cAMP-dependent, long-lasting inhibition of a K+ current in mammalian neurons.

Abstract

We report the long-term modulation of K+ channels by cAMP in cultured murine colliculi neurons. A short (1-2 s) application of 8-Br-cAMP induced a long-lasting broadening of the action potential, a loss of after-hyperpolarization, and a reduction in spike accommodation. In agreement with these changes, 8-Br-cAMP produced a longlasting (2 hr) inhibition of a K+ current. These effects were also observed after a short activation of the pituitary adenylyl cyclase-activating polypeptide, 6-adrenergic, and 5-hydroxytryptamine type 4 (5-HT4) receptors, all known to increase cAMP. A transient activation of the cAMP-dependent protein kinase and a long-lasting inhibition of phosphatases (up to 2 hr) were detected. The blockade of the K+ current resulting from a brief application of 8-Br-cAMP or 5-hydroxytryptamine was prolonged from 2 to 4 hr when proteinserine/threonine phosphatases 1 and 2A were inhibited with 10 nM okadaic acid. The critical steps following the cAMPdependent protein kinase activation and resulting in a longterm blockade of phosphatases are discussed in this report.Neurotransmitters and hormones elevate the intracellular level of cAMP through their interaction at the membrane surface with specific receptors which are positively coupled to adenylyl cyclase via a stimulatory guanine nucleotide-binding protein (Gs protein). The cellular responses to an increase in intracellular cAMP are determined by the cell-specific substrate proteins that are phosphorylated by the cAMPdependent protein kinase (PKA). These neuronal substrate proteins include proteins involved in transmitter release, neurotransmitter synthesis, and ion channel function. Ion channels known to be phosphorylated by PKA include the glutamate and nicotinic acetylcholine receptors and voltage-dependent Na+, Ca2+, and K+ channels such as transient (IA), delayed, and Ca2+-activated K+ channels (1, 2).We have recently identified a cAMP-inhibited K+ current in cultured colliculi neurons isolated from the embryonic mouse (3). However, the physiological significance of this receptormediated K+ channel modulation remains to be determined. One physiological consequence of cAMP-mediated inhibition of K+ channels is the prolonged enhancement of transmitter release which underlies the long-term sensitization of the Aplysia californica gill withdrawal reflex (4, 5). Although some forms of long-term potentiation of synaptic transmission in the mammalian brain have been shown to involve either K+ channels (6) or cAMP (7-9), there is no direct evidence for an implication of cAMP-regulated K+ channels. We addressed this question by examining the long-term effects of a brief elevation of the intracellular concentration in cAMP on excitability and K+ current activity in cultured colliculi neurons For protein phosphatase assays, 100 ,uM 8-Br-cAMP or 10 p.M serotonin (5-hydroxytryptamine, 5-HT) was added to the culture medium for 5 min at 37°C. The neurons were then washed three times with culture medium. The neuron suspension was immediately homogenized a...

Clinically Relevant Doses of Fluoxetine and Reboxetine Induce Changes in the TrkB Content of Central Excitatory Synapses

“…The delayed decrease of TrkB in PSDs and synaptic membranes observed here may result as a consequence of increased BDNF release following antidepressant treatment. Flx acutely inhibits neuronal reuptake of serotonin, and this monoamine is able to potentiate the action of excitatory stimuli on cortical pyramidal neurons, for example, through inhibition of Ca-activated K channels (Aghajanian, 1995;Ansanay et al, 1995;Araneda and Andrade, 1991;Foehring et al, 2002;Goaillard and Vincent, 2002;Torres et al, 1995;Wallén et al, 1989) or inhibition of voltage-activated K channels (Aghajanian, 1995;Andrade and Nicoll, 1987;Beck, 1992). Similar proexcitatory effects have been also described for norepinephrine (Larkman and Kelly, 1992;Lee and McCormick, 1996;Madison and Nicoll, 1986a, b;McCormick et al, 1991).…”

Attenuated Response to Stress and Novelty and Hypersensitivity to Seizures in 5-HT4Receptor Knock-Out Mice

“…This result suggests a tonic inhibitory influence of 5-HT 4 receptors on the excitability of neurons either in adulthood or during development. Several studies have described a 5-HT 4 receptor-mediated increase in neuronal excitability via the blockade of K ϩ channels in cortical and hippocampal neurons (Dumuis et al, 1988;Bockaert et al, 1990;Chaput et al, 1990;Fagni et al, 1992;Monferini et al, 1993;Ansanay et al, 1995). This could ultimately contribute to the facilitation of neuromediator release.…”

5-Hydroxytryptamine Induces a Protein Kinase A/Mitogen-Activated Protein Kinase-Mediated and Macromolecular Synthesis-Dependent Late Phase of Long-Term Potentiation in the Amygdala

“…This effect could be blocked by the 5-HT 4 antagonist and PKA inhibitor recombinant protein-cAMP (Svenningsson et al, 2002). Activation of the 5-HT 4 receptor increases neuronal excitability in hippocampal neurons (Andrade and Chaput, 1991;Ansanay et al, 1995;Torres et al, 1996;Chapin et al, 2002;Spencer et al, 2004;Kemp and Vaughan, 2005) and in prefrontal cortex (Beique et al, 2004). 5-HT 4 receptor agonists enhance the magnitude of population spikes recorded in the CA1 region of hippocampus (Siarey et al, 1995;Spencer et al, 2004).…”

“…In neurons of colliculus and hippocampus the activation of the 5-HT 4 receptor can inhibit K ϩ current and reduce afterhyperpolarization, which increases neuronal excitability and neurotransmitter release. These effects occur via the mediation of cAMP/ PKA (Ansanay et al, 1995;Eglen et al, 1995;Heine et al, 2002).…”