Temporal effects of fornix transection on brain tryptophan hydroxylase activity and plasma corticosterone levels

The effect of fornix transection on plasma corticosterone and on midbrain (MID), septum/preoptic (S/POA) or hippocampal (HIP) tryptophan hydroxylase activity (THA) was studied in adult male rats. A mild stress resulted in higher levels of plasma corticosterone in operated as compared to sham-operated rats 6 and 40 h post-transection; however, at 30 days post-operation no difference was found. The response of THA to fornix transection was region-specific. No significant change in the S/POA region was found at any time. THA in the HIP, a terminal area of 5-HT fibers, showed a progressive fall over time to values 80% below normal levels. This result suggests that most of the 5-HT fibers to the HIP had been severed. A 28 h half-life for HIP THA was calculated. THA in the MID, an area known to contain the majority of 5-HT cell bodies with ascending fibers, was significantly reduced compared to sham controls at 6 h, 40 h, and 8 days posttransection. However, at 30 days post-operation no difference was found. The depression in MID THA by its rapid onset, the distance from the fornix transection site, and its return to normal after 30 days, is thought to be due to a transneuronal effect on the serotonin-containing neurons in MID raphe. The fall in MID THA at a time when plasma corticosterone levels are increased in fornix-transected rats may be compared with the situation in normal, stressed and adrenalectomized rats where MID THA and plasma corticosterone levels change in the same direction. The data suggest that the glucocorticoid effects on MID 5-HT-containing neurons are mediated transneuronally through the hormone concentrating cells in the HIP.