Ionic currents influencing spontaneous firing and pacemaker frequency in dopamine neurons of the ventrolateral periaqueductal gray and dorsal raphe nucleus (vlPAG/DRN): A voltage-clamp and computational modelling study

Abstract

Dopamine (DA) neurons of the ventrolateral periaqueductal gray (vlPAG) and dorsal raphe nucleus (DRN) fire spontaneous action potentials (APs) at slow, regular patterns in vitro but a detailed account of their intrinsic membrane properties responsible for spontaneous firing is currently lacking. To resolve this, we performed a voltage-clamp electrophysiological study in brain slices to describe their major ionic currents and then constructed a computer model and used simulations to understand the mechanisms behind autorhythmicity in silico. We found that vlPAG/DRN DA neurons exhibit a number of voltage-dependent currents activating in the subthreshold range including, a hyperpolarization-activated cation current (IH), a transient, A-type, potassium current (IA), a background, ‘persistent’ (INaP) sodium current and a transient, low voltage activated (LVA) calcium current (ICaLVA). Brain slice pharmacology, in good agreement with computer simulations, showed that spontaneous firing occurred independently of IH, IA or calcium currents. In contrast, when blocking sodium currents, spontaneous firing ceased and a stable, non-oscillating membrane potential below AP threshold was attained. Using the DA neuron model we further show that calcium currents exhibit little activation (compared to sodium) during the interspike interval (ISI) repolarization while, any individual potassium current alone, whose blockade positively modulated AP firing frequency, is not required for spontaneous firing. Instead, blockade of a number of potassium currents simultaneously is necessary to eliminate autorhythmicity. Repolarization during ISI is mediated initially via the deactivation of the delayed rectifier potassium current, while a sodium background ‘persistent’ current is essentially indispensable for autorhythmicity by driving repolarization towards AP threshold.Electronic supplementary materialThe online version of this article (doi:10.1007/s10827-017-0641-0) contains supplementary material, which is available to authorized users.

Effectiveness in the Block by Honokiol, a Dimerized Allylphenol from Magnolia Officinalis, of Hyperpolarization-Activated Cation Current and Delayed-Rectifier K+ Current

“…Previous studies have demonstrated that the magnitude of I h in electrically excitable cells might greatly influence the rising phase of pacemaker potential or abrupt spike depolarizations [ 24 , 34 , 44 , 47 , 48 ]. Thus, next, we intended to determine how HNK exerted any perturbations on the deactivating I h in response to the upsloping ramp pulse from −120 to −40 mV with varying durations.…”

“…Previous studies have demonstrated that the magnitude of I h in electrically excitable cells might greatly influence the rising phase of pacemaker potential or abrupt spike depolarizations [24,34,44,47,48].…”

“…This occurrence thus reflected the fact that, as the duration of upsloping ramp pulse increased, the density of deactivating I h would be exponentially decreased, and that the presence of HNK decreased the current magnitude in a time-dependent fashion. Previous studies have demonstrated that the magnitude of Ih in electrically excitable cells might greatly influence the rising phase of pacemaker potential or abrupt spike depolarizations [24,34,44,47,48]. Thus, next, we intended to determine how HNK exerted any perturbations on the deactivating Ih in response to the upsloping ramp pulse from −120 to −40 mV with varying durations.…”

Molecular and Circuit Architecture of Social Hierarchy

“…Barium currents were recorded as a proxy for calcium currents to get better signal-to-noise ratio and to minimize inactivation (Dougalis et al, 2017; Hille, 2001), and were recorded in the presence of compounds to block sodium and potassium currents in modified oxygenated ACSF with composition (in mM): 95 NaCl, 2.5 KCl, 10 HEPES, 1.25 NaH2PO4, 1 MgCl2, 20 glucose, 0.5 BaCl2, 4 4-AP, 20 TEA and 1μm TTX (osmolarity 290, pH 7.35). Neurons were held −70 mV, hyperpolarized to −110 mV, and then depolarized in 5 mV increments to 30 mV.…”

Neural mechanisms of social homeostasis

“…Another component of the midbrain dopamine system—the DRN dopamine neurons—also exhibits acute isolation‐induced adaptations. These dopamine neurons were historically considered a caudal extension of the VTA, but accumulating evidence has revealed distinct downstream projections and functional roles . In adult male mice, 24‐h social isolation potentiated glutamatergic synapses onto DRN dopamine neurons, and also heightened their activity in vivo in response to a novel mouse .…”