Neuroprotective effects of hibernation-regulating substances against low-temperature-induced cell death in cultured hamster hippocampal neurons

The neuroprotective effects of hibernation-regulating substances (HRS) such as adenosine (ADO), opioids, histamine and thyrotropin-releasing hormone (TRH) on low-temperature-induced cell death (LTCD) were examined using primary cultured hamster hippocampal neurons. LTCD was induced when cultures were maintained at <22 °C for 7 days. ADO (10–100 μM) protected cultured neurons from LTCD in a dose-dependent manner. The neuroprotective effects of ADO were reversed by both 8-cyclopenthyltheophilline (CPT; A1 receptor antagonist) and 3,7-dimethyl-1-propargylxanthine (DMPX; A2 receptor antagonist). Morphine (a non-selective opioid receptor agonist) was also effective in attenuating LTCD at an in vitro dose range of 10–100 μM. The neuroprotective effects of morphine were antagonized by naloxone (a non-selective opioid receptor antagonist). In addition, although [D-Ala2, N-Me-Phe4, Gly-ol5]-enkephalin (DAMGO; μ-opioid receptor agonist), [D-Pen2,5]-enkephalin (DPDPE; δ-opioid receptor agonist) and U-69593 (κ-opioid receptor agonist) were also effective, LTCD of cultured hippocampal neurons was not affected by TRH. Furthermore, histamine produced hypothermia in Syrian hamsters and protected hippocampal neurons in vitro at 100 μM. The neuroprotective effect of histamine was reversed by pyrilamine (H1 receptor antagonist). Apoptosis was probably involved in LTCD. These results suggest that ADO protected hippocampal neurons in vitro via its agonistic actions on both A1 and A2 receptors, whereas morphine probably elicited its neuroprotective effects via agonistic effects on the μ-, δ- and κ-opioid receptors. In addition, histamine also protected hippocampal neurons via its agonistic action on the H1 receptor. Thus, HRS-like adenosine-, opioid- and histamine-like hypothermic actions would most likely induce neuroprotective effects against LTCD in vitro.