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