Functional neurochemical evidence for the presence of presynaptic nicotinic acetylcholine receptors at the terminal region of myenteric motoneurons: a study with epibatidine

Endogenous cholinergic neurotransmission contributes to behavioral sensitization to morphine

Neuroplasticity in the mesolimbic dopaminergic system is critical for behavioral adaptations associated with opioid reward and addiction. These processes may be influenced by cholinergic transmission arising from the laterodorsal tegmental nucleus (LDTg), a main source of acetylcholine to mesolimbic dopaminergic neurons. To examine this possibility we asked if chronic systemic morphine administration affects expression of genes in ventral and ventrolateral periaqueductal gray at the level of the LDTg using rtPCR. Specifically, we examined gene expression changes in the area of interest using Neurotransmitters and Receptors PCR array between chronic morphine and saline control groups. Analysis suggested that chronic morphine administration led to changes in expression of genes associated, in part, with cholinergic neurotransmission. Furthermore, using a quantitative immunofluorescent technique, we found that chronic morphine treatment produced a significant increase in immunolabeling of the cholinergic marker (vesicular acetylcholine transporter) in neurons of the LDTg. Finally, systemic administration of the nonselective and noncompetitive neuronal nicotinic antagonist mecamylamine (0.5 or 2 mg/kg) dose-dependently blocked the expression, and to a lesser extent the development, of locomotor sensitization. The same treatment had no effect on acute morphine antinociception, antinociceptive tolerance or dependence to chronic morphine. Taken together, the results suggest that endogenous nicotinic cholinergic neurotransmission selectively contributes to behavioral sensitization to morphine and this process may, in part, involve cholinergic neurons within the LDTg.

α7 nicotinic acetylcholine receptors and their role in cognition

The precise role of nicotinic acetylcholine receptors (nAChRs) in central cognitive processes still remains incompletely understood almost 150 years after its initial discovery. Central nAChRs are activated by acetylcholine, which functions in the extracellular space as a nonsynaptic messenger. Recently, a novel concept in the nAChR mode of operation has been described as a fast-type nonsynaptic transmission. In this review, we attempt to summarise the experimental findings that support the role of one of the most distributed receptor subtypes, the α7 nAChRs, and particularly focus on its procognitive effects following receptor activation. The basic characteristics of α7 nAChRs are discussed, from receptor homology to cellular-level functions. Synaptic plasticity is often implicated with α7 nAChRs on the basis of several diverse studies. Here, we provide a summary of the plastic features of the α7 receptor subtype and its role in higher level cognitive function. Finally, recent clinical evidence is reviewed, which demonstrates with increasing confidence the promise α7 nAChRs as a molecular target in future pharmacotherapy to prevent cognitive decline in various types of dementia, specifically, via the development of positive allosteric modulator compounds.

Role of presynaptic nicotinic acetylcholine receptors in the regulation of gastrointestinal motility

Presynaptic nicotinic acetylcholine receptors (nAChRs) located on cholinergic terminals facilitate the release of acetylcholine (ACh), thereby constituting a fail-safe mechanism at strategic locations, such as the neuromuscular junction, where reliable transmission is vital. Accumulating data indicate that myenteric neurons in the enteric nervous system possess not only somatodendritic nAChRs, which mediate cholinergic transmission between neurons, but also presynaptic nAChRs. Functional evidence shows that these receptors mediate a positive feedback with respect to ACh release from myenteric motoneurons, and might therefore play an important role in the regulation of gastrointestinal motility. These presynaptic nAChRs were found to be more sensitive to nicotinic ligands than somatodendritic nAChRs and could therefore be primary targets of exogenous compounds, such as nicotine. This interaction might provide a neurochemical basis for the effect of smoking on gastrointestinal motility. Another important human pharmacological implication is based on our recent observation that monoamine uptake inhibitor-type antidepressant drugs are able to inhibit presynaptic nAChRs in the enteric nervous system. The disruption of the nAChR-mediated positive feedback modulation by antidepressants might explain the frequent occurrence of constipation, a common side effect, attributed to these drugs. Clarification of the role of presynaptic nAChRs in feedback mechanisms in the enteric nervous system might be instrumental in the development of new drugs affecting gastrointestinal motility.

Inhibitory effect of hemicholinium-3 on presynaptic nicotinic acetylcholine receptors located on the terminal region of myenteric motoneurons

Previously we have demonstrated the presence of presynaptic nicotinic acetylcholine receptors on the terminals of myenteric neurons in Auerbach's plexus of guinea-pig ileum. During these studies we observed, that the presence of hemicholinium-3, an inhibitor of the high affinity choline uptake significantly influences the contraction of the longitudinal muscle strip preparation. Our aim was to investigate the neurochemical background of this effect and quantitatively characterize the action of HC-3. We studied the effect of HC-3 on epibatidine- and electrical stimulation-evoked contraction and release of [3H]acetylcholine from the guinea-pig longitudinal muscle strip preparation. We found that in the presence of tetrodotoxin, when the contribution of somatodendritic nicotinic acetylcholine receptors to the response was prevented due to the inhibition of axonal conduction, HC-3 inhibited the epibatidine-evoked contraction and [3H]acetylcholine release in the submicromolar range (IC50 = 897 nM and IC50 = 693 nM, respectively), whereas the electrical stimulation-evoked contraction was not affected by HC-3, and the release of [3H]acetylcholine was apparently enhanced. Our data indicate that HC-3 inhibits the presynaptic nicotinic acetylcholine receptors of myenteric neurons. Since these receptors play an important role in the regulation of cholinergic neurotransmission in the enteric nervous system, the use of HC-3 in [3H]acetylcholine release experiments might bias the interpretation of data.