Nucleus accumbens medium spiny neurons target non-dopaminergic neurons in the ventral tegmental area

Abstract

The midbrain ventral tegmental area (VTA) projection to the nucleus accumbens (NAc) is implicated in motivation and reinforcement. A significant number of NAc medium spiny neurons (MSNs) project back to the VTA, although the nature of this projection is essentially unknown. For example, do NAc MSNs directly target accumbens-projecting dopamine neurons and do they act via the GABA(A) or GABA(B) receptor? To address these issues, we expressed the light-sensitive channel rhodopsin-2 in the rat NAc and made electrophysiological recordings from VTA neurons ex vivo. We found that the NAc directly targets non-dopaminergic VTA neurons, including some that project back to the NAc. These MSN GABAergic terminals are opioid sensitive and act via GABA(A) receptors.

Figure 1.

ChR2-expression in the NAc. A, Confocal image of ChR2–tdtomato expression (red) in NAc neurons. Scale bar, 50 ìm. B, Diagram of experimental design: light stimulation (blue) of ChR2 expressing NAc MSN (red). C, Example recording from an MSN showing responses to 1 ms pulses of light at three different intensities. D, MSNs can follow trains of light pulses up to 100 Hz. E, Average probability of firing an action potential (AP) for each stimulus in a train for three different frequencies (n = 6). F, Example recording of a light-evoked IPSC in a neighboring uninfected MSN (current is inward when holding MSN at −75 mV). This IPSC was entirely blocked after the application of picrotoxin (PTX). G, IPSCs evoked in response to a 50 Hz train of light.

Figure 2.

ChR2-expressing MSNs project to the VTA. A, Confocal image of ChR2–tdtomato expression (red) in axon terminals within the VTA near TH⁺ neurons (green) after microinjection into the NAc. Scale bar, 50 ìm. B, Diagram of experimental design: light stimulates ChR2-expressing NAc MSN terminals (red) while recording from uninfected VTA neurons. C, Example recording from VTA neuron showing responses to two 3 ms pulses of light, 50 ms apart. This IPSC was entirely blocked after application of picrotoxin (PTX). Bottom graph shows time course of pharmacological effect.

Figure 3.

Lack of light-evoked GABA_B response in VTA neurons. A, Average of five consecutive traces in an NAc-connected neuron after the GABA_A response was blocked with 100 μm picrotoxin and after application of the GABA_B receptor antagonist CGP-55845 [(2S)-3-[(15)-1-(3,4-dichlorophenyl)ethyl]amino-2-hydroxypropyl)(phenylmethyl)phosphinic acid] shows no response to 100 Hz light stimulation. B, The amplitude of electrically evoked GABA_B IPSCs depends on the stimulation frequency. C, The addition of NNC-711 (10 μm) does not reveal a light-evoked GABA_B IPSC. D, NNC-711 enhanced electrically evoked GABA_B IPSCs. E, Bath application of 20 μm baclofen produced a small outward current in a neuron that received input from the NAc (top) compared with the response of a control TH (+) neuron (bottom). F, Connected neurons (n = 5) showed significantly smaller baclofen responses than TH (+) control neurons (n = 9), which had larger responses to baclofen than identified TH (−) neurons (n = 5). *p < 0.05, **p < 0.01.

Figure 4.

NAc MSNs target VTA non-dopamine neurons. A, Proportion of connected and unconnected neurons that express the I_h. B, Among I_h-expressing neurons, there was no difference in the magnitude of the I_h in connected and unconnected neurons. C, Example of a biocytin-filled TH (−) neuron that responded to light stimulation with an IPSC. Biocytin, Red; TH, green. Inset shows summary of TH immunocytochemistry for connected and unconnected neurons. *p < 0.01.

Figure 5.

NAc MSNs target NAc-projecting VTA neurons. A, Triple-labeled VTA slice showing TH (green), DiI (violet), and ChR2 (red) after microinjection of DiI and ChR2 into the NAc. Scale bar, 20 ìm. B, Diagram of experimental design. Light stimulation (blue) of ChR2-expressing NAc MSN terminals (red) while recording from retrogradely labeled VTA neurons (violet). C, Ten consecutive light-evoked IPSCs from a retrogradely labeled VTA neuron. D, Distribution of action potential durations from connected (blue) and unconnected (red) NAc-projecting neurons. Inset shows average of 10 consecutive action potentials (AP; from example in C). Dashed line indicates where action potential duration was measured.

Figure 6.

NAc projections to the VTA are inhibited by μ-opioids. Bath application of the μ-opioid receptor agonist DAMGO (1 μm) inhibited light-evoked IPSCs arising from the NAc (n = 9). Top traces show average of five consecutive IPSCs from control (left) and in DAMGO (right).

Figure 7.

Circuit diagram of NAc connections within the VTA MSNs of the NAc synapse onto non-dopaminergic neurons in the VTA, including some that project back to the NAc. NAc MSNs do not synapse onto VTA dopamine neurons but may indirectly modulate their activity through local connections within the VTA.