Effect of Spine Density on Excitability in Accumbal Medium Spiny Neurons-A Computational Approach

The nucleus accumbens (NAc), the major subdivision of the ventral striatum plays an important role in the reward pathway. GABAergic Medium Spiny Neurons (MSNs) are the principal cell type of NAc. These neurons receive excitatory synaptic inputs over the numerous spines which are present on their complex dendritic arbours. Alterations in spine density and morphology can affect the integrative properties of MSNs. We developed a biophysically realistic, spiny model of MSN. We found that inclusion of spines in an existing aspiny model changed passive as well as active properties of the cell. The spiny model was tuned to match its properties with that of the earlier aspiny model. We found that a total of 192 inputs from middle and distal dendrites were required to generate a characteristic bimodal behaviour of the membrane potential. Using this model, we investigated the effect of loss of spines on the excitability of the cell. We found that with no spine loss, when only the number of activated inputs was reduced by 15%, spike frequency of the cell reduced to zero, rendering the cell completely inexcitable. However, spine loss of 15% along with 15% reduction in activated synaptic inputs decreased the spike frequency to 1.1 Hz. Our results suggest that when spines are lost along with synaptic inputs, excitability of the cell is not abolished completely, although this might happen when only synaptic inputs are lost. Instead, in such a case the excitability can be increased by slightly enhancing the input connections.