L-histidine enhances learning in stressed zebrafish.

The aim of the present study was to determine the effect of the histaminergic precursor L-histidine and the H3 receptor antagonist thioperamide on the learning process of zebrafish submitted or not to confinement stress. On each of the 5 consecutive days of experiment (D1, D2, D3, D4, D5), animals had to associate an interruption of the aquarium air supply with food offering. Non-stressed zebrafish received an intraperitoneal injection of 100 mg/kg L-histidine, 10 mg/kg thioperamide or saline after training. Stressed animals received drug treatment and then were submitted to confinement stress for 1 h before the learning procedure. Time to approach the feeder was measured (in seconds) and was considered to be indicative of learning. A decrease in time to approach the feeder was observed in the saline-treated group (D1 = 141.92 +/- 13.57; D3 = 55 +/- 13.54), indicating learning. A delay in learning of stressed animals treated with saline was observed (D1 = 217.5 +/- 25.66). L-histidine facilitated learning in stressed (D1 = 118.68 +/- 13.9; D2 = 45.88 +/- 8.2) and non-stressed (D1 = 151.11 +/- 19.20; D5 = 62 +/- 14.68) animals. Thioperamide inhibited learning in non-stressed (D1 = 110.38 +/- 9.49; D4 = 58.79 +/- 16.83) and stressed animals (D1 = 167.3 +/- 26.39; D5 = 172.15 +/- 27.35). L-histidine prevented the increase in blood glucose after one session of confinement (L-histidine = 65.88 +/- 4.50; control = 53 +/- 3.50 mg/dL). These results suggest that the histaminergic system enhances learning and modulates stress responses in zebrafish.