Presynaptic dopamine autoreceptors and second messengers controlling tyrosine hydroxylase activity in rat brain

α-Synuclein-induced dysregulation of neuronal activity contributes to murine dopamine neuron vulnerability

Pathophysiological damages and loss of function of dopamine neurons precede their demise and contribute to the early phases of Parkinson's disease. The presence of aberrant intracellular pathological inclusions of the protein α-synuclein within ventral midbrain dopaminergic neurons is one of the cardinal features of Parkinson's disease. We employed molecular biology, electrophysiology, and live-cell imaging to investigate how excessive α-synuclein expression alters multiple characteristics of dopaminergic neuronal dynamics and dopamine transmission in cultured dopamine neurons conditionally expressing GCaMP6f. We found that overexpression of α-synuclein in mouse (male and female) dopaminergic neurons altered neuronal firing properties, calcium dynamics, dopamine release, protein expression, and morphology. Moreover, prolonged exposure to the D2 receptor agonist, quinpirole, rescues many of the alterations induced by α-synuclein overexpression. These studies demonstrate that α-synuclein dysregulation of neuronal activity contributes to the vulnerability of dopaminergic neurons and that modulation of D2 receptor activity can ameliorate the pathophysiology. These findings provide mechanistic insights into the insidious changes in dopaminergic neuronal activity and neuronal loss that characterize Parkinson's disease progression with significant therapeutic implications.

Unique Effects of Acute Aripiprazole Treatment on the Dopamine D2 Receptor Downstream cAMP-PKA and Akt-GSK3β Signalling Pathways in Rats

Aripiprazole is a wide-used antipsychotic drug with therapeutic effects on both positive and negative symptoms of schizophrenia, and reduced side-effects. Although aripiprazole was developed as a dopamine D₂ receptor (D₂R) partial agonist, all other D₂R partial agonists that aimed to mimic aripiprazole failed to exert therapeutic effects in clinic. The present in vivo study aimed to investigate the effects of aripiprazole on the D₂R downstream cAMP-PKA and Akt-GSK3β signalling pathways in comparison with a D₂R antagonist – haloperidol and a D₂R partial agonist – bifeprunox. Rats were injected once with aripiprazole (0.75mg/kg, i.p.), bifeprunox (0.8mg/kg, i.p.), haloperidol (0.1mg/kg, i.p.) or vehicle. Five brain regions – the prefrontal cortex (PFC), nucleus accumbens (NAc), caudate putamen (CPu), ventral tegmental area (VTA) and substantia nigra (SN) were collected. The protein levels of PKA, Akt and GSK3β were measured by Western Blotting; the cAMP levels were examined by ELISA tests. The results showed that aripiprazole presented similar acute effects on PKA expression to haloperidol, but not bifeprunox, in the CPU and VTA. Additionally, aripiprazole was able to increase the phosphorylation of GSK3β in the PFC, NAc, CPu and SN, respectively, which cannot be achieved by bifeprunox and haloperidol. These results suggested that acute treatment of aripiprazole had differential effects on the cAMP-PKA and Akt-GSK3β signalling pathways from haloperidol and bifeprunox in these brain areas. This study further indicated that, by comparison with bifeprunox, the unique pharmacological profile of aripiprazole may be attributed to the relatively lower intrinsic activity at D₂R.

Neurotoxicity following acute inhalation exposure to the oil dispersant COREXIT EC9500A

Consequent to the 2010 Deepwater Horizon oil spill in the Gulf of Mexico, there is an emergent concern about the short- and long-term adverse health effects of exposure to crude oil, weathered-oil products, and oil dispersants among the workforce employed to contain and clean up the spill. Oil dispersants typically comprise of a mixture of solvents and surfactants that break down floating oil to micrometer-sized droplets within the water column, thus preventing it from reaching the shorelines. As dispersants are generally sprayed from the air, workers are at risk for exposure primarily via inhalation. Such inhaled fractions might potentially permeate or translocate to the brain via olfactory or systemic circulation, producing central nervous system (CNS) abnormalities. To determine whether oil dispersants pose a neurological risk, male Sprague-Dawley rats were exposed by whole-body inhalation exposure to a model oil dispersant, COREXIT EC9500A (CE; approximately 27 mg/m(3) × 5 h/d × 1 d), and various molecular indices of neural dysfunction were evaluated in discrete brain areas, at 1 or 7 d postexposure. Exposure to CE produced partial loss of olfactory marker protein in the olfactory bulb. CE also reduced tyrosine hydroxylase protein content in the striatum. Further, CE altered the levels of various synaptic and neuronal intermediate filament proteins in specific brain areas. Reactive astrogliosis, as evidenced by increased expression of glial fibrillary acidic protein, was observed in the hippocampus and frontal cortex following exposure to CE. Collectively, these findings are suggestive of disruptions in olfactory signal transduction, axonal function, and synaptic vesicle fusion, events that potentially result in an imbalance in neurotransmitter signaling. Whether such acute molecular aberrations might persist and produce chronic neurological deficits remains to be ascertained.

Dopamine transporter cell surface localization facilitated by a direct interaction with the dopamine D2 receptor

Altered synaptic dopamine levels have been implicated in several neurological/neuropsychiatric disorders, including drug addiction and schizophrenia. However, it is unclear what precipitates these changes in synaptic dopamine levels. One of the key presynaptic components involved in regulating dopaminergic tone is the dopamine transporter (DAT). Here, we report that the DAT is also regulated by the dopamine D2 receptor through a direct protein-protein interaction involving the DAT amino-terminus and the third intracellular loop of the D2 receptor. This physical coupling facilitates the recruitment of intracellular DAT to the plasma membrane and leads to enhanced dopamine reuptake. Moreover, mice injected with peptides that disrupt D2-DAT interaction exhibit decreased synaptosomal dopamine uptake and significantly increased locomotor activity, reminiscent of DAT knockout mice. Our data highlight a novel mechanism through which neurotransmitter receptors can functionally modulate neurotransmitter transporters, an interaction that can affect the synaptic neurotransmitter levels in the brain.

Clozapine Modulates Aromatic l-Amino Acid Decarboxylase Activity in Mouse Striatum

Clozapine is efficacious for treating dopaminergic psychosis in Parkinson's disease and ameliorates l-DOPA-induced motor complications. Based on its pharmacology and reported enhancing effects on dopamine metabolism and tyrosine hydroxylase activity, we investigated whether it could modulate the activity of aromatic l-amino acid decarboxylase (AAAD), the second enzyme for the biosynthesis of catecholamines and indoleamines. A single dose of clozapine increased AAAD activity of striatum in a dose- and time-dependent manner. At 1 h, enhanced enzyme activity was characterized by an increased V(max) for substrate and cofactor and was accompanied by elevated levels of protein in striatum and mRNA in substantia nigra, ventral tegmental area, locus coeruleus, and raphe nuclei. Acute clozapine increased tyrosine hydroxylase activity in striatum but with differing temporal patterns from AAAD and heightened dopamine metabolism. Interestingly, the response of the dopaminergic markers to clozapine was greater following a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) lesion. Chronically administered clozapine increased AAAD activity and protein and dopamine metabolism in striatum without affecting tyrosine hydroxylase. Exogenous l-DOPA decarboxylation was accelerated in the striatum of intact and MPTP-lesioned mice following acute clozapine, and the effect was exaggerated in the MPTP mice. To identify receptors involved, antagonists of receptors occupied by clozapine were employed. D4, 5-HT1(A), and 5-HT2(A), in addition to D1, D2, and D3, antagonists, augmented AAAD activity in striatum, whereas 5-HT2(C), 5-HT3, muscarinic, and alpha-1 and alpha-2 adrenergic antagonists were ineffective. For the first time, these studies provide evidence that clozapine modulates AAAD activity in the brain and suggests that dopamine and serotonin receptors are involved.

Lack of induction of substance P gene expression by hypoxia and absence of neurokinin 1-receptor mRNAs in the rat carotid body

Peripheral chemoreceptors are commonly thought to respond to hypoxia by releasing neurotransmitters from the type 1 cells of the carotid body; these molecules then bind to post-synaptic receptors on the carotid sinus nerve. The tachykinin substance P (SP) may act as an important neurotransmitter/neuromodulator in hypoxic chemotransmission in peripheral arterial chemoreceptors. In order to elucidate the role of SP in modulating hypoxic chemotransmission, we have used quantitative in situ hybridization histochemistry, to determine the effect of hypoxia on SP gene induction, and the localization of neurokinin 1 (NK-1) receptor mRNA in the carotid body and petrosal ganglia complex in rats at 21 days post-natal age. For comparison, we also determined: (1) the effect of hypoxia on tyrosine hydroxylase (TH) gene induction and (2) the localization of the mRNA encoding the D2-dopamine receptor. SP mRNA was not detected in the rat carotid body during normoxia and its expression was not induced after a 1 h of exposure to hypoxia (10% O2/90% N2), a stimulus that was sufficient to cause a significant increase (P < 0.01) in TH mRNA levels in the carotid body. Both SP and TH mRNAs were abundantly expressed in multiple cells in the petrosal and the jugular ganglia. However, these mRNAs were not co-localized and SP and TH mRNA levels were not affected by hypoxia in these ganglia. Although D2-dopamine receptor mRNA was abundantly expressed in the rat carotid body, we found no evidence of NK-1 receptor mRNA in the carotid body. In contrast, both NK-1 receptor mRNA and D2-dopamine receptor mRNA were present in petrosal ganglion cells. In the rat, SP does not appear to modulate hypoxic chemotransmission by being made in and released from type 1 cells in the carotid body, and neither does SP modulate the activity of type 1 cells by binding to NK-1 receptors on these cells.

Developmental expression of tyrosine hydroxylase, D2-dopamine receptor and substance P genes in the carotid body of the rat

Alterations in the level of putative neurotransmitters/neuromodulators and corresponding receptors may be a possible mechanism involved in changes in chemosensitivity of peripheral chemoreceptors in the carotid body during development. Using quantitative in situ hybridization histochemistry, levels of messenger RNAs encoding tyrosine hydroxylase, the rate-limiting enzyme for dopamine synthesis, the D2-dopamine receptor and substance P of newborn rats at postnatal days 0, 2, 14 and 21 were determined. For comparison, during the same time points during development, we also determined the level of expression of these messenger RNAs in the cells of the superior cervical ganglion which are not chemosensitive. Tyrosine hydroxylase and D2-dopamine receptor messenger RNAs were co-localized in many of the cells in both the carotid body and the superior cervical ganglion. In the carotid body, the level of tyrosine hydroxylase messenger RNA expression was greatest at birth, significantly decreased by 48 h postnatal age and remained decreased at 14 and 21 postnatal days. In contrast, D2-dopamine receptor messenger RNA levels significantly increased with postnatal age in the carotid body. This profile of an D2-dopamine receptor was not observed in the superior cervical ganglion where tyrosine hydroxylase and D2-dopamine receptor messenger RNAs levels did not significantly change from postnatal days 0 to 21. Lastly, in the rat carotid body, substance P messenger RNA was not detected. However, substance P messenger RNA was abundant in the nodose and petrosal ganglion. The increasing contribution of carotid body on ventilation with increasing postnatal age is associated with changes in levels of gene expression for tyrosine hydroxylase and D2-dopamine receptor in the carotid body.