Reduced inhibition and sensitivity to neurosteroids in hippocampus of mice lacking the GABA(A) receptor delta subunit.
The delta subunit of the gamma-aminobutyric acid (A) receptor (GABA(A)R) is expressed postnatally mostly in the cerebellum, thalamus, and dentate gyrus. Previous studies in mice with a targeted disruption of the delta subunit revealed a considerable attenuation of behavioral responses to neuroactive steroids but not to other neuromodulatory drugs. Here we show that delta subunit loss leads to a concomitant reduction in hippocampal alpha4 subunit levels. These changes were accompanied by faster decay of evoked inhibitory postsynaptic potentials (IPSPs) in dentate granule neurons of -/- mutants (decay tau = 25 ms) compared with +/+ controls (tau = 50 ms). Furthermore, the GABA(A)R-mediated miniature inhibitory postsynaptic currents (mIPSCs) also decayed faster in delta-mutants (tau = 6.3 ms) than controls (tau = 7.2 ms) and had decreased frequency (controls, 10.5 Hz; mutants, 6.6 Hz). Prolongation of mIPSCs by the neuroactive steroid anesthetic, alphaxalone (1-10 microM), was smaller in delta-mutants (at 10 microM, 65% increase) compared with +/+ littermates (308% increase). In competition binding experiments, alphaxalone (0.03-1 microM) modulation of [35S]t-butylbicyclophosphorothionate binding was reduced in delta-mutant brain homogenates, indicating that the decreased alphaxalone effects on mIPSCs were due to changes in the GABA(A)R protein. Faster decay of evoked IPSPs and mIPSCs in delta-mutants suggests presence of the delta subunit at both synaptic and extrasynaptic GABA(A)Rs. Decreased synaptic and extrasynaptic inhibition likely contributes to the pro-epileptic phenotype of delta-mutants. Reduced neurosteroid sensitivity might also contribute to seizure susceptibility. While the simplest explanation is that delta subunit-containing GABA(A)Rs represent the actual target of neurosteroids, it is possible that the behavioral and physiological sensitivity to neuroactive steroids is indirectly altered in the delta -/- mice.
Spigelman
Igor
I
Division of Oral Biology and Medicine, UCLA School of Dentistry, Los Angeles, California 90095-1668, USA. igor@ucla.edu
Li
Zhiwei
Z
Liang
Jing
J
Cagetti
Elisabetta
E
Samzadeh
Sepideh
S
Mihalek
Robert M
RM
Homanics
Gregg E
GE
Olsen
Richard W
RW
D000777:Anesthetics / Q000494:pharmacology; D000818:Animals; D015153:Blotting, Western; D004594:Electrophysiology; D005260:Female; D006624:Hippocampus / Q000187:drug effects / Q000502:physiology*; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D009433:Neural Inhibition / Q000187:drug effects / Q000235:genetics; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D011277:Pregnanediones / Q000494:pharmacology; D011963:Receptors, GABA-A / Q000172:deficiency* / Q000187:drug effects / Q000235:genetics / Q000502:physiology*
D000777:Anesthetics; D011277:Pregnanediones; D011963:Receptors, GABA-A; C006477:alphaxalone