Henrike Hartung

Alpha6-containing nicotinic acetylcholine receptors dominate the nicotine control of dopamine neurotransmission in nucleus accumbens.

Modulation of striatal dopamine (DA) neurotransmission plays a fundamental role in the reinforcing and ultimately addictive effects of nicotine. Nicotine, by desensitizing β2 subunit-containing (β2*) nicotinic acetylcholine receptors (nAChRs) on striatal DA axons, significantly enhances how DA is released by reward-related burst activity compared to nonreward-related tonic activity. This action provides a synaptic mechanism for nicotine to facilitate the DA-dependent reinforcement. The subfamily of β2*-nAChRs responsible for these potent synaptic effects could offer a molecular target for therapeutic strategies in nicotine addiction. We explored the role of α6β2*-nAChRs in the nucleus accumbens (NAc) and caudate-putamen (CPu) by observing action potential-dependent DA release from synapses in real-time using fast-scan cyclic voltammetry at carbon-fiber microelectrodes in mouse striatal slices. The α6-specific antagonist α-conotoxin-MII suppressed DA release evoked by single and low-frequency action potentials and concurrently enhanced release by high-frequency bursts in a manner similar to the β2*-selective antagonist dihydro-β-erythroidine (DHβE) in NAc, but less so in CPu. The greater role for α6*-nAChRs in NAc was not due to any confounding regional difference in ACh tone since elevated ACh levels (after the acetylcholinesterase inhibitor ambenonium) had similar outcomes in NAc and CPu. Rather, there appear to be underlying differences in nAChR subtype function in NAc and CPu. In summary, we reveal that α6β2*-nAChRs dominate the effects of nicotine on DA release in NAc, whereas in CPu their role is minor alongside other β2*-nAChRs (eg α4*), These data offer new insights to suggest striatal α6*-nAChRs as a molecular target for a therapeutic strategy for nicotine addiction.

Serotonin-dependent depression in Parkinson’s disease: A role for the subthalamic nucleus?

Depression is the most common neuropsychiatric co-morbidity in Parkinsons disease (PD). The underlying mechanism of depression in PD is complex and likely involves biological, psychosocial and therapeutic factors. The biological mechanism may involve changes in monoamine systems, in particular the serotonergic (5-hydroxytryptamine, 5-HT) system. It is well established that the 5-HT system is markedly affected in the Parkinsonian brain, with evidence including pathological loss of markers of 5-HT axons as well as cell bodies in the dorsal and median raphe nuclei of the midbrain. However, it remains unresolved whether alterations to the 5-HT system alone are sufficient to confer vulnerability to depression. Here we propose low 5-HT combined with altered network activity within the basal ganglia as critically involved in depression in PD. The latter hypothesis is derived from a number of recent findings that highlight the close interaction between the basal ganglia and the 5-HT system, not only in motor but also limbic functions. These findings include evidence that clinical depression is a side effect of deep brain stimulation (DBS) of the subthalamic nucleus (STN), a treatment option in advanced PD. Further, it has recently been demonstrated that STN DBS in animal models inhibits 5-HT neurotransmission, and that this change may underpin depressive-like side effects. This review provides an overview of 5-HT alterations in PD and a discussion of how these changes might combine with altered basal ganglia network activity to increase depression vulnerability.

Electrical stimulation alleviates depressive-like behaviors of rats: investigation of brain targets and potential mechanisms.

Deep brain stimulation (DBS) is a promising therapy for patients with refractory depression. However, key questions remain with regard to which brain target(s) should be used for stimulation, and which mechanisms underlie the therapeutic effects. Here, we investigated the effect of DBS, with low- and high-frequency stimulation (LFS, HFS), in different brain regions (ventromedial prefrontal cortex, vmPFC; cingulate cortex, Cg; nucleus accumbens (NAc) core or shell; lateral habenula, LHb; and ventral tegmental area) on a variety of depressive-like behaviors using rat models. In the naive animal study, we found that HFS of the Cg, vmPFC, NAc core and LHb reduced anxiety levels and increased motivation for food. In the chronic unpredictable stress model, there was a robust depressive-like behavioral phenotype. Moreover, vmPFC HFS, in a comparison of all stimulated targets, produced the most profound antidepressant effects with enhanced hedonia, reduced anxiety and decreased forced-swim immobility. In the following set of electrophysiological and histochemical experiments designed to unravel some of the underlying mechanisms, we found that vmPFC HFS evoked a specific modulation of the serotonergic neurons in the dorsal raphe nucleus (DRN), which have long been linked to mood. Finally, using a neuronal mapping approach by means of c-Fos expression, we found that vmPFC HFS modulated a brain circuit linked to the DRN and known to be involved in affect. In conclusion, HFS of the vmPFC produced the most potent antidepressant effects in naive rats and rats subjected to stress by mechanisms also including the DRN.

A combined in vivo neurochemical and electrophysiological analysis of the effect of high-frequency stimulation of the subthalamic nucleus on 5-HT transmission

Movement disability in advanced Parkinsons disease (PD) can be treated by high frequency stimulation (HFS) of the subthalamic nucleus (STN) but some patients experience psychiatric side-effects including depression, which is strongly linked to decreases in 5-hydroxytryptamine (5-HT). The current study investigated the effect of bilateral STN HFS on extracellular 5-HT in brain regions of anesthetized and freely moving rats as measured with microdialysis. Parallel in vivo electrophysiological experiments allowed a correlation of changes in extracellular 5-HT with the firing of 5-HT neurons. Bilateral STN HFS decreased (by up to 25%) extracellular levels of 5-HT in both striatum and medial prefrontal cortex of anesthetized rats. STN HFS also decreased extracellular 5-HT in the medial prefrontal cortex of freely moving rats. This decrease in extracellular 5-HT persisted after turning off the stimulation, and was present in dopamine-denervated rats. As with changes in extracellular 5-HT, in anesthetized rats STN HFS evoked a decrease in the in vivo firing of midbrain raphe 5-HT neurons that also persisted after cessation of stimulation. These data provide neurochemical evidence for an inhibition of 5-HT neurotransmission by STN HFS, which may contribute to its psychiatric side effects and guide therapeutic options.

Nitric oxide donors enhance the frequency dependence of dopamine release in nucleus accumbens.

Dopamine (DA) neurotransmission in the nucleus accumbens (NAc) is critically involved in normal as well as maladaptive motivated behaviors including drug addiction. Whether the striatal neuromodulator nitric oxide (NO) influences DA release in NAc is unknown. We investigated whether exogenous NO modulates DA transmission in NAc core and how this interaction varies depending on the frequency of presynaptic activation. We detected DA with cyclic voltammetry at carbon-fiber microelectrodes in mouse NAc in slices following stimuli spanning a full range of DA neuron firing frequencies (1–100 Hz). NO donors 3-morpholinosydnonimine hydrochloride (SIN-1) or z-1-[N-(3-ammoniopropyl)-N-(n-propyl)amino]diazen-1-ium-1,2-diolate (PAPA/NONOate) enhanced DA release with increasing stimulus frequency. This NO-mediated enhancement of frequency sensitivity of DA release was not prevented by inhibition of soluble guanylyl cyclase (sGC), DA transporters, or large conductance Ca2+-activated K+ channels, and did not require glutamatergic or GABAergic input. However, experiments to identify whether frequency-dependent NO effects were mediated via changes in powerful acetylcholine–DA interactions revealed multiple components to NO modulation of DA release. In the presence of a nicotinic receptor antagonist (dihydro-β-erythroidine), NO donors increased DA release in a frequency-independent manner. These data suggest that NO in the NAc can modulate DA release through multiple GC-independent neuronal mechanisms whose net outcome varies depending on the activity in DA neurons and accumbal cholinergic interneurons. In the presence of accumbal acetylcholine, NO promotes the sensitivity of DA release to presynaptic activation, but with reduced acetylcholine input, NO will promote DA release in an activity-independent manner through a direct action on dopaminergic terminals.

Striatal dopamine transmission is reduced after chronic nicotine with a decrease in α6-nicotinic receptor control in nucleus accumbens.

Nicotine directly regulates striatal dopamine (DA) neurotransmission via presynaptic nicotinic acetylcholine receptors (nAChRs) that are α6β2 and/or α4β2 subunit‐containing, depending on region. Chronic nicotine exposure in smokers upregulates striatal nAChR density, with some reports suggesting differential impact on α6‐ or α4‐containing nAChRs. Here, we explored whether chronic nicotine exposure modifies striatal DA transmission, whether the effects of acute nicotine on DA release probability persist and whether there are modifications to the regulation of DA release by α6‐subunit‐containing (*) relative to non‐α6* nAChRs in nucleus accumbens (NAc) and in caudate‐putamen (CPu). We detected electrically evoked DA release at carbon‐fiber microelectrodes in striatal slices from mice exposed for 4–8 weeks to nicotine (200 μg/mL in saccharin‐sweetened drinking water) or a control saccharin solution. Chronic nicotine exposure subtly reduced striatal DA release evoked by single electrical pulses, and in NAc enhanced the range of DA release evoked by different frequencies. Effects of acute nicotine (500 nm) on DA release probability and its sensitivity to activity were apparent. However, in NAc there was downregulation of the functional dominance of α6‐nAChRs (α6α4β2β3), and an emergence in function of non‐α6* nAChRs. In CPu, there was no change in the control of DA release by its α6 nAChRs (α6β2β3) relative to non‐α6. These data suggest that chronic nicotine subtly modifies the regulation of DA transmission, which, in NAc, is through downregulation of function of a susceptible population of α6α4β2β3 nAChRs. This imbalance in function of α6:non‐α6 nAChRs might contribute to DA dysregulation in nicotine addiction.

High-frequency stimulation of the subthalamic nucleus inhibits the firing of juxtacellular labelled 5-HT-containing neurones.

High-frequency stimulation (HFS) of the subthalamic nucleus (STN) is an established neurosurgical therapy for movement disability in advanced Parkinsons disease (PD), but some patients experience psychiatric side-effects like depression. In a previous electrophysiological study, we observed that HFS of the STN inhibited a population of neurones in the rat dorsal raphe nucleus (DRN), with firing properties characteristic of 5-HT neurones. The present study extended these findings to a second population of neurones, and combined extracellular recording with juxtacellular-labelling to investigate the chemical identity of the neurones affected by HFS. Bilateral HFS (130 Hz, 100-200 μA, 5 min) of the STN inhibited (26.0±2.9%) the firing of 37/74 DRN neurones displaying a slow, regular firing pattern. Slower firing neurones were more strongly inhibited than those firing faster. Importantly, 10 inhibited DRN neurones were juxtacellular-labelled with neurobiotin, and all neurones contained 5-HT as shown by post-mortem 5-HT immunocytochemistry. A minority of slow firing DRN neurones (18/74) were activated by STN HFS (37.9±8.3%) which was not observed previously. Of these neurones, three were juxtacellular-labelled and one was 5-HT immunopositive. Also a small number of DRN neurones (19/74) did not respond to HFS, four of which were juxtacellular-labelled and all contained 5-HT. These data show that individual chemically-identified 5-HT-containing neurones in the DRN were modulated by STN HFS, and that the majority were inhibited but some were activated and some failed to respond. These data extend previous findings of modulation of the 5-HT system by STN HFS but suggest a destabilisation of the 5-HT system rather than simple inhibition as indicated previously. Although the mechanism is not yet known, such changes may contribute to the psychiatric side-effects of STN stimulation in some PD patients.

Thalamic and Entorhinal Network Activity Differently Modulates the Functional Development of Prefrontal–Hippocampal Interactions

Precise information flow during mnemonic and executive tasks requires the coactivation of adult prefrontal and hippocampal networks in oscillatory rhythms. This interplay emerges early in life, most likely as an anticipatory template of later cognitive performance. At neonatal age, hippocampal theta bursts drive the generation of prefrontal theta-gamma oscillations. In the absence of direct reciprocal interactions, the question arises of which feedback mechanisms control the early entrainment of prefrontal–hippocampal networks. Here, we demonstrate that prefrontal–hippocampal activity couples with discontinuous theta oscillations and neuronal firing in both lateral entorhinal cortex and ventral midline thalamic nuclei of neonatal rats. However, these two brain areas have different contributions to the neonatal long-range communication. The entorhinal cortex mainly modulates the hippocampal activity via direct axonal projections. In contrast, thalamic theta bursts are controlled by the prefrontal cortex via mutual projections and contribute to hippocampal activity. Thus, the neonatal prefrontal cortex modulates the level of hippocampal activation by directed interactions with the ventral midline thalamus. Similar to the adult task-related communication, theta-band activity ensures the feedback control of long-range coupling in the developing brain. SIGNIFICANCE STATEMENT Memories are encoded by finely tuned interactions within large-scale neuronal networks. This cognitive performance is not inherited, but progressively matures in relationship with the establishment of long-range coupling in the immature brain. The hippocampus initiates and unidirectionally drives the oscillatory entrainment of neonatal prefrontal cortex, yet feedback interactions that precisely control this early communication are still unresolved. Here, we identified distinct roles of entorhinal cortex and ventral midline thalamus for the functional development of prefrontal–hippocampal interactions. While entorhinal oscillations modulate the hippocampal activity by timing the neuronal firing via monosynaptic afferents, thalamic nuclei act as a relay station routing prefrontal activation back to hippocampus. Understanding the mechanisms of network maturation represents the prerequisite for assessing circuit dysfunction in neurodevelopmental disorders.

From Shortage to Surge: A Developmental Switch in Hippocampal-Prefrontal Coupling in a Gene-Environment Model of Neuropsychiatric Disorders.

Cognitive deficits represent a major burden of neuropsychiatric disorders and result in part from abnormal communication within hippocampal–prefrontal circuits. While it has been hypothesized that this network dysfunction arises during development, long before the first clinical symptoms, experimental evidence is still missing. Here, we show that pre-juvenile mice mimicking genetic and environmental risk factors of disease (dual-hit GE mice) have poorer recognition memory that correlates with augmented coupling by synchrony and stronger directed interactions between prefrontal cortex and hippocampus. The network dysfunction emerges already during neonatal development, yet it initially consists in a diminished hippocampal theta drive and consequently, a weaker and disorganized entrainment of local prefrontal circuits in discontinuous oscillatory activity in dual-hit GE mice when compared with controls. Thus, impaired maturation of functional communication within hippocampal–prefrontal networks switching from hypo- to hyper-coupling may represent a mechanism underlying the pathophysiology of cognitive deficits in neuropsychiatric disorders.

High-frequency stimulation of the substantia nigra induces serotonin-dependent depression-like behavior in animal models.

To the Editor: H igh-frequency stimulation (HFS; deep brain stimulation) of the subthalamic nucleus (STN) is a popular neurosurgical therapy to treat motor disability in advanced Parkinson’s disease (PD) (1). In some patients, psychiatric effects, including depression and increased risk of suicide, are a recognized postoperative problem (2,3). Some authors, including ourselves, have linked these effects to interactions between the STN and limbic circuitry, and specifically the midbrain serotonin (5-HT; 5-hydroxytryptamine) system, which, when acutely lowered, can trigger low mood in individuals at risk of depression (4 –7). Others, however, have suggested that current spread or even direct stimulation of the ventrally located substantia nigra pars reticulata (SNR) through misplaced electrodes contributes to the psychiatric effects of STN stimulation. In this respect, three case reports described acute depression caused by accidental electrode placement and stimulation in the SNR, by a still unknown mechanism (8-10). Here we report the induction of acute depression-like behavior by HFS of the SNR by a 5-HT– dependent mechanism in animal models. Naïve male Sprague-Dawley rats (270 g; Harlan, Bicester, United Kingdom; Charles River, Rotterdam, The Netherlands) underwent bilateral stereotaxic implantation of stimulation electrodes into the SNR (bregma: anteroposterior (AP) –5.3 mm, mediolateral (ML) 2.5 mm, dorsoventral (DV) – 8.2 mm [11,12]). After 1 week of recovery, each electrode was connected to a current isolator (DS100 or A360, WPI Europe, Berlin, Germany) driven by a pulse generator (DS8000, WPI Europe or Master 8, AMPI Israel, Jerusalem, Israel). For stimulation a clinically relevant paradigm (100 A, 60 sec, and 130 Hz) was applied (12). Nonstimulated controls were connected to the stimulator without stimulation being performed. Motor and nonmotor behavior were evaluated in SNR HFS treated rats and nonstimulated controls (n 8 –10 per group). Spontaneus locomotor activity was measured in the open field during a 0-minute trial (5). Anxiety behavior was analyzed by the time spent n the open and closed arms of the elevated zero maze during 5 inutes (13). Motivation and hedonia-like behavior were evaluated ith food consumption and sucrose intake tests, respectively 13,14). In the food consumption test, rats were food deprived for 4 hours followed by presentation of 60 g food pellets for 2 hours. In he sucrose intake test, rats were food and water deprived for 14 ours after which sucrose intake was measured over a 1-hour peiod. The food consumption and sucrose intake tests were repeated fter 7 days treatment with the 5-HT reuptake inhibitor escitaloram (5 mg/kg subcutaneously). The effect of HFS of the SNR on xtracellular 5-HT in medial prefrontal cortex (mPFC) was investiated by in vivo microdialysis (n 4 per group). Bilateral SNR HFS lectrodes and a single mPFC microdialysis probe were surgically mplanted (bregma: AP 3.2 mm, ML – 0.6 mm, DV –5.4 mm [11]) nder general anesthesia as described (4), and 1 week later microialysis was performed in awake freely moving rats. Perfusates were ollected every 5 minutes and analyzed using high-pressure liquid hromatography with electrochemical detection for 5-HT (4). Postortem verification of electrode (Figure 1A) and microdialysis robe placements was performed. Analysis of the behavioral data showed that HFS of the SNR did ot alter spontaneous locomotor activity in the open field (Figure B). Moreover, anxiety behavior in the elevated zero maze was omparable between animals with stimulation and nonstimulated a