Intrinsic synapses in the ventromedial nucleus of the hypothalamus: an ultrastructural study

To test the ideas that neurons intrinsic to a hypothalamic region might participate in steroid hormone effects and that intrinsic synapses might have unusual morphological features, we deafferented the ventromedial nucleus of the hypothalamus (VMH) in ovariectomized female rats with a Halasz knife. Some rats were given estrogen; others were endocrine controls. The main result of this study was that a large number of synapses in VMH survived the circumscribing cut. In fact, they comprised between one-half and two-thirds the number counted in unoperated controls. These were apparently from neuron cell bodies intrinsic to the medial basal hypothalamic island. Their morphology was not clearly different from that expected from unoperated controls or from other brain regions. A significant estrogen effect, an increase in the number of axodendritic synapses per unit area as seen in animals without surgical transections, was not present in the deafferented groups. This suggests that the hormone effect is due to synapses on VMH neurons from cell groups outside the medial basal hypothalamic island. If functional effects of estrogen mediated through VMH do not rely on intrinsic neurons, they must use estrogen-concentrating neurons that project to other brain regions.