Orchidectomy does not significantly affect spine synapse density in the CA3 hippocampal subfield in St. Kitts vervet monkeys (Chlorocebus aethiops sabaeus)

- Effects of orchidectomy on hippocampal synapses in CA3 and the dentate gyrus were examined in St. Kitts vervet monkeys.
- Orchidectomy significantly reduced spine synapses in the dentate gyrus, but not in CA3.
- Compensatory increases of BDNF in mossy fibers may limit synaptic loss in CA3 after orchidectomy.

Gonadal hormones induce significant changes in cognitive function, associated with alterations in the structure of the hippocampus. We have previously shown that androgens increase the number of spine synapses in the CA1 stratum radiatum of the monkey hippocampus. Recent evidence, however, suggests that loss of testicular hormone production may have variable effects on neuroplasticity in different regions of the hippocampus. To test this hypothesis, we examined the effects of orchidectomy in the dentate gyrus and CA3 subfield of the hippocampus in male St. Kitts vervet monkeys (Chlorocebus aethiops sabaeus). Spine synapse density was significantly reduced (39%) after orchidectomy in the dentate gyrus, consistent with previously published reports in CA1 (40%). However, in CA3 orchidectomy induced a much smaller (22%) reduction in synapse density, which did not reach the limits of statistical significance. These results suggest that orchidectomy exerts heterogeneous effects on hippocampal spine synapse density, the CA3 subfield being relatively spared compared to CA1 and the dentate gyrus. This heterogeneity may contribute to the mixed functional responses observed in males following loss of testicular hormone secretions.