Effects of supplemental choline on extracellular acetylcholine in the nucleus accumbens during normal behavior and pharmacological acetylcholine depletion

Long-lasting nicotinic modulation of GABAergic synaptic transmission in the rat nucleus accumbens associated with behavioural sensitization to amphetamine

A robust increase in dopaminergic transmission in the nucleus accumbens (NAc) shell has been reported to be consistently associated with the long-term expression of behavioural sensitization to drugs of abuse. However, little is known about how this affects the neuronal network of the NAc. We made cellular recordings in NAc slices of saline- and amphetamine-pretreated adult rats and found that expression of behavioural sensitization was associated with long-lasting changes in the basal firing pattern of cholinergic interneurons up to 3 weeks after the last drug injection. Consequently, upon amphetamine sensitization, an inhibiting effect of the nicotinic receptor blocker mecamylamine on the amplitudes of spontaneous GABAergic synaptic currents as well as on the failure rate of electrically evoked GABAergic currents was found that was not present under control conditions. Thus, behavioural sensitization to amphetamine is associated with an up-regulation of the endogenous activation of nicotinic receptors that, in turn, stimulate the GABAergic synaptic transmission within the NAc shell. This is a new mechanism by which drugs of abuse may induce alterations in the processing and integration of NAc inputs involved in psychomotor sensitization.

Cholinergic modulation of dopaminergic reward areas: upstream and downstream targets of nicotine addiction

Nicotine reinforces smoking behaviour by activating nicotinic acetylcholine receptors in the midbrain dopaminergic reward centres. Upstream of the dopaminergic neurons nicotine induces long-term potentiation of the excitatory input to dopamine cells in the ventral tegmental area, and depresses inhibitory inputs. Both effects of nicotine were shown to last much longer than the nicotine exposure and together will activate the dopaminergic ventral tegmental area projection toward the nucleus accumbens. However, downstream of dopamine, effects of nicotine are also likely to occur. Cholinergic interneurons within the nucleus accumbens are important in the tonic control of the gamma-amino buteric acid (GABA) nucleus accumbens output neurons, which project back to the ventral tegmental area. The nicotinic acetylcholine receptors that mediate this control are likely to desensitise upon preexposure to the nicotine concentrations found in the blood of smokers. Thus, synaptic mechanisms both upstream and downstream of dopamine release are potentially important factors contributing to the etiology of nicotine addiction.

Cholinergic modulation of nucleus accumbens medium spiny neurons

The rat nucleus accumbens contains acetylcholine-releasing interneurons, presumed to play a regulatory role in the electrical activity of medium spiny output neurons. In order to examine this issue in detail, we made electrophysiological recordings in rat nucleus accumbens slices. These experiments showed that gamma-aminobutyric acid-mediated inhibition of the output neurons might be facilitated by activation of nicotinic acetylcholine receptors, in addition to being suppressed via activation of muscarinic acetylcholine receptors. In contrast, glutamatergic excitation of output neurons appeared to be inhibited by activation of muscarinic acetylcholine receptors and to be insensitive to activation of nicotinic acetylcholine receptors. The spontaneous firing frequency of cholinergic neurons appeared to be under control of both a muscarinic and a nicotinic pathway in a bi-directional manner. Finally, we made paired recordings in which the functional connection between cholinergic neurons and output neurons was monitored. Driving the cholinergic neurons at physiological firing frequencies stimulated gamma-aminobutyric acid-mediated inhibition of the output neurons, via activation of nicotinic acetylcholine receptors. The onset of this effect was slow and lacked a fixed delay. These data indicate that activation of nicotinic acetylcholine receptors in rat nucleus accumbens may mediate the facilitation of gamma-aminobutyric acid-mediated inhibition of medium spiny output neurons. Possible mechanisms of neurotransmission, mediating this cholinergic modulation are discussed.

Application of in vivo microdialysis to the study of cholinergic systems

The application of in vivo microdialysis to the study of acetylcholine (ACh) release has contributed greatly to our understanding of cholinergic brain systems. This article reviews standard experimental procedures for dialysis probe selection and implantation, perfusion parameters, neurochemical detection, and data analysis as they relate to microdialysis assessments of cholinergic function. Particular attention is focused on the unique methodological considerations that arise when in vivo microdialysis is dedicated expressly to the recovery and measurement of ACh as opposed to other neurotransmitters. Limitations of the microdialysis technique are discussed, as well as methodological adaptations that may prove useful in overcoming these limitations. This is followed by an overview of recent studies in which the application of in vivo microdialysis has been used to characterize the basic pharmacology and physiology of cholinergic neurons. Finally, the usefulness of the microdialysis approach for testing hypotheses regarding the cholinergic systems' involvement in cognitive processes is examined. It can be concluded that, in addition to being a versatile and practical method for studying the neurochemistry of cholinergic brain systems, in vivo microdialysis represents a valuable tool in our efforts to better comprehend ACh's underlying role in a variety of behavioral processes.

Glucose enhancement of scopolamine-induced increase of hippocampal high-affinity choline uptake in mice: relation to plasma glucose levels

The administration of glucose has been shown to improve memory for various learning tasks in rodents. In humans, glucose also increases declarative memory performance in elderly people and in some patients with mild Alzheimer's disease. One of the possible physiological bases for the effect of glucose on memory processes is a facilitation of cholinergic function through increased synthesis. In support of this hypothesis, glucose was shown to attenuate the amnesia induced by scopolamine and, in similar conditions, glucose increased extracellular levels of acetylcholine following a scopolamine injection. To further examine the interaction between glucose and cholinergic function, the present experiment measured the effects of combined injections of glucose and scopolamine on hippocampal sodium-dependent high-affinity choline uptake, an indirect index of cholinergic activity. Results showed that the injection of 3 g/kg glucose enhanced the increase in high affinity choline uptake in hippocampal synaptosomes produced by scopolamine. A regression analysis revealed the existence of a positive correlation between plasma blood glucose level and hippocampal choline uptake particularly in the animals receiving a combined injection of scopolamine and glucose. These data further support the hypothesis that glucose administration can facilitate acetylcholine synthesis under certain conditions and that this action could explain how glucose attenuates scopolamine-induced amnesia.