Automatic classification of interference patterns in driven event series: application to single sympathetic neuron discharge forced by mechanical ventilation.

This study proposes a method for the automatic classification of nonlinear interactions between a strictly periodical event series modelling the activity of an exogenous oscillator working at a fixed and well-known rate and an event series modelling the activity of a self-sustained oscillator forced by the exogenous one. The method is based on a combination of several well-known tools (probability density function of the cyclic relative phase, probability density function of the count of forced events per forcing cycle, conditional entropy of the cyclic relative phase sequence and a surrogate data approach). Classification is reached via a sequence of easily applicable decision rules, thus rendering classification virtually user-independent and fully reproducible. The method classifies four types of dynamics: full uncoupling, quasiperiodicity, phase locking and aperiodicity. In the case of phase locking, the coupling ratio (i.e. n: m) and the strength of the coupling are calculated. The method, validated on simulations of simple and complex phase-locking dynamics corrupted by different levels of noise, is applied to data derived from one anesthetized and artificially ventilated rat to classify the nonlinear interactions between mechanical ventilation and: (1) the discharges of two (contemporaneously recorded) single postganglionic sympathetic neurons innervating the caudal ventral artery in the tail and (2) arterial blood pressure. Under central apnea, the activity of the underlying sympathetic oscillators is perturbed by means of five different lung inflation rates (0.58, 0.64, 0.76, 0.95, 1.99 Hz). While ventilation and arterial pressure are fully uncoupled, ventilation is capable of phase locking sympathetic discharges, thus producing 40% of phase-locked patterns (one case of 2:5, 1:1, 3:2 and 2:2) and 40% of aperiodic dynamics. In the case of phase-locked patterns, the coupling strength is low, thus demonstrating that this pattern is sliding. Non-stationary interactions are observed in 20% of cases. The two discharges behave differently, suggesting the presence of a population of sympathetic oscillators working at different frequencies.

Rostral ventromedial medulla and the control of cutaneous vasoconstrictor activity following i.c.v. prostaglandin E(1).

Neurones within the rostral ventromedial medulla project to the intermediolateral cell column of the spinal cord where they may influence sympathetic preganglionic neuronal activity controlling cutaneous vascular beds. Here we assess whether such neurones contribute to cutaneous sympathetic vasoconstrictor activity in a fever-like state induced by i.c.v. injection of E-series prostaglandin. In urethane-anaesthetised rats, we recorded population sympathetic activity to the tail (an index of vasoconstrictor discharge regulating cutaneous thermoregulatory circulations). A survey of the effects of GABA microinjections (200 mM; 60-80 nl; 111 sites in 57 rats) demonstrated that those into the rostral ventromedial medulla (in the region of raphe pallidus and magnus; approximately bregma -10 to -12 mm) markedly decreased (51%-100%) population sympathetic cutaneous vasoconstrictor activity during "normothermic control." In contrast, injections at sites dorsal and lateral to this region tended to produce either a smaller decrease or have no effect. In heat-clamp (nine animals: body temperature 40-41 degrees C) cutaneous vasoconstrictor activity was decreased by 83+/-5%. I.c.v. prostaglandin E(1) (100 ng and above) restored activity to, or above, control levels in these animals and where body temperature was maintained at control levels (12 animals: body temperature 35.5-36.5 degrees C). The depressant action of GABA was sustained in both conditions. GABA did not significantly influence concurrently recorded splanchnic nerve activity and heart rate in any condition although both were increased following i.c.v. prostaglandin E(1) (500 ng). This study is the first to demonstrate that inhibition of neuronal activity within the rostral ventromedial medulla decreases sympathetic cutaneous vasoconstrictor activity during normothermic control and following i.c.v. prostaglandin E(1) (both with and without heat-clamp). Therefore, sympathetic premotor neurones in this area contribute to vasoconstrictor drive in these conditions. In contrast, we were unable to demonstrate that the same area had a substantial involvement in the control of splanchnic nerve activity or heart rate, even when these were enhanced following i.c.v. prostaglandin E(1).

Cutaneous sympathetic motor rhythms in the decerebrate rat.

Investigation of rhythmic discharges may provide insights into integrative mechanisms underlying nervous system control of effectors. We have previously shown that, in CNS-intact, anesthetized rats, cutaneous sympathetic vasoconstrictor neurones innervating thermoregulatory circulations exhibit a robust rhythmicity in the 0.4-1.2-Hz frequency range (T-rhythm). Here we examined whether the neural circuitry required to generate this rhythm remained intact in decerebrate (at collicular level), paralyzed and artificially ventilated preparations with cervical vagotomy, ligation of common carotid arteries and pneumothorax. Population sympathetic activity was recorded from the ventral collector nerve (VCN) of the tail in nine animals, while monitoring central respiratory drive. We found that rhythmic activity remained a robust feature and that activity behaved in a comparable manner to that previously described in the intact anesthetized preparation. Manifest as peaks in the autospectra, the dominant rhythm was either at the frequency of (f) lung inflation cycle (fLIC), central respiratory drive (fCRD) or in the 'free-run' T-rhythm frequency range. Through manipulation of fLIC we could alter the dominant rhythm of discharges. We show a significant relationship between fLIC and the likelihood of the dominant rhythm in VCN discharges being at fLIC or at a frequency that was neither fLIC nor fCRD. At fLIC of 1 Hz: in seven of nine animals the VCN dominant rhythm was 1 Hz, zero of nine displayed a dominant T-rhythm; at fLIC of 2 Hz: two of nine had a dominant VCN rhythm at 2 Hz and five of nine a T-rhythm. Furthermore, CRD was never observed to entrain to fLIC. These experiments demonstrate that the network underlying the generation of the T-rhythm is located below the collicular level of the neuraxis and that in this preparation LIC-related modulation of discharges may be mediated by spinal (sympathetic) afferents.

Cutaneous sympathetic motor rhythms during a fever-like response induced by prostaglandin E(1).

Neuronal population discharges within the CNS and in somatic and sympathetic motor nerves often display oscillations. Peripheral oscillations may provide a window into central mechanisms, as they often show coherence with population activity of subsets of central neurones. The reduction in heat loss through the cutaneous circulation during fever may be mediated via sympathetic premotor neurones not utilised during normal temperature regulation. Consequently, here we assessed, in anaesthetised rats, whether the frequency signature of population sympathetic discharge observed in neurones innervating the tail (thermoregulatory) circulation changed during a fever-like response induced by intracerebroventricular injection of prostaglandin E(1). We found that when core temperature was raised to 38.8-40.5 degrees C sympathetic activity was abolished. Following administration of prostaglandin (400 ng or 1 microg per rat), activity was restored to levels seen prior to heating (154+/-53.5%; n=10). Injection of vehicle had no effect (n=7). Prior to heating when most animals were in central apnoea (14/18) two peaks were observed in autospectra of sympathetic activity: one at 0.68-0.93 Hz (T-peak) and another at the frequency of ventilation (2 Hz). Central respiratory drive was recruited during hyperthermia where it was 1:2 locked to the frequency of ventilation and following prostaglandin administration, an additional peak in sympathetic autospectra was seen at this frequency. Time-evolving spectra indicated that this peak resulted from the dynamic locking of the 'T-peak' to central respiratory drive. Our data show that during a fever-like response the dominant oscillations in sympathetic activity controlling a thermoregulatory circulation and their dynamic coupling to respiratory-related inputs are similar to those seen under normal conditions. Therefore, during this fever-like response, the neural substrate(s) underlying the oscillations is not reconfigured and remains capable of sculpturing the pattern of sympathetic neuronal discharge that may be regulated by several descending pathways.

Resetting of sympathetic rhythm by somatic afferents causes post-reflex coordination of sympathetic activity in rat.

1. We have proposed previously that graded synchronous activity is produced by periodic inputs acting on weakly coupled or uncoupled oscillators influencing the discharges of a population of cutaneous vasoconstrictor sympathetic postganglionic neurones (PGNs) in anaesthetized rats. 2. Here we investigated the effects of somatic afferent (superficial radial nerve, RaN) stimulation, on the rhythmic discharges of this population. We recorded (1) at the population level from the ventral collector nerve and (2) from single PGNs focally from the caudal ventral artery of the tail. 3. Following RaN stimulation we observed an excitatory response followed by a period of reduced discharge and subsequent rhythmical discharges seemingly phase-locked to the stimulus. 4. We suggest that the rhythmical discharges following the initial excitatory response (conventional reflex) result from a resetting of sympathetic rhythm generators such that rhythmic PGN activity is synchronized transiently. We also demonstrate that a natural mechanical stimulus can produce a similar pattern of response. 5. Our results support the idea that in sympathetic control, resetting of multiple oscillators driving the rhythmic discharges of a population of PGNs may provide a mechanism for producing a sustained and coordinated response to somatic input.

Multiple oscillators, dynamic synchronization and sympathetic control.

1. Intermittent bursts of activity are a robust feature of the discharges of sympathetic nerves. There are at least two major mechanisms producing such discharges: (i) phasic inputs influencing sympathetic circuits; and (ii) oscillators embedded within sympathetic networks. The functional significance of patterned and synchronized activity underlying bursts of population activity may reside in their influence on information transfer between excitable cells. At the level of the single neuron, firing pattern appears to be an important determinant of synaptic/neuroeffector function (e.g. the probability of transmitter release, the types of transmitter released, the types of receptor activated and plasticity). Synchronization of inputs at a target favours summation and, therefore, may influence response (short term and long term). 2. In the present paper, I review the work from my laboratory that has focused on furthering understanding of the potential functional importance of pattern and synchrony coding in sympathetic nervous control of cardiovascular function. Because the rat tail artery has been used extensively as a model for studying neuroeffector transmission, in our investigations we have recorded from its sympathetic innervation. 3. In the anaesthetized preparation, under steady state conditions, we have established that the discharges of these sympathetic neurons have a distinct rhythm (frequency approximately 0.8 Hz). This can be detected both at single neuron and population levels. 4. A family of oscillators appears to control their discharge such that under some conditions all neurons do not have the same frequency of rhythmical activity. However, these weakly coupled or uncoupled oscillators can be synchronized dynamically by various inputs, such as central respiratory drive, lung inflation cycle-related inputs and inputs arising from visceral and somatic afferents. 5. The potential functional significance of dynamic synchronization of sympathetic oscillators in relation to sympathetic pattern generation and neuroeffector transmission is discussed.

Multiple oscillators provide metastability in rhythm generation.

Biological rhythms such as cardiac and circadian rhythms arise from activity of multiple oscillators with dispersed intrinsic frequencies. It has been proposed that a stable population rhythm, fundamental to normal physiological processes, can be achieved in these systems by synchronization, through mutual entrainment, of individual oscillators. Mutual entrainment, however, is unlikely to be the mechanism underlying the generation of a stable rhythm in a population of multiple weakly coupled or uncoupled oscillators. We have recently identified such a population that is involved in the sympathetic regulation of vascular tone in a thermoregulatory circulation. In this paper, we investigate the stability of the output rhythm of these sympathetic oscillators by subjecting the system to a periodic driving force (the lung inflation cycle-related activity). We show that a population rhythm coupled to the drive can remain stable over a much wider driving frequency range compared with that of any one of its constituent oscillators. This population rhythmicity still exists despite the fact that the dominant frequencies of individual oscillators are not necessarily 1:1 frequency-locked to the drive. We provide evidence to show that this population metastability is achieved through linear and nonlinear dynamic interactions between the driving force and single sympathetic oscillators. Our study suggests that the generation of a stable population rhythm can exist even in the absence of mutual entrainment of its constituents, and this allows the population to generate a stable and flexible patterned response.

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.

Sympathetic neuronal oscillators are capable of dynamic synchronization.

In this paper we show that the discharges of sympathetic neurons innervating an identified peripheral target are driven by multiple oscillators that undergo dynamic synchronization when an entraining force, central respiratory drive (CRD), is increased. Activity was recorded from postganglionic sympathetic neurons (PGNs) innervating the caudal ventral artery of the rat tail: (1) at the population level from the ventral collector nerve (VCN); and (2) from pairs of single PGNs recorded simultaneously using a focal recording technique. Autospectral analysis of VCN activity revealed a more prominent rhythmical component in the presence of CRD than in its absence, suggesting that (1) multiple oscillators drive the discharges of PGNs and (2) these oscillators can be entrained and therefore synchronized by CRD. This interpretation was supported by analysis of the firing behavior of PGN pairs. Autocorrelation and cross-correlation analysis showed that pairs were not synchronized in the absence of CRD but showed significant synchronization when CRD was enhanced. Time-evolving spectral analysis and raster plots demonstrated that the temporal stability of PGN-to-PGN and CRD-to-PGN interactions at a given level of CRD were also dynamic in nature, with stable constant phase relationships predominating as CRD was increased. This is the first reported example of dynamic synchronization in populations of single postganglionic sympathetic neurons, and we suggest that, as in sensory processing and motor control, temporal pattern coding may also be an important feature of neuronal discharges in sympathetic pathways.

Rhythmic sympathetic discharges and 'escape behaviour'.

The characteristic rhythmical discharges of single postganglionic sympathetic neurones (PSNs) innervating the caudal ventral tail artery (CVA) of anaesthetised rats can still be recorded following the sectioning of afferents arising from the tail and hindquarters. Consequently, we suggest that such rhythmical discharges are neither a 'local sign' sympathetic response nor a sympathetic correlate of 'escape behaviour'.

Effects of aortic nerve stimulation on discharges of sympathetic neurons innervating rat tail artery and vein.

Activity was recorded from postganglionic sympathetic neurons (PSNs) innervating either the caudal ventral artery (CVA) or a lateral vein (LV) of the tail circulation of anesthetized rats. The study sought to determine whether sympathetic activity directed at the CVA and LV was influenced by cardiovascular mechanoreceptor afferents and whether this effect was differential. Cardiac rhythmicity was not a robust component of either CVA PSN activity or LV PSN activity. Stimulation of an aortic nerve with short trains was followed by a decreased probability of discharge in both CVA and LV PSNs that was followed by a series of peaks that showed a constant periodicity that was not significantly different from that revealed by autocorrelogram analysis over the same data set. The latter dominant periodicity is referred to in this and related previous publications as the T rhythm. Furthermore, blood volume expansion and long-train aortic nerve stimulation produced a significant decrease in the frequency of the T rhythm. It is concluded that the CVA and LV sympathetic activity can be influenced by inputs from cardiovascular mechanoreceptors and that this effect is mediated in part by a modulation of the T rhythm.

CNS cell groups projecting to sympathetic outflow of tail artery: neural circuits involved in heat loss in the rat.

In the rat, approximately 20% of total body heat-loss occurs by sympathetically mediated increases in blood flow through an elaborate system of arteriovenous anastomoses in the skin of its tail. In this study, the CNS cell groups that regulate this sympathetic outflow were identified by the viral transneuronal labeling method. Pseudorabies virus was injected into the wall of the ventral tail artery in rats that had their cauda equina transected to eliminate the somatic innervation of the tail. After 4-7 days survival, the pattern of CNS transneuronal labeling was studied. Sympathetic preganglionic neurons in the T11-L2 (mainly L1) levels of the intermediolateral cell column (IML) were labeled by 4 days. After 5 days, sympathetic pre-motor neurons (i.e., supraspinal neurons that project to the IML) were identified near the ventral medullary surface; some of these contained serotonin immunoreactivity. Additional groups of the sympathetic premotor areas were labeled by 6 days post-injection, including the rostral ventrolateral medulla (C1 adrenergic neurons), rostral ventromedial medulla, caudal raphe nuclei (serotonin neurons in the raphe pallidus and magnus nuclei), A5 noradrenergic cell group, lateral hypothalamic area and paraventricular hypothalamic area (oxytocin-immunoreactive neurons). Seven days after the PRV injections, additional cell groups in the telencephalon (viz., bed nucleus of the stria terminalis, medial and lateral preoptic areas and medial preoptic nucleus), diencephalon (viz., subincertal nucleus, zona incerta as well as dorsal, dorsomedial, parafascicular, posterior and ventromedial hypothalamic nuclei) and midbrain (viz., periaqueductal gray matter, precommissural nucleus, Edinger-Westphal nucleus and ventral tegmental area) were labeled. The discussion is focused on the CNS cell groups involved in the control of body temperature and fever.

Focally recorded single sympathetic postganglionic neuronal activity supplying rat lateral tail vein.

1. In anaesthetized rats, using a focal recording technique, activity was recorded from single sympathetic postganglionic neurones innervating the lateral tail veins. On-going activity was examined in order to determine whether it had similar or different characteristics to those recorded from the caudal ventral artery in a previous study. 2. Animals were artificially ventilated, vagotomized, paralysed and given a pneumothorax. 3. The discharges of fourteen out of seventeen sympathetic postganglionic neurones were rhythmic. Such units had a mean firing frequency of 1.62 +/- 0. 70 Hz. The mean frequency of the dominant sympathetic rhythm under control conditions was 0.82 +/- 0.05 Hz. 4. The frequency of the dominant sympathetic rhythm was different from that of the phrenic rhythm in nine out of fourteen cases. 5. The mean frequency of the dominant sympathetic rhythm was: (i) not influenced significantly by hypocapnic apnoea, (ii) decreased by hyperthermia, which increased the frequency of the phrenic rhythm, (iii) in all cases different from that of the artificial ventilation cycle. 6. The above characteristics are similar to those recorded from the sympathetic supply to the caudal ventral artery of the same vascular bed under comparable conditions.

Segmental origin of sympathetic preganglionic neurones regulating the tail circulation in the rat.

The spinal segments of origin of the sympathetic preganglionic neurones (SPNs) influencing the activity of sympathetic postganglionic neurones innervating the tail have been studied using a neurophysiological approach. Activity was recorded from the ventral collector nerve that carries 70% of the sympathetic fibres innervating targets within the tail and provides 80% of the innervation of the caudal ventral artery. When recording activity from the ventral collector nerve at the tail base, the largest responses were evoked following electrical stimulation within spinal segments lumbar (L) 1 and 2 and smaller responses from thoracic (T) 13 (n = 5). Although similar responses to those recorded from the tail base were elicited from spinal segments L1 and L2, when activity was recorded from mid-tail only minimal responses were evoked from T13 (n = 6). On average robust responses were never elicited following stimulation beyond these segments. Responses had latencies compatible with conduction over C-fibre axons and were absent following ganglionic blockade. It is concluded that SPNs influencing the tail circulation reside mainly in L1 and L2 spinal segments and there is also a substantial but lesser contribution arising from segment T13.

Stimulation within the rostral ventrolateral medulla can evoke monosynaptic GABAergic IPSPs in sympathetic preganglionic neurons in vitro.

The inhibitory responses of identified sympathetic preganglionic neurons (SPNs) to stimulation within the rostral ventrolateral medulla (RVLM) were studied to determine their nature and pharmacology. Whole cell patch-clamp recordings were made from 36 SPNs in the upper thoracic segments of the spinal cord in a neonatal rat brain stem-spinal cord preparation. Neurons were identified as SPNs on the basis of their antidromic activation after stimulation of the ipsilateral segmental ventral root and their morphology and location in the intermediolateral cell column and intercalated nucleus. In all SPNs, electrical stimulation of the RVLM evoked fast excitatory postsynaptic potentials (EPSPs) that were mediated by non-N-methyl-D-aspartate (NMDA) and NMDA receptors. These excitatory responses were the most prominent response in control artificial cerebrospinal fluid and have been studied previously. In 22 of the SPNs, RVLM stimulation also elicited fast inhibitory postsynaptic potentials (IPSPs), which increased in amplitude as the membrane was depolarized. Five of these neurons were not studied further as they responded occasionally with IPSPs that had highly variable onset latencies indicating the involvement of a polysynaptic pathway. In the remaining SPNs (n = 17), the evoked IPSPs persisted in the presence of the excitatory amino acid antagonists 6-cyano-7-nitroquinoxaline-2,3,-dione and D,L-2-amino-5-phosphonopentanoic acid. In eight of these SPNs, it was necessary to block the EPSPs to reveal the IPSPs. In the 7 SPNs tested, the onset latencies of the IPSPs were not significantly different from the onset latencies of the fast EPSPs. The low sweep-to-sweep fluctuations in onset latency of individual IPSPs (absolute average deviation: 0.4 ms) indicated that the IPSPs were elicited by activation of a monosynaptic pathway. The amplitudes of the IPSPs decreased in amplitude as the membrane was hyperpolarized and reversed in polarity at -70.3 +/- 1.7 mV (mean +/- SD), which was close to the equilibrium potential for chloride ions. In addition, in seven SPNs, bath applications of 5 microM bicuculline, a gamma-aminobuturic acid-A (GABAA) antagonist, abolished or reduced the evoked IPSPs. Five SPNs also were studied that displayed ongoing IPSPs. The amplitudes of these IPSPs increased with membrane depolarization and were blocked by bath applications of 5 microM bicuculline, suggesting that they also were mediated by activation of GABAA receptors. These results demonstrate the existence of a bulbospinal GABAergic pathway impinging directly onto SPNs. This pathway may be tonically active in the neonatal rat brain stem-spinal cord preparation.

On the dominant rhythm in the discharges of single postganglionic sympathetic neurones innervating the rat tail artery.

1. In anaesthetized rats, using a focal recording technique, activity was recorded from single sympathetic postganglionic neurones innervating the caudal ventral artery of the tail. The following hypotheses were tested: (i) that the frequency of the dominant rhythmic discharge of the neurones can be different from the frequency of the central respiratory rhythm (as indicated by rhythmic phrenic discharge); and (ii) that the dominant sympathetic rhythm is not reliant on afferent feedback carried in aortic, sinus and vagus nerves. 2. Four types of preparation were used: spontaneously breathing (group 1), artificially ventilated (group 2), artificially ventilated with vagi cut (group 3), and artificially ventilated with vagus and sino-aortic denervation (group 4). 3. The frequencies of the dominant sympathetic rhythm under control conditions were: group 1, 0.91 +/- 0.12 Hz (mean +/- S.E.M., n = 5); group 2, 0.81 +/- 0.04 Hz (n = 18); group 3, 0.83 +/- 0.03 Hz (n = 17); group 4, 0.95 +/- 0.06 Hz (n = 11). The frequency of the dominant sympathetic rhythm was different from that of the phrenic rhythm in thirty-five out of fifty-one cases. 4. The mean frequency of the dominant sympathetic rhythm was not influenced significantly by hypocapnic apnoea. 5. Hyperthermia increased the frequency of the phrenic rhythm whilst decreasing that of the dominant sympathetic rhythm. 6. In all cases the frequency of the dominant sympathetic rhythm was different from that of the artificial ventilation cycle. 7. It is concluded that the frequency of the dominant sympathetic rhythm can be different from that of central respiratory drive and that it is not "driven' by afferent feedback relayed via sinus, aortic and vagus nerves. 8. It is proposed that the dominant sympathetic rhythm is unlikely to be generated by a central respiratory oscillator.

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

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

Sympathoexcitatory influence of a fast conducting raphe-spinal pathway in the rat.

Experiments were carried out on 20 pentobarbitone sodium (alpha-chloralose supplemented)-anesthetized, artificially ventilated, and paralyzed rats. The possibility was explored that raphe-spinal neurons with myelinated axons arising in the rostral part of raphe obscurus provide excitatory drive to sympathetic neurons. Electrical stimulation within obscurus was observed to evoke an "early" sympathoexcitatory response compatible with its conduction over such a pathway. The effect of the microinjection of excitatory and inhibitory amino acids [DL-homocysteic acid (DLH) and gamma-aminobutyric acid (GABA), respectively] on the evoked response was studied at the sites of electrical stimulation. The size of the early response was increased by 91.7 +/- 24.4% (n = 7) and depressed by -48 +/- 4.8% (n = 7) by DLH and GABA, respectively. Saline was without effect (-14.5 +/- 12.2%, n = 6). The evoked responses were decreased when blood pressure was raised by administration of phenylephrine (2-6 micrograms/kg iv) and totally suppressed by an increase in blood pressure of 19.3 +/- 4.3 mmHg (baseline 89.1 +/- 2.5 mmHg, n = 7). It is concluded that some cell bodies located in rostral raphe obscurus that project to the spinal cord relay excitatory drive to sympathetic neurons.

A study of sympathetic preganglionic neuronal activity in a neonatal rat brainstem-spinal cord preparation.

Extracellular recordings were made from 46 sympathetic preganglionic neurones (SPNs) in a neonatal rat brainstem-spinal cord preparation. Neurones were identified as SPNs as they were: (i) activated at constant latencies (2-10 ms) following stimulation of the ventral root, which indicated antidromic activation and (ii) recorded at sites located either in the intermediolateral cell column or the intercalated nucleus of the thoracic spinal cord. Over one-third of the neurones (n = 17) recorded displayed ongoing activity with firing frequencies of 0.3-5 Hz. Of the neurones analyzed only one showed a very obvious phasic firing pattern. Dorsal root stimulation evoked firing in 16 of 26 SPNs recorded from the same spinal segment (6 of 10 with ongoing activity). The types of responses observed varied between neurones. The excitation of all neurones was characterised by a response occurring at a latency of 6-50 ms. In addition, SPNs in 'spinalised' preparations (n = 2) responded with latencies of 10-40 ms, similar to those observed in the intact preparation. The latencies of responses in SPNs were longer and more variable than those observed in ventral horn motor neurones. This indicates that a spinal polysynaptic pathway was involved in mediating these responses. In 7 SPNs dorsal root stimulation also elicited longer latency responses which were observed up to 1000 ms after stimulation. These responses may involve activation of bulbospinal and/or propriospinal pathways. These results show that the neonatal rat brainstem-spinal cord preparation is viable for studying SPNs and that dorsal root-SPN reflexes are intact.