Photically responsive neurons in the hypothalamus of a diurnal ground squirrel

The suprachiasmatic nuclei (SCN) function as a circadian pacemaker. Entrainment to the external light-dark cycle is mediated by the retina which gives rise to both direct and indirect projections to the SCN. The hypothalamic targets of the retinohypothalamic tract (RHT) were investigated in thirteen-lined ground squirrels (Spermophilus tridecemlineatus) following WGA-HRP injections. The results indicate that retinal fibers project to the entire SCN. Extracellular single-unit activity was recorded in and near the SCN of thiopental sodium anesthetized squirrels while the eyes were photically stimulated. A small population of hypothalamic cells were responsive to retinal illumination. About half of these cells were activated by light while the others were light-suppressed. The majority of these cells responded in a sustained way to light pulses. Light intensities of at least 1000 lux appeared necessary to induce a sustained response to light. No differences in light responsiveness were observed between visual cells inside and outside the SCN. The visual properties of SCN cells have previously been investigated in hamsters and rats, both nocturnal species. Hypothalamic cells in all 3 species were similar in that they showed predominantly sustained responses to retinal illumination. The diurnal squirrel differed from the other two species in that there was a higher proportion of photically suppressed cells in the squirrel, and in that higher light intensities were required to stimulate photically responsive neurons.