Further in vitro and in vivo studies with the putative presynaptic dopamine agonist N,N-dipropyl-7-hydroxy-2-aminotetralin

Stimulus properties of 7-OH-DPAT versus auto- and postsynaptic receptor-specific doses of quinpirole

The five types of dopamine (DA) receptor subtypes have been grouped into two families, the D(1)-like (D(1) and D(5) receptors) and D(2)-like (D(2), D(3), and D(4) receptors). Experimental evidence indicates that D(2)-like receptors can be located either presynaptically, where they modulate the synthesis and release of DA, or postsynaptically. Controversy exists, however, over the precise location and role of the D(3) subtype of DA receptor. To investigate this issue, rats were trained using standard operant drug discrimination procedures to discriminate 0.10 mg/kg of the putatively D(3) receptor-preferring agonist R(+)-7-hydroxy-N,N,-di-n-propyl-2-aminotetralin (7-OH-DPAT) from saline. Patterns of generalization to D-amphetamine, AMPT, and SCH 23390 indicated a presynaptic action of 7-OH-DPAT, while apomorphine generalization patterns suggested a postsynaptic action; quinpirole generalization suggested both a pre- and postsynaptic action of 7-OH-DPAT. The ability of spiperone, eticlopride, SCH 23390, and UH 232 to partially antagonize the 7-OH-DPAT stimulus attests to its lack of receptor subtype specificity. These results suggest both pre- and postsynaptic actions of 7-OH-DPAT along with a lack of specificity of the various pharmacological compounds for the D(3) receptor.

The dopamine D3/D2 receptor agonist 7-OH-DPAT induces cognitive impairment in the marmoset

Previous work has shown that dopaminergic systems are involved in cognitive function in the common marmoset. The present study investigated the role of dopamine D3 receptors in cognitive performance in the marmoset. The effects of the putative dopamine D3 receptor agonist, 7-OH-DPAT, on performance of a same-day reversal visual object discrimination task were assessed using a miniature Wisconsin General Test Apparatus (WGTA). Within the same test session marmosets acquired a two-choice object discrimination initial task and a reversal task to criterion. 7-OH-DPAT (6-10 microg/kg) significantly impaired reversal task performance only, without affecting acquisition of the initial task. A higher dose of 25 microg/kg 7-OH-DPAT impaired initial task acquisition as well as reversal task acquisition, possibly as a consequence of a nonspecific influence on motor function. The dopamine D2 receptor antagonist (-)sulpiride (5-10 microg/kg) and the alpha2-receptor antagonist yohimbine (50 microg/kg) failed to attenuate the effects of 7-OH-DPAT (6 microg/kg) in this task. In contrast, the dopamine D2/D3 receptor antagonist raclopride (50 microg/kg) significantly attenuated the 7-OH-DPAT-induced impairment of reversal task performance. These results suggest that activation of dopamine D3 receptors produces a selective impairment of aspects of cognitive function in the marmoset.

Anxiolytic effects of dopamine receptor ligands: I. Involvement of dopamine autoreceptors

The anxiolytic-like properties of dopamine agonists and antagonists with different receptor profiles were investigated in the ultrasonic vocalization test in rats after subcutaneous administration. Only dopamine D2 receptor agonists inhibited ultrasonic vocalization with the following ED50 values: apomorphine (0.07 mg/kg), quinelorane (0.01 mg/kg), quinpirole (0.04 mg/kg), pramipexole (0.09 mg/kg), roxindole (0.04 mg/kg), talipexole (0.04 mg/kg), (+/-)-7-OH-DPAT (0.05 mg/kg), (+/-)-PPHT (0.03 mg/kg), (-)-TNPA (0.06 mg/kg), PD128907 (0.13 mg/kg). The D2 antagonists haloperidol, mazapertine, raclopride, remoxipride, L745870, U99194A, U101958 and S(-)-DS121, the partial agonists PD143188 and preclamol, the selective D1 agonist R(+)-SKF38393 and the D1 antagonist SCH23390, and the uptake inhibitors GBR12909, GBR12935 and indatraline lacked significant inhibitory effects on ultrasonic vocalization. Because at least some of the D2 receptor agonists investigated have selectivity for dopamine autoreceptors, it is speculated that the dopamine autoreceptor may be a target for the development of new antianxiety drugs.

Regional CNS densities of serotonin and dopamine receptors in high alcohol-drinking (HAD) and low alcohol-drinking (LAD) rats

The densities of subtypes of serotonin (5-HT) and dopamine (DA) receptors were determined in the CNS of male alcohol-naive HAD and LAD lines of rats. Autoradiographic studies were undertaken to measure the densities of (a) 5-HT1A sites labelled with 2 nM [3H]8-OH DPAT, (b) 5-HT2A sites labelled with 2 nM [3H] ketanserin, (c) D1 sites labelled with 1 nM [3H]SCH23390, and (d) D2 sites labelled with 20 nM [3H]sulpiride. Membrane binding, using tissue combined from the olfactory bulb, olfactory tubercle, and nucleus accumbens, was carried out to determine Kd and Bmax values for the binding of 0.25-8.0 nM [3H]7-OH DPAT to D3 sites. Among the 14 regions measured for densities of 5-HT1A sites, no interline differences were found in the cerebral cortical regions or in the septal nuclei; however, within the hippocampus, 15-20% lower binding of [3H]8-OH DPAT was observed in the posterior dorsal CA3 and dentate gyrus of the HAD line. There were no interline differences in any of the 10 regions examined for [3H]ketanserin binding to 5-HT2A sites, or in the densities of D1 and D2 sites in the mesolimbic and nigrostriatal DA systems, except for a 35% higher density of D2 sites in the substantia nigra pars compacta of the HAD line. There were no interline differences in the Kd or Bmax values for [3H]7-OH DPAT binding to D3 sites. Overall, these results indicate that no marked interline differences are evident in the densities of 5-HT1A, 5-HT2A, D1, D2, and D3 receptors within the mesolimbic system that could be associated with the disparate alcohol drinking behaviors of the HAD and LAD rats.

Regional CNS densities of monoamine receptors in alcohol-naive alcohol-preferring P and -nonpreferring NP rats

The densities of subtypes of serotonin (5-HT) and dopamine (DA) receptors were determined in the CNS of alcohol-naive alcohol-preferring P and -nonpreferring NP lines of rats. Autoradiography studies were undertaken to measure the densities of 5-HT1B sites labelled with 100 pM [125I](-)-iodocyanopindolol, 5-HT3 sites labelled with 2 nM [3H]LY 278584, and D1 sites labelled with 1 nM[3H]SCH 23390. Membrane binding, using tissue combined from the olfactory bulb, olfactory tubercle, and nucleus accumbens, was carried out to determine Kd and B max values for the binding of 0.25-8.0 nM[3H]7-OH DPAT to D3 sites. Among the 48 regions measured for differences in 5-HT1B recognition sites, statistically significant differences (p < 0.05) were found only in the cingulate and retrosplenial cortices, in the lateral and medial septum, and in the lateral nucleus of the amygdala, with lower values being found in the P than the NP line. There were no significant differences in the regional CNS densities of D1 or 5-HT3 sites between the P and NP lines. There were also no differences between the rat lines in the Kd or Bmax values for [3H]7-OH DPAT binding to D3 sites. The lower densities of 5-HT1B sites in the CNS of the P compared to the NP rats may be a result of reduced numbers of 5-HT1B presynaptic autoreceptors as well as postsynaptic receptors in the P line. The observation that there are no differences in the amount of radioligand binding to D1, 5-HT3, and D3 sites between the P and NP lines suggests that the disparate alcohol drinking behaviors of these two lines is not associated with an innate alteration in the densities of these receptor subtypes.

Upregulation of (+)-7-hydroxy-N,N-di-n-[3H]propyl-2-aminotetralin binding following intracerebroventricular administration of a nitric oxide generator

Nitric oxide modulation of dopamine D2 and D3 receptor binding was examined using [125I]epidepride (D2) and (+)7-hydroxy-N,N-di-n-[3H]propyl-2-aminotetralin ([3H](+)-7-OH-DPAT, D3). Nitric oxide, generated by i.c.v. injection of S-nitroso-N-acetyl-penicillamine (SNAP; 5 microg or 10 microg), significantly increased the density of [3H](+)-7-OH-DPAT binding sites (39% and 134%, respectively) in the striatum 24 hours post-injection in the absence of Gpp(NH)p, representing an upregulation of either D3 receptors or high affinity D2 receptors. In the presence of 10 microM Gpp(NH)p, D3 receptor upregulation was maintained in both the 5 microg (increased 35%) and 10 microg SNAP (increased 44%) groups. [3H](+)-7-OH-DPAT binding was reduced in both striatum and nucleus accumbens in the presence of 10 microM Gpp(NH)p compared to binding in the absence of Gpp(NH)p, suggesting an upregulation of D3 receptors. Administration of SNAP did not alter total specific [125I]epidepride binding in either brain region. These data suggest that; 1) D3 receptor density is modified following nitric oxide generation, and 2) the density of high affinity D2 receptors identified by [3H](+)-7-OH-DPAT increases in the striatum, but decreases in the nucleus accumbens.

Aminotetralin drugs and D3 receptor functions. What may partially selective D3 receptor ligands tell us about dopamine D3 receptor functions?

The dopamine D3 receptor gene was identified by Sokoloff and colleagues in 1990. This finding rapidly gained the interest of the scientific community because this unexpected dopamine receptor subtype may play an important role in the antipsychotic activity of neuroleptic drugs. It recognizes most neuroleptics with a high affinity, and its brain distribution is restricted mainly to the ventral part of the striatal complex. However, the characterization and the subsequent identification of functions of the D3 receptor were hampered initially by at least four important factors that are still partially unresolved: (1) the absence of selective drugs that can discriminate between the D2 and D3 receptor subtype functions in vivo, (2) the lack of apparent coupling with GTP-dependent proteins, (3) the absence of effects on second messenger systems, and (4) the low level of expression of this receptor in brain tissue; these factors have contributed to tempering the interest of scientists. However, this situation has begun to change with the identification of [3H]7-hydroxy-N,N-(di-n-propyl)-2-aminotetralin ([3H]7-OH-DPAT), the first selective ligand for the dopamine D3 receptor. Although its binding selectivity for the D3 versus the D2 receptor is somewhat artificial, the potentially important impact of identification of a function for the D3 receptor encouraged scientists to use this aminotetralin compound for in vivo studies with, however, limited success. This commentary is focused on the impact and controversies generated by the use of 7-OH-DPAT and its congeners, on new conceptual views that may arise from this research, and on what partially selective D3 receptor ligands may tell us about dopamine D3 receptor functions.

In vivo evidence for preferential role of dopamine D3 receptor in the presynaptic regulation of dopamine release but not synthesis

Brain microdialysis was used to investigate the effects of the putative dopamine D3 receptor agonist (+/-)-7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) on dopamine release, metabolism and synthesis in the dorsal striatum and nucleus accumbens of awake rats. The drug administered i.p. dose dependently decreased the release, metabolism and synthesis of dopamine in both brain areas. The potency of 7-OH-DPAT to decrease dopamine release was found to be higher in the nucleus accumbens than in the dorsal striatum (ED50 for nucleus accumbens 0.0096 mg/kg, i.p.; for dorsal striatum 0.068 mg/kg, i.p.). Dopamine metabolism, assessed by measuring 3,4-dihydroxyphenylacetic acid extracellular levels, and dopamine synthesis, determined as 3,4-dihydroxyphenylalanine output following perfusion with the L-aromatic acid decarboxylase inhibitor 3-hydroxybenzylhydrazine (10(-5) M), were decreased at higher dose ranges of 7-OH-DPAT (ED50 for decrease of 3,4-dihydroxyphenylalanine output in nucleus accumbens 0.124 mg/kg, i.p.; in dorsal striatum 0.101 mg/kg, i.p.). The hypomotility of rats induced by 7-OH-DPAT in doses of 0.002-0.25 mg/kg, i.p., was shown to correlate with the decreased dopamine release in the nucleus accumbens. Pretreatment of animals with 7-OH-DPAT at the putative dopamine D3 receptor 'selective' dose of 0.05 mg/kg, i.p., was found to prevent the increase of dopamine release but not the increase in metabolism in the dorsal striatum of freely moving rats induced by (+)-AJ76, cis (+)-(1S,2R)-5-methoxy-1-methyl-1-2-(n-propylamino)tetralin HCI (7 mg/kg, i.p.) and haloperidol (0.1 mg/kg, i.p.). Local application of 7-OH-DPAT by addition into the perfusing medium also resulted in a preferential decrease of dopamine release in the nucleus accumbens as compared with the dorsal striatum (EC50 for nucleus accumbens 1.9 nM; for dorsal striatum 11.3 nM). The present results give further support to the hypothesis that the dopamine D3 autoreceptor is preferentially involved in the presynaptic regulation of dopamine release, while the D2 autoreceptor controls dopamine synthesis.

The locomotor effects of a putative dopamine D3 receptor agonist in developing rats

Dopamine receptors have been categorized into subfamilies D1 and D2, each with separate roles in dopamine-mediated behaviors. Of the D2 subfamily, the dopamine D3 receptor has been cloned, but the behavioral effects of selectively stimulating the D3 receptor are largely unknown. The purpose of this study was to quantify the locomotor responses of developing rats to the putative dopamine D3 receptor agonist, 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT). One of three doses of 7-OH-DPAT (0.01, 0.10, 1.00 mg/kg) or saline was injected subcutaneously into rats at the age of 10,20,30, or 60 days. Five minutes after the injection, rats were placed in automated activity monitors which recorded locomotor behavior at 5 min intervals for 2 h. The high dose of 7-OH-DPAT increased locomotor activity in rats of all ages. The medium and low doses increased activity in 10- and 20-day-old rats but not in 30- or 60-day-old rats. The level of drug-induced activation peaked at 20 days of age. In 30- and 60-day-old rats, but not 10- and 20-day-old rats, a period of locomotor suppression preceded the activation in response to the high dose of 7-OH-DPAT. In rats aged 20 days and older, the middle and low doses decreased locomotion early in the test session, but activation did not ensue. This dose-response pattern across ontogeny closely resembles that induced by quinpirole, an agonist at the dopamine D2 receptor subfamily.

Behavioural effects in the rat of the putative dopamine D3 receptor agonist 7-OH-DPAT: comparison with quinpirole and apomorphine

This study assessed the effects of IP injections of (+/-) 7-hydroxy-2(N,N-di-n-propylamino)tetralin (7-OH-DPAT), a dopamine agonist that has been reported to have preferential affinity for the dopamine D3 sub-type of receptor, on four behavioural procedures in the rat: 1) spontaneous locomotion, 2) electrical self-stimulation of the ventral tegmental area (VTA), using the curve-shift procedure 3) operant responding for food under a progressive-ratio (PR) schedule and 4) induction of stereotypies. The effects of (+/-) 7-OH-DPAT were compared to the effects of apomorphine, a non-specific DA agonist, and quinpirole, a selective D2/D3 agonist. All three dopamine agonists decreased locomotor activity at low doses (0.01-0.3 mg/kg), and only apomorphine had clear locomotor stimulant effects at the highest dose tested (3 mg/kg). The three drugs dose-dependently depressed VTA self-stimulation in a similar way, with low doses inducing a fairly parallel rightward shift of the frequency/rate curves and higher doses flattening the curves. In contrast, responding for food under the PR schedule appeared to be differentially affected by the three agonists: 7-OH-DPAT induced a biphasic effect, with a maximal decrease in lever-pressing at 0.1 mg/kg, followed by a return to baseline levels with increasing doses (0.3-3 mg/kg); quinpirole showed a tendency to decrease responding over the whole dose-range tested with a maximal effect of about 50% of baseline between 0.25 and 1 mg/kg, and apomorphine dose-dependently decreased responding, with rats ceasing to respond at 0.3 mg/kg. All three DA agonists induced stereotypies, but there was a difference in the maximal stereotypy score induced by each of the ligands: 7-OH-DPAT produced a lower maximal effect than quinpirole or apomorphine. This indicates that each of the three dopamine agonists preferentially induced different types of stereotypies. Together, these data suggest that the putative dopamine D3 agonist 7-OH-DPAT, at low doses, has depressant effects similar to those induced by low doses of the other two DA agonists. Differences in the behavioural effects of higher doses were, however, mostly observed in two procedures, PR responding and induction of stereotypies.

Effects of 7-hydroxy-N,N-di-n-propylaminotetralin on behaviour and blood pressure of spontaneously hypertensive rats

The in vivo effects of administration of the putative dopamine D3 receptor agonist 7-hydroxy-N,N-di-n-propylaminotetralin (7-OH-DPAT) were investigated in spontaneously hypertensive rats (SHR) and normotensive wistar-Kyoto controls (WKY). The i.p. injection of 7-OH-DPAT induced hyperactivity in WKY at 10 mg/kg, but only an inhibition of exploratory locomotor activity was observed in SHR at 1 mg/kg. In WKY and SHR with unilateral lesions of the nigrostriatal system, s.c. injection of 0.01-1 mg/kg of 7-OH-DPAT induced dose-dependent contralateral turning behaviour. This response was more pronounced in SHR than in WKY. The s.c. injection of 0.03, but not of 0.01 or 0.1 mg/kg, of 7-OH-DPAT induced yawning in WKY and SHR. The i.v. injection of 0.1 or 1 mg/kg of 7-OH-DPAT induced an immediate rise in blood pressure in both WKY and SHR. Pretreatment with the dopamine receptor antagonist haloperidol partially prevented this pressor response and, in addition, unmasked a late fall in blood pressure in SHR. The s.c. injection of 1 mg/kg of 7-OH-DPAT induced a decrease in body temperature, which was more pronounced in SHR than in WKY. This effect could be inhibited by pretreatment with haloperidol, but a residual hypothermia remained in SHR. These results suggest that 7-OH-DPAT induces a variety of effects in vivo, many of which may be mediated by dopamine D2 receptors or non-dopaminergic receptors. Thus, more selective dopamine D3 receptor agonists or -antagonists are needed to further explore the role of dopamine D3 receptors in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

[3H]7-OH-DPAT labels both dopamine D3 receptors and sigma sites in the bovine caudate nucleus

The binding of [3H]7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) to membranes of the bovine caudate nucleus was characterized and shown to be heterogenous. While [3H]7-OH-DPAT labels the dopamine D3 receptor in this tissue (Ki = 4.8 nM), as shown by its sensitivity to dopamine and (-)-quinpirole, it also binds with high affinity (Ki = 48 nM) to sigma recognition sites. The present results demonstrate that [3H]7-OH-DPAT is a useful radioligand to label native D3 receptors in the bovine caudate, although care must be taken to assure its selectivity.

Identification, characterization, and localization of the dopamine D3 receptor in rat brain using 7-[3H]hydroxy-N,N-di-n-propyl-2-aminotetralin

We have identified 7-[3H]hydroxy-N,N-di-n-propyl-2-aminotetralin ([3H]7-OH-DPAT) as a selective probe for the recently cloned dopamine D3 receptor and used it to assess the presence of this receptor and establish its distribution and properties in brain. In transfected Chinese hamster ovary (CHO) cells, it binds to D3 receptors with subnanomolar affinity, whereas its affinity is approximately 100-, 1000-, and 10,000-fold lower at D2, D4, and D1 receptors, respectively. Specific [3H]7-OH-DPAT binding sites, with a Kd of 0.8 nM and a pharmacology similar to those at reference D3 receptors of CHO cells, were identified in rat brain. D3 receptors differ from D2 receptors in brain by their lower abundance (2 orders of magnitude) and distribution, restricted to a few mainly phylogenetically ancient areas--e.g., paleostriatum and archicerebellum--as evidenced by membrane binding are autoradiography studies. Native D3 receptors in brain are characterized by an unusually high nanomolar affinity for dopamine and a low modulatory influence of guanyl nucleotides on agonist binding. These various features suggest that D3 receptors are involved in a peculiar mode of neurotransmission in a restricted subpopulation of dopamine neurons.

Stimulation and inhibition of food intake by the selective dopamine D2 agonist, N-0437: a meal pattern analysis

Feeding and drinking responses were recorded in home-caged rats over 24-hour periods, and the data were analysed in terms of meal patterns. The highly selective dopamine D2 receptor agonist, N-0437, was injected IP at the start of the night phase. At the smallest dose (0.3 mg/kg), N-0437 increased food intake as a result of increases in meal size and duration. The rate of feeding during meals was not increased. This effect lasted 1-2 hours in the night phase. At higher doses (1.0 and 3.0 mg/kg), N-0437 inhibited food intake for 3-6 hours. The inhibition was due to a substantial reduction in meal size, with no change in meal frequency. Rate of feeding during meals tended to be reduced. A biphasic dose-effect relationship was not apparent in the drinking data, and there was not a significant effect of N-0437 on drinking responses. The meal pattern analysis results indicate the stimulation of D2 receptors produces increases and decreases in food intake and meal size. These results are discussed in terms of dose-dependent stimulation of presynaptic (autoreceptor) and postsynaptic D2 receptors, and the relationship to satiety within meals.

Pharmacological profile of a chromanamine analogue (DP-6OH-3CA) of the selective presynaptic dopamine agonist N,N-dipropyl-7-hydroxy-2-aminotetralin

The pharmacological profile of an oxygen isostere of the selective presynaptic dopamine agonist DP-7OH-AT, i.e. dipropyl-6-hydroxy-3-chromanamine (DP-6OH-3CA) has been evaluated in various receptor binding, neurobiochemical and behavioural experiments. The chromanamine displaced the 3H-labelled dopamine ligands, 5,6-DPAT and N-0437, with Ki values of 106 and 143 nM, respectively. In in-vivo biochemical models for presynaptic activity the chromanamine induced a half-maximal effect in the gamma-butyrolactone reversal test at 6.8 mumol kg-1 and had an ED70 value of 40 mumol kg-1 for HVA decrease in the striatum. In behavioural models for postsynaptic dopaminergic activity a half-maximal effect for the induction of stereotypy was reached at 100 mumol kg-1 and reversal of the effects of reserpine to a level of 200 counts was induced at 11 mumol kg-1. On comparison of these results with the results obtained with the carbon analogue of DP-6OH-3CA, i.e. DP-7OH-AT, it is apparent that the chromanamine has a reduced potency for dopamine D2 receptors in in-vitro and in-vivo models. The selectivity for presynaptic dopamine receptors was lower than with DP-7OH-AT and the isomeric chromanamine, DP-8OH-3CA, indicating that the optimal position of the hydroxyl group for presynaptic selectivity is in the 8 and not in the 6 position for the chromanamines.