Muscarinic M1, M3, Nicotinic,GABAA and GABAB Receptor ...

Most of the nerve fibers enter the pancreas along the arteries (Miller, 1981; Woods
& Porte, 1974). Unmyelinated nerve fibers are found in the neighborhood of all
islet cell types at the periphery and within the islet. At some distance from the
islets, glial Schwann cells often form a thin sheet around nerve fibers on their
travel toward and within the islet. In the vicinity of islet cells, however, it is not
rare to see some nerve fibers lacking this glial protection and coming close to or
ending blindly 20–30 nm from the endocrine cells (Bock, 1986; Fujita &
Kobayashi, 1979; Legg, 1967; Radke & Stach, 1986; Shorr & Bloom, 1970;
Watari, 1968).
The preganglionic fibers of the parasympathetic limb originate from
perikarya located in the dorsal motor nucleus of the vagus (Berthoud et al., 1990;
Berthoud & Powley, 1991; Chen et al., 1996) and possibly also in the nucleus
ambiguus (Luiten et al., 1986) which are both under the control of the
hypothalamus. They are organized in well separated branches traveling within the
vagus nerves (cranial nerve X) and through the hepatic, gastric (Berthoud et al.,
1990; Berthoud & Powley, 1991) and possibly celiac branches of the vagus
(Kinami et al., 1997). They reach intrapancreatic ganglia that are dispersed in the
exocrine tissue. These ganglia send unmyelinated postganglionic fibers toward the
islets. Preganglionic vagal fibers release acetylcholine that binds to nicotinic
receptors on intraganglionic neurons. Postganglionic vagal fibers release several
neurotransmitters: acetylcholine, Vasoactive Intestinal Peptide (VIP), gastrinreleasing
peptide (GRP), nitric oxide (NO) and pituitary adenylate cyclaseactivating
polypeptide (PACAP) (Havel et al., 1997, Ahren et al., 1999; Love &
Szebeni, 1999; Wang et al., 1999; Ahren, 2000; Myojin et al., 2000). Cholinergic
terminals are found in the neighborhood of all islet cell types at the periphery and
within the islet (Van der Zee et al., 1992). The importance of the cholinergic
innervation of the endocrine pancreas is attested by the presence of a 10-fold
higher activity of choline acetyltransferase and acetylcholine esterase (the
enzymes involved in the synthesis and the degradation of acetylcholine
15