Segment Specific Effects of FMRFamide on Membrane Properties of Heart
Interneurons in the Leech.
Schmidt, Joachim, Siglinde Gramoll, Ronald L. Calabrese.
Department of Biology, Emory University, 1510 Clifton Road, Atlanta, GA 30322.
APStracts 2:0177N, 1995.
SUMMARY AND CONCLUSIONS
1. The effects of FMRFamide (10 -6 M) on membrane properties of heart
interneurons in the 3rd, 4th and 5th segmental ganglia, HN(3), HN(4), and
HN(5) cells respectively, of the leech were studied using discontinuous
current clamp and single electrode voltage clamp techniques. FMRFamide was
focally applied onto the soma of the cell under investigation. 2. Application
of FMRFamide depolarized HN(3) and HN(4) interneurons by evoking an inward
current. These responses were subject to pronounced desensitization. The
inward currents evoked by application of FMRFamide were associated with an
increase in membrane conductance and appeared to be voltage-dependent.
Currents were enhanced at more depolarized potentials. 3. The responsiveness
of the HN(3) and HN(4) interneurons was not affected when the Ca 2+
concentration in the bathing saline was reduced from normal 1.8 mM to 0.1 mM.
The depolarizing response upon application of FMRFamide was blocked when Co 2+
substituted for Ca 2+ . 4. HN(3) and HN(4) interneurons did not respond to
FMRFamide application in Na + -free solution. Inward currents were largely
reduced when bathing saline with 30% of the normal Na + concentration was
used. When Li + was substituted for Na + in the saline, application of
FMRFamide still evoked depolarizing responses in HN(3) and HN(4) cells. 5. We
conclude that focal application of FMRFamide onto the somata of heart
interneurons HN(3) and HN(4) evokes a voltage-dependent inward current,
carried largely by Na + . 6. Focal application of FMRFamide onto somata of
heart interneurons HN(5) hyperpolarized these cells by activating a voltage-
dependent outward current. 7. HN(5) interneurons were loaded with Cl - until
inhibitory postsynaptic potentials carried by Cl - reversed. Cl - -loaded
cells still responded with a hyperpolarization when FMRFamide was applied onto
their somata. Therefore the outward current evoked by FMRFamide appears to be
mediated by a K + conductance increase. 8. Application of FMRFamide onto the
somata of HN(5) interneurons enhanced outward currents that were evoked by
depolarizing voltage steps from a holding potential of -45 mV. 9. We conclude
that the hyperpolarizing response of HN(5) interneurons to focal application
of FMRFamide onto their somata is the result of an up-regulation of a voltage-
dependent K + current.
Received 9 January 1994; accepted in final form 6 June 1995.
APS Manuscript Number J18-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 6 July 1995.