Cholinergic interneuron characteristics and nicotinic properties in the striatum

Dopamine neurons mediate a fast excitatory signal via their glutamatergic synapses

Dopamine neurons are thought to convey a fast, incentive salience signal, faster than can be mediated by dopamine. A resolution of this paradox may be that midbrain dopamine neurons exert fast excitatory actions. Using transgenic mice with fluorescent dopamine neurons, in which the axonal projections of the neurons are visible, we made horizontal brain slices encompassing the mesoaccumbens dopamine projection. Focal extracellular stimulation of dopamine neurons in the ventral tegmental area evoked dopamine release and early monosynaptic and late polysynaptic excitatory responses in postsynaptic nucleus accumbens neurons. Local superfusion of the ventral tegmental area with glutamate, which should activate dopamine neurons selectively, produced an increase in excitatory synaptic events. Local superfusion of the ventral tegmental area with the D2 agonist quinpirole, which should increase the threshold for dopamine neuron activation, inhibited the early response. So dopamine neurons make glutamatergic synaptic connections to accumbens neurons. We propose that dopamine neuron glutamatergic transmission may be the initial component of the incentive salience signal.

Donepezil modulates nicotinic receptors of substantia nigra dopaminergic neurones

1. The effects of donepezil, one of the most common cholinesterase inhibitors used for treatment of Alzheimer's disease, were studied on nicotinic receptors (nAChRs)-mediated postsynaptic currents, in dopaminergic neurons of the substantia nigra pars compacta, using the patch-clamp recording technique in slice preparations. 2. Donepezil (10-100 microM) selectively and reversibly depressed nicotine currents, induced by brief puffer pulses, through a glass micropipette positioned above the slice. 3. The peak amplitude fading of the responses generated by repeated test applications of low doses of nicotine was accelerated by donepezil, while it slowed the recovery of nicotine currents after a large, desensitising, dose of the same agonist. 4. Donepezil depressed even maximal responses to nicotine, revealing a noncompetitive mechanism of action; moreover, the inhibition of nAChRs was voltage and time independent. 5. Pretreatment with vesamicol or methamidophos did not prevent the reduction of nicotine-induced currents. The data indicated direct effect on nAChR, independent from the activity of donepezil as cholinesterase inhibitor.

Model of autism: increased ratio of excitation/inhibition in key neural systems

Autism is a severe neurobehavioral syndrome, arising largely as an inherited disorder, which can arise from several diseases. Despite recent advances in identifying some genes that can cause autism, its underlying neurological mechanisms are uncertain. Autism is best conceptualized by considering the neural systems that may be defective in autistic individuals. Recent advances in understanding neural systems that process sensory information, various types of memories and social and emotional behaviors are reviewed and compared with known abnormalities in autism. Then, specific genetic abnormalities that are linked with autism are examined. Synthesis of this information leads to a model that postulates that some forms of autism are caused by an increased ratio of excitation/inhibition in sensory, mnemonic, social and emotional systems. The model further postulates that the increased ratio of excitation/inhibition can be caused by combinatorial effects of genetic and environmental variables that impinge upon a given neural system. Furthermore, the model suggests potential therapeutic interventions.

Vesicular localization and activity-dependent trafficking of presynaptic choline transporters

Presynaptic synthesis of acetylcholine (ACh) requires a steady supply of choline, acquired by a plasma membrane, hemicholinium-3-sensitive (HC-3) choline transporter (CHT). A significant fraction of synaptic choline is recovered from ACh hydrolyzed by acetylcholinesterase (AChE) after vesicular release. Although antecedent neuronal activity is known to dictate presynaptic CHT activity, the mechanisms supporting this regulation are unknown. We observe an exclusive localization of CHT to cholinergic neurons and demonstrate that the majority of CHTs reside on small vesicles within cholinergic presynaptic terminals in the rat and mouse brain. Furthermore, immunoisolation of presynaptic vesicles with multiple antibodies reveals that CHT-positive vesicles carry the vesicular acetylcholine transporter (VAChT) and synaptic vesicle markers such as synaptophysin and Rab3A and also contain acetylcholine. Depolarization of synaptosomes evokes a Ca2+-dependent botulinum neurotoxin C-sensitive increase in the Vmax for HC-3-sensitive choline uptake that is accompanied by an increase in the density of CHTs in the synaptic plasma membrane. Our study leads to the novel hypothesis that CHTs reside on a subpopulation of synaptic vesicles in cholinergic terminals that can transit to the plasma membrane in response to neuronal activity to couple levels of choline re-uptake to the rate of ACh release.

Plastic Control of Striatal Glutamatergic Transmission by Ensemble Actions of Several Neurotransmitters and Targets for Drugs of Abuse

Long-lasting alterations in the efficacy of glutamatergic synapses, such as long-term potentiation (LTP) and long-term depression (LTD), are prominent models for mechanisms of information storage in the brain. It has been suggested that exposure to drugs of abuse produces synaptic plasticity at glutamatergic synapses that shares many features with LTP and LTD, and that these synaptic changes may play roles in addiction. We have examined the involvement of particular neurotransmitters in synaptic plasticity at glutamatergic synapses within the striatum, a brain region with prominent roles in initiation and sequencing of actions, as well as habit formation. Our studies indicate that multiple neurotransmitters interact to produce striatal synaptic plasticity, and that the relative strength and patterning of the afferent inputs that release the various neurotransmitters determines whether LTP or LTD is activated. Drugs of abuse interact with glutamatergic synaptic plasticity in multiple ways, including alterations in dopamine release and more direct effects on glutamate release and glutamate receptors. We hypothesize that these effects contribute to addiction by facilitating the formation of new, drug-centered habits, and by disruption of more adaptive behaviors.

Nicotine is highly effective at producing desensitization of rat alpha4beta2 neuronal nicotinic receptors

We examined desensitization by acetylcholine (ACh) and nicotine at the rat alpha4beta2 neuronal nicotinic receptor stably expressed in HEK cells. For both agonists, the decay in response due to desensitization ('onset') was best fitted by the sum of two exponentials with the fast component dominant at concentrations > 1 microM. The time constants for onset were similar for both agonists, and showed little concentration dependence over the range of 0.1-100 microM. Recovery from desensitization also showed two exponential components. In contrast to the similarity in onset, nicotine produced longer lasting desensitization, resulting from an increase in the proportion of receptors in the slowly recovering population and from an increase in the time constant for the slow recovery process. The proportion of receptors in the slowly recovering population increased as the duration of the desensitizing pulse increased. Desensitization was also induced by low concentrations of agonist, with no apparent macroscopic response. A 100 s application of 10 nM nicotine desensitized 70 % of the peak response, while 100 s of 10 nM ACh desensitized only 15 %. At higher concentrations of agonist, which result in a macroscopic response, desensitization in the absence of activation also can occur. Nicotine is a very potent and efficacious desensitizing agent at this neuronal nicotinic receptor.

Distribution of high affinity choline transporter immunoreactivity in the primate central nervous system

A mouse monoclonal antibody (clone 62-2E8) raised against a human recombinant high-affinity choline transporter (CHT)-glutathione-S-transferase fusion protein was used to determine the distribution of immunoreactive profiles containing this protein in the monkey central nervous system (CNS). Within the monkey telencephalon, CHT-immunoreactive perikarya were found in the striatum, nucleus accumbens, medial septum, vertical and horizontal limb nuclei of the diagonal band, nucleus basalis complex, and the bed nucleus of the stria terminalis. Dense fiber staining was observed within the islands of Calleja, olfactory tubercle, hippocampal complex, amygdala; moderate to light fiber staining was seen in iso- and limbic cortices. CHT-containing fibers were also present in sensory and limbic thalamic nuclei, preoptic and hypothalamic areas, and the floccular lobe of the cerebellum. In the brainstem, CHT-immunoreactive profiles were observed in the pedunculopontine and dorsolateral tegmental nuclei, the Edinger-Westphal, oculomotor, trochlear, trigeminal, abducens, facial, ambiguus, dorsal vagal motor, and hypoglossal nuclei. In the spinal cord, CHT-immunoreactive ventral horn motoneurons were seen in close apposition to intensely immunoreactive C-terminals at the level of the cervical spinal cord. CHT immunostaining revealed a similar distribution of labeled profiles in the aged human brain and spinal cord. Dual fluorescent confocal microscopy revealed that the majority of CHT immunoreactive neurons contained the specific cholinergic marker, choline acetyltransferase, at all levels of the monkey CNS. The present observations indicate that the present CHT antibody labels cholinergic structures within the primate CNS and provides an additional marker for the investigation of cholinergic neuronal function in aging and disease.

Impairment of reward-related learning by cholinergic cell ablation in the striatum

The striatum in the basal ganglia-thalamocortical circuitry is a key neural substrate that is implicated in motor balance and procedural learning. The projection neurons in the striatum are dynamically modulated by nigrostriatal dopaminergic input and intrastriatal cholinergic input. The role of intrastriatal acetylcholine (ACh) in learning behaviors, however, remains to be fully clarified. In this investigation, we examine the involvement of intrastriatal ACh in different categories of learning by selectively ablating the striatal cholinergic neurons with use of immunotoxin-mediated cell targeting. We show that selective ablation of cholinergic neurons in the striatum impairs procedural learning in the tone-cued T-maze memory task. Spatial delayed alternation in the T-maze learning test is also impaired by cholinergic cell elimination. In contrast, the deficit in striatal ACh transmission has no effect on motor learning in the rota-rod test or spatial learning in the Morris water-maze test or on contextual- and tone-cued conditioning fear responses. We also report that cholinergic cell elimination adaptively up-regulates nicotinic ACh receptors not only within the striatum but also in the cerebral cortex and substantia nigra. The present investigation indicates that cholinergic modulation in the local striatal circuit plays a pivotal role in regulation of neural circuitry involving reward-related procedural learning and working memory.

Differential desensitization and distribution of nicotinic acetylcholine receptor subtypes in midbrain dopamine areas

Although many psychopharmacological factors contribute to nicotine addiction, midbrain dopaminergic systems have received much attention because of their roles in reinforcement and associative learning. It is generally thought that the mesocorticolimbic dopaminergic system is important for the acquisition of behaviors that are reinforced by the salient drives of the environment or by the inappropriate stimuli of addictive drugs. Nicotine, as obtained from tobacco, can activate nicotinic acetylcholine receptors (nAChRs) and excite midbrain neurons of the mesocorticolimbic system. Using midbrain slices from rats, wild-type mice, and genetically engineered mice, we have found differences in the nAChR currents from the ventral tegmental area (VTA) and the substantia nigra compacta (SNc). Nicotinic AChRs containing the alpha7 subunit (alpha7* nAChRs) have a low expression density. Electrophysiological analysis of nAChR currents, autoradiography of [125I]-alpha-bungarotoxin binding, and in situ hybridization revealed that alpha7* nAChRs are more highly expressed in the VTA than the SNc. In contrast, beta2* nAChRs are move evenly distributed at a higher density in both the VTA and SNc. At the concentration of nicotine obtained by tobacco smokers, the slow components of current (mainly mediated by beta2* nAChRs) become essentially desensitized. However, the minority alpha7* component of the current in the VTA/SNc is not significantly desensitized by nicotine in the range < or =100 nm. These results suggest that nicotine, as obtained from tobacco, can have multiple effects on the midbrain areas by differentially influencing dopamine neurons of the VTA and SNc and differentially desensitizing alpha7* and non-alpha7 nAChRs.