Michael Gilbey

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.

Sympathetic activity recorded from the rat caudal ventral artery in vivo.

1. In twenty‐five sodium pentobarbitone (alpha‐chloralose supplemented)‐anaesthetized, artificially ventilated and paralysed rats, postganglionic sympathetic single unit activity was recorded at the level of the adventitia of the caudal ventral artery of the tail using a focal recording technique. 2. Ten units were identified as being sympathetic in nature, as they were activated following electrical stimulation of the lumbar sympathetic chain. The on‐going activity of seven of these was blocked by hexamethonium (6‐12 mg kg‐1). 3. The units were not under tonic baroreceptor modulation, as indicated by the lack of pulse modulation of discharge. Respiratory modulation was apparent, with neurones firing mainly during expiration (phrenic silence), and activity was influenced also by the lung inflation cycle. Whole‐body warming decreased unit activity. 4. Interspike interval and autocorrelation analysis showed that unit discharge was dominated by the respiratory rhythm and that units tended to discharge in bursts (often duplets). It is suggested that the intraburst interval may be determined by a hypothetical sympathetic oscillator. 5. This study presents the first analysis of single unit activity recorded in vivo from sympathetic fibres innervating an identified blood vessel.

CENTRAL RESPIRATORY DRIVE-RELATED ACTIVITY IN SYMPATHETIC-NERVES OF THE RAT - THE REGIONAL DIFFERENCES

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.

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.

Characteristics of sympathetic preganglionic neurones in the lumbar spinal cord of the cat.

1. In anaesthetized cats extracellular recordings have been made from antidromically identified sympathetic preganglionic neurones, located in the 2nd and 3rd lumbar segments, with axons projecting into the left lumbar sympathetic chain beyond the L4 ganglion. Sympathetic preganglionic neurones have been characterized with respect to: axonal conduction velocities, firing patterns in relation to ECG and phrenic nerve activity, responses to noxious stimuli applied to the ipsilateral hindlimb and ionophoretically applied 5‐HT. 2. Two hundred and ninety‐seven sympathetic preganglionic neurones were studied. Their axonal conduction velocities (0.5‐13.9 m/s) were in the B and C fibre range. Sixty‐eight had on‐going activity and the remainder were quiescent. Of the 229 quiescent sympathetic preganglionic neurons, 111 were activated by the ionophoretic application of glutamate. 3. Of the 100 sympathetic preganglionic neurones analysed for an ECG‐related pattern of discharge, forty‐nine had no, and fifty‐one had an ECG‐related pattern of discharge. Both sympathetic preganglionic neurones with on‐going activity and glutamate activated cells exhibited ECG‐related patterns of discharge. 4. Only six of fifty sympathetic preganglionic neurones had a respiratory‐related activity pattern. Three had maximal discharge during expiration and three during inspiration. 5. Forty‐one sympathetic preganglionic neurones were examined for their response to noxious stimulation of the ipsilateral hindlimb. Ten had their activity decreased (seven glutamate‐activated, three with on‐going activity), seven had their activity increased (four glutamate‐activated and three with on‐going activity) and twenty‐four were unaffected. These results demonstrate that both sympathetic preganglionic neurones with on‐going activity and glutamate‐activated neurones can be influenced by noxious input. Ten sympathetic preganglionic neurones had properties consistent with them having a skin vasoconstrictor function and three with muscle vasoconstrictor function. 6. Ionophoretic application of 5‐HT in the vicinity of fifty‐one sympathetic preganglionic neurones caused increases in the discharge of 53%, decreases in the firing of 12% and did not affect the discharge of 35%. 7. Sympathetic preganglionic neurones which had excitatory responses to 5‐HT showed only decreased discharge or no response to noxious stimulation of the ipsilateral hindlimb. Conversely, sympathetic preganglionic neurones which had discharge decreased by 5‐HT had primarily excitatory responses to noxious inputs. 8. It is concluded that lumbar sympathetic preganglionic neurones consist of a heterogeneous population with respect to their physiological properties and their responses to ionophoretically applied 5‐HT: both may be related to function.

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.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.

[3H]Prazosin binding in the intermediolateral cell column and the effects of iontophoresed methoxamine on sympathetic preganglionic neuronal activity in the anaesthetized cat and rat

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.

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.

Evidence for the ability of central 5-HT1A receptors to modulate the vagal bradycardia induced by stimulating the upper airways of anesthetized rabbits with smoke

Cigarette smoke passed through the nasal cavity of atenolol pre-treated anesthetized spontaneously breathing rabbits caused a bradycardia which was significantly modified by intracisternal application of the 5-HT1A receptor ligands 8-OH-DPAT (50 micrograms.kg-1) and buspirone (200 micrograms.kg-1). 8-OH-DPAT attenuated while buspirone potentiated the bradycardia. These results support the view that 5-HT1A receptors play an important role in modulating the excitability of cardiac vagal motoneurones.

Effects of aortic nerve stimulation on cervical sympathetic preganglionic neurones in the rat.

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.