CNS distribution of D1 receptors: Use of a new specific D1 receptor antagonist, [3H]SCH39166

Dopaminergic regulation of hippocampal plasticity, learning, and memory

The hippocampus is responsible for encoding behavioral episodes into short-term and long-term memory. The circuits that mediate these processes are subject to neuromodulation, which involves regulation of synaptic plasticity and local neuronal excitability. In this review, we present evidence to demonstrate the influence of dopaminergic neuromodulation on hippocampus-dependent memory, and we address the controversy surrounding the source of dopamine innervation. First, we summarize historical and recent retrograde and anterograde anatomical tracing studies of direct dopaminergic projections from the ventral tegmental area and discuss dopamine release from the adrenergic locus coeruleus. Then, we present evidence of dopaminergic modulation of synaptic plasticity in the hippocampus. Plasticity mechanisms are examined in brain slices and in recordings from in vivo neuronal populations in freely moving rodents. Finally, we review pharmacological, genetic, and circuitry research that demonstrates the importance of dopamine release for learning and memory tasks while dissociating anatomically distinct populations of direct dopaminergic inputs.

Dopamine-induced functional activation of Gαq mediated by dopamine D1-like receptor in rat cerebral cortical membranes

Although multiple roles of dopamine through D₁-like (D₁ and D₅) and D₂-like (D₂, D₃, and D₄) receptors are initiated primarily through stimulation or inhibition of adenylyl cyclase via G_s/olf or G_i/o, respectively, there have been many reports indicating diverse signaling mechanisms that involve alternative G protein coupling. In this study, dopamine-induced Gα_q activation in rat brain membranes was investigated. Agonist-induced Gα_q activation was assessed by increase in guanosine-5'-O-(3-[³⁵S]thio)triphosphate ([³⁵S]GTPγS) binding to Gα_q determined by [³⁵S]GTPγS binding/immunoprecipitation assay in rat brain membranes. Dopamine-stimulated Gα_q functionality was highest in cortex as compared to hippocampus or striatum. In cerebral cortical membranes, this effect was mimicked by benzazepine derivatives with agonist properties at dopamine D₁-like receptors, that is, SKF83959, SKF83822, R(+)-SKF81297, R(+)-SKF38393, and SKF82958, but not by the compounds with dopamine D₂-like receptor agonist properties except for aripiprazole. Against expectation, stimulatory effects were also induced by SKF83566, R(+)-SCH23390, and pergolide. The pharmacological profiling by using a series of antagonists indicated that dopamine-induced response was mediated through dopamine D₁-like receptor, which was distinct from the receptor involved in 5-HT-induced response (5-HT_2A receptor). Conversely, the responses induced by SKF83566, R(+)-SCH23390, and pergolide were most likely mediated by 5-HT_2A receptor, but not by dopamine D₁-like receptor. Caution should be paid when interpreting the experimental data, especially in behavioral pharmacological research, in which SKF83566 or R(+)-SCH23390 is used as a standard selective dopamine D₁-like receptor antagonist. Also, possible clinical implications of the agonistic effects of pergolide on 5-HT_2A receptor has been mentioned.

Anticlockwise or clockwise? A dynamic Perception-Action-Laterality model for directionality bias in visuospatial functioning

Orientation bias and directionality bias are two fundamental functional characteristics of the visual system. Reviewing the relevant literature in visual psychophysics and visual neuroscience we propose here a three-stage model of directionality bias in visuospatial functioning. We call this model the 'Perception-Action-Laterality' (PAL) hypothesis. We analyzed the research findings for a wide range of visuospatial tasks, showing that there are two major directionality trends in perceptual preference: clockwise versus anticlockwise. It appears these preferences are combinatorial, such that a majority of people fall in the first category demonstrating a preference for stimuli/objects arranged from left-to-right rather than from right-to-left, while people in the second category show an opposite trend. These perceptual biases can guide sensorimotor integration and action, creating two corresponding turner groups in the population. In support of PAL, we propose another model explaining the origins of the biases - how the neurogenetic factors and the cultural factors interact in a biased competition framework to determine the direction and extent of biases. This dynamic model can explain not only the two major categories of biases in terms of direction and strength, but also the unbiased, unreliably biased or mildly biased cases in visuosptial functioning.

The external globus pallidus: progress and perspectives

The external globus pallidus (GPe) of the basal ganglia is in a unique and powerful position to influence processing of motor information by virtue of its widespread projections to all basal ganglia nuclei. Despite the clinical importance of the GPe in common motor disorders such as Parkinson's disease, there is only limited information about its cellular composition and organizational principles. In this review, recent advances in the understanding of the diversity in the molecular profile, anatomy, physiology and corresponding behaviour during movement of GPe neurons are described. Importantly, this study attempts to build consensus and highlight commonalities of the cellular classification based on existing but contentious literature. Additionally, an analysis of the literature concerning the intricate reciprocal loops formed between the GPe and major synaptic partners, including both the striatum and the subthalamic nucleus, is provided. In conclusion, the GPe has emerged as a crucial node in the basal ganglia macrocircuit. While subtleties in the cellular makeup and synaptic connection of the GPe create new challenges, modern research tools have shown promise in untangling such complexity, and will provide better understanding of the roles of the GPe in encoding movements and their associated pathologies.

Effect of dopamine D1 and D2 receptor antagonism in the lateral hypothalamus on the expression and acquisition of fructose-conditioned flavor preference in rats

The attraction to sugar-rich foods is influenced by conditioned flavor preferences (CFP) produced by the sweet taste of sugar (flavor-flavor learning) and the sugar's post-oral actions (flavor-nutrient) learning. Brain dopamine (DA) circuits are involved in both types of flavor learning, but to different degrees. This study investigated the role of DA receptors in the lateral hypothalamus (LH) on the flavor-flavor learning produced the sweet taste of fructose. In an acquisition study, food-restricted rats received bilateral LH injections of a DA D1 receptor antagonist (SCH23390), a D2 antagonist (RAC, raclopride) or vehicle prior to 1-bottle training sessions with a flavored 8% fructose+0.2% saccharin solution (CS+/F) and a less-preferred flavored 0.2% saccharin solution (CS-). Drug-free 2-bottle tests were then conducted with the CS+ and CS- flavors presented in saccharin. The fructose-CFP did not differ among groups given vehicle (76%), 12 nmol SCH (78%), 24 nmol (82%) or 24 nmol RAC (90%) during training. In an expression study with rats trained drug-free, LH injections of 12 or 24 nmol SCH or 12-48 nmol RAC prior to 2-bottle tests did not alter CS+ preferences (77-90%) relative to vehicle injection (86%). Only a 48 nmol SCH dose suppressed the CS+ preference (61%). The minimal effect of LH DA receptor antagonism upon fructose flavor-flavor conditioning differs with the ability of LH SCH injections to block the acquisition of glucose flavor-nutrient learning.

Lateral hypothalamus dopamine D1-like receptors and glucose-conditioned flavor preferences in rats

This study examined the role of dopamine D1-like receptor transmission in the lateral hypothalamus (LH) in flavor preference learning induced by intragastric (IG) infusions of glucose. Rats fitted with gastric catheters were injected daily in the LH with either saline or SCH23390 (12 nmol/brain), 10 min prior to training sessions with a flavor (CS+) paired with IG infusions of 8% glucose and a different flavor (CS-) paired with IG water infusions. In a post-training two-bottle test, SCH-treated rats preferred the CS+ to the CS- although their preference was weaker than that of the Control rats (61% vs. 87%). The same dose of SCH23390 reduced CS+ intake of the Control rats in a subsequent test but did not suppress their CS+ preference (90%). These results show that D1-like receptor activation in the lateral hypothalamus modulates the acquisition, but not the expression of flavor preference learning induced by the post-oral reinforcing properties of glucose.

In vivo autoradiography of [3H]SCH 39166 in rat brain: selective displacement by D1/D5 antagonists

The purpose of this study was to examine the receptor occupancy of D1/D5 antagonists for D1-like dopamine receptors in rat brain using [3H]SCH 39166, a highly selective D1/D5 antagonist with low affinity for 5HT2 receptors. A single concentration of triated SCH 39166 was administered to rats, with or without competing doses of the Dl/D5 antagonist SCH 23390 and unlabeled SCH 39166. the D2-like antagonists haloperidol or the 5-HT, antagonist ketanserin. The bound radioactivity in the cortex, striatum, nucleus accumbens and olfactory tubercle was then quantified using an in vivo autoradiographic procedure. The results indicated that [3H]SCH 39166 was dose dependently displaced by the Dl/D5 antagonists in regions associated with both the nigro-striatal pathway and the mesolimbic dopamine pathway, particularly the nucleus accumbens. Neither haloperidol nor ketanserin displaced [3H]SCH 39166 in any of the regions examined. The data were compared with previously published data examining the in vivo binding of [3H]SCH 39166 in rat brain homogenates. The relative values obtained were comparable to values detected in rat brain homogenates after in vivo binding of [3H]SCH 39166.