Medial preoptic islands in the rat brain: electron microscopic evidence for intrinsic synapses

Neurons intrinsic to the preoptic area might participate in the control of neuroendocrine or behavioral events. To determine their existence and features, we deafferented the preoptic area of female rats, using completely circumscribing cuts with a Halasz knife. Despite obvious signs of degeneration of synapses originating from nerve cell bodies outside the preoptic island, some synapses survived complete deafferentation. We saw synaptic contacts not only on the neuronal cell body, but also on the dendritic shaft and spine. There were no peculiar morphological features, as might suggest unique physiologic functions of these intrinsic synapses. The prominence of intrinsic synapses in the preoptic area suggests that, in addition to hormone effects on preoptic neurons, and long ascending afferents, intrinsic synapses might play significant roles in neuroendocrine controls.