Fig. 3.

Models of DN1 function. (A) Model 1: Distinct signaling molecules generate a peak and trough in SLR and MSDR, respectively. WT males exhibit robust morning and evening activity peaks in SLR as well as precisely timed troughs at dusk in MSDR. Rhythmically released PDF (octagon) produces morning peaks (SLR) directly and synchronizes CLK/CYC activity in DN1s. SM1 (pentagon) and SM2 (square) are produced in and released from DN1s and are necessary to generate evening peaks (SLR) and troughs of courtship activity (MSDR) at dusk, respectively. Cells in red and green indicate high and low levels of CYC/CLK, respectively. Different colors of MSDR traces represent individual males. Enhancing and suppressing functions are indicated by arrows and blocked lines, respectively. The interrupted line indicates extended release of SM2 because of lack of CLK/CYC in DN1s. (B) Model 2: Distinct levels of a single signaling molecule are required to generate the evening peak and trough in SLR and MSDR, respectively. In WT males, tightly controlled release of SM2 acts on two distinct neuronal targets. In the absence of female stimuli, a high concentration of SM2 leads to an increase in locomotor activity at dusk (evening peak), whereas in the presence of female signals, SM2 suppresses directly or indirectly the courtship circuit, leading to the courtship trough. Lack of CLK/CYC cycling leads to extended release of SM2, and hence lower SM2 levels sufficient only to trigger a courtship trough but not an evening peak in SLR.