Lanthanum‐mediated modification of GABA_A receptor deactivation, desensitization and inhibitory synaptic currents in rat cerebellar neurons

Abstract

We investigated La3+ effects on recombinant and native γ‐aminobutyric acid A (GABAA) receptors using rapid agonist applications and on inhibitory synaptic currents (IPSCs) in granule and stellate neurons of rat cerebellar slices. Rapid desensitization of currents elicited by 200 ms pulses of 1 mM GABA to small lifted cells transfected with α1β3γ2 cDNAs was greatly decreased by the coapplication of 100 μm LaCl3. GABA responses were unaffected when coapplication lasted only 2 ms. In contrast, with LaCl3 pre‐perfusion, a significant slowing of deactivation in response to 2 ms applications was observed. LaCl3 pre‐perfusion also prolonged the duration of responses to 20 mM taurine. Outside‐out patches excised from cells transfected with α1β3γ2 subunit cDNAs were briefly exposed to a saturating concentration of GABA, eliciting a transient activation of single channel currents with a main conductance of 30 pS. Opening and burst durations increased by pre‐equilibration of patches with LaCl3. LaCl3 depressed the peak amplitude without affecting the slow deactivation and desensitization of GABA responses in cells transfected with α6β3γ2 and α6β3δ cDNAs. No significant difference in La3+ modulation of GABA‐gated currents was observed between α1β3γ2 and α1β3δ receptors. The effects of LaCl3 on deactivation and desensitization of GABA responses observed in nucleated patches excised from rat cerebellar granule and stellate neurons were comparable to those in the cells transfected with α1β3γ2 cDNAs. In addition, La3+ clearly prolonged the spontaneous IPSC time course without changing the amplitude. Our results indicate that La3+ has a dual action on GABA‐gated currents: it decreases desensitization and increases channel opening duration. These actions depend on receptor subunit composition and contribute to the prolongation of IPSCs.

Short-term steroid treatment increases δ GABAA receptor subunit expression in rat CA1 hippocampus: Pharmacological and behavioral effects

“…In addition, the maximal THIP-gated current was greater than the maximal GABA-gated current following 48 h steroid treatment, another unique pharmacological characteristic of α4[6]βδ GABAR (Brown et al, 2002) but not other GABAR subunit combinations, where THIP is a partial GABA agonist. In addition, La 3+ produced a nearly complete inhibition of THIP-generated current following steroid treatment, which is also characteristic of α4[6]βδ GABAR (Brown et al, 2002;Saxena et al, 1997;Zhu et al, 1998). Although La 3+ effects are not known at all GABAR subtypes, potent inhibition by La 3+ is not observed at α1βδ, α1βγ2 or α4βγ2 subtypes (Brown et al, 2002;Saxena et al, 1997;Zhu et al, 1998), and thus allows us to partially distinguish between these receptor subtypes.…”

“…In addition, La 3+ produced a nearly complete inhibition of THIP-generated current following steroid treatment, which is also characteristic of α4[6]βδ GABAR (Brown et al, 2002;Saxena et al, 1997;Zhu et al, 1998). Although La 3+ effects are not known at all GABAR subtypes, potent inhibition by La 3+ is not observed at α1βδ, α1βγ2 or α4βγ2 subtypes (Brown et al, 2002;Saxena et al, 1997;Zhu et al, 1998), and thus allows us to partially distinguish between these receptor subtypes. We have reported (Sundstrom-Poromaa et al, 2002) similar pharmacological characteristics of CA1 hippocampal pyramidal cells after withdrawal from P, when α4βδ GABAR expression is increased in hippocampus.…”

“…A lower concentration of THIP (5 μM) produced a 70-80 pA current following steroid treatment, while the THIP-generated current recorded under control conditions was barely detectable. La 3+ blocks current gated by α4[6]βδ GABAR (Brown et al, 2002;Saxena et al, 1997;Zhu et al, 1998). Therefore, the level of La 3+ -sensitive THIP-evoked current would be an estimate of current generated by α4βδ GABAR in the slice preparation.…”

“…For instance, the GABA partial agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) or gaboxadol generates a current greater than maximum concentrations of GABA only at α4 [6]βδ GABAR (Brown et al, 2002) but not at other αβγ2 subtypes (Ebert et al, 1997). In addition, the trivalent cation lanthanum (La 3+ ) is a potent inhibitor of current gated by α4[α6]βδ GABAR (Brown et al, 2002;Saxena et al, 1997;Zhu et al, 1998), while it produces minimal inhibition of α4 [6]βγ2 GABAR, and potentiates α1βγ2 and α1βδ GABAR (Saxena et al, 1997;Zhu et al, 1998).…”

Ethanol effects on GABA-gated current in a model of increased α4βδ GABAA receptor expression depend on time course and preexposure to low concentrations of the drug

“…These findings are characteristic of increased expression of α4βδ GABAR, for which gaboxadol produces a greater maximal current than GABA and which are nearly completely inhibited by La 3+ (Brown et al, 2002). In contrast, most other receptors are either unaffected or potentiated by La 3+ (Saxena et al, 1997;Zhu et al, 1998). The increased response of native GABAR to 1-3 mM ethanol after short-term steroid treatment in the present study is nearly identical to what we have observed in recombinant α4β2δ receptors as well as following steroid withdrawal (Sundstrom-Poromaa et al, 2002), when hippocampal α4βδ GABAR expression is increased.…”