IN-VITRO CIRCADIAN-RHYTHMS OF THE MAMMALIAN SUPRACHIASMATIC NUCLEI - COMPARISON OF MULTIUNIT AND SINGLE-UNIT NEURONAL-ACTIVITY RECORDINGS

The circadian clock in the mammalian suprachiasmatic nuclei (SCN) expr esses 24-h rhythms when isolated in vitro. Numerous studies have demon strated that recordings of SCN single-unit neuronal activity (SUA), wh en expressed as a population rhythm, can be used to reliably estimate SCN circadian clock phase in vitro. The main disadvantage of this tech nique is its laborious nature. Thus, the present experiments were desi gned 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 s how that several serotonergic agents previously shown to consistently advance the SUA rhythm 2 to 3 h when applied during the day induce app arent 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 treatme nts was an advance of about 1 h. Finally, the results show that glutam ate and optic chasm stimulation applied during early subjective night can induce apparent delays in the MUA rhythm. The results of these exp eriments 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 exhib ited more variability than that generally seen in SUA experiments. Tak en together, these results indicate that MUA recordings of the SCN exh ibit significantly more variability than do SUA recordings. The extent of this variability leads to the conclusion that, using the technique s and equipment outlined here, MUA recordings are not an adequate subs titute for SUA recordings when trying to estimate the phase of the SCN circadian clock.