Role of GIRK channels on the noradrenergic transmission in vivo: an electrophysiological and neurochemical study on GIRK2 mutant mice

Abstract

Dysfunctional noradrenergic transmission is related to several neuropsychiatric conditions, such as depression. Nowadays, the role of G protein-coupled inwardly rectifying potassium (GIRK)2 subunit containing GIRK channels controlling neuronal intrinsic excitability in vitro is well known. The aim of this study was to investigate the impact of GIRK2 subunit mutation on the central noradrenergic transmission in vivo. For that purpose, single-unit extracellular activity of locus coeruleus (LC) noradrenergic neurons and brain monoamine levels using the HPLC technique were measured in wild-type and GIRK2 mutant mice. Girk2 gene mutation induced significant differences among genotypes regarding burst activity of LC neurons. In fact, the proportion of neurons displaying burst firing was increased in GIRK2 heterozygous mice as compared to that recorded from wild-type mice. Furthermore, this augmentation was even greater in the homozygous genotype. However, neither the basal firing rate nor the coefficient of variation of LC neurons was different among genotypes. Noradrenaline and serotonin basal levels were altered in the dorsal raphe nucleus from GIRK2 heterozygous and homozygous mice, respectively. Furthermore, noradrenaline levels were increased in LC projecting areas such as the hippocampus and amygdale from homozygous mice, although not in the prefrontal cortex. Finally, potency of clonidine and morphine inhibiting LC activity was reduced in GIRK2 mutant mice, although the efficacy remained unchanged. Altogether, the present study supports the role of GIRK2 subunit-containing GIRK channels on the maintenance of tonic noradrenergic activity in vivo. Electric and neurochemical consequences derived from an altered GIRK2-dependent signalling could facilitate the understanding of the neurobiological basis of pathologies related to a dysfunctional monoaminergic transmission.

Depressive-like behavior observed with a minimal loss of locus coeruleus (LC) neurons following administration of 6-hydroxydopamine is associated with electrophysiological changes and reversed with precursors of norepinephrine

“…Three weeks after the bilateral LC administration of either vehicle or 6-OHDA (5 μg/μl) (Figure 1A), single-unit extracellular recordings were performed as previously described (Torrecilla et al, 2013). Mice were anesthetized with chloral hydrate (400 mg/ kg i.p.)…”

“…This activity was processed using computer software (Spike2 software; Cambridge Electronic Design, UK) and the following patterns were calculated: firing rate; the coefficient of variation (percentage ratio of standard deviation to the mean interval value of an interspike time-interval histogram); percentage of spikes in burst; mean spikes/burst; percentage of cells exhibiting burst firing and number of active neurons per track. Basal electrophysiological activity was measured for 180 s. Burst-firing of LC neurons was detected as a train of at least two spikes with the first interspike interval <80 ms and a termination interval >160 ms (Miguelez et al, 2011; Torrecilla et al, 2013). The number of spontaneously active noradrenergic neurons was determined in 5–10 stereotaxic electrode tracks 50 μm concentrically to the initial track.…”

The Roles of Gβγ and Gα in Gating and Regulation of GIRK Channels

“…Heterologously expressed GIRK1-containing subunits have a large Gβγ-dependent component of I basal , whereas homomeric GIRK2a has low and largely Gβγ-independent I basal . I basal observed in hippocampal neurons, where GIRK1/2 is the most prominent channel composition (Luscher & Slesinger, 2010), is also quite sizeable (Chen & Johnston, 2005;Farhy Tselnicker et al, 2014;Torrecilla et al, 2013;Wiser et al, 2006), though its relative magnitude compared to I evoked is smaller than in Xenopus oocytes (I. Farhy Tselnicker & N. D., unpublished results). Thus, heterologous studies on GIRK1/2 appear to be relevant to neuronal cells.…”

“…Whatever the nature of I basal in the brain, it certainly carries important physiological functions. GIRK's basal activity and the balance between I basal and I evoked emerge as important determinants of the level of excitability and resting membrane potential in neurons (Chen & Johnston, 2005;Luscher et al, 1997;Torrecilla, Fernandez-Aedo, Arrue, Zumarraga, & Ugedo, 2013;Torrecilla et al, 2002;Wiser et al, 2006), bistability of some neuronal networks (Sanders, Berends, Major, Goldman, & Lisman, 2013), neuronal plasticity through depotentiation of long-term potentiation (Chung, Ge, et al, 2009;Chung, Qian, Ehlers, Jan, & Jan, 2009;Cooper et al, 2012), depressive behavior (Llamosas, Bruzos-Cidón, Rodríguez, Ugedo, & Torrecilla, 2015), dendritic integration (Makara & Magee, 2013), and possibly in working memory (Sanders et al, 2013), and have been proposed to be related to effects of Li + , a drug used in the treatment of bipolar disorder (Farhy Tselnicker et al, 2014).…”

GIRK Channels

“…Chronic exposure to opioids increases the excitability of neurons in the locus coeruleus (Kogan, Nestler, & Aghajanian, 1992), a mechanism that is thought to drive some of the symptoms of opioid withdrawal. GIRK channels mediate the intrinsic excitability of neurons in the locus coeruleus, as well as the effects of acute opioids on these neurons (Torrecilla, Fernandez-Aedo, Arrue, Zumarraga, & Ugedo, 2013;Torrecilla et al, 2002). Additionally, the morphine withdrawal-induced increase in the spontaneous firing of locus coeruleus neurons is absent in Girk2/3 À/À mice, and the behavioral withdrawal symptoms are attenuated (Cruz et al, 2008).…”