Physiological Properties Of Horizontal Cells And Neural System Temporal Information Processing

Calcium is one of the most versatile intracellular second messengers,playing crucial roles in many intracellular signaling pathways.It was recently reported that NMDA receptors, which are highly permeable to Ca²⁺,are expressed in carp retinal H1-type horizontal cells.To understand the functional role of NMDA receptors for intracellular free calcium concentration([Ca²⁺_i]dynamics of H1 cells,[Ca²⁺]_i changes initiated by the activation of NMDA receptors were studied in H1 cells using both experimental and computational techniques.Fura-2 fluorescence calcium imaging showed that H1 subtype horizontal cells responded to exogenously applied NMDA with a transient[Ca²⁺]_i increase which decayed to a sustained,but elevated level of[Ca²⁺]_i.Contributions of different Ca²⁺ flux pathways underlying the time course of this increment of[Ca²⁺]_i were further explored.Intracellular calcium stores were suggested to play crucial role for the initial transient dynamics of [Ca²⁺]_i.Ca²⁺ is known to play crucial roles in the chemical synapses for neuronal communica-tion.Retinal H1 cells is also called as luminosity-type horizontal cells(LHC) according to its light response properties.The second part of this thesis introduces our work on the chem-ical synapses properties between LHC and photoreceptors in the outer retina.Previous work in our laboratory indicated that repetitive red flashes progressively strengthened the synap-tic connection between red-cone and LHC,whereas weakened that between green-cone and LHC.On the other hand,repetitive green flashes remarkably depressed the LHC's red re-sponse,but caused little changes in the cell's green response.This phenomena indicated that the chemical synapses between LHC and photoreceptors in the outer retina are highly modi-fiable.We tried to explore the underlying mechanisms of this phenomenon by computational model.The results suggested that:(1) the auto-enhancement effect might be induced by the Ca²⁺-dependent process on the post-synaptic AMPA receptors,which could lead to changes of the ionic channel's properties;(2) the asymmetric response to red-and green-flashes and the mutual-chromatic suppression effects might be attributed to the regulatory effects on the pre-synaptic glutamate release. Neurons in the central nervous system(CNS) communicate with each other through both chemical and electrical synapses.We tried to explore the possible roles of electrical synapses in temporal information processing by constructing neural network models.Physiological observations indicate that neurons in the sensory levels of CNS do not respond selectively to the temporal properties of external stimuli.On the other hand,neurons which show selective response to specific temporal properties,especially the duration content,have been reported in the cortex of many species.Temporal information is therefore suggested to be transformed into the spatially distributed neuronal activities in the cortex.Results in our work show that electrical synapse can substantially contribute to the temporal-to-spatial transformation of neuronal activities,and the neuronal activities in such networks can potentially represent the durations of external stimuli.