Serotonin depletion does not alter lipopolysaccharide-induced activation of the rat paraventricular nucleus

Abstract

We have investigated the effects of serotonin depletion on immune-mediated activation of the hypothalamopituitary-adrenal (HPA) axis. Corticotrophin-releasing factor (CRF) mRNA, c-fos mRNA and Fos peptide responses in the paraventricular nucleus (PVN) together with circulating levels of corticosterone were assessed in response to i.p. injections of three doses of lipopolysaccharide (LPS) both in control animals and animals pretreated with p-chlorophenylalanine (PCPA).Conscious animals received either an i.p. injection of 0·5 ml saline or 200 mg/kg PCPA in 0·5 ml saline on 2 consecutive days. This treatment resulted in a 93% depletion of serotonin on the fourth day. On day 4, animals received i.p. injections of LPS (2·5 mg/0·5 ml saline, 250 µg/0·5 ml or 50 µg/0·5 ml; E. coli 055:B5), or saline injections as controls.Pretreatment with PCPA had no effect on the basal levels of corticosterone, or on the elevated levels induced by the three doses of LPS. Fos peptide and c-fos mRNA were undetectable in control animals, and Fos-like immunoreactivity increased in a dosedependent manner following i.p. LPS in both control and PCPA-pretreated animals. C-fos mRNA expression induced by LPS was unaffected by serotonin depletion. Following the lowest dose of LPS, CRF mRNA did not change above control levels, however, the medium and high doses of LPS produced a significant (P<0·05) increase in CRF mRNA levels in both depleted and intact animals.To confirm the temporal effects of serotonin depletion on activation of the HPA axis we collected plasma at 30 min, 1, 2, 3, 4, 5 and 6 h after LPS in both intact and serotonin-depleted animals. No significant differences in plasma corticosterone levels were found at any of the time points between intact and depleted animals.It appears that, at least under these experimental conditions, serotonergic inputs do not seem to play a major role in mediating the effects of LPS on changes in mRNA levels in the PVN or on the subsequent activation of the HPA axis.

Central LPS-Induced <i>c-fos</i> Expression in the PVN and the A1/A2 Brainstem Noradrenergic Cell Groups Is Altered by Adrenalectomy

“…injection of LPS [1, 9, 11]. Previous results from our group have demonstrated that peripheral administration of this dose, from this particular batch of LPS is insufficient to cause elevations in CRH mRNA within the PVN, thus suggesting that its actions are mediated via central mechanisms in this study [21]. We have further noted a significant increase in both c-fos mRNA and Fos protein in the PVN and the A1 and A2 regions of the brainstem, confirming previous results concerning activation of the HPA axis and c-fos mRNA and Fos peptide following central injection of immune modulators [11, 29, 30].…”

“…In intact animals, following acute stress, there is a peak in c-fos mRNA and plasma CORT concentration after 30 min. This is in contrast to the response to LPS which has a later onset and is more sustained, with peaks in CORT and c-fos expression occuring around 3 h after injection [21, 29, 30, 31]. In a previous study we were unable to determine the presence of c-fos mRNA within the PVN 30 min after LPS, but this increase was apparent after 3 h [21].…”

“…This is in contrast to the response to LPS which has a later onset and is more sustained, with peaks in CORT and c-fos expression occuring around 3 h after injection [21, 29, 30, 31]. In a previous study we were unable to determine the presence of c-fos mRNA within the PVN 30 min after LPS, but this increase was apparent after 3 h [21]. The presence of c-fos mRNA within the PVN suggests that the neurons activated may be CRH or vasopressin (AVP)-containing CRH neurons, but LPS could not induce a further increase in CRH mRNA above levels seen following ADX.…”

“…The dose of LPS administered centrally was based on preliminary experiments, whereby maximal activation of the HPA axis was obtained. The same dose, from the same batch when administered peripherally did not affect CRH mRNA within the PVN [21]. The remaining animals received vehicle i.c.v.…”

Modulatory Role of the Epinergic System in the Neuroendocrine-Immune System Function

“…The mechanisms whereby normal adrenal response to LPS was found in SKF-64139-treated rats remain to be elucidated; however, depletion of endogenous serotonin, a positive modulator compound of the HPA axis activity [33], in rats resulted in impaired ACTH secretion after LPS [34] accompanied by a normal adrenal response [34,35]. Also, we found that inhibition of the central PNMT activity resulted in enhanced hypothalamic vasopressinergic activity (from parvicellular or magnocellular or both origins) which could contribute, as a compensatory mechanism under sustained EPI inhibition, to a normal glucocorticoid release during acute endotoxemia by stimulating adrenal V1a receptors [36].…”