DIRECT AND INDIRECT RETINOHYPOTHALAMIC PROJECTIONS TO THE SUPRAOPTIC NUCLEUS IN THE FEMALE ALBINO-RAT

Earlier studies have shown that retinohypothalamic projections termina te extensively within the hypothalamus of the rat. Recently, we identi fied a light retinal projection to the supraoptic nucleus as well as a larger, well-focused projection resulting in a peri-supraoptic nucleu s terminal field. In this study, we employed a double labeling method with cholera toxin conjugated to horseradish peroxidase (CT-HRP) and p seudorabies virus, a transsynaptic neural tracer, to evaluate retinore cipient neurons in both the supraoptic nucleus and peri-supraoptic nuc leus terminal field. In addition, we looked for evidence that cells in the peri-supraoptic nucleus terminal field project into the supraopti c nucleus. Three strains of pseudorabies virus were compared. A direct retinosupraoptic nucleus circuit was confirmed with all three strains . Retinorecipient neurons in the peri-supraoptic nucleus terminal fiel d were also confirmed. However, there was a strain-based difference in the identification of these neurons. The wild-type Becker strain labe led cells in the peri-supraoptic nucleus terminal field in a manner pa ralleling the early, intermediate and late stages of infection of the suprachiasmatic nucleus. This parallel time course suggests that retin al ganglion cells terminate directly on cells in the peri-supraoptic n ucleus terminal field. Conversely, the Bartha and PRV-91 strains showe d appreciable labeling of peri-supraoptic neurons only at long surviva l times. This longer time course suggests that these mutant strains la bel neurons in the peri-supraoptic nucleus terminal field indirectly, after passing through additional neurons. In addition, experiments wit h monocular injection of CT-HRP and posterior pituitary injection of p seudorabies virus showed retrogradely labeled second-order cells in th e peri-supraoptic nucleus amidst the CT-HRP labeled terminal field of the retinohypothalamic tract. These results demonstrate a direct proje ction from the retina to the supraoptic nucleus and provide evidence f or an indirect circuit from the retina to the supraoptic nucleus via n eurons located in the peri-supraoptic nucleus terminal field. The stra in-based differences imply that those retinal ganglion cells that proj ect to the peri-supraoptic nucleus terminal field differ from those th at project to the suprachiasmatic nucleus. In addition, these results suggest a neuroanatomic basis for photic effects on physiological mech anisms that are not mediated by the circadian timing system. (C) 1994 Wiley-Liss, Inc.