Responses of the circadian system of rats to conditioned and unconditionedstimuli

The circadian systems of rodents respond to light pulses presented during t he subjective night with phase shifts and altered cellular activity in the suprachiasmatic nuclei (SCN), including expression of immediate-early genes (IEGs) such as c-fos. A recent study showed that a nonphotic stimulus tan air disturbance generated by a fan) that does not normally induce the expre ssion of c-fos-like immunoreactivity in the SCN of rats can be made to do s o after being paired repeatedly with a light pulse in a Pavlovian condition ing paradigm. Furthermore, after conditioning (but not after noncontingent exposure to these stimuli), the fan also induced phase shifts in activity a nd body temperature rhythms comparable to those produced by light. The auth ors performed three experiments designed to replicate and extend these find ings in rats. In experiment 1, rats were tested for conditioning effects of repeated pairings of a light pulse with a neutral air disturbance under a full photoperiod. In experiment 2, a modified conditioning paradigm was use d in which a skeleton photoperiod served as both the entraining zeitgeber a nd the unconditioned stimulus. Animals in the paired and unpaired training conditions were exposed to both the light pulse and the air disturbance, bu t the air disturbance signaled the onset of light in the paired condition o nly. Phase shifts of wheel-running activity rhythms and gene expression in the SCN, intergeniculate leaflet, and paraventricular nucleus of the thalam us were assessed in animals following either of the training conditions or the control procedures. Experiment 3 assessed whether the air disturbance c ould entrain the circadian activity rhythms of rats with or without previou s pairing with light in a classical conditioning paradigm. No evidence for classical conditioning, nor for unconditioned effects of the air disturbanc e on the circadian system, was found in these studies.