Glutamic acid decarboxylase 67 (GAD67) gene expression in discrete regions of the rostral preoptic area change during the oestrous cycle and with age

Gamma-aminobutyric acid (GABA), an inhibitory neurotransmitter, is important to the timing and amplitude of the gonadotrophin-releasing hormone (GnRH)-induced luteinizing hormone (LH) surge on pro-oestrus. Data suggest that GABA input in the preoptic area must decrease for a normal LH surge to occur in young rats. We have previously found that ageing alters the timing and amplitude of the LH surge. Therefore, this study focused on changes in GAD₆₇ gene expression, a reflection of GABA synthesis, in two regions of the rostral preoptic area, the organum vasculosum of the lamina terminalis (OVLT) and the anteroventral periventricular nucleus (AVPV) during the oestrous cycle and with age. We assessed the possibility that the expression of GAD₆₇ mRNA in these regions displays time-related and age-dependent changes on pro-oestrus. Our results demonstrate that, with age, overall expression of GAD₆₇ mRNA decreases in the area surrounding the OVLT and in the AVPV. Young rats display a diurnal rhythm in GAD₆₇ mRNA in both regions. GAD₆₇ mRNA expression is high during the early morning hours of pro-oestrus and then declines around the time of the GnRH-induced LH surge. In addition, the diurnal rhythm disappears in the AVPV and is attenuated in the area surrounding the OVLT of middle-aged proestrous rats. These findings suggest that a loss of rhythmicity in GAD₆₇ gene expression and maintenance of inhibitory tone on proestrous afternoon may alter the timing and amplitude of the LH surge, as previously observed in middle-aged rats.