Presynaptic opioid and nicotinic receptor modulation of dopamine overflow in the nucleus accumbens

Abstract

Behaviorally relevant stimuli prompt midbrain dopamine (DA) neurons to switch from tonic to burst firing patterns. Similar shifts to burst activity are thought to contribute to the addictive effects of opiates and nicotine. The nucleus accumbens DA overflow produced by these drugs is a key element in their pathological effects. Using electrochemical techniques in brain slices, we explored the effects of opioids on single-spike and burst stimuli-evoked DA overflow in the dorsal and ventral striatum. In specific subregions of the nucleus accumbens, mu-opioids inhibit DA overflow elicited with single-spike stimuli while leaving that produced by burst stimuli unaffected. This is similar to published effects of nicotinic receptor blockade or desensitization, and is mediated by opioid receptor-induced inhibition of cholinergic interneurons. Whereas delta-opioids have similar effects, kappa-opioids inhibit evoked DA overflow throughout the striatum in a manner that is not overcome with high-frequency stimuli. These observations reveal remarkable mechanistic overlap between the effects of nicotine and opiates within the dopamine reward pathway.

Figure 1.

μ- and δ-opioid receptor agonists inhibit single-spike-evoked DA overflow. a, Single-spike-evoked DA responses constructed using fast-scan cyclic voltammetry before and after successive 10 min bath applications of the μ-opioid EM-1 (1 μm) and the μ-opioid receptor antagonist naloxonazine (1 μm). The insets are subtracted voltammograms obtained at the peak of evoked DA responses in each condition. b, Summary of the effect of 1 μm EM-1 on evoked DA responses in the dorsal medial NAc shell (n = 24). Each data point reflects a single stimulus pulse, unless noted by 2P, 4P, or 6P, indicating two, four, or six stimulus pulses, respectively, delivered at 25 Hz (in this and subsequent figures). Average DA concentration elicited by single spikes during control conditions was normalized to 1 for each recording. EM-1 significantly inhibited single-spike-evoked DA overflow (t = 17.6, p < 0.001, n = 24) and naloxonazine induced significant recovery (t = −12.1, p < 0.001, n = 24). Baseline single-spike dopamine levels during control averaged 0.59 ± 0.10 μm. c, The difference between DA release from burst and single stimuli were determined by subtracting the peak DA overflow responses to single spikes from that measured after two, four, or six stimuli at 25 Hz. Statistical comparison by repeated-measures ANOVA revealed significant effects of pulse number (F_(2,69) = 25.6, p < 0.0001), EM-1 (F_(1,69) = 31.1, p < 0.0001), and an interaction (F_(2,69) = 4.4, p = 0.0157). d, Summary of the effect of 1 μm deltorphin II, a δ-opioid receptor agonist, followed by 1 μm BNTX, an δ₁-opioid receptor antagonist, on evoked DA responses in the NAc shell (n = 6). The 25 Hz burst stimuli of two, four, or six stimulus pulses are indicated. Deltorphin II induced significant inhibition of single-spike-evoked DA overflow (t = 8.9, p < 0.001, n = 6) and BNTX induced significant recovery (t = −2.5, p < 0.05, n = 6).

Figure 2.

μ-Opioid receptor effects on single and burst stimulation induced DA overflow, as well as short-term presynaptic facilitation of DA overflow. a, Effect of the number of 25 Hz stimulus pulses on peak DA overflow under control conditions and after treatment with EM-1, deltorphin II, or nicotine (n = 24, 24, 6, 8, respectively). b, DA responses to single and 25 Hz paired pulses demonstrate EM-1-induced transformation of paired-pulse depression to paired-pulse facilitation. P₂ is calculated by subtracting P₁ from P₁ + P₂. c, Paired-pulse ratios in control conditions and in the presence of 1 μm EM-1, deltorphin II, or nicotine (n = 24, 6, 8, respectively). A one-way ANOVA followed by Fisher's PLSD post hoc test found the control condition was significantly different from each drug condition (F_(3,57) = 6.7, *p < 0.02).

Figure 3.

μ- and δ-opioid responses are not uniform throughout the striatum. a, The prevalence of μ- and δ-opioid effects on DA responses in the NAc shell, NAc core, and the dorsal striatum. The highest prevalence of the μ-opioid effect was seen in a subregion of the NAc shell, identified here as the “hotspot,” which is located in the dorsal half of the NAc shell between 2 and 3 mm rostral to bregma. b, In the NAc core and dorsal striatum, EM-1 is ineffective in modulating evoked DA overflow (n = 19 and 12, respectively). Note that in these areas, burst stimuli only slightly augment the DA overflow over that produced with single pulses. Baseline single-spike dopamine levels during control averaged 1.57 ± 0.28 μm in the NAc Core, and 2.50 ± 0.24 μm in the dorsal striatum. c, Concentration–effect relationship for the effects of EM-1 on single-spike-evoked DA overflow from recordings in the NAc shell hotspot region (n = 4–15). d, Schematic of the NAc hot spot, where EM-1 most consistently affected DA overflow. The red portion of each diagram denotes the hot spot region. D, Dorsal; R, rostral; C, caudal. Adapted from Paxinos and Watson (1998).

Figure 4.

μ-Opioids modulate DA responses by inhibiting cholinergic interneuron activity. a, A saturating concentration of EM-1 (1 μm) occludes further modulation of DA overflow by nicotine. Peak DA responses elicited by single pulses or, where labeled, 25 Hz burst stimuli showing that nicotine (1 μm) is not effective in modulating evoked DA overflow when applied subsequent to EM-1. b, On-cell electrophysiological recordings revealed that EM-1 (1 μm) suppresses the firing rate of cholinergic interneurons in the NAc shell hotspot region (n = 6). Frequency data from each cell was normalized to baseline for averaging (average baseline firing rate, 3.9 ± 1.7 Hz). c, An example whole-cell recording from a cholinergic interneuron in the NAc shell hotspot region before and after a 2 min bath application of 1 μm EM-1. d, Whole-cell voltage-clamp recordings from cholinergic interneurons in the NAc shell hotpot region reveal that EM-1 produces an outward current. Top, Example trace from a single cell. Bottom, Summary data (n = 5). e, Voltage ramps from −120 to −60 mV were applied during current response to EM-1 in cholinergic interneurons from the NAc shell hotspot. Currents were averaged and the difference current was obtained by subtracting control from EM-1 to produce the current–voltage relationship for each cell. The largest inward current was normalized to −1 and the traces were averaged across all cells. The reversal potential (−104 ± 10 mV) was similar to calculated Nernst potential for potassium (−107 mV; n = 5).

Figure 5.

κ-Opioid receptor activation uniformly inhibits evoked DA overflow. a, Single-spike-evoked DA responses constructed using fast-scan cyclic voltammetry before and after successive 10 min bath applications of a κ-opioid receptor agonist, BRL52537 (1 μm), and antagonist nBI (1 μm). The insets are subtracted voltammograms obtained at the peak of evoked DA responses in each condition. b, Summary of the effect of 1 μm BRL52537 on evoked DA responses in recordings throughout the NAc (n = 10). Each data point reflects a single stimulus pulse, unless noted by 2P, 4P, or 6P, indicating two, four, or six stimulus pulses, respectively, delivered at 25 Hz. BRL52537 inhibited single-spike-evoked DA overflow (t = 14.2, p < 0.001, n = 10) and nBI induced recovery (t = −8.2, p < 0.001, n = 10). Baseline single-spike dopamine levels during control averaged 1.17 ± 0.17 μm. c, Effect of number of 25 Hz stimulus pulses on peak DA responses under control and BRL52537-treated conditions. To compare the relative effects of multiple stimuli on DA overflow, single-spike-evoked DA levels in the presence of BRL52537 were normalized to 1.

Figure 6.

Effects of κ-opioids on DA overflow are independent of nAChR function and DA reuptake. a, A saturating concentration of nicotine (1 μm) does not occlude further modulation of DA overflow by κ-opioid receptor activation. Peak DA responses elicited by single pulses or, where labeled, 100 Hz burst stimuli from amperometric recordings show that BRL52537 (1 μm) modulates evoked DA overflow despite a preceding application of nicotine (n = 5). Amperometric recordings have faster kinetics and the stimulus artifact leads to underestimation of the peak magnitude during burst stimuli. b, Representative amperometric traces obtained with single pulse stimuli in the presence and absence of BRL52537 (1 μm) and the DAT inhibitor nomifensine (1 μm) (top). To assess drug-induced changes in decay rates, the portion of the currents with similar magnitudes were overlayed (bottom). c, Summary of the effects of BRL52537 (n = 8) and nomifensine (n = 3) on DA clearance rates, measured by fitting the current decay with a single exponential (τ). *p < 0.001.

Figure 7.

Schematic representation of opioid receptor modulation of DA release in the NAc shell. Activation of μ-opioid receptors on cholinergic interneurons suppresses ACh release and, in turn, decreases nAChR activity on DA terminals. κ-Opioid receptors are located directly on DA terminals where they suppress DA release directly, but their downstream effectors are unknown.