Jamie Day

Lack of tolerance to nicotine-induced dopamine release in the nucleus accumbens

The extent to which repeated administration produces tolerance to nicotine-induced increases in dopamine transmission in the nucleus accumbens was investigated in rats. In vivo microdialysis was used to sample extracellular dopamine and metabolites after a nicotine challenge (0.35 mg/kg) in (1) naive rats, (2) acutely pretreated rats (1 prior nicotine injection), and (3) chronically pretreated rats (12-15 prior daily nicotine injections, 0.35 mg/kg per injection). Nicotine increased extracellular DA and its metabolites, and these increases were not significantly altered by either acute or chronic prior exposure to the drug. The failure to find evidence of tolerance is compatible with the hypothesis that the mesolimbic dopaminergic system is a substrate for the reinforcing properties of chronically administered nicotine.

Enhanced acetylcholine release in hippocampus and cortex during the anticipation and consumption of a palatable meal

In rats trained for 14 days to consume a palatable liquid chocolate meal (Sustacal), in vivo brain microdialysis was used to measure release of acetylcholine in the frontal cortex and hippocampus during anticipation and consumption of the meal. Rats were trained in an experimental chamber in which they were separated from the Sustacal by a screen for 20 min (trained, rewarded group). The screen was then removed and the rats were allowed 20 min of access to the meal. Two control groups were run concurrently: these groups consisted of rats (i) that were trained over 14 days but only had access to water in the experimental chamber (trained, non-rewarded), or (ii) that were introduced into the experimental chamber for the first time on the final test (i.e. dialysis) session, and presented with Sustacal (naive). Different results were obtained in the hippocampus and frontal cortex. In the hippocampus there were no group differences with respect to acetylcholine release. Thus, in all three groups acetylcholine release increased to about 220% of basal values when animals were placed in the experimental chamber. In the frontal cortex, acetylcholine release also increased significantly in all three groups. However, the extent of this increase was significantly greater in the trained, rewarded group, reaching approximately 300% of basal values during the anticipatory and consummatory components of the task. The significant increases in acetylcholine release which occurred in both the hippocampus and frontal cortex of each of the three groups are consistent with an involvement of cholinergic basal forebrain neurons in the regulation of arousal or attention. In addition, however, acetylcholine release in the frontal cortex can be further selectively enhanced by the animals past training experience, perhaps being associated with the anticipation of reward.

Behavioral Pharmacology and Biochemistry of Central Cholinergic Neurotransmission

Systemically administered cholinergic (muscarinic) receptor antagonists can impair the acquisition and post-acquisition performance of a variety of learned behaviors. acquisition performance of a variety of learned behaviors. At present, there is no consensus about the psychological mechanisms underlying these deficits. Behavioral inhibition, working (short-term) memory, reference (long-term) memory, attention, movement and strategy selection, and stimulus processing are among the constructs that have been proposed as underlying the effects of muscarinic receptor blockade. On the basis of neuroanatomical and neuropharmacological considerations it is contended that debates about the nature of the mediating events are pointless because they are on an anatomy that does not exist. Specifically, given that cholinergic neurons innervate almost the entire neuraxis and that muscarinic cholinergic receptors are distributed throughout the central nervous system, it is virtually certain that systemically applied antimuscarinic drugs will influence a broad spectrum of brain functions. In addition, the nature of the deficits produced by scopolamine and atropine, which are competitive antagonists, will depend on the regional endogenous rate of acetylcholine release, which may in turn be influenced by the particular environment and/or level of training imposed on the animal. As the literature seems to indicate, therefore, the effects of competitive antagonists will vary as a function of both the behavioral test and the level of training. Accordingly, attempts at unitary formulations of central cholinergic function are ill-conceived and illusory. Another approach to understanding central cholinergic function has been based on the use of local injections of excitotoxins into brain regions such as the basal forebrain that contain cholinergic neurons. Recent published reports indicate, that many of the behavioral deficits observed after ibotenic acid lesions of the basal forebrain are due primarily to the loss of non-cholinergic neurons. The inherent limitations of the excitotoxin lesion approach for unravelling the functions of central cholinergic systems are such that they cannot produce definitive information and might best, therefore, be abandoned. At present, a reliable selective toxin for cholinergic neurons is not available and urgently required. Until such a compound is identified, local intracerebral applications of antimuscarinic agents may be the preferred procedure for studying the behavioral correlates of regional blockade of cholinergic activity. Brain microdialysis in freely moving animals also holds considerable promise with respect to defining the circumstances under which acetylcholine is released in discrete regions of the central nervous system. At present, the function of central cholinergic systems and the possible role of each in learning and memory remain poorly understood.

Dopaminergic Regulation of Septohippocampal Cholinergic Neurons

Abstract: The extent to which acetylcholine (ACh) release in the hippocampus is regulated by dopaminergic mechanisms was assessed using in vivo microdialysis in freely moving rats. Systemic administration of the dopamine (DA) receptor agonist apomorphine (1.0 mg/kg) or the specific D1 agonist CY 208–243 (1.0 mg/kg) increased microdialysate concentrations of ACh in the hippocampus. The D2 receptor agonist quinpirole (0.5 mg/kg) produced a small but statistically significant decrease in hippocampal ACh release. d‐Amphetamine (2.0 mg/kg) increased ACh release, an effect that was blocked by the D1 receptor antagonist SCH 23390 (0.3 mg/kg) but not by the D2 antagonist raclopride (1.0 mg/kg). These findings suggest that endogenous DA stimulates septo‐hippocampal cholinergic neurons primarily via actions at D1 receptors. In addition, these results are similar to previous findings regarding the dopaminergic regulation of cortical ACh release, and suggest that the anatomical continuum formed by basal forebrain cholinergic neurons that project to the cortex and hippocampus acts as a functional unit, at least with respect to its regulation by DA.

Dopaminergic regulation of cortical acetylcholine release: Effects of dopamine receptor agonists

The regulation of the basal forebrain cholinergic system by D1 and D2 dopamine receptors was assessed in the rat using in vivo microdialysis of cortical acetylcholine. The D1 agonist CY 208-243 significantly increased cortical acetylcholine release; in contrast, the D2 agonists quinpirole and (+)-4-propyl-9-hydroxynaphthoxazine were without significant effects. Moreover, when administered in combination with CY 208-243, quinpirole failed to potentiate the D1 agonist-induced increases in cortical acetylcholine release. The non-selective dopamine receptor agonist apomorphine also increased cortical acetylcholine release, and this was completely blocked by the selective D1 receptor antagonist SCH 23390 and slightly, but not significantly attenuated by the D2 antagonist raclopride. The present results indicate that stimulation of D1 receptors activates cortically-projecting cholinergic neurons; however, a minor contribution of D2 receptors cannot be excluded.

Serotonergic regulation of acetylcholine release in rat frontal cortex.

Abstract: The extent to which serotonin regulates the activity of cortically projecting cholinergic neurons was studied using in vivo microdialysis to monitor interstitial concentrations of acetylcholine in the frontal cortex of freely moving rats. Systemic administration of the serotonin release‐inducing agent fenfluramine (3 or 10 mg/kg, i.p.) increased acetylcholine release by 110–130%. The fenfluramine‐induced increase in acetylcholine release was significantly attenuated by pretreatment with the selective serotonin uptake inhibitor fluoxetine (10 mg/kg, i.p.). Pretreatment with the selective dopamine D1 receptor antagonist SCH‐23390 (0.3 mg/kg, s.c.) failed to prevent the fenfluramine‐induced increase in acetylcholine release. In contrast, the serotonin 5‐HT2A receptor antagonist ketanserin (5 mg/kg, i.p.) blocked fenfluramine‐induced increases in acetylcholine release. In contrast to previous studies that have concluded that serotonin has inhibitory actions on cortical acetylcholine release, the present results indicate that fenfluramine increases cortical acetylcholine release in vivo by its ability to enhance serotonin transmission and that serotonin produces these effects at least in part via actions at serotonin 5‐HT2A receptors.

Neurotrophins differentially enhance acetylcholine release, acetylcholine content and choline acetyltransferase activity in basal forebrain neurons

Several lines of evidence indicate that nerve growth factor is important for the development and maintenance of the basal forebrain cholinergic phenotype. In the present study, using rat primary embryonic basal forebrain cultures, we demonstrate the differential regulation of functional cholinergic markers by nerve growth factor treatment (24–96 h). Following a 96‐h treatment, nerve growth factor (1–100 ng/mL) increased choline acetyltransferase activity (168–339% of control), acetylcholine content (141–185%), as well as constitutive (148–283%) and K+‐stimulated (162–399%) acetylcholine release, but increased release was not accompanied by increased high‐affinity choline uptake. Enhancement of ACh release was attenuated by vesamicol (1 µm), suggesting a vesicular source, and was abolished under choline‐free conditions, emphasizing the importance of extracellular choline as the primary source for acetylcholine synthesized for release. A greater proportion of acetylcholine released from nerve growth factor‐treated cultures than from nerve growth factor‐naïve cultures was blocked by voltage‐gated Ca2+ channel antagonists, suggesting that nerve growth factor modified this parameter of neurotransmitter release. Cotreatment of NGF (20 ng/mL) with K252a (200 nm) abolished increases in ChAT activity and prevented enhancement of K+‐stimulated ACh release beyond the level associated with K252a, suggesting the involvement of TrkA receptor signaling. Also, neurotrophin‐3, neurotrophin‐4 and brain‐derived neurotrophic factor (all at 5–200 ng/mL) increased acetylcholine release, although they were not as potent as nerve growth factor and higher concentrations were required. High brain‐derived neurotrophic factor concentrations (100 and 200 ng/mL) did, however, increase release to a level similar to nerve growth factor. In summary, long‐term exposure (days) of basal forebrain cholinergic neurons to nerve growth factor, and in a less‐potent fashion the other neurotrophins, enhanced the release of acetylcholine, which was dependent upon a vesicular pool and the availability of extracellular choline.

Dopamine depletion attenuates amphetamine-induced increases of cortical acetylcholine release

The extent to which the d-amphetamine (2.0 mg/kg)-induced increase in cortical acetylcholine release is mediated by dopamine and/or noradrenaline was assessed using in vivo microdialysis in freely moving rats. Unilateral 6-hydroxydopamine lesions of the mesotelencephalic dopaminergic system, which depleted forebrain dopamine by 99% on the lesioned side, significantly attenuated the effect of d-amphetamine on cortical acetylcholine release compared to a surgical control group (160% baseline vs. 270%), suggesting that dopamine at least in part mediates this effect of d-amphetamine. In contrast, bilateral 6-hydroxydopamine lesions of the dorsal noradrenergic bundle which depleted forebrain noradrenaline by at least 95% had no effect on d-amphetamine-stimulated cortical acetylcholine release. These results point to an important role for forebrain dopamine in the regulation of cortically projecting cholinergic neurons and fail to support the hypothesis that the ascending noradrenergic projections of the locus coeruleus are significantly involved.