Anesthetics disrupt growth cone guidance cue sensing through actions on the GABAA α2 receptor mediated by the immature chloride gradient.

BACKGROUND: General anesthetics (GAs) may exert harmful effects on the developing brain by disrupting neuronal circuit formation. Anesthetics that act on γ-aminobutyric acid (GABA) receptors can interfere with axonal growth cone guidance, a critical process in the assembly of neuronal circuitry. Here we investigate the mechanism by which isoflurane prevents sensing of the repulsive guidance cue, Semaphorin 3A (Sema3A). METHODS: Growth cone sensing was assayed by measuring growth cone collapse in dissociated neocortical cultures exposed to recombinant Sema3A in the presence or absence of isoflurane and/or a panel of reagents with specific actions on components of the GABA receptor and chloride ion systems. RESULTS: Isoflurane exposure prevents Sema3A induced growth cone collapse. A GABAA α2 specific agonist replicates this effect (36.83 ±â€¯3.417% vs 70.82 ±â€¯2.941%, in the Sema3A induced control group, p < 0.0001), but an α1-specific agonist does not. Both a Na-K-Cl cotransporter 1 antagonism (bumetanide, BUM) and a chloride ionophore (IONO) prevent isoflurane from disrupting growth cone sensing of Sema3A. (65.67 ±â€¯3.775% in Iso + BUM group vs 67.45 ±â€¯3.624% in Sema3A induced control group, 65.34 ±â€¯1.678% in Iso + IONO group vs 68.71 ±â€¯2.071% in Sema3A induced control group, no significant difference) (n = 96 growth cones per group). CONCLUSION: Our data suggest that the effects of isoflurane on growth cone sensing are mediated by the α2 subunit of the GABAA receptor and also that they are dependent on the developmental chloride gradient, in which Cl- exhibits a depolarizing effect. These findings provide a rationale for why immature neurons are particularly susceptible to anesthetic toxicity.