Excitation of rat striatal large neurons by dopamine and/or glutamate released from nerve terminals via presynaptic nicotinic receptor (A4beta2 type) stimulation

Cholinergic control of striatal neurons to modulate L-dopa-induced dyskinesias

L-dopa induced dyskinesias (LIDs) are a disabling motor complication of L-dopa therapy for Parkinson's disease (PD) management. Treatment options remain limited and the underlying network mechanisms remain unclear due to a complex pathophysiology. What is well-known, however, is that aberrant striatal signaling plays a key role in LIDs development. Here, we discuss the specific contribution of striatal cholinergic interneurons (ChIs) and GABAergic medium spiny projection neurons (MSNs) with a particular focus on how cholinergic signaling may integrate multiple striatal systems to modulate LIDs expression. Enhanced ChI transmission, altered MSN activity and the associated abnormal downstream signaling responses that arise with nigrostriatal damage are well known to contribute to LIDs development. In fact, enhancing M4 muscarinic receptor activity, a receptor favorably expressed on D1 dopamine receptor-expressing MSNs dampens their activity to attenuate LIDs. Likewise, ChI activation via thalamostriatal neurons is shown to interrupt cortical signaling to enhance D2 dopamine receptor-expressing MSN activity via M1 muscarinic receptors, which may interrupt ongoing motor activity. Notably, numerous preclinical studies also show that reducing nicotinic cholinergic receptor activity decreases LIDs. Taken together, these studies indicate the importance of cholinergic control of striatal neuronal activity and point to muscarinic and nicotinic receptors as significant pharmacological targets for alleviating LIDs in PD patients.

The striatal cholinergic system in L-dopa-induced dyskinesias

Cholinergic signaling plays a key role in regulating striatal function. The principal source of acetylcholine in the striatum is the cholinergic interneurons which, although low in number, densely arborize to modulate striatal neurotransmission. This modulation occurs via strategically positioned nicotinic and muscarinic acetylcholine receptors that influence striatal dopamine, GABA and other neurotransmitter release. Cholinergic interneurons integrate multiple striatal synaptic inputs and outputs to regulate motor activity under normal physiological conditions. Consequently, an imbalance between these systems is associated with basal ganglia disorders. Here, we provide an overview of how striatal cholinergic interneurons modulate striatal activity under normal and pathological conditions. Numerous studies show that nigrostriatal damage such as that occurs with Parkinson's disease affects cholinergic receptor-mediated striatal activity. This altered cholinergic signaling is an important contributor to Parkinson's disease as well as to the dyskinesias that develop with L-dopa therapy, the gold standard for treatment. Indeed, multiple preclinical studies show that cholinergic receptor drugs may be beneficial for the treatment of L-dopa-induced dyskinesias. In this review, we discuss the evidence indicating that therapeutic modulation of the cholinergic system, particularly targeting of nicotinic cholinergic receptors, may offer a novel approach to manage this debilitating side effect of dopamine replacement therapy for Parkinson's disease.

Evaluation of AZD1446 as a Therapeutic in DYT1 Dystonia

DYT1 dystonia is an early-onset, hyperkinetic movement disorder caused by a deletion in the gene TOR1A, which encodes the protein torsinA. Several lines of evidence show that in animal models of DTY1 dystonia, there is impaired basal dopamine (DA) release and enhanced acetylcholine tone. Clinically, anticholinergic drugs are the most effective pharmacological treatment for DYT1 dystonia, but the currently used agents are non-selective muscarinic antagonists and associated with side effects. We used a DYT1 ∆GAG knock-in mouse model (DYT1 KI) to investigate whether nicotine and/or a non-desensitizing nicotinic agonist, AZD1446, would increase DA output in DYT1 dystonia. Using in vivo microdialysis, we found that DYT1 KI mice showed significantly increased DA output and greater sensitivity to nicotine compared to wild type (WT) littermate controls. In contrast, neither systemic injection (0.25–0.75 mg/kg) or intrastriatal infusion (30 μM–1 mM) of AZD1446 had a significant effect on DA efflux in WT or DYT1 KI mice. In vitro, we found that AZD1446 had no effect on the membrane properties of striatal spiny projection neurons (SPNs) and did not alter the spontaneous firing of ChI interneurons in either WT or DYT1 KI mice. We did observe that the firing frequency of dopaminergic neurons was significantly increased by AZD1446 (10 μM), an effect blocked by dihydro-beta-erythroidine (DHβE 3 μM), but the effect was similar in WT and DYT1 KI mice. Our results support the view that DYT1 models are associated with abnormal striatal cholinergic transmission, and that the DYT1 KI animals have enhanced sensitivity to nicotine. We found little effect of AZD1446 in this model, suggesting that other approaches to nicotinic modulation should be explored.

Striatal cholinergic interneuron regulation and circuit effects

The striatum plays a central role in motor control and motor learning. Appropriate responses to environmental stimuli, including pursuit of reward or avoidance of aversive experience all require functional striatal circuits. These pathways integrate synaptic inputs from limbic and cortical regions including sensory, motor and motivational information to ultimately connect intention to action. Although many neurotransmitters participate in striatal circuitry, one critically important player is acetylcholine (ACh). Relative to other brain areas, the striatum contains exceptionally high levels of ACh, the enzymes that catalyze its synthesis and breakdown, as well as both nicotinic and muscarinic receptor types that mediate its postsynaptic effects. The principal source of striatal ACh is the cholinergic interneuron (ChI), which comprises only about 1-2% of all striatal cells yet sends dense arbors of projections throughout the striatum. This review summarizes recent advances in our understanding of the factors affecting the excitability of these neurons through acute effects and long term changes in their synaptic inputs. In addition, we discuss the physiological effects of ACh in the striatum, and how changes in ACh levels may contribute to disease states during striatal dysfunction.

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.

Involvement of α7- and α4β2-type postsynaptic nicotinic acetylcholine receptors in nicotine-induced excitation of dopaminergic neurons in the substantia nigra: a patch clamp and single-cell PCR study using acutely dissociated nigral neurons

The receptor subtypes, which mediate nicotine-induced excitation of dopaminergic neurons in the substantia nigra, were investigated by whole-cell patch clamp studies and single-cell RT-PCR using acutely dissociated nigral neurons. Three types of current were observed when acetylcholine (1 mM) was applied to the neurons in the presence of atropine (1 microM) by the U-tube system, which allowed the rapid application of drugs. In 50% of neurons examined, acetylcholine (1 mM) plus atropine (1 microM) evoked a current with a rapidly desensitizing decay phase (designated as type Ia current). In 14% of neurons tested, the current induced by acetylcholine plus atropine had a decay phase with slow desensitization (designated as type II current). The third type of response, which had both characteristics of type Ia and II currents, was evoked in 36% of neurons tested (designated as type Ib currents). Nicotine (1 mM) also induced three types of inward currents which were similar to those induced by acetylcholine (1 mM) plus atropine (1 microM). In all three types of current, nicotine (0.1 microM-1 mM)-evoked inward currents were dose-dependent. Type Ia and II currents were inhibited by methyllycaconitine (MLA, 0.01 microM), a selective nicotinic alpha7 receptor antagonist, and dihydro-beta-erythroidine (DHbetaE, 0.1 microM), an antagonist for alpha4beta2 receptor, respectively. In type Ib currents, a fast-decaying phase was inhibited by MLA (0.01 microM), while a slow-decaying phase was blocked by DHbetaE (0.1 microM). After recording the type Ib current, single-cell RT-PCR analysis was performed using aspirated cytoplasm as total RNA templates. The results revealed that mRNAs for alpha7 nicotinic receptor subunit and tyrosine hydroxylase were detected in the same single neuron tested, which confirms the existence of alpha7-type nicotinic acetylcholine receptor in dopaminergic neurons of this area. These results suggest that nicotine directly acts on postsynaptic alpha7- and alpha4beta2-type nicotinic acetylcholine receptors and induces inward current, which result in the excitation of dopaminergic neurons in the substantia nigra.

Effects of acute and repeated administration of a cholinesterase inhibitor on timing behaviour

It has been hypothesised that a leftward shift in the response distribution obtained in the peak interval (PI) procedure is a characteristic of cognitive enhancement in which mental processes are speeded. Metrifonate, a cholinesterase inhibitor with reported cognitive enhancing properties in many animal models of learning and memory, was tested in the PI procedure. Acute administration of 3 and 60 mg/kg but not 1 and 30 mg/kg in fully trained rats shifted the response distribution to the right, whereas subchronic administration of 10, 30 or 50 mg/kg during task acquisition had no effect on timing behaviour. On the basis of the present data, it can be concluded that the effects of a cognition enhancer in the PI procedure cannot be predicted from the scalar expectancy theory (SET). Furthermore, SET does not appear to be an appropriate tool for analysing the acquisition of timing behaviour.

Electrophysiological characterization of nicotine-induced excitation of dopaminergic neurons in the rat substantia nigra

The electrophysiological characteristics of nicotine-induced excitation of dopaminergic neurons in the rat substantia nigra was investigated using the whole-cell patch clamp technique under the voltage-clamp mode. Nicotine (0.01 - 100 microM) induced inward currents corresponding to nicotine-induced depolarization with an increase in firing in a dose-dependent manner. This current was inhibited by dihydro-beta-erythroidine, a selective antagonist for the alpha4beta2 type neuronal nicotinic receptor. Nicotine directly acts on postsynaptic alpha4beta2 type nicotinic receptors and induces inward currents, resulting in excitation of dopaminergic neurons in the substantia nigra and subsequent enhancement of dopamine release in the corpus striatum.

Postsynaptic α4β2 and α7 type nicotinic acetylcholine receptors contribute to the local and endogenous acetylcholine-mediated synaptic transmissions in nigral dopaminergic neurons

The local and endogenous nicotinic neuronal transmissions of dopaminergic neurons in the substantia nigra were confirmed electrophysiologically using a slice-patch technique. After identifying dopaminergic neurons based on their electrophysiological characteristics, miniature postsynaptic inward currents were recorded in the presence of atropine (a muscarinic acetylcholine receptor antagonist), bicuculline (a GABA receptor antagonist) and L-glutamic acid diethyl ester (GDEE) (a non-selective glutamate receptor antagonist). Under conditions that eliminated muscarinic, GABAergic and glutamatergic synaptic transmissions, we found miniature currents that were inhibited by the specific neuronal nicotinic receptor antagonists, dihydro-beta-erythroidine (DHbetaE) and/or methyllycaconitine (MLA) (selective alpha4beta2 and/or alpha7 nicotinic acetylcholine receptor antagonists, respectively). Under the same extracellular conditions, local stimulations in the vicinity of a target neuron evoked excitatory postsynaptic inward currents (EPSCs). These EPSCs were elicited in an extracellular Ca(2+) dependent manner and were also blocked by DHbetaE and/or MLA. These results suggest that dopaminergic neurons in the substantia nigra receive excitatory cholinergic inputs that are mediated via at least two types of postsynaptic nicotinic receptors, namely alpha7 and alpha4beta2 subtypes.

Modulatory action of acetylcholine on striatal neurons: microiontophoretic study in awake, unrestrained rats

Cholinergic interneurons innervate virtually all medium spiny striatal cells, but the relevance of this input in regulating the activity and afferent responsiveness of these cells remains unclear. Studies in anaesthetized animals and slice preparations have shown that iontophoretic acetylcholine (ACh) either weakly excites or inhibits striatal neurons. These differential responses may reflect cholinergic receptor heterogeneity but may be also related to the different activity states of recorded units and different afferent inputs specific in each preparation. Single-unit recording was combined with iontophoresis in awake, unrestrained rats to examine the effects of ACh and selective muscarinic (oxotremorine M or Oxo-M) and nicotinic agonists (nicotine or NIC) on dorsal and ventral striatal neurons. These effects were tested on naturally silent, spontaneously active and glutamate-stimulated units. We found that iontophoretic ACh primarily inhibited spontaneously active and glutamate-stimulated units; the direction of the ACh response, however, was dependent on the firing rate. The effects of ACh were generally mimicked by Oxo-M and, surprisingly, by NIC, which is known to excite units in most central structures, including striatal neurons in anaesthetized preparation. Given that NIC receptors are absent on striatal cells but located primarily on dopamine terminals, we assessed the effects of NIC after complete blockade of dopamine receptors induced by systemic administration of a mixture of D1 and D2 antagonists. During dopamine receptor blockade the number of NIC-induced inhibitions dramatically decreased and NIC had mainly excitatory effects on striatal neurons. Thus, our data suggest that under physiologically relevant conditions ACh acts as a state-dependent neuromodulator, and its action involves not only postsynaptic but also presynaptic cholinoreceptors located on dopamine- and glutamate-containing terminals.