Medial prefrontal cortex inversely regulates toluene-induced changes in markers of synaptic plasticity of mesolimbic dopamine neurons

Abstract

Toluene is a volatile solvent that is intentionally inhaled by children, adolescents, and adults for its intoxicating effects. Although voluntary use of toluene suggests that it possesses rewarding properties and abuse potential, it is unknown whether toluene alters excitatory synaptic transmission in reward-sensitive dopamine neurons like other drugs of abuse. Here, using a combination of retrograde labeling and slice electrophysiology, we show that a brief in vivo exposure of rats to a behaviorally relevant concentration of toluene vapor enhances glutamatergic synaptic strength of dopamine (DA) neurons projecting to nucleus accumbens core and medial shell neurons. This effect persisted for up to 3 d in mesoaccumbens core DA neurons and for at least 21 d in those projecting to the medial shell. In contrast, toluene vapor exposure had no effect on synaptic strength of DA neurons that project to the medial prefrontal cortex (mPFC). Furthermore, infusion of GABAergic modulators into the mPFC before vapor exposure to pharmacologically manipulate output, inhibited, or potentiated toluene's action on mesoaccumbens DA neurons. Together, the results of these studies indicate that toluene induces a target-selective increase in mesolimbic DA neuron synaptic transmission and strongly implicates the mPFC as an important regulator of drug-induced plasticity of mesolimbic dopamine neurons.

Figure 1.

Analysis of toluene vapor concentration by gas chromatography. A, Schematic of experimental design. Toluene exposures are marked by the gray boxes. B, Sample chromatogram from 5000 PPM. H, Hexane; T, toluene; X, o-xylene. C, Standard curve showing the ratio of toluene and o-xylene area under the curve (r² = 0.991; n = 5 for each). Open circles represent the measured concentrations from 4 and 8% toluene exposures in the vapor chamber (n = 5 for each). D, Representative chromatogram from an air sample from the vapor chamber when 8% toluene is flowing at a rate of 4 L/min for 10 min.

Figure 2.

Immunohistochemistry for TH. A, Examples of TH+ and TH− VTA neurons. Green indicates TH; and red, biocytin. Scale bar, 20 μm. B, Proportion of TH+ recorded neurons per region that received the retrograde tracer microinjection; blue, TH+; red TH−. NAc core: 125 TH+; 18 TH−; NAc medial shell: 41 TH+; 5 TH−; mPFC: 11 TH+; 5 TH−.

Figure 3.

Toluene vapor enhances the AMPA/NMDA ratio in mesoaccumbens core and medial shell DA neurons. A, Retrograde tracer injections into the NAcc result in retrobead expression in midbrain dopamine neurons. Left panels, Serial sections of both brain regions with a heat map of retrobead locations from 6 subjects. Values indicate distance from bregma (mm). Right panels, An example of retrobead expression (red) in a midbrain DA neuron. B, Retrograde tracer injections into NAcs and retrobead expression in midbrain DA neuron. C, AMPA/NMDA ratios 24 h after vapor chamber exposure in VTA DA neurons that project to the NAcc. *The 5700 PPM AMPA/NMDA ratio is higher than the ratio for the air control (p < 0.05, Kruskal–Wallis, Dunn's multiple-comparison test). D, Time course of toluene effect on AMPA/NMDA ratio in mesoaccumbens core DA neurons. *AMPA/NMDA ratios 1 and 3 d after 5700 PPM toluene exposure are greater than air controls (p < 0.05, Kruskal–Wallis, Dunn's multiple-comparison test). E, Time course of toluene effect on AMPA/NMDA ratio in mesoaccumbens medial shell neurons. *AMPA/NMDA ratios 1, 3, and 21 d after 5700 PPM toluene exposure are greater than air controls (p < 0.05, one-way ANOVA, Bonferroni's multiple-comparison test). F, Example traces are AMPA-mediated (gray) and NMDA-mediated (black) currents in mesoaccumbens core neurons from subjects exposed to air, 2850 PPM toluene, or 5700 PPM toluene. Calibration: 20 pA, 10 ms. G, Example traces from subjects exposed to air or 5700 PPM toluene. Calibration: 20 pA, 10 ms.

Figure 4.

Toluene vapor has no effect on mesocortical DA neurons. A, Retrograde tracer injections into mPFC and retrobead expression in a midbrain DA neuron. B, Summary of effects of toluene on AMPA/NMDA ratio in mesocortical DA neurons. C, Example traces from subjects exposed to air or 5700 PPM toluene. Calibration: 20 pA, 10 ms.

Figure 5.

In vivo toluene exposure increases sEPSC amplitude, but not frequency, in mesoaccumbens core DA neurons. A, Averaged sEPSC traces from air-exposed (gray) and toluene-exposed subjects (black) (calibration: 5 ms, 5 pA) and an extended EPSC recording (calibration: 0.5 s, 25 pA, 0.5 s). B, Cumulative probability chart of sEPSC amplitude from animals exposed to 5700 PPM toluene or air. ***Significant amplitude × vapor treatment interaction (p < 0.001, mixed ANOVA). Inset: Average sEPSC amplitude for subjects exposed to air or toluene. *p < 0.05 (t test). C, Cumulative probability chart of sEPSC IEI for toluene vapor and air controls. Inset, Average sEPSC IEI for toluene and air control.

Figure 6.

In vivo toluene exposure alters AMPA receptor rectification in mesoaccumbens core DA neurons. A, I–V relationship for evoked EPSC normalized peak amplitudes at varying holding potentials in subjects exposed to toluene vapor and air. ***Current output at +40 mV holding potential is different between groups (p < 0.001, mixed ANOVA, Bonferroni multiple-comparison test). B, Sample AMPA traces at +40 mV and −70 mV holding potentials for air controls (gray) and toluene-exposed subjects (black). Calibration: 5 ms, 25 pA. C, Summary of toluene effect on rectification index. *AMPA rectification index for toluene-treated subjects is different from air control (p < 0.05, t test).

Figure 7.

PFC output regulates toluene's effect on AMPA/NMDA ratio in mesoaccumbens core DA neurons. A, Microinjection cannula placements in the medial prefrontal cortex. Values are distance from bregma (mm). B, Summary of effects of an intra-mPFC picrotoxin infusion (100 ng) before vapor chamber exposure. **Saline mPFC infusion/5700 PPM toluene group is different from all other groups (p < 0.01 for each comparison, two-way ANOVA, Bonferroni post hoc test). C, Summary of effects of an intra-mPFC muscimol/baclofen infusion (3.5 mg/130 ng) before vapor chamber exposure. *Muscimol/baclofen mPFC infusion/2850 PPM toluene group is different from all other groups (p < 0.05 for each comparison, two-way ANOVA, Bonferroni post hoc test).