Immunohistochemical localization of nicotinic acetylcholine receptors in the guinea pig bowel and pancreas

Abstract

Although nicotinic acetylcholine receptors (nAChRs) are known to be present on neural elements in both the bowel and the pancreas, the precise location of these receptors has not previously been determined. Immunocytochemistry, by using a rat monoclonal antibody (mAb35), which recognizes alpha-bungarotoxin (alpha-Bgt)-insensitive nAChRs, and a polyclonal antibody raised against the alpha-Bgt-sensitive receptor subunit, alpha7, was used to locate receptor protein in guinea pig gut and pancreas. mAb35-receptor (mAb35-R) immunoreactivity was abundant in both enteric plexuses, enterochromaffin cells, and pancreatic ganglia. Immunostaining was associated with the cell membrane, and clusters of mAb35-R were observed on cell somas and dendrites. Receptor immunoreactivity was also observed on terminals and axons, suggesting that a subset of nAChRs is presynaptic. Internal sites of mAb35-R were observed in permeabilized ganglia. Cells expressing the receptors were closely associated with ChAT-immunoreactive nerve fibers. In addition, the majority of ChAT-positive neurons expressed both cell surface and internal stores of mAb35-R. In the bowel, clusters of mAb35-R were present on the soma and dendrites of Dogiel type I motorneurons and secretomotor neurons. Receptors were detected at the plasma membrane of calbindin-immunoreactive myenteric neurons. In contrast, calbindin-immunoreactive submucosal neurons did not express cell surface mAb35-R, supporting the idea that they are sensory neurons. A subset of enteric neurons expressed both mAb35-R and glutamate receptor (GluR1) immunoreactivity. In the pancreas, mAb35-R immunoreactivity was only observed in ganglia. Alpha7-immunoreactivity was found on both enteric cell bodies and nerve fibers. Based on these results, it appears that visceral nAChRs are composed of at least four subunits and that both pre- and postsynaptic nAChRs are present in the gut and pancreas.

Nicotinic mechanisms in the autonomic control of organ systems

“…composition of nAChRs in the gut. Immunostaining of guinea pig ganglia suggests the presence of ␣3, ␣5, ␣7, ␤2, and ␤4 nAChR subunits in both enteric plexuses (Kirchgessner and Liu, 1998;Obaid et al, 1999), and a recent study reports the presence of ␣3 in the human gut (Richardson et al, 2001). The presence of ␤4* nAChRs in S type neurons is also suggested by electrophysiology experiments .…”

“…Neuronal nAChRs have been localized on both pre-and postsynaptic terminals of enteric neurons (Kirchgessner and Liu, 1998;Obaid et al, 1999;Schneider and Galligan, 2000). As in the sympathetic and parasympathetic division of the ANS, the presynaptic nAChRs may mediate the release of neurotransmitters such as substance P and/or neurokinin A , and the postsynaptic receptors mediate fEPSPs.…”

Synaptic Transmission at Functionally Identified Synapses in the Enteric Nervous System: Roles for Both Ionotropic and Metabotropic Receptors

“…Immunohistochemical and molecular analyses have identified several different subunits of nAChR in the ENS (Table 1) [40,63]. It is generally agreed that the 7 subunit is expressed by some enteric neurons, but it is unclear whether this receptor has a role in synaptic transmission.…”

Concomitant alpha7 and beta2 nicotinic AChR subunit deficiency leads to impaired energy homeostasis and increased physical activity in mice

“…The secretory activity of pancreatic islets is a highly regulated process, under the control of nutrition, hormones and the nervous system involving cholinergic signaling. The muscarinic receptor is classically described as the terminal effector of the cholinergic signaling in pancreatic β-cells [34][35][36] and the function of nAChRs in the context of pancreatic islets has long been limited to their role in ganglionic autonomic vagal neurotransmission [37][38][39][40]. Nevertheless, paracrine cholinergic signaling sensitizes the glucoseinduced β-cell response in human islets [41] and nicotinic cholinergic stimulation dampens insulin secretion [15,42] whereas antagonism of the α7nAChR increases insulin release [43,44] according to previous studies.…”