Role of cyclic AMP in dopamine modulation of potassium channels on rat striatal neurons: regulation of a subconductance state

REVIEW ■ : ATP-sensitive Potassium Channels: Diverse Functions in the Central Nervous System

ATP-sensitive potassium channels open when cytoplasmic levels of ATP drop, thus linking membrane potential to the metabolic state of the cell. Cloning studies have suggested that these channels are related structurally to the inward rectifier family of potassium channels, with two putative membrane-spanning regions. Sulfonylurea drugs, which are used in the treatment of diabetes, inhibit these channels by binding to an associated membrane protein. Other drugs, including some vasodilators, activate ATP-sensitive potassium channels. Diverse neurotransmitter and hormone receptors can modulate these channels, in some cases through interactions with guanyl nucleotide binding proteins. There appear to be multiple subtypes of these channels, differing in electrical properties as well as in drug sensitivities. In the brain, these channels appear to play a role in mediating satiety after feeding. They also function in neurons to protect against excitotoxicity, by counteracting the membrane depolarization associated with metabolic stress. Brain dopamine receptors appear to modulate a novel subtype of ATP-sensitive potassium channel. The association of dopamine receptors with a mechanism involved in protection against neurodegeneration may have implications for the causes of diseases in which dopaminergic regions of brain undergo structural changes, possibly including schizophrenia. NEUROSCIENTIST 2:145-152, 1996

Dopaminergic modulation of axonal potassium channels and action potential waveform in pyramidal neurons of prefrontal cortex

Voltage-gated K(+) (KV) channels play critical roles in shaping neuronal signals. KV channels distributed in the perisomatic regions and thick dendrites of cortical pyramidal neurons have been extensively studied. However, the properties and regulation of KV channels distributed in the thin axons remain unknown. In this study, by performing somatic and axonal patch-clamp recordings from layer 5 pyramidal neurons of prefrontal cortical slices, we showed that the rapidly inactivating A-currents mediated the transient K(+) currents evoked by action potential (AP) waveform command (KAP) at the soma, whereas the rapidly activating but slowly inactivating KV1-mediated D-currents dominated the KAP at the axon. In addition, activation of D1-like receptors for dopamine decreased the axonal K(+) currents, as a result of an increase in the activity of cAMP-PKA pathway. In contrast, activation of D2-like receptors showed an opposite effect on the axonal K(+) currents. Further experiments demonstrated that functional D1-like receptors were expressed at the main axon trunk and their activation could broaden the waveforms of axonal APs. Together, these results show that axonal KV channels were subjected to dopamine modulation, and this modulation could regulate the waveforms of propagating APs at the axon, suggesting an important role of dopaminergic modulation of axonal KV channels in regulating neuronal signalling.

Dopamine D(2) receptor modulation of K(+) channel activity regulates excitability of nucleus accumbens neurons at different membrane potentials

The nucleus accumbens (NAc) is a forebrain area in the mesocorticolimbic dopamine (DA) system that regulates many aspects of drug addiction. Neuronal activity in the NAc is modulated by different subtypes of DA receptors. Although DA signaling has received considerable attention, the mechanisms underlying D(2)-class receptor (D(2)R) modulation of firing in medium spiny neurons (MSNs) localized within the NAc remain ambiguous. In the present study, we performed whole cell current-clamp recordings in rat brain slices to determine whether and how D(2)R modulation of K(+) channel activity regulates the intrinsic excitability of NAc neurons in the core region. D(2)R stimulation by quinpirole or DA significantly and dose-dependently decreased evoked Na(+) spikes. This D(2)R effect on inhibiting evoked firing was abolished by antagonism of D(2)Rs, reversed by blockade of voltage-sensitive, slowly inactivating A-type K(+) currents (I(As)), or eliminated by holding membrane potentials at levels in which I(As) was inactivated. It was also mimicked by inhibition of cAMP-dependent protein kinase (PKA) activity, but not phosphatidylinositol-specific phospholipase C (PI-PLC) activity. Moreover, D(2)R stimulation also reduced the inward rectification and depolarized the resting membrane potentials (RMPs) by decreasing "leak" K(+) currents. However, the D(2)R effects on inward rectification and RMP were blocked by inhibition of PI-PLC, but not PKA activity. These findings indicate that, with facilitated intracellular Ca(2+) release and activation of the D(2)R/G(q)/PLC/PIP(2) pathway, the D(2)R-modulated changes in the NAc excitability are dynamically regulated and integrated by multiple K(+) currents, including but are not limited to I(As), inwardly rectifying K(+) currents (I(Kir)), and "leak" currents (I(K-2P)).

Altered gating of opiate receptor-modulated K+ channels on amygdala neurons of morphine-dependent rats

The molecular mechanism of tolerance to opiate drugs is poorly understood. We have used single-channel patch-clamp recordings to study opiate receptor effects on dissociated neurons from rat amygdala, a limbic region implicated in addiction processes. A 130-pS inwardly rectifying K(+)-preferring cation channel was activated by mu opioid receptors in a membrane-delimited manner. After chronic treatment of the rats with morphine, channel gating changed markedly, with an approximately 100-fold decrease in open probability at a given morphine concentration. The change in channel gating correlated both in time course and in dose of morphine treatment with the development of functional opiate dependence and appeared to arise at a step after G-protein activation and before channel permeation by K(+). This decreased receptor-channel coupling appears to be large enough to account quantitatively for opiate tolerance and may represent one of the mechanisms through which tolerance occurs.

Permeant ion binding affinity in subconductance states of an L-type Ca2+ channel expressed in Xenopus laevis oocytes

1. The relationship between single-channel conductance and ion binding affinity in Ca2+ channels was investigated by measuring differences in the apparent binding affinity (K'D) for Ca2+ among naturally occurring conductance states of an L-type (alpha1C) Ca2+ channel heterologously expressed in Xenopus oocytes. Using cell-attached patch recordings, three or more conductance levels were observed when Ca2+, Ba2+ or Li+ was used as the permeating ion. 2. With Li+ as the charge carrier, low concentrations of Ca2+ (0.1-3.0 microM) produced discrete blocking events in all conductance states. Measurements of open and blocked times as a function of Ca2+ concentration were used to calculate rates of block and unblock. 3. K'D was calculated for three of the conductance levels. Binding affinity for Ca2+ increased as conductance decreased (K'D: large = 7.5 microM, medium = 4.0 microM, small = 2.7 microM). The lower K'D values of the smaller conductance states arose from a combination of larger on-rates and smaller off-rates. 4. These results imply that permeant ions such as Ca2+ have both easier access to, and longer dwell time in, the Ca2+ binding locus in the pore when the channel opens to a subconductance level as compared to the fully open level. 5. The difference in K'D between the large and small conductance levels corresponds to a small difference in the free energy of binding, DeltaDeltaG approximately 1kBT, where kB is Boltzmann's constant and T is absolute temperature (kelvin). Nonetheless, an Eyring model of Ca2+ channel permeation incorporating the state-specific on- and off-rate constants for Ca2+ was able to reproduce the large difference in channel conductance, indicating that small differences in binding energy may be able to account for large differences in amplitude between conductance states.

Expression of a dopamine D2 receptor-activated K+ channel on identified striatopallidal and striatonigral neurons

One view of the efferent circuitry of the basal ganglia holds that dopamine D1 and D2 receptors are segregated to striatonigral and striatopallidal neurons, respectively. The present studies investigated whether functional D2-like receptors are, in fact, restricted to striatopallidal neurons. Single, freshly dissociated cells from rat striatum were identified as either striatonigral or striatopallidal projection neurons by fluorescence retrograde labeling. By using cell-attached patch-clamp recordings, neurons of each efferent group were evaluated for the presence of a D2-like receptor-activated 85-pS K+ channel as a measure of receptor function. We now report the presence of this D2 receptor-mediated response on both striatal efferent populations, but we observed an approximately 2-fold higher likelihood of encountering the channel on pallidal- versus nigral-projecting neurons. The channel's conductance properties appeared identical in both groups of neurons, but there was a significantly greater open probability for channels detected on striatopallidal neurons. These results indicate that functional D2 receptors are not segregated to striatopallidal neurons, but may be expressed in a higher proportion of, or at a higher density and/or efficiency of coupling on, pallidal- versus nigral-projecting striatal efferents.

Phe-Met-Arg-Phe-amide activates a novel voltage-dependent K+ current through a lipoxygenase pathway in molluscan neurones

The neuropeptide Phe-Met-Arg-Phe-amide (FMRFa) dose dependently (ED50 = 23 nM) activated a K+ current in the peptidergic caudodorsal neurones that regulate egg laying in the mollusc Lymnaea stagnalis. Under standard conditions ([K+]o = 1.7 mM), only outward current responses occurred. In high K+ salines ([K+]o = 20 or 57 mM), current reversal occurred close to the theoretical reversal potential for K+. In both salines, no responses were measured below -120 mV. Between -120 mV and the K+ reversal potential, currents were inward with maximal amplitudes at approximately -60 mV. Thus, U-shaped current-voltage relations were obtained, implying that the response is voltage dependent. The conductance depended both on membrane potential and extracellular K+ concentration. The voltage sensitivity was characterized by an e-fold change in conductance per approximately 14 mV at all [K+]o. Since this result was also obtained in nearly symmetrical K+ conditions, it is concluded that channel gating is voltage dependent. In addition, outward rectification occurs in asymmetric K+ concentrations. Onset kinetics of the response were slow (rise time approximately 650 ms at -40 mV). However, when FMRFa was applied while holding the cell at -120 mV, to prevent activation of the current but allow activation of the signal transduction pathway, a subsequent step to -40 mV revealed a much more rapid current onset. Thus, onset kinetics are largely determined by steps preceding channel activation. With FMRFa applied at -120 mV, the time constant of activation during the subsequent test pulse decreased from approximately 36 ms at -60 mV to approximately 13 ms at -30 mV, confirming that channel opening is voltage dependent. The current inactivated voltage dependently. The rate and degree of inactivation progressively increased from -120 to -50 mV. The current is blocked by internal tetraethylammonium and by bath- applied 4-aminopyridine, tetraethylammonium, Ba2+, and, partially, Cd2+ and Cs+. The response to FMRFa was affected by intracellular GTPgammaS. The response was inhibited by blockers of phospholipase A2 and lipoxygenases, but not by a cyclo-oxygenase blocker. Bath-applied arachidonic acid induced a slow outward current and occluded the response to FMRFa. These results suggest that the FMRFa receptor couples via a G-protein to the lipoxygenase pathway of arachidonic acid metabolism. The biophysical and pharmacological properties of this transmitter operated, but voltage-dependent K+ current distinguish it from other receptor-driven K+ currents such as the S-current- and G-protein-dependent inward rectifiers.