Yawning produced by dopamine agonists in rhesus monkeys

The scalar property during isochronous tapping is disrupted by a D2-like agonist in the nonhuman primate

Dopamine, and specifically the D2 system, has been implicated in timing tasks where the absolute duration of individual time intervals is encoded discretely, yet the role of D2 during beat perception and entrainment remains largely unknown. In this type of timing, a beat is perceived as the pulse that marks equally spaced points in time and, once extracted, produces the tendency in humans to entrain or synchronize their movements to it. Hence, beat-based timing is crucial for musical execution. In this study we investigated the effects of systemic injections of quinpirole (0.005–0.05 mg/kg), a D2-like agonist, on the isochronous rhythmic tapping of rhesus monkeys, a classical task for the study of beat entrainment. We compared the rhythmic timing accuracy, precision, and the asynchronies of the monkeys with or without the effects of quinpirole, as well as their reaction times in a control serial reaction time task (SRTT). The results showed a dose-dependent disruption in the scalar property of rhythmic timing due to quinpirole administration. Specifically, we found similar temporal variabilities as a function of the metronome tempo at the largest dose, instead of the increase in variability across durations that is characteristic of the timing Weber law. Notably, these effects were not due to alterations in the basic sensorimotor mechanism for tapping to a sequence of flashing stimuli, because quinpirole did not change the reaction time of the monkeys during SRTT. These findings support the notion of a key role of the D2 system in the rhythmic timing mechanism, especially in the control of temporal precision.

NEW & NOTEWORTHY Perceiving and moving to the beat of music is a fundamental trait of musical cognition. We measured the effect of quinpirole, a D2-like agonist, on the precision and accuracy of rhythmic tapping to a metronome in two rhesus monkeys. Quinpirole produced a flattening of the temporal variability as a function of tempo duration, instead of the increase in variability across durations that is characteristic of the scalar property, a hallmark property of timing.

Yawning elicited by intravenous ethanol in rhesus monkeys with experience self-administering cocaine and ethanol: Involvement of dopamine D3 receptors

Characterization of the effects of long-term alcohol consumption on the brain would be aided by the development of behavioral assays that are relatively easy to implement in animal models of alcohol use disorders. Assessing unconditioned behaviors, such as drug-elicited yawning in models that permit long-term alcohol ingestion, may be a valuable complement to more invasive and costly procedures. The present studies investigated previous unexpected findings of ethanol-induced yawning in nonhuman primates. Subjects were adult male rhesus monkeys (n = 8), all of which had experience self-administering intravenous cocaine for several years. Four monkeys also had experience consuming 2.0 g/kg ethanol over 1 h per day, 5 days per week, for 6.8-12.0 months. All monkeys received saline or ethanol (0.25-1.0 g/kg) infused intravenously (i.v.) over 10 min, and the number of yawns elicited during the infusion was counted. A second experiment in the ethanol-experienced monkeys examined whether ethanol-induced yawning could be blocked by PG01037 (1.0, 3.0 mg/kg, i.v.), a selective antagonist at dopamine D3 receptors (D3R). Ethanol significantly and dose-dependently increased yawns in the ethanol-experienced animals, but not the ethanol-naïve animals. In the ethanol-experienced monkeys, this effect of ethanol was blocked by the D3R antagonist. The pharmacology of yawning is complex and a good deal of model development remains to be performed to characterize the potential involvement of other neurotransmitter systems. Nonetheless, drug-elicited yawning may be a useful unconditioned behavioral assay to assess the effects of long-term alcohol consumption in established nonhuman primate models.

Differential effects of the dopamine D3 receptor antagonist PG01037 on cocaine and methamphetamine self-administration in rhesus monkeys

The dopamine D3 receptor (D3R) has been shown to mediate many of the behavioral effects of psychostimulants associated with high abuse potential. This study extended the assessment of the highly selective D3R antagonist PG01037 on cocaine and methamphetamine (MA) self-administration to include a food-drug choice procedure. Eight male rhesus monkeys (n=4/group) served as subjects in which complete cocaine and MA dose-response curves were determined daily in each session. When choice was stable, monkeys received acute and five-day treatment of PG01037 (1.0-5.6 mg/kg, i.v.). Acute administration of PG01037 was effective in reallocating choice from cocaine to food and decreasing cocaine intake, however, tolerance developed by day 5 of treatment. Up to doses that disrupted responding, MA choice and intake were not affected by PG01037 treatment. PG01037 decreased total reinforcers earned per session and the behavioral potency was significantly greater on MA-food choice compared to cocaine-food choice. Furthermore, the acute efficacy of PG01037 was correlated with the sensitivity of the D3/D2R agonist quinpirole to elicit yawning. These data suggest (1) that efficacy of D3R compounds in decreasing drug choice is greater in subjects with lower D3R, perhaps suggesting that it is percent occupancy that is the critical variable in determining efficacy and (2) differences in D3R activity in chronic cocaine vs. MA users. Although tolerance developed to the effects of PG01037 treatment on cocaine choice, tolerance did not develop to the disruptive effects on food-maintained responding. These findings suggest that combination treatments that decrease cocaine-induced elevations in DA may enhance the efficacy of D3R antagonists on cocaine self-administration.

Further characterization of quinpirole-elicited yawning as a model of dopamine D3 receptor activation in male and female monkeys

The dopamine (DA) D3 receptor (D3R) has been associated with impulsivity, pathologic gambling, and drug addiction, making it a potential target for pharmacotherapy development. Positron emission tomography studies using the D3R-preferring radioligand [(11)C]PHNO ([(11)C](+)-propyl-hexahydro-naphtho-oxazin) have shown higher binding potentials in drug abusers compared with control subjects. Preclinical studies have examined D3R receptor activation using the DA agonist quinpirole and the unconditioned behavior of yawning. However, the relationship between quinpirole-elicited yawning and D3R receptor availability has not been determined. In Experiment 1, eight drug-naive male rhesus monkeys were scanned with [(11)C]PHNO, and the ability of quinpirole (0.01-0.3 mg/kg i.m.) to elicit yawning was examined. Significant positive (globus pallidus) and negative (caudate nucleus, putamen, ventral pallidum, and hippocampus) relationships between D3R receptor availability and quinpirole-induced yawns were noted. Experiment 2 replicated earlier findings that a history of cocaine self-administration (n = 11) did not affect quinpirole-induced yawning and extended this to examine monkeys (n = 3) with a history of methamphetamine (MA) self-administration and found that monkeys with experience self-administering MA showed greater potency and significantly higher quinpirole-elicited yawning compared with controls. Finally, quinpirole-elicited yawning was studied in drug-naive female monkeys (n = 6) and compared with drug-naive male monkeys (n = 8). Sex differences were noted, with quinpirole being more potent and eliciting significantly more yawns in males compared with females. Taken together these findings support the use of quinpirole-elicited yawning as a behavioral tool for examining D3R activation in monkeys and that both drug history and sex may influence individual sensitivity to the behavioral effects of D3R compounds.

Influence of cocaine history on the behavioral effects of Dopamine D(3) receptor-selective compounds in monkeys

Although dopamine D(3) receptors have been associated with cocaine abuse, little is known about the consequences of chronic cocaine on functional activity of D(3) receptor-preferring compounds. This study examined the behavioral effects of D(3) receptor-selective 4-phenylpiperazines with differing in vitro functional profiles in adult male rhesus monkeys with a history of cocaine self-administration and controls. In vitro assays found that PG 619 (N-(3-hydroxy-4-(4-(2-methoxyphenyl)piperazin-1-yl)butyl)-4-(pyridin-2-yl)benzamide HCl) was a potent D(3) antagonist in the mitogenesis assay, but a fully efficacious agonist in the adenylyl cyclase assay, NGB 2904 (N-(4-(4-(2,3-dichlorophenyl)piperazin-1-yl)butyl)-9H-fluorene-2-carboxamide HCl) was a selective D(3) antagonist, whereas CJB 090 (N-(4-(4-(2,3-dichlorophenyl)piperazin-1-yl)butyl)-4-(pyridin-2-yl)benzamide HCl) exhibited a partial agonist profile in both in vitro assays. In behavioral studies, the D(3) preferential agonist quinpirole (0.03-1.0 mg/kg, i.v.) dose-dependently elicited yawns in both groups of monkeys. PG 619 and CJB 090 elicited yawns only in monkeys with an extensive history of cocaine, whereas NGB 2904 did not elicit yawns, but did antagonize quinpirole and PG 619-elicited yawning in cocaine-history monkeys. In another experiment, doses of PG 619 that elicited yawns did not alter response rates in monkeys self-administering cocaine (0.03-0.3 mg/kg per injection). Following saline extinction, cocaine (0.1 mg/kg) and quinpirole (0.1 mg/kg), but not PG 619 (0.1 mg/kg), reinstated cocaine-seeking behavior. When given before a cocaine prime, PG 619 decreased cocaine-elicited reinstatement. These findings suggest that (1) an incongruence between in vitro and in vivo assays, and (2) a history of cocaine self-administration can affect in vivo efficacy of D(3) receptor-preferring compounds PG 619 and CJB 090, which appear to be dependent on the behavioral assay.

Characterization of the dopamine receptor system in adult rhesus monkeys exposed to cocaine throughout gestation

RATIONALE

Cocaine use during pregnancy is associated with alterations in the dopamine (DA) system in the fetal brain. However, little is known about the effects of prenatal cocaine exposure on the postnatal dopaminergic system.

OBJECTIVES

The objective of the study was to examine DA receptor function in adult monkeys that were prenatally exposed to cocaine.

MATERIALS AND METHODS

Male and female rhesus monkeys (approximately 13 years old) that had been prenatally exposed to cocaine (n = 10) and controls (n = 10) were used in all studies. First, DA D2-like receptor availability was assessed using positron emission tomography and the D2-like receptor radiotracer [(18)F]fluoroclebopride (FCP). Next, D(3) receptor function was assessed by measuring quinpirole-induced yawning (0.03-0.3 mg/kg). Finally, D1-like receptor function was examined by measuring eye blinking elicited by the high-efficacy D1-like receptor agonist SKF81297 (0.3-3.0 mg/kg).

RESULTS

There were no differences between groups or sexes in D2-like receptor availability in the caudate nucleus, putamen or amygdala. However, quinpirole elicited significantly more yawns in prenatally cocaine-exposed monkeys compared with control monkeys. A significant correlation between gestational dose of cocaine and peak effects of quinpirole was observed. In all monkeys, administration of SKF81297 elicited dose-dependent increases in eye blinks that did not differ between groups.

CONCLUSIONS

These findings suggest that prenatal cocaine exposure can have long-term effects on DA D(3) receptor function in adults.

Effects of Two Novel D3-Selective Compounds, NGB 2904 [N-(4-(4-(2,3-Dichlorophenyl)piperazin-1-yl)butyl)-9H-fluorene-2-carboxamide] and CJB 090 [N-(4-(4-(2,3-Dichlorophenyl)piperazin-1-yl)butyl)-4-(pyridin-2-yl)benzamide], on the Reinforcing and Discriminative Stimulus Effects of Cocaine in Rhesus Monkeys

The present study examined the effects of two novel dopamine D3 receptor compounds, NGB 2904 [N-(4-(4-(2,3-dichlorophenyl)piperazin-1-yl)butyl)-9H-fluorene-2-carboxamide], an antagonist, and CJB 090 [N-(4-(4-(2,3-dichlorophenyl)piperazin-1-yl)butyl)-4-(pyridin-2-yl)benzamide], a partial agonist, in two models of cocaine abuse in rhesus monkeys. To establish a dose range and time course of effects, both compounds were shown to block quinpirole-induced yawning when administered i.m. 15, 30, or 120 min before quinpirole. Next, rhesus monkeys were trained to discriminate i.m. injections of saline (0.5 ml) and cocaine (0.3 mg/kg). Neither D3 compound (0.03-3.0 mg/kg; n=3) substituted for cocaine in any monkey. When given in combination with cocaine, CJB 090 but not NGB 2904 attenuated the discriminative stimulus effects of cocaine, shifting the cocaine dose-response curve to the right. In a separate group of monkeys, responding was maintained under a second-order schedule of either food (1.0-g pellets; n=3) or cocaine (0.1 mg/kg/injection; n=4) presentation. When responding was stable, a dose of NGB 2904 (1.0-5.6 mg/kg i.v.) or CJB 090 (0.3-3.0 mg/kg i.v.) was administered for 5 consecutive days, immediately before the session. CJB 090, but not NGB 2904, decreased cocaine- and food-maintained responding. These data indicate that compounds with relatively high affinity and selectivity for the D3 receptor can attenuate the discriminative and reinforcing stimulus effects of cocaine while not producing cocaine-like effects. The present findings support the continued examination of D3 compounds as pharmacological tools for better understanding the role of this receptor subtype in cocaine addiction and as potential lead compounds for novel therapeutic agents.

Enhanced binding of dopamine D1receptors in caudate-putamen subregions in High-Yawning Sprague-Dawley rats

Previous reports have shown that the inbred High-Yawning (HY) and Low-Yawning (LY) rats differ in several behavioral characteristics related to mesolimbic and nigrostriatal dopamine (DA) function. To determine if differential expression of DA receptors or DA transporter may mediate the behavioral differences in these two sublines of the Sprague-Dawley rat, we performed a quantitative autoradiography study of the DA D1-like, D2-like, and DA-transporter binding in the basal ganglia and nucleus accumbens. The results show that levels of the D1 binding in the caudate-putamen of the HY rat were higher than in the LY animals, whereas no significant differences in the DA D2 receptors and DA transporter were noted in these sublines. These data suggest that the differences in DA receptors in D1 binding in HY rats may in part have contributed to the behavioral differences related to DA functions such as grooming and penile erection. Our findings are consistent with previous reports showing a decrease in the behavioral responses after systemic administration of DA agonist in LY compared to HY rats.

Effect of time-of-day on the yawning response to apomorphine in normal subjects

The yawning response to the dopamine (DA) receptor agonist apomorphine HCl (Apo, 7 microg/kg s.c.) and placebo (physiological saline) were examined in two groups of normal men. One group (n = 11) was investigated in the morning and the other group (n = 16) in the afternoon. The frequency of yawning was polygraphically monitored for 60 min following injection. Apo increased yawning compared with placebo when given in the morning (p < 0.02), but not when given in the afternoon. Yawning frequency was increased after both Apo (p < 0.01) and placebo (p < 0.025) when given in the morning compared with responses in the afternoon. These results suggest that yawning frequency with both Apo and placebo is influenced by time of day, possibly as a result of diurnal variation in DA receptor sensitivity.