GABA(A) receptor epsilon-subunit may confer benzodiazepine insensitivity to the caudal aspect of the nucleus tractus solitarii of the rat

1. Benzodiazepines (BZ) and barbiturates both potentiate chloride currents through GABAA receptors to enhance inhibition. However, unlike barbiturates BZ do not impair autonomic control of heart rate. We hypothesised that BZ might not significantly potentiate GABAergic transmission in the caudal nuc leus of the solitary tract (cNTS), which is critically important for mediat ing the baroreceptor reflex. 2. In rat brain shoes the BZ agonists chlordiazepoxide and midazolam. (2 an d 50 did not significantly enhance currents evoked by GABA in voltage-clamp ed cNTS neurones. Chlordiazepoxide (50 muM) reversibly increased electrical ly evoked IPSPs in 5/10 rostral NTS (rNTS) neurones but only in 2/10 cNTS n eurones. Pentobarbitone (50-100 muM) was effective in enhancing GABAA-media ted responses in all NTS neurones. An inverse BZ agonist, methyl 6,7-dimeth oxy-4-ethyl-beta -carboline-3-carboxylate (DMCM; 1 or 10 muM), failed to de press GABA-induced currents in the cNTS. 3. Microinjections. of midazolam. (10 and 100 mum solutions) into the cNTS did not affect the baroreceptor reflex (P > 0.2) while pentobarbitone (100 muM) significantly and reversibly depressed it (gain decrease to 53 +/- 11 % of control, P < 0.01). 4. Reverse transcriptase polymerase chain reaction revealed the presence of alpha (1), alpha (2), beta (2), beta (3) nd gamma (2) GABAA receptor subun it mRNA in the cNTS. No alternatively spliced variants of the alpha (1)- an d gamma (2)-subunits were. revealed. Moreover, GABA(A) epsilon -subimit mRN A was found in both the cNTS and rNTS as two alternatively spliced transcri pts. 5. Immunocytochemical analysis revealed numerous GABAA epsilon -subunit-pos itive neurones within the cNTS with significantly fewer epsilon -subunit-po sitive cells in the rNTS. 6. As incorporation of the epsilon -subunit in recombinant GABA(A) receptor s may confer BZ insensitivity we propose that the paucity of BZ actions in the cNTS is due to a high level of epsilon -subunit expression. This is the first demonstration of a possible physiological impact of the epsilon -sub unit on native GABAA receptors.