The effects of bilirubin on evoked potentials and long-term potentiation in rat hippocampus in vivo

Neonatal jaundice is a common condition that could potentially lead to severe neurotoxicity. In this electrophysiological study we observed the effects of a short-term bilirubin injection on evoked potentials (population spike, PS) and long-term potentiation (LTP) in the hippocampal CA, region of Sprague Dawley rats in vivo. The animal received a bolus i.v. injection of either 60 mg/kg, or 30 mg/kg of bilirubin, or an equivalent volume of bilirubin-free vehicle in 5 min. The results showed that both bilirubin-treated groups had a dose-independent prolongation of peak latencies and decrease of slopes of the PS at all measured time points following injection (1, 3, 5, 10, 15, 30, 45, 60, 90, 120 min), while the amplitudes of the PS did not change significantly. The peak latency, slope, and the amplitude of PS stayed unchanged in the control group. Furthermore, while LTP could be induced by high-frequency stimulation in control animals, this phenomenon was absent in both bilirubin-treated groups. The amplitudes of the PS in the two treated groups after stimulation were also smaller than those of the control animals at every time points. These findings are in accordance with previous observations showing significant depressive effects of bilirubin on the nervous system. Our novel finding that short-term exposure to bilirubin can inhibit the induction of LTPs in the hippocampus, is compatible with the suggestion that neonatal hyperbilirubinemia can impact on learning and memory.