An ultradian rhythm in adrenal secretion of corticosterone has been described in awake rats using intra-adrenal microdialysis. To determine the role of the autonomic innervation of the adrenal on the expression of the corticosterone rhythm, adrenal extracellular fluid was sampled by intra-adrenal microdialysis in intact (CTRL) and splanchnicectomized (SPLNX) rats 5-7 h before (light period) and after dark onset (dark period). Experiments conducted 1, 2, or 5 days after surgical insertion of the microdialysis probe consisted of continuous collection of dialysate at intervals of 10 min. Time domain pulse detection using PC-PULSAR showed that 5 days after surgery, SPLNX decreased interpulse interval (IPI) during the light period, but had no effect during the dark period, resulting in the loss of the diurnal rhythm in corticosterone secretion. Although diurnal modulation of both pulse amplitude and pulse frequency was observed, only the frequency was altered by SPLNX. In CTRL animals IPI increased at 5 days postsurgery, relative to 1 and 2 days, but the amplitude of normalized secretory pulses did not change. The decrease in IPI caused by SPLNX was observed 5 days, but not 1 or 2 days after surgery, suggesting that surgical stress obscures the inhibitory effect of splanchnic neural activity. Power spectral analysis showed significant periodicities in corticosterone secretion rate in individual CTRL and SPLNX animals at 1, 2, and 5 days. One day after surgery, SPLNX reduced the frequency of the ultradian rhythm detected by power spectral analysis. This finding suggests that splanchnic neural activity may increase pulse frequency in stressed rats, in opposition to the effect seen after extended recovery from surgery. In conclusion, our data suggest that the nadir of the diurnal rhythm in corticosterone secretion results in part from neural inhibitory control. Splanchnic neural innervation may also have an excitatory role in the adrenocortical stress response.

To establish whether adrenal corticosterone secretion is episodic and whether episodes are random or periodic events, adrenal extracellular fluid was sampled in awake rats by intra-adrenal microdialysis. Experiments conducted 1 or 2 days postsurgery between 1000 and 1830 h consisted of continuous collection of dialysate at intervals of 10 min. Pulses detected by PC-Pulsar had an interpulse interval of 54.1 +/- 4.5 and 51.3 +/- 5.0 min on days 1 and 2, respectively. Fourier analysis was used to determine whether significant periodicity was present. Six of seven of the day 1 animals had significant secretory rhythmicity, with a predominant periodicity at 60-80 min. In day 2 animals, in addition to a period near 50-80 min, a lower-frequency rhythm with a period near 213 min was identified. Autocorrelation analysis produced results qualitatively similar to Fourier analysis. Secretion rates were normalized based on maximal adrenal responses to adrenocorticotropic hormone and averaged at each time point. When the composite series was subjected to Fourier analysis, a significant rhythmicity (period 58 min) was still detectable in the day 1 animals. When composite data for the day 2 animals were analyzed, in addition to a component near 60-80 min, a lower frequency component near 320 min was present. The persistence of periodicity after time averaging suggests that corticosterone pulses are synchronized between animals. Cross-correlation analysis of individual pairs of data series supported synchronization in many cases. These results suggest that pulsatile secretion of corticosterone in rats has a primary periodicity near 60 min and is synchronized across animals. The mechanism responsible for this synchronization is unknown.

Awake rats demonstrate an ultradian rhythm in adrenal secretion of corticosterone. Splanchnic denervation in unstressed rats increases the frequency of corticosterone pulses, revealing an inhibitory function of adrenal innervation. In contrast, one day after surgical stress, adrenal denervation reduces adrenal pulsatility, suggesting a stimulatory function of adrenal innervation. To test whether neural modulation of pulsatile secretion was due to a direct effect of the splanchnic nerve on adrenal sensitivity to adrenocorticotropic hormone (ACTH), rats treated with dexamethasone were administered repetitive pulses of ACTH, and the amplitude of corticosterone responses was determined. Intact or control (C) and splanchnicotomized (SPLNX) rats were tested at 2 or 5 days after surgery. Five days after surgery, adrenal responsiveness in C animals was reduced compared with SPLNX animals. However, no differences were seen 2 days after surgery. To determine whether the reduction in adrenal responsiveness involved a cellular or organ level mechanism, dispersed adrenal cortical cells isolated from intact or denervated adrenal glands were stimulated with ACTH, and corticosterone secretion was determined. Consistent with in vivo results, denervation increased the responsiveness of adrenal cells obtained 5, but not 1 or 2, days after surgery. These findings support a neurally mediated inhibition of adrenal sensitivity to ACTH in unstressed rats.