This document discusses the role of SNX27, a protein involved in trafficking G-protein-gated inwardly rectifying potassium (GIRK) channels, in midbrain dopamine neurons. It finds that SNX27 regulates GABABR- and D2R-dependent GIRK signaling in both ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) dopamine neurons. Deletion of SNX27 from these neurons leads to increased sensitivity to cocaine-induced locomotor sensitization, and this effect can be reversed by exogenous expression of the GIRK2a subunit in VTA neurons. Therefore, SNX27 regulates the response to cocaine by trafficking G
Rifken et al. 2018 - GIRK currents in VTA dopamine neurons control the sensitivity of mice to cocaine-induced locomotor sensitization
1.
2. Acute or repeated exposure to psychostimulants can produce long-term neuronal adaptations in
mesocorticolimbic dopamine (DA) circuits which promote drug taking despite negative consequences
For a review see Sultzer, 2011 Neuron
The classic understanding of VTA neuronal subtypes
describes…
GABA
DA
GABADA
Ih – “funny” current
Hyperpolarization
-activated cyclic
nucleotide–gated
(HCN) channels
6. G-protein coupling of GIRK channels in VTA neurons
At membrane
potentials negative to
K+ reversal potential,
GIRK channels can
carry significant
inward current,
At membrane
potentials positive to
K+ reversal potential
GIRKs will conduct a
small-moderate
outward current
DA
DA
D2R
D2R
9. PDZ motif
In most neurons, GIRK1, GIRK2, and GIRK3 subunits
are expressed together. In contrast, VTA DA neurons express
only GIRK2c and GIRK3 subunits and SNc DA neurons express
only two splice variants of GIRK2, GIRK2a, and GIRK2c SNX27
associates directly with a C-terminal PDZ motif (i.e.,
ESKV), present in GIRK2c and GIRK3
VTA DA
neurons
SNc DA
neurons
SNX27
Involved in retrograde transport from endosome to plasma
membrane, prevents entry into lysosomal pathway
12. Designer allele recombinase
Vs.
BAC vector
BAC vector to drive correct
expression in transgenic mice
is a result of their ability to
accommodate dispersed
regulatory sequences across
relatively large regions of the
genome (tens to hundreds of
kilobases) for a given gene
Recently, some concern has been raised for the selection of
Cre-driver lines for targeting midbrain DA neurons. Therefore,
we also used a Bac-transgenic TH-Cre+/ line, backcrossed
more than five generations into C57BL/6, to breed with
SNX27fl/fl mice (i.e., SNX27TH KO).
13. retrograding adeno-associated
virus 5 (AAV5) that expresses
Cre-dependent eYFP
(AAV.DIO.eYFP)
NAc lateral shell, which is the
primary target of “conventional”
Ih+ and D2R-expressing DA
neurons in the VTA
14. DS projecting SNc
DA neurons
SNX27 is required for maintaining GABABRGIRK
and D2R-GIRK signaling in both ventral and DS projecting
DA neurons. Furthermore, SNX27 appears to regulate
GABABR-GIRK and D2R-GIRK signaling in the absence of
the GIRK3 subunit, because SNc DA neurons appear to lack
GIRK3
Cre-dependent eYFP
(AAV.DIO.eYFP)
15. VTA NAc SNc DS
SNX27 plays an important role in regulating GABABR dependent
inhibition of firing, with little change in resting neuronal excitability (Vrest)
These cell type- and projection-specific findings in SNX27TH KO mice
suggest that GIRK3 is not required for SNX27-dependent regulation of
GIRK channels in midbrain DA neurons in vivo
17. In addition to GIRK2c/GIRK3
channels, SNX27 regulates
trafficking of other signaling
proteins—for example,
glutamate receptors and β-
adrenergic receptors raising
the possibility that some of the
behavioral changes observed
in SNX27TH KO may not be due
to changes in regulation of
GIRK channels.
SNX27
18. These findings demonstrate that,
irrespective of the diverse binding targets
of SNX27, the behavioral effects of its
deletion from midbrain DA neurons on
locomotor sensitization to cocaine can be
fully reversed by exogenous expression
of GIRK2a in primarily VTA DA neurons.
Thus, the role of SNX27 in VTA DA
neurons in changing the sensitivity to
locomotor sensitization with cocaine is
mediated primarily by SNX27-dependent
regulation of GIRK channels.Direct injection leads to GIRK2a expression
in all VTA neurons which may express to
other regions (BLA, mPFC, etc.)
