Transgenic mouse models of dopamine deficiency

Intestinal Absorption of Bile Acids in Health and Disease

The intestinal reclamation of bile acids is crucial for the maintenance of their enterohepatic circulation. The majority of bile acids are actively absorbed via specific transport proteins that are highly expressed in the distal ileum. The uptake of bile acids by intestinal epithelial cells modulates the activation of cytosolic and membrane receptors such as the farnesoid X receptor (FXR) and G protein-coupled bile acid receptor 1 (GPBAR1), which has a profound effect on hepatic synthesis of bile acids as well as glucose and lipid metabolism. Extensive research has focused on delineating the processes of bile acid absorption and determining the contribution of dysregulated ileal signaling in the development of intestinal and hepatic disorders. For example, a decrease in the levels of the bile acid-induced ileal hormone FGF15/19 is implicated in bile acid-induced diarrhea (BAD). Conversely, the increase in bile acid absorption with subsequent overload of bile acids could be involved in the pathophysiology of liver and metabolic disorders such as fatty liver diseases and type 2 diabetes mellitus. This review article will attempt to provide a comprehensive overview of the mechanisms involved in the intestinal handling of bile acids, the pathological implications of disrupted intestinal bile acid homeostasis, and the potential therapeutic targets for the treatment of bile acid-related disorders. Published 2020. Compr Physiol 10:21-56, 2020.

The paradox of hyperdopaminuria in aromatic L-amino Acid deficiency explained

Aromatic L-amino acid decarboxylase (AADC) decarboxylates 3,4-L-dihydroxylphenylalanine (L-dopa) to dopamine, and 5-hydroxytryptophan to serotonin. In AADC deficiency, dopamine and serotonin deficiency leads to a severe clinical picture with mental retardation, oculogyric crises, hypotonia, dystonia, and autonomic dysregulation. However, despite dopamine deficiency in the central nervous system, urinary dopamine excretion in AADC-deficient patients is normal to high.In human, renal AADC-activity is very high compared to other tissues including brain tissue. Plasma L-dopa levels are increased in AADC deficiency. In this study, the hypothesis that in AADC deficiency relatively high-residual renal AADC-activity combined with high substrate availability of L-dopa leads to normal or elevated levels of urinary dopamine is tested and verified using 24-h urine collection of two AADC-deficient patients.Renal dopamine is a major regulator of natriuresis and plays a crucial role in the maintenance of sodium homeostasis. Therefore, the preservation of sufficient renal AADC-activity in AADC deficiency might be crucial for survival of AADC-deficient patients.In this study, we underpinned an empirical finding with theory, thereby putting a clinical observation into its physiological context. Our study stresses the difference - not qualitatively but quantitatively - between dopamine production in the central nervous system and peripheral organs. Furthermore, this study clarifies the so far unexplained observation that neurotransmitter profiles in urine should be interpreted with extreme caution in the diagnostic work-up of patients suspected to suffer from neurometabolic disorders.

Joining the dots: neurobiological links in a functional analysis of depression

Depression is one of the major contributors to the Total Disease Burden and afflicts about one-sixth of Western populations. One of the most effective treatments for depression focuses upon analysis of causal chains in overt behaviour, but does not include brain-related phenomena as steps along these causal pathways. Recent research findings regarding the neurobiological concomitants of depressive behaviour suggest a sequence of structural and functional alterations to the brain which may also produce a beneficial outcome for the depressed individual--that of adaptive withdrawal from uncontrollable aversive stressors. Linking these brain-based explanations to models of observable contingencies for depressive behaviour can provide a comprehensive explanation of how depressive behaviour occurs and why it persists in many patients.

Altered neurocircuitry in the dopamine transporter knockout mouse brain

The plasma membrane transporters for the monoamine neurotransmitters dopamine, serotonin, and norepinephrine modulate the dynamics of these monoamine neurotransmitters. Thus, activity of these transporters has significant consequences for monoamine activity throughout the brain and for a number of neurological and psychiatric disorders. Gene knockout (KO) mice that reduce or eliminate expression of each of these monoamine transporters have provided a wealth of new information about the function of these proteins at molecular, physiological and behavioral levels. In the present work we use the unique properties of magnetic resonance imaging (MRI) to probe the effects of altered dopaminergic dynamics on meso-scale neuronal circuitry and overall brain morphology, since changes at these levels of organization might help to account for some of the extensive pharmacological and behavioral differences observed in dopamine transporter (DAT) KO mice. Despite the smaller size of these animals, voxel-wise statistical comparison of high resolution structural MR images indicated little morphological change as a consequence of DAT KO. Likewise, proton magnetic resonance spectra recorded in the striatum indicated no significant changes in detectable metabolite concentrations between DAT KO and wild-type (WT) mice. In contrast, alterations in the circuitry from the prefrontal cortex to the mesocortical limbic system, an important brain component intimately tied to function of mesolimbic/mesocortical dopamine reward pathways, were revealed by manganese-enhanced MRI (MEMRI). Analysis of co-registered MEMRI images taken over the 26 hours after introduction of Mn²⁺ into the prefrontal cortex indicated that DAT KO mice have a truncated Mn²⁺ distribution within this circuitry with little accumulation beyond the thalamus or contralateral to the injection site. By contrast, WT littermates exhibit Mn²⁺ transport into more posterior midbrain nuclei and contralateral mesolimbic structures at 26 hr post-injection. Thus, DAT KO mice appear, at this level of anatomic resolution, to have preserved cortico-striatal-thalamic connectivity but diminished robustness of reward-modulating circuitry distal to the thalamus. This is in contradistinction to the state of this circuitry in serotonin transporter KO mice where we observed more robust connectivity in more posterior brain regions using methods identical to those employed here.

Expression of Npc1 in glial cells corrects sterility in Npc1(-/-) mice

Niemann-Pick type C1 (NPC) disease is an autosomal recessive neurodegenerative disorder. One feature of the mouse model of NPC1 is it's infertility. We have made transgenic mice which express the Npc1 protein exclusively in fibrillary astrocytes, using the glial fibrillary acidic protein (GFAP) promoter. This selective expression of Npc1 corrects sterility in GFAP-Npc1(-/-), Npc1(-/-) mice. Counts of acidophils in the pituitary of GFAP-Npc1E, Npc1(-/-) mice, as compared Npc1(-/-) mice, and measurements of dopamine D2 receptor (DRD2) mRNA in the pituitary, suggest mechanisms for fertility enhancement. We conclude that the correction of sterility in GFAP-Npc1E, Npc1(-/-) mice is a result of restoring hypothalamic control of the pituitary.

The phenotypic spectrum of paediatric neurotransmitter diseases and infantile parkinsonism

Paediatric neurotransmitter diseases are a group of inherited disorders attributable to a disturbance of neurotransmitter metabolism. The monoamines, catecholamines and serotonin, also called biogenic amines, are neurotransmitters with multiple roles including psychomotor function, hormone secretion, cardiovascular, respiratory and gastrointestinal control, sleep mechanisms, body temperature and pain. Given the multiple functions of monoamines, disorders of their metabolism comprise a wide spectrum of manifestations, with motor dysfunction being the most prominent clinical feature. The severity of the clinical manifestations ranges from mild to severe. Patients with severe and intermediate phenotypes may present with infantile parkinsonism that differs in a number of aspects from the parkinsonism in nigrostriatal degeneration. Analysis of monoamine metabolites and pterins in spinal fluid assists in the diagnosis of these disorders. Treatment options include tetrahydrobiopterin supplementation, L: -dopa, 5-hydroxytryptophan, and medications that potentiate monoamine transmission. Response to treatment is variable.

Clinical Utility of Monoamine Neurotransmitter Metabolite Analysis in Cerebrospinal Fluid

Background: Measurements of monoamine neurotransmitters and their metabolites in plasma and urine are commonly used to aid in the detection and monitoring of neuroblastoma and pheochromocytoma and the evaluation of hypotension or hypertension. Measurements of these neurotransmitters and metabolites can also be helpful in the investigation of disorders that primarily affect the central nervous system, but only when the measurements are made in cerebrospinal fluid (CSF).

Content: I describe CSF profiles of monoamine metabolites in the primary and secondary defects affecting serotonin and catecholamine metabolism. I outline the methods required to analyze these metabolites together with details of specific sample handling requirements, sample stability, and interfering compounds, and I emphasize a need for age-related reference intervals.

Summary: Measured values of monoamine metabolites in CSF provide only a single-time snapshot of the overall turnover of the monoamine neurotransmitters within the brain. Because these measurements reflect the average concentrations accumulated from all brain regions plus the regional changes that occur within the spinal cord, they may miss subtle abnormalities in particular brain regions or changes that occur on a minute-to-minute or diurnal basis. Clearly defined diagnosed disorders are currently limited to those affecting synthetic and catabolic pathways. In many cases, abnormal monoamine metabolite concentrations are found in CSF and an underlying etiology cannot be found. Molecular screening of candidate genes related to steps in the neurotransmission process, including storage in presynaptic nerve vesicles, release, interaction with receptors, and reuptake, might be a fruitful endeavor in these cases.

Metabolic interactions between glutamatergic and dopaminergic neurotransmitter systems are mediated through D(1) dopamine receptors

Interactions between the dopaminergic and glutamatergic neurotransmission systems were investigated in the adult brain of wild-type (WT) and transgenic mice lacking the dopamine D(1) or D(2) receptor subtypes. Activity of the glutamine cycle was evaluated by using (13)C NMR spectroscopy, and striatal activity was assessed by c-Fos expression and motor coordination. Brain extracts from (1,2-(13)C(2)) acetate-infused mice were prepared and analyzed by (13)C NMR to determine the incorporation of the label into the C4 and C5 carbons of glutamate and glutamine. D(1)R(-/-) mice showed a significantly higher concentration of cerebral (4,5-(13)C(2)) glutamine, consistent with an increased activity of the glutamate-glutamine cycle and of glutamatergic neurotransmission. Conversely, D(2)R(-/-) mice did not show any significant changes in (4,5-(13)C(2)) glutamate or (4,5-(13)C(2)) glutamine, suggesting that alterations in glutamine metabolism are mediated through D(1) receptors. This was confirmed with D(1)R(-/-) and WT mice treated with reserpine, a dopamine-depleting drug, or with reserpine followed by L-DOPA, a dopamine precursor. Exposure to reserpine increased (4,5-(13)C(2)) glutamine in WT to levels similar to those found in untreated D(1)R(-/-) mice. These values were the same as those reached in the reserpine-treated D(1)R(-/-) mice. Treatment of WT animals with L-DOPA returned (4,5-(13)C(2)) glutamine levels to normal, but this was not verified in D(1)R(-/-) animals. Reserpine impaired motor coordination and decreased c-Fos expression, whereas L-DOPA restored both variables to normal values in WT but not in D(1)R(-/-). Together, our results reveal novel neurometabolic interactions between glutamatergic and dopaminergic systems that are mediated through the D(1), but not the D(2), dopamine receptor subtype.

Altered corticostriatal neurotransmission and modulation in dopamine transporter knock-down mice

Dopamine (DA) modulates glutamate neurotransmission in the striatum. Abnormal DA modulation has been implicated in neurological and psychiatric disorders. The development of DA transporter knock-down (DAT-KD) mice has permitted modeling of these disorders and has shed new light on DA modulation. DAT-KD mice exhibit increased extracellular DA, hyperactivity, and alterations in habituation. We used whole cell patch-clamp recordings from visually identified striatal neurons in slices to examine the effects of DAT-KD on corticostriatal transmission. Electrophysiological recordings from medium-sized spiny neurons in the dorsal striatum revealed alterations in both amplitude and frequency, of spontaneous glutamate receptor-mediated synaptic currents in cells from DAT-KD mice. Furthermore, kinetic analyses revealed that these currents had shorter half-amplitude durations and faster decay times. In contrast, GABA-receptor-mediated synaptic currents were not altered. Striatal neurons from DAT-KD mice also responded differently to amphetamine, cocaine, and DA D2-receptor agonists or antagonists compared with wildtype (WT) littermate controls. In WTs amphetamine and cocaine reduced the frequency of spontaneous glutamate currents and these effects appeared to be mediated by activation of D2 receptors. In contrast, in DAT-KD mice either no changes or only small increases in frequency occurred. D2-receptor agonists or antagonists also had opposing effects in WT and DAT-KD mice. Together, these results indicate that chronically increased extracellular DA produces long-lasting changes in corticostriatal communication that may be mediated by changes in D2-receptor function. These findings have implications for understanding mechanisms underlying attention deficit hyperactivity disorder and Tourette's syndrome and may provide insights into novel therapeutic approaches.

Murine models of inherited monoaminergic and GABAergic neurotransmitter disorders

Monoamine and amino acid neurotransmitters perform diverse biological functions in mammals, including the regulation of inhibitory/excitatory neurotransmission in the brain and spinal cord, movement and sleep, autonomic function, mood and reward, and numerous other processes. The primary transmitters involved include dopamine, serotonin, epinephrine, norepinephrine and γ-aminobutyric acid (GABA). With the exception of the amino acid transmitter GABA, the cofactor integrating these systems is tetrahydrobiopterin, an oxidizable intermediate found in high concentrations in dopaminergic neurons. With growing awareness of the clinical phenotypes, expanding numbers of patients with monoaminergic and GABAergic neurotransmitter disorders are being identified. For some people, therapeutic intervention demonstrates remarkably positive benefits; conversely, for most other disorders therapy offers limited efficacy. Decoding of the complete mouse genome, coupled with methodology capable of ablating specific genes, has revolutionized how geneticists understand and treat human genetic disease. This is well-exemplified in the disorders covered in this review, which focuses predominantly on monoaminergic (tetrahydrobiopterin-dependent) and GABAergic signaling neurotransmitter disorders.

Dopamine-independent locomotor actions of amphetamines in a novel acute mouse model of Parkinson disease

Brain dopamine is critically involved in movement control, and its deficiency is the primary cause of motor symptoms in Parkinson disease. Here we report development of an animal model of acute severe dopamine deficiency by using mice lacking the dopamine transporter. In the absence of transporter-mediated recycling mechanisms, dopamine levels become entirely dependent on de novo synthesis. Acute pharmacological inhibition of dopamine synthesis in these mice induces transient elimination of striatal dopamine accompanied by the development of a striking behavioral phenotype manifested as severe akinesia, rigidity, tremor, and ptosis. This phenotype can be reversed by administration of the dopamine precursor, L-DOPA, or by nonselective dopamine agonists. Surprisingly, several amphetamine derivatives were also effective in reversing these behavioral abnormalities in a dopamine-independent manner. Identification of dopamine transporter- and dopamine-independent locomotor actions of amphetamines suggests a novel paradigm in the search for prospective anti-Parkinsonian drugs.

Identification of dopamine transporter- and dopamine- independent locomotor actions of amphetamines suggests a novel paradigm in the search for prospective anti-Parkinsonian drugs.

Foundation Review: Transgenic animals and their impact on the drug discovery industry

The ability to direct genetic changes at the molecular level has resulted in a revolution in biology. Nowhere has this been more apparent than in the production of transgenic animals. Transgenic technology lies at the junction of several enabling techniques in such diverse fields as embryology, cell biology and molecular genetics. A host of techniques have been used to effect change in gene expression and develop new pharmaceutical and nutraceutical compounds cost-effectively. Scientific advances gained by transgenic capabilities enable further understanding of basic biological pathways and yield insights into how changes in fundamental processes can perturb programmed development or culminate in disease pathogenesis.