Olivier Waroux

SK channels control the firing pattern of midbrain dopaminergic neurons in vivo

A vast body of experimental in vitro work and modelling studies suggests that the firing pattern and/or rate of a majority of midbrain dopaminergic neurons may be controlled in part by Ca2+‐activated K+ channels of the SK type. However, due to the lack of suitable tools, in vivo evidence is lacking. We have taken advantage of the development of the water‐soluble, medium potency SK blocker N‐methyl‐laudanosine (CH3‐L) to test this hypothesis in anaesthetized rats. In the lateral ventral tegmental area, CH3‐L iontophoresis onto dopaminergic neurons significantly increased the coefficient of variation of their interspike intervals and the percentage of spikes generated in bursts as compared to the control condition. The effect of CH3‐L persisted in the presence of a specific GABAA antagonist, suggesting a direct effect. It was robust and reversible, and was also observed in the substantia nigra. Control experiments demonstrated that the effect of CH3‐L could be entirely ascribed to its blockade of SK channels. On the other hand, the firing pattern of noradrenergic neurons was much less affected by CH3‐L. We provide here the first demonstration of a major role of SK channels in the control of the switch between tonic and burst firing of dopaminergic neurons in physiological conditions. This study also suggests a new strategy to develop modulators of the dopaminergic (DA) system, which could be of interest in the treatment of Parkinsons disease, and perhaps other diseases in which DA pathways are dysfunctional.

Kv7 channels: interaction with dopaminergic and serotonergic neurotransmission in the CNS.

Neuronal Kv7 channels (also termed KCNQ channels) are the molecular correlate of the M‐current. The Kv7 channels activate at rather negative membrane potentials (≤ 60 mV), thereby ‘fine‐tuning’ the resting membrane potential. The Kv7 channels are widely expressed in the brain with the Kv7.2, Kv7.3 and Kv7.5 channels being the most abundant. The Kv7.4 subunit has the most restricted brain regional expression being present in discrete nuclei of brainstem only. Kv7 channels are expressed at different subcellular locations, being on both somatodendritic, axonal and terminal sites. This complex subcellular distribution of Kv7 channels enables them to participate in both pre‐ and postsynaptic modulation of basal and stimulated excitatory neurotransmission. Activation of neuronal Kv7 channels limits repetitive firing thereby potentially limiting the generation of long bursts, with subsequent inhibition of monoaminergic neurotransmitter release. In this review, we focus on the influence of Kv7 channels on dopaminergic and serotonergic neurotransmission. The data suggest a novel action of Kv7 channel openers which could translate into having therapeutic value in the treatment of disease states characterized by overactivity of dopaminergic (e.g. schizophrenia and drug abuse) and serotonergic neurotransmission (e.g. anxiety).

The KCNQ Channel Opener Retigabine Inhibits the Activity of Mesencephalic Dopaminergic Systems of the Rat

Homo- and heteromeric complexes of KCNQ channel subunits are the molecular correlate of the M-current, a neuron-specific voltage-dependent K+ current with a well established role in control of neural excitability. We investigated the effect of KCNQ channel modulators on the activity of dopaminergic neurons in vitro and in vivo in the rat ventral mesencephalon. The firing of dopaminergic neurons recorded in mesencephalic slices was robustly inhibited in a concentration-dependent manner by the KCNQ channel opener N-(2-amino-4-(4-fluorobenzylamino)-phenyl) carbamic acid ethyl ester (retigabine). The effect of retigabine persisted in the presence of tetrodotoxin and simultaneous blockade of GABAA receptors, small-conductance calcium-activated K+ (SK) channels, and hyperpolarization-activated (Ih) channels, and it was potently reversed by the KCNQ channel blocker 4-pyridinylmethyl-9(10H)-anthracenone (XE991), indicating a direct effect on KCNQ channels. Likewise, in vivo single unit recordings from dopaminergic neurons revealed a prominent reduction in spike activity after systemic administration of retigabine. Furthermore, retigabine inhibited dopamine synthesis and c-Fos expression in the striatum under basal conditions. Retigabine completely blocked the excitatory effect of dopamine D2 autoreceptor antagonists. Again, the in vitro and in vivo effects of retigabine were completely reversed by preadministration of XE991. Dual immunocytochemistry revealed that KCNQ4 is the major KCNQ channel subunit expressed in all dopaminergic neurons in the mesolimbic and nigrostriatal pathways. Collectively, these observations indicate that retigabine negatively modulates dopaminergic neurotransmission, likely originating from stimulation of mesencephalic KCNQ4 channels.

SK Channel blockade promotes burst firing in dorsal raphe serotonergic neurons

Previous in vivo studies have shown that blockade of small‐conductance Ca2+‐activated potassium (SK) channels enhances burst firing in dopaminergic neurons. As bursting has been found to be physiologically relevant for the synaptic release of serotonin (5‐HT), we investigated the possible role of SK channels in the control of this firing pattern in 5‐HT neurons of the dorsal raphe nucleus. In these cells, bursts are usually composed of doublets consisting of action potentials separated by a small interval (< 20 ms). Both in vivo and in vitro extracellular recordings were performed, using anesthetized rats and rat brain slices, respectively. In vivo, the specific SK blocker UCL 1684 (200 μm) iontophoresed onto presumed 5‐HT neurons significantly increased the production of bursts in 13 out of 25 cells. Furthermore, the effect of UCL 1684 persisted in the presence of both the GABAA antagonist SR 95531 (10 mm) and the GABAB antagonist CGP 35348 (10 mm), whereas these agents by themselves did not significantly influence the neuronal firing pattern. In vitro, bath superfusion of the SK channel blocker apamin (300 nm) induced bursting in only three out of 18 neurons, although it increased the coefficient of variation of the interspike intervals in all the other cells. Our results suggest that SK channel blockade promotes bursting activity in 5‐HT neurons via a direct action. An input which is present only in vivo seems to be important for the induction of this firing pattern in these cells.

M-type channels selectively control bursting in rat dopaminergic neurons

Midbrain dopaminergic neurons in the substantia nigra, pars compacta and ventral tegmental area are critically important in many physiological functions. These neurons exhibit firing patterns that include tonic slow pacemaking, irregular firing and bursting, and the amount of dopamine that is present in the synaptic cleft is much increased during bursting. The mechanisms responsible for the switch between these spiking patterns remain unclear. Using both in‐vivo recordings combined with microiontophoretic or intraperitoneal drug applications and in‐vitro experiments, we have found that M‐type channels, which are present in midbrain dopaminergic cells, modulate the firing during bursting without affecting the background low‐frequency pacemaker firing. Thus, a selective blocker of these channels, 10,10‐bis(4‐pyridinylmethyl)‐9(10H)‐anthracenone dihydrochloride, specifically potentiated burst firing. Computer modeling of the dopamine neuron confirmed the possibility of a differential influence of M‐type channels on excitability during various firing patterns. Therefore, these channels may provide a novel target for the treatment of dopamine‐related diseases, including Parkinson’s disease and drug addiction. Moreover, our results demonstrate that the influence of M‐type channels on the excitability of these slow pacemaker neurons is conditional upon their firing pattern.