Our studies have addressed three research areas. 1. Pain mechanisms in PD. Although primarily known as a movement-related disorder, Parkinsons disease (PD) has several non-motor symptoms, such as pain, that have gained increasing attention. High prevalence of pain and increased pain sensitivity have been observed in human PD patients and animal models of PD. Studies have shown the ventromedial thalamic nucleus (VM) to relay nociceptive information from the medullary subnucleus reticular dorsalis to both the BG and a cortical region known to be involved in pain processing, the anterior cingulate cortex (ACC). As recent work from our lab has shown that the ACC and VM, components of the BG-thalamocortical circuit, exhibit pathological beta activity in a parkinsonian rat model, we have hypothesized that the excessive beta oscillations will disrupt pain processing. We have used 6-hydroxydopamine to lesion dopamine neurons in one hemisphere, to induce a unilateral rat model of PD. Electrode bundles are chronically implanted into the subthalamic nucleus (STN), ACC, and VM of the dopamine cell lesioned left hemisphere of rats. To induce a pain response, 1.5% concentration of the toxin formalin was subcutaneously injected into the planar surface of the right hind paw. The formalin test produces a biphasic response consisting of a 3-5 minute interval of pain behavior immediately after injection, a 10-15 minute quiescent interphase during which the rats will not display nociceptive behavior, and finally a 20-40 minute period of inflammatory pain. Behavioral pain response, spiking and LFP activity were recorded for the two hours immediately after formalin injection. Our studies, in collaboration with Dr. Yarimar Carrasquillo, show that the lesioned rats injected with 1.5% formalin have greater alpha (12-19 Hz) LFP power in the VM than sham lesioned rats. Further analysis is ongoing. Additional study into pain mechanisms may lead to the development of better treatment options for PD patients experiencing pain. 2. Cognitive function in PD. Investigation of changes in the ACC cognitive function have provided the basis of a PhD thesis recently successfully submitted by Alex Weiss (NIH Cambridge/Oxford PhD program). These studies are currently being further analyzed and written up for publication and presentation in a poster at the Society for Neuroscience in Nov. PD patients are known to express dopamine-dependent cognitive impairments, implying effects of dopamine loss on PFC function. The electrophysiological correlates of these cognitive symptoms are not well understood. His recent study under the Oxford arm of this project compared electrophysiological data from the BG of PD and non-PD patients during a cognitive task and showed the presence of electrophysiological abnormalities associated with cognitive function in the BG that may be used as biomarkers to help understand the basis of PD cognitive impairment. In light of the ACCs development of parkinsonian exaggerated beta activity, we have investigated the involvement of BG thalamocortical circuits in dopamine-impaired and healthy rats in response to a salient cue that predicts the onset of either tone-to-treadmill-induced walking (an expected event) or tone to-no-treadmill-induced walking (an unexpected event). Rats were trained to expect epochs of treadmill walking after a tone, with subsequent epochs manipulating expectancy through tone-to-walk and tone-to-no-walk epochs. Changes in LFP power, spiking, and coherence data from several areas of the BG thalamocortical circuit involved in decision-making were recorded during epochs surrounding auditory stimuli and treadmill walking in control animals and after dopamine cell lesion. Thus far, results provided a series of observations supporting the view that the BG thalamocortical circuit develops both exaggerated oscillatory activity and spike-to-LFP phase locking during cognition in the hemiparkinsonian rat model of advanced stage PD. Theta frequency in the ACC has been linked with error prediction. Our results show significant increase in theta power the ACC during the first rule reversal in the control rats but not the lesioned rats on day 7 post-lesion. We also observed significant increase of coherence in the theta range between the ACC and the VM from day 7 to day 21 post-lesion during the first rule reversal, as well as significant decrease in the coherence between ACC and the STN. Dopamine lesioned rats exhibited higher ACC alpha power than non-lesioned rats during unexpected outcomes, which may suggest poor memory function or attention in the lesioned animals. In the beta range, LFP power was significantly increased in all areas during the walk epochs and first rule reversal epoch with the exception of the STN during the rule reversal. These results will be further analyzed, and could provide clues to biomarkers for alteration of executive control in hemiparkinsonian rats. As cognitive symptoms in PD patients become an increasingly recognized issue, preclinical animal models for the study of PD cognitive impairment have become increasingly necessary. Despite the limitations of the model, hemiparkinsonian rats display some of the most important cognitive symptoms seen in PD patients, including impairment of executive function, and our results so far have been able to reproduce some of the electrophysiological dysfunctions in the same anatomical sites as observed in human subjects. 3. Role of Parafascicular thalamic nucleus (PF) in PD. The PF nucleus receives inputs from the BG, cortex, and cerebellum and provides feedback to the subthalamic nucleus (STN) and striatum (STR) and thus could be implicated in the pathophysiology of PD. Unlike the VM, however, we have found that exaggerated beta oscillations are not evident in recordings from the PF in the behaving, hemiparkinsonian rat. Instead, our latest results call attention to the potential role of PF output in tonic modulation of STR and STN activity and support the idea that reductions in PF activity may have a therapeutic effect on motor dysfunction in PD. In the present study, one group of hemiparkinsonian rats received infusion into the PF of the inhibitory DREADD virus: AAV2-hSyn-hM4D(Gi)-mCherry (hM4D) and a second group received infusion the CRE-dependent virus (pAAV-hSyn-dF-HA-KORD-IRES-mCitrine) into PF in conjunction with infusion of the retrograde Cre-recombinase virus (AAV pmSyn1-EBFP-Cre) into dorsolateral striatum, allowing for retrograde expression of Cre in the PF neurons projecting to the striatum and subsequent expression of the KORD Cre-dependent virus in the PF nucleus. Histological studies are underway to determine if the KORD virus is selectively expressed in the striatum, as projections from PF neurons could also impact STN activity directly. Recording electrodes were implanted targeting motor cortex (MCx), STR, and SNpr. Histological analyses revealed modest DREADD virus expression in the PF with some spread to surrounding areas. Similar to previous studies, exaggerated high beta power in the MCx and SNpr and coherence between these regions were observed during treadmill walking. At 3-4 weeks post-surgery, clozapine-N-oxide (1-5 mg/kg, ip.) and salvinorin B (1-2 mg/kg, sc.), both of which activate the receptors expressed by the corresponding DREADD virus, substantially improved circular treadmill walking in the clockwise direction without modifying cortical or nigral high beta oscillations. These results suggest that inactivation of PF terminals on PF output neurons reduces motor deficit in a rodent model of PD, similar to the effect of the inhibitory agonist muscimol. Ongoing DREADDs experiments are further exploring the contribution of the PF to high beta oscillations in BG-cortical motor circuits as well as spiking activity in PF, STR, SNpr and MCx.
