Nanomolar concentrations of pregnenolone sulfate enhance striatal dopamine overflow in vivo

Abstract

The balance between GABA-mediated inhibitory and glutamate-mediated excitatory synaptic transmission represents a fundamental mechanism for controlling nervous system function, and modulators that can alter this balance may participate in the pathophysiology of neuropsychiatric disorders. Pregnenolone sulfate (PS) is a neuroactive steroid that can modulate the activity of ionotropic glutamate and GABA(A) receptors either positively or negatively, depending upon the particular receptor subtype, and modulates synaptic transmission in a variety of experimental systems. To evaluate the modulatory effect of PS in vivo, we infused PS into rat striatum for 20 min via a microdialysis probe while monitoring local extracellular dopamine (DA) levels. The results demonstrate that PS at low nanomolar concentrations significantly increases extracellular DA levels. The PS-induced increase in extracellular DA is antagonized by the N-methyl-d-aspartate (NMDA) receptor antagonist, d-AP5 [d-(-)-2-amino-5-phosphonopentanoic acid], but not by the sigma receptor antagonist, BD 1063 [1(-)[2-(3,4-dichlorophenyl)-ethyl]-4-methylpiperazine]. The results demonstrate that exogenous PS, at nanomolar concentrations, is able to increase DA overflow in the striatum through an NMDA receptor-mediated pathway.

Figure 1

Histological verification of microdialysis probe placement in the striatum. Lines indicate the placement of the microdialysis probes in the striatum, and numbers indicate distance from bregma (mm) according to the atlas of Paxinos and Watson (Paxinos and Watson, 1997).

Figure 2

PS induces an increase in extracellular DA in the rat striatum. A, time course showing DA levels with or without infusion of PS (10 nM) through the dialysis probe (horizontal bar indicates period of PS infusion). Analysis by mixed factors ANOVA with repeated measures over time revealed a significant effect of time [F(8,88) = 4.593; p < 0.0001] and a significant time × treatment interaction [F(8,88) = 4.074; p < 0.0004]. Number of animals is 8 for aCSF group, 6 for PS group. B, concentration dependence of PS-induced increase in striatal extracellular DA. Bars indicate mean ± SEM DA content of the sample collected during 20-min perfusion with the indicated concentration of PS, expressed as percentage of DA content of baseline samples. Statistical analysis revealed a significant main effect of treatment [F(7,69) = 6.789; p < 0.0001]. * Denotes significant differences from baseline values (Bonferroni, p < 0.003). Number of animals is given in parentheses.

Figure 3

The PS-induced DA increase is inhibited by D-AP5. Bars indicate mean DA content of dialysate collected while perfusing PS and/or D-AP5 through the dialysis probe for 20 min, expressed as percentage of baseline. There was a significant main effect on extracellular DA levels [F(3,25) = 9.948; p < 0.0002]. * Denotes significant differences from baseline values (Bonferroni, p < 0.05). # Denotes significant difference from 50 nM PS (Bonferroni, p < 0.002). Number of animals per group is given in parentheses.

Figure 4

PS metabolites do not increase extracellular DA. Statistical analysis revealed no significant differences in DA recovery compared with control (F(8,33) = 1.075; p < 0.404). Data are mean ± SEM DA content of dialysate collected during 20 min perfusion with the indicated compound, expressed as percentage of baseline. Number of animals per group is given in parentheses.

Figure 5

PHS increases extracellular DA. A, time course showing effect of PHS (50 nM) on DA levels. Number of animals is 6 per group. Horizontal bar indicates period of PHS infusion. B, concentration dependence of PHS induced increase in extracellular DA. PHS perfusion resulted in a significant increase in dopamine recovery [F(4, 25) = 3.369; p < 0.025]. * Denotes significant differences from baseline values (Bonferroni, p < 0.05). Data are mean ± SEM DA content of dialysate collected during infusion of PHS (20 min), presented as percentages of baseline values.

Figure 6

DA overflow is unaffected by σ receptor ligands. PS and/or BD 1063 or SKF 10,047 was perfused through the dialysis probe. Bars indicate mean ± SEM DA content of the sample collected while perfusing 50 nM PS and/or 30 μM BD 1063 or 100 μM SKF 10,047 through the dialysis probe for 20 min, expressed as percentage of baseline. Statistical analysis revealed a significant main effect of drug treatment [F(4,28) = 3.934, p < 0.011]. Post-hoc analyses revealed a significant increase in DA levels following PS and PS + BD 1063, indicating that BD 1063 does not block the PS-induced increase in DA overflow. SKF 10,047 administration into the striatum through the dialysis probe did not significantly affect extracellular DA levels compared with control. * Denotes significant differences from baseline values (Bonferroni, p < 0.03). Number of animals is 6-8 per group.