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 H(3) 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 1h 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.