Identification of Rat Ventral Tegmental Area GABAergic Neurons

Abstract

The canonical two neuron model of opioid reward posits that mu opioid receptor (MOR) activation produces reward by disinhibiting midbrain ventral tegmental area (VTA) dopamine neurons through inhibition of local GABAergic interneurons. Although indirect evidence supports the neural circuit postulated by this model, its validity has been called into question by growing evidence for VTA neuronal heterogeneity and the recent demonstration that MOR agonists inhibit GABAergic terminals in the VTA arising from extrinsic neurons. In addition, VTA MOR reward can be dopamine-independent. To directly test the assumption that MOR activation directly inhibits local GABAergic neurons, we investigated the properties of rat VTA GABA neurons directly identified with either immunocytochemistry for GABA or GAD65/67, or in situ hybridization for GAD65/67 mRNA. Utilizing co-labeling with an antibody for the neural marker NeuN and in situ hybridization against GAD65/67, we found that 23±3% of VTA neurons are GAD65/67(+). In contrast to the assumptions of the two neuron model, VTA GABAergic neurons are heterogeneous, both physiologically and pharmacologically. Importantly, only 7/13 confirmed VTA GABA neurons were inhibited by the MOR selective agonist DAMGO. Interestingly, all confirmed VTA GABA neurons were insensitive to the GABAB receptor agonist baclofen (0/6 inhibited), while all confirmed dopamine neurons were inhibited (19/19). The heterogeneity of opioid responses we found in VTA GABAergic neurons, and the fact that GABA terminals arising from neurons outside the VTA are inhibited by MOR agonists, make further studies essential to determine the local circuit mechanisms underlying VTA MOR reward.

Neurosteroid Agonist at GABAA Receptor Induces Persistent Neuroplasticity in VTA Dopamine Neurons

“…Both cell types exhibited GABAergic inhibitory activity that varied in amplitude and event frequency, and some recorded cells expressed no tonic current. These data are in agreement with the idea that VTA neurons belong to different subtypes and receive different innervation (Margolis et al, 2012), showing marked heterogeneity within the DA and GABA neuronal populations.…”

“…Importantly, we used two mouse lines, GAD67-GFP knockin mice and Th-EGFP BAC transgenic mice (Gong et al, 2003;Tamamaki et al, 2003), which helped us to identify the VTA GABA and DA neurons, respectively, independently of their electrophysiological characteristics (Margolis et al, 2012). Despite expression of tonic inhibition in both VTA DA and GABA neurons in the presence 500 nM GABA (in the range of physiological extracellular GABA concentration (Murphy and Maidment, 1999)), we believe that tonic inhibition is more effective in VTA GABA interneurons as these neurons have a higher input resistance than DA neurons, allowing the same charge transfer to more effectively change their membrane potential (Margolis et al, 2012). We observed tonic currents with amplitudes between 10 and 30 pA, in line with studies in various brain areas (Marowsky et al, 2012;Stell et al, 2003).…”

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

“…A main effect of dose (F (3,54) ϭ 103.4, p Ͻ 0.0001) and genotype (F (3,18) ϭ 3.2, p Ͻ 0.05) was observed, but there was no dose/genotype interaction (F (9,54) ϭ 1.8, p ϭ 0.1). **p Ͻ 0.01 versus wild-type; cent of findings in the rat (Margolis et al, 2012). Similarly, only one-third of the RMTg neurons projecting to the ventral midbrain (VTA and susbstantia nigra) were hyperpolarized by MOR agonists (Matsui and Williams, 2011).…”

Differentiation and molecular heterogeneity of inhibitory and excitatory neurons associated with midbrain dopaminergic nuclei

“…Inhibitory signals to DA neurons, located in the ventral tegmental area (VTA), substantia nigra pars compacta (SNpc) and retrorubral field (RRF), arrive from neighbouring GABAergic neurons in the VTA, substantia nigra pars reticulata (SNpr) and rostromedial tegmental nucleus (RMTg, also called the tail of VTA) (Perrotti et al, 2005;Jhou et al, 2009a,b;Kaufling et al, 2009;Zhou and Lee, 2011;Cohen et al, 2012;Lammel et al, 2012;Margolis et al, 2012;Yetnikoff et al, 2015). Collectively, these GABAergic neurons are referred to as dopaminergic neuron-associated GABAergic (D-GABA) neurons .…”