For example in the hippocampus, estradiol potentiates excitatory synaptic transmission and suppresses inhibitory synaptic transmission on a timescale of DPO-1 minutes. Ovarian estrogens are unlikely to be the physiological ligands that activate these acute effects in vivo, as circulating estrogens fluctuate on a timescale of days. An alternative idea is that aromatase within brain areas such as the hippocampus generates local and possibly rapid fluctuations in estrogens that acutely modulate synaptic function. For this to be the case, however, aromatase expressed in the hippocampus must be capable of synthesizing estrogens. Studies using a ribonuclease protection assay demonstrated that there are two forms of aromatase mRNA in the rat brain: a long form and a short form, and suggested that the long form generates the active enzyme. Among the brain regions that were examined, expression of long-form aromatase exhibited regional differences that correlated with aromatase activity, whereas short-form did not. These initial observations were confirmed using an RNA probe that targeted the long form of aromatase specifically. The presence of two forms of aromatase may AM095 explain mismatch in the distribution of aromatase protein in the brain detected by immunohistochemistry and aromatase activity measured by enzyme assays. To investigate the distribution of aromatase in the rat brain, and avoid caveats of antibody specificity, we used quantitative realtime PCR to measure relative mRNA levels specifically of long-form aromatase in the amygdala, BNST, POA, dorsal hippocampus, cingulate cortex, brainstem and cerebellum in both male and female rats. We also evaluated possible hormonal regulation of long-form aromatase in each sex. We were particularly interested to determine whether the female rat hippocampus expresses long-form aromatase, as this has not been shown previously, yet is predicted by the observation that E2 acutely modulates synaptic transmission in the hippocampus of females. The present study investigated relative expression of mRNA for the long form of aromatase in distinct regions of the male and female rat brain and the effect of gonadal/hormonal status on aromatase expression in both sexes.