Involvement of opioid and GABA systems in the ventrolateral periaqueductal gray on analgesia associated with tonic immobility

Ventrolateral periaqueductal gray (VL-PAG) contains key neuronal circuits related to the analgesic effect involved in integrated defensive behaviors such as immobility response (IR). The latter is characterized by a reversible state of motor inhibition that can be elicited in rats under several conditions including restriction of movements (tonic immobility: TI). It is known that IR-induced analgesia can be elicited by manipulations or drugs acting on the central nervous system (CNS) at different levels. The aim of this study was to assess the role of the opioid and the GABA systems in TI-elicited analgesia. After inducing TI in naïve rats by neck clamping, the analgesic effect was evaluated by the tail-flick (TF) test. Compared to the control group, rats with TI had increased TF latency evidencing an analgesic effect. An opioid receptor agonist and antagonist were injected systemically, as well as microinjected locally in VL-PAG, as well as GABA_A receptor agonist and antagonist were microinjected into VL-PAG. Under both injection schemes, morphine increased TF latency and TI duration, while naloxone blocked TI-induced analgesia. Muscimol reduced TF latency and TI duration while bicuculline increased TF latency but not TI duration. This suggests that TI-elicited analgesia was mediated by opioids at different levels of the CNS especially in the VL-PAG by inhibition of intrinsic tonic GABAergic activity. There were no additive analgesic effects of morphine or bicuculline with tonic immobility, which probably means reach a certain upper limit under such conditions.