Functional characterization of GABA_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_A receptors are formed from a number of distinct subunits, designated α, β, γ, δ, ε, and θ, many of which have multiple isoforms. Little data exist, however, on the functional characteristics of the GABA_A, receptors present on hypothalamic neurons. To gain insight into which GABA_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_A receptor-mediated currents were detected in all neurons tested and had an average EC₅₀ of 20 ± 1.6 μM. Hypothalamic GABA_A receptors were modulated by diazepam (EC₅₀ = 0.060 μM), zolpidem (EC₅₀ = 0.19 μM), loreclezole (EC₅₀ = 4.4 μM), methyl-6,7-dimethoxy-4-ethyl-β-carboline (EC₅₀ = 7.7 μM), and 5α-pregnan-3α-hydroxy-20-one (3α-OH-DHP). Conversely, these receptors were inhibited by Zn²⁺ (IC₅₀ = 70.5 μM), dehydroepiandrosterone sulfate (IC₅₀ = 16.7 μM), and picrotoxin (IC₅₀ = 2.6 μM). The α4/6-selective antagonist furosemide (10-1,000 μM) was ineffective in all hypothalamic neurons tested. The results of our pharmacological analysis suggest that hypothalamic neurons express functional GABA_A receptor subtypes that incorporate al and/or α2 subunits, β2 and/or β3 subunits, and the γ2 subunit. Our results suggest receptors expressing α3-α6, β1, γ1, and δ, if present, represent a minor component of functional hypothalamic GABA_A receptors.