HCN2 and HCN1 Channels Govern the Regularity of Autonomous Pacemaking and Synaptic Resetting in Globus Pallidus Neurons

Abstract

The globus pallidus (GP) is a critical component of the basal ganglia circuitry controlling motor behavior. Dysregulation of GP activity has been implicated in a number of psychomotor disorders, including Parkinson's disease (PD), in which a cardinal feature of the pathophysiology is an alteration in the pattern and synchrony of discharge in GP neurons. Yet the determinants of this activity in GP neurons are poorly understood. To help fill this gap, electrophysiological, molecular, and computational approaches were used to identify and characterize GABAergic GP neurons in tissue slices from rodents. In vitro, GABAergic GP neurons generate a regular, autonomous, single-spike pacemaker activity. Hyperpolarization-activated, cyclic nucleotide-gated cation (HCN) channels make an important contribution to this process: their blockade with ZD7288 significantly slowed discharge rate and decreased its regularity. HCN currents evoked by somatic voltage clamp had fast and slow components. Single-cell RT-PCR and immunohistochemical approaches revealed robust expression of HCN2 subunits as well as significant levels of HCN1 subunits in GABAergic GP neurons. Transient activation of striatal GABAergic input to GP neurons led to a resetting of rhythmic discharge that was dependent on HCN currents. Simulations suggested that the ability of transient striatal GABAergic input to reset pacemaking was dependent on dendritic HCN2/HCN1 channels. Together, these studies show that HCN channels in GABAergic GP neurons are key determinants of the regularity and rate of pacemaking as well as striatal resetting of this activity, implicating HCN channels in the emergence of synchrony in PD.

Enhancement of Excitatory Synaptic Integration by GABAergic Inhibition in the Subthalamic Nucleus

“…To assess their involvement in the firing of STN neurons, we used perforated patch-clamp recordings of STN neurons in the presence of synaptic transmission blockers (50 M APV, 20 M CNQX, 20 M GABAzine, and 1-2 M CGP55845) and during perfusion of 2 mM external Cs ϩ to block HCN channels. In contrast to GP neurons (Chan et al, 2004) and in agreement with previous findings (Bevan and Wilson, 1999;Beurrier et al, 2000;Do and Bean, 2003), blockade of HCN channels with Cs ϩ did not affect the firing rate of STN neurons [WSR test;control,6.3 Ϯ 2.8 Hz; Cs ϩ , 6.3 Ϯ 3.3 Hz; n ϭ 6; p ϭ 0.84 ( In STN and GP neurons, stimulation of GABAergic IPSPs transiently hyperpolarizes the membrane potential and produces a pause in firing and a partial or complete reset of the phase of the autonomous oscillation (Bevan et al, 2002a;Chan et al, 2004). In GP neurons, blockade of HCN channels disrupted phase resetting.…”

“…In sensory thalamic neurons and GP neurons, HCN channels are important contributors to pacemaking activity (McCormick and Bal, 1997;Robinson and Siegelbaum, 2003;Chan et al, 2004) and to the resetting of rhythmic firing by GABAergic synaptic inputs (McCormick and Bal, 1997;Chan et al, 2004). To assess their involvement in the firing of STN neurons, we used perforated patch-clamp recordings of STN neurons in the presence of synaptic transmission blockers (50 M APV, 20 M CNQX, 20 M GABAzine, and 1-2 M CGP55845) and during perfusion of 2 mM external Cs ϩ to block HCN channels.…”

“…In addition to Na v channels, other potential contributors to activity and synaptic integration are hyperpolarization-activated cyclic nucleotidegated (HCN) and class 3 voltage-dependent Ca 2ϩ (Ca v 3) channels (Huguenard, 1996;Williams and Stuart, 2003;Chan et al, 2004), which underlie prominent inward currents in STN neurons (Beurrier et al, 2000;Song et al, 2000). Single/low-frequency stimulation of putative GP fibers elicits GABA A receptor-mediated IPSPs in STN neurons that produce a pause and reset the phase of autonomous oscillation (Bevan et al, 2002a).…”

Mechanisms and Targets of Deep Brain Stimulation in Movement Disorders

“…Several possibilities have been proposed, including depolarization block due to an increase in potassium current 28 or an inactivation of sodium channels, 29,30 presynaptic depression of excitatory afferents, 31 and stimulation-induced activation of inhibitory afferents. 32,33 Support for the depolarization block hypothesis comes primarily from in vitro experiments. Magariños-Ascone et al, 34 for example, reported that STN cells in rat brain slices increased their instantaneous firing rate during the initial stimulation period, after which these neurons failed to respond.…”

“…Hyperpolarization-activated cation (HCN) and low-threshold calcium (T-type) channel currents, for example, may be activated during HFS resulting in an excitatory time-locked rebound. 31,33 The contribution of network and reentrant loops 35 also warrants further investigation with in vivo experiments that incorporate local infusion of specific antagonists during HFS.…”