Abstracts

Although Zn²⁺ is a potent and efficacious biphasic modulator of [alpha][sub]2[/sub]-containing strychnine sensitive glycine receptors (GlyRs), the physiological role of this modulation is unknown. Therefore, we examined whether potentiating Zn²⁺ concentrations enhance GlyR mediated inhibition of neuronal bursting induced by a Mg²⁺ free extracellular solution., Current clamp recordings from cultured embryonic mouse hippocampal neurons were obtained using the whole-cell patch clamp technique. Bursts consisting of 2-10 second depolarizing envelopes with superimposed action potentials were induced by omitting extracellular Mg²⁺ ([ldquo]0[rdquo] mM Mg²⁺). The basal Zn²⁺ concentration in our extracellular solution was 0.8 [mu]M. A nominally Zn²⁺ free extracellular solution ([ldquo]0[rdquo] [mu]M Zn²⁺) was obtained by adding 10 mM tricine, a Zn²⁺ chelator. Ten [mu]M D-serine was added to the extracellular solution to saturate the NMDA receptor glycine binding site. Statistical comparisons were made by ANOVA., In an extracellular solution containing 1 mM Mg²⁺ + [ldquo]0[rdquo] [mu]M or 0.8 [mu]M Zn²⁺, no bursts occurred and action potentials occurred at 0.2 [plusmn] 0.1 Hz (S.E). In [ldquo]0[rdquo] mM Mg²⁺ + 0.8 [mu]M Zn²⁺, 11 [plusmn] 0.8 bursts occurred per minute, and the action potential frequency was 3.5 [plusmn] 0.4 Hz (Figure). Omitting Mg²⁺ and Zn²⁺ from the extracellular solution did not change burst or action potential frequency, which were 12 [plusmn] 0.8 per minute and 2.7 [plusmn] 0.4 Hz, respectively. In [ldquo]0[rdquo] [mu]M Mg²⁺ + 0.8 [mu]M Zn²⁺, 20 [mu]M glycine decreased burst frequency from 11 [plusmn] 1.3 per minute to 1.4 [plusmn] 0.6 per minute (p [lt] 0.05) and decreased action potential frequency from 2.9 [plusmn] 0.8 Hz to 0.3 [plusmn] 0.1 Hz (p [lt] 0.05) (Figure, top panel). In [rdquo]0[rdquo] Mg²⁺ + [ldquo]0[rdquo] [mu]M Zn²⁺, 20 [mu]M glycine decreased burst frequency from 13 [plusmn] 0.8 per minute to 7.8 [plusmn] 0.8 per minute (p [lt] 0.05) and decreased action potential frequency from 2.6 [plusmn] 0.5 Hz to 1.9 [plusmn] 0.5 Hz (p [lt] 0.05) (Figure, bottom panel). Although baseline burst and action potential frequency in 0.8 [mu]M and [ldquo]0[rdquo] [mu]M Zn²⁺ did not differ, 20 [mu]M glycine decreased burst and action potential frequency in 0.8 [mu]M Zn²⁺ more than in [ldquo]0[rdquo] [mu]M Zn²⁺ (p [lt] 0.05). Similar results were obtained with 200 [mu]M taurine. These inhibitory effects of glycine and taurine were blocked by 1 [mu]M strychnine., These results indicate that potentiating Zn²⁺ concentrations enhance GlyR mediated depression of neuronal hyperexcitability in pathological conditions. Thus, designing drugs that modulate GlyRs directly or via Zn²⁺ may lead to novel anticonvulsants.[figure1], (Supported by NIH and Washington University Department of Neurology.)