Clinical studies have shown that schizophrenia is accompanied by hypoalgesia. Accordingly, we have previously reported that a chronic schizophrenia-related rat substrain (Wisket) showed decreased acute heat pain sensitivity. The aim of the present study was to determine the mechanical pain sensitivity and the effects of opioid ligands in a chronic osteoarthritic pain model generated using Wisket rats. Our previous molecular biological studies indicated that the impairment in opioid and cannabinoid receptor functions observed in these animals did not explain their altered pain sensitivity. Therefore, we aimed to investigate another endogenous antinociceptive system, i.e., the oxytocinergic system (which is also implicated in schizophrenia) via the determination the brain-region specific oxytocin receptor mRNA expression in Wisket rats. Osteoarthritis was induced in male adult control Wistar rats without any interventions and in Wisket rats after juvenile social isolation and ketamine treatment. The degree of allodynia and the effects of systemic morphine or intrathecal endomorphin-1 administration were determined. Furthermore, the expression of the oxytocin receptor mRNA was assessed in different brain structures (prefrontal cortex, striatum, diencephalon, brainstem, and olfactory bulb). A lower degree of allodynia was observed in the Wisket group compared with control animals 1 and 2 weeks after the induction of osteoarthritis, which was accompanied by a comparable degree of edema. Systemically or intrathecally applied opioids caused similar time-response curves in both groups, with apparently shorter effects in Wisket animals. The expression of the oxytocin receptor mRNA was lower in most of the brain regions (with the exception of the diencephalon) investigated in Wisket rats vs. the control animals. In summary, both acute and chronic hypoalgesia (as nonspecific symptoms in patients with schizophrenia) can be simulated in Wisket animals as endophenotypes despite the impairment of the endogenous antinociceptive systems evaluated. Thus, this model might be an appropriate tool for further investigation of the molecular basis of altered pain perception in schizophrenia.
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