INTRODUCTION
Cocaine is the most used psychostimulant illicit drug worldwide (UNODC,
2019), causing severe health problems that include the development of
cocaine addiction in around 15-16% of cocaine users (O’Brien and
Anthony, 2005). Cocaine addiction is a complex psychiatric disorder that
results from the interaction of genetic, epigenetic, and environmental
risk factors (Pierce et al., 2018). The heritability of cocaine
addiction is one of the highest among psychiatric disorders, estimated
around 65% for women (Kendler et al., 2000) and 79% for men (Kendler
and Prescott, 1998). However, the genetic factors and mechanisms that
underlie the transition from drug use to addiction and its establishment
remain unknown.
In a previous study, we identified a single nucleotide polymorphism in
the PLCB1 gene associated with drug dependence (rs1047383), and
especially with a subgroup of cocaine-addicted patients, that was
replicated in an independent clinical sample (Cabana-Domínguez et al.,
2017). Also, genetic variants in this gene were found nominally
associated with an illegal substance and cocaine addiction in two GWAS
performed in European-American samples (Drgon et al., 2012). On the
other hand, we found that cocaine increased the expression ofPLCB1 both in human dopaminergic neuron-like cells
(differentiated SH-SY5Y cells) after acute cocaine exposure and in
postmortem samples of the nucleus accumbens (NAc) of cocaine abusers
(Cabana-Domínguez et al., 2017). Interestingly, this gene was also found
over-expressed in the same brain region in mice after cocaine
administration for 7 days and also during withdrawal (Eipper-Mains et
al., 2013). All this evidence suggest that PLCB1 may play a role
in cocaine addiction.
The PLCB1 gene encodes phospholipase C beta 1, and it is highly
expressed in the brain, mainly in the frontal cortex, basal ganglia
(caudate, putamen, and NAc) and hippocampus. These brain regions are
crucial for drug reward and the formation of drug-context associations,
both contributing to the development and maintenance of addiction
(Bisagno et al., 2016; Castilla-Ortega et al., 2016, 2017; Kutlu and
Gould, 2016; Pitts et al., 2016; Cooper et al., 2017). Several
neurotransmitters activate this protein, including dopamine through DRD1
and DRD2 (Lee et al., 2004; Rashid et al., 2007), serotonin by 5-HT2A
and 2C receptors (Hagberg et al., 1998; Chang et al., 2000) and
glutamate by mGluR1 (Conn and Pin, 1997; Hannan et al., 2001). Thus,
PLCB1 might be a point of convergence of neurotransmitter systems that
play an essential role in the development of addiction (recently
reviewed (Howell and Negus, 2014; Moretti et al., 2020)). The activation
of PLCB1 produces the cleavage of
phosphatidylinositol
4,5-bisphosphate (PIP2) into the second messengers
diacylglycerol
(DAG) and
inositol
1,4,5-trisphosphate (IP3), responsible for intracellular
signal
transduction. Alterations in PLCB1-mediated signaling in the brain have
been associated with other neuropsychiatric disorders such as epilepsy,
schizophrenia, and bipolar disorder (Yang et al., 2016).
Here we studied the contribution of the PLCB1 gene to cocaine
addiction and dissected its participation in the different aspects of
the addictive process. We used heterozygous knockout (KO) mice
(Plcb1+/- ), as the homozygous KO (Plcb1 -/-) showed seizure
attacks and low viability after birth (Kim et al., 1997). In addition,
the use of a constitutive heterozygous KO mouse model allowed us to
assess Plcb1 haploinsufficiency during neurodevelopment in mice
similarly to humans, where inherited genetic risk or protective variants
can modulate the susceptibility to addiction (Cabana-Domínguez et al.,
2017). First, we performed a general phenotypic characterization using a
battery of behavioral tests, including memory, anxiety, locomotor
activity, coordination, food and water intake, and sucrose preference.
Then, we evaluated cocaine operant self-administration, extinction, and
cue-induced reinstatement. Finally, we studied transcriptional
alterations to further understand the molecular mechanisms involved.