Expression of glutamic acid decarboxylase messenger RNA in rat medial preoptic area neurones during the oestrous cycle and after ovariectomy

Abstract

Evidence suggests that medial preoptic area (MPOA) neurones containing γ-aminobutyric acid (GABA) are modulated directly by oestrogen. We have used an alkaline phosphatase-labelled antisense oligonucleotide probe to examine glutamic acid decarboxylase 67 (GAD) mRNA expression within individual cells of the MPOA, diagonal band of Broca (DBB) and parietal cortex in rats killed at noon on each day of the oestrous cycle and after ovariectomy ( n = 4−5). As a fall in extracellular GABA concentrations occurs in the MPOA on the afternoon of proestrus, the GAD 67 mRNA content of cells was also examined in proestrous rats at 15:00h immediately prior to the pre-ovulatory luteinising hormone (LH) surge. The MPOA was found to have an intermediate number of GAD 67 mRNA-containing cells compared with the DBB and cortex ( P < 0.01) but expressed the lowest mean hybridisation signal ( P < 0.01). The parietal cortex had significantly fewer ( P < 0.01) GAD mRNA-containing cells than either the MPOA or DBB but these contained highest mean density of signal ( P < 0.01). The hybridisation signal for GAD mRNA was abolished by either ribonuclease pre-treatment or the use of excess non-labelled probe. No significant ( P > 0.05) differences in GAD 67 mRNA were detected in animals killed at noon throughout the oestrous cycle or after ovariectomy. On the afternoon of proestrus (15:00h) there was a significant 40% reduction in mean GAD 67 mRNA content within cells of only the MPOA compared with noon ( P < 0.05). The numbers of cells in the MPOA expressing GAD 67 mRNA were not significantly different. No change in GAD 67 mRNA signal was detected in diestrous animals killed at 15:00h compared with noon. These results suggest that ovarian steroid hormones do not modulate the preoptic GABA system by altering GAD 67 mRNA content directly. However, the reduction in GAD 67 mRNA content within MPOA neurones on the afternoon of proestrus may be responsible for the fall in preoptic extracellular GABA concentrations implicated in the induction of the LH surge.