Effects of dopamine D(2)-like receptor agonists in mice trained to discriminate cocaine from saline: influence of feeding condition

Olfactory regulation by dopamine and DRD2 receptor in the nose

Despite the identification of neural circuits and circulating hormones in olfactory regulation, the peripheral targets for olfactory modulation remain relatively unexplored. Here we show that dopamine D2 receptor (DRD2) is expressed in the cilia and somata of mature olfactory sensory neurons (OSNs), while nasal dopamine (DA) is mainly released from the sympathetic nerve terminals, which innervate the mouse olfactory mucosa (OM). We further demonstrate that DA-DRD2 signaling in the nose plays important roles in regulating olfactory function using genetic and pharmacological approaches. Moreover, the local DA synthesis in mouse OM is reduced during hunger, which contributes to starvation-induced olfactory enhancement. Altogether, we demonstrate that nasal DA and DRD2 receptor can serve as the potential peripheral targets for olfactory modulation.

Olfactory behavior is important for animal survival, and olfactory dysfunction is a common feature of several diseases. Despite the identification of neural circuits and circulating hormones in olfactory regulation, the peripheral targets for olfactory modulation remain relatively unexplored. In analyzing the single-cell RNA sequencing data from mouse and human olfactory mucosa (OM), we found that the mature olfactory sensory neurons (OSNs) express high levels of dopamine D2 receptor (Drd2) rather than other dopamine receptor subtypes. The DRD2 receptor is expressed in the cilia and somata of mature OSNs, while nasal dopamine is mainly released from the sympathetic nerve terminals, which innervate the mouse OM. Intriguingly, genetic ablation of Drd2 in mature OSNs or intranasal application with DRD2 antagonist significantly increased the OSN response to odorants and enhanced the olfactory sensitivity in mice. Mechanistic studies indicated that dopamine, acting through DRD2 receptor, could inhibit odor-induced cAMP signaling of olfactory receptors. Interestingly, the local dopamine synthesis in mouse OM is down-regulated during starvation, which leads to hunger-induced olfactory enhancement. Moreover, pharmacological inhibition of local dopamine synthesis in mouse OM is sufficient to enhance olfactory abilities. Altogether, these results reveal nasal dopamine and DRD2 receptor as the potential peripheral targets for olfactory modulation.

Discriminative stimulus effects of 3,4-methylenedioxypyrovalerone (MDPV) and structurally related synthetic cathinones

The 3,4-methylenedioxypyrovalerone (MDPV), and other structurally related synthetic cathinones, are popular alternatives to prototypical illicit psychostimulants, such as cocaine and methamphetamine. These drugs are often referred to as 'bath salts' and function either as cocaine-like inhibitors of monoamine uptake, or amphetamine-like substrates for dopamine, norepinephrine and serotonin transporters. These studies used male Sprague-Dawley rats trained to discriminate MDPV from saline to evaluate the substitution profiles of structurally related synthetic cathinones, cocaine, and other direct-acting dopamine and noradrenergic receptor agonists in order to characterize the relative contributions of dopamine, norepinephrine and serotonin to the discriminative stimulus effects of MDPV. As expected, each of the cathinones and cocaine dose-dependently increased MDPV-appropriate responding, with a rank-order potency that was positively correlated with their potency to inhibit dopamine and norepinephrine, but not serotonin, a relationship that is consistent with the rank order to maintain self-administration. The dopamine D2/3 receptor-preferring agonist quinpirole produced a modest increase in MDPV-appropriate responding, whereas the dopamine D1/5 receptor agonist, SKF 82958, nonselective dopamine receptor agonist, apomorphine, as well as the α-1, and α-2 adrenergic receptor agonists, phenylephrine and clonidine, respectively, failed to increase MDPV-appropriate responding at doses smaller than those that suppressed responding altogether. Although these studies do not support a role for serotonergic or adrenergic systems in mediating/modulating the discriminative stimulus effects of MDPV, convergent evidence is provided to suggest that the discriminative stimulus effects of MDPV are primarily mediated by its capacity to inhibit dopamine uptake, and the subsequent activation of dopamine D2 or D3 receptors.

Hypothalamic control of interoceptive hunger

While energy balance is critical to survival, many factors influence food intake beyond caloric need or "hunger." Despite this, some neurons that drive feeding in mice are routinely referred to as "hunger neurons," whereas others are not. To understand how specific hypothalamic circuits control interoceptive hunger, we trained mice to discriminate fasted from sated periods. We then manipulated three hypothalamic neuronal populations with well-known effects on feeding while mice performed this task. While activation of ARC^AGRP neurons in sated mice caused mice to report being food-restricted, LH^VGAT neuron activation or LH^VGLUT2 neuron inhibition did not. In contrast, LH^VGAT neuron inhibition or LH^VGLUT2 neuron activation in fasted mice attenuated natural hunger, whereas ARC^AGRP neuron inhibition did not. Each neuronal population evoked distinct effects on food consumption and reward. After satiety- or sickness-induced devaluation, ARC^AGRP neurons drove calorie-specific feeding, while LH^VGAT neurons drove calorie-indiscriminate food intake. Our data support a role for ARC^AGRP neurons in homeostatic feeding and implicate them in driving a hunger-like internal state that directs behavior toward caloric food sources. Moreover, manipulations of LH circuits did not evoke hunger-like effects in sated mice, suggesting that they may govern feeding more related to reward, compulsion, or generalized consumption than to energy balance, but also that these LH circuits can be powerful negative appetite modulators in fasted mice. This study highlights the complexity of hypothalamic feeding regulation and can be used as a framework to characterize how other neuronal circuits affect hunger and identify potential therapeutic targets for eating disorders.

Discriminative stimulus properties of the atypical antipsychotic amisulpride: comparison to its isomers and to other benzamide derivatives, antipsychotic, antidepressant, and antianxiety drugs in C57BL/6 mice

RATIONALE

Racemic (RS)-amisulpride (Solian^®) is an atypical antipsychotic drug used to treat schizophrenia and dysthymia. Blockade of dopamine D₂/D₃ and/or serotonin 5-HT₇ receptors is implicated in its pharmacological effects. While the (S)-amisulpride isomer possesses a robust discriminative cue, discriminative stimulus properties of (RS)-amisulpride have not been evaluated.

OBJECTIVES

The present study established (RS)-amisulpride as a discriminative stimulus and assessed amisulpride-like effects of amisulpride stereoisomers, other benzamide derivatives, and antipsychotic, antidepressant, and anxiolytic drugs.

METHODS

Adult, male C57BL/6 mice were trained to discriminate 10 mg/kg (RS)-amisulpride from vehicle in a two-lever food-reinforced operant conditioning task.

RESULTS

(RS)-Amisulpride's discriminative stimulus was dose-related, time-dependent, and stereoselective. (S)-Amisulpride (an effective dose of 50% (ED₅₀) = 0.21 mg/kg) was three times more potent than (RS)-amisulpride (ED₅₀ = 0.60 mg/kg) or (R)-amisulpride (ED₅₀ = 0.68 mg/kg). (RS)-Amisulpride generalized fully to the structurally related atypical antipsychotic/antidysthymia drug sulpiride (Sulpor^®; ED₅₀ = 7.29 mg/kg) and its (S)-enantiomer (ED₅₀ = 9.12 mg/kg); moderate to high partial generalization [60-75% drug lever responding (%DLR)] occurred to the benzamide analogs tiapride (Tiapridal^®) and raclopride, but less than 60% DLR to metoclopramide (Reglan^®), nemonapride (Emilace^®), and zacopride. Antipsychotic, antidepressant, and antianxiety drugs from other chemical classes (chlorpromazine, quetiapine, risperidone, and mianserin) produced 35-55% amisulpride lever responding. Lastly, less than 35% DLR occurred for clozapine, olanzapine, aripiprazole imipramine, chlordiazepoxide, and bupropion.

CONCLUSIONS

(RS)-Amisulpride generalized to some, but not all benzamide derivatives, and it failed to generalize to any other antipsychotic, antidepressant, or antianxiety drugs tested. Interestingly, the (R)-isomer shared very strong stimulus properties with (RS)-amisulpride. This finding was in contrast to findings from Donahue et al. (Eur J Pharmacol 734:15-22, 2014), which found that the (R)-isomer did not share very strong stimulus properties when the (S)-isomer was the training drug.

High ambient temperature facilitates the acquisition of 3,4-methylenedioxymethamphetamine (MDMA) self-administration

RATIONALE

MDMA alters body temperature in rats with a direction that depends on the ambient temperature (T_A). The thermoregulatory effects of MDMA and T_A may affect intravenous self-administration (IVSA) of MDMA but limited prior reports conflict.

OBJECTIVE

To determine how body temperature responses under high and low T_A influence MDMA IVSA.

METHODS

Male Sprague-Dawley rats were trained to IVSA MDMA (1.0mg/kg/infusion; 2-h sessions; FR5 schedule of reinforcement) under T_A 20°C or 30°C. Radiotelemetry transmitters recorded body temperature and activity during IVSA.

RESULTS

MDMA intake increased under both T_A during acquisition, but to a greater extent in the 30°C group. The magnitude of hypothermia was initially equivalent between groups but diminished over training in the 30°C group. Within-session activity was initially lower in the 30°C group, but by the end of acquisition and maintenance, activity was similar for both groups. When T_A conditions were swapped, the hot-trained group increased MDMA IVSA under 20°C T_A and a modest decrease in drug intake was observed in the cold-trained group under 30°C T_A. Subsequent non-contingent MDMA (1.0-5.0mg/kg, i.v.) found that rats with higher MDMA IVSA rates showed blunted hypothermia compared with rats with lower IVSA levels; however, within-session activity did not differ by group. High T_A increased intracranial self-stimulation thresholds in a different group of rats and MDMA reduced thresholds below baseline at low, but not high, T_A.

CONCLUSIONS

High T_A appears to enhance acquisition of MDMA IVSA through an aversive effect and not via thermoregulatory motivation.

Effects of Acute and Chronic Treatments with Dopamine D2 and D3 Receptor Ligands on Cocaine versus Food Choice in Rats

Dopamine D₃ receptor ligands are potential medications for psychostimulant addiction. Medication assessment may benefit from preclinical studies that evaluate chronic medication effects on choice between an abused drug and an alternative, nondrug reinforcer. This study compared acute and chronic effects of dopamine D₂- and D₃-preferring ligands on choice between intravenous cocaine and palatable food in rats. Under baseline conditions, cocaine maintained dose-dependent increases in cocaine choice and reciprocal decreases in food choice. Acutely, the D₂ agonist R-(-)-norpropylapomorphine (NPA) and antagonist L-741,626 [3-[[4-(4-chlorophenyl)-4-hydroxypiperidin-l-yl]methyl-1H-indole] produced leftward and rightward shifts in cocaine dose-effect curves, respectively, whereas the partial agonist terguride had no effect. All three drugs dose-dependently decreased food-maintained responding. Chronically, the effects of R-(-)-norpropylapomorphine and L-741,626 on cocaine self-administration showed marked tolerance, whereas suppression of food-reinforced behavior persisted. Acute effects of the D₃ ligands were less systematic and most consistent with nonselective decreases in cocaine- and food-maintained responding. Chronically, the D₃ agonist PF-592,379 [5-[(2R,5S)-5-methyl-4-propylmorpholin-2-yl]pyridin-2-amine] increased cocaine choice, whereas an intermediate dose of the D₃ antagonist PG01037 [N-[(E)-4-[4-(2,3-dichlorophenyl)piperazin-1-yl]but-2-enyl]-4-pyridin-2-ylbenzamide] produced a therapeutically desirable decrease in cocaine choice early in treatment; however, tolerance to this effect developed, and lower and higher doses were ineffective. D₃ ligands failed to significantly modify total cocaine intake but caused persistent decreases in food intake. Thus, D₂-and D₃-preferring ligands showed distinct profiles, consistent with different pharmacological actions. In addition, these results highlight the role of acute versus chronic treatment as a determinant of test drug effects. With the possible exception of the D₃ antagonist PG01037, no ligand was promising in terms of cocaine addiction treatment.

Novel Bivalent Ligands Based on the Sumanirole Pharmacophore Reveal Dopamine D2 Receptor (D2R) Biased Agonism

The development of bivalent ligands has attracted interest as a way to potentially improve the selectivity and/or affinity for a specific receptor subtype. The ability to bind two distinct receptor binding sites simultaneously can allow the selective activation of specific G-protein dependent or β-arrestin-mediated cascade pathways. Herein, we developed an extended SAR study using sumanirole (1) as the primary pharmacophore. We found that substitutions in the N-1- and/or N-5-positions, physiochemical properties of those substituents, and secondary aromatic pharmacophores can enhance agonist efficacy for the cAMP inhibition mediated by G_i/o-proteins, while reducing or suppressing potency and efficacy toward β-arrestin recruitment. Compound 19 was identified as a new lead for its selective D₂ G-protein biased agonism with an EC₅₀ in the subnanomolar range. Structure-activity correlations were observed between substitutions in positions N-1 and/or N-5 of 1 and the capacity of the new bivalent compounds to selectively activate G-proteins versus β-arrestin recruitment in D₂R-BRET functional assays.

Opposing effects of dopamine D1- and D2-like agonists on intracranial self-stimulation in male rats

Dopamine acts through dopamine Type I receptors (comprising D1 and D5 subtypes) and dopamine Type II receptors (comprising D2, D3, and D4 subtypes). Intracranial self-stimulation (ICSS) is 1 experimental procedure that can be used to evaluate abuse-related effects of drugs targeting dopamine receptors. This study evaluated effects of dopamine receptor ligands on ICSS in rats using experimental procedures that have been used previously to examine abused indirect dopamine agonists such as cocaine and amphetamine. Male Sprague-Dawley rats responded under a fixed-ratio 1 schedule for electrical stimulation of the medial forebrain bundle, and frequency of stimulation varied from 56-158 Hz in 0.05 log increments during each experimental session. Drug potency and time course were determined for the D1 ligands A77636, SKF82958, SKF38393, fenoldopam, and SCH39166 and the D2/3 ligands sumanirole, apomorphine, quinpirole, PD128907, pramipexole, aripiprazole, eticlopride, and PG01037. The high-efficacy D1 agonists A77636 and SKF82958 produced dose-dependent, time-dependent, and abuse-related facilitation of ICSS. Lower efficacy D1 ligands and all D2/3 ligands failed to facilitate ICSS at any dose or pretreatment time. A mixture of SKF82958 and quinpirole produced a mixture of effects produced by each drug alone. Quinpirole also failed to facilitate ICSS after regimens of repeated treatment with either quinpirole or cocaine. These studies provide more evidence for divergent effects of dopamine D1- and D2-family agonists on ICSS procedure in rats and suggest that ICSS may be a useful complement to other approaches for preclinical abuse potential assessment, in part because of the reproducibility of results. (PsycINFO Database Record

Novel Analogues of (R)-5-(Methylamino)-5,6-dihydro-4H-imidazo[4,5,1-ij]quinolin-2(1H)-one (Sumanirole) Provide Clues to Dopamine D2/D3 Receptor Agonist Selectivity

Novel 1-, 5-, and 8-substituted analogues of sumanirole (1), a dopamine D2/D3 receptor (D2R/D3R) agonist, were synthesized. Binding affinities at both D2R and D3R were higher when determined in competition with the agonist radioligand [(3)H]7-hydroxy-N,N-dipropyl-2-aminotetralin (7-OH-DPAT) than with the antagonist radioligand [(3)H]N-methylspiperone. Although 1 was confirmed as a D2R-preferential agonist, its selectivity in binding and functional studies was lower than previously reported. All analogues were determined to be D2R/D3R agonists in both GoBRET and mitogenesis functional assays. Loss of efficacy was detected for the N-1-substituted analogues at D3R. In contrast, the N-5-alkyl-substituted analogues, and notably the n-butyl-arylamides (22b and 22c), all showed improved affinity at D2R over 1 with neither a loss of efficacy nor an increase in selectivity. Computational modeling provided a structural basis for the D2R selectivity of 1, illustrating how subtle differences in the highly homologous orthosteric binding site (OBS) differentially affect D2R/D3R affinity and functional efficacy.