Abstract
Olfactory receptor neurons (ORNs) of the hawkmoth Manduca sexta sensitize via cAMP- and adapt via cGMP-dependent mechanisms. Perforated patch clamp recordings distinguished 11 currents in these ORNs. Derivatives of cAMP and/or cGMP antagonistically affected three of five K+ currents and two non-specific cation currents. The Ca2+-dependent K+ current I-K(Ca())2+ and the sensitive ...
Abstract
Olfactory receptor neurons (ORNs) of the hawkmoth Manduca sexta sensitize via cAMP- and adapt via cGMP-dependent mechanisms. Perforated patch clamp recordings distinguished 11 currents in these ORNs. Derivatives of cAMP and/or cGMP antagonistically affected three of five K+ currents and two non-specific cation currents. The Ca2+-dependent K+ current I-K(Ca())2+ and the sensitive pheromone-dependent K+ current IK(cGMP-), which both express fast kinetics, were inhibited by 8bcGMP, while a slow K+ current, IK(cGMP+), was activated by 8bcGMP. Furthermore, application of 8bcAMP blocked slowly activating, zero mV-reversing, non-specific cation currents, I-LL and I-cat(PKC?), which remained activated in the presence of 8bcGMP. Their activations pull the membrane potential towards their 0-mV reversal potentials, in addition to increasing intracellular Ca2+ levels voltage- and I-LL-dependently. Twenty minutes after application, 8bcGMP blocked a TEA-independent K+ current, I-K(noTEA), and a fast cation current, I-cat(nRP), which both shift the membrane potential to negative values. We conclude that conditions of sensitization are maintained at high levels of cAMP, via specific opening/closure of ion channels that allow for fast kinetics, hyperpolarized membrane potentials, and low intracellular Ca2+ levels. In contrast, adaptation is supported via cGMP, which antagonizes cAMP, opening Ca2+-permeable channels with slow kinetics that stabilize depolarized resting potentials. The antagonistic modulation of peripheral sensory neurons by cAMP or cGMP is reminiscent of pull-push mechanisms of neuromodulation at central synapses underlying metaplasticity.
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