Figure 3.

Stimulus- and time-of-day specific phase-shifts of SCN neuronal activity are accompanied by stimulus- and time-of-day-specific peptide release. Horizontal suprachiasmatic brain slices of the mediobasal hypothalamus and preserving the ONs, which contain retinal afferents to the SCN, were monitored in vitro. (A) The SCN displays a circadian rhythm in neuronal firing rate that peaks at mid-subjective day, CT 6. ONs were stimulated bilaterally via suction electrodes with specific current, frequency, and pulse duration parameters at circadian time points (arrowheads). During daytime (CT 6) and late subjective nighttime (CT 19), ON stimulation evokes phase advance of the neuronal activity rhythm (B, D, and E), while effective parameters at early subjective nighttime (CT 14) trigger phase delay of SCN firing rhythm (C). MALDI-TOF-MS analyses of releasate collected from the SCN exhibit time-of-day and stimulus-specific peptide release profiles (H-L). Some peptide peaks, such as arginine vasopressin (a) and galanin (f), are observed at every timepoint, while release of other peptides, such as neurokinin-B (b), somatostatin-28 (c), angiotensin (d), and little SAAS (e) is restricted to specific time-of-day and stimulus conditions. Effective parameters at CT 19 are ineffective in phase shift (F) and peptide release (M) at CT 14. Transectional ON crush prevents stimulus-evoked phase shifting (G)and peptide release (N). Phase-shifting responses to ON stimulation depend upon unique, time-of-day-effective stimulus parameters (O). **, p < 0.001, one-way ANOVA.