Changes of μ-opioid receptors and GABA in visual cortex of chronic morphine treated rats

Electrophysiological and biochemical studies have indicated that GABAergic modulation is involved in the opioid-induced altering of response properties of visual cortical cells and impairing of short-term synaptic plasticity in the geniculo-cortical visual pathway. The aim of the current study was to examine whether there were changes in the localization and density of μ-opioid receptor subtype (MOR1) and GABA in the visual cortex of morphine-dependent and abstinent rats. Immunofluorescence histochemical method was applied to display MOR1 and GABA distribution. We found that MOR1-like immunoreactive neurons were significantly lowered in layer I-VI of visual cortex of rats sacrificed immediately after the last injection (defined as morphine-dependent (MD)) than saline-control group. In rats sacrificed just before the last injection (defined as morphine-abstinent (MA)), the density of μ-opioid receptor was higher than that in dependent group in layer I-V neurons of visual cortex, but remained lower than those in control group. Three hours after the last morphine injection (defined as 3 h after morphine-abstinent (3 h)), MOR1-like immunoreactive neurons in layer I and layer IV of visual cortex were still significantly lower than control. As to GABA-like immunoreactive neurons, they were significantly decreased in abstinent group compared to dependent group. These results provide morphological evidence that opioid-induced altering of response properties of visual cortical cells might be due to the change of visual cortical GABAergic system induced by opioid via μ-opioid receptor, which result in disinhibition of visual cortex projection neurons and their abnormal firing.