Properties and opioid inhibition of mesolimbic dopamine neurons vary according to target location

Abstract

The mesolimbic dopamine system, which mediates the rewarding properties of nearly all drugs of abuse, originates in the ventral tegmental area (VTA) and sends major projections to both the nucleus accumbens (NAc) and the basolateral amygdala (BLA). To address whether differences occur between neurons that project to these separate areas, retrograde microspheres were injected to either the BLA or the NAc of DBA/2J mice. Whole-cell recordings were made from labeled VTA dopamine neurons. We found that identified neurons that projected to the BLA and NAc originated within different quadrants of the VTA with neither group exhibiting large-amplitude h-currents. Neurons that projected to the NAc exhibited a greater outward current in response to the kappa-opioid agonist (5alpha,7alpha,8alpha)-(+)-N-methyl-N-[7-(pyrrolidinyl)-1-oxaspiro [4,5]dec-8-yl]-benzeneacetamide (U69593; 200 nM), whereas neurons that projected to the BLA exhibited greater inhibition to the mu/delta opioid agonist [Met5] enkephalin (ME; 3 microM). In addition, we found that the presynaptic inhibition of GABAergic transmission at both GABAA and GABAB receptors was differentially regulated by U69593 between the two groups. When dopamine IPSCs were examined, U69593 caused a greater inhibition in NAc- than BLA-projecting neurons. ME had no effect on either. Finally, the regulation of extracellular dopamine by dopamine uptake transporters was equal across the VTA. These results suggest that opioids differentially inhibit mesolimbic neurons depending on their target projections. Identifying the properties of projecting mesolimbic VTA dopamine neurons is crucial to understanding the action of drugs of abuse.

Figure 1.

VTA neurons that project to the BLA and the NAc originate from distinct anatomical locations within the VTA. A, B, Bright-field images identifying the anatomical location of injection sites of retrograde tracers into either the BLA or the NAc [overlay adapted from (Paxinos and Franklin (2001)]. C, D, Horizontal confocal sections (10×) imaged after staining for tyrosine hydroxylase (red) to identify DA neurons from mice in which microspheres (overlaid in white) were retrogradely transported from the BLA (C) or NAc (D). Ant, Anterior; Med, medial. E, F, High-power 60× single-plane confocal sections (0.1 μm) illustrating the presence of microspheres (green) and DA neurons stained for TH (red). The arrows illustrate representative neurons in which yellow indicates the colocalization of microspheres and TH. G, Example of a neurobiotin-filled neuron from a mouse in which tracer was injected into the BLA. Triple staining for neurobiotin, tracer, and TH confirms that this recorded BLA-projecting neuron was DAergic.

Figure 2.

Amplitude of I_h is different among BLA- and NAc-projecting VTA neurons and SNc neurons. A–C, Typical families of whole-cell currents evoked by −10 mV square-wave step pulses from V_h of −60 to −120 mV recorded from NAc- (A), and BLA- (B) projecting and unlabeled SNc (C) neurons. D, Summary of observed mean I_h amplitudes from NAc-projecting (n = 45), BLA-projecting (n = 47), and unlabeled SNc neurons (n = 11). E, Summary of observed input resistances and capacitances from BLA- and NAc-projecting neurons. Error bars signify SEM. * signifies significance at p < 0.05.

Figure 3.

Action potentials wider than 1.2 ms are a marker of principle DA neurons. A, Stacked (0.1 μm section, 1 μm step, 10 stacked images) horizontal confocal image of a neuron that was filled with 0.3% neurobiotin. Neurobiotin was visualized by treating slices with Alexa-568-conjugated streptavidin (green). Neurons were also stained for TH (red). The overlay illustrates the colocalization (yellow) of the filled cell with TH. B, Histogram illustrating the spike width distribution of microsphere-labeled VTA DA neurons and control, unlabeled secondary neurons. Note the lack of overlap between the two populations of neurons. C, D, Cell-attached extracellularly recorded APs recorded from the principle DA neuron illustrated in A and an unlabeled control secondary neuron (D).

Figure 4.

Opioid inhibition of mesolimbic VTA neurons is dependent on target location. A, B, Whole-cell currents produced in representative NAc- (A) and BLA- (B) projecting neurons in response to the KOR agonist U69593 (200 nm), the M/DOR agonist ME (3 μm), and the 5-HT₁ agonist 5-CT (100 nm). The nonspecific opioid receptor antagonist naloxone (NLX; 1 μm) was used to reverse the effect caused by U69593. C–E, Scatter plots illustrating both the peak-amplitude whole-cell currents from NAc- and BLA-projecting neurons in response to U69593 (200 nm; C), ME (3 μm; D), and 5-CT (100 nm; E). The numbers in parentheses signify the proportion of total cells recorded that responded to drug application with a measurable outward current. The box and whisker plot illustrates the mean ± SEM. Note the opposing actions of the KOR and M/DOR agonists on BLA- and NAc-projecting cells. 5-CT failed to produce an outward current in all recorded neurons. * signifies significance at p < 0.05.

Figure 5.

KORs inhibit GABA_A IPSCs from mesolimbic-projecting DA neurons. A–C, Normalized summary data and typical recordings of GABA_A IPSCs illustrating the effects of U69593 (200 nm; A), ME (3 μm; B), and 5-CT (100 nm; C) on BLA- (○) and NAc- (•) projecting VTA neurons (n = 6–11). Illustrated GABA_A IPSCs are the averages of three to five traces before and during drug application. The calibration in C applies to all traces. Error bars indicate SEM. * signifies significance at p < 0.05. NLX, Naloxone.

Figure 6.

Inhibition of GABA_B IPSCs by opioids and 5-CT from identified BLA- and NAc-projecting neurons. A–C, Normalized summary data and typical recordings of GABA_B IPSCs illustrating the effects of U69593 (200 nm; A), ME (3 μm; B), and 5-CT (100 nm; C) on BLA- (○) and NAc- (•) projecting VTA neurons (n = 6–8). Illustrated GABA_B IPSCs are the averages of two to four traces before and during drug application. IPSCs were evoked with monopolar glass electrodes by a train of five pulses (duration, 0.5 ms; 40 Hz) every 60 s. The calibration in the middle panel of C applies to all traces from NAc-projecting cells; the calibration in the right panel of C applies to all traces from BLA-projecting cells. Error bars indicate SEM. * signifies significance at p < 0.05. NLX, Naloxone.

Figure 7.

Inhibition of the D₂ IPSC by KORs, but not M/DORs or 5-HT-Rs, is greater in NAc- than BLA-projecting VTA neurons. A–C, Normalized summary data and typical recordings of D₂ IPSCs illustrating the effects of U69593 (200 nm; A), ME (3 μm; B), and 5-CT (100 nm; C) on BLA- (○) and NAc- (•) projecting VTA neurons (n = 6–8). Illustrated D₂ IPSCs are the averages from two to four traces before and during drug application. IPSCs were evoked with monopolar glass electrodes by a train of five pulses (duration, 0.5 ms; 40 Hz) every 60 s. The calibration in C applies to all traces. Error bars indicate SEM. * signifies significance at p < 0.05. NLX, Naloxone.

Figure 8.

Dopamine uptake transporters regulate extracellular dopamine equally at the somatodendritic level across the VTA. A, Representative synaptic and exogenous currents recorded from NAc- and BLA-projecting neurons. IPSCs were evoked with monopolar glass electrodes by a train of five pulses (duration, 0.5 ms; 40 Hz). Iontophoretic currents were evoked by ejecting DA as a cation with a single pulse (20–150 nA; 25–100 ms) once every 60 s. Note the equal increase in amplitudes and the slowing of kinetics of both currents by the application of the nonspecific monoamine uptake inhibitor cocaine (1 μm) in both BLA- and NAc-projecting neurons. B, Summary data (normalized percentage change; mean ± SEM) from BLA- (n = 5) and NAc- (n = 7) projecting neurons. C, Summary of the kinetics of D₂ IPSCs recorded in BLA- (n = 6) and NAc- (n = 9) projecting VTA neurons. Stim, Stimulation.