Studies on the effects of somatostatin treatment on the visual cortical processing

Abstract

In the cerebral cortex, Somatostatin (SST) is a neuropeptide that is known to be expressed and released by a type of GABAergic interneurons. Despite the abundance of the peptide and its receptors in the brain, its precise roles in modulating cortical activity are not yet fully fathomed. In this study, the effects of the exogenously applied SST are illustrated through various experimental approaches on the cortical visual processing in mice. Firstly, a T-maze visual discrimination task was designed with drift-grating visual stimuli so that the subject mice have to choose between two directions depending on the stimuli. When lower contrast levels were used to test the visual discriminability of the subject mice, the injection of SST into the primary visual cortex (V1) was shown to improve the performance, while that of SST blocker cyclosomatostatin decreased the performance. Secondly, in order to understand how such behavioral improvement occurred, a slice whole-cell electrophysiology experiment was done to observe the ex-vivo responses at the cellular level. It was illustrated that the presence of SST reduces the amplitude and the frequency of spontaneous excitatory postsynaptic current (sEPSC) for V1 layer 2/3 parvalbumin-positive GABAergic interneurons, specifically. Lastly, in order to see how the neurons behave in vivo, in vivo recording was done at V1 with anesthetized mice. Two types of visual stimuli (contrast-variant checkerboards, and contrast-variant, orientation-variant sinusoidal drift-grating) were used to see if any changes in response can be observed with treatment of SST on the surface. Only for the drift-grating stimulus, there was a clear improvement in the orientation selectivity of the cells in V1. Hence, exogenous application of SST can improve the visual perception by improving the orientation selectivity of the cells through reducing inputs into specific interneurons.