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* To whom correspondence should be addressed. E-mail: j.midtgaard{at}mfi.ku.dk.
Granule cells excitability in the turtle olfactory bulb was analyzed using whole cell recordings in current and voltage clamp mode. Low-threshold spikes (LTSs) were evoked at potentials that are subthreshold for Na spikes in normal medium. The LTSs were evoked from rest but hyperpolarization of the cell usually increased their amplitude so that they more easily boosted Na spike initiation. The LTS persisted in the presence of tetrodotoxin (TTX), but was antagonized by blockers of T-type calcium channels. The voltage dependence, kinetics and inactivation properties of the LTS were characteristic of a low-threshold calcium spike. The threshold of the LTS was slightly above the resting potential but well below the Na spike threshold and often the LTS was evoked in isolation in normal medium. Tetraethylammonium (TEA) and 4-aminopyridine (4-AP) had only minimal effects on the LTS but revealed the presence of a high threshold Ca2+ spike (HTS), which was antagonized by Cd2+. The LTS displayed paired-pulse attenuation, with a timescale for recovery from inactivation of about two seconds at resting membrane potential. The LTS strongly boosted Na-spike initiation; with repetitive stimulation the long recovery of the LTS governed Na spike initiation. Thus the olfactory granule cells possess an LTS, with intrinsic kinetics which contribute to sub- and supra-threshold responses on a timescale of seconds. This adds a new mechanism to the early processing of olfactory input.
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