Journal of Neuroscience. 2015 January, 35(3):1211–1216.
Patterned but not tonic optogenetic stimulation in motor thalamus improves reaching in acute drug-induced parkinsonian rats
Sonja Seeger-Armbruster1, Clémentine Bosch-Bouju2, Shane TC Little2, Roseanna A Smither1, Stephanie M Hughes3, Brian I Hyland1 and Louise C Parr-Brownlie2
Departments of Physiology1, Anatomy2 and Biochemistry3, Otago School of Medical Science, Brain Health Research Centre, Brain Research New Zealand, University of Otago, Dunedin, New Zealand
High frequency deep brain stimulation (DBS) in motor thalamus (Mthal) ameliorates tremor but not akinesia in Parkinson’s disease. The aim of this study was to investigate if there are effective methods of Mthal stimulation to treat akinesia. Glutamatergic Mthal neurons, transduced with channelrhodopsin-2 by injection of lentiviral vector (pLenti.CamKII.hChR2(H134R).mCherry), were selectively stimulated with blue light (473 nm) via a chronically implanted fibre-optic probe. Rats performed a reach-to-grasp task in either acute drug-induced parkinsonian akinesia (0.03-0.07 mg/kg haloperidol, s.c.) or control (vehicle injection) conditions and the number of reaches was recorded for 5 minutes before, during and after stimulation. We compared the effect of DBS using complex physiological patterns previously recorded in the Mthal of a control rat during reaching or exploring behaviour, with tonic DBS delivering the same number of stimuli per second (rate-control 6.2 or 1.8 Hz, respectively) and with stimulation patterns commonly used in other brain regions to treat neurological conditions (tonic 130 Hz, theta burst (TBS), and tonic 15 Hz rate-control for TBS). Control rats typically executed >150 reaches per 5 minutes, which was unaffected by any of the stimulation patterns. Acute parkinsonian rats executed <20 reaches, displaying marked akinesia, which was significantly improved by stimulating with the physiological reaching pattern or TBS (both p<0.05), whereas the exploring and all tonic patterns failed to improve reaching. Data indicate that the Mthal may be an effective site to treat akinesia, but the pattern of stimulation is critical for improving reaching in parkinsonian rats.
KEYWORDS: Parkinson’s disease; motor thalamus; optogenetic stimulation; patterns of stimulation; reaching task; theta burst stimulation
Parkinson’s disease (PD) is a movement disorder caused by degeneration of dopamine cells in the midbrain and accummulation of the protein alpha-synuclein throughout many brain regions. The greatest risk factor for PD is age. With an aging population in most Western societies the prevalence of PD is expected to double by 2040. While current drug and surgical treatments are helpful, they cause unwanted side-effects in many patients. Therefore, there is a critical need to develop novel treatments that improve the quality of patients’ lives and also have fewer side-effects.
|Figure 1. A rat reaching for a food reward while the VA thalamus is being stimulated with blue light.|
To discover new treatments, we have been recording cell activity in movement areas of the brain to examine if changes occur in a rat model of PD. We are interested in knowing if changes in cell activity are related to the movement deficits of PD; slowness of movement (bradykinesia), difficulty initiating movements (akinesia) and postural rigidity. We use a skilled reaching task to examine effects because the muscles involved and the order they are activated are comparable between humans and rats, and the deficits experienced by patients and the PD model rats are also similar. Briefly, rats reach through a narrow tube to obtain a food reward (Figure 1).
Using this approach we recorded activity in the ventroanterior (VA) motor thalamus in control and PD model rats1. The VA thalamus receives information from the basal ganglia, the location of dopamine neurons that degenerate in PD, and also projects information to the motor cortex, the last site in the brain where a motor plan is developed before it descends the spinal cord and controls muscle contractions (Figure 2). Few studies had investigated changes in VA thalamus activity, with half reporting no changes and the remaining studies reporting conflicting results. Key to our study was that we investigated changes in cell activity when PD model rats moved, but movements were impaired – previous studies had been conducted in anesthetised animals or animals so affected they did not move at all. We found that VA thalamus firing rate and pattern was impaired when the rats rested quietly. More importantly, PD model rats showed discrete deficits in firing rate and pattern during specific phases of the reaching movement1. VA cell firing rate in control rats had a triphasic pattern; a significant increase (peak or excitation) at the time the rat obtained the food and significant decreases (inhibitions) before and after the peak. The late inhibition displayed the largest and most consistent modulation. In PD model rats, inhibitory modulations were lost1, which is consistent with altered input from the basal ganglia. For firing pattern, VA thalamus cells in control rats exhibited a high frequency (>100 Hz) burst of activity (low threshold calcium spike bursts) that is characteristic of thalamic cells, but had not been previously reported in the awake state or during movement. During the task, this high frequency burst decreased in control rats after they had obtained the food whereas this modulation was lost in PD model rats1. This paper revealed that changes in VA motor thalamus activity underlie the movement deficits of PD, which raises the possibility that VA thalamus might be a good site to target treatments for PD.
|Figure 2. Motor pathway affected by Parkinson’s disease.|
We wanted to test if the PD-induced changes in VA thalamus cell activity were key for producing the parkinsonian movement deficits. To explore this and also determine if stimulation targeted in the VA thalamus might effectively treat symptoms of PD, we stimulated the PD rat brain using the timing of the previously recorded cell activity from a control rat reaching (reaching pattern). We also stimulated the VA thalamus with a theta-burst pattern (15 stimuli per s, applied in 5 bursts with stimuli within bursts at 50 Hz) and tonically applied stimuli between 2-130 Hz. We wanted to precisely control the type of brain cells activated and know the maximum area of the brain that was stimulated to ensure that stimulation was restricted to only VA thalamus. To do this, we used optogenetic technology by injecting a lentiviral vector containing the light responsive channel (channelrhodopsin, ChR2) and the mCherry reporter flurophore into the VA thalamus2. Only glutamatergic cells became light responsive because expression of ChR2-mCherry was controlled by the promoter CaMKIIalpha. We injected the vector directly into the VA thalamus and at the end of experiments we checked that that ChR2-mCherry expression only occurred in the VA thalamus. To activate cells, blue light (473 nm) from a laser was connected to a fibre optic probe implanted into the VA thalamus. The timing of blue light pulses was recorded using video and behavioural analysis programmes so that we could correlate the stimulation pattern and reaching performance.
We found that stimulation in VA thalamus could recover movements. Prior to stimulation, parkinsonian rats executed less than 3 reaches per minute, whereas control rats executed more than 30 reaches per minute. We applied light pulses for 20 minutes. Only the reaching and theta-burst patterns improved reaching movements; with the reaching pattern producing the best and most consistent effect. At the end of the 20 minutes of stimulation, PD model rats executed 11 reaches per minute. Blue light stimulation with a consistent interval was ineffective, including the rate-controls (mean rate) for reaching and theta-burst patterns. These experiments show that the VA motor thalamus can be used to treat PD, but complex patterns of stimulation need to be applied. We believe that modifying the stimulation parameters will improve treatment outcomes; enhance the anti-akinetic effect and also improve bradykinesia and postural rigidity. In addition, our experiments support the notion that the timing of neural events is key for controlling movements. Deviations from the normal pattern of brain activity underlie movement deficits.
Our findings provide hope that there are new sites and ways to stimulate the brain. We predict that optogenetic sitmulation will produce fewer side-effects for PD patients than traditional methods. Optogenetic stimulation is not currently used on PD patients; many more experiments need to be done before a clinical trial will be approved. In addition, the human brain is significantly larger than the rat brain, which means that the technology must be modified for optogenetic stimulation to be a feasible treatment for patients. Furthermore, devices need to be developed to enable chronic light stimulation. We, and others, are working on solutions. We envisage that optogenetic stimulation may become an treatment option for PD in about 10 years.
Figure 3. The talented young investigators involved in this project. L to R, Shane Little, Louise Parr-Brownlie, Sonja Seeger-Armbruster, Roseanna Smither, Clementine Bosch-Bouju. Inset photos of Stephanie Hughes and Brian Hyland.
- Bosch-Bouju C, Smither RA, Hyland BI, Parr-Brownlie LC (2014) Reduced reach-related modulation of motor thalamus neural activity in a rat model of Parkinson’s disease. Journal of Neuroscience, 34 (48), 15836-15850.
- Zhang F, Aravanis AM, Adamantidis A, de Lecea L, Deisseroth K (2007) Circuit-breakers: optical technologies for probing neural signals and systems. Nature Reviews Neuroscience 8, 577-581.
Louise Parr-Brownlie, PhD
Department of Anatomy
University of Otago
PO Box 913
+64 3 479 4960