Bicuculline enhances the corticosterone secretion induced by lipopolysaccharide and interleukin-1 alpha in male rats

Lipopolysaccharide (LPS)-induced inflammatory stress activates the hypothalamus-pituitary-adrenal (HPA) function. Interleukin-I (IL-1) is one of the key factors during this event; however, the mechanisms mediating IL-1 stimulation of HPA axis are still unclear. The present study evaluated the possible involvement of gamma-aminobutyric acid (GABA) in LPS-induced activation of HPA axis in adult male rats. In addition, the possible existence of diurnal changes of LPS-induced HPA axis activity was also investigated. Bicuculline (0.8 mg/kg BW), a GABA-A receptor antagonist and GABA (1 g/kg BW) were intraperitoneally (ip) injected 15 min before LPS (2 mg/kg BW, ip) or recombinant human IL-1 alpha (microgram/rat) administration in intact rats. Control animals received an equivalent volume of 0.9% saline. Rats were sacrificed at 60 min or 90 min after LPS, or IL-1 alpha or saline injection. Plasma corticosterone levels were measured by radioimmunoassay. Results showed that pretreatment with bicuculline enhanced both LPS- and IL-1-induced corticosterone secretion; while pretreatment with GABA significantly reduced the LPS-stimulated corticosterone release (p < 0.05, vs LPS alone). The effect is dependent on the time of sampling and such effect of bicuculline or GABA was not observed when rats were stimulated in the evening. In addition, the maximal changes of plasma corticosterone following LPS administration in the evening were significantly lower than in the morning (p < 0.01). The present study provides evidence that GABA is involved, at least in part, in the neuroendocrine regulation of LPS/interleukin-1a-induced corticosterone secretion via GABA-A receptor in rats. In addition, the response of plasma corticosterone to LPS has a diurnal variation, which corresponds to a diurnal change of GABAergic modulation of the immunoneuroendocrine response.