Imaging apomorphine stimulation of brain arachidonic acid signaling <Emphasis Type="Italic">via</Emphasis> D<Subscript>2</Subscript>-like receptors in unanesthetized rats

Rationale and objective Because of the important role of dopamine in neurotransmission, it would be useful to be able to image brain dopamine receptor-mediated signal transduction in animals and humans. Administering the D₁–D₂ receptor agonist apomorphine may allow us to do this, as the D₂-like receptor is reported to be coupled to cytosolic phospholipase A₂ activation and arachidonic acid (AA) release from membrane phospholipid. Methods Unanesthetized adult rats were given intraperitoneally apomorphine (0.5 mg/kg) or saline, with or without pretreatment with 6 mg/kg intravenous raclopride, a D₂/D₃ receptor antagonist. [1–¹⁴C]AA was injected intravenously, then AA incorporation coefficients k*—brain radioactivity divided by integrated plasma radioactivity—markers of AA signaling, were measured using quantitative autoradiography in 62 brain regions. Results Apomorphine significantly elevated k* in 26 brain regions, including the frontal cortex, motor and somatosensory cortex, caudate-putamen, thalamic nuclei, and nucleus accumbens. Raclopride alone did not change baseline values of k*, but raclopride pretreatment prevented the apomorphine-induced increments in k*. Conclusions A mixed D₁–D₂ receptor agonist, apomorphine, increased the AA signal by activating only D₂-like receptors in brain circuits containing regions with high D₂-like receptor densities. Thus, apomorphine might be used with positron emission tomography to image brain D₂-like receptor-mediated AA signaling in humans in health and disease.     

Effects of age on dopamine D2 receptor availability in striatal subdivisions: A high-resolution positron emission tomography study

The purpose of the present study was to examine the relationship between age and dopamine D₂ receptor availability in striatal subdivisions of young and middle-aged healthy subjects using high-resolution positron emission tomography (PET) with [¹¹C]raclopride to better characterize the nature of age-related decrements in striatal D₂ receptor availability. Twenty-four healthy volunteers completed 3-Tesla magnetic resonance imaging and high-resolution [¹¹C]raclopride PET scans. The analyses using linear and exponential models revealed that age had a significant negative correlation with D₂ receptor availability in the post-commissural putamen (postPU) and that D₂ receptor binding in the postPU decreased significantly more with age than in the ventral striatum, suggesting subregional differences in age-related changes in D₂ receptor binding. The postPU, which belongs to the sensorimotor striatum, may be particularly vulnerable to the effects of age in young and middle-aged subjects.     

Self-administration of the D1 agonist SKF 82958 is mediated by D1, not D2, receptors

We have previously reported that two D₁ dopamine agonists, SKF 82958 and SKF 77434, are readily self-administered by rats. However, due to the limited selectivities of these agents, it was not possible to attribute their reinforcing effects exclusively to their D₁ actions. To assess the relative involvement of D₁ and D₂ receptors in SKF 82958 reinforcement, rats were pretreated 30 min before self-administration sessions with either the D₁-selective antagonist (+)SCH 23390 or the D₂-selective antagonist raclopride. The D₁ antagonist (+)SCH 23390 (5–20 µg/kg, SC) produced significant, dose-related (compensatory) increases in SKF 82958; in contrast, the D₂ antagonist raclopride (25–400 µg/kg, SC) did not significantly increase SKF 82958 self-administration, although raclopride did increase cocaine self-administration. Compensatory increases in self-administration rates are thought to reflect antagonist-induced reductions in drug reinforcement. Hence, we conclude that SKF 82958 self-administration depends on activation of a D₁-regulated reinforcement substrate.This work was supported by U.S.P.H.S. grants DA-05107, DA-05379, and DA-07747. All animal procedures were performed in accordance with the Guide for the Care and Use of Laboratory Animals (NIH pub. no. 86-23, revised 1985).     

The involvement of dopamine D2 receptors, but not D3 or D4 receptors, in the rewarding effect of brain stimulation in the rat

To identify the subtype of dopamine receptors critically involved in the rewarding effect of brain stimulation, four dopamine antagonists were intracranially injected in 25 rats. The importance of dopamine D1 receptors had been demonstrated previously by using SCH 23390, a highly selective D1 antagonist. Rats were implanted with electrodes into the medial forebrain bundle and cannulae into either one of the following structures: the nucleus accumbens, the vicinity of the islands of Calleja, or the ventral tegmental area, all ipsilateral to the electrodes. The animals were trained to press a bar for electrical stimulation, and the frequency-response functions were plotted before and after injection of each dopamine antagonist through the cannulae. Raclopride and haloperidol, which have high affinities for D2 receptors, reduced the rewarding effect after injection into any one of the three cannula sites. Neither (+)-UH232, a selective D3 antagonist, nor clozapine, a D4 antagonist, influenced the rewarding effect. The results suggest that dopamine D2, but not D3 or D4, receptors are critically involved in producing the rewarding effect of brain stimulation.     

Dopaminergic regulation of cortical acetylcholine release.

The extent to which the activity of basal forebrain cholinergic neurons is influenced by dopamine (DA) was investigated using in vivo microdialysis of cortical acetylcholine (ACh). Systemic administration of the DA receptor agonist apomorphine significantly increased dialysate concentrations of ACh. Systemic, but not local, administration of d-amphetamine produced similar effects. Both D1 (SCH 23390) and D2 (haloperidol, raclopride) DA receptor antagonists attenuated the amphetamine-induced increase in cortical ACh release; however, only the D1 antagonist significantly reduced basal output of cortical ACh. These findings suggest that the activity of cortically projecting cholinergic neurons in the nucleus basalis is regulated in an excitatory manner by central dopaminergic neurons and that both D1 and D2 receptors are involved.     

Radial-arm maze performance in rats is impaired by a combination of nicotinic-cholinergic and D2 dopaminergic antagonist drugs

Performance on the radial-arm maze depends on the integrity of both cholinergic and dopaminergic systems. We have previously found that administration of either the nicotinic-cholinergic antagonist, mecamylamine, or the muscarinic-cholinergic antagonist, scopolamine, impairs choice accuracy in the radial-arm maze. Co-administration of the dopaminergic antagonist, haloperidol, ameliorated the performance deficit caused by scopolamine but exacerbated the deficit caused by mecamylamine. Furthermore, antagonism of the effect of scopolamine is due specifically to blockade of D₁ receptors. In the present experiment behaviorally subthreshold doses of mecamylamine and the D₂ antagonist raclopride impaired maze performance when administered together. No interactive effects were observed between mecamylamine and the D₁ antagonist SCH 23390. Although several of the drug treatments studied significantly increased choice latency, an index of motor behavior, there was no perfect relationship between choice accuracy and choice latency. These data indicate that nicotinic-cholinergic and muscarinic-cholinergic systems interact selectively and differentially with D₁ and D₂ dopaminergic systems.     

Dopamine D2 receptors mapped in rat brain with [3H](+)PHNO

Previous work with membrane preparations had demonstrated that the agonist (+)-4-propyl-9-hydroxynaphthoxazine (PHNO) labels the high-affinity state of dopamine D₂ receptors with 67-fold selectivity over D₁ sites. In this study, quantitative autoradiography was used to examine the binding of [³H](+)PHNO to rat brain sections. Highest binding densities were found in caudate-putamen, accumbens, and olfactory tubercles, as expected, and also in specific layers of the olfactory bulb. In addition, a second group of brain regions, including lateral septum, entorhinal cortex, molecular layer of hippocampus, and several brainstem structures showed low but consistent levels of binding. In all brain regions [³H](+)PHNO binding (2 nM) was completely displaced by 10 μM sulpiride (>99%). Addition of 150 μM guanilylimidodiphosphate, which normally converts D2 receptors from high to low affinity states, abolished [³H](+)PHNO binding in all brain regions (>99%), except for the islands of Callejas. This is likely to reflect binding to D₃ sites in this area. Omission of preincubation in binding assays decreased [³H](+)PHNO binding in a regionally dependent manner, with strongest effects (22%) seen in high-density areas. These preincubation results confirm that (+)PHNO may have limitations for in vivo imaging studies. On the other hand, [³H](+)PHNOs negligible levels of non-specific binding compared to other agonists and overall selectivity would make it an excellent tool for in vitro autoradiographic monitoring of the high affinity state of D₂ receptors. © 1994 Wiley-Liss. Inc.     

Examination of drug-induced and isolation-induced disruptions of prepulse inhibition as models to screen antipsychotic drugs

Prepulse inhibition (PPI) of an acoustic startle response is impaired in schizophrenics. PPI can also be studied in the rat, and is impaired by dopamine (DA) D_2/3 receptor agonists such as apomorphine. This disruption is reversed by DA antagonists, leading to proposals that this approach may be a useful means to identify novel antipsychotics. There is also evidence to suggest a role of serotonergic (5-HT) and glutamatergic systems in schizophrenia, and accordingly PPI can be disrupted by the 5-HT₂ agonist DOI, and the non-competitive NMDA antagonist, dizocilpine. In the present study we have examined the effect of four antipsychotic drugs, haloperidol (0.1–0.3 mg/kg), raclopride (0.03–0.3 mg/kg), risperidone (0.3–3 mg/kg) and clozapine (0.0001–10 mg/kg), against the PPI disruptions induced by apomorphine (0.5 mg/kg), DOI (3 mg/kg) and dizocilpine (0.15 mg/kg). Furthermore, these drugs have been examined for their ability to restore a PPI deficit produced by housing rats under conditions of social isolation. All drugs except clozapine reversed an apomorphine-induced disruption. However, clozapine and risperidone, but not raclopride and haloperidol, reversed a DOI-induced disruption. Only risperidone was effective in restoring a PPI deficit produced by dizocilpine. In contrast to the drug-induced disruptions which were differentially sensitive to the various neuroleptics, isolation-induced disruptions were restored by each drug. These results support the idea that non-drug induced disruptions of PPI, such as social isolation, may be a more viable approach to identify novel antipsychotics.     

Microcatalepsy and disruption of forelimb usage during operant behavior: differences between dopamine D1 (SCH-23390) and D2 (raclopride) antagonists

In an experiment designed to distinguish between the behavioral consequences of treatment with SCH-23390, a D₁ dopamine receptor blocker, and raclopride, a D₂ antagonist, rats were trained to perform a water-reinforced forelimb operant response. Response rate and the duration of each forelimb contact with the operandum were recorded. In addition, the durations of the rat's visits to the reward well were detected by a photobeam which was blocked by the rat's muzzle as it remained at the reward well. In a between-groups dosing design, separate groups of rats (11–13 rats/group) received SCH-23390 (0, 0.01, 0.02, 0.04, 0.08, 0.12 mg/kg, IP, 30 min) or raclopride (0. 0.05, 0.1, 0.2, 0.4, 0.8 mg/kg, IP, 30 min) for 21 consecutive days. Quantitative analyses indicated that for comparable amounts of operant rate reduction, raclopride had a significantly greater tendency than SCH-23390 to increase the duration of operant responses and to increase the maximum muzzle entry duration (i.e., to induce microcatalepsy). The results support the idea that at relatively low doses D₂ antagonism is more likely than D₁ antagonism to produce effects identified preclinically with extrapyramidal side effects.     

Potentiation of the effects of raclopride on sucrose consumption by the 5-HT2 antagonist ritanserin

We examined the effects on sucrose consumption of a dopamine (DA) D₂/D₃ antagonist (raclopride, 0–0.3 mg/kg), a serotonin 5-HT_2/2c antagonist (ritanserin, 0–0.4 mg/kg), and raclopride (0.15 mg/kg) combined with ritanserin (0–0.4 mg/kg). Three different concentrations of sucrose solution (0.7%, 7.0% and 34%) were tested. The concentration-intake function in drug-free rats was an inverted-U, with 7.0% sucrose supporting the highest intake. Raclopride (0.3 mg/kg) inhibited intake of 7.0% sucrose, but its effect on 0.7% sucrose fell short of significance (P<0.06) and intake of 34% sucrose was unchanged. Ritanserin did not affect sucrose intake when given on its own, but synergized with a previously ineffective dose of raclopride (0.15 mg/kg) so as to cause a significant inhibition of 7.0% sucrose intake and increase in 34% sucrose intake. These data indicate that 5-HT₂ antagonism potentiated the behavioural effects of raclopride and we speculate that this interaction might contribute to the efficacy of atypical neuroleptic drugs.