Activation of alpha-adrenoceptors indirectly facilitates sodium pumping in frog motoneurons.
The effects of clonidine on Na+ pumping in motoneurons of the isolated frog spinal cord was investigated using sucrose gap recordings from ventral roots. Na+ pump activity, induced in motoneurons either by tetanizing the dorsal root or by rapidly exposing the cord to normal medium following 30 min in K(+)-free Ringer's solution (K(+)-activated hyperpolarizations), was increased by application of clonidine (100 microM). These actions of clonidine were blocked by the preferential alpha 2-adrenergic antagonist yohimbine, but not by alpha 1-adrenergic antagonist prazosin or the beta-blocker propranolol. Clonidine's effects on Na+ pumping appeared to be indirect (presumably via interneurons) because its effects on K(+)-activated hyperpolarizations were reduced by tetrodotoxin (TTX) or high concentrations of Mg2+. This indirect mechanism involved activation of non-NMDA excitatory amino acid receptors. Thus, in the presence of clonidine, CNQX, but not APH, limited the ability of clonidine to enhance K(+)-activated hyperpolarizations. The AMPA receptor may play a role in the process, K(+)-activated hyperpolarizations were augmented by the presence of AMPA; NMDA had no effect. The present results are consistent with the idea that activation of alpha 2-adrenoceptors produces the following: the release of excitatory amino acids from interneurons; the activation of non-NMDA receptors on motoneurons; increased Na+ influx and loading and increased Na+ pump activity.
Shope
S B
SB
Department of Neurology, University of Miami School of Medicine, FL.
Hackman
J C
JC
Holohean
A M
AM
Davidoff
R A
RA
D015763:2-Amino-5-phosphonovalerate / Q000031:analogs & derivatives*; D018750:6-Cyano-7-nitroquinoxaline-2,3-dione; D000596:Amino Acids / Q000378:metabolism / Q000494:pharmacology; D000818:Animals; D003000:Clonidine / Q000494:pharmacology*; D009046:Motor Neurons / Q000187:drug effects*; D016202:N-Methylaspartate / Q000494:pharmacology; D009638:Norepinephrine / Q000494:pharmacology; D011224:Prazosin / Q000494:pharmacology; D011433:Propranolol / Q000494:pharmacology; D011810:Quinoxalines / Q000494:pharmacology; D011894:Rana pipiens; D000254:Sodium-Potassium-Exchanging ATPase / Q000187:drug effects*; D013116:Spinal Cord / Q000166:cytology / Q000187:drug effects*; D013779:Tetrodotoxin / Q000494:pharmacology; D018350:alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid / Q000494:pharmacology
D000596:Amino Acids; D011810:Quinoxalines; D013779:Tetrodotoxin; D016202:N-Methylaspartate; D018750:6-Cyano-7-nitroquinoxaline-2,3-dione; D015763:2-Amino-5-phosphonovalerate; D018350:alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; D011433:Propranolol; D000254:Sodium-Potassium-Exchanging ATPase; D003000:Clonidine; C031231:2-amino-7-phosphonoheptanoic acid; D009638:Norepinephrine; D011224:Prazosin