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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 °C) cutaneous vasoconstrictor activity was decreased by 83±5%. I.c.v. prostaglandin E1 (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 °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 E1 (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 E1 (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 E1. [Copyright &y& Elsevier]

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.

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.

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.

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.

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.

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.

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.

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'., (Copyright 1998 Elsevier Science B.V.)

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.

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.

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.

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.

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.

Sympathetic activity is modulated by central respiratory drive. Bulbospinal neurones arising in the ventrolateral medulla and A5 region probably contribute to this modulation. In the present investigation the involvement of caudal raphe-spinal neurones in relaying respiratory-related inputs to sympathetic preganglionic neurones was investigated. Experiments were carried out on anaesthetized, vagotomized, paralysed and artificially ventilated rats. Extracellular recordings were made from the cell bodies of 53 caudal raphe neurones activated antidromically by stimulating the spinal cord between T1 and T3. The axonal conduction velocities ranged from 0.7-9.1 m/s (median = 3.8 m/s). Thirty-six of 53 neurones (consisting of neurones with on-going activity and quiescent neurones activated with glutamate) were held long enough for detailed analysis. Of those recorded 26 were in the region of raphe obscurus, nine in raphe pallidus and one in raphe magnus. Twenty-five of 36 neurons had firing patterns related to phrenic nerve discharge. Of the four firing patterns defined: seven neurones had the highest probability of firing during inspiration (inspiratory-related), 10 neurones had the highest probability of firing during expiration (expiratory-related), 3 had the highest probability of firing during post-inspiration (post-inspiratory-related) and 5 had lowest levels of firing during early- and post-inspiratory phases (early and post-inspiratory depressed). Of 27 neurones with axonal projections through or to the region of the intermediolateral cell column in the upper thoracic cord 19 had a respiratory-related discharge pattern. For respiratory-modulated neurones with on-going activity the median of the modal inter-spike intervals was 0.08 s. None of the neurones had an ECG-related firing pattern. The findings of this study also indicate a species difference between rats and cats regarding the physiological properties of some raphe-spinal neurones; i.e., an absence of ECG-related activity in the rats. The characteristics of the neurones recorded in this study are not those of 'typical' 5-HT-containing neurones with reference to axonal conduction velocities and discharge characteristics.

1. Medullary raphe neurons are involved in the control of sympathetic activity during desynchronized sleep (DS). Eserine sulfate induces a state with the somatic and visceral signs of DS in decerebrate animals. The rabbit and rat display diverse hemodynamic patterns during DS. 2. To determine whether eserine sulfate provokes different responses in the medullary raphe neuron population of these different species, the drug (100 mg/kg, i.v.) was administered to urethane-anesthetized (1.2 g/kg, i.v.) rabbits (1.5-3.0 kg) and rats (Wistar, 260-310 g). 3. Extracellular activity was recorded from 66 neurons in 30 rats. Cholinergic stimulation provoked an increase in discharge rate (DR) in 45 neurons (68%), a decrease in 8 neurons (12%) and no change in 13 neurons (20%). Recordings were obtained from 30 neurons in 11 rabbits. Stimulation of these cells provoked an increase in DR in 17 neurons (57%), a decrease in DR in 7 neurons (23%) and no change in 6 neurons (20%). Interspike interval and auto-correlation analysis was performed on 28 rat and rabbit neurons. No significant difference was found between the rat and the rabbit with respect to the number of the neurons which were either inhibited or excited by cholinergic stimulation (P > 0.05). Similarly, unit response to eserine was not related to whether the unit displayed regular or irregular DR. 4. Therefore, we suggest that the diverse hemodynamic patterns during DS and the distinct cardiovascular responses to raphe nuclei stimulation are not due to differences in the organization of the raphe nuclei themselves but to differences in their axonal projections or in the postsynaptic receptors activated in the intermediolateral cell column or other postsynaptic targets.

Investigations continue into the central neuronal circuitry involved in cardiovascular control, the activity of sympathetic efferents, cardiovascular responses, and experimental hypertension. Neurons within the ventrolateral medulla are integral to several reflex pathways. The activation of neurons within the A5 cell group can generate complex patterns of sympathetic and cardiovascular response. The type of respiratory modulation of a sympathetic neuron may be associated with its function. Opioid mechanisms within the central nervous system are important in cardiovascular responses to hemorrhage. The central actions of angiotensin II and atrial natriuretic peptide may blunt the baroreceptor reflex in spontaneously hypertensive rats.

The sympathetic nervous system consists of efferent neurones supplying the viscera. The cell bodies of preganglionic neurones are located in four areas in the thoracolumbar cord; however, the majority are found in the IML. Various tracing techniques have provided information concerning the location of the cell bodies of sympathetic preganglionic neurones projecting into various nerves and ganglia and regulating the adrenal gland, the kidney and the sympathetic supply to skeletal muscle. Numerous supraspinal neurones project to the neuropil surrounding sympathetic preganglionic neurones and may form synaptic contacts with these neurones. The areas of the brain that project to the IML appear to be part of a network of reciprocally connected supraspinal cell groups. Although much emphasis has been placed on the importance of the RVLM in the mediation of tonic and phasic inputs to sympathetic preganglionic neurones, it appears that other areas are of significant import; the RVLM should not be considered to be 'the vasomotor centre'. Spinal and cranial afferents influence the sympathetic nervous system. Baroreceptor afferents terminate in the NTS and may utilize an excitatory amino acid as their neurotransmitter. However, a number of neuropeptides are also associated with these afferents. Neurones within the NTS project to a number of brain stem areas thought to be involved in the regulation of sympathetic activity; consequently the baroreceptor reflex may be mediated over a number of parallel pathways involving both supraspinal and spinal sites of inhibition. Many neurotransmitters are thought to regulate the activity of sympathetic preganglionic neurons: monoamines, peptides and amino acids. Matching the chemical content of the cell bodies of neurones within a particular cell group with physiological characteristics is a challenging task; some barosensitive neurones of the RVLM do not appear to be adrenergic although they are in the midst of the C1 adrenergic cell group. Besides acetylcholine and noradrenaline, neurotransmission in the periphery appears to involve numerous peptides and ATP.

The autoradiographic localization of [3H]prazosin (alpha 1-adrenoceptor ligand) binding sites was determined in cat spinal cord sections. High levels of [3H]prazosin binding were found in the intermediolateral cell column (IML) at thoracic and lumbar levels. The iontophoresis of the alpha 1-adrenoceptor agonist methoxamine onto sympathetic preganglionic neurones (SPNs) in anaesthetized cats and rats caused excitation of 8 cat SPNs and 13 rat SPNs. These results suggest an excitatory role for some of the catecholaminergic innervation of the IML.

In halothane-anaesthetized, vagotomized, SA-denervated rats, the activity of various sympathetic nerves has been analyzed with respect to phrenic nerve discharge (an indicator of central respiratory drive (CRD)). The cervical and lumbar sympathetic nerves had maximal activity following, and were least active during phrenic nerve discharge. In contrast, the splanchnic, cardiac, renal and adrenal nerves exhibited their activity peak during phrenic nerve discharge (i.e. inspiration). Similar activity profiles were observed after ganglion blockade in the mixed pre- and postganglionic fibre preparations. These observations indicate that it is the subpopulations of preganglionic neurones and the proportional contribution of each to whole-nerve activity which give rise to the differences in CRD-related activity profiles between nerves.

The possibility that the putative transmitter 5-hydroxytryptamine (5-HT) is involved in the mediation of long latency to onset raphe-spinal inhibition of sympathetic preganglionic neurones was investigated in anaesthetized cats by stimulating sites located in nucleus raphe pallidus and obscurus and recording sympathetic discharge in T3 or T10 white rami evoked either reflexively or by intraspinal stimulation at cervical level. Several putative 5-HT antagonists were administered intravenously (i.v.) or topically to the spinal cord. In 7 cats lysergic acid diethylamide (LSD) in a dose range 25-50 microgram/kg i.v. or 0.6 microgram topically, reversibly reduced the raphe spinal inhibition by 40-100%. Topical application was more effective than i.v. administration. In 5 cats stimulating within the ventromedial reticular formation at sites unlikely to involved 5-HT neurones produced a short latency to onset inhibition which was unaffected by LSD. Methysergide, cinanserin and cyproheptadine depressed sympathetic discharge in the absence of brain stimulation in cats with CNS intact and in unanaesthetized decerebrate spinal cats. The results are discussed in the light of the known actions of the putative 5-HT antagonists.

In chloralose-urethane anaesthetized cats and in one unanesthetized decerebrate cat, the possibility of a baroreceptor pathway exerting an inhibitory effect directly on the sympathetic preganglionic neurone (SPN) was examined. Stimulation of excitatory points in the brain stem and spinal cord evoked responses in T3 white ramus (T3WR) which could be markedly depressed (80-100%) by raising the pressure in an isolated carotid sinus. Conditioning-testing experiments indicated that the onset of this inhibition had a long latency (100-250 msec). Responses in T3WR evoked by stimulating both 'fast' (5 m/sec) and 'slow' (2-3 m/sec) bulbospinal pathways were similarly affected by baroreceptor stimulation. This inhibition could be much reduced (40-100%) by topical application to T3 segment of phentolamine (1-2 microgram). Baroreceptor activation also caused a complete cessation of the discharge evoked by microelectrophoretic application of glutamate to single antidromically identified SPN. The data support the suggestion of a direct action on SPN of a bulbospinal baroreceptor inhibitory pathway.

The distribution of adrenoceptors and opiate receptors in the nucleus of the tractus solitarius and the intermediolateral cell column of the thoracic spinal cord of the cat have been investigated using an in vitro autoradiographic technique. Specific binding of [3H]yohimbine and [3H]rauwolscine (alpha 2-adrenoceptor ligands) was seen within the intermediolateral cell column but no obvious binding of [3H]prazosin, an alpha 1-ligand, was observed. No evidence of a significant population of opiate receptors was obtained in the intermediolateral cell column. Within the nucleus of the tractus solitarius a marked binding of [3H]yohimbine and [3H]rauwolscine was accompanied, however, by a more restricted binding of [3H]naloxone and [3H]dihydromorphine indicating the presence of both alpha 2-adrenoceptors and opiate receptors. As with the intermediolateral cell column no evidence of [3H]prazosin binding was seen. These observations may have particular relevance for the physiology and pharmacology of cardiovascular control. In the case of the intermediolateral cell column it is consistent with evidence of a catecholamine innervation originating from the brainstem. With regard to the nucleus of the tractus solitarius the location of the receptor groups is discussed in the light of the anatomy and physiology of its afferent innervation.

In anaesthetized rats, neurones were found in the ventrolateral medulla that responded to antidromic stimulation of their axons in the thoracic spinal cord. These neurones were identified as being antidromic, according to various established electrophysiological criteria. A total of 44 antidromically identified neurones were found, 23 had conduction velocities below 2.0 m/s. Many (70%) had ongoing activity with a slow firing rate (0.2-5.0 Hz). Catecholamine-containing cells were visualized in this ventrolateral region using a modified Mg+-catalyzed glyoxylic acid technique and revealed multipolar, small diameter cells (17-20 microns) which were diffusely scattered (as demonstrated in previous histofluorescence studies). The Pontamine sky blue-marked recording sites of 7 units (0.3-1.1 m/s) showed close apposition to a CA-fluorescent cell whilst a further 4 (2.0-2.7 m/s) could not be correlated with the presence of a fluorescent cell. The results are discussed in the light of recent data in the rat, suggesting that the spinal cord catecholamine innervation arises from brainstem cell groups other than A1.

The effects of aortic nerve stimulation on the activity of single cervical sympathetic preganglionic neurones have been studied. Based upon the relationship of their firing patterns to central respiratory drive the neurones were categorized as either expiratory-related, inspiratory-related or non-modulated. Aortic nerve stimulation depressed the activity of each type of neurone indicating that irrespective of their respiratory modulation they are amenable to baroreceptor control.

In anaesthetized rats sympathetic preganglionic neurones (SPN) were identified by their antidromic response to stimulation of the ipsilateral cervical sympathetic trunk (CST). Units were recorded at a depth of 0.75-1.1 mm from the dorsal surface of the spinal cord. The majority of SPN had axonal conduction velocities less than 1 m/s. Units could be routinely held for periods of up to 1 h. Spontaneously active SPN had discharge rates within the range 0.3-8.0 Hz. Application of horseradish peroxidase (HRP) to the central cut end of the CST resulted in the labelling of neurones only on the ipsilateral side, mainly in the intermediolateral cell column (IML) and the lateral funiculus and between the first and third thoracic (T) segments.

In anaesthetized cats the effect on antidromically identified single sympathetic preganglionic neurones (SPN) in the third thoracic segment of microelectrophoretically applied monoamines, amino acids and acetyl choline was examined. 5-Hydroxytryptamine (5-HT) creatinine sulphate and bimaleate excited a majority of SPN. A few cells were inhibited by 5-HT creatinine sulphate. These effects were observed on spontaneously active SPN (cardiac and non-cardiac type) and on silent SPN. Noradrenaline, adrenaline and dopamine inhibited all 'types' of SPN, including spontaneously active neurons, silent neurones activated by glutamate or DL-homocysteic acid and neurones synaptically activated by electrically stimulating a brain stem excitatory region. Acetyl choline had no effect on different types of SPN.

In anaesthetized rats the effect of two procedures was studied on antidromically identified sympathetic preganglionic neurones (SPN) in the second thoracic (T) segment of the spinal cord: the application of iontophoresed oxytocin and vasopressin, and bipolar electrical stimulation of the paraventricular nucleus of the hypothalamus (PVN). In the majority of cases (16/23) oxytocin inhibited SPN firing, 1/23 being excited. Vasopressin inhibited 8/14 neurones and excited 4/14. PVN stimulation inhibited SPN apparently by an action on the membrane of SPN. The possibility that oxytocin and vasopressin act as transmitters in the paraventriculo-spinal pathway, and their possible involvement in the mediation of PVN evoked inhibition of SPN activity has been discussed.