Pharmacological identification of the K(+) currents mediating the hypoglycemic hyperpolarization of rat midbrain dopaminergic neurones.
Hypoglycemia (zero glucose) initially depolarized the membrane and increased the spontaneous firing of rat midbrain dopaminergic neurones (more than 50%) intracellularly recorded in an in vitro slice preparation. Under single-electrode voltage-clamp mode (V(h) -55 mV), this transient phase correlated with an inward current of -18 pA. In all the cells tested (n=30), an inhibition fully developed over 16.9 min of hypoglycemia and was associated with a hyperpolarization of the membrane (7.7 mV) or outward current (95.6 pA). Upon re-application of a control solution (glucose 10 mM) a rebound hyperpolarization/outward current developed. The depression of firing was only seen when the artificial cerebrospinal fluid (ACSF) contained less than 1 mM glucose. In addition, the period of time required to block the spontaneous activity decreased, by diminishing the extracellular concentration of glucose from 1 to 0 mM. The hypoglycemia-induced outward current was associated with an increase in membrane conductance and reversed polarity at -100.4 mV, close to the reversal potential of K(+). The post-hypoglycemic outward current was not associated with an increase in membrane conductance and did not reverse. The K(+)-ATP channel blockers, tolbutamide (300 microM-1 mM) and glibenclamide (3-30 microM) reduced the hypoglycemia-induced inhibition. In addition, the blocker of the Ca(++)-activated K(+)-channels, charybdotoxin (100-400 nM) partially counteracted the hypoglycemic hyperpolarization. Furthermore, barium (100-300 microM) fully antagonized the hypoglycemia-induced inhibition. The post-hypoglycemic hyperpolarization/outward current was not observed in cells treated with the Na(+)/K(+) ATPase pump inhibitor strophanthidin (1-3 microM). Our data suggest that midbrain dopaminergic cells respond to glucose deprivation with a hyperpolarization generated by the opening of several K(+) channels (sulphonylurea-sensitive, charybdotoxin-sensitive and sulphonylurea and charybdotoxin-insensitive) and by the activation of the Na(+)/K(+) ATPase pump after the hypoglycemic period.
Marinelli
S
S
IRCCS, Clinica Santa Lucia, via Ardeatina 306, 00179 Roma and Clinica Neurologica, Universita' di Roma "Tor Vergata", via O. Raimondo 8, 00173, Rome, Italy.
Bernardi
G
G
Giacomini
P
P
Mercuri
N B
NB
D000818:Animals; D001464:Barium / Q000494:pharmacology; D018999:Charybdotoxin / Q000494:pharmacology; D004298:Dopamine / Q000378:metabolism*; D005110:Extracellular Space / Q000378:metabolism; D005947:Glucose / Q000172:deficiency*; D005905:Glyburide / Q000494:pharmacology; D007004:Hypoglycemic Agents / Q000494:pharmacology; D066298:In Vitro Techniques; D008636:Mesencephalon / Q000166:cytology / Q000502:physiology*; D009474:Neurons / Q000378:metabolism / Q000502:physiology*; D018408:Patch-Clamp Techniques; D026902:Potassium Channel Blockers; D015221:Potassium Channels / Q000502:physiology*; D051381:Rats; D017208:Rats, Wistar; D013327:Strophanthidin / Q000494:pharmacology; D014044:Tolbutamide / Q000494:pharmacology
D007004:Hypoglycemic Agents; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D018999:Charybdotoxin; D001464:Barium; D013327:Strophanthidin; D014044:Tolbutamide; D005947:Glucose; D005905:Glyburide; D004298:Dopamine