Journal of Neurophysiology, Vol 74, Issue 4 1639-1651, Copyright © 1995 by APS
Neurotransmitter regulation of the heart in the nudibranch Archidoris montereyensis
B. L. Wiens and P. H. Brownell
Department of Zoology, Oregon State University, Corvallis 97331-2914, USA.
1. The heart of the nudibranch mollusc Archidoris montereyensis is uniquely
responsive to regulation by identifiable cardiac motor neurons. The
neurotransmitters mediating the strong excitatory and inhibitory actions of
the neurons are unknown. 2. In this study we developed an infused, in vitro
preparation of the Archidoris heart to determine which of several
cardioactive transmitters described in mollusks could affect changes in the
rate, amplitude, or tonus of cardiac contractions. Several
neurotransmitters we tested increased the rate and amplitude of heart
contractions, including serotonin (threshold < 10 nM), dopamine (100
nM), and the neuropeptides R15 alpha 2 (3 nM), small cardioactive peptide B
(10 nM), and FMRFamide (20 microM). Myomodulin also excited the heart (0.8
microM) and potentiated the cardioexcitatory action of serotonin at
subthreshold concentrations. 3. Only acetylcholine (10 nM) inhibited the
heart, decreasing the rate, amplitude, and tonus of contraction. Glycine
and the peptides substance P and R15 alpha 1 had no effect on the heart. 4.
Antisera against the active neurotransmitters labeled central neurons and
nerves innervating the heart in a pattern consistent with their putative
cardioregulatory functions. 5. Thus, despite the simplicity of the cardiac
motor circuit in Archidoris, contractile activity of the heart appears to
be regulated by several neurotransmitters, each with subtly different modes
and thresholds of action.
