The Effect Of Chronic Morphine Exposure And GABA On Visual Neuronal Responses In Cats

1. Chronic morphine exposure affects contrast response functions of V1neurons in catsDrug abuse is a serious social and medical problem in the world. Chronic exposure to opiates eventually leads to physiological and psychological dependence and causes severe damage to the nervous system. It has been indicated that the visual system is rich in opiate receptors. Morphological and electrophysiological studies reveal that chronic morphine administration leads to the prominently structural modification in the primary visual cortex. Many visual response properties of the neurons in LGN and V1, such as response modulation, orientation and direction selectivity and the visual response latency, are found to be significantly affected by chronic morphine exposure.Coding of contrast plays an important role in the visual information processing. Psychophysical studies indicate that human motion perception is contrast-dependent, such as speed perception, orientation discrimination and figure-ground segregation. It has als.o been suggested that variation of stimulus contrast can influence the neuronal tuning for other parameters, like spatial frequency and speed. Up to now, few studies have focused on the effects of opiates on contrast response function of the neurons in the visual cortexIn the present study, we have studied and compared contrast response functions of neurons in primary visual cortex (V1) in chronic morphine-treated cats (MCs) and saline-treated cats (SCs)(control) using extracellular single-unit recording techniques. We found that C50(half-saturation contrast) of both groups were normally distributed and significant differences were seen between these two distributions. C50in MCs varied with a wider range (from0.10to0.77) than those in SCs (from0.07to0.32), More than half of the cells (51.3%,41of80) in MCs showed low contrast sensitivity (C50≥0.3) while only7.0%of the cells (4of57) in SCs exhibited similar response property. On the other hand, the percentage of the cells that were highly contrast sensitive (C50<0.1) is greater in SCs (8.8%,5of57) than in MCs (2.5%,2of80) These findings indicate a substantial decline in contrast sensitivity induced by chronic morphine exposure. We also analyzed the maximum visual responses (Rmax), baseline responses (M) and the signal-to-noise ratios (SNR) of all recorded neurons in MCs and SCs. We found that cells in MCs exhibited significantly higher maximum visual responses and baseline responses than those in SCs. Signal-to-noise ratios of neurons in MCs were much smaller than those in SCs.Our results that significant changes exist in contrast response function between MCs and SCs provide the first evidence of the influence of chronic morphine exposure on neuronal contrast encoding, which is an important response property of the neurons in the visual pathway. These findings provide new neurobiology evidence that chronic morphine exposure induces functional degradation of vision system and could be helpful for better understanding of the mechanism by which chronic morphine treatment impairs many aspects of visual perception,2. GABA affects the neural representation of speed in CatsThe perception of speed is important for vision and plays a key role in perceptual judgments and motion actions. The speed of a grating stimulus is given by its temporal frequency divided by its spatial frequency. If the preferred speed is the same at all spatial frequency, then the temporal frequency tuning must vary as a function of spatial frequency:we say that the neuron is "speed tuned". If the preferred temporal frequency does not vary as a function of spatial frequency, then the preferred speed must vary with spatial frequency:we say that the neuron has "separable tuning" for spatial and temporal frequency. So far the speed tuning properties of neurons in cats are largely unknown.GABAnergic system is critical for maintaining normal visual function. A lot of studies have demonstrated that GABA and GABAa receptor agonist could improve impaired neuronal functions in visual cortex. However, it is still unknown whether GABAnergic system affects speed tuning of neurons in cats.In the present study, we used extracellular single-unit recording techniques to study the effect of spatial frequency on the preferred speed of neurons in areas V1and PMLS of anesthetized cats. Furthermore, we also studied the effects of electrophoretic application of the inhibitory transmitter GABA and the GABAA antagonist bicuculline on speed tuning properties of V1and PMLS neurons. We found that the preferred speed of V1and PMLS neurons became less dependent on spatial frequency following GABA treatment. On the contrary, bicuculline resulted in increased dependence of preferred speed of V1and PMLS neurons on spatial frequency. In addition, we also found the speed tuning of PMLS neurons depended less on spatial frequency than that of V1neurons. In conclusion, the results of this study show that the speed tuning properties of V1and PMLS neurons are different and GABAnergic system affects speed tuning of neurons in cats.The present study also has implications for understanding speed tuning properties of neurons and visual motion processing.