DOPAMINE ENHANCEMENT AND DEPRESSION OF GLUTAMATE-REGULATED CALCIUM AND ELECTRICAL-ACTIVITY IN HYPOTHALAMIC NEURONS

1. The neurotransmitter dopamine is found throughout the hypothalamus both in cell bodies and in axons originating from intra- and extrahypo thalamic sources. To study the mechanisms of action of dopamine on cul tured rat hypothalamic neurons, particularly in relation to Ca2+ regul ation, we used Ca2+ digital imaging with fura-2 and whole cell patch-c lamp recording. We focused on the modulatory actions of dopamine on gl utamate. 2. Dopamine administration had little or no independent effec t on intracellular Ca2+. However, in the presence of tetrodotoxin to b lock action potentials and action-potential-dependent transmitter rele ase, dopamine (10 mu M for 2-3 min) caused an increase in glutamate-ev oked Ca2+ rises in 22% of 64 neurons and depressed glutamate-evoked Ca 2+ rises in an equal number of neurons. Shorter exposure to dopamine r educed the number of responding cells. 3. Dopamine application to neur ons with an elevated Ca2+ due to synaptic release of glutamate (in the absence of tetrodotoxin) generally caused a decrease in Ca2+ levels ( 40% of 106 neurons), but sometimes increased cytosolic Ca2+ (10% of 10 6 neurons). That dopamine influenced cells differently in conditions o f spontaneous activity compared with evoked activity may be due to dop amine effects on presynaptic receptors detected under conditions of on going synaptic release of glutamate. 4. Dopamine modulation of glutama te responses was detected at early stages of neuronal development (emb ryonic day 18 after 2 days in vitro) and also after 60 days in vitro. 5. The D1, D2, and D3 dopamine receptor agonists SKF38393. quinpirole, and 7-OH-DPAT (+/-7 hydroxy-dipropylaminotetralin) caused a reduction in Ca2+ levels raised by endogenous glutamate release or evoked by ex ogenous glutamate application. 6. To block the actions of dopamine rel eased by hypothalamic neurons. D1 and D2 dopamine receptor antagonists were used. As with dopamine, dopamine antagonists had no effect on in tracellular Ca2+ during glutamate receptor blockade. In the absence of glutamate receptor block, the DI antagonist SCH23390 ( 1 mu M) reduce d Ca2+ in responding cells; in contrast, the D2 antagonist eticlopride (1 mu M) generated a delayed increase in Ca2+ levels. 7. Dopamine is known to activate second messengers through G proteins independent of changes in membrane potential or input resistance. Whole cell recordin g was used to demonstrate that, parallel to the modulation of Ca2+, do pamine exerted a dramatic change in glutamate-mediated electrical acti vity, generally depressing activity and hyperpolarizing the membrane p otential (8 of 15 neurons). 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 mod ulation of glutamate actions. Dopamine depressed glutamate-evoked curr ents in a dose-dependent fashion, with Hill slopes in individual neuro ns ranging from 0.3 to 0.6. Dopamine could also evoke a direct hyperpo larizing action on hypothalamic neurons in the presence of tetrodotoxi n or glutamate receptor blockers, at least in part by opening K+ chann els. 9. Glutamate plays an important role as a primary excitatory tran smitter within the hypothalamus. Our data support the hypothesis that a major mechanism of dopamine's influence on hypothalamic neurons invo lves the modulation of glutamate's excitatory action, mostly by inhibi tion. This is consistent with the hypothesis that modulation of glutam ate activity may be an important mechanism of dopamine action througho ut the nervous system.