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In
vertebrates, receptors for the neurotransmitter acetylcholine are
divided into two classes, nicotinic and muscarinic receptors. These
receptors can be classified based on their differential responsiveness
to selective agonists and antagonists. For example, nicotine preferentially
activates nicotinic over muscarinic receptors, while the converse
is true of muscarine. The transduction mechanisms utilized by the
two classes of acetylcholine receptors are different. Nicotinic
receptors are an integral part of a ligand-gated ion channel. When
acetylcholine binds to the receptor, the ion channel opens, causing
a depolarization of the cell. In contrast, muscarinic receptors
are metabotropic receptors, causing the production of a second messenger(s)
when activated. Depending on the particular subtype of muscarinic
receptor considered, almost any second messenger system could be
activated.
The muscular pharynx of the medicinal leech responds to the application
of acetylcholine by producing a dose-dependent contraction. The
goal of this work was to determine if the acetylcholine receptor(s)
on the pharyngeal muscles could be classified as a nicotinic-like
or muscarinic-like receptor. The responses of the pharynx to acetylcholine
(a nonselective agonist), nicotine (a nicotinic agonist) and McN
A 343 (a muscarinic agonist) were used to classify the acetylcholine
receptor. The responses were quantified by measuring the peak tension
produced in response to the agonist and the integrated area under
the contraction curve. Concentration-response relationships for
both peak tension and integrated area were generated for each agonist
when applied in concentrations ranging from 0.1 nM to 1 mM. The
pharynx was most responsive to nicotine, followed by McN A 343,
and was least sensitive to acetylcholine. The relative lack of responsiveness
to acetylcholine is probably due to the presence of a very active
cholinesterase in the pharynx. Based on the relative responsiveness
of the pharynx to these agonists, the acetylcholine receptor on
the pharyngeal muscles appears to be a nicotinic-like receptor.
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