Increased numerical density of synapses in CA3 region of hippocampus and molecular layer of motor cortex after self-stimulation rewarding experience.

Self-stimulation has been considered as an intensely rewarding behavioural experience, being perhaps even more influential than feeding or sexual behaviour. Our earlier studies have demonstrated a self-stimulation rewarding experience-induced increase in dendritic branching points, intersections and spine densities in CA3 hippocampal and layer V motor cortical pyramidal neurons. In the present study, we report self-stimulation-induced alterations in the numerical density of synapses in the hippocampus and motor cortex. A self-stimulation experience was provided 1 h daily for a period of 10 days through bipolar electrodes, implanted bilaterally in the lateral hypothalamus and substantia nigra-ventral tegmental area, stereotaxically. The results revealed a significant (P < 0.001) increase in the number of synapses in the CA3 region of hippocampus and the molecular layer of the motor cortex in self-stimulation-experienced rats. The increased synaptic number may be due to the activation of afferent pathways to the hippocampus and motor cortex following self-stimulation, which may lead to the induction of long-term potentiation. Long-term potentiation is known to cause structural changes by strengthening the existing synapses or resulting in the formation of new synapses. These changes may be related to the improved cognitive functions observed in self-stimulation-experienced rats.