CASTRATION DECREASES SINGLE-CELL LEVELS OF MESSENGER-RNA ENCODING GLUTAMIC-ACID DECARBOXYLASE IN THE DIAGONAL BAND OF BROCA AND THE SEXUALLY DIMORPHIC NUCLEUS OF THE PREOPTIC AREA

Using quantitative in situ hybridization histochemistry (ISHH), we det ermined the effect of castration on single cell levels of glutamic aci d decarboxylase (GAD) mRNA in discrete hypothalamic regions of the mal e rat brain associated with the control of gonadotropin secretion, A 4 8-base oligodeoxynucleotide probe was used to detect with equal affini ty the two isoforms of GAD message, GAD(65) and GAD(67), GAD message a lso was quantitated in a number of selected areas of the brain to cont rast GAD gene expression amongst several populations of GABAergic neur ons, Comparison of 11 brain regions demonstrated a 9.3-fold range in t he quantity of single cell GAD mRNA with levels being highest. in the amygdala and the diagonal band of Broca, moderate in the piriform cort ex, caudate nucleus, substantia innominata, globus pallidus, cingulate cortex and medial septal nucleus, and lowest in the lateral septal nu cleus and the medial preoptic nucleus (MPN), Castration markedly reduc ed single cell GAD mRNA levels in the DBB and the MPN, two discrete hy pothalamic structures known to contain dendritic fields, cell bodies, and axons of GnRH neurons projecting to the median eminence, A strikin g finding was a dense core of steroid-sensitive GABAergic neurons with in the MPN comprising the sexually dimorphic nucleus of the preoptic a rea (SDN-POA), Similar to the MPN as a whole, the amount of GAD mRNA e xpressed by cells in the SDN-POA of sham operated control rats was gre ater than in castrated animals, GAD mRNA levels were inversely related to serum LH titers, suggesting a role for these neurons in the mechan ism controlling gonadal steroid negative feedback on LH secretion, Thi s report provides the basis for future work to determine if GAD(65), G AD(67) or whether both isoforms are affected by gonadal steroid input.