REMOVAL OF THE OLFACTORY BULBS DELAYS PHOTIC REENTRAINMENT OF CIRCADIAN ACTIVITY RHYTHMS AND MODIFIES THE REPRODUCTIVE AXIS IN MALE OCTODON-DEGUS

The diurnal rodent, Octodon degus, exhibits robust sex differences in several circadian measures, including circadian period (tau) and reent rainment rates to photic and nonphotic (social) zeitgebers. The neural substrates underlying such physiological differences remain unknown. In female degus, olfactory bulbectomies (BX) inhibit socially-facilita ted reentrainment, but do not alter photic reentrainment, entrained me asures, or tau in constant darkness (DD). This experiment investigated the effects of EX in male degus on (i) photic reentrainment rates of circadian rhythms following a 6-h phase advance of the Light-dark (LD) cycle; (ii) photic entrainment; (iii) tau of free-running activity rh ythms in DD; and (iv) body weight, paired testis weight, and the repro ductive hormones, testosterone, androstenedione and follicle stimulati ng hormone (FSH). EX significantly delayed photic reentrainment rates. They did not, however, modify tau, the phase of activity onset or off set, amplitude or duration (alpha) of the activity rhythm, mean daily locomotor activity levels, or body weight. FSH, testosterone and andro stenedione were unaffected by EX, whereas paired testis weights were s ignificantly greater in EX degus compared with shams. Thus, the olfact ory bulbs influence photic reentrainment of circadian rhythms and mode stly affect the reproductive axis in male degus. Our results suggest t hat the olfactory bulbs may be a neural source of observed sex differe nces in photic reentrainment in degus, and highlight interspecies vari ation in the olfactory bulbs' effects on entrained and free-running ci rcadian rhythms and on reproduction. (C) 1998 Elsevier Science B.V.