Slices of the nucleus accumbens, the terminus of the mesolimbic dopaminergic system and the striatum, the terminus of the nigrostriatal dopaminergic system obtained from rats, were used for analyzing the dopamine releasing effects of TRH and its analog DN-1417 (γ-butyrolactone-γ-carbonyl-L-histidyl-L-prolinamide citrate). (1) Dopamine release: The addition of DN-1417 (5×10
-5 M) or TRH (5×10
-4 M) stimulated the release of prelabelled [
3H]-dopamine (DA) from the superfused nucleus accumbens slices, and 100 or 20 times higher concentrations of each compound stimulated DA release from the striatal slices. The omission of Ca
2+ from the superfusion medium or the addition of ouabain (5×10
-3 M), a (Na
++K
+) stimulated adenosine triphosphate (ATP) phosphohydrolase [(Na
++K
+)-ATPase] inhibitor, almost completely abolished the DN-1417 or TRH-induced DA releasing effect. (2)
45Ca
2+ uptake: The addition of DN-1417 (10
-4 M) or TRH (10
-4 M) stimulated
45Ca
2+ uptake into the nucleus accumbens slices in a time-dependent manner from 1 to 5 min at 30°C. On the other hand, both drugs (10
-4 M) had no effect on
45Ca
2+ uptake into the striatal slices. A Ca
2+ ionophore, A-23187 (10
-6 M), stimulated
45Ca
2+ uptake into slices of the nucleus accumbens and striatum. Ouabain abolished the DN-1417-, TRH- and A-23187-induced effects. (3) Cyclic AMP formation: The addition of DN-1417 (10
-4 M) or TRH (10
-4 and 10
-3 M) accelerated cyclic AMP formation in the nucleus accumbens, but not in the striatal slices. A Ca
2+ antagonist, methoxyverapamil (D-600, 10
-6 M) almost completely abolished the DN-1417 and TRH-effects. (4) (Na
++K
+)-ATPase activity: The addition of DN-1417 (10
-5 and 10
-4 M) or TRH (10
-4 M) activated (Na
++K
+)-ATPase in the P
1 fraction (cell debris, nuclei) of the nucleus accumbens, but had little effect on the same activity in the striatum. (5) [
3H]-TRH binding: The addition of 10
-6 to 10
-4 M of DN-1417 and TRH inhibited concentration-dependently [
3H]-TRH binding in the nucleus accumbens and striatal slices. Ouabain (5×10
-3 M) almost completely abolished [
3H]-TRH binding in both types of slices. These results suggest that DN-1417 and TRH activate (Na
++K
+)-ATPase and stimulate Ca
2+ uptake in the presynaptic sites and release DA which stimulates cyclic AMP formation in the postsynaptic sites of the mesolimbic dopaminergic system mainly and causes several central nervous actions.
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