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mise à jour du
19 mai 2005
Eur J Pharmacology
1997; 323; 27-36
Locomotor inhibition, yawning and vacuous chewing induced by a novel dopamine D2 post-synaptic receptor agonist
Smith HP, Nichols DE, et al.
University North Carolina, USA


The development of novel ligands with high selectivity for dopamine receptor isoforms has been important for understanding brain dopaminergic function in normal and disease states (e.g., Parkinson's disease and schizophrenia). Dopamine receptors comprise a subset of the super-family of G-protein coupled receptors.
Currently there are two known pharmacologically similar families of dopamine receptors, usually categorized as D1 and D2. At least five genes code for unique dopamine receptors, some having splice variants. Each molecular subtype has a unique regional distribution in the brain, the functional significance of which is, at present, poorly understood. The five genes can be divided into two families that are often referred to as D1-like and D2-like. D1-like dopamine receptors (the D1A and D1B or D5) have intron-less genes, are expressed as proteins having a relatively short third intracellular loop and a relatively long carboxy tail, and show high affinity for phenyl-tetrahydrobenzazepines such as SCH23390 (k( + )-7-chioro-8-hydroxy-3-methyl- l -phenyl-2.3.4,5-tetrahydro- 1 H-3-benza-zepine) and SKF38393 (R( + )- I -phenyl-7.8-dihydroxy2,3.4.5-tetrahydro-( 1 H )-3-benzazepine). D2-like dopamine receptors (D2çhOrt and D210, splice variants, D3, and D4) are genes having multiple introns. are expressed as proteins having a long third intracellular loop and a short carboxy tail, and show high affinity for butyrophenones (e.g.. spiperone) and benzamides (e.g.,( - )-sulpiride). Currently available ligands have limited ability to discriminate between molecular isoforms within each family. Throughout this paper, our references to dopamine D1 and D2 receptors will be to each family, although if specific information is available about a particular receptor isoform. (e.g., D2 vs. D3), this will be stated explicitly.
Dopamine D2 receptors are expressed both as-autoreceptors on dopamine neurons and terminals, and as postsynaptic receptors on target cells. Systemically administered dopamine receptor agonists acting at dopamine D2 receptors are known to have a biphasic dose-response effect on unconditioned behaviors in rodents: low doses inhibit spontaneous locomotor activity, whereas high doses increase locomotor activity and elicit oral stereotypies. This biphasic dose response effect has been attributed to the ability of low agonist doses to stimulate selectively dopamine D2 autoreceptors on the dopamine neuron, thereby decreasing synaptic concentrations of dopamine via down-regulation of neural firing rate, dopamine synthesis, and dopamine release. Consistent with an autoreceptor hypothesis of decreased locomotor activity, several atypical dopamine receptor agonists displaying functional selectivity for dopamine D2 autoreceptors are observed to dose-dependently inhibit locomotor activity in rodents, even at high doses.
Yet not all reports are consistent with this autoreceptor hypothesis (e.g., Stahle). The most recent challenge to this notion offers the alternative hypothesis that a post-synaptic subpopulation of dopamine D2 receptors (possibly the D3 molecular isoform of the receptor) can mediate decreases in spontaneous locomotor activity. This is based on the evidence that purported antagonists with selectivity for the dopamine D3 (i.e.. at least in vitro) vs. the D2 receptor increase locomotor activity without affecting dopamine release or utilization.
Several years ago, we reported on dihydrexidine ((± )trans- 10,11 -dihydroxy-5,6,6 a,7,8, 12 b-hexahydrobenzo[ a]phenanthridine), the parent compound of a novel class of dopamine receptor agonists. It is now known that both the D1 and D2 affinity and functional potency reside in the (+ )-enantiomer. Although originally designed as a ligand for the dopamine D1-like receptor, it was found that dihydrexidine and several of its analogs also were high affinity ligands for dopamine D2-like receptors as well. Of particular interest was the fact that dihydrexidine and other structural analogs could functionally activate post-synaptic dopamine D2 receptors in striatum and pituitary, yet displayed little or no activity at release- or synthesis-modulating terminal dopamine autoreceptors. The unique post-synaptically selective pharmacology of hexahydrobenzo[a]phenanthridine ligands is supported further by a lack of agonist effects at impulseregulating D2 autoreceptors located on the somatodendritic membranes of dopamine neurons. These differential functional effects occur despite equivalent binding affinity for pre- and post-synaptic receptor sites.
The present work used the N-n-propyl analog of (± )dihydrexidine to assess the effects of selective activation of post-synaptic dopamine D2 receptors on unconditioned behaviors in rats. Binding studies had shown that (± )-n-propyl-dihydrexidine has 10-fold selectivity for native dopamine D2 receptors vs. D1 receptors in rat striatum, and also has high affinity for the cloned dopamine D3 receptor. The purpose of the present study was to determine whether the unique functional selectivity of (± )-N-n-propyl-dihydrexidine for post-synaptic vs. pre-synaptic dopamine D2 receptors would induce a linear dose-dependent increase in spontaneous locomotor activity and oral stereotypy as predicted by the autoreceptor hypothesis of dopamine receptor agonist effects on unconditioned behaviors.
To our surprise, we observed just the opposite effect: systemic administration of (± )-N-n-propyl-dihydrexidine in unhabituateD2 drug-naive rats induced a linear dose-dependent decrease in spontaneous locomotor activity, and did not induce oral stereotypy typically seen after administration of high doses of dopamine receptor agonists. In addition, we observed significant dose-dependent increases in yawning and vacuous chewing, behaviors usually observed after activation of dopamine D2 autoreceptors and D1 receptors, respectively. These data support the hypothesis that a sub-population of post-synaptic dopamine D2 receptors (possibly the D3 receptor) mediate suppression of spontaneous locomotor activity. The activation of these receptors also appears to be important in potentiating yawning and vacuous chewing behaviors.
dopamine D3
The present findings provide a comprehensive description of the behavioral effects of the novel dopamine receptor agonist (± )-N-n-propyl-dihydrexidine. Administration of this compound induced marked increases in inactivity and vacuous chewing and decreases in a number of other specific behavioral elements, including locomotion, rearing and repetitive sniffing. Increased yawning was seen at the highest dose of ( ± )-N-n-propyl-dihydrexidine. Oral stereotypies (licking and gnawing) were notably absent. The use of dopamine receptor subtype-selective antagonists confirmed the dopaminergic basis for the prominent behavioral effects of this novel agonist. The increased inactivity induced by (± )-N-n-propyl-dihydrexidine was mediated by dopamine D2 receptors, while both dopamine D1 and D2 receptors contributed to the observed increases in vacuous chewing. Taken together. the behavioral effects observed with (± )-N-n-propyl-dihydrexidine resembled those obtained with a low dose of the traditional dopamine D2 receptor agonist (- )-apomorphine.
The behavioral profile observed with (± )-N-n-propyldihydrexidine is of special relevance for evaluating traditional concepts concerning the neurobiological substrate for dopamine receptor-mediated behaviors. The modification of spontaneous locomotion and oral behaviors in rats by dopamine D2 receptor agonists is typically biphasic, as is the case for (-)-apomorphine. Electrophysiological data indicating that dopamine D2 autoreceptors are functionally more sensitive to low doses of dopamine agonists than are post-synaptic dopamine D2 receptors initiated an autoreceptor hypothesis to explain the biphasic dose effects of dopamine receptor agonists on unconditioned behavior. This hypothesis has been supported further by data indicating that agonists with functional selectivity for dopamine D2 autoreceptors induce monotonic decreases in locomotor activity.
When viewed against this background, the behavioral effects obtained with N-n-propyl-dihydrexidine would be considered as indicative of actions exclusively at dopamine D2 autoreceptors. Yet this conclusion is at odds with other data indicating that hexahydrobenzo[alphenanthridine hgands (e.g., (± )-N-n-propyl-dihydrexidine) are dopamine receptor agonists that have little functional activity at dopamine D2 autoreceptors while having potent functional activity at post-synaptic dopamine D2 receptors. For example, administration of either (± )-dihydrexidine or its N-npropyl analog reduces serum prolactin levels to an extent equivalent to that of the full agonist (- )-quinpirole, actions that are mediated by dopamine D2 heteroreceptors located on pituitary lactotrophs. Likewise, members of the hexahydrobenzo[a]phenanthridine class produce full inhibition of the enzyme adenylate cyclase in models that reflect dopamine D2 postsynaptic receptor function primarily (e.g., cAMP efflux in superfused striatal slices). Conversely. these same compounds display little or no efficacy at dopamine D2 receptors controlling dopamine release or cell firing in substantia nigra. For example, N-n-propyl dihydrexidine has minimal effects on dopamine release as measured using in vivo voltammetry in striatal slices. Finally, it is of special interest that the differing functional profiles of this novel class of agonists at dopamine D2 prevs. post-synaptic receptors occur despite equal binding affinity to each receptor population.
Given the evidence that (± )-N-n-propyl-dihydrexidine does not activate any of the autoreceptor-linked processes leading to decreased dopaminergic transmission, the dose effects of this agonist on unconditioned behavior are not consistent with an autoreceptor hypothesis. Decreased locomotor activity was shown to be dopamine D2 receptormediateD2 therefore we conclude that decreases in spontaneous locomotor activity can be induced by post-synaptic dopamine D2 receptor activation. This is important since drug effects on spontaneous locomotor activity are routinely used as a pharmacological screen for determining autoreceptor vs. post-synaptic dopamine D2 receptor activation; our data advocate extreme caution in using locomotor activity as a measure of functional selectivity for novel agonists.
There have been some data in the literature to suggest that post-synaptic dopamine D3 receptors might have functional significance in decreasing spontaneous locomotor activity in rats. The evidence for this is limited for two reasons: (I) there are no highly selective dopamine D3 receptor ligands, yet there is a relatively larger expression of dopamine D2 receptors in the relevant brain regions, thus complicating interpretation, particularly in vivo and (2) the same inavailability of a highly selective dopamine D3 ligand has prevented learning about the biochemical endpoints that dopamine D3 receptor activation mediates in vivo.
Nevertheless, some classes of dopamine agonists with high affinity for the cloned dopamine D3 receptor have been shown to decrease spontaneous locomotor activity over a larger dose range than other agonist classes (e.g., (± )-7-hydroxy-N, N-di-n-propyl-2-aminotetralin (7OHDPAT). The decreased locomotor activity caused by (±)7-OHDPAT occurs over a dose range at which terminal dopamine release is not affected, suggesting post-synaptic mechanisms. Yet other structural classes having equivalent high dopamine D3 receptor affinity show discretely biphasic effects on locomotor activity ((-)-quinpirole. A novel ligand (U99194A) determined to be a dopamine D3 receptor antagonist based on its affinity in vitro and the steep slope of its binding curve, has been shown to increase locomotor activity in rats without altering terminal dopamine release, consistent with the hypothesis that post-synaptic dopamine D3 receptors have functional significance in decreasing locomotor activity. The high affinity of (± )-N-n-propyl-dihydrexidine for the D3 receptor, and its functional selectivity for post-synaptic vs. pre-synaptic dopamine D2 receptors, are consistent with a hypothesis that post-synaptic dopamine D3 receptors functionally mediate decreases in spontaneous locomotor activity.
Yawning behavior also has been hypothesized to be mediated by selective stimulation of dopamine D2 autoreceptors. Our data indicate that high doses of (±)-N-n-propyl-dihy-drexidine, functionally activating post-synaptic dopamine D2 receptors. significantly increase the frequency of yawning observed in rats. This supports other evidence that yawning is not exclusively associated with the activation of dopamine D2 autoreceptors. (±)-7-OHDPAT has been shown to increase yawning frequency within the same dose range as it decreases terminal dopamine release. Taken together with our data, this suggests that both autoreceptors and post-synaptic dopamine D2/D3 receptors have functional significance in mediating yawning.
An unexpected result was the potent dose-dependent increase in vacuous chewing observed with (± )-N-n-propyl-dihydrexidine. This became a high-intensity behavior (i.e., nearly continuous throughout observation time) at doses equal to or higher than 2 mg/kg. Vacuous chewing is not in the category of oral stereotypies typically observed in the high dose range of dopamine D-, receptor agonists. Moreover, vacuous chewing is typically associated with acute administration of dopamine D1 receptor agonists or the chronic administration of dopamine D2 receptor antagonists (for review, see Waddington, 1990). Interestingly, our antagonist studies for (± )-N-n-propyl-dihydrexidine effects on vacuous chewing behavior provide evidence that both dopamine D1 and D2 receptors contribute to the potentiation of this behavior. This is in distinct contrast to most of the literature on acutely induceD2 dopamine D1 receptor-mediated vacuous chewing, in which an antagonistic functional interaction between dopamine D1 and D2 receptors is observed. Currently available pharmacological ligands have limited selectivity for distinguishing between molecular isoforms of the dopamine D1 (D1 and D5) and D2 (D2, D3 and D4) receptor families. With the introduction of new ligands that are highly selective for molecular subtypes of the receptor, conflicting issues of dopamine D1-D2 receptor functional interaction may be clarified by a discovery that certain sub-populations of the dopamine receptor families have antagonistic, while others have cooperative/synergistic, functional interactions.
In summary, these findings demonstrate that locomotor inhibition and yawning, often considered as markers of dopamine D2 presynaptic receptor activation, can occur following administration of a novel dopamine receptor agonist that does not produce the neurochemical changes (e.g., changes in dopamine release) that have been linked to activation of dopamine D2 presynaptic receptors. Although the mechanism for the unusual profile of this compound has not been elucidateD2 a role for postsynaptic dopamine D2-like receptor subpopulations (D receptors) is hypothesized. These findings lend weight to recent challenges to established ideas about the behavioral consequences of dopamine D2 presynaptic receptor function. In a similar vein, the unexpected emergence of vacuous chewing observed with N n-propyl-dihydrexidine should prompt efforts to reformulate current concepts concerning the role of the dopamine D1 and D2 receptor subtypes in the generation of this behavior.
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