Neural regulation of inflammation: no neural connection from the vagus to splenic sympathetic neurons.

The 'inflammatory reflex' acts through efferent neural connections from the central nervous system to lymphoid organs, particularly the spleen, that suppress the production of inflammatory cytokines. Stimulation of the efferent vagus has been shown to suppress inflammation in a manner dependent on the spleen and splenic nerves. The vagus does not innervate the spleen, so a synaptic connection from vagal preganglionic neurons to splenic sympathetic postganglionic neurons was suggested. We tested this idea in rats. In a preparatory operation, the anterograde tracer DiI was injected bilaterally into the dorsal motor nucleus of vagus and the retrograde tracer Fast Blue was injected into the spleen. On histological analysis 7-9 weeks later, 883 neurons were retrogradely labelled from the spleen with Fast Blue as follows: 89% in the suprarenal ganglia (65% left, 24% right); 11% in the left coeliac ganglion; but none in the right coeliac or either of the superior mesenteric ganglia. Vagal terminals anterogradely labelled with DiI were common in the coeliac but sparse in the suprarenal ganglia, and confocal analysis revealed no putative synaptic connection with any Fast Blue-labelled cell in either ganglion. Electrophysiological experiments in anaesthetized rats revealed no effect of vagal efferent stimulation on splenic nerve activity or on that of 15 single splenic-projecting neurons recorded in the suprarenal ganglion. Together, these findings indicate that vagal efferent neurons in the rat neither synapse with splenic sympathetic neurons nor drive their ongoing activity.

Are pre-ganglionic neurones recruited in a set order?

AIM: The idea that, like somatic motor neurones, sympathetic pre-ganglionic neurones are engaged to fire in a pre-determined recruitment order, was investigated in chloralose-anaesthetized cats. METHOD: Ongoing pre-ganglionic spike activity was recorded from fine filaments of otherwise intact thoracic white rami, while post-ganglionic activity was recorded from the whole inferior cardiac nerve (ICN). Spikes of individual pre-ganglionic fibres were extracted from few-fibre recordings by spike shape analysis. Presumed cardiac pre-ganglionic fibres were further selected by the spike-triggered average of ICN activity, which showed a clear peak when triggered by their spikes. RESULTS: To test whether particular pre-ganglionic neurones were recruited to fire in a set time sequence, the spontaneous spike trains of fibres in the same white ramus were compared by cross correlation. In all 24 cases the cross correlograms showed a central peak (width 163 +/- 15 ms), indicating that the two neurones tended to fire together. In 23 of the 24 cases that peak spanned the zero point on the time axis, showing that each neurone could fire either first or second. To test whether pre-ganglionic neurones were recruited in a set order with respect to burst amplitude, the firing of individual pre-ganglionic neurones was compared with the strength of the corresponding post-ganglionic burst discharge, on a heartbeat-by-heartbeat basis. Pre-ganglionic neurone firing was probabilistic: each neurone fired with only a minority of post-ganglionic bursts. Firing probability increased linearly with burst amplitude (30 of 30 cases). The slope of the relation varied between units, but its intercept was always close to the origin (zero pre-ganglionic firing probability at zero post-ganglionic burst size). CONCLUSION: The data indicate that, at least under these conditions, sympathetic pre-ganglionic neurones follow no set recruitment sequence in either their firing times or with respect to the strength of the autonomic motor output.

Activity patterns of cardiac vagal motoneurons in rat nucleus ambiguus.

Extracellular recordings were made in the right nucleus ambiguus of urethane-anesthetized rats from 33 neurons that were activated at constant latency from the craniovagal cardiac branch. Their calculated conduction velocities were in the B-fiber range (1.6-13.8 m/s, median 4.2), and most (22/33) were silent. Active units were confirmed as cardiac vagal motoneurons (CVM) by the collision test for antidromic activation and by the presence of cardiac rhythmicity in their resting discharge (9/9). Brief arterial pressure rises of 20-50 mmHg increased the activity in five of five CVM by 0.1 +/- 0.02 spikes. s(-1). mmHg(-1) from a resting 3.8 +/- 1.2 spikes/s; they also recruited activity in two of four previously silent cardiac branch-projecting neurons. CVM firing was modulated by the central respiratory cycle, showing peak activity during inspiration (8/8). Rat CVM thus show firing properties similar to those in other species, but their respiratory pattern is distinct. These findings are discussed in relation to mechanisms of respiratory sinus arrhythmia.

Analysis of firing correlations between sympathetic premotor neuron pairs in anesthetized cats.

The activity of sympathetic premotor neurons in the rostral ventrolateral medulla (subretrofacial nucleus) supports sympathetic vasomotor tone, but the factors that drive these premotor neurons' activity have not been determined. This study examines whether either direct interconnections between subretrofacial neurons or synchronizing common inputs to them are important for generating their tonic activity. Simultaneous extracellular single-unit recordings were made from 32 pairs of sympathetic premotor neurons in the subretrofacial nucleus of chloralose-anesthetized cats. Paired spike trains were either separated by spike shape from a single-electrode recording (14 pairs) or recorded from two electrodes less than 250 microm apart (18 pairs). All neurons were inhibited by carotid baroreceptor stimulation and most had a spinal axon proven by antidromic stimulation from the spinal cord. Autocorrelation, inter-spike interval, and cardiac cycle-triggered histograms were constructed from the spontaneous activity of each neuron, and cross-correlation histograms covering several time scales were generated for each neuron pair. No significant peaks or troughs were found in short-term cross-correlation histograms (2 ms bins, +/-100 ms range), providing no support for important local synaptic interactions. On an intermediate time scale (20 ms bins, +/-1 s range), cross-correlation revealed two patterns indicating shared, synchronizing inputs. Repeating peaks and troughs (19/32 pairs) were due to the two neurons' common cardiac rhythmicity, of presumed baroreceptor origin. Single, zero time-spanning peaks of 40--180 ms width were seen in 5/32 cases. Calculations based on the prevalence and strength of these synchronizing inputs indicate that most of the ensemble spike activity of the subretrofacial neuron population is derived from asynchronous sources (be they intrinsic or extrinsic). If synchronizing sources such as neuronal oscillators were responsible for more than a minor part of the drive, they would be multiple, dispersed, and weak.