Hypothalamic circuits and circadian rhythms: effects of knife cuts vary with their placement within the suprachiasmatic area.

The suprachiasmatic nucleus (SCN) of the hypothalamus and its efferent projections are necessary for the generation of circadian rhythms. To further investigate the role of SCN connections in the generation of behavioral rhythms, intact and blinded-castrated male rats housed in constant conditions were given horizontal knife cuts aimed dorsal to the SCN, or sham surgery. Rhythms of locomotor activity and drinking behavior were monitored using a microcomputer. Cuts that spared the SCN failed to abolish rhythms. Effects of cuts that damaged the SCN ranged from changes in period length of the rhythms to abolition of drinking rhythms. These and previous results indicate that connections between the SCN and the paraventricular nucleus of the hypothalamus are not necessary for the expression of behavioral rhythms.