons controlling different effectors occupy wide and overlapping ranges of adjacent segments in the spinal cord (cardiac: T1-T7, vertebral: T2-T8). Because, however, the majority of preganglionic neurons are silent at resting states, the present study was designed to estimate the segmental map of subsets of these neurons including only those active at rest using simultaneous recordings from the inferior cardiac and vertebral nerves, under chloralose-urethan or urethan anesthesia. In 22 cats, thoracic white rami T1-T8 were cut in a sequential manner. Three-minute-long data segments were recorded between sectionings and analyzed in the frequency domain using the fast Fourier transform. We found that cardiac and vertebral active maps involved segments T3-T5 and T4-T8, respectively. In individual experiments, however, most of the power of rhythmic activity originated from only one or two segments and the dominant segments for the two nerves never overlapped. Moreover, the separation between dominant segments generating cardiac and vertebral nerve discharges was wider and the distribution of tonically active preganglionic neurons projecting to each nerve was narrower under urethan than chloralose-urethan anesthesia. We conclude that the proportion of active to quiescent preganglionic neurons regulating cardiac and vertebral nerve discharges varies from spinal segment to segment and that active neurons projecting to these nerves are nonoverlapping.

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.

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.

Organization of NADPH-diaphorase-reactive neurons and catecholaminergic fibers in human intermediolateral cell column

We studied the distribution of NADPH-d-reactive sympathetic preganglionic neurons (SPNs) and tyrosine hydroxylase (TH) immunoreactive fibers at T1, T4, T8, and T10 of human thoracic cord. NADPH-d-reactive SPNs were present at all segments. TH-immunoreactive fibers were distributed within the NADPH-d neuropil and appeared to contact SPNs. These interactions may be important for normal and pathological control of arterial pressure.

The spinally mediated 10-Hz rhythm in the sympathetic nerve activity of cats

To examine the origin of the so-called '10-Hz rhythm' in the sympathetic nerves, the mass discharges of the white ramus of the third thoracic segment (T3WR) and the inferior cardiac nerve (ICN) and the activities of single postganglionic neurons in the stellate ganglion were recorded in spinal cats. During the chemical or electrical stimulation of the spinal cord, the time of peak of discharges in the sympathetic nerves was analyzed. Both intrathecal administration of N-methyl-D-aspartic acid (NMDA; 3-10 mM) and continuous high frequency (80-200 Hz) electrical stimulation of the dorsolateral funiculus at the second cervical level increased activity of the sympathetic nerves in a similar fashion. In these conditions, modes of the inter-peak interval histograms (IPIH) were about 100 ms (range; 90-130 ms), the inverse of about 10 Hz, but no correlation was observed in autocorrelograms of these peaks of discharges. Therefore, this 100-ms interval activity might have some significance for the 10-Hz rhythm. In order to make this point clear, we stimulated the dorsolateral funiculus with intermittent trains of electrical pulses (0.2-ms duration, 10-35 pulses of 80-200 Hz frequency, in every 300-800 ms). While intermittent trains of pulses were applied, multiple peaks of discharges were evoked in the sympathetic nerves. IPIHs of the nerves were multimodal. The first mode (shortest interval) was about 100 ms. The first mode depended on none of the stimulus parameters but the probability of the about 100-ms interval activity depended on the interval of trains of pulses and the stimulus intensity. With this intermittent stimulation, the autocorrelogram of the peaks revealed the 100-ms interval rhythm. To confirm that the peak of discharges in the ICN was composed of synchronized spikes of postganglionic fibers, single neuronal activities of postganglionic neurons were recorded during the intermittent stimulation. Inter-spike interval histograms showed almost same profile as the IPIHs of the ICN. These results can be explained if the following two assumptions are valid; (i) There are mechanisms that limit minimum firing interval of most preganglionic neurons to about 100 ms. (ii) Simultaneously a interneuron in the spinal cord resets the spike generation of multiple preganglionic neurons. Similarity of the spike activities of the sympatho-excitatory reticulospinal neurons to the intermittent stimulation can explain the 10-Hz rhythm in the peripheral sympathetic nerves in intact spinal cord animals. It is not necessary to postulate the specific 10-Hz rhythm generator in the brain stem for the sympathetic nervous system.

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.

Synapses on axons of sympathetic preganglionic neurons in rat and rabbit thoracic spinal cord

Axosomatic and axodendritic synapses occur on sympathetic preganglionic neurons, but it is not yet known whether their axons receive synaptic input, which could be particularly effective at regulating sympathetic outflow. Here, we examined retrogradely labelled sympathetic preganglionic axons to see if they received synapses. Cholera toxin B subunit (CTB) or CTB conjugated to horseradish peroxidase (CTB-HRP) was used to label neurons projecting to the rat or rabbit superior cervical ganglion, the rat adrenal medulla, or the rabbit stellate ganglion. At the light microscopic level, small groups of CTB-immunoreactive axons travelled through the ventral horn near its lateral boundary, with occasional axons taking a more medial course. The axons passed through the ventrolateral funiculus to exit at the ventral roots. In parasagittal section, a few axons branched within the ventral horn, sending processes rostrally and caudally for short distances before they turned ventrally to exit the spinal cord. At the ultrastructural level, CTB-immunoreactive rat and rabbit sympathetic preganglionic axons were almost exclusively unmyelinated. In contrast, labelling with CTB-HRP revealed both myelinated and unmyelinated axons in the ventral horn, the ventrolateral white matter, and the ventral roots. CTB-HRP also allowed the detection of the initial segment of a sympathetic preganglionic axon. Synapses, with vesicles clustered presynaptically and membrane specializations postsynaptically, were found on some unmyelinated CTB-immunoreactive axons. Occasional axons received several synapses. Synapses were most common on CTB-containing axons just ventral to the intermediolateral cell column. One synapse was found on an axon within 2 microns of its origin from a proximal dendrite. Rare synapses were found several hundred micrometers ventral to the intermediolateral cell column. One branching axon had synapses just below the branch point on both the main axon and the axonal branch. These findings indicate an extensive synaptic input to the axons of at least some sympathetic preganglionic neurons. These axoaxonic synapses could have a profound effect on sympathetic activity.

Arrangement of dendrites and morphological characteristics of sympathetic preganglionic neurones projecting to the superior cervical ganglion and adrenal medulla in adult cat

Sympathetic preganglionic neurones (SPN) projecting to the superior cervical ganglion (SCG) and adrenal medulla (AM) in the adult cat were retrogradely labelled with cholera B horseradish peroxidase (CBHRP). Labelled neurones were found in 4 sub-nuclei: the nucleus intermediolateralis thoracolumbalis pars principalis (ILp), the nucleus intermediolateralis pars funicularis (ILf), the nucleus intercalatus spinalis (IC) and the nucleus pars paraependymatis (ICpe). The majority of SPN were found in the ILp (75%). Each group of target specified SPN had a different segmental distribution. SCG-SPN between cervical 8 (C8) and thoracic 6 (T6) and AM-SPN between thoracic 3 (T3) and lumbar 2 (L2). Fusiform and round bodied neurones were the most common shapes found, a third longitudinal type was occasionally found. SCG and AM-SPN exhibited a dense rostrocaudal dendritic projection extending along the length of the ILp. There was also a lateral projection into the ILf and a medial one projecting towards the central canal. This dendritic arrangement gave the ILp the appearance of being an 'open nucleus'. The dendrites branched at their distal ends and all along their lengths swellings could be seen. It was concluded that contrary to previous descriptions the arrangement of SPN in the adult cat is not too dissimilar to that in the adult rat.

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.

The spinal organisation of the baroreceptor reflex

Baroreceptor inhibition of a spinally evoked response in a renal nerve was studied following removal of excitatory drive from the rostral ventrolateral medulla (RVLM), by microinjecting glycine into this region (RVLM block). Activation of arterial baroreceptors was still able to inhibit a spinally evoked response after RVLM block and this effect was abolished by intrathecal strychnine. Intrathecal bicuculline also was shown to reduce the magnitude of the baroreceptor inhibition but only when the RVLM was intact indicating that bicuculline was removing a facilitation. Both strychnine and bicuculline antagonised an NTS induced inhibition of activity in single sympathetic preganglionic neurones. It is concluded that arterial baroreceptor reflex regulation of vasomotor activity occurs at a spinal as well as a supraspinal site and GABA and glycine are the likely inhibitory mediators at both sites.

Integrative properties of sympathetic preganglionic neurones within the thoracic spinal cord

The discharge pattern of sympathetic preganglionic neurones (SPNs) in the lateral horn is shaped by the interplay of synaptic inputs, membrane properties and local factors within the spinal cord. Intracellular recordings in vivo and in vitro have clarified the importance of some of these factors. Pacemaker activity can be recorded in vitro, but does not contribute to the generation of action potentials in vivo where spikes are solely generated from synaptic potentials. Synaptic potentials occur in phase with either the cardiac or the respiratory cycle or at irregular intervals. Postsynaptic interaction of these various inputs at the level of SPNs as well as presynaptic gating mechanisms in relation to the respiratory cycle have been observed. The discharge pattern is also modified by specific membrane properties which function to limit their discharge rate in the absence of axon collaterals. Finally the discharge of SPNs is affected by local factors: Since asphyxia causes a strong sympathetic activation when synaptic inputs to other neurones are already non-functioning synapses on SPNs are resistant to hypoxia or changes in the extracellular fluid somehow influence the activity of these neurones.

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.

A comparison between the adult rat and neonate rat of the architecture of sympathetic preganglionic neurones projecting to the superior cervical ganglion, stellate ganglion and adrenal medulla

Sympathetic preganglionic neurones (SPN) projecting to the superior cervical ganglion (SCG) and adrenal medulla (AM) in the neonate (< 14 days) and SCG, stellate ganglion (SG) and AM in the adult rat (> 3 months) were retrogradely labelled with cholera B horseradish peroxidase (CBHRP). Labelled neurones were found in 4 four distinct nuclei: the nucleus intermediolateralis thoracolumbalis pars principalis (ILp), a nucleus equivalent to the intemediolateral cell column (IML); the nucleus intermediolateralis thoracolumbalis pars funicularis (ILf); the nucleus intercalatus spinalis (IC) and the nucleus intercalatus pars paraependymatis (ICpe) or central autonomic area (CA). These were represented to a similar extent in both neonate and adult. Neonate and adult SCG, SG and AM-SPN had a similar segmental distribution cervical 8 (C8) to thoracic 5 (T5) for SCG-SPN and thoracic 3 (T3) to thoracic (T13) for AM-SPN whereas adult SG-SPN were distributed over segments C8 to T9. Most labelled neurones (70%) were located in the ILp with one segment containing the highest proportion of SPN. Three morphologically distinct neurones were evident. Fusiform and roundbodied were the most common. Fusiform somata of the ILp were orientated both mediolaterally and rostrocaudally in the neonate but only rostrocaudally in the adult. Dendrites of the SPN in the adult and neonate extended in a dense rostrocaudal band along the ILp, more diffusely into the white matter of the Ilf and in bundles medially towards the central canal (CC). The neonate showed some significant differences. In the ILp, the cell bodies were less tightly packed into a narrow band and into clusters and the dendrites were more diffuse. It was concluded that at 12 days postnatally the organisation of the sympathetic nuclei had still nor reached the adult form. However, there is no extensive realignment of dendrites in the adult so the ILp remains an 'open' nucleus like the neonate.

Membrane properties and dendritic arborization of the intermediolateral nucleus neurons in the guinea-pig thoracic spinal cord in vitro

The morphological and electrophysiological properties of neurons in the intermediolateral nucleus (IML) were studied in the transverse and longitudinal slice of guinea-pig thoracic spinal cord (T2-T3) using intracellular staining and recording techniques. Two morophologically different types of neurons were observed: fusiform cells with craniocaudally oriented dendrites, and multipolar cells with dendrites diffusely extending in the IML. The ratio of fusiform to multipolar cells was 4:1. The fusiform cells were identified as sympathetic preganglionic neurons (SPNs) by their antidromic responses to stimulation of the ventral root exit zone, while the multipolar cells were not antidromically activated by stimulation of this site. Both cell types showed similar resting membrane potential and input resistance. The tonic responses of these neurons to hyperpolarizing current pulses were characteristically different: the SPNs had a marked hyperpolarizing sag at the break of the pulse, caused by an A current, while the unidentified neurons showed no A current. In addition, the SPNs had much longer duration of spike and afterhyperpolarization, as well as lower frequency of spontaneous or current-evoked firing, than the unidentified neurons. These observations suggest that, in the absence of the criterion of antidromic activation by stimulation of the axon, it is still possible to differentiate SPNs from other IML neurons on the basis of morphological and electrophysiological properties of the neuron.

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.

Pattern of irregular dorsal root discharge in the spinal cat

Pattern of irregular type of dorsal root discharge (DRD) in non-anaesthetized spinal cats was inferred from the distribution of its interspike intervals. Interspike interval histograms were compiled from antidromic spike potentials of irregular DRD which were recorded in the central ends of 33 single dorsal root fibres of L7 dorsal root. The majority of histograms was unimodal. The mean preferred interval of irregular DRD which indicated the most frequently occurring interspike interval was 6.1 +/- 0.5 ms. The shortest and longest intervals were 3.7 +/- 0.3 ms and 45.9 +/- 4.4 ms, respectively. The coefficient of symmetry of the histogram was 0.06, indicating highly unsymmetrical distribution of intervals of irregular DRD. Conduction velocity of dorsal root fibres carrying irregular antidromic discharge amounted to 52.3 +/- 5.3 m/s (n = 23). There were no significant correlations between three analysed interspike intervals and conduction velocity. The results are discussed in view of the hypothesis of the modulating effect of antidromic discharge on the afferent inflow. Comparison of preferred interspike interval of irregular DRD with the existing data on the rate of orthodromic activity in afferent nerve fibres suggests that irregular antidromic discharge on many occasions is able to block othodromic impulses by collision.

Classification of preganglionic neurones projecting into the cat cervical sympathetic trunk

1. The spontaneous and reflex activity patterns of 167 single preganglionic axons dissected from the cervical sympathetic trunk were examined in chloralose-anaesthetized cats. Each neurone was classified into one of four major groups, on the basis of three principal criteria: the presence or absence of significant cardiac rhythmicity of the activity, the response to noxious stimulation of the skin, and the coupling of its activity to central inspiratory drive (phrenic nerve activity). Most neurones were also subjected to additional tests, which included carotid chemoreceptor stimulation, nasopharyngeal probing, systemic hypercapnia (ventilation with 8% CO2), hyperventilation, adrenaline-induced blood pressure rises and retinal illumination. 2. Group I neurones (n = 69; 41%) showed significant cardiac rhythmicity, indicating strong baroreceptor control. Most (54/69) were excited by noxious stimuli, the rest being unaffected. Their activity showed variable degrees of excitatory coupling to the central inspiratory drive, and was enhanced by hypercapnia (35/39). Their responses to stimulation of arterial chemoreceptors (12/15) and nasopharyngeal receptors (24/35) were excitatory. 3. Group II neurones (n = 39; 23%) were inhibited by noxious stimulation of skin. With nine exceptions, they showed no significant cardiac rhythmicity, although they were weakly inhibited by an adrenaline-induced blood pressure rise. Their coupling to central inspiratory drive was weak or absent, and their responses to hypercapnia and hyperventilation were variable. By contrast to other groups, they were inhibited by both chemoreceptor stimulation (9/10) and nasopharyngeal stimulation (17/18). 4. Group III neurones (n = 33; 20%) showed no significant cardiac rhythmicity, but their activity was closely coupled to central inspiratory drive. They were inhibited by hyperventilation (9/9) and excited by hypercapnia (20/21), but only fired during the central inspiratory phase and sometimes during late expiration. Their responses to noxious stimulation (28/33), chemoreceptor stimulation (8/11) and nasopharyngeal probing (24/24) were excitatory, but the induced activity was 'gated' by the respiratory cycle, occurring primarily during inspiration and avoiding the postinspiratory phase. 5. Group IV neurones (n = 26; 16%) showed no significant cardiac or respiratory related activity and were either excited (n = 22) or unaffected (n = 4) by noxious stimuli. One of the latter and three group II neurones were inhibited by retinal illumination; thirty-one other neurones of all classes were unaffected. 6. Approximately 45% of thoracic sympathetic neurones were silent under the experimental conditions. About 25% of these could be recruited by systemic hypercapnia leaving 34% without spontaneous and reflex activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Whole-cell recordings from sympathetic preganglionic neurons in rat spinal cord slices

Whole-cell patch-clamp recordings (WCR) were made from sympathetic preganglionic neurons (SPN) in neonate rat spinal cord slices. SPN were identified histologically by filling them with the fluorescent dye Lucifer Yellow contained within the patch pipette solution. Current clamp recordings were obtained from SPN with a potassium based pipette solution. The cells exhibited many of the characteristic properties of SPN seen previously with intracellular recordings in both the rat and the cat. However, we found an order of magnitude increase in both cell input resistance (950 M omega) and time constant (118 ms) over those seen with conventional recordings. We believe these values approximate better the situation in intact cells, and will have a vital bearing upon how SPN integrate inputs. We conclude that WCR in spinal cord slices provides a powerful tool for investigating the cellular properties of SPN.

Light and electron microscopic analyses of intraspinal axon collaterals of sympathetic preganglionic neurons

Experiments were performed in pigeons (Columba livia). Sympathetic preganglionic neurons (SPNs) in the first thoracic spinal cord segment (T1) were identified electrophysiologically using antidromic activation and collision techniques and then intracellularly labeled with horseradish peroxidase (HRP). In 6 of 10 HRP-labeled SPNs, the site of axon origin and intraspinal axonal trajectory could be specified. In 2 of the 6 HRP-labeled axons, the peripherally projecting process branched intraspinally. The presence or absence of SPN intraspinal axonal collateralization did not correlate with parent perikaryal subnuclear location or dendritic alignment. None of the collaterals were recurrent onto the SPN of origin. Light microscopically, the collateral branches appeared to end with punctate, bulbous swellings. The spinal regions of the terminal end swellings for the two axons did not overlap one another. In one instance the entire terminal field was confined within the principal preganglionic cell column (column of Terni). The other axon had collateral branches which terminated in the lateral white matter and in a ventrolateral region of lamina VII. A serial section, electron microscopic reconstructive analysis of the entire intraspinal collateral terminal field within the column of Terni revealed that: (a) the primary collateral process was unmyelinated and arose at a node of Ranvier; (b) after issuance of the collateral branch, the myelinated parent axon continued to increase its myelin wrapping throughout the spinal gray; (c) the bulbous swellings observed light microscopically corresponded to axon terminal boutons and regions of synaptic contact; (d) the axon collateral terminals were exclusively presynaptic to small caliber dendrites and formed only asymmetric specializations; and (e) the collateral terminals contained numerous mitochondria, and densely packed, electron-lucent, spherical vesicles.

Relative effects of different spinal autonomic nuclei on cardiac sympathoexcitatory function

Mean arterial pressure and heart rate were monitored in immobilized and artificially ventilated male Wistar rats either anesthetized with pentobarbital or decerebrated at midcollicular level. The rate of increase in the left ventricular pressure was also monitored in order to compute contractility index. L-glutamate (1.77 nmole) was microinjected (10 nl) into the following autonomic nuclei of the spinal cord at C8 to T4 levels: 1) intermediolateral column (IML), 2) n. intercalatus spinalis (IC) and 3) n. intercalatus pars paraependymalis (ICpe); this region is commonly known as the central autonomic area (CA). The site of microinjection was marked by injection of a dye; these studies suggested that microinjections of glutamate into the IML are likely to encompass the neurons in the nucleus (n.) intermediolateralis thoracolumbalis pars principalis (ILp) and n. intermediolateralis thoracolumbalis pars funicularis (ILf). Sympathoexcitatory cardiac responses to glutamate microinjections were elicited from T1 to T3 levels; these responses could not be evoked at C8 and T4 levels. In each of these segments, maximum responses were obtained from the IML while the responses evoked from the IC and the CA were minimal. These results suggest that at T1 to T3 levels of the spinal cord, IML is the main cell group regulating sympathetic cardiac function; CA and IC may play a relatively minor role in this function.

Spontaneous rhythmic activity in the intermediolateral cell nucleus of the neonate rat thoracolumbar spinal cord in vitro

Intracellular recordings from the intermediolateral cell nucleus of the neonate rat thoracolumbar spinal cord slice preparation revealed a population of neurons which displayed three types of spontaneous rhythmic activity: burst firing, tonic beating and membrane oscillations. Most neurons displayed more than one of these types of activity. Neurons had mean resting potentials of -59 mV and input resistances ranging from 10 to 48 m omega. Spontaneous oscillations which were observed either independently or following hyperpolarization of neurons displaying tonic beating or bursting behaviour had a mean peak amplitude and frequency of approximately 14 mV and 1 Hz respectively. Oscillations were not obviously reversible as they were still apparent at potentials as negative as -120 to -140 mV. This suggests that the oscillations had a site of generation distant to the recording electrode. Neurons displaying tonic beating activity were characterized by low frequency firing activated at the peak of the depolarizing phase of the underlying oscillation and these neurons could be induced to exhibit burst behaviour by membrane depolarization. The frequency of firing in tonic beating neurons ranged from 0.1 to 8.8 Hz. Burst firing was characterized by: bursts of 3-17 action potentials; burst cycle frequency of approximately 1 Hz; an afterdepolarization potential mainly observed at the termination of a burst. Burst firing was abolished by cobalt and membrane hyperpolarization but not by barium, low calcium or tetraethylammonium chloride. The switch from tonic beating to burst firing may, in part, involve activation of a voltage- and calcium-dependent afterdepolarization potential. We conclude that a population of neurons in the lateral horn of the spinal cord are capable of rhythmic activity with underlying spontaneous pacemaker-like oscillations.

Inhibitory postsynaptic potentials in neonatal rat sympathetic preganglionic neurones in vitro

1. Intracellular recordings were made from antidromically identified sympathetic preganglionic neurones (SPNs) in transverse sections of thoraco-lumbar spinal cord from neonatal (12-22 day) rats. 2. Two types of hyperpolarizing (inhibitory) postsynaptic potentials (IPSPs) were recorded in the SPNs. The first type, which we have termed unitary IPSPs, were small, discrete IPSPs that occurred spontaneously and also following chemical or electrical stimulation applied to the spinal cord slices. The second type IPSP was a hyperpolarizing response evoked by either dorsal or ventral root stimulation. 3. Spontaneously occurring unitary IPSPs had an amplitude of 1 to 5 mV, and reversal potential of -60 to -75 mV; they were reversibly abolished by low Ca2+, tetrodotoxin (TTX) or strychnine but not by bicuculline and picrotoxin. 4. Pressure application of N-methyl-D-aspartate (NMDA), an excitatory amino SPNs; these were abolished by either strychnine or by the NMDA receptor antagonist D-2-amino-5-phosphonovalerate. Furthermore, electrical stimulation of dorsal rootlets elicited in several SPNs the discharge of strychnine-sensitive unitary IPSPs. 5. Electrical stimulation applied to dorsal or ventral rootlets elicited in nineteen and eight SPNs, respectively, an IPSP of larger amplitude (5 to 15 mV). The IPSP exhibited a reversal potential of -60 to 75 mV; it was changed to a depolarizing response in a low [Cl-]o solution, but was not significantly affected in a low [K+]o. Strychnine but not bicuculline or picrotoxin reversibly blocked the IPSPs in nearly all the SPNs. Additionally, hexamethonium and d-tubocurarine antagonized the IPSPs evoked by ventral but not by dorsal root stimulations. 6. Our results suggest that unitary and evoked IPSPs recorded in SPNs are due primarily to an increase of Cl- conductance by glycine or a glycine-like substance, released from interneurones, that can be activated by NMDA. Furthermore, IPSPs evoked by ventral root stimulation appear to represent a disynaptic event whereby nicotinic activation of a glycine-releasing interneurone results in a release of the inhibitory transmitter; this is then analogous to the Renshaw cell circuitry of the spinal motoneurones.

Ventrolateral medullary neurones: effects on magnitude and rhythm of discharge of mesenteric and renal nerves in cats

1. Discharge of whole mesenteric and renal nerves was recorded in eighteen chloralose-anaesthetized, artificially respired cats. 2. Inhibition of tonic activity of neurones within the rostral ventrolateral medulla (RVLM blockade) by bilateral application of glycine caused significant reductions in discharge of renal and mesenteric nerves, arterial blood pressure and heart rate. The decrease in discharge of renal nerves was significantly greater than that of mesenteric nerves. 3. During the response to glycine application, the spinal cord was transected at the first cervical segment. The magnitude of renal nerve discharge after transection was not different from that during blockade of the RVLM. On the other hand, mesenteric nerve activity increased following spinal cord transection, returning to control levels. 4. Power spectral analysis revealed that mesenteric and renal nerves discharged with periodicities ranging from 1 to 6 Hz. Application of glycine to the RVLM reduced the slow rhythm in firing of mesenteric and renal nerves similarly. Transection of the spinal cord resulted in further reduction in the rhythmicity in discharge of both nerves. 5. The results indicate that excitatory drive from the RVLM is crucial for the maintenance of on-going discharge of renal, but not of mesenteric nerves. However, such inputs are apparently essential to maintain the slow rhythm in firing of both nerves.

Rostrocaudal location of sympathetic preganglionic neurones within the third thoracic segment of the cat spinal cord investigated by the retrograde transport of horseradish peroxidase and by recording of antidromic field potentials

The rostrocaudal location of sympathetic preganglionic neurones (SPNs) in the intermediolateral cell column of the third thoracic segment was studied in the cat by the retrograde transport of horseradish peroxidase and by recording of antidromic field potentials in the spinal cord in response to stimulation of white ramus T3. By both methods, the position of the rostral and caudal border of SPNs was determined in relation to the entry of segmental dorsal roots. It was found that SPN's are confined in the spinal cord to the length of one segment (9494 +/- 823 micron), but are shifted rostrally by about 3 mm with respect to the point of entry of the dorsal roots of segment T3.

A transient outward rectification in the cat sympathetic preganglionic neuron

Intracellular recordings were made from sympathetic preganglionic neurons of the intermediolateral nucleus, in the slice of the T3 segment of the cat spinal cord. Depolarization to -60 mV from membrane potentials more negative than -75 to -80 mV caused a transient outward rectification which decayed in 0.5-2.0s. The rectification was sensitive to changes in K concentration, was abolished by 4AP (2 mM), and was unaffected by low Ca (0.25 mM), Co (2 mM), Ba (1 mM), TEA (20 mM) or intracellular Cs. These properties suggest that the rectification is due to a transient K-current similar to the A-current described in other neurons.