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Ca²⁺-dependent ion channels underlying spontaneous activity in insect circadian pacemaker neurons
1FB 10, Biology, Animal Physiology, University of Kassel, Heinrich Plett Str. 40, 34132 Kassel, Germany.
The European Journal of Neuroscience
|July 24, 2012
Summary
Cockroach circadian pacemaker neurons exhibit gamma frequency activity, crucial for temporal encoding. This study identifies novel ion channels, including low-voltage-activated calcium and hyperpolarization-activated cation channels, driving this fast rhythmic activity.
Area of Science:
- Neuroscience
- Chronobiology
- Insect Physiology
Background:
- Gamma frequency electrical activity is vital for temporal encoding in vertebrate brains.
- Circadian pacemaker neurons in cockroaches also display fast rhythmic activity in the gamma band (20-70 Hz).
- The specific ionic conductances responsible for this fast activity in cockroach pacemakers remain largely unknown.
Purpose of the Study:
- To investigate the ion channels underlying spontaneous rhythmic activity in the circadian pacemaker neurons of the adult accessory medulla in *Rhyparobia maderae*.
- To elucidate the ionic mechanisms driving fast spontaneous calcium transients and action potentials in these neurons.
- To differentiate the channels involved in spontaneous pacemaker activity from those in input-dependent activity.
Main Methods:
- Dispersed circadian pacemaker neurons from adult *Rhyparobia maderae* were utilized.
- Calcium (Ca2+) imaging techniques were employed to monitor neuronal activity.
- Pharmacological agents, including tetrodotoxin (TTX), mibefradil, DK-AH269, and nifedipine, were used to block specific ion channels.
- Spontaneous action potentials and calcium transients were analyzed in conjunction with pharmacological manipulations.
Main Results:
- Fast spontaneous Ca2+ transients in pacemaker neurons were found to accompany tetrodotoxin (TTX)-blockable spontaneous action potentials.
- Contrary to vertebrate pacemakers, spontaneous depolarizations were rarely initiated by TTX-sensitive sustained sodium (Na+) channels.
- Activity was predominantly driven by mibefradil-sensitive, low-voltage-activated Ca2+ channels and DK-AH269-sensitive hyperpolarization-activated, cyclic nucleotide-gated cation channels.
- Rhythmic depolarizations activated voltage-gated Na+ channels and nifedipine-sensitive high-voltage-activated Ca2+ channels, leading to Ca2+ rises.
- These Ca2+ rises activated small-conductance, Ca2+-dependent K+ channels for repolarization.
Conclusions:
- The study reveals that low-voltage-activated Ca2+ channels and hyperpolarization-activated cation channels are key drivers of spontaneous gamma frequency activity in cockroach circadian pacemaker neurons.
- These findings contrast with the known mechanisms in vertebrate pacemakers, highlighting divergent evolutionary pathways for circadian rhythm generation.
- P/Q-type Ca2+ channels coupled to large-conductance Ca2+-dependent K+ channels are hypothesized to be involved in input-dependent neuronal activity, distinct from spontaneous rhythm generation.
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