Non-technical summary The lower mid-brain of rodent is home to addiction and hedonism of substances of abuse. Due to overlapping physiological properties among different types of nerve cell within this brain region, it has been difficult to selectively study the physiology of a single type of nerve cell. Here, using gene technology together with selective antibody detection, we have exclusively identified the two dominant populations of nerve cells and found dopamine-containing cells five times more abundant. We found clear non-overlapping cellular characteristics between the two types that are unequivocal predictors for selection of nerve cells, whilst other previously employed cellular criteria were found to be less useful.Abstract The midbrain ventral tegmental area (VTA) contains neurons largely with either a dopaminergic (DAergic) or GABAergic phenotype. Physiological and pharmacological properties of DAergic neurons have been determined using tyrosine hydroxylase (TH) immunohistochemistry but many properties overlap with non-DAergic neurons presumed to be GABAergic. This study examined properties of GABAergic neurons, non-GABAergic neurons and TH-immunopositive neurons in VTA of GAD67-GFP knock-in mice. Ninety-eight per cent of VTA neurons were either GAD-GFP or TH positive, with the latter being five times more abundant. During cell-attached patch-clamp recordings, GAD-GFP neurons fired brief action potentials that could be completely distinguished from those of non-GFP neurons. Pharmacologically, the μ-opioid agonist DAMGO inhibited firing of action potentials in 92% of GAD-GFP neurons but had no effect in non-GFP neurons. By contrast, dopamine invariably inhibited action potentials in non-GFP neurons but only did so in 8% of GAD-GFP neurons. During whole-cell recordings, the narrower width of action potential in GAD-GFP neurons was also evident but there was considerable overlap with non-GFP neurons. GAD-GFP neurons invariably failed to exhibit the potassium-mediated slow depolarizing potential during injection of positive current that was present in all non-GFP neurons. Under voltage-clamp the cationic current, I h , was found in both types of neurons with considerable overlap in both amplitude and kinetics. These distinct cellular properties may thus be used to confidently discriminate GABAergic and DAergic neurons in VTA during in vitro electrophysiological recordings.

Chronic morphine treatment produces behavioral and cellular opioid tolerance that has been proposed to be caused by attenuated -opioid receptor (MOR) recovery from desensitization (resensitization). The process of MOR resensitization is thought to require ␤arrestin-2 (␤arr-2)-dependent trafficking of desensitized receptors to endosomal compartments, followed by recycling of resensitized receptors back to the plasma membrane. However, there is little direct evidence for this, particularly in native neurons. This study used whole-cell patch-clamp recording in locus ceruleus (LC) neurons from wild-type (w.t.) and ␤arr-2 knock-out (k.o.) mice to examine whether ␤arr-2/dynamin-dependent trafficking is required for MOR resensitization in neurons from opioid-naive and morphine-treated mice. Surprisingly, recovery of MOR from acute desensitization in LC neurons does not require ␤arr-2-or dynamin-dependent trafficking. To the contrary, MOR resensitization was accelerated by disruption of either ␤arr-2 or dynamin function. Chronic morphine treatment caused cellular MOR tolerance and concurrently impaired MOR resensitization in neurons from w.t. mice, as expected from previous studies, but neither occurred in neurons from ␤arr-2 k.o. mice. Moreover, the impairment of MOR resensitization caused by chronic morphine was reversed in w.t. neurons when G-protein-coupled receptor kinase-2 (GRK2) or dynamin function was disrupted. Together, these results establish that ␤arr-2/dynamin-dependent receptor regulation is not required for MOR resensitization in LC neurons. Furthermore, chronic morphine treatment modifies GRK2-␤arr-2-dynamin-dependent MOR trafficking to impair receptor resensitization, thereby contributing to opioid tolerance in LC neurons by reducing the number of functional receptors on the surface membrane.

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