Mechanisms of neuroprotection and retain the function of central nervous system during hibernation in Syrian hamsters

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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℃ 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, A_1 receptor antagonist) and 3,7-dimethyl-1-propargybcanthine (DMPX; A_2 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-Ala^2, N-Me-Phe^4, Gly-ol^5]-enkephalin (DAMGO; μ-opioid receptor agonist), [D-Pen^<25>]-enkephalin (DPDPE; δ-opioid receptor agoni … More st) and U-69593 (k-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 (H_1 receptor antagonist). Apoptosis was probably involved in LTCD. These results suggest that ADO protected hippocampal neurons in vitro via its agonistic actions on both A_1 and A_2 receptors, while morphine probably elicited its neuroprotective effects via agonistic effects on the μ-, δ- and k-opioid receptors. In addition, histamine also protected hippocampal neurons via its agonistic action on the H_1 receptor. Thus, HRS-like adenosine-, opioid- and histamine-like hypothermic actions would most likely induce neuroprotective effects against LTCD in vitro.Endogenous opioid peptides involved in the central regulatory system of body temperature (Tb) during hibernation in Syrian hamsters. The torpor was classified into i) entrance, ii) maintenance and iii) arousal phases according to Tb changes. In previous study, we demonstrated that central opioid system involved in the Tb regulatory system during maintenance phase. Immunostaining of c-Fos and β-endorphin was performed by using avidin-biotin-peroxidase procedure. Intracerebroventricular (icv) injection of naloxonazine (μ1-opioid receptor antagonist) was effective in arousing hamster from torpor. In contrast, neither 10 nM of naltrindole (δ-opioid receptor antagonist) nor nor-BNI (k-opioid receptor antagonist) interrupted the torpor. Moreover, icv injection of anti-β-endorphin antibody was effective in interrupting the torpor, but anti-endomorphin-1 and-2 antibodies were ineffective. The number of c-Fos positive neuron increased in arcuate nucleus (ARC) at 1 hr after torpor onset compared with before torpor. Furthermore, although the expression level of proopiomelanocortin (POMC) mRNA did not change, the endoprotease-2 mRNA incresed in ARC. Β-Endorphin-like immunoreactivity (β-End IR) was observed in ARC. With progress of hibernation, the β-End IR decreased with ARC and increased with organum vasculosum lamina terminalis (OVLT) . These results suggest that the β-endorphin was transported to OVLT from ARC by axonal flow and then regulates body temperature during maintenance phase of torpor via μ1-opioid receptors. Less