Comparison of responses of ventral posterolateral and posterior complex thalamic neurons in naïve rats and rats with hindpaw inflammation: μ-opioid receptor mediated inhibitions

The aim of the present study was to compare the effects of morphine on thalamic neuronal responses in naïve rats and rats with carrageenan-induced hindpaw inflammation. Multiple single unit ventral posterolateral (VPL) and posterior complex (Po) activity was recorded and mechanically- (7 g, 14 g, 21 g, 60 g and 80 g) evoked responses of VPL and Po neurones were measured in naïve rats and rats with carrageenan (100 μl, 2%)-induced hindpaw inflammation. Effects of systemic (0.5 mg kg−1) and intra-thalamic (66 μM, 250 nL) morphine on neuronal responses were determined. Mechanically-evoked (60 g) nociceptive responses of VPL neurones were significantly larger in inflamed rats (29 ± 4 spikes s−1) compared to naïve rats (19 ± 2 spikes s−1, P < 0.05). Systemic morphine inhibited 7 g-evoked responses of VPL neurones in inflamed (24 ± 8% control, P < 0.01), but not in naïve rats (123 ± 3% control). Frank noxious-evoked responses of VPL neurones in inflamed rats were less sensitive to the effects of systemic and intra-thalamic morphine, compared to naïve rats (P < 0.05 for both). These data provide evidence for altered evoked responses of neurones at the level of VPL, but not at Po, during hindpaw inflammation and suggest that thalamic sites of action contribute to the effects of systemic morphine.