Contribution of a calcium-activated non-specific conductance to NMDA receptor-mediated synaptic potentials in granule cells of the frog olfactory bulb.
We studied granule cells (GCs) in the intact frog olfactory bulb (OB) by combining whole-cell recordings and functional two-photon Ca(2+) imaging in an in vitro nose-brain preparation. GCs are local interneurones that shape OB output via distributed dendrodendritic inhibition of OB projection neurones, the mitral-tufted cells (MTCs). In contrast to MTCs, GCs exhibited a Ca(2+)-activated non-specific cation conductance (I(CAN)) that could be evoked through strong synaptic stimulation or suprathreshold current injection. Photolysis of the caged Ca(2+) chelator o-nitrophenol-EGTA resulted in activation of an inward current with a reversal potential within the range -20 to +10 mV. I(CAN) in GCs was suppressed by the intracellular Ca(2+) chelator BAPTA (0.5-5.0 mM), but not by EGTA (up to 5 mM). The current persisted in whole-cell recordings for up to 1.5 h post-breakthrough, was observed during perforated-patch recordings and was independent of ionotropic glutamate and GABA(A) receptor activity. In current-clamp mode, GC responses to synaptic stimulation consisted of an initial AMPA-mediated conductance followed by a late-phase APV-sensitive plateau (100-500 ms). BAPTA-mediated suppression of I(CAN) resulted in a selective reduction of the late component of the evoked synaptic potential, consistent with a positive feedback relationship between NMDA receptor (NMDAR) current and I(CAN). I(CAN) requires Ca(2+) influx either through voltage-gated Ca(2+) channels or possibly NMDARs, both of which have a high threshold for activation in GCs, predicting a functional role for this current in the selective enhancement of strong synaptic inputs to GCs.
Hall
Benjamin J
BJ
Simon Fraser University, Burnaby, BC, Canada V5A 1S6. bhall7@jhmi.edu
Delaney
Kerry R
KR
D015763:2-Amino-5-phosphonovalerate / Q000494:pharmacology; D000200:Action Potentials / Q000187:drug effects / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002614:Chelating Agents / Q000494:pharmacology; D004533:Egtazic Acid / Q000031:analogs & derivatives* / Q000494:pharmacology; D004553:Electric Conductivity; D004558:Electric Stimulation; D005071:Evoked Potentials / Q000502:physiology; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D008297:Male; D009474:Neurons / Q000502:physiology; D009830:Olfactory Bulb / Q000166:cytology / Q000502:physiology*; D018408:Patch-Clamp Techniques; D010777:Photochemistry; D011894:Rana pipiens; D016194:Receptors, N-Methyl-D-Aspartate / Q000378:metabolism*; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology*
D002614:Chelating Agents; D018691:Excitatory Amino Acid Antagonists; D016194:Receptors, N-Methyl-D-Aspartate; D004533:Egtazic Acid; D015763:2-Amino-5-phosphonovalerate; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; D002118:Calcium