Generation of a physiological sympathetic motor rhythm in the rat following spinal application of 5-HT

Abstract

When applied in vitro to various CNS structures 5-HT and/or NMDA have been observed to generate rhythmic nervous activity. In contrast, reports of similar in vivo actions are relatively rare. Here we describe a physiological sympathetic motor rhythm regulating the thermoregulatory circulation of the rat tail (T-rhythm; 0.40-1.20 Hz) that can be elicited following intrathecal (i.t.) application of 5-HT to an in situ'isolated' spinal cord preparation (anaesthetized rats spinalized at T10-T11 and cauda equina cut). i.t. injections were delivered to L1 as sympathetic neuronal activity to the tail (SNAT) arises from preganglionic neurones at T11-L2. SNAT was abolished after spinal transection (n = 18) and it did not return spontaneously. The administration of 5-HT (250 nmol) generated rhythmic sympathetic discharges (n = 6). The mean frequency of the T-like rhythm during the highest level of activity was 0.88 +/- 0.04 Hz which was not significantly different from the T-rhythm frequency observed in intact animals (0.77 +/- 0.02 Hz; P > 0.05 n = 16). In contrast, NMDA (1 micromol) generated an irregular tonic activity, but it failed to generate a T-like rhythm (n = 9), even though the mean levels of activity were not significantly different to those produced by 5-HT. However, 5-HT (250 nmol) applied after NMDA generated a T-like rhythm (0.95 +/- 0.11 Hz, n = 6). Our observations support the idea that 5-HT released from rostral ventromedial medullary neurones, known to innervate sympathetic preganglionic neurones, can induce sympathetic rhythmic activity.

Figure 1. Intrathecal application of 5-HT (250 nmol) in a rat with spinal cord transection (SCT) induced robust rhythmic sympathetic activity similar to the physiological T-rhythm

A, raw and B, rectified and smoothed (time constant 100 ms) DCN neurograms. C, DCN mean level of activity (time constant 10 s). The data were divided into 300 s subsections. D, time-evolving autospectra of the DCN from the same animal and across the same time periods as in C. Spectral analysis was performed on each data set. The relative power density across time is coded by a 64-grade colour scale. E, 300 s data set power spectra in the intact animal and after spinal cord transection (SCT). The symbols highlight the power spectra taken at 4 specific data sets during the response to 5-HT, i.e. first data set with robust rhythmicity (▴), highest mean level of activity (▪), last data set with rhythmicity (♦) and no rhythmicity (•). In the intact animal the DCN neurogram shows burst discharges with variable frequency and amplitude, and the autospectra are dominated by a band and a peak at 0.78 Hz (and a lung inflation cycle-related component at 2 Hz). SNAT was completely abolished after SCT; mean level of activity dropped to zero and rhythmical components are no longer observed in the autospectra. The i.t. administration of 5-HT induced robust rhythmic discharges. Note that DCN frequency changed with regard to the mean level of activity: the first data set with rhythmicity (▴) and the data set with the highest level of activity (▪) were dominated by a band and a peak in the autospectra at 0.78 Hz with its first harmonic peak at 1.56 Hz. The last data set with rhythmicity (♦) showed a peak at 0.48 Hz and its first harmonic at 0.96 Hz. Rhythmic components were no longer observed in the following data set (•).

Figure 2. Intrathecal application of NMDA (1 μmol) in a rat with spinal cord transection (SCT) induced tonic sympathetic activity

The same as Fig. 1. The symbols highlight the power spectra taken at 5 specific data sets during the response to NMDA, i.e. highest and stable mean level of activity (• and ▪, respectively) and 5-HT, i.e. first data set with robust rhythmicity (▴), last data set with rhythmicity (♦) and no rhythmicity (▪). In the intact animal the DCN neurogram shows burst discharges with variable frequency and amplitude, and the autospectra is dominated by a band and a peak at 0.83 Hz (and a lung inflation cycle-related component at 2 Hz). Mean level of activity dropped to zero after SCT and rhythmical components were no longer observed in the autospectra. The i.t. administration of NMDA generated an irregular tonic activity. The mean level of activity increased 152% (•), and dropped rapidly, remaining stable at 29% (▪). DCN autospectra showed a peak with very low frequency (0.07 Hz) during these two phases. A dose of 5-HT (250 nmol, i.t.) was administered 30 min after NMDA-induced activity was stable; mean level of activity remained unchanged after 5-HT administration but tonic activity became rhythmic during 6 data sets. The

Figure 3. T-like rhythm frequency with relation to the mean level of activity

Response to 5-HT and NMDA + 5-HT in animals with spinal cord transection (SCT). A, 5-HT response. The frequency of the T-like rhythm was similar during the first data set with rhythmicity and the highest mean level of activity. Note that the frequency in the last data set with rhythmicity is significantly slower even though the level of activity is similar to the first data set. B, NMDA + 5-HT response. Spectral analysis showed no rhythmic components in response to NMDA application. The mean levels of activity induced by NMDA and 5-HT alone (A) were not significantly different (highest versus highest and stable versus first/last data set with rhythmicity, respectively). However, a T-like rhythm appeared when 5-HT was administered without producing significant changes in the mean level of activity. The frequency of the T-rhythm was stable and no significant differences were observed in the first and last data sets with rhythmicity. Results are expressed as mean ±s.e.m. One-way ANOVA was used to analyse statistical differences in T-like rhythm frequency between data sets. *P < 0.05. first (▴) and the last (♦) data sets with rhythmicity showed a peak and a band in autospectra at 0.53 Hz and 0.63 Hz, respectively. Rhythmic components were no longer observed in the following data sets (▪) and activity returned to the tonic state and remained stable for 3 h until the animal was killed.