RELATIVE VULNERABILITY OF DOPAMINE AND GABA NEURONS IN MESENCEPHALIC CULTURE TO INHIBITION OF SUCCINATE-DEHYDROGENASE BY MALONATE AND 3-NITROPROPIONIC ACID AND PROTECTION BY NMDA RECEPTOR BLOCKADE

The effects of different severities of metabolic stress on dopamine (D A) and gamma-aminobutyric acid (GABA) cell loss were examined in rat m esencephalic culture. Partial metabolic inhibition was induced in 12-d ay-old cultures by a 24-hr treatment with various concentrations of 3- nitropropionic acid(3-NPA, 0.1-0.5 mM) or malonate (10-50 mM), irrever sible and reversible inhibitors of the Krebs cycle enzyme, succinate d ehydrogenase, Cell damage to the DA and GABA populations was assessed after a 48-hr recovery period by simultaneous measurement of high affi nity uptake for H-3-DA and C-14-GABA. 3-NPA or malonate caused a dose- dependent loss of DB uptake (EC(50) 0.21 or 42 mM, respectively). 3-NP A treatment was equally detrimental to the GABA population, whereas ma lonate exposure did not cause any significant loss of GABA uptake. The presence of the NMDA antagonist, MK-801 (1 mu M), during 24 hr of 3-N PA or malonate treatment fully protected against DA and GABA loss with 50 mM malonate or 0.25 mM 3-NPA and partially protected versus 0.5 mM 3-NPA To determine the degree of metabolic stress imposed by 3-NPA an d malonate, 12-day-old cultures were treated with 0.5 mM 3-NPA or 50 m M malonate for 3 hr and the rate of lactate formation was measured. La ctate was increased nearly 2-fold at 3 hr of treatment with 3-NPA, but was not significantly elevated above basal with malonate treatment. S DH activity was decreased by 48 or 58% after 3 hr of treatment with 0. 25 and 0.5 mM 3-NPA, respectively. 3-NPA (0.5 mM) did not produce an i ncrease in extracellular glutamate after 3 hr of exposure. However, th is length of exposure to 3-NPA was sufficient to result in an 84% redu ction in DA uptake measured after 48 hr of recovery. Consistent with m alonate producing a much milder metabolic stress, a 3-hr exposure to 5 0 mM malonate did not result in significant toxicity when assayed afte r 48 hr of recovery. These studies indicate that DA neurons in vitro d isplay a relative vulnerability to mild metabolic stress as compared w ith mesencephalic GABAergic neurons, which results in toxicity that is mediated by NMDA receptors. NMDA receptor involvement was not correla ted with a rise in extracellular glutamate during partial metabolic in hibition and further suggests that under these conditions, factors oth er than increased extracellular glutamate may be responsible for NMDA receptor activation.