Chronic morphine exposure affects contrast response functions of V1 neurons in cats

Opiates disrupt neural functions in many brain areas, including visual cortex. Previous studies have indicated substantial changes of many neuronal response properties induced by chronic morphine exposure in the visual information processing system. However, it remains unclear whether neuronal contrast coding is also affected. To investigate this issue, we measured the contrast response functions (CRFs) of V1 neurons in chronic morphine-treated and saline-treated cats by using extra-cellular single-unit recording techniques. Our results indicated significantly lower contrast sensitivity in morphine-treated cats than in saline-treated cats and V1 neurons in morphine-treated cats exhibited enhanced maximum visual responses, higher baseline responses and lower signal-to-noise ratios compared with saline-treated cats. These findings provide some neurobiological evidence for the morphine-mediated degenerations of the visual cortex, which could underlie the opiate-induced deficits in visual function.