Abstract
The circadian clock in the mammalian suprachiasmatic nuclei (SCN) expresses 24-h rhythms when isolated in vitro. Numerous studies have demonstrated that recordings of SCN single-unit neuronal activity (SUA), when expressed as a population rhythm, can be used to reliably estimate SCN circadian clock phase in vitro. The main disadvantage of this technique is its laborious nature. Thus, the present experiments were designed to investigate whether in vitro multi-unit neuronal activity (MUA) recordings from the SCN could reliably substitute for SUA recordings. The results show that an MUA rhythm can be recorded from rat SCN for 3 days in vitro but that this rhythm is extremely variable; times of peak MUA in control experiments vary by 7 to 9 h each day. They also show that several serotonergic agents previously shown to consistently advance the SUA rhythm 2 to 3 h when applied during the day induce apparent advances in the MUA rhythm in some experiments; in other cases, however, there appears to be a delay or no change in the phase of the rhythm. Thus, the mean change in time of peak seen after these treatments was an advance of about 1 h. Finally, the results show that glutamate and optic chiasm stimulation applied during early subjective night can induce apparent delays in the MUA rhythm. The results of these experiments were less variable, so that the overall effect was a delay in peak MUA of 2.5 to 3.5 h. Nevertheless, these experiments still exhibited more variability than that generally seen in SUA experiments. Taken together, these results indicate that MUA recordings of the SCN exhibit significantly more variability than do SUA recordings. The extent of this variability leads to the conclusion that, using the techniques and equipment outlined here, MUA recordings are not an adequate substitute for SUA recordings when trying to estimate the phase of the SCN circadian clock.