Zinc modulation of ionic currents in the horizontal limb of the diagonal band of Broca.
We examined modulation of ionic currents by Zn2+ in acutely dissociated neurons from the rat's horizontal limb of the diagonal band of Broca using the whole-cell patch-clamp technique. Application of 50 microM Zn2+ increased the peak amplitude of the transiently activated potassium current, I(A) (at + 30 mV), from 2.20+/-0.08 to 2.57+/-0.11 nA (n = 27). This response was reversible and could be repeated in 0 Ca2+/1 microM tetrodotoxin (n = 15). Zn2+ shifted the inactivation curve to the right, resulting in a shift in the half-inactivation voltage from 76.4+/-2.2 to -53.4+/-2.0 mV (n = 11), with no effect on the voltage dependence of activation gating (n = 15). There was no significant difference in the time to peak under control conditions (7.43+/-0.35 ms, n = 14) and in the presence of Zn2+ (8.20+/-0.57 ms, n = 14). Similarly, the time constant of decay of I(A) (tau(d)) at + 30 mV showed no difference (control: 38.68+/-3.68 ms, n = 15; Zn2+: 38.48+/-2.85 ms, n = 15). I(A) was blocked by 0.5-1 mM 4-aminopyridine. In contrast to its effects on I(A), Zn2+ reduced the amplitude of the delayed rectifier potassium current (I(K)). The reduction of outward K+ currents was reproducible when cells were perfused with 1 microM tetrodotoxin in a 0 Ca2+ external solution. The amplitude of the steady-state outward currents at +30 mV under these conditions was reduced from 6.40+/-0.23 (control) to 5.76+/-0.18 nA in the presence of Zn2+ (n = 16). The amplitudes of peak sodium currents (INa) were not significantly influenced (n = 10), whereas barium currents (I(Ba)) passing through calcium channels were potently modulated. Zn2+ reversibly reduced I(Ba) at -10 mV by approximately 85% from -2.06+/-0.14 nA under control conditions to -0.30+/-0.10 nA in the presence of Zn2+ (n = 14). Further analyses of Zn2+ effects on specific calcium channels reveals that it suppresses all types of high-voltage-activated Ca2+ currents. Under current-clamp conditions, application of Zn2+ resulted in an increase in excitability and loss of accommodation (n = 13), which appears to be mediated through its effects on Ca2+-dependent conductances.
Easaw
J C
JC
Department of Medicine (Neurology), University of Alberta, Edmonton, Canada.
Jassar
B S
BS
Harris
K H
KH
Jhamandas
J H
JH
D015761:4-Aminopyridine / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002413:Cations, Divalent / Q000494:pharmacology; D020667:Diagonal Band of Broca / Q000187:drug effects / Q000502:physiology*; D005071:Evoked Potentials / Q000187:drug effects*; D066298:In Vitro Techniques; D008297:Male; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D009553:Nimodipine / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000378:metabolism; D013779:Tetrodotoxin / Q000494:pharmacology; D015032:Zinc / Q000494:pharmacology*; D020960:omega-Conotoxins / Q000494:pharmacology
D002121:Calcium Channel Blockers; D002413:Cations, Divalent; D015221:Potassium Channels; D020960:omega-Conotoxins; D013779:Tetrodotoxin; D009553:Nimodipine; D012964:Sodium; D015761:4-Aminopyridine; D015032:Zinc; D002118:Calcium