Research Article: NMDA Receptors on Non-Dopaminergic Neurons in the VTA Support Cocaine Sensitization

Date Published: August 16, 2010

Publisher: Public Library of Science

Author(s): Yu Luo, Cameron H. Good, Oscar Diaz-Ruiz, YaJun Zhang, Alexander F. Hoffman, Lufei Shan, Serena Y. Kuang, Nasir Malik, Vladimir I. Chefer, Andreas C. Tomac, Carl R. Lupica, Cristina M. Bäckman, Georges Chapouthier.

Abstract: The initiation of behavioral sensitization to cocaine and other psychomotor stimulants is thought to reflect N-methyl-D-aspartate receptor (NMDAR)-mediated synaptic plasticity in the mesolimbic dopamine (DA) circuitry. The importance of drug induced NMDAR mediated adaptations in ventral tegmental area (VTA) DA neurons, and its association with drug seeking behaviors, has recently been evaluated in Cre-loxp mice lacking functional NMDARs in DA neurons expressing Cre recombinase under the control of the endogenous dopamine transporter gene (NR1DATCre mice).

Using an additional NR1DATCre mouse transgenic model, we demonstrate that while the selective inactivation of NMDARs in DA neurons eliminates the induction of molecular changes leading to synaptic strengthening, behavioral measures such as cocaine induced locomotor sensitization and conditioned place preference remain intact in NR1DATCre mice. Since VTA DA neurons projecting to the prefrontal cortex and amygdala express little or no detectable levels of the dopamine transporter, it has been speculated that NMDA receptors in DA neurons projecting to these brain areas may have been spared in NR1DATCre mice. Here we demonstrate that the NMDA receptor gene is ablated in the majority of VTA DA neurons, including those exhibiting undetectable DAT expression levels in our NR1DATCre transgenic model, and that application of an NMDAR antagonist within the VTA of NR1DATCre animals still blocks sensitization to cocaine.

These results eliminate the possibility of NMDAR mediated neuroplasticity in the different DA neuronal subpopulations in our NR1DATCre mouse model and therefore suggest that NMDARs on non-DA neurons within the VTA must play a major role in cocaine-related addictive behavior.

Partial Text: Midbrain dopamine (DA) neurons in the ventral tegmental area (VTA) represent a common substrate for drugs of abuse and mediate the engagement of addictive behaviors. Glutamatergic transmission within the VTA has been shown to be particularly important since local injection of glutamate antagonists during repeated drug administration blocks behavioral sensitization and conditional place preference (CPP) [1], [2], [3], [4], [5], [6]. Previous work has also shown that abused drugs evoke N-methyl-D-aspartate receptor (NMDAR)-dependent, long-term potentiation (LTP) of glutamatergic transmission in DA neurons [7], [8], [9]. Therefore, NMDAR-dependent LTP might represent an essential component of the neural basis of sensitization, and the development of compulsive drug-seeking behavior. The role of VTA NMDARs in addictive behavior was recently investigated using DA cell-specific ablation of the gene coding for subunit 1 of the NMDA receptor (NR1), by expressing Cre recombinase using the DAT promoter [10], [11]. This causes a selective absence of functional NMDARs on DA neurons expressing DAT, and represents a significant advance because it permits an evaluation of NMDAR function in these cells, compared to the non-selective effects of antagonists on multiple neuron subtypes. Surprisingly, these prior studies demonstrated that despite the elimination of NMDAR-dependent LTP in DA neurons in these NR1 knockout (KO) mice, cocaine sensitization developed normally [10], [11], suggesting that locomotor sensitization does not require NMDAR-dependent neuronal plasticity.

The involvement of local glutamatergic synapses within the VTA in learning and the development of adaptive processes in animal models of drug abuse has been widely documented [25]. The fact that both, single or repeated administration of an addictive drug directly into the VTA initiates sensitization to subsequent systemic drug challenge, together with the observation that NMDAR antagonists delivered directly to the VTA block the development of sensitization [1], [4], [5], suggests a role for glutamatergic transmission within the VTA for the initiation of addictive behaviors. However, the precise substrates through which altered glutamatergic transmission results in drug modified synaptic plasticity and the development of addictive behaviors remain to be elucidated. Our results extend previous findings by using a transgenic mouse lacking the NR1 receptor subunit in DA neurons (NR1DATCre), and by confirming the absence of NMDARs in virtually all VTA DA neurons. Since our approach, like previous studies [6], [10], [11], [21], relied upon DAT expression for the selective elimination of NMDAR function in DA neurons, we considered it important to determine whether NR1 gene deletion occurred in mesostriatal and mesocortical circuits, because the latter is characterized by DA neurons with low or undetectable DAT protein levels [12]. It has been speculated that remaining NMDAR function in these neuronal circuits might account for the initiation of behavioral sensitization in NR1 KO animals [26]. Our study provides the first direct evidence that the NR1 coding region in VTA DA neurons of NR1DATCre mice is absent in nearly all DA neurons, even those with undetectable or low DAT protein levels. This suggests that the low activity of the DAT promoter in these cells is sufficient to drive Cre-mediated recombination. Alternatively, a transient elevation of DAT expression levels during development [27] may have induced the permanent deletion of NR1 that we observe in adult NR1DATCre animals.