Dopamine enhancement and depression of glutamate-regulated calcium and
electrical activity in hypothalamic neurons.
Van den Pol, Anthony N., Vinh Cao and Andrei B.Belousov.
5 March 1996,5 July 1996..
APStracts 3:0159N, 1996.
SUMMARY AND CONCLUSIONS
1. The neurotransmitter dopamine is found throughout the hypothalamus in both
cell bodies and in axons originating from intra- and extrahypothalamic
sources. To study the mechanisms of action of dopamine on cultured rat
hypothalamic neurons, particularly related to Ca 2+ regulation, Ca 2+ digital
imaging with fura-2 and whole-cell patch clamp recording were used. We focused
on the modulatory actions of dopamine on glutamate. 2. Dopamine administration
had little or no independent effect on intracellular Ca 2+ . However, in the
presence of tetrodotoxin to block action potentials and action potential-
dependent transmitter release, dopamine (10 uM for 2-3 min) caused an increase
in glutamate evoked Ca 2+ rises in 22% of 64 neurons and depressed glutamate
evoked Ca 2+ rises in an equal number of neurons. Shorter exposure to dopamine
reduced the number of responding cells. 3. Dopamine application to neurons
with an elevated Ca 2+ due to synaptic release of glutamate (in the absence of
tetrodotoxin) generally caused a decrease in Ca 2+ levels (40% of 106
neurons), but sometimes increased cytosolic Ca 2+ (10% of 106 neurons). That
dopamine influenced cells differently in conditions of spontaneous activity
compared with evoked activity may be due to dopamine effects on presynaptic
receptors detected under conditions of ongoing synaptic release of glutamate.
4. Dopamine modulation of glutamate responses was detected at early stages of
neuronal development (embryonic day 18 after 2 days in vitro) and also after
60 days in vitro. 5. D1-, D2-, and D3-dopamine receptor agonists SKF38393,
quinpirole, and 7 OH-DPAT caused a reduction in Ca 2+ levels raised by
endogenous glutamate release or evoked by exogenous glutamate application. 6.
To block the actions of dopamine released by hypothalamic neurons, D1- and D2-
dopamine receptor antagonists were used. As with dopamine, dopamine
antagonists had no effect on intracellular Ca 2+ during glutamate receptor
blockade. In the absence of glutamate receptor block, the D1 antagonist
SCH23390 (1 uM) reduced Ca 2+ in responding cells; in contrast, the D2
antagonist eticlopride (1 uM) generated a delayed increase in Ca 2+ levels. 7.
Dopamine is known to activate second messengers through G-proteins independent
of changes in membrane potential or input resistance. Whole cell recording was
used to demonstrate that, parallel to the modulation of Ca 2+ , dopamine
exerted a dramatic change in glutamate mediated electrical activity, generally
depressing activity and hyperpolarizing the membrane potential (8 of 15). In a
smaller number of neurons (5 of 15) dopamine enhanced glutamate-mediated
excitatory activity. 8. Dopamine-evoked changes in membrane potential were in
part mediated through modulation of glutamate actions. Dopamine depressed
glutamate-evoked currents in a dose dependent fashion, with Hill slopes in
individual neurons ranging from 0.3 to 0.6. Dopamine could also evoke a direct
hyperpolarizing action on hypothalamic neurons in the presence of tetrodotoxin
or glutamate receptor blockers, at least in part by opening K + channels. 9.
Glutamate plays an important role as a primary excitatory transmitter within
the hypothalamus. Our data support the hypothesis that a major mechanism of
dopamine's influence on hypothalamic neurons involves the modulation of
glutamate's excitatory action, mostly by inhibition. This is consistent with
the hypothesis that modulation of glutamate activity may be an important
mechanism of dopamine action throughout the nervous system.
Received 5 March 1996; accepted in final form 5 July 1996.
APS Manuscript Number J175-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 21 August 1996