Studies of the neural mechanisms of deep brain stimulation in rodent models of Parkinson's disease

Bidirectional Modulation of Extinction of Drug Seeking by Deep Brain Stimulation of the Ventral Striatum

BACKGROUND

Recent research in humans and rodents has explored the use of deep brain stimulation (DBS) of the ventral capsule/ventral striatum (VS) as a possible treatment for drug addiction. However, the optimum electrode placement and optimum DBS parameters have not been thoroughly studied. Here we varied stimulation sites and frequencies to determine whether DBS of the VS could facilitate the extinction of morphine-induced conditioned place preference in rats.

METHODS

Rats were implanted with DBS electrodes in the dorsal or ventral subregions of the VS and trained to the morphine conditioned place preference. Subsequently, rats received extinction sessions over 9 days, combined with 60 min of either high- (130 Hz) or low- (20 Hz) frequency DBS. To study circuit-wide activations after DBS of the VS, c-fos immunohistochemistry was performed in regions involved in the extinction of drug-seeking behaviors.

RESULTS

High-frequency DBS of the dorsal-VS impaired both extinction training and extinction memory, whereas high-frequency DBS of the ventral-VS had no effect. In contrast, low-frequency DBS of the dorsal-VS strengthened extinction memory when tested 2 or 9 days after the cessation of stimulation. Both DBS frequencies increased c-fos expression in the infralimbic prefrontal cortex, but only low-frequency DBS increased c-fos expression in the basal amygdala and the medial portion of the central amygdala.

CONCLUSIONS

Our results suggest that low-frequency (rather than high-frequency) DBS of the dorsal-VS strengthens extinction memory and may be a potential adjunct for extinction-based therapies for treatment-refractory opioid addiction.

No Effect of Subthalamic Deep Brain Stimulation on Intertemporal Decision-Making in Parkinson Patients

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) is a widely used treatment for the motor symptoms of Parkinson's disease (PD). DBS or pharmacological treatment is believed to modulate the tendency to, or reverse, impulse control disorders. Several brain areas involved in impulsivity and reward valuation, such as the prefrontal cortex and striatum, are linked to the STN, and activity in these areas might be affected by STN-DBS. To investigate the effect of STN-DBS on one type of impulsive decision-making--delay discounting (i.e., the devaluation of reward with increasing delay until its receipt)--we tested 40 human PD patients receiving STN-DBS treatment and medication for at least 3 months. Patients were pseudo-randomly assigned to one of four groups to test the effects of DBS on/off states as well as medication on/off states on delay discounting. The delay-discounting task consisted of a series of choices among a smaller. sooner or a larger, later monetary reward. Despite considerable effects of DBS on motor performance, patients receiving STN-DBS did not choose more or less impulsively compared with those in the off-DBS group, as well as when controlling for risk attitude. Although null results have to be interpreted with caution, our findings are of significance to other researchers studying the effects of PD treatment on impulsive decision-making, and they are of clinical relevance for determining the therapeutic benefits of using STN-DBS.

Deep brain stimulation of the ventral striatum enhances extinction of conditioned fear

Deep brain stimulation (DBS) of the ventral capsule/ventral striatum (VC/VS) reduces symptoms of intractable obsessive-compulsive disorder (OCD), but the mechanism of action is unknown. OCD is characterized by avoidance behaviors that fail to extinguish, and DBS could act, in part, by facilitating extinction of fear. We investigated this possibility by using auditory fear conditioning in rats, for which the circuits of fear extinction are well characterized. We found that DBS of the VS (the VC/VS homolog in rats) during extinction training reduced fear expression and strengthened extinction memory. Facilitation of extinction was observed for a specific zone of dorsomedial VS, just above the anterior commissure; stimulation of more ventrolateral sites in VS impaired extinction. DBS effects could not be obtained with pharmacological inactivation of either dorsomedial VS or ventrolateral VS, suggesting an extrastriatal mechanism. Accordingly, DBS of dorsomedial VS (but not ventrolateral VS) increased expression of a plasticity marker in the prelimbic and infralimbic prefrontal cortices, the orbitofrontal cortex, the amygdala central nucleus (lateral division), and intercalated cells, areas known to learn and express extinction. Facilitation of fear extinction suggests that, in accord with clinical observations, DBS could augment the effectiveness of cognitive behavioral therapies for OCD.

Deep brain stimulation mechanisms: beyond the concept of local functional inhibition

Deep brain electrical stimulation has become a recognized therapy in the treatment of a variety of motor disorders and has potentially promising applications in a wide range of neurological diseases including neuropsychiatry. Behavioural observation that electrical high-frequency stimulation of a given brain area induces an effect similar to a lesion suggested a mechanism of functional inhibition. In vitro and in vivo experiments as well as per operative recordings in patients have revealed a variety of effects involving local changes of neuronal excitability as well as widespread effects throughout the connected network resulting from activation of axons, including antidromic activation. Here we review current data regarding the local and network activity changes induced by high-frequency stimulation of the subthalamic nucleus and discuss this in the context of motor restoration in Parkinson's disease. Stressing the important functional consequences of axonal activation in deep brain stimulation mechanisms, we highlight the importance of developing anatomical knowledge concerning the fibre connections of the putative therapeutic targets.

Invasive circuitry-based neurotherapeutics: stereotactic ablation and deep brain stimulation for OCD

Psychiatric neurosurgery, specifically stereotactic ablation, has continued since the 1940s, mainly at a few centers in Europe and the US. Since the late 1990s, the resurgence of interest in this field has been remarkable; reports of both lesion procedures and the newer technique of deep brain stimulation (DBS) have increased rapidly. In early 2009, the US FDA granted limited humanitarian approval for DBS for otherwise intractable obsessive-compulsive disorder (OCD), the first such approval for a psychiatric illness. Several factors explain the emergence of DBS and continued small-scale use of refined lesion procedures. DBS and stereotactic ablation have been successful and widely used for movement disorders. There remains an unmet clinical need: current drug and behavioral treatments offer limited benefit to some seriously ill people. Understandings of the neurocircuitry underlying psychopathology and the response to treatment, while still works in progress, are much enhanced. Here, we review modern lesion procedures and DBS for OCD in the context of neurocircuitry. A key issue is that clinical benefit can be obtained after surgeries targeting different brain structures. This fits well with anatomical models, in which circuits connecting orbitofrontal cortex (OFC), medial prefrontal cortex (mPFC), basal ganglia, and thalamus are central to OCD pathophysiology and treatment response. As in movement disorders, dedicated interdisciplinary teams, here led by psychiatrists, are required to implement these procedures and maintain care for patients so treated. Available data, although limited, support the promise of stereotactic ablation or DBS in carefully selected patients. Benefit in such cases appears not to be confined to obsessions and compulsions, but includes changes in affective state. Caution is imperative, and key issues in long-term management of psychiatric neurosurgery patients deserve focused attention. DBS and contemporary ablation also present different patterns of potential benefits and burdens. Translational research to elucidate how targeting specific nodes in putative OCD circuitry might lead to therapeutic gains is accelerating in tandem with clinical use.

Evolution of Deep Brain Stimulation: Human Electrometer and Smart Devices Supporting the Next Generation of Therapy

Deep Brain Stimulation (DBS) provides therapeutic benefit for several neuropathologies including Parkinson's disease (PD), epilepsy, chronic pain, and depression. Despite well established clinical efficacy, the mechanism(s) of DBS remains poorly understood. In this review we begin by summarizing the current understanding of the DBS mechanism. Using this knowledge as a framework, we then explore a specific hypothesis regarding DBS of the subthalamic nucleus (STN) for the treatment of PD. This hypothesis states that therapeutic benefit is provided, at least in part, by activation of surviving nigrostriatal dopaminergic neurons, subsequent striatal dopamine release, and resumption of striatal target cell control by dopamine. While highly controversial, we present preliminary data that are consistent with specific predications testing this hypothesis. We additionally propose that developing new technologies, e.g., human electrometer and closed-loop smart devices, for monitoring dopaminergic neurotransmission during STN DBS will further advance this treatment approach.

Monophasic but not biphasic pulses induce brain tissue damage during monopolar high-frequency deep brain stimulation

OBJECTIVE

Electrical high-frequency stimulation (HFS) of deep brain structures has been successfully used as a treatment for patients with movement disorders. The mechanisms of HFS allowing therapeutic clinical effects remain unclear, which justifies experimental studies to address these questions. These experiments require an external stimulator, which may offer the possibility to deliver a current with monophasic or biphasic pulses. The aim of the present study was to quantify the evolution of a potentially deleterious effect of HFS according to the duration and/or intensity in monophasic and biphasic conditions.

METHODS

In all rats, HFS was performed with monophasic pulses in deep brain structures of 1 hemisphere and with biphasic pulses symmetrically in the other hemisphere. The effect of HFS was tested, first for various durations of HFS at a constant intensity (100 microA) and, second, for measuring the effect of various current intensities of HFS at constant duration (10 minutes). At the end of each stimulation test, the volume of lesion was determined and analyzed.

RESULTS

In all hemispheres in which stimulation using biphasic pulses was delivered, we never found any relevant lesions. Conversely, monophasic electrical stimulation always created a lesion: at 100 microA, a minimal duration of HFS of 5 minutes induced a tissue damage volume of 0.0055 +/- 0.0015 mm(3). For 10 minutes of HFS, a minimal intensity of 100 microA induced a tissue damage volume of 0.0062 +/- 0.0017 mm(3). Regression analysis showed that the extent of lesion increased linearly with the intensity and duration.

CONCLUSION

In conclusion, this study proved that HFS using monophasic pulses systematically created tissue damage after 5 minutes of stimulation at 100 microA. HFS is safe when biphasic pulses are used for intensities as high as 2 mA and durations as long as 120 minutes. Monophasic pulses can be safely used only during short stimulation and at low intensities.

Deep brain stimulation of the nucleus accumbens reduces ethanol consumption in rats

Recent studies have shown that deep brain stimulation (DBS) of the nucleus accumbens (NAcc) has an inhibitory effect on drug-seeking behaviors including reinstatement responding for cocaine. The objective of the present study was to expand on these findings by assessing the effects of DBS on behaviors related to alcohol consumption. The specific aim of this study was to determine whether DBS delivered to either the shell or core of the NAcc would reduce ETOH intake in rats using a two-bottle choice limited access procedure. Long Evans rats were induced to drink a 10% ethanol solution using a saccharin fading procedure. Bipolar electrodes were implanted bilaterally into either the core or shell of the NAcc. During testing animals received DBS 5 min prior to and during a 30-minute test session in which both ETOH and water bottles were accessible. Current was delivered at amplitudes ranging from 0 to 150 microA. ETOH consumption was significantly reduced from baseline levels at the 150 microA current for both shell and core electrode placements. A significant current effect was not found for water consumption for either site. These results provide evidence that DBS delivered either to the nucleus accumbens core or shell subregions can significantly reduce ethanol intake in the rat.

High frequency stimulation or elevated K+ depresses neuronal activity in the rat entopeduncular nucleus

High frequency stimulation (HFS) is applied to many brain regions to treat a variety of neurological disorders/diseases, yet the mechanism(s) underlying its effects remains unclear. While some studies showed that HFS inhibits the stimulated nucleus, others report excitation. In this in vitro study, we stimulated the rat globus pallidus interna (entopeduncular nucleus, EP), a commonly stimulated area for Parkinson's disease, to investigate the effect of HFS-induced elevation of extracellular potassium (K(+)(e)) on rat EP neuronal activity. Whole-cell patch-clamp recordings and [K(+)](e) measurements were obtained in rat EP brain slices before, during and after HFS. After HFS (150 Hz, 10 s), [K(+)](e) increased from 2.5-9.6+/-1.4 mM, the resting membrane potential of EP neurons depolarized by 11.1+/-2.5 mV, spiking activity was significantly depressed, and input resistance decreased by 25+/-6%. The GABA(A) receptor blocker, gabazine, did not prevent these effects. The bath perfusion of 6 or 10 mM K(+), with or without synaptic blockers, mimicked the HFS-mediated effects: inhibition of spike activity, a 20+/-9% decrease in input resistance and a 17.4+/-3.0 mV depolarization. This depolarization exceeded predicted values of elevated [K(+)](e) on the resting membrane potential. A depolarization block did not fully account for the K(+)-induced inhibition of EP neuronal activity. Taken together, our results show that HFS-induced elevation of [K(+)](e) decreased EP neuronal activity by the activation of an ion conductance resulting in membrane depolarization, independent of synaptic involvement. These findings could explain the inhibitory effects of HFS on neurons of the stimulated nucleus.