Studies On Aging-related Neuronal Function Degradation And Axon Initial Segment Plasticity In The Primary Visual Cortex Of Rats

Aging causes serious decline of many visual functions,including identification of visual signal's orientation and motion direction,detection of object moving velocity and luminance contrast,and etc.Numerous research reports show that this aging-related visual capacity deterioration is mainly attributed to an improved neuronal excitation in the aged visual cortex,which lead to functional degradation in visual signal detection.Some studies in recent years indicate that aging-related functional decline of visual cortical neurons is related to reduction of intracortical inhibition,especially GABAergic inhibition.However,evidences from recent studies on the expression of GABAergic molecular markers during aging are controversial,and whether this age-dependent neuronal function degeneration is coupled with GABAergic inhibition alterations is not confirmed.On the other hand,recent investigations show that the structure of axon initial segment?AIS?of neurons exhibits high plasticity in response to neuronal activity changes,so as to maintain stability of neuronal excitation.Whether aging-dependent enhancement of neuronal excitation will trigger AIS's structural changes is unknown.To answer these questions,we carried out a systematic study on senescent and young adult rats using combined techniques of in vivo electrophysiological recording,fluorescent double-labeling and Western blot.Firstly,the V1 neurons'spontaneous activity and visually-evoked response to different orientations and motion directions were compared so as to determine the effects of aging on neuronal response properties and functions;Secondly,the expression level of GABAergic markers in the V1 of young adult and old rats were examined in order to evaluate the impacts of senescence on GABAergic inhibition;Finally,the AIS length and the distribution of Nav1.6,a type of Na⁺channels with specific location along AIS and a key role in action potential?AP?initiation,of V1 neurons were measured and compared so as to assess plastic changes of AISs evoked by an increased neuronal excitation during senescence.Our results found that:?1?The mean spontaneous activity and visually evoked response of V1 neurons in the old group of rats was significantly higher than that in young adult rats,and the signal-to-noise ratio of V1 neurons was significantly lower in old rats relative to young ones,which demonstrated that aging caused a significantly enhanced neuronal excitation in the V1 area.Additionally,the increased amplitude of neuronal response to stimuli with non-preferred orientation and motion directions in old rats was larger than that to the preferred orientation and motion direction,which resulted in reducing response selectivity to visual stimuli in old rats relative to young adult rats.This result demonstrates that V1 neurons in old individuals have a weaker ability to distinguish between different visual signals.?2?Compared with that in young adult rats,the proportion of GABAergic neurons in each cortical layer of V1 in old rats was significantly reduced,and concurrently the content of GAD67,a key GABA synthesizing enzyme,in the V1 in old rats was significantly lowered as shown by an evident weakening of immunofluorescence and a reduced protein expression of GAD67,although the immunofluorescence intensity and protein expression of GAD65,another GABA synthesizing enzyme,showed no significant or marginal difference between the two age groups.In addition,the immunofluorescence intensity and protein expression level of GABA_AR?₁,an important GABA_A receptor subunit,in the V1 of old rats were also significantly decreased relative to young adult rats.These results indicate that aging may result in reduction of GABAergic inhibition.?3?Accompanied with the increased neuronal excitation in the V1 of old rats,the mean AIS length of neurons in II-III and V cortical layer was significantly shortened and the expression of Nav1.6 protein was significantly reduced in old rats relative to young rats.Further,the mean fluorescence intensity of Nav1.6,especially Nav1.6fluorescence at the proximal region of AISs,was specifically lowered in old rats compared with that in young rats,suggesting that the proportion of AIS with Na⁺distribution or,in other words,the number of Na⁺channels in old rats was evidently reduced during aging process.This will cause a fewer Na⁺entry during AP initiation in V1 neurons,reduce ATP consumption by Na⁺-K⁺ion pumps during AP repolarization,and thus compensate over energy expenditure caused by elevated discharging rate of V1 neurons during senescence.In conclusion,this study found that aging caused V1 neurons of rats a significantly enhanced excitation and significant function decline in visual signal detection,and this aging-related neuronal function degradation was accompanied by a reduced expression of GABAergic markers,which proved that an intracortical decrease of inhibitory GABAergic neurotransmitter system could be an important factor that underlie functional deterioration of cortical neurons.Additionally,the present results also indicated that aging-related neuronal function degeneration also elicited adaptative compensatory mechanism through AIS structural plasticity to maintain the balance between energy input and ouput.