Evaluation of Urea-Based Inhibitors of the Dopamine Transporter Using the Experimental Autoimmune Encephalomyelitis Model of Multiple Sclerosis.

The dopaminergic system is involved in the regulation of immune responses in various homeostatic and disease conditions. For conditions such as Parkinson's disease and multiple sclerosis (MS), pharmacological modulation of dopamine (DA) system activity is thought to have therapeutic relevance, providing the basis for using dopaminergic agents as a treatment of relevant states. In particular, it was proposed that restoration of DA levels may inhibit neuroinflammation. We have recently reported a new class of dopamine transporter (DAT) inhibitors with high selectivity to the DAT over other G-protein coupled receptors tested. Here, we continue their evaluation as monoamine transporter inhibitors. Furthermore, we show that the urea-like DAT inhibitor (compound 5) has statistically significant anti-inflammatory effects and attenuates motor deficits and pain behaviors in the experimental autoimmune encephalomyelitis model mimicking clinical signs of MS. To the best of our knowledge, this is the first study reporting the beneficial effects of DAT inhibitor-based treatment in animals with induced autoimmune encephalomyelitis, and the observed results provide additional support to the model of DA-related neuroinflammation.

Kappa opioid receptor activation in the amygdala disinhibits CRF neurons to generate pain-like behaviors.

Recent evidence suggests that kappa opioid receptors (KOR) in limbic brain regions such as the amygdala contribute to pain conditions, but underlying mechanisms remain to be determined. The amygdala is an important player in averse-affective aspects of pain and pain modulation. The central nucleus (CeA) serves output functions through projection neurons that include corticotropin releasing factor (CRF) expressing neurons. The CeA is also rich in KOR. Here we tested the novel hypothesis that KOR activation in the CeA generates pain-like behaviors through a mechanism that involves inhibition of synaptic inhibition (disinhibition) of CRF neurons. Intra-CeA administration of a KOR agonist (U-69,593) increased vocalizations of naïve rats to noxious stimuli, and induced anxiety-like behaviors in the open field test (OFT) and avoidance in the conditioned place preference test, without affecting mechanosensory thresholds. Optogenetic silencing of CeA-CRF neurons blocked the facilitatory effects of systemically applied U-69,593 in naïve rats. Patch-clamp recordings of CRF neurons in rat brain slices found that U-69,593 decreased feedforward inhibitory transmission evoked by optogenetic stimulation of parabrachial afferents, but had no effect on monosynaptic excitatory transmission. U-69,593 decreased frequency, but not amplitude, of inhibitory synaptic currents, suggesting a presynaptic action. Multiphoton imaging of CeA-CRF neurons in rat brain slices showed that U-69,593 increased calcium signals evoked by electrical stimulation of presumed parabrachial input. This study shows for the first time that KOR activation increases activity of amygdala CRF neurons through synaptic disinhibition, resulting in averse-affective pain-like behaviors. Blocking KOR receptors may therefore represent a novel therapeutic strategy.