CIRCADIAN VARIATION OF EAAC1 GLUTAMATE TRANSPORTER MESSENGER-RNA IN THE RAT SUPRACHIASMATIC NUCLEI

Using in situ hybridization, we examined temporal changes of the EAAC1 glutamate transporter mRNA within the suprachiasmatic nuclei (SCN) of rats in constant darkness. Film autoradiographs showed that the SCN a nd supraoptic nuclei (SON) contained a marked density of hybridization signal. Analysis of silver grains per cell in emulsion-dipped section s indicated that cellular expression of EAAC1 mRNA in the SCN was elev ated during the latter part of the subjective night and at the beginni ng of the subjective day, with a peak at circadian time 23.1 as determ ined by cosinor analysis. The times at which EAAC1 mRNA is highest cor respond to the time points at which extracellular glutamate, a neurotr ansmitter that putatively mediates photic entrainment, has been report ed to be low within the SCN. The presence of EAAC1 mRNA in the SCN and SON may partially explain the resistance of these nuclei to glutamate receptor-mediated excitotoxins; furthermore, the raised level precedi ng subjective dawn in the SCN may ensure sub-toxic levels of extracell ular glutamate at the onset of photic stimulation during the LD cycle. In contrast, cellular expression of EAAC1 mRNA in the cingulate corte x and reticular thalamus remained constant at all time points studied. These results suggest that there is circadian control of the EAAC1 mR NA by the clock intrinsic to the SCN.