Journal of Neuroscience, Vol 7, 3953-3960, Copyright © 1987 by Society for Neuroscience
Slow oscillations of membrane potential in interneurons that control heartbeat in the medicinal leech
EA Arbas and RL Calabrese
Biological Laboratories, Harvard University, Cambridge, Massachusetts 02138.
In the preceding paper (Arbas and Calabrese, 1987), we identified several
properties that contribute to the activity of neurons (HN cells) that
control heartbeat in the medicinal leech. Premotor HN (7) interneurons,
which do not generate the heartbeat rhythm, exhibit Na+- dependent fast
action potentials, Ca2+-mediated plateau potentials in the absence of Na+,
and hyperpolarization-activated "restorative" changes in membrane potential
that depolarize the membrane potential on hyperpolarization due to injected
currents or synaptic inhibition. HN interneurons of ganglia 3 and 4 (i.e.,
timing oscillator interneurons) exhibit all of the properties described for
HN (7) interneurons and have the additional characteristic that they are
connected in oscillatory circuits. Reciprocal oscillations in membrane
potential occurred in the bilateral HN interneurons (3) and (4) in the
presence of elevated Ca2+ that were independent of Na+ -mediated action
potentials. Their ability to oscillate in this way is based on 3
parameters: (1) production of a regenerative plateau potential by one of
the pair of HN neurons in either ganglion, (2) inhibition of the
contralateral HN neuron by the HN neuron in plateau, and (3) a phase
transition mediated by escape from inhibition by the hyperpolarized HN
neuron. The conductances responsible for restorative membrane potential
shifts activated by hyperpolarization during synaptic inhibition may
mediate the escape from inhibition that times the phase transition of the 2
HN neurons.