Plasticity in GABAA receptor subunit mRNA expression by hypothalamic magnocellular neurons in the adult rat

The magnocellular hypothalamic neurons exhibit a substantial degree of structural and functional plasticity over the time of pregnancy, parturition, and lactation. This study has used in situ hybridization techniques to examine whether the content of alpha 1, alpha 2, beta 2, gamma 2 GABAA receptor subunit mRNAs expressed by these cells fluctuates over this period. A process of regional, followed by cellular and then topographical, analyses within the supraoptic (SON) and posterior paraventricular (PVN) nuclei revealed that an increase in magnocellular alpha 1 subunit mRNA content occurred during the course of pregnancy up to day 19, after which a decline in expression was detected on the day of parturition. Significant fluctuations of this nature were observed only in the oxytocin neuron-enriched regions of the SON and PVN. The expression of alpha 2, beta 2, and gamma 2 subunit mRNAs in the SON and PVN and of all subunit mRNAs in the cingulate cortex did not change over this period. During lactation, gamma 2 subunit mRNA content within the PVN increased significantly on day 14 of lactation as compared with day 7, and topographical analysis suggested that it involved principally magnocellular vasopressin neurons. These results demonstrate the cell-and subunit-specific regulation of GABAA receptor mRNA expression within the hypothalamic magnocellular system. In particular, they suggest that fluctuations in alpha 1 subunit expression may contribute to the marked variations in electrical activity exhibited by magnocellular oxytocin neurons at the time of parturition. More generally, they provide evidence in support of GABAA receptor plasticity within a physiological context in the adult rat brain.