Collateral input to the paraventricular and supraoptic nuclei in rat. I. Afferents from the subfornical organ and the anteroventral third ventricle region.

Injections of two fluorescent retrograde tracers were used to investigate the existence of collateral branching of input to the hypothalamic magnocellular neuroendocrine neurons. Injection of one tracer (either Fluoro-Gold or rhodamine-labeled microspheres) into the supraoptic nucleus and the other tracer into the ipsilateral paraventricular nucleus produced labeled neurons within the subfornical organ and the anteroventral third ventricle area. Some labeled cells were found to contain both fluorescent tracers (double-labeled cells), suggesting that they project to both the paraventricular and supraoptic nuclei via branching axons. Most double-labeled cells were found within the subfornical organ. Fewer of these cells were located within the nucleus medianus preopticus, and still fewer were distributed in the organum vasculosum lamina terminalis, the bed nucleus of the stria terminalis, and the medial and the lateral preoptic areas. These data present the first direct evidence that single cells may provide input to more than one magnocellular neuroendocrine nucleus. Hypothetically, hormonal release would require coordinated firing of many magnocellular cells. Thus, the branched input to these neurons may assist in the organization and the timely activation of this system in response to physiological stimuli.