Functional Characterization of GABA sub(A) Receptors in Neonatal Hypothalamic Brain Slice

The hypothalamus influences a number of autonomic functions. The activity of hypothalamic neurons is modulated in part by release of the inhibitory neurotransmitter GABA onto these neurons. GABA sub(A) receptors are formed from a number of distinct subunits, designated alpha , beta , gamma , delta , epsilon , and theta , many of which have multiple isoforms. Little data exist, however, on the functional characteristics of the GABA sub(A) receptors present on hypothalamic neurons. To gain insight into which GABA sub(A) receptor subunits are functionally expressed in the hypothalamus, we used an array of pharmacologic assessments. Whole cell recordings were made from thin hypothalamic slices obtained from 1- to 14-day-old rats. GABA sub(A) receptor-mediated currents were detected in all neurons tested and had an average EC sub(50) of 20 plus or minus 1.6 mu M. Hypothalamic GABA sub(A) receptors were modulated by diazepam (EC sub(50) = 0.060 mu M), zolpidem (EC sub(50) = 0.19 mu M), loreclezole (EC sub(50) = 4.4 mu M), methyl-6,7-dimethoxy-4-ethyl- beta -carboline (EC sub(50) = 7.7 mu M), and 5 alpha -pregnan-3 alpha -hydroxy-20-one (3 alpha -OH-DHP). Conversely, these receptors were inhibited by Zn super(2+) (IC sub(50) = 70.5 mu M), dehydroepiandrosterone sulfate (IC sub(50) = 16.7 mu M), and picrotoxin (IC sub(50) = 2.6 mu M). The alpha 4/6-selective antagonist furosemide (10-1,000 mu M) was ineffective in all hypothalamic neurons tested. The results of our pharmacological analysis suggest that hypothalamic neurons express functional GABA sub(A) receptor subtypes that incorporate alpha 1 and/or alpha 2 subunits, beta 2 and/or beta 3 subunits, and the gamma 2 subunit. Our results suggest receptors expressing alpha 3- alpha 6, beta 1, gamma 1, and delta , if present, represent a minor component of functional hypothalamic GABA sub(A) receptors.