Gregory T. Collins

Yawning and hypothermia in rats: effects of dopamine D3 and D2 agonists and antagonists.

RationaleIdentification of behaviors specifically mediated by the dopamine D2 and D3 receptors would allow for the determination of in vivo receptor selectivity and aide the development of novel therapeutics for dopamine-related diseases.ObjectivesThese studies were aimed at evaluating the specific receptors involved in the mediation of D2/D3 agonist-induced yawning and hypothermia.Materials and methodsThe relative potencies of a series of D2-like agonists to produce yawning and hypothermia were determined. The ability of D3-selective and D2-selective antagonists to inhibit the induction of yawning and hypothermia were assessed and a series of D2/D3 antagonists were characterized with respect to their ability to alter yawning induced by a low and high dose of PD-128,907 and sumanirole-induced hypothermia.ResultsD3-preferring agonists induced yawning at lower doses than those required to induce hypothermia and the D2-preferring agonist, sumanirole, induced hypothermia at lower doses than were necessary to induce yawning. The rank order of D3 selectivity was pramipexole > PD-128,907 = 7-OH-DPAT = quinpirole = quinelorane > apomorphine = U91356A. Sumanirole had only D2 agonist effects. PG01037, SB-277011A, and U99194 were all D3-selective antagonists, whereas haloperidol and L-741,626 were D2-selective antagonists and nafadotride’s profile of action was more similar to the D2 antagonists than to the D3 antagonists.ConclusionsD3 and D2 receptors have specific roles in the mediation of yawning and hypothermia, respectively, and the analysis of these effects allow inferences to be made regarding the selectivity of D2/D3 agonists and antagonists with respect to their actions at D2 and D3 receptors.

Food Restriction Alters N′-Propyl-4,5,6,7-tetrahydrobenzothiazole-2,6-diamine dihydrochloride (Pramipexole)-Induced Yawning, Hypothermia, and Locomotor Activity in Rats: Evidence for Sensitization of Dopamine D2 Receptor-Mediated Effects

Food restriction enhances sensitivity to the reinforcing effects of a variety of drugs of abuse including opiates, nicotine, and psychostimulants. Food restriction has also been shown to alter a variety of behavioral and pharmacological responses to dopaminergic agonists, including an increased sensitivity to the locomotor stimulatory effects of direct- and indirect-dopamine agonists, elevated extracellular dopamine levels in responses to psychostimulants, as well as suppression of agonist-induced yawning. Behavioral and molecular studies suggest that augmented dopaminergic responses observed in food-restricted animals result from a sensitization of the dopamine D2 receptor; however, little is known about how food restriction affects dopamine D3 receptor function. The current studies were aimed at better defining the effects of food restriction on D2 and D3 receptor function by assessing the capacity of N′-propyl-4,5,6,7-tetrahydrobenzothiazole-2,6-diamine dihydrochloride (pramipexole) to induce yawning, penile erection (PE), hypothermia, and locomotor activity in free-fed and food-restricted rats. Food restriction resulted in a suppression of pramipexole-induced yawning, a sensitized hypothermic response, and an enhanced locomotor response to pramipexole, effects that are suggestive of an enhanced D2 receptor activity; no effect on pramipexole-induced PE was observed. Antagonist studies further supported a food restriction-induced enhancement of the D2 receptor activity because the D2 antagonist 3-[4-(4-chlorophenyl)-4-hydroxypiperidin-l-yl]methyl-1H-indole (L741,626) recovered pramipexole-induced yawning to free-fed levels, whereas yawning and PE were suppressed following pretreatment with the D3 antagonist N-{4-[4-(2,3-dichlorophenyl)-piperazin-1-yl]-trans-but-2-enyl}-4-pyridine-2-yl-benzamide hydrochloride (PG01037). The results of the current studies suggest that food restriction sensitized rats to the D2-mediated effects of pramipexole while having no effect on the D3-mediated effects of pramipexole.

Cocaine Esterase Prevents Cocaine-Induced Toxicity and the Ongoing Intravenous Self-Administration of Cocaine in Rats

Cocaine esterase (CocE) is a naturally occurring bacterial enzyme, is a very efficient protein catalyst for the hydrolysis of cocaine, and has previously been shown to protect rodents from the lethal effects of cocaine. The current studies were aimed at evaluating the capacity of a longer acting mutant form (CocE T172R/G173Q; DM CocE) of CocE to protect against the lethal effects of cocaine, and alter ongoing intravenous cocaine self-administration in rats. A dose-response analysis revealed a dose-dependent suppression of cocaine-reinforced responding with 1.0 mg of CocE T172R/G173Q producing saline-like rates of responding. The effects of 1.0 mg of CocE T172R/G173Q on cocaine-reinforced responding were then compared with responding when saline was available for injection, whereas the selectivity of CocE T172R/G173Qs effects was assessed by evaluating the effects of 1.0 mg of CocE T172R/G173Q on (−)-2β-carbomethoxy-3β-phenyltropane (WIN-35065-2)- and food-reinforced responding. Although 1.0 mg of CocE T172R/G173Q suppressed responding maintained by 0.1 mg/kg/injection cocaine, a significant increase in responding was observed when responding was maintained by 1.0 mg/kg/injection cocaine, resulting in a 10-fold rightward shift in the dose-response curve for cocaine self-administration at a dose that did not significantly alter responding maintained by either WIN-35065-2 or food. These findings demonstrate that a long-acting form of CocE is effective at abruptly reducing the ongoing self-administration of low doses of cocaine, and provides a robust antagonism of cocaines reinforcing effects. Furthermore, these studies provide strong evidence for the potential usefulness of a suitable, stable, and long-acting form of CocE as a pharmacotherapy for cocaine abuse in humans.

Drug and Reinforcement History as Determinants of the Response-Maintaining Effects of Quinpirole in the Rat

The present study examined the effect of drug and reinforcement history on quinpirole-maintained responding in rats. Quinpirole (0.01, 0.032, or 0.1 mg/kg per injection) was assessed as a reinforcer in experimentally naive rats, as well as in rats trained to self-administer cocaine, remifentanil, ketamine, or food under a fixed ratio 1 schedule of reinforcement. Quinpirole failed to maintain responding in experimentally naive rats, or in ketamine- or food-trained rats. However, robust responding was maintained in rats with a history of cocaine reinforcement, and modest levels of responding were observed in rats with a history of responding for remifentanil. In a second set of studies, the effects of protracted drug histories on quinpirole-maintained responding in food-trained rats were assessed. Rats were maintained with food reinforcement, and different groups of rats were then allowed to respond for saline, quinpirole, and response-contingent cocaine or were administered noncontingent cocaine; all rats were subsequently allowed to respond for quinpirole. Only rats that previously responded for cocaine showed quinpirole-maintained responding; all other conditions failed to establish quinpirole-maintained responding. Although the high levels of quinpirole-maintained responding observed when quinpirole was substituted for cocaine are suggestive of positive reinforcing effects, these response-maintaining effects were highly dependent upon both drug and reinforcement history, suggesting that quinpirole may only function as a reinforcer under very specific conditions. The behavioral effects of quinpirole under these situations represent a novel constellation of actions relative to other drug reinforcers, and they suggest that the direct effects of self-administered quinpirole may be important in establishing the response-maintaining effects.

Proerectile effects of dopamine D2-like agonists are mediated by the D3 receptor in rats and mice.

Dopamine D2-like agonists induce penile erection (PE) and yawning in a variety of species, effects that have been suggested recently to be specifically mediated by the D4 and D3 receptors, respectively. The current studies were aimed at characterizing a series of D2, D3, and D4 agonists with respect to their capacity to induce PE and yawning in the rat and the proerectile effects of apomorphine [(R)-(-)-5,6,6a,7-tetrahydro-6-methyl-4H-dibenzo-[de,g]quinoline-10,11-diol hydrochloride] in wild-type and D4 receptor (R) knockout (KO) mice. All D3 agonists induced dose-dependent increases in PE and yawning over a similar range of doses, whereas significant increases in PE or yawning were not observed with any of the D4 agonists. Likewise, D2, D3, and D4 antagonists were assessed for their capacity to alter apomorphine- and pramipexole (N′-propyl-4,5,6,7-tetrahydrobenzothiazole-2,6-diamine dihydrochloride)-induced PE and yawning. The D3 antagonist, PG01037 [N-{4-[4-(2,3-dichlorophenyl)-piperazin-1-yl]-trans-but-2-enyl}-4-pyridine-2-yl-benzamide hydrochloride], inhibited the induction of PE and yawning, whereas the D2 antagonist, L-741,626 [3-[4-(4-chlorophenyl)-4-hydroxypiperidin-l-yl]methyl-1H-indole], reversed the inhibition of PE and yawning observed at higher doses. The D4 antagonist, L-745,870 [3-(4-[4-chlorophenyl]piperazin-1-yl)-methyl-1H-pyrrolo[2,3-b]pyridine trihydrochloride], did not alter apomorphine- or pramipexole-induced PE or yawning. A role for the D3 receptor was further supported because apomorphine was equipotent at inducing PE in wild-type and D4RKO mice, effects that were inhibited by the D3 antagonist, PG01037, in both wild-type and D4R KO mice. Together, these studies provide strong support that D2-like agonist-induced PE and yawning are differentially mediated by the D3 (induction) and D2 (inhibition) receptors. These studies fail to support a role for the D4 receptor in the regulation of PE or yawning by D2-like agonists.

Neurophamacology of Yawning

Yawning is a common behavioral event that is observed in humans, as well as other mammals, birds and reptiles. In humans, yawning often occurs just before bed and upon waking up, and is also associated with tedious or boring situations. Although the physiologic roles of yawning have yet to be fully elucidated, the past 50 years of research has led to a much greater understanding of the neuropharmacologic regulation of yawning. While many of the early studies concluded that yawning was primarily driven by changes in cholinergic neurotransmission, we now know that numerous neurotransmitters and neurohormones are involved in the mediation of yawning, including acetylcholine, dopamine, glutamate, serotonin, oxytocin, GABA, opioids, adrenergics, nitric oxide, as well as the proopiomelanocortin-derived peptides ACTH and alpha-MSH. Furthermore, antagonist interaction studies have clearly defined at least 3 distinct neural pathways involved in the induction of yawning, as well as the hierarchical order through which these different neurotransmitter systems interact to regulate yawning. The following sections will discuss the state of knowledge for each of the major neurotransmitters and neurohormones involved in the regulation of yawning, their interactions with one another, and their place in the hierarchical organization of yawning.

Yawning and locomotor behavior induced by dopamine receptor agonists in mice and rats

Dopaminergic (DA) agonist-induced yawning in rats seems to be mediated by DA D3 receptors, and low doses of several DA agonists decrease locomotor activity, an effect attributed to presynaptic D2 receptors. Effects of several DA agonists on yawning and locomotor activity were examined in rats and mice. Yawning was reliably produced in rats, and by the cholinergic agonist, physostigmine, in both the species. However, DA agonists were ineffective in producing yawning in Swiss–Webster or DA D2R and DA D3R knockout or wild-type mice. The drugs significantly decreased locomotor activity in rats at one or two low doses, with activity returning to control levels at higher doses. In mice, the drugs decreased locomotion across a 1000–10 000-fold range of doses, with activity at control levels (U-91356A) or above control levels [(±)-7-hydroxy-2-dipropylaminotetralin HBr, quinpirole] at the highest doses. Low doses of agonists decreased locomotion in all mice except the DA D2R knockout mice, but were not antagonized by DA D2R or D3R antagonists (L-741 626, BP 897, or PG01037). Yawning does not provide a selective in-vivo indicator of DA D3R agonist activity in mice. Decreases in mouse locomotor activity by the DA agonists seem to be mediated by D2 DA receptors.

Design, Synthesis and Evaluation of Potent and Selective Ligands for the Dopamine 3 (D3) Receptor with a Novel in vivo Behavioral Profile

A series of compounds structurally related to pramipexole were designed, synthesized, and evaluated as ligands for the dopamine 3 (D3) receptor. Compound 12 has a K(i) value of 0.41 nM to D3 and a selectivity of >30000- and 800-fold over the D1-like and D2 receptors, respectively. Our in vivo functional assays showed that this compound is a partial agonist at the D3 receptor with no detectable activity at the D2 receptor.

Influence of conditioned reinforcement on the response-maintaining effects of quinpirole in rats.

D2-like agonists, such as quinpirole, maintain responding in monkeys, rats, and mice when they are substituted for cocaine. This study examined the influence of operant history and cocaine-paired stimuli (CS) on quinpirole-maintained responding in rats trained to nose poke for cocaine. Upon acquisition of responding for cocaine, substitutions were performed in the presence or absence of injection–CS pairings. Although cocaine maintained responding regardless of whether injections were accompanied by CS, quinpirole maintained responding only when CS were paired with injections. To assess the influence of operant history, injections of cocaine, quinpirole, remifentanil, nicotine, or saline were made available on a previously inactive lever, while nose pokes continued to result in CS presentation. Although responding was reallocated from the nose poke to the lever when cocaine or remifentanil was available, lever presses remained low, and nose poking persisted when quinpirole or nicotine was made contingent upon lever presses. Finally, quinpirole pretreatments resulted in high rates of nose poking when nose pokes resulted in CS presentation alone, but failed to maintain nose poking when the CS was omitted. Taken together, these results suggest that the response-maintaining effects of quinpirole are primarily mediated by an enhancement of the conditioned reinforcing effects of earlier CS, rather than by a reinforcing effect of quinpirole.

Effects of pramipexole on the reinforcing effectiveness of stimuli that were previously paired with cocaine reinforcement in rats

RationaleDopamine D2-like agonists maintain responding when substituted for cocaine in laboratory animals. However, these effects appear to be mediated by an interaction with stimuli that were previously paired with cocaine reinforcement (CS).ObjectivesTo evaluate the extent to which the pramipexole-maintained and pramipexole-induced responding are influenced by cocaine-paired stimuli.MethodsRats were trained to nosepoke for cocaine under fixed ratio 1 (FR1) or progressive ratio (PR) schedules of reinforcement. In FR1-trained rats, pramipexole was substituted for cocaine with injections either paired with CSs, or delivered in their absence. The capacity of experimenter-administered pramipexole to induce FR1 and PR responding for CS presentation was evaluated. The effects of altering stimulus conditions, as well as pretreatments with D2- (l-741,626) and D3-preferring (PG01037) antagonists on pramipexole-induced PR responding were also evaluated.ResultsWhen substituted for cocaine, pramipexole maintained responding at high rates when injections were paired with CSs, but low rates when CSs were omitted. Similarly, experimenter-administered pramipexole induced dose-dependent increases in FR1 or PR responding, with high rates of responding observed when the CS was presented, and low rates of responding when CS presentation was omitted. D2 and D3 antagonists differentially affected pramipexole-induced PR responding, with l-741,626 and PG01037 producing rightward, and downward shifts in the dose–response curve for CS-maintained responding, respectively.ConclusionsThese data indicate that pramipexole is capable of enhancing the reinforcing effectiveness of conditioned stimuli, and raise the possibility that similar mechanisms are responsible for the increased occurrence of impulse control disorders in patients being treated with pramipexole.