Gonadal hormones are responsible for maintaining the integrity of spine synapses in the CA1 hippocampal subfield of female nonhuman primates.

It is well established that gonadal hormonal manipulation results in morphologic changes in the rat hippocampus. The great similarities in the hippocampal formation between nonhuman primates and humans, as well as the differences in this structure between humans and rats, led to this investigation of whether hormonal manipulation in female subhuman primates influences pyramidal cell spine density in the CA1 hippocampal subfield, as it does in rats. African green monkeys (Cercopithecus aethiops sabaeus) were ovariectomized, and half of the animals received estrogen replacement therapy. One month later, the monkeys were killed. In the first group of experiments, pyramidal cell spines were analyzed on Golgi-impregnated material taken from the CA1 hippocampal subfield. In the second experiment, unbiased electron microscopic stereologic calculations were performed to estimate the volumetric density of spine synapses in the same hippocampal subfield. Analysis of the Golgi-impregnated material showed that the spine density of CA1 pyramidal cells is much lower in the ovariectomized animals than in ovariectomized and estrogen-replaced monkeys. The unbiased, electron microscopic, stereologic calculation confirmed the light microscopic observation. The volumetric density (number of spine synapses/microm(3)) of spine synapses was significantly lower (43.33%) in the ovariectomized animals than in ovariectomized and estrogen-replaced monkeys. Because the hippocampus is involved in specific mnemonic functions, this observation highlights the importance of hormone replacement therapy in postmenopausal conditions.