Loop G in the GABA_A receptor α1 subunit influences gating efficacy

KEY POINTS: The functional importance of residues in loop G of the GABA(A) receptor has not been investigated. D43 and T47 in the α1 subunit are of particular significance as their structural modification inhibits activation by GABA. While the T47C substitution had no significant effect, non‐conservative substitution of either residue (D43C or T47R) reduced the apparent potency of GABA. Propofol potentiated maximal GABA‐evoked currents mediated by α1(D43C)β2γ2 and α1(T47R)β2γ2 receptors. Non‐stationary variance analysis revealed a reduction in maximal GABA‐evoked P (open), suggesting impaired agonist efficacy. Further analysis of α1(T47R)β2γ2 receptors revealed that the efficacy of the partial agonist THIP (4,5,6,7‐tetrahydroisoxazolo[5,4‐c]pyridine‐3‐ol) relative to GABA was impaired. GABA‐, THIP‐ and propofol‐evoked currents mediated by α1(T47R)β2γ2 receptors deactivated faster than those mediated by α1β2γ2 receptors, indicating that the mutation impairs agonist‐evoked gating. Spontaneous gating caused by the β2(L285R) mutation was also reduced in α1(T47R)β2(L285R)γ2 compared to α1β2(L285R)γ2 receptors, confirming that α1(T47R) impairs gating independently of agonist activation. ABSTRACT: The modification of cysteine residues (substituted for D43 and T47) by 2‐aminoethyl methanethiosulfonate in the GABA(A) α1 subunit loop G is known to impair activation of α1β2γ2 receptors by GABA and propofol. While the T47C substitution had no significant effect, non‐conservative substitution of either residue (D43C or T47R) reduced the apparent potency of GABA. Propofol (1 μm), which potentiates sub‐maximal but not maximal GABA‐evoked currents mediated by α1β2γ2 receptors, also potentiated maximal currents mediated by α1(D43C)β2γ2 and α1(T47R)β2γ2 receptors. Furthermore, the peak open probabilities of α1(D43C)β2γ2 and α1(T47R)β2γ2 receptors were reduced. The kinetics of macroscopic currents mediated by α1(D43C)β2γ2 and α1(T47R)β2γ2 receptors were characterised by slower desensitisation and faster deactivation. Similar changes in macroscopic current kinetics, together with a slower activation rate, were observed with the loop D α1(F64C) substitution, known to impair both efficacy and agonist binding, and when the partial agonist THIP (4,5,6,7‐tetrahydroisoxazolo[5,4‐c]pyridine‐3‐ol) was used to activate WT or α1(T47R)β2γ2 receptors. Propofol‐evoked currents mediated by α1(T47R)β2γ2 and α1(F64C)β2γ2 receptors also exhibited faster deactivation than their WT counterparts, revealing that these substitutions impair gating through a mechanism independent of orthosteric binding. Spontaneous gating caused by the introduction of the β2(L285R) mutation was also reduced in α1(T47R)β2(L285R)γ2 compared to α1β2(L285R)γ2 receptors, confirming that α1(T47R) impairs gating independently of activation by any agonist. These findings implicate movement of the GABA(A) receptor α1 subunit's β1 strand during agonist‐dependent and spontaneous gating. Immobilisation of the β1 strand may provide a mechanism for the inhibition of gating by inverse agonists such as bicuculline

Mode of action of ICS 205,930, a novel type of potentiator of responses to glycine in rat spinal neurones

1. The effect of a novel potentiator of glycine responses, ICS 205,930, was studied by whole-cell recordings from spinal neurones, and compared with that of other known potentiators, in an attempt to differentiate their sites of action. 2. The ability of ICS 205,930 (0.2 μM) to potentiate glycine responses persisted in the presence of concentrations of Zn(2+) (5–10 μM) that were saturating for the potentiating effect of this ion. 3. Preincubation with 10 μM Zn(2+) before application of glycine plus Zn(2+) had an inhibitory effect, which did not result from Zn(2+) entry into the neurone, since it persisted with either 10 mM internal EGTA or 10 μM internal Zn(2+). To test whether the potentiating effects of ICS 205,930 and Zn(2+) interact, both compounds were applied without preincubation. 4. The potentiating effect of ICS 205,930 was similar for responses to glycine and for responses to glycine plus Zn(2+), provided the concentrations of agonist were adjusted so as to induce control responses of identical amplitudes. 5. ICS 205,930 remained able to potentiate glycine responses in the presence of ethanol (200 mM). 6. ICS 205,930 also retained its potentiating effect in the presence of the anaesthetic propofol (30–90 μM), which strongly potentiated glycine responses but, in contrast with ICS 205,930, also markedly increased the resting conductance. 7. The anticonvulsant chlormethiazole (50–100 μM) neither potentiated glycine responses nor prevented the effect of ICS 205,930, even though it increased the resting conductance and potentiated GABA(A) responses. 8. The mechanism of action of ICS 205,930 appears to be different from those by which Zn(2+), propofol or ethanol potentiate glycine responses

A tyrosine kinase regulates propofol-induced modulation of the beta-subunit of the GABA(A) receptor and release of intracellular calcium in cortical rat neurones

Propofol, an intravenous anaesthetic, has been shown to interact with the beta -subunit of the gamma -amino butyric acid(A) (GABA(A) ) receptor and also to cause changes in [Ca2+ ](i) . The GABA(A) receptor, a suggested target for anaesthetics, is known to be regulated by kinases. We have investigated if tyrosine kinase is involved in the intracellular signal system used by propofol to cause anaesthesia. We used primary cell cultured neurones from newborn rats, pre-incubated with or without a tyrosine kinase inhibitor before propofol stimulation. The effect of propofol on tyrosine phosphorylation and changes in [Ca2+ ](i) were investigated. Propofol (3 mu g mL(-1) , 16.8 mu M) increased intracellular calcium levels by 122 +/- 34% (mean +/- SEM) when applied to neurones in calcium free medium. This rise in [Ca2+ ](i) was lowered by 68% when the cells were pre-incubated with the tyrosine kinase inhibitor herbimycin A before exposure to propofol (P < 0.05). Propofol caused an increase (33 +/- 10%) in tyrosine phosphorylation, with maximum at 120 s, of the beta -subunit of the GABA(A) -receptor. This tyrosine phosphorylation was decreased after pre-treatment with herbimycin A (44 +/- 7%, P < 0.05), and was not affected by the absence of exogenous calcium in the medium. Tyrosine kinase participates in the propofol signalling system by inducing the release of calcium from intracellular stores and by modulating the beta -subunit of the GABA(A) -receptor