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

Hemodynamic orthostatic dizziness/vertigo: Diagnostic criteria

This paper presents the diagnostic criteria for hemodynamic orthostatic dizziness/vertigo to be included in the International Classification of Vestibular Disorders (ICVD). The aim of defining diagnostic criteria of hemodynamic orthostatic dizziness/vertigo is to help clinicians to understand the terminology related to orthostatic dizziness/vertigo and to distinguish orthostatic dizziness/vertigo due to global brain hypoperfusion from that caused by other etiologies. Diagnosis of hemodynamic orthostatic dizziness/vertigo requires: A) five or more episodes of dizziness, unsteadiness or vertigo triggered by arising or present during upright position, which subsides by sitting or lying down; B) orthostatic hypotension, postural tachycardia syndrome or syncope documented on standing or during head-up tilt test; and C) not better accounted for by another disease or disorder. Probable hemodynamic orthostatic dizziness/vertigo is defined as follows: A) five or more episodes of dizziness, unsteadiness or vertigo triggered by arising or present during upright position, which subsides by sitting or lying down; B) at least one of the following accompanying symptoms: generalized weakness/tiredness, difficulty in thinking/concentrating, blurred vision, and tachycardia/palpitations; and C) not better accounted for by another disease or disorder. These diagnostic criteria have been derived by expert consensus from an extensive review of 90 years of research on hemodynamic orthostatic dizziness/vertigo, postural hypotension or tachycardia, and autonomic dizziness. Measurements of orthostatic blood pressure and heart rate are important for the screening and documentation of orthostatic hypotension or postural tachycardia syndrome to establish the diagnosis of hemodynamic orthostatic dizziness/vertigo.

Intrinsic chemosensitivity of rostral ventrolateral medullary sympathetic premotor neurons in the in situ arterially perfused preparation of rats

Brainstem hypoperfusion is a major excitant of sympathetic activity triggering hypertension, but the exact mechanisms involved remain incompletely understood. A major source of excitatory drive to preganglionic sympathetic neurons originates from the ongoing activity of premotor neurons in the rostral ventrolateral medulla (RVLM sympathetic premotor neurons). The chemosensitivity profile of physiologically characterized RVLM sympathetic premotor neurons during hypoxia and hypercapnia remains unclear. We examined whether physiologically characterized RVLM sympathetic premotor neurons can sense brainstem ischaemia intrinsically. We addressed this issue in a unique in situ arterially perfused preparation before and after a complete blockade of fast excitatory and inhibitory synaptic transmission. During hypercapnic hypoxia, respiratory modulation of RVLM sympathetic premotor neurons was lost, but tonic firing of most RVLM sympathetic premotor neurons was elevated. After blockade of fast excitatory and inhibitory synaptic transmission, RVLM sympathetic premotor neurons continued to fire and exhibited an excitatory firing response to hypoxia but not hypercapnia. This study suggests that RVLM sympathetic premotor neurons can sustain high levels of neuronal discharge when oxygen is scarce. The intrinsic ability of RVLM sympathetic premotor neurons to maintain responsivity to brainstem hypoxia is an important mechanism ensuring adequate arterial pressure, essential for maintaining cerebral perfusion in the face of depressed ventilation and/or high cerebral vascular resistance.

Role of the rostral ventrolateral medulla (RVLM) in the patterning of vestibular system influences on sympathetic nervous system outflow to the upper and lower body

Research on animal models as well as human subjects has demonstrated that the vestibular system contributes to regulating the distribution of blood in the body through effects on the sympathetic nervous system. Elimination of vestibular inputs results in increased blood flow to the hindlimbs during vestibular stimulation, because it attenuates the increase in vascular resistance that ordinarily occurs in the lower body during head-up tilts. Additionally, the changes in vascular resistance produced by vestibular stimulation differ between body regions. Electrical stimulation of vestibular afferents produces an inhibition of most hindlimb vasoconstrictor fibers and a decrease in hindlimb vascular resistance, but an initial excitation of most upper body vasoconstrictor fibers accompanied by an increase in upper body vascular resistance. The present study tested the hypothesis that neurons in the principal vasomotor region of the brainstem, the rostral ventrolateral medulla (RVLM), whose projections extended past the T10 segment, to spinal levels containing sympathetic preganglionic neurons regulating lower body blood flow, respond differently to electrical stimulation of the vestibular nerve than RVLM neurons whose axons terminate rostral to T10. Contrary to our hypothesis, the majority of RVLM neurons were excited by vestibular stimulation, despite their level of projection in the spinal cord. These findings indicate that the RVLM is not solely responsible for establishing the patterning of vestibular-sympathetic responses. This patterning apparently requires the integration by spinal circuitry of labyrinthine signals transmitted from the brainstem, likely from regions in addition to the RVLM.

Late histological and functional changes in the P23H rat retina after photoreceptor loss

Several strategies have been proposed to restore useful vision following photoreceptor degeneration. However, a very few studies have investigated late anatomical changes and functional state of residual retinal neurons after complete photoreceptor loss. We investigated the progressive degeneration of retinal ganglion cells (RGCs) in P23H rats. The RGC multielectrode array recordings indicated lower firing rates, disappearance of broad-scale, and maintenance of short-scale pairwise correlations. Up to 11% of RGCs displayed repetitive and often correlated spike discharges, reminiscent of developmental rhythmic activity, which could be reversibly suppressed by blockade of the AMPA/kainite glutamate receptors. RGCs in P23H rats remain sensitive to local electrical stimulation, generating short-latency responses as in the normal retina. These results provide evidence that, despite the demonstrated RGC degeneration, remaining active RGCs maintain their basic physiological and network properties with some emerging functional changes such as the spontaneous rhythmic activity in late stages of the degenerative disease.

Biophysical and histological determinants underlying natural firing behaviors of splanchnic sympathetic preganglionic neurons in neonatal rats

Isolated thoracic spinal cords of neonatal rats spontaneously generate splanchnic sympathetic nerve discharge (SND) with a quasiperiodic rhythm approximately 1-Hz. Using in vitro nerve-cord preparations that retained T6-T12 spinal segments, we investigated whether the natural firing behavior of sympathetic preganglionic neurons (SPNs) encoded the SND rhythm and what were the main biophysical and histological determinants of SPN firing. Under extracellular recording conditions, electrical stimulation of splanchnic nerves elicited antidromic responses in 212 SPNs. Among them, 92 SPNs were quiescent; 120 active SPNs had an average firing rate of 0.72+/-0.04 Hz, which was close to the quasiperiodic rhythm of SND. SPNs with rhythmic burst firing were rare. Probability plots of interspike intervals were constructed to extract mathematical features underlying SPN firing. Most active SPNs (88%) had a firing well described by unimodal Gaussian, suggesting a predominantly tonic pattern with normal variations. Biophysical properties of 112 SPNs were measured under whole-cell recording conditions. The charging time constant, tau, is positively correlated with the average firing rate. Histological properties were examined in 45 SPNs with intracellular diffusion of Lucifer Yellow or biocytin. SPNs with pyramidal somata and multipolar dendrites tend to be spontaneously active. In contrast, those with bipolar somata and fewer dendritic branches were quiescent in firing. These observations suggest that activity levels of SPNs are correlated with their capacity for temporal and spatial summation of synaptic inputs. How the seemingly tonic firing of individual SPNs is integrated into whole-nerve SND with quasiperiodic rhythms is discussed.

Simulation of the neuronal interactions and connection of neuronal activity with changes of heart rhythm and myocardial electrophysiological properties

The purpose was to create the software for analysis of temporal relationship between spike trains of several neurons and connection of neuronal activity with changes of heart rhythm and some electrocardiogram parameters. The designed software represents an integrated estimation of the cross-correlograms and cardiac-cycle triggered correlograms combination. It permits the study of the nonrandom temporal relationship between spike trains of neurons using experimental data and the estimation of the roles of one neuron and a group of them and the role of their discharges patterns in changes of heart rhythm and electrophysiological properties of the myocardium.

The cardiac-related rhythm in preganglionic sympathetic activities of developing piglets

This investigation was performed to determine whether partial spectral analysis of preganglionic sympathetic nerve discharges would reveal age-related differences in the distribution of baroreceptor afferent information to brainstem sympathetic-related neurons. Any influence of baroreceptor afferent activity on ordinary spectra of cervical sympathetic and splanchnic nerves was removed by partialization using the arterial blood pressure signal which represented baroreceptor activity. An absence of statistically significant coherence in partialized nerve spectra would indicate that sympathetic-related neurons receive peripheral baroreceptor afferent input, but are not interconnected, whereas the presence of significant coherence would mean that these neurons are interconnected. Ordinary spectral analysis did not demonstrate age-related differences in the relationship between nerve activity and baroreceptor afferent input. In many animals, large peaks, located at cardiac frequencies (range 2.75-5.6 Hz), were noted in ordinary nerve autopower spectra, and were significantly correlated in ordinary coherence spectra. Partialization of nerve spectra eliminated or reduced cardiac-related peaks in autopower spectra regardless of age, and, in 8 of 10 animals, reduced coherence estimates to non-significant values. In two animals, 19 and 36 days old, significant coherence values remained after partialization. These results demonstrated that cardiac-related peaks in coherence in spectra of preganglionic splanchnic and cervical sympathetic nerves were dependent upon peripheral afferent baroreceptor input in most animals. Further, the finding that significant residual coherence was absent in most cases suggested a paucity of intrabulbar pathways connecting brainstem sympathetic-related neurons.

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.

Functional connectivity between neurons generating resting discharge in renal sympathetic neurons in the rabbit

In anaesthetised rabbits, we analysed the cross-correlations of resting discharges in pairs of simultaneously recorded renal neurons. The study of 428 significant cross-correlations showed three pure types of neuronal co-ordination and two combined effects. Pure shared input was observed in 202 pairs of neurons (47.2%). The width of cross-correlogram peak was 222.9 +/- 7.2 ms. The peak height of shared input was 4.8 +/- 0.2 spikes s(-1) and was significantly related with correlogram width. Pure excitatory connection was found in 24 neuronal pairs (5.6%). Its peak width was 7.9 +/- 1.9 ms and it was shifted from time zero by 13.3 +/- 3.4 ms. The amplitude of the peak was 19.6 +/- 4.6 spikes s(-1). In 165 pairs of neurons (38.6%), excitatory connection was combined with shared input component. Pure reciprocal activation was seen in only 5 pairs of neurons (1.1%) while in 32 pairs (7.5%) it was accompanied by shared input co-ordination. The distance between narrow peaks measured in the combined type of co-ordination was 32.5 +/- 3.5 ms and the mean peak height was 4.1 +/- 0.7 spikes s(-1). Shared input in pure form and that accompanying both excitatory connection and reciprocal activation were significantly related to the frequency of discharge of neurons contributing to the cross-correlogram. The incidence and basic properties of the encountered types of neural coordination may indicate a pattern of interconnections between cells generating vasomotor tone in renal sympathetic neurons.