FLUCTUATIONS IN RELATIVE LEVELS OF CHOLINE-ACETYLTRANSFERASE MESSENGER-RNA IN DIFFERENT REGIONS OF THE RAT BASAL FOREBRAIN ACROSS THE ESTROUS-CYCLE - EFFECTS OF ESTROGEN AND PROGESTERONE

Quantitative in situ hybridization techniques were used to compare rel ative cellular levels of choline acetyltransferase (ChAT) mRNA in diff erent regions of the female rat basal forebrain at different stages of the estrous cycle and at different time points after the administrati on of physiological levels of estrogen and progesterone. Significant f luctuations in relative levels of ChAT mRNA were detected during the c ourse of the estrous cycle. In the medial septum (MS) and striatum, th e highest levels of ChAT mRNA were detected on diestrus 1. Fluctuation s in the nucleus basalis magnocellularis (NBM) were highly variable, w ith the highest levels detected on diestrus 2. In ovariectomized anima ls, significant increases in ChAT mRNA were detected in the MS, NBM, a nd striatum within 1-3 d after a single administration of estradiol. I n addition, the effects of estradiol on ChAT mRNA expression in the NB M and striatum were significantly enhanced by the subsequent administr ation of progesterone. The magnitude and timing of the effects of ster oid replacement were consistent with the magnitude and time course of the fluctuations detected during the course of the estrous cycle. Thes e data demonstrate that estrogen and progesterone can increase basal f orebrain levels of ChAT mRNA significantly in specific regions of the rat basal forebrain, that the magnitude and time course of the effects vary between different subpopulations of cholinergic neurons, and tha t the effects are associated with changes in the functioning of specif ic basal forebrain cholinergic neurons across the estrous cycle.