Beth Levant

Decreased brain docosahexaenoic acid during development alters dopamine-related behaviors in adult rats that are differentially affected by dietary remediation.

Docosahexaenoic acid (DHA) is a major component of neuronal membranes. In rats, low brain levels of DHA during development produce alterations in the mesocortical and mesolimbic dopamine systems. In this study, male Long-Evans rats (n = 6-7 per group) were raised from conception on diets with (control) or without alpha-linolenic acid, the dietary precursor of DHA. The deficient diet reduced brain DHA to 80% of that of control animals at maturity. At weaning (P21), half of the pups from each deficient litter were changed to a remediation diet enriched in DHA by the addition of fish oil. Beginning at P56, rats were subjected to a battery of behavioral tests. Catalepsy induced by haloperidol (3mg/kg, i.p.) in deficient animals was 49% of that observed in control animals (P < 0.05). Catalepsy in remediated animals was similar to that observed for control animals. In a test of locomotor activity in a novel environment, deficient animals exhibited 187% of the activity of control animals (P < 0.05). Locomotor activity stimulated by d-amphetamine (0.5mg/kg, s.c.) in deficient animals was 145% of that observed in control animals (P < 0.05). Basal and amphetamine-stimulated activity of remediated animals was similar to that of deficient animals. Response to a thermal stimulus was not different between groups. These findings demonstrate that rats exposed from conception to a diet that produces a relatively modest decrease in brain DHA content exhibit alterations in adult behavior indicative of altered dopaminergic function. Some of these behavioral alterations were reversed by dietary remediation initiated at weaning.

Differential distribution of D3 dopamine receptors in the brains of several mammalian species

The D3 dopamine receptor has been proposed as a potential target for the treatment of schizophrenia and drug abuse. This study compares the distribution of D3 sites in mouse, rat, guinea pig, and rabbit brain, and dog and human cerebellum using quantitative autoradiography with the putatively selective D3 receptor radioligand [3H]PD 128907. In the mouse, rat, guinea pig, and rabbit, specific [3H]PD 128907 binding was heterogeneously distributed with highest densities observed in the islands of Calleja, followed by the nucleus accumbens. Moderate densities of [3H]PD 128907 binding were observed in the anteroventral and dorsomedial caudate nucleus. Dense [3H]PD 128907-labelled sites were observed in the dorsal thalamus, posterior mamilliary nucleus, and dorsomedial interpeduncular nucleus of the rabbit that were not detected in the other species studied. Moderately dense []PD 128907 binding was also observed in the molecular layer of cerebellar lobule X of the rat but not in the mouse, guinea pig, rabbit, dog, or human. These observations indicate significant inter-species differences in the distribution of D3 receptors.

The D3 dopamine receptor in cellular and organismal function

Abstract The D3 dopamine receptor is a member of the family of D2-like dopamine receptors. Since the cloning and identification of the D3 receptor in 1990, considerable progress has been made towards understanding the function of this novel site. Although some avenues of investigation have yielded more definitive results than others, studies to date indicate the D3 receptor is localized preferentially in limbic brain areas and affects locomotion and perhaps reinforcement and reward. A subpopulation of the receptors appear to be autoreceptors which modulate dopamine synthesis, release, and neuronal activity. These observations have led to the hypothesis that the D3 receptor may be an appropriate target in the treatment of neuropsychiatric disorders such as schizophrenia and drug addiction. The role of D3 sites in disease, however, remains to be established. Genetic association of D3 receptor polymorphisms with neuropsychiatric disorders have been proposed. Alterations in expression of D3 sites may occur in some diseases. Although the study of this receptor is clearly in the early stages, these findings lay the foundation for future investigation. In this review, dopamine D3 receptor brain localization, cellular signaling mechanisms, and associated behavior will be discussed. The potential role of the D3 site in neuropsychiatric disorders and as a therapeutic target is also addressed.

D3 dopamine receptors in rat spinal cord: implications for sensory and motor function

Quantitative autoradiography was used to determine the distribution of D(3) receptors in rat spinal cord and compare it with the distribution of D(1)-like and D(2) (and D(4)) receptors. [(3)H]PD 128907-labeled D(3) sites were observed in roughly 6-fold lower density than [(3)H]spiperone-labeled D(2) (D(4)) sites and 60-fold lower density than [(3)H]SCH 23390-labeled D(1)-like sites. Highest densities of D(3) binding were observed in the superficial layers of the dorsal horn at cervical and lumbar levels followed by the pars centralis and dorsal horn. Lowest densities of D(3) sites were detected in the ventral horn. These observations suggest that spinal D(3) receptors may play a role in sensory and/or motor function or contribute to the pharmacological effects of dopaminergic drugs.

Decreased brain docosahexaenoic acid content produces neurobiological effects associated with depression: Interactions with reproductive status in female rats

Decreased tissue levels of docosahexaenoic acid (DHA; 22:6n-3) are implicated in the etiologies of non-puerperal and postpartum depression. With the aim of determining neurobiological sequelae of decreased brain DHA content, this study examined the effects of a loss of brain DHA content and concurrent reproductive status in adult female Long-Evans rats. An alpha-linolenic acid-deficient diet and breeding protocols were used to produce virgin and parous female rats with cortical phospholipid DHA levels 23-26% lower than virgin and parous rats fed a control diet containing adequate alpha-linolenic acid. Parous dams were tested/euthanized at weaning (postnatal day 20) of the second litter; virgin females, during diestrus. Decreased brain DHA was associated with decreased hippocampal BDNF gene expression and increased relative corticosterone response to an intense stressor, regardless of reproductive status. In virgin females with decreased brain DHA, serotonin content and turnover in frontal cortex were decreased compared to virgin females with normal brain DHA. In parous dams with decreased brain DHA, the density of 5-HT(1A) receptors in the hippocampus was increased, corticosterone response to an intense stressor was increased, and the latency to immobility in the forced swim test was decreased compared to parous dams with normal DHA. These findings demonstrate neurobiological alterations attributable to decreased brain DHA or an interaction of parous status and brain DHA level. Furthermore, the data are consistent with findings in depressed humans, and thus support a role for DHA as a factor in the etiologies of depressive illnesses, particularly postpartum depression.

Binding of [3H]PD 128907, a Putatively Selective Ligand for the D3 Dopamine Receptor, in Rat Brain: A Receptor Binding and Quantitative Autoradiographic Study

[3H]PD 128907 has been proposed as a selective ligand for the D3 dopamine receptor. This study characterizes the binding of this radioligand in rat brain using in vitro radioligand binding and autoradiographic methods. In radioligand binding studies, [3H]PD 128907 exhibited 0.3 nmol/L affinity for a single, low density site in ventral striatal membranes. The pharmacological profile for [3H]PD 128907 was similar to that of [3H](+)-7-OH-DPAT with the rank order of potency for dopamine agonists being PD 128907 ≈ 7-OH-DPAT ≈ quinpirole ≥ dopamine; for antagonists, spiperone > (+)-butaclamol ≈ domperidone ≥ haloperidol > SCH 23390. Guanyl nucleotides had no effect on the binding of either ligand. These observations indicate labeling of a dopaminergic site with characteristics consistent with the D3 receptor. In autoradiographic studies, highest densities of [3H]PD 128907–labeled sites were observed in islands of Calleja followed by the nucleus accumbens, nucleus of the horizontal limb of the diagonal band, the molecular layer of cerebellar lobule X, and the ventral caudate/putamen.

Reduced numbers of dopamine neurons in the substantia nigra pars compacta and ventral tegmental area of rats fed an n-3 polyunsaturated fatty acid-deficient diet: a stereological study.

Inadequate dietary n-3 polyunsaturated fatty acid (PUFA) content is associated with altered function of the CNS dopamine systems. In this study, the effects of dietary n-3 PUFA content were determined on dopamine cell number and morphology. Adult (postnatal day 70), male, Long-Evans rats were raised from conception on diets containing adequate (control) or negligible n-3 PUFAs. The number and morphology of tyrosine hydroxylase-positive cells in the substantia nigra pars compacta and ventral tegmental area were determined stereologically. The number of tyrosine hydroxylase-positive cells in rats fed the n-3 PUFA-deficient diet was 33.9% lower in the substantia nigra pars compacta and 33.7% lower in the ventral tegmental area than in those fed the control diet (P<0.05); however, the volume of tyrosine hydroxylase-positive cell bodies was not different between diet groups in either brain region. Rats fed the n-3 PUFA-deficient diet also exhibited dendritic depletion and isolation of tyrosine hydroxylase-positive cells compared to rats fed the control diet, which had clustering of tyrosine hydroxylase-positive cells and extensive dendritic arborization. These findings support a role for n-3 PUFAs in the survival of dopamine neurons and suggest that altered dopamine cell number, as well as function, contributes to the behavioral effects observed in rats raised on n-3 PUFA-deficient diets.

Reduced brain DHA content after a single reproductive cycle in female rats fed a diet deficient in N-3 polyunsaturated fatty acids.

BACKGROUND Low levels of n-3 polyunsaturated fatty acids (PUFAs), particularly docosahexaenoic acid (DHA, 22:6n3), are implicated in postpartum depression. METHODS The effects of pregnancy and lactation on brain phospholipid fatty acid content were determined in female rats fed diets containing sufficient (control) or negligible (deficient) alpha-linolenic acid (18:3n-3), the dietary precursor of DHA, beginning at conception. Female virgins, fed the diets for 6 weeks, served as control animals. Whole brain total phospholipid composition was determined at weaning by GC. RESULTS Brain DHA content of postpartum dams fed the deficient diet was decreased by 21% compared with age-matched virgin control animals, with a reciprocal increase in docosapentaenoic acid (22:5n6) to 243%. CONCLUSIONS Under dietary conditions supplying inadequate n-3 PUFAs, maternal brain DHA content can be reduced after a single reproductive cycle. This depletion may affect neuronal function and thus the sensitivity of the postpartum organism to stress.

In vivo occupancy of D2 dopamine receptors by nafadotride.

Nafadotride has been proposed as a selective antagonist for the D3 dopamine receptor. This drug has been shown to exhibit selectivity between D2 and D3 dopamine receptors in in vitro assay systems; however, the in vivo D2/D3 selectivity of the compound has not been determined. In this study, protection against inactivation by EEDQ was used as a measure of in vivo occupancy of D2 receptors by behaviorally relevant doses of nafadotride (0.1–10 mg/kg, SC and IP) in adult, male Sprague-Dawley rats. Ex vivo [3H]spiperone binding was then determined in striatal membranes. l-Nafadotride (10 mg/kg) protected 71% of D2 receptors after SC administration; 40% after IP administration. Protection of 13% of D2 receptors was observed at a dose of 3 mg/kg (SC). These data suggest that blockade of D2 receptors contributes to the pharmacological effects of nafadotride when administered at doses above 1 mg/kg (SC) or 3 mg/kg (IP).

Dopamine D3 Receptors

The discovery of dopamine receptor subtypes provides an opportunity to develop receptor subtype-specific drugs which could improve the therapeutic index (TI) associated with dopamine replacement therapy in Parkinson’s disease. The first generation ergot-derivative direct-acting dopamine receptor agonists are fairly nonspecific agonists of all dopamine receptors. A number of second generation direct-acting dopamine receptor agonists have recently received marketing approval. Among these, ropinirole and pramipexole show appreciable affinity for D3 receptors; indeed, pramipexole appears to be D3 receptor-preferring. Using these agents, it may be possible to determine if D3 receptor agonists confer a therapeutic advantage over the first-generation dopamine receptor agonists.D3 receptors are located in many of the same areas of the brain as D2 receptors, although they are most abundant in the limbic system. This may explain the results from animal studies revealing behavioural actions of D3 receptor stimulation consistent with functions traditionally associated with the nucleus accumbens. However, D3 receptors produce both postsynaptic and presynaptic autoreceptor effects, similar to those of D2 receptors, in the nigrostriatal pathway.A comparison of the first- and second-generation dopamine receptor agonists does not appear to reveal any significant differences in the adverse effect profile of these drugs, which is likely to reflect the fact that they both have D2 receptor agonist effects. However, data from tissue culture and animal studies may suggest an enhanced neuroprotective benefit associated with D3 receptor-preferring agents. In addition, the second-generation agents are full intrinsic agonists at dopamine receptors and are apparently more efficacious than first-generation drugs in the treatment of the motor dysfunction that accompanies Parkinson’s disease. Until agonists with greater selectivity and specificity at D3 receptors can be developed, however, it will be difficult to determine if a D3 receptor agonist would possess a higher TI than the drugs currently available for the treatment of Parkinson’s disease.