Faculty Bibliography

BACKGROUND: Pain and natural rewards such as food elicit different behavioral effects. Both pain and rewards, however, have been shown to alter synaptic activities in the nucleus accumbens (NAc), a key component of the brain reward system. Mechanisms by which external stimuli regulate plasticity at NAc synapses are largely unexplored. Medium spiny neurons (MSNs) from the NAc receive excitatory glutamatergic inputs and modulatory dopaminergic and cholinergic inputs from a variety of cortical and subcortical structures. Glutamate inputs to the NAc arise primarily from prefrontal cortex, thalamus, amygdala, and hippocampus, and different glutamate projections provide distinct synaptic and ultimately behavioral functions. The family of vesicular glutamate transporters (VGLUTs 1-3) plays a key role in the uploading of glutamate into synaptic vesicles. VGLUT1-3 isoforms have distinct expression patterns in the brain, but the effects of external stimuli on their expression patterns have not been studied. RESULTS: In this study, we use a sucrose self-administration paradigm for natural rewards, and spared nerve injury (SNI) model for chronic pain. We examine the levels of VGLUTs (1-3) in synaptoneurosomes of the NAc in these two behavioral models. We find that chronic pain leads to a decrease of VGLUT1, likely reflecting decreased projections from the cortex. Pain also decreases VGLUT3 levels, likely representing a decrease in projections from GABAergic, serotonergic, and/or cholinergic interneurons. In contrast, chronic consumption of sucrose increases VGLUT3 in the NAc, possibly reflecting an increase from these interneuron projections. CONCLUSION: Our study shows that natural rewards and pain have distinct effects on the VGLUT expression pattern in the NAc, indicating that glutamate inputs to the NAc are differentially modulated by rewards and pain.

The mechanisms by which natural rewards such as sugar affect synaptic transmission and behavior are largely unexplored. Here, we investigate regulation of nucleus accumbens synapses by sucrose intake. Previous studies have shown that AMPA receptor (AMPAR) trafficking is a major mechanism for regulating synaptic strength, and that in vitro, trafficking of AMPARs containing the GluA1 subunit takes place by a two-step mechanism involving extrasynaptic and then synaptic receptor transport. We report that in rat, repeated daily ingestion of a 25% sucrose solution transiently elevated spontaneous locomotion and potentiated accumbens core synapses through incorporation of Ca(2+)-permeable AMPA receptors (CPARs), which are GluA1-containing, GluA2-lacking AMPARs. Electrophysiological, biochemical, and quantitative electron microscopy studies revealed that sucrose training (7 d) induced a stable (>24 h) intraspinous GluA1 population, and that in these rats a single sucrose stimulus rapidly (5 min) but transiently (<24 h) elevated GluA1 at extrasynaptic sites. CPARs and dopamine D1 receptors were required in vivo for elevated locomotion after sucrose ingestion. Significantly, a 7 d protocol of daily ingestion of a 3% solution of saccharin, a noncaloric sweetener, induced synaptic GluA1 similarly to 25% sucrose ingestion. These findings identify multistep GluA1 trafficking, previously described in vitro, as a mechanism for acute regulation of synaptic transmission in vivo by a natural orosensory reward. Trafficking is stimulated by a chemosensory pathway that is not dependent on the caloric value of sucrose.

BACKGROUND: Chronic pain is associated with depression. In rodents, pain is often assessed by sensory hypersensitivity, which does not sufficiently measure affective responses. Low-dose ketamine has been used to treat both pain and depression, but it is not clear whether ketamine can relieve depression associated with chronic pain and whether this antidepressant effect depends on its antinociceptive properties. METHODS: The authors examined whether the spared nerve injury model of neuropathic pain induces depressive behavior in rats, using sucrose preference test and forced swim test, and tested whether a subanesthetic dose of ketamine treats spared nerve injury-induced depression. RESULTS: Spared nerve injury-treated rats, compared with control rats, showed decreased sucrose preference (0.719 +/- 0.068 (mean +/- SEM) vs. 0.946 +/- 0.010) and enhanced immobility in the forced swim test (107.3 +/- 14.6s vs. 56.2 +/- 12.5s). Further, sham-operated rats demonstrated depressive behaviors in the acute postoperative period (0.790 +/- 0.062 on postoperative day 2). A single subanesthetic dose of ketamine (10 mg/kg) did not alter spared nerve injury-induced hypersensitivity; however, it treated spared nerve injury-associated depression-like behaviors (0.896 +/- 0.020 for ketamine vs. 0.663 +/- 0.080 for control rats 1 day after administration; 0.858 +/- 0.017 for ketamine vs. 0.683 +/- 0.077 for control rats 5 days after administration). CONCLUSIONS: Chronic neuropathic pain leads to depression-like behaviors. The postoperative period also confers vulnerability to depression, possibly due to acute pain. Sucrose preference test and forced swim test may be used to compliment sensory tests for assessment of pain in animal studies. Low-dose ketamine can treat depression-like behaviors induced by chronic neuropathic pain

PARK8/LRRK2 (leucine-rich repeat kinase 2) was recently identified as a causative gene for autosomal dominant Parkinson's disease (PD), with LRRK2 mutation G2019S linked to the most frequent familial form of PD. Emerging in vitro evidence indicates that aberrant enzymatic activity of LRRK2 protein carrying this mutation can cause neurotoxicity. However, the physiological and pathophysiological functions of LRRK2 in vivo remain elusive. Here we characterize two bacterial artificial chromosome (BAC) transgenic mouse strains overexpressing LRRK2 wild-type (Wt) or mutant G2019S. Transgenic LRRK2-Wt mice had elevated striatal dopamine (DA) release with unaltered DA uptake or tissue content. Consistent with this result, LRRK2-Wt mice were hyperactive and showed enhanced performance in motor function tests. These results suggest a role for LRRK2 in striatal DA transmission and the consequent motor function. In contrast, LRRK2-G2019S mice showed an age-dependent decrease in striatal DA content, as well as decreased striatal DA release and uptake. Despite increased brain kinase activity, LRRK2-G2019S overexpression was not associated with loss of DAergic neurons in substantia nigra or degeneration of nigrostriatal terminals at 12 months. Our results thus reveal a pivotal role for LRRK2 in regulating striatal DA transmission and consequent control of motor function. The PD-associated mutation G2019S may exert pathogenic effects by impairing these functions of LRRK2. Our LRRK2 BAC transgenic mice, therefore, could provide a useful model for understanding early PD pathological events

AMPA receptors are excitatory glutamate receptors that are critical for synaptic transmission. Not surprisingly, they serve an important function at the synapse between a peripheral nociceptive neuron and a dorsal horn neuron in the spinal cord. Furthermore, a subset of AMPA receptors, calcium permeable AMPA receptors, by allowing calcium influx, is capable of activating calcium-dependent signaling pathways. The activation of these signaling pathways, in turn, leads to long-term changes at the spinal synapses and may even play a role in central sensitization. Several antagonists for AMPA receptors have been developed for preclinical studies of stroke, seizure, amyotropic lateral sclerosis, Alzheimer's disease, addiction, and pain. Many of these agents have shown promise as potential targeted therapeutic interventions for these diseases, and clinical trials are ongoing for many specific AMPA antagonists. In the near future, these AMPA antagonists may emerge as newer analgesics with fewer side effects