GIRK Channels Modulate Opioid-Induced Motor Activity in a Cell Type- and Subunit-Dependent Manner

Abstract

G-protein-gated inwardly rectifying Kϩ (GIRK/Kir3) channel activation underlies key physiological effects of opioids, including analgesia and dependence. GIRK channel activation has also been implicated in the opioid-induced inhibition of midbrain GABA neurons and consequent disinhibition of dopamine (DA) neurons in the ventral tegmental area (VTA). Drug-induced disinhibition of VTA DA neurons has been linked to reward-related behaviors and underlies opioid-induced motor activation. Here, we demonstrate that mouse VTA GABA neurons express a GIRK channel formed by GIRK1 and GIRK2 subunits. Nevertheless, neither constitutive genetic ablation of Girk1 or Girk2, nor the selective ablation of GIRK channels in GABA neurons, diminished morphine-induced motor activity in mice. Moreover, direct activation of GIRK channels in midbrain GABA neurons did not enhance motor activity. In contrast, genetic manipulations that selectively enhanced or suppressed GIRK channel function in midbrain DA neurons correlated with decreased and increased sensitivity, respectively, to the motor-stimulatory effect of systemic morphine. Collectively, these data support the contention that the unique GIRK channel subtype in VTA DA neurons, the GIRK2/GIRK3 heteromer, regulates the sensitivity of the mouse mesolimbic DA system to drugs with addictive potential.

Selective Ablation of GIRK Channels in Dopamine Neurons Alters Behavioral Effects of Cocaine in Mice

“…We recently demonstrated that VTA DA neurons in DATCre(+):Girk2 flox/flox mice (GIRK2 DA KO mice) exhibited diminished inhibitory somatodendritic current responses to GABA B R activation (Kotecki et al, 2015). To test whether autoreceptor-mediated signaling in VTA DA neurons from GIRK2 DA KO mice was similarly blunted, we measured currents evoked by a saturating concentration (20 μM) of the D 2/3 R agonist quinpirole.…”

“…The generation of Girk2 flox/flox , DATCre(+/ − ):Girk2 flox/flox , and Pitx3-eGFP(+)/DATCre(+/ − ):Girk2 flox/flox mice was described previously (Kotecki et al, 2015). DATCre (B6.SJLSlc6a3 tm1.1(cre)Bkmn ) and Drd1a-tdTomato (B6.Cg-Tg(Drd1a-tdTomato)6Calak/J) lines were purchased from The Jackson Laboratory (Bar Harbor, ME), and the Drd2-eGFP strain (Tg (Drd2-EGFP)S118Gsat) was obtained from the Mutant Mouse Regional Resource Center.…”

“…Horizontal slices (225 μm) of the mouse VTA (5-7 weeks) were prepared as described (Kotecki et al, 2015). Neurons medial to the medial terminal nucleus of the accessory optic tract, and identified via GFP expression driven by the Pitx3 promoter, were targeted for analysis.…”

“…Constitutive Girk2 -/-mice are hyperactive, a phenotype normalized by D 1 DA receptor (D 1 R) blockade (Blednov et al, 2002). Moreover, Girk2 -/-mice exhibit enhanced locomotor activation in response to morphine and cocaine (Arora et al, 2010;Kotecki et al, 2015). The many phenotypes and adaptations associated with global Girk2 ablation, however, confound interpretation of these data (Lujan et al, 2014).…”

“…For example, VTA DA neurons, NAc medium spiny neurons (MSNs), and layer 5/6 medial prefrontal cortex (mPFC) pyramidal neurons from Girk2 -/-mice exhibit elevated AMPA receptor-mediated neurotransmission (Arora et al, 2010;Hearing et al, 2013). The recent availability of mice lacking GIRK2 in DA neurons permits a more precise evaluation of the role of GIRK-dependent signaling in modulating DA neuron excitability and cocaine-induced behaviors (Kotecki et al, 2015). Here we report that GIRK channel ablation in DA neurons reduces the autoreceptor-mediated inhibition of VTA DA neurons and alters behavioral sensitivity to cocaine.…”