Dopamine inhibits mammalian photoreceptor Na+,K+-ATPase activity via a selective effect on the alpha3 isozyme

Illuminating and Sniffing Out the Neuromodulatory Roles of Dopamine in the Retina and Olfactory Bulb

In the central nervous system, dopamine is well-known as the neuromodulator that is involved with regulating reward, addiction, motivation, and fine motor control. Yet, decades of findings are revealing another crucial function of dopamine: modulating sensory systems. Dopamine is endogenous to subsets of neurons in the retina and olfactory bulb (OB), where it sharpens sensory processing of visual and olfactory information. For example, dopamine modulation allows the neural circuity in the retina to transition from processing dim light to daylight and the neural circuity in the OB to regulate odor discrimination and detection. Dopamine accomplishes these tasks through numerous, complex mechanisms in both neural structures. In this review, we provide an overview of the established and emerging research on these mechanisms and describe similarities and differences in dopamine expression and modulation of synaptic transmission in the retinas and OBs of various vertebrate organisms. This includes discussion of dopamine neurons’ morphologies, potential identities, and biophysical properties along with their contributions to circadian rhythms and stimulus-driven synthesis, activation, and release of dopamine. As dysregulation of some of these mechanisms may occur in patients with Parkinson’s disease, these symptoms are also discussed. The exploration and comparison of these two separate dopamine populations shows just how remarkably similar the retina and OB are, even though they are functionally distinct. It also shows that the modulatory properties of dopamine neurons are just as important to vision and olfaction as they are to motor coordination and neuropsychiatric/neurodegenerative conditions, thus, we hope this review encourages further research to elucidate these mechanisms.

Cell biology and dynamics of Neuronal Na+/K+-ATPase in health and diseases

Neuronal Na⁺/K⁺-ATPase is responsible for the maintenance of ionic gradient across plasma membrane. In doing so, in a healthy brain, Na⁺/K⁺-ATPase activity accounts for nearly half of total brain energy consumption. The α3-subunit containing Na⁺/K⁺-ATPase expression is restricted to neurons. Heterozygous mutations within α3-subunit leads to Rapid-onset Dystonia Parkinsonism, Alternating Hemiplegia of Childhood and other neurological and neuropsychiatric disorders. Additionally, proteins such as α-synuclein, amyloid-β, tau and SOD1 whose aggregation is associated to neurodegenerative diseases directly bind and impair α3-Na⁺/K⁺-ATPase activity. The review will provide a summary of neuronal α3-Na⁺/K⁺-ATPase functional properties, expression pattern, protein-protein interactions at the plasma membrane, biophysical properties (distribution and lateral diffusion). Lastly, the role of α3-Na⁺/K⁺-ATPase in neurological and neurodegenerative disorders will be discussed. This article is part of the special issue entitled 'Mobility and trafficking of neuronal membrane proteins'.

Regulation of Dopamine Uptake by Vasoactive Peptides in the Kidney

Considering the key role of renal dopamine in tubular sodium handling, we hypothesized that c-type natriuretic peptide (CNP) and Ang-(1-7) may regulate renal dopamine availability in tubular cells, contributing to Na(+), K(+)-ATPase inhibition. Present results show that CNP did not affect either (3)H-dopamine uptake in renal tissue or Na(+), K(+)-ATPase activity; meanwhile, Ang-(1-7) was able to increase (3)H-dopamine uptake and decreased Na(+), K(+)-ATPase activity in renal cortex. Ang-(1-7) and dopamine together decreased further Na(+), K(+)-ATPase activity showing an additive effect on the sodium pump. In addition, hydrocortisone reversed Ang-(1-7)-dopamine overinhibition on the enzyme, suggesting that this inhibition is closely related to Ang-(1-7) stimulation on renal dopamine uptake. Both anantin and cANP (4-23-amide) did not modify CNP effects on (3)H-dopamine uptake by tubular cells. The Mas receptor antagonist, A-779, blocked the increase elicited by Ang-(1-7) on (3)H-dopamine uptake. The stimulatory uptake induced by Ang-(1-7) was even more pronounced in the presence of losartan, suggesting an inhibitory effect of Ang-(1-7) on AT1 receptors on (3)H-dopamine uptake. By increasing dopamine bioavailability in tubular cells, Ang-(1-7) enhances Na(+), K(+)-ATPase activity inhibition, contributing to its natriuretic and diuretic effects.

Bioelectrical function and structural assessment of the retina in patients with early stages of Parkinson's disease (PD)

Purpose

To determine bioelectrical function and structural changes of the retina in patients with early stages of Parkinson’s disease (PD).

Materials and methods

Thirty-eight eyes of 20 patients with early idiopathic PD and 38 eyes of 20 healthy age- and sex-matched controls were ophthalmologically examined, including assessment of distance best-corrected visual acuity (DBCVA), slit lamp examination of the anterior and posterior segment of the eye, evaluation of the eye structures: paramacular retinal thickness (RT) and retinal nerve fiber layer (RNFL) thickness with the aid of OCT, and the bioelectrical function by full-field electroretinogram (ERG). Additionally, PD patients were interviewed as to the presence of dopamine-dependent visual functions abnormalities.

Results

In patients with early PD, statistically significant changes in comparison with the control group were observed in ERG. They contained a reduction in mean amplitudes of the scotopic a-wave (rod–cone response), the scotopic oscillatory potentials (OPs)—OP2 and OP3, the photopic b-wave, and a reduction in the overall index (OP1 + OP2 + OP3) and a prolongation of mean peak times of the scotopic OP1, OP2, OP3, OP4 (p < 0.05). A questionnaire concerning abnormalities of dopamine-dependent visual functions revealed that PD patients with abnormal peak times of OP1, OP2, and OP3 reported non-specific visual disturbances more frequently in comparison with PD patients with normal peak times of OPs. Other analyzed parameters of ERG, DBCVA, RT, and RNFL did not significantly differ between patients with PD and the control group.

Conclusion

In patients with early PD, bioelectrical dysfunction of the retina was observed in the ERG test, probably as a result of dopamine deficiency in the retina. The results of our study indicate that ERG may also be a useful tool for understanding the reason for non-specific visual disturbances occurring in PD patients.

Retinal and visual system: occupational and environmental toxicology

Occupational chemical exposure often results in sensory systems alterations that occur without other clinical signs or symptoms. Approximately 3000 chemicals are toxic to the retina and central visual system. Their dysfunction can have immediate, long-term, and delayed effects on mental health, physical health, and performance and lead to increased occupational injuries. The aims of this chapter are fourfold. First, provide references on retinal/visual system structure, function, and assessment techniques. Second, discuss the retinal features that make it especially vulnerable to toxic chemicals. Third, review the clinical and corresponding experimental data regarding retinal/visual system deficits produced by occupational toxicants: organic solvents (carbon disulfide, trichloroethylene, tetrachloroethylene, styrene, toluene, and mixtures) and metals (inorganic lead, methyl mercury, and mercury vapor). Fourth, discuss occupational and environmental toxicants as risk factors for late-onset retinal diseases and degeneration. Overall, the toxicants altered color vision, rod- and/or cone-mediated electroretinograms, visual fields, spatial contrast sensitivity, and/or retinal thickness. The findings elucidate the importance of conducting multimodal noninvasive clinical, electrophysiologic, imaging and vision testing to monitor toxicant-exposed workers for possible retinal/visual system alterations. Finally, since the retina is a window into the brain, an increased awareness and understanding of retinal/visual system dysfunction should provide additional insight into acquired neurodegenerative disorders.

The rat with oxygen-induced retinopathy is myopic with low retinal dopamine

PURPOSE

Dopamine (DA) is a neurotransmitter implicated both in modulating neural retinal signals and in eye growth. Therefore, it may participate in the pathogenesis of the most common clinical sequelae of retinopathy of prematurity (ROP), visual dysfunction and myopia. Paradoxically, in ROP myopia the eye is usually small. The eye of the rat with oxygen-induced retinopathy (OIR) is characterized by retinal dysfunction and short axial length. There have been several investigations of the early maturation of DA in rat retina, but little at older ages, and not in the OIR rat. Therefore, DA, retinal function, and refractive state were investigated in the OIR rat.

METHODS

In one set of rats, the development of dopaminergic (DAergic) networks was evaluated in retinal cross-sections from rats aged 14 to 120 days using antibodies against tyrosine hydroxylase (TH, the rate-limiting enzyme in the biosynthesis of DA). In another set of rats, retinoscopy was used to evaluate spherical equivalent (SE), electoretinography (ERG) was used to evaluate retinal function, and high-pressure liquid chromatography (HPLC) was used to evaluate retinal contents of DA, its precursor levodopamine (DOPA), and its primary metabolite 3,4-dihydroxyphenylacetic acid (DOPAC).

RESULTS

The normally rapid postnatal ramification of DAergic neurons was disrupted in OIR rats. Retinoscopy revealed that OIR rats were relatively myopic. In the same eyes, ERG confirmed retinal dysfunction in OIR. HPLC of those eyes' retinae confirmed low DA. Regression analysis indicated that DA metabolism (evaluated by the ratio of DOPAC to DA) was an important additional predictor of myopia beyond OIR.

CONCLUSIONS

The OIR rat is the first known animal model of myopia in which the eye is smaller than normal. Dopamine may modulate, or fail to modulate, neural activity in the OIR eye, and thus contribute to this peculiar myopia.

Effects of strobe light stimulation on postnatal developing rat retina

The nature and intensity of visual stimuli have changed in recent years because of television and other dynamic light sources. Although light stimuli accompanied by contrast and strength changes are thought to have an influence on visual system development, little information is available on the effects of dynamic light stimuli such as a strobe light on visual system development. Thus, this study was designed to evaluate changes caused by dynamic light stimuli during retinal development. This study used 80 Sprague-Dawley rats. From eye opening (postnatal day 14), half of the rats were maintained on a daily 12-h light/dark cycle (control group) and the remaining animals were raised under a 12-h strobe light (2 Hz)/dark cycle (strobe light-reared group). Morphological analyses and electroretinogram (ERG) were performed at postnatal weeks 3, 4, 6, 8, and 10. Among retinal neurons, tyrosine hydroxylase-immunoreactive (TH-IR, dopaminergic amacrine cells) cells showed marked plastic changes, such as variations in numbers and soma sizes. In whole-mount preparations at 6, 8, and 10 weeks, type I TH-IR cells showed a decreased number and larger somata, while type II TH-IR cells showed an increased number in strobe-reared animals. Functional assessment by scotopic ERG showed that a-wave and b-wave amplitudes increased at 6 and 8 weeks in strobe-reared animals. These results show that exposure to a strobe light during development causes changes in TH-IR cell number and morphology, leading to a disturbance in normal visual functions.

Na(+),K (+)-ATPase as a docking station: protein-protein complexes of the Na(+),K (+)-ATPase

The Na(+),K(+)-ATPase, or sodium pump, is well known for its role in ion transport across the plasma membrane of animal cells. It carries out the transport of Na(+) ions out of the cell and of K(+) ions into the cell and thus maintains electrolyte and fluid balance. In addition to the fundamental ion-pumping function of the Na(+),K(+)-ATPase, recent work has suggested additional roles for Na(+),K(+)-ATPase in signal transduction and biomembrane structure. Several signaling pathways have been found to involve Na(+),K(+)-ATPase, which serves as a docking station for a fast-growing number of protein interaction partners. In this review, we focus on Na(+),K(+)-ATPase as a signal transducer, but also briefly discuss other Na(+),K(+)-ATPase protein-protein interactions, providing a comprehensive overview of the diverse signaling functions ascribed to this well-known enzyme.

Bcl-xL-mediated remodeling of rod and cone synaptic mitochondria after postnatal lead exposure: electron microscopy, tomography and oxygen consumption

PURPOSE

Postnatal lead exposure produces rod-selective and Bax-mediated apoptosis, decreased scotopic electroretinograms (ERGs), and scotopic and mesopic vision deficits in humans and/or experimental animals. Rod, but not cone, inner segment mitochondria were considered the primary site of action. However, photoreceptor synaptic mitochondria were not examined. Thus, our experiments investigated the structural and functional effects of environmentally relevant postnatal lead exposure on rod spherule and cone pedicle mitochondria and whether Bcl-xL overexpression provided neuroprotection.

METHODS

C57BL/6N mice pups were exposed to lead only during lactation via dams drinking water containing lead acetate. The blood [Pb] at weaning was 20.6±4.7 µg/dl, which decreased to the control value by 2 months. To assess synaptic mitochondrial structural differences and vulnerability to lead exposure, wild-type and transgenic mice overexpressing Bcl-xL in photoreceptors were used. Electron microscopy, three-dimensional electron tomography, and retinal and photoreceptor synaptic terminal oxygen consumption (QO(2)) studies were conducted in adult control, Bcl-xL, lead, and Bcl-xL/lead mice.

RESULTS

The spherule and pedicle mitochondria in lead-treated mice were swollen, and the cristae structure was markedly changed. In the lead-treated mice, the mitochondrial cristae surface area and volume (abundance: measure correlated with ATP (ATP) synthesis) were decreased in the spherules and increased in the pedicles. Pedicles also had an increased number of crista segments per volume. In the lead-treated mice, the number of segments/crista and fraction of cristae with multiple segments (branching) similarly increased in spherule and pedicle mitochondria. Lead-induced remodeling of spherule mitochondria produced smaller cristae with more branching, whereas pedicle mitochondria had larger cristae with more branching and increased crista junction (CJ) diameter. Lead decreased dark- and light-adapted photoreceptor and dark-adapted photoreceptor synaptic terminal QO(2). Bcl-xL partially blocked many of the lead-induced alterations relative to controls. However, spherules still had partially decreased abundance, whereas pedicles still had increased branching, increased crista segments per volume, and increased crista junction diameter. Moreover, photoreceptor and synaptic QO(2) were only partially recovered.

CONCLUSIONS

These findings reveal cellular and compartmental specific differences in the structure and vulnerability of rod and cone inner segment and synaptic mitochondria to postnatal lead exposure. Spherule and pedicle mitochondria in lead-exposed mice displayed complex and distinguishing patterns of cristae and matrix damage and remodeling consistent with studies showing that synaptic mitochondria are more sensitive to Ca(2+) overload, oxidative stress, and ATP loss than non-synaptic mitochondria. The lead-induced decreases in QO(2) likely resulted from the decreased spherule cristae abundance and smaller cristae, perhaps due to Bax-mediated effects as they occurred in apoptotic rod inner segments. The increase in pedicle cristae abundance and CJ diameter could have resulted from increased Drp1-mediated fission, as small mitochondrial fragments were observed. The mechanisms of Bcl-xL-mediated remodeling might occur via interaction with formation of CJ protein 1 (Fcj1), whereas the partial protection of synaptic QO(2) might result from the enhanced efficiency of energy metabolism via Bcl-xL's direct interaction with the F1F0 ATP synthase and/or regulation of cellular redox status. These lead-induced alterations in photoreceptor synaptic terminal mitochondria likely underlie the persistent scotopic and mesopic deficits in lead-exposed children, workers, and experimental animals. Our findings stress the clinical and scientific importance of examining synaptic dysfunction following injury or disease during development, and developing therapeutic treatments that prevent synaptic degeneration in retinal and neurodegenerative disorders even when apoptosis is blocked.

Dopamine D₄ receptor activation controls circadian timing of the adenylyl cyclase 1/cyclic AMP signaling system in mouse retina

In the mammalian retina, dopamine binding to the dopamine D₄ receptor (D₄R) affects a light-sensitive pool of cyclic AMP by negatively coupling to the type 1 adenylyl cyclase (AC1). AC1 is the primary enzyme controlling cyclic AMP production in dark-adapted photoreceptors. A previous study demonstrated that expression of the gene encoding AC1, Adcy1, is downregulated in mice lacking Drd4, the gene encoding the D₄R. The present investigation provides evidence that D₄R activation entrains the circadian rhythm of Adcy1 mRNA expression. Diurnal and circadian rhythms of Drd4 and Adcy1 mRNA levels were observed in wild-type mouse retina. Also, rhythms in the Ca²⁺-stimulated AC activity and cyclic AMP levels were observed. However, these rhythmic activities were damped or undetectable in mice lacking the D₄R. Pharmacologically activating the D₄R 4 h before its normal stimulation at light onset in the morning advances the phase of the Adcy1 mRNA expression pattern. These data demonstrate that stimulating the D₄R is essential in maintaining the normal rhythmic production of AC1 from transcript to enzyme activity. Thus, dopamine/D₄R signaling is a novel zeitgeber that entrains the rhythm of Adcy1 expression and, consequently, modulates the rhythmic synthesis of cyclic AMP in mouse retina.

Gestational lead exposure selectively decreases retinal dopamine amacrine cells and dopamine content in adult mice

Gestational lead exposure (GLE) produces supernormal scotopic electroretinograms (ERG) in children, monkeys and rats, and a novel retinal phenotype characterized by an increased number of rod photoreceptors and bipolar cells in adult mice and rats. Since the loss of dopaminergic amacrine cells (DA ACs) in GLE monkeys and rats contributes to supernormal ERGs, the retinal DA system was analyzed in mice following GLE. C57BL/6 female mice were exposed to low (27 ppm), moderate (55 ppm) or high (109 ppm) lead throughout gestation and until postnatal day 10 (PN10). Blood [Pb] in control, low-, moderate- and high-dose GLE was ≤ 1, ≤ 10, ~25 and ~40 μg/dL, respectively, on PN10 and by PN30 all were ≤ 1 μg/dL. At PN60, confocal-stereology studies used vertical sections and wholemounts to characterize tyrosine hydroxylase (TH) expression and the number of DA and other ACs. GLE dose-dependently and selectively decreased the number of TH-immunoreactive (IR) DA ACs and their synaptic plexus without affecting GABAergic, glycinergic or cholinergic ACs. Immunoblots and confocal revealed dose-dependent decreases in retinal TH protein expression and content, although monoamine oxidase-A protein and gene expression were unchanged. High-pressure liquid chromatography showed that GLE dose-dependently decreased retinal DA content, its metabolites and DA utilization/release. The mechanism of DA selective vulnerability is unknown. However, a GLE-induced loss/dysfunction of DA ACs during development could increase the number of rods and bipolar cells since DA helps regulate neuronal proliferation, whereas during adulthood it could produce ERG supernormality as well as altered circadian rhythms, dark/light adaptation and spatial contrast sensitivity.

Light-dependent translocation of arrestin in rod photoreceptors is signaled through a phospholipase C cascade and requires ATP

Partitioning of cellular components is a critical mechanism by which cells can regulate their activity. In rod photoreceptors, light induces a large-scale translocation of arrestin from the inner segments to the outer segments. The purpose of this project is to elucidate the signaling pathway necessary to initiate arrestin translocation to the outer segments and the mechanism for arrestin translocation. Mouse retinal organotypic cultures and eyes from transgenic Xenopus tadpoles expressing a fusion of GFP and rod arrestin were treated with both activators and inhibitors of proteins in the phosphoinositide pathway. Confocal microscopy was used to image the effects of the pharmacological agents on arrestin translocation in rod photoreceptors. Retinas were also depleted of ATP using potassium cyanide to assess the requirement for ATP in arrestin translocation. In this study, we demonstrate that components of the G-protein-linked phospholipase C (PLC) pathway play a role in initiating arrestin translocation. Our results show that arrestin translocation can be stimulated by activators of PLC and protein kinase C (PKC), and by cholera toxin in the absence of light. Arrestin translocation to the outer segments is significantly reduced by inhibitors of PLC and PKC. Importantly, we find that treatment with potassium cyanide inhibits arrestin translocation in response to light. Collectively, our results suggest that arrestin translocation is initiated by a G-protein-coupled cascade through PLC and PKC signaling. Furthermore, our results demonstrate that at least the initiation of arrestin translocation requires energy input.

Essential roles of dopamine D4 receptors and the type 1 adenylyl cyclase in photic control of cyclic AMP in photoreceptor cells

Light and dopamine regulate many physiological functions in the vertebrate retina. Light exposure decreases cyclic AMP formation in photoreceptor cells. Dopamine D(4) receptor (D(4)R) activation promotes light adaptation and suppresses the light-sensitive pool of cyclic AMP in photoreceptor cells. The key signaling pathways involved in regulating cyclic AMP in photoreceptor cells have not been identified. In the present study, we show that the light- and D(4)R-signaling pathways converge on the type 1 Ca(2+)/calmodulin-stimulated adenylyl cyclase (AC1) to regulate cyclic AMP synthesis in photoreceptor cells. In addition, we present evidence that D(4)R activation tonically regulates the expression of AC1 in photoreceptors. In retinas of mice with targeted deletion of the gene (Adcy1) encoding AC1, cyclic AMP levels and Ca(2+)/calmodulin-stimulated adenylyl cyclase activity are markedly reduced, and cyclic AMP accumulation is unaffected by either light or D(4)R activation. Similarly, in mice with disruption of the gene (Drd4) encoding D(4)R, cyclic AMP levels in the dark-adapted retina are significantly lower compared to wild-type retina and are unresponsive to light. These changes in Drd4-/- mice were accompanied by significantly lower Adcy1 mRNA levels in photoreceptor cells and lower Ca(2+)/calmodulin-stimulated adenylyl cyclase activity in retinal membranes compared with wild-type controls. Reduced levels of Adcy1 mRNA were also observed in retinas of wild-type mice treated chronically with a D(4)R antagonist, L-745870. Thus, activation of D(4)R is required for normal expression of AC1 and for the regulation of its catalytic activity by light. These observations illustrate a novel mechanism for cross-talk between dopamine and photic signaling pathways regulating cyclic AMP in photoreceptor cells.

Low-level human equivalent gestational lead exposure produces supernormal scotopic electroretinograms, increased retinal neurogenesis, and decreased retinal dopamine utilization in rats

BACKGROUND

Postnatal lead exposure in children and animals produces alterations in the visual system primarily characterized by decreases in the rod-mediated (scotopic) electroretinogram (ERG) amplitude (subnormality). In contrast, low-level gestational Pb exposure (GLE) increases the amplitude of scotopic ERGs in children (supernormality).

OBJECTIVES

The goal of this study was to establish a rat model of human equivalent GLE and to determine dose-response effects on scotopic ERGs and on retinal morphology, biochemistry, and dopamine metabolism in adult offspring.

METHODS

We exposed female Long-Evans hooded rats to water containing 0, 27 (low), 55 (moderate), or 109 (high) ppm of Pb beginning 2 weeks before mating, throughout gestation, and until postnatal day (PND) 10. We measured maternal and litter indices, blood Pb concentrations (BPb), retinal Pb concentrations, zinc concentrations, and body weights. On PND90, we performed the retinal experiments.

RESULTS

Peak BPb concentrations were < 1, 12, 24, and 46 microg/dL in control, low-, moderate- and high-level GLE groups, respectively, at PNDs 0-10. ERG supernormality and an increased rod photoreceptor and rod bipolar cell neurogenesis occurred with low- and moderate-level GLE. In contrast, high-level GLE produced ERG subnormality, rod cell loss, and decreased retinal Zn levels. GLE produced dose-dependent decreases in dopamine and its utilization.

CONCLUSIONS

Low- and moderate-level GLE produced persistent scotopic ERG supernormality due to an increased neurogenesis of cells in the rod signaling pathway and/or decreased dopamine utilization, whereas high-level GLE produced rod-selective toxicity characterized by ERG subnormality. The ERG is a differential and noninvasive biomarker of GLE. The inverted U-shaped dose-response curves reveal the sensitivity and vulnerability of the developing retina to GLE.

Ionic dysregulatory phenotyping of pathologic retinal thinning with manganese-enhanced MRI

PURPOSE

To test the hypothesis that manganese-enhanced MRI (MEMRI) provides a sensitive and robust measure of an important retinal ionic dysregulatory phenotype in pathologic retinal thinning.

METHODS

Four hours after intraperitoneal MnCl(2) injection, high-resolution MEMRI data were collected from overnight dark-adapted male control Sprague-Dawley and albino Royal College of Surgeons rats before (at development stage postnatal day [P] 17) and during photoreceptor degeneration (P36 and P57). In separate experiments, control rats, with and without repetitive hypoxic preconditioning, were subjected to high IOP (100 mm Hg) for 60 minutes followed by 24 hours or 7 days of reperfusion (e.g., ischemia/reperfusion). Central retinal thickness and intraretinal ion activity were measured from the MEMRI data. Histology examination was also performed to confirm retinal damage.

RESULTS

In two different neurodegenerative models, MEMRI revealed first-time evidence for changes (P < 0.05) in intraretinal ion regulation before and during pathologic, but not (P > 0.05) developmental, retinal thinning. This phenotype was significantly altered by a neuroprotective repetitive hypoxic preconditioning protocol.

CONCLUSIONS

MEMRI and a nontoxic systemic dose of MnCl(2) provided an objective, noninvasive measure of an ionic deregulatory phenotype that appears useful for improved early diagnosis and treatment prognosis in a range of neurodegenerative diseases and their treatment.

Dopamine D4 receptors regulate intracellular calcium concentration in cultured chicken cone photoreceptor cells: relationship to dopamine receptor-mediated inhibition of cAMP formation

Dopamine is a retinal neuromodulator secreted from amacrine and interplexiform cells. Activation of dopamine D4 receptors on photoreceptor cells reduces a light-sensitive pool of cAMP. The aim of the present study was to evaluate the role of dopamine receptors and cAMP in the regulation of intracellular Ca(2+) concentrations ([Ca(2+)](i)) in photoreceptor cells of chick retina. Retinal cells from 6 day-old chicken embryos were isolated and cultured for 5-7 days prior to experiments. Cone photoreceptors were the predominant cell type in these cultures. Dopamine and agonists of dopamine D4 receptors suppressed K(+)-stimulated uptake of (45)Ca(2+) and [Ca(2+)](i), measured with the Ca(2+)-sensitive fluorescent dye fura-2AM. The effects of the agonists were blocked by dopamine D2/D4 receptor antagonists or by pertussis toxin. 8Br-cAMP, a cell-permeable analog of cAMP, had no effect on inhibition of K(+)-stimulated (45)Ca(2+) influx or [Ca(2+)](i) by dopamine D2/D4 receptor agonists. Quinpirole inhibited the increase in cAMP level elicited by K(+), which requires Ca(2+) influx through voltage-gated Ca(2+) channels, but not that induced by the calcium ionophore A23187. Moreover, dopamine had no effect on either forskolin-stimulated or Ca(2+)/calmodulin-stimulated adenylyl cyclase activity in cell membranes prepared from the cultured cells. These data indicate that the decrease of cAMP elicited by dopamine D4 receptor stimulation may be secondary to decreased [Ca(2+)](i).

Differential expression of Na+/K+-ATPase alpha-subunits in mouse hippocampal interneurones and pyramidal cells

The sodium pump (Na+/K+-ATPase), maintains intracellular and extracellular concentrations of sodium and potassium by catalysing ATP. Three sodium pump alpha subunits, ATP1A1, ATP1A2 and ATP1A3, are expressed in brain. We compared their role in pyramidal cells and a subset of interneurones in the subiculum. Interneurones were identified by their expression of GFP under the GAD-65 promoter. We used the sensitivity to the cardiac glycoside, ouabain, to discriminate between different alpha subunit isoforms. GFP-positive interneurones were depolarized by nanomolar doses of ouabain, but higher concentrations were needed to depolarize pyramidal cells. Comparison of pump currents in these cells revealed a current sensitive to low doses of ouabain in interneurones, while micromolar doses of ouabain were needed to suppress the pump current in subicular pyramidal cells. As predicted, nanomolar doses of ouabain increased the frequency but not the amplitudes of IPSPs in pyramidal cells. Immunostaining confirmed a differential distribution of alpha-subunits of the Na+/K+-ATPase in subicular interneurones and pyramidal cells. In conclusion, these data suggest that while ATP1A3-isoforms regulate sodium and potassium homeostasis in subicular interneurones, ATP1A1-isoforms assume this function in pyramidal cells. This differential expression of sodium pump isoforms may contribute to differences in resting membrane potential of subicular interneurones and pyramidal cells.

Spatiotemporal regulation of ATP and Ca2+ dynamics in vertebrate rod and cone ribbon synapses

PURPOSE

In conventional neurons, Ca2+ enters presynaptic terminals during an action potential and its increased local concentration triggers transient exocytosis. In contrast, vertebrate photoreceptors are nonspiking neurons that maintain sustained depolarization and neurotransmitter release from ribbon synapses in darkness and produce light-dependent graded hyperpolarizing responses. Rods transmit single photon responses with high fidelity, whereas cones are less sensitive and exhibit faster response kinetics. These differences are likely due to variations in presynaptic Ca2+ dynamics. Metabolic coupling and cross-talk between mitochondria, endoplasmic reticulum (ER), plasma membrane Ca2+ ATPase (PMCA), and Na+-Ca2+ exchanger (NCX) coordinately control presynaptic ATP production and Ca2+ dynamics. The goal of our structural and functional studies was to determine the spatiotemporal regulation of ATP and Ca2+ dynamics in rod spherules and cone pedicles.

METHODS

Central retina tissue from C57BL/6 mice was used. Laser scanning confocal microscopy (LSCM) experiments were conducted on fixed-frozen vertical sections. Primary antibodies were selected for their tissue/cellular specificity and ability to recognize single, multiple or all splice variants of selected isoforms. Electron microscopy (EM) and 3-D electron tomography (ET) studies used our standard procedures on thin- and thick-sectioned retinas, respectively. Calibrated fluo-3-Ca2+ imaging experiments of dark- and light-adapted rod and cone terminals in retinal slices were conducted.

RESULTS

Confocal microscopy showed that mitochondria, ER, PMCA, and NCX1 exhibited distinct retinal lamination patterns and differential distribution in photoreceptor synapses. Antibodies for three distinct mitochondrial compartments differentially labeled retinal areas with high metabolic demand: rod and cone inner segments, previously undescribed cone juxtanuclear mitochondria and the two plexiform layers. Rod spherule membranes uniformly and intensely stained for PMCA, whereas the larger cone pedicles preferentially stained for NCX1 at their active zones and PMCA near their mitochondria. EM and ET revealed that mitochondria in rod spherules and cone pedicles differed markedly in their number, location, size, volume, and total cristae surface area, and cristae junction diameter. Rod spherules had one large ovoid mitochondrion located near its active zone, whereas cone pedicles averaged five medium-sized mitochondria clustered far from their active zones. Most spherules had one ribbon synapse, whereas pedicles contained numerous ribbon synapses. Fluo-3 imaging studies revealed that during darkness rod spherules maintained a lower [Ca2+] than cone pedicles, whereas during light adaptation pedicles rapidly lowered their [Ca2+] below that observed in spherules.

CONCLUSIONS

These findings indicate that ATP demand and mitochondrial ATP production are greater in cone pedicles than rod spherules. Rod spherules employ high affinity/low turnover PMCA and their mitochondrion to maintain a relatively low [Ca2+] in darkness, which increases their sensitivity and signal-to-noise ratio. In contrast, cone pedicles utilize low affinity/high turnover NCX to rapidly lower their high [Ca2+] during light adaptation, which increases their response kinetics. Spatiotemporal fluo-3-Ca2+ imaging results support our immunocytochemical results. The clustering of cone pedicle mitochondria likely provides increased protection from Ca2+ overload and permeability transition. In summary, these novel studies reveal that several integrated cellular and subcellular components interact to regulate ATP and Ca2+ dynamics in rod and cone synaptic terminals. These results should provide a greater understanding of in vivo photoreceptor synaptic terminal exocytosis/endocytosis, Ca2+ overload and therapies for retinal degenerations.

Involvement of dopamine system in regulation of Na+,K+-ATPase in the striatum upon activation of opioid receptors by morphine

The striatum is believed to be a crucial brain region associated with drug reward. Adaptive alteration of neurochemistry in this area might be one potential mechanism underlying drug dependence. It has been proposed that the dysfunction of Na+,K+-ATPase function is involved in morphine tolerance and dependence. The present study, therefore, was undertaken to study the adaptation of the striatal Na+,K+-ATPase activity in response to morphine treatment. The results demonstrated that in vivo short-term morphine treatment stimulated Na+,K+-ATPase activity in a dose-dependent manner. This action could be significantly inhibited by D2-like dopamine receptor antagonist S(-)-3-chloro-5-ethyl-N-[(1-ethyl-2-pyrrolidinyl)methyl]-6-hydroxy-2-methoxybenzamine (eticlopride). Contrary to shortterm morphine treatment, long-term morphine treatment significantly suppressed Na+,K+-ATPase activity. This effect could be significantly inhibited by D(1)-like dopamine receptor antagonist R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH 23390). However, both short-term and long-term morphine treatment-induced changes in Na+,K+-ATPase activity could be reversed by opioid receptor antagonist naltrexone. It was further found that cAMP-dependent protein kinase (PKA) was crucially involved in regulating Na+,K+-ATPase activity by morphine. Different regulation of the phosphorylation levels of the alpha3 subunit of Na+,K+-ATPase by PKA was related to the distinct modulations of Na+,K+-ATPase by short-term and long-term morphine treatment. Short-term morphine treatment inhibited PKA activity and then decreased the phosphorylation of Na+,K+-ATPase, leading to increase in enzyme activity. These effects were sensitive to eticlopride or naltrexone. Conversely, long-term morphine treatment stimulated PKA activity and then increased the phosphorylation of Na+,K+-ATPase, leading to the reduction of enzyme activity. These effects were sensitive to SCH 23390 or naltrexone. These findings demonstrate that dopamine receptors are involved in regulation of Na+,K+-ATPase activity after activation of opioid receptors by morphine.

Circadian organization of the mammalian retina

The mammalian retina contains an endogenous circadian pacemaker that broadly regulates retinal physiology and function, yet the cellular origin and organization of the mammalian retinal circadian clock remains unclear. Circadian clock neurons generate daily rhythms via cell-autonomous autoregulatory clock gene networks, and, thus, to localize circadian clock neurons within the mammalian retina, we have studied the cell type-specific expression of six core circadian clock genes in individual, identified mouse retinal neurons, as well as characterized the clock gene expression rhythms in photoreceptor degenerate rd mouse retinas. Individual photoreceptors, horizontal, bipolar, dopaminergic (DA) amacrines, catecholaminergic (CA) amacrines, and ganglion neurons were identified either by morphology or by a tyrosine hydroxylase (TH) promoter-driven red fluorescent protein (RFP) fluorescent reporter. Cells were collected, and their transcriptomes were subjected to multiplex single-cell RT-PCR for the core clock genes Period (Per) 1 and 2, Cryptochrome (Cry) 1 and 2, Clock, and Bmal1. Individual horizontal, bipolar, DA, CA, and ganglion neurons, but not photoreceptors, were found to coordinately express all six core clock genes, with the lowest proportion of putative clock cells in photoreceptors (0%) and the highest proportion in DA neurons (30%). In addition, clock gene rhythms were found to persist for >25 days in isolated, cultured rd mouse retinas in which photoreceptors had degenerated. Our results indicate that multiple types of retinal neurons are potential circadian clock neurons that express key elements of the circadian autoregulatory gene network and that the inner nuclear and ganglion cell layers of the mammalian retina contain functionally autonomous circadian clocks.

Involvement of cAMP/cAMP-dependent protein kinase signaling pathway in regulation of Na+,K+-ATPase upon activation of opioid receptors by morphine

The depolarization of neurons induced by impairment of Na+,K+-ATPase activity after long-term opiate treatment has been shown to involve the development of opioid dependence. However, the mechanisms underlying changes in Na+,K+-ATPase activity after opioid treatment are unclear. The best-established molecular adaptation to long-term opioid exposure is up-regulation of the cAMP/cAMP-dependent protein kinase (PKA) signaling pathway; this study, therefore, was undertaken to investigate the role of up-regulation of cAMP/PKA signaling pathway in alteration of the mouse hippocampal Na+,K+-ATPase activity. The results demonstrated that short-term morphine treatment dose dependently stimulated Na+,K+-ATPase activity. This action could be significantly suppressed by adenylyl cyclase activator 7beta-acetoxy-8,13-epoxy-1alpha,6beta,9alpha-trihydroxylabd-14-en-11-one (forskolin), or the cAMP analog dibutyryl-cAMP. Contrary to short-term morphine treatment, long-term treatment significantly inhibited Na+,K+-ATPase activity. Moreover, an additional decrease in Na+,K+-ATPase activity was observed by naloxone precipitation. The effects of both short- and long-term morphine treatment on Na+,K+-ATPase activity were naltrexone-reversible. The regulation of Na+,K+-ATPase activity by morphine was inversely correlated with intracellular cAMP accumulation. N-[2-(4-Bromocinnamylamino)ethyl]-5-isoquinoline (H89), a specific PKA inhibitor, mimicked the stimulatory effect of short-term morphine but antagonized the inhibitory effect of long-term morphine treatment on Na+,K+-ATPase activity. However, okadaic acid, a protein phosphatase inhibitor, suppressed short-term morphine stimulation but potentiated long-term morphine inhibition of Na+,K+-ATPase activity. The regulation of Na+,K+-ATPase activity by morphine treatment seemed to associate with the alteration in phosphorylation level but not to be relevant to the change in abundance of Na+,K+-ATPase. These findings strongly demonstrate that cAMP/PKA signaling pathway involves regulation of Na+,K+-ATPase activity after activation of opioid receptors.

DARPP-32-like immunoreactivity in AII amacrine cells of rat retina

Previous studies demonstrated that the dopamine- and adenosine 3',5'-monophosphate-regulated phosphatase inhibitor known as "DARPP-32" is present in rat, cat, monkey, and human retinas. We have followed up these studies by asking what specific cell subtypes contain DARPP-32. Using a polyclonal antibody directed against a peptide sequence of human DARPP-32, we immunostained adult rat retinas that were either transretinally sectioned or flat mounted and found DARPP-32-like immunoreactivity in some cells of the amacrine cell layer across the entire retinal surface. We report here, based on the shape and spatial distribution of these cells, their staining by an anti-parvalbumin antibody, and their juxtaposition with processes containing tyrosine hydroxylase, that DARPP-32-like immunoreactivity is present in AII amacrine cells of rat retina. These results suggest that the response of AII amacrine cells to dopamine is not mediated as simply as previously supposed.

Na+, K+-ATPase: the new face of an old player in pathogenesis and apoptotic/hybrid cell death

The Na(+), K(+)-ATPase is a ubiquitous membrane transport protein in mammalian cells, responsible for establishing and maintaining high K(+) and low Na(+) in the cytoplasm required for normal resting membrane potentials and various cellular activities. The ionic homeostasis maintained by the Na(+), K(+)-ATPase is also critical for cell growth, differentiation, and cell survival. Although the toxic effects of blocking the Na(+), K(+)-ATPase by ouabain and other selective inhibitors have been known for years, the mechanism of action remained unclear. Recent progress in two areas has significantly advanced our understanding of the role and mechanism of Na(+), K(+)-ATPase in cell death. Along with increased recognition of apoptosis in a wide range of disease states, Na(+), K(+)-ATPase deficiency has been identified as a contributor to apoptosis and pathogenesis. More importantly, accumulating evidence now endorses a close relationship between ionic homeostasis and apoptosis, namely the regulation of apoptosis by K(+) homeostasis. Since Na(+), K(+)-ATPase is the primary system for K(+) uptake, dysfunction of the transport enzyme and resultant disruption of ionic homeostasis have been re-evaluated for their critical roles in apoptosis and apoptosis-related diseases. In this review, instead of giving a detailed description of the structure and regulation of Na(+), K(+)-ATPase, the author will focus on the most recent evidence indicating the unique role of Na(+), K(+)-ATPase in cell death, including apoptosis and the newly recognized "hybrid death" of concurrent apoptosis and necrosis in the same cells. It is also hoped that discussion of some seemingly conflicting reports will inspire further debate and benefit future investigation in this important research field.

Pharmacological strategies to block rod photoreceptor apoptosis caused by calcium overload: a mechanistic target-site approach to neuroprotection

PURPOSE

Photoreceptor apoptosis and resultant visual deficits occur in humans and animals with inherited, and disease-, injury- and chemical-induced retinal degeneration. Our aims were three-fold: 1) to determine the kinetics of rod apoptosis and Ca2+ overload in Pde6b9rd1) mice and developmentally lead-exposed rats, 2) to establish a pathophysiologically-relevant model of Ca2+ overload/rod-selective apoptosis in isolated rat retina and 3) to examine different mechanistic based neuroprotective strategies that would abrogate or mollify rod Ca2+ overload/apoptosis.

METHODS

Retinal morphometry and elemental calcium content ([Ca]) determined the kinetics of rod apoptosis and Ca2+ overload. A multiparametric analysis of apoptosis including rod [Ca], a live/dead assay, rod oxygen consumption, cytochrome c immunoblots and caspase assays was combined with pharmacological studies of an isolated rat retinal model of rod-selective Ca2+ overload/apoptosis.

RESULTS

Ca2+ overload preceded rod apoptosis in mice and rats, although the extent and kinetics in each differed significantly. The isolated rat model of rod Ca2+ overload/apoptosis showed that blockade of Ca2+ entry through rod cGMP-activated channels with L-cis diltiazem was partially neuroprotective, whereas blockade of Ca2+ entry into rods through L-type Ca2+ channels with D-cis diltiazem or verapamil provided no protection. Inhibition of the mitochondrial Na+/Ca2+ exchanger with D-cis diltiazem provided no protection. CsA and NIM811, mitochondrial permeability transition pore (mPTP) inhibitors, blocked all Ca(2+)-induced apoptosis, whereas the caspase-3 inhibitor DEVD-fmk only blocked the downstream cytochrome c-induced apoptosis.

CONCLUSIONS

The successful pharmacological neuroprotective strategies for rod Ca2+ overload/apoptosis targeted the rod cGMP-activated channels or mPTP, but not the rod L-type Ca2+ channels.

Bcl-xL overexpression blocks bax-mediated mitochondrial contact site formation and apoptosis in rod photoreceptors of lead-exposed mice

Photoreceptor apoptosis and resultant visual deficits occur in humans and animals with inherited and disease-, injury-, and chemical-induced retinal degeneration. A clinically relevant mouse model of progressive rod photoreceptor-selective apoptosis was produced by low-level developmental lead exposure and studied in combination with transgenic mice overexpressing Bcl-x(L) only in the photoreceptors. A multiparametric analysis of rod apoptosis and mitochondrial structure-function was performed. Mitochondrial cristae topography and connectivity, matrix volume, and contact sites were examined by using 3D electron tomography. Lead-induced rod-selective apoptosis was accompanied by rod Ca(2+) overload, rhodopsin loss, translocation of Bax from the cytosol to the mitochondria, decreased rod mitochondrial respiration and membrane potential, mitochondrial cytochrome c release, caspase-3 activation, and an increase in the number of mitochondrial contact sites. These effects occurred without mitochondrial matrix swelling, outer membrane rupture, caspase-8 activation, or Bid cleavage. Bcl-x(L) overexpression completely blocked all apoptotic events, except Ca(2+) overload, and maintained normal rod mitochondrial function throughout adulthood. This study presents images of mitochondrial contact sites in an in vivo apoptosis model and shows that Bcl-x(L) overexpression blocks increased contact sites and apoptosis. These findings extend our in vitro retinal studies with Pb(2+) and Ca(2+) and suggest that developmental lead exposure produced rod-selective apoptosis without mitochondrial swelling by translocating cytosolic Bax to the mitochondria, which likely sensitized the Pb(2+) and Ca(2+) overloaded rod mitochondria to release cytochrome c. These results have relevance for therapies in a wide variety of progressive retinal and neuronal degenerations where Ca(2+) overload, lead exposure, andor mitochondrial dysfunction occur.

Rhythmic changes in metabolism of dopamine in the chick retina: the importance of light versus biological clock

Rhythmic changes in dopamine (DA) content and metabolism were studied in retinas of chicks that were adapted to three different lighting conditions: 12-h light : 12-h dark (LD), constant darkness (DD) and continuous light (LL). Retinas of chicks kept under LD conditions exhibited light-dark-dependent variations in the steady-state level of DA and the two metabolites of DA, i.e. 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanilic acid (HVA). Concentrations of DA, DOPAC and HVA were high in light hours and low in dark hours of the LD illumination cycle. In retinas of chicks kept under DD, the content of DA, DOPAC and HVA oscillated in a rhythmic manner for 2 days, with higher values during the subjective light phase than during the subjective dark phase. The amplitudes of the observed oscillations markedly and progressively declined compared with the amplitudes recorded under the LD cycle. In retinas of chicks kept under LL conditions, levels of DA, DOPAC and HVA were similar to those found during the light phase of the LD cycle. Changes in the retinal contents of DA and HVA did not exhibit pronounced daily oscillations, while on the first day of LL the retinal concentrations of DOPAC were significantly higher during the subjective light phase than during the subjective dark phase. Acute exposure of chicks to light during the dark phase of the LD cycle markedly increased DA and DOPAC content in the retina. In contrast, light deprivation during the day decreased the retinal concentrations of DA and DOPAC. It is suggested that of the two regulatory factors controlling the level and metabolism of DA in the retina of chick, i.e. light and biological clock, environmental lighting conditions seem to be of major importance, with light conveying a stimulatory signal for the retinal dopaminergic cells.

Adenosine A(2a) receptor-mediated inhibition of rod opsin mRNA expression in tiger salamander

The neuromodulator adenosine mediates dark-adaptive changes in retinal photoreceptors through A(2a) receptors. In cold-blooded vertebrates, opsin mRNA expression is lower at night than during the day. In the present study, we tested whether adenosine could inhibit opsin mRNA expression in cultured rod cells and if endogenous adenosine acts to suppress opsin mRNA in the intact retina at night. Semi-quantitative in situ hybridization showed that treatment with 100 nm of the A(2a)/A(2b) agonist N(6)-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine (DPMA) reduced opsin mRNA 41% in cultured rod cells. The effect of DPMA was blocked by 10 microm of the A(2a) antagonist 8-(3-chlorostyryl)caffeine (CSC) but not by 10 microm of the A(2b) antagonist alloxazine. One micromolar adenosine alone had no effect on opsin mRNA. However, in the presence of the adenosine deaminase inhibitor erythro-9-(2-hydroxy-3-nonyl)adenine hydrochloride (EHNA), 1 microm adenosine reduced opsin mRNA 61%. EHNA alone reduced opsin mRNA by 26%. Consistent with an A(2a) receptor mechanism, 100 nm forskolin (adenylate cyclase agonist) decreased opsin mRNA 34%. Finally, northern blots showed that intravitreal injection of 10 microm CSC at night increased opsin I mRNA 38%. Thus, endogenous adenosine suppresses rod opsin I mRNA expression at night; in vitro results indicate this reduction occurs through A(2a)-like receptor binding and stimulation of adenylate cyclase activity.

Dysfunctional light-evoked regulation of cAMP in photoreceptors and abnormal retinal adaptation in mice lacking dopamine D4 receptors

Dopamine is a retinal neuromodulator that has been implicated in many aspects of retinal physiology. Photoreceptor cells express dopamine D4 receptors that regulate cAMP metabolism. To assess the effects of dopamine on photoreceptor physiology, we examined the morphology, electrophysiology, and regulation of cAMP metabolism in mice with targeted disruption of the dopamine D4 receptor gene. Photoreceptor morphology and outer segment disc shedding after light onset were normal in D4 knock-out (D4KO) mice. Quinpirole, a dopamine D2/D3/D4 receptor agonist, decreased cAMP synthesis in retinas of wild-type (WT) mice but not in retinas of D4KO mice. In WT retinas, the photoreceptors of which were functionally isolated by incubation in the presence of exogenous glutamate, light also suppressed cAMP synthesis. Despite the similar inhibition of cAMP synthesis, the effect of light is directly on the photoreceptors and independent of dopamine modulation, because it was unaffected by application of the D4 receptor antagonist l-745,870. Nevertheless, compared with WT retinas, basal cAMP formation was reduced in the photoreceptors of D4KO retinas, and light had no additional inhibitory effect. The results suggest that dopamine, via D4 receptors, normally modulates the cascade that couples light responses to adenylyl cyclase activity in photoreceptor cells, and the absence of this modulation results in dysfunction of the cascade. Dark-adapted electroretinogram (ERG) responses were normal in D4KO mice. However, ERG b-wave responses were greatly suppressed during both light adaptation and early stages of dark adaptation. Thus, the absence of D4 receptors affects adaptation, altering transmission of light responses from photoreceptors to inner retinal neurons. These findings indicate that dopamine D4 receptors normally play a major role in regulating photoreceptor cAMP metabolism and adaptive retinal responses to changing environmental illumination.

Dysfunctional light-evoked regulation of cAMP in photoreceptors and abnormal retinal adaptation in mice lacking dopamine D4 receptors

Dopamine is a retinal neuromodulator that has been implicated in many aspects of retinal physiology. Photoreceptor cells express dopamine D4 receptors that regulate cAMP metabolism. To assess the effects of dopamine on photoreceptor physiology, we examined the morphology, electrophysiology, and regulation of cAMP metabolism in mice with targeted disruption of the dopamine D4 receptor gene. Photoreceptor morphology and outer segment disc shedding after light onset were normal in D4 knock-out (D4KO) mice. Quinpirole, a dopamine D2/D3/D4 receptor agonist, decreased cAMP synthesis in retinas of wild-type (WT) mice but not in retinas of D4KO mice. In WT retinas, the photoreceptors of which were functionally isolated by incubation in the presence of exogenous glutamate, light also suppressed cAMP synthesis. Despite the similar inhibition of cAMP synthesis, the effect of light is directly on the photoreceptors and independent of dopamine modulation, because it was unaffected by application of the D4 receptor antagonist l-745,870. Nevertheless, compared with WT retinas, basal cAMP formation was reduced in the photoreceptors of D4KO retinas, and light had no additional inhibitory effect. The results suggest that dopamine, via D4 receptors, normally modulates the cascade that couples light responses to adenylyl cyclase activity in photoreceptor cells, and the absence of this modulation results in dysfunction of the cascade. Dark-adapted electroretinogram (ERG) responses were normal in D4KO mice. However, ERG b-wave responses were greatly suppressed during both light adaptation and early stages of dark adaptation. Thus, the absence of D4 receptors affects adaptation, altering transmission of light responses from photoreceptors to inner retinal neurons. These findings indicate that dopamine D4 receptors normally play a major role in regulating photoreceptor cAMP metabolism and adaptive retinal responses to changing environmental illumination.

D2-like dopamine receptors promote interactions between calcium and chloride channels that diminish rod synaptic transfer in the salamander retina

Activation of D2-like dopamine receptors in rods with quinpirole stimulates L-type calcium currents (ICa). This result appears inconsistent with studies showing that D2-like dopamine receptor activation diminishes rod signals in second-order retinal neurons. Since small reductions in [Cl-] can inhibit photoreceptor ICa, we tested the hypothesis that enhancement of ICa with the D2/D4 receptor agonist, quinpirole, increases calcium-activated chloride currents (ICl(Ca)) causing an efflux of Cl- from rods that would provide a negative feedback inhibition of ICa. In agreement with studies from Xenopus, quinpirole reduced rod input to second-order neurons of tiger salamander retina without significantly altering rod voltage responses. Quinpirole also diminished the amplitude of depolarization-evoked increases in [Ca2+]i measured with Fura-2 in rods, a finding consistent with inhibition of synaptic transmission from rods. Electrophysiological and Cl(-)-imaging experiments indicated ECl in rods is approximately -20 mV. Quinpirole enhanced ICl(Ca) and elicited an efflux of Cl- at the resting potential. A similar Cl- efflux was produced by extracellular replacement of 24 mM Cl- with CH3SO4- and this low Cl- solution inhibited Ca2+ responses to a similar degree as quinpirole did. When ICl(Ca) was inhibited with niflumic acid, quinpirole enhanced both ICa and depolarization-evoked increases in [Ca2+]i. Furthermore, with niflumic acid, quinpirole no longer inhibited rod inputs into horizontal and bipolar cells. These results suggest an initial enhancement of ICa by quinpirole is followed by a stimulation of Cl- currents, including ICl(Ca). The net result is a Cl- efflux that inhibits depolarization-evoked increases in [Ca2+]i and synaptic transmission from rods.

Circadian clock regulation of pH in the rabbit retina

Although it is generally accepted that the acid-base ratio of tissue, as represented by the pH, is strictly regulated to maintain normal function, recent studies in the mammalian nervous system have shown that neuronal activity can result in significant shifts in pH. In the mammalian retina, many cellular phenomena, including neuronal activity, are regulated by a circadian clock. We thus investigated whether a clock regulates retinal pH, using pH-sensitive microelectrodes to measure the extracellular pH (pH(o)) of the in vitro rabbit retina in the subjective day and night, that is, under conditions of constant darkness. These measurements demonstrated that a circadian clock regulates the pH(o) of the rabbit retina so that the pH(o) is lower at night than in the day. This day/night difference in retinal pH(o) was observed when the rabbits were maintained on a normal light/dark cycle and after they were maintained on a light/dark cycle that was phase-delayed by 9 hr. Continuous recordings of retinal pH(o) around subjective dusk indicated that the change from daytime to nighttime pH(o) is relatively fast and suggested that the clock that regulates pH(o) is located in the retina. The lowest pH(o) recorded in the retina in both the day and night was in the vicinity of the inner segments of photoreceptor cells, supporting the idea that photoreceptors serve as the primary source of protons. The circadian-induced shift in pH(o) was several times greater than light-induced pH(o) changes. These findings suggest that a circadian clock in the mammalian retina regulates retinal pH.

Inhibitors of Na(+)/H(+) and Na(+)/Ca(2+) exchange potentiate methamphetamine-induced dopamine neurotoxicity: possible role of ionic dysregulation in methamphetamine neurotoxicity

Although the neurotoxic potential of methamphetamine (METH) is well established, underlying mechanisms have yet to be identified. In the present study, we sought to determine whether ionic dysregulation was a feature of METH neurotoxicity. In particular, we reasoned that if METH impairs the function of Na(+)/H(+) and/or Na(+)/Ca(2+) antiporters by compromising the inward Na(+) gradient [via prolonged DA transporter (DAT) activation and Na(+)/K(+) ATPase inhibition], then amiloride (AMIL) and other inhibitors of Na(+)/H(+) and/or Na(+)/Ca(2+) exchange would potentiate METH neurotoxicity. To test this hypothesis, mice were treated with METH alone or in combination with AMIL or one of its analogs; 1 week later, the animals were killed for studies of dopamine (DA) neuronal integrity. AMIL markedly potentiated the toxic effect of METH on DA neurons. Potentiation was not caused by increased core temperature, enhanced DAT activity or higher METH brain levels. The DAT inhibitor, WIN-35,428, protected completely against METH-induced DA neurotoxicity in AMIL pretreated animals, suggesting that the potentiating effects of AMIL require a METH/DAT interaction. Findings with METH and AMIL were extended to six other AMIL analogs (MIA, EIPA, DIMA, BENZ, BEP, DiCBNZ), another species (rats), and neuronal type (5-HT neurons). These results support the notion that ionic dysregulation may play a role in METH neurotoxicity.

Circadian clock regulation of pH in the rabbit retina

Although it is generally accepted that the acid-base ratio of tissue, as represented by the pH, is strictly regulated to maintain normal function, recent studies in the mammalian nervous system have shown that neuronal activity can result in significant shifts in pH. In the mammalian retina, many cellular phenomena, including neuronal activity, are regulated by a circadian clock. We thus investigated whether a clock regulates retinal pH, using pH-sensitive microelectrodes to measure the extracellular pH (pH(o)) of the in vitro rabbit retina in the subjective day and night, that is, under conditions of constant darkness. These measurements demonstrated that a circadian clock regulates the pH(o) of the rabbit retina so that the pH(o) is lower at night than in the day. This day/night difference in retinal pH(o) was observed when the rabbits were maintained on a normal light/dark cycle and after they were maintained on a light/dark cycle that was phase-delayed by 9 hr. Continuous recordings of retinal pH(o) around subjective dusk indicated that the change from daytime to nighttime pH(o) is relatively fast and suggested that the clock that regulates pH(o) is located in the retina. The lowest pH(o) recorded in the retina in both the day and night was in the vicinity of the inner segments of photoreceptor cells, supporting the idea that photoreceptors serve as the primary source of protons. The circadian-induced shift in pH(o) was several times greater than light-induced pH(o) changes. These findings suggest that a circadian clock in the mammalian retina regulates retinal pH.

Dopamine D4 receptor-mediated regulation of rod opsin mRNA expression in tiger salamander

Light stimulates dopamine release in the retina and has been shown to rapidly up-regulate rod opsin mRNA. In the present study, we tested the effect of dopamine on rod opsin mRNA expression and examined the hypothesis that dopamine can mediate a light-evoked increase in opsin gene expression. Northern blots showed that a 30-min light-exposure increased rod opsin mRNA expression 27%. In situ hybridization on isolated rods showed that 500 nM dopamine and 1 microM quinpirole (dopamine D2/D3/D4 agonist) increased opsin mRNA 45% and 26%, respectively. The effect of quinpirole was selectively blocked by the D4 antagonist, L750,667 (20 microM). In very low density cultures, quinpirole increased opsin expression 46%, suggesting a direct effect on rod photoreceptors. Consistent with a dopamine D4 receptor mechanism, 1 microM H-89 (protein kinase A inhibitor) increased opsin mRNA 39%. Finally, intravitreal injection of quinpirole increased opsin mRNA 21% whereas injection of L750,667 (10 microM) blocked the light-evoked increase in opsin expression. These data show that rod opsin mRNA is up-regulated by dopamine binding a D4-like receptor on rods, possibly through inhibition of protein kinase A, and that endogenous dopamine can mediate the light-evoked increase in opsin mRNA expression.

Diurnal metabolism of dopamine in dystrophic retinas of homozygous and heterozygous retinal degeneration slow (rds) mice

Dopamine metabolism was studied in dystrophic retinal degeneration slow (rds) mice which carry a mutation in the rds/peripherin gene. RDS mutations in humans cause several forms of retinal degeneration. Dopamine synthesis and utilization were analyzed at various time points in the diurnal cycle in homozygous rds/rds retinas which lack photoreceptor outer segments and heterozygous rds/+ retinas which have short malformed outer segments. Homozygous retinas exhibited depressed dopamine synthesis and utilization while the heterozygous retina retained a considerable level of activity which was, nevertheless, significantly lower than that of normal retinas. By one year, heterozygous rds/+ retinas which had lost half of the photoreceptors still maintained significant levels of dopamine metabolism. Normal characteristics of dopamine metabolism such as a spike in dopamine utilization at light onset were observed in mutant retinas. However, light intensity-dependent changes in dopamine utilization were observed in normal but not rds/+ retinas. The findings of this study suggest that human patients with peripherin/rds mutations, or other mutations that result in abnormal outer segments that can still capture light, might maintain light-evoked dopamine metabolism and dopamine-dependent retinal functions during the progression of the disease, proportional to remaining levels of light capture capabilities. However, visual deficits due to reduced light-evoked dopamine metabolism and abnormal patterns of dopamine utilization could be expected in such diseased retinas.

Differential modulation of rod and cone calcium currents in tiger salamander retina by D2 dopamine receptors and cAMP

Synaptic transmission from vertebrate photoreceptors involves activation of L-type calcium currents (ICa). Dopamine is an important circadian neuromodulator in the retina and photoreceptors possess D2 dopamine receptors. We examined modulation of ICa by dopamine and cAMP in retinal slices and isolated cells of larval tiger salamander. Results show that dopamine and a D2 agonist, quinpirole, enhanced ICa in rods and red-, blue- and UV-sensitive small single cones but inhibited ICa in red-sensitive large single cones. A D1 agonist, SKF-38393, was without effect. Quinpirole effects were blocked by pertussis toxin (PTx) pretreatment indicating involvement of PTx-sensitive G-proteins. Like dopamine, inhibition of cAMP-dependent protein kinase (PKA) by Rp-cAMPS enhanced ICa in rods and small single cones, but inhibited ICa in large single cones. In contrast, forskolin and Sp-cAMPS, which stimulate PKA, inhibited ICa in rods and small single cones but enhanced ICa in large single cones. Sp-cAMPS also occluded effects of quinpirole. These results suggest that D2 receptors modulate ICa via inhibition of cAMP. Differences among the responses of photoreceptors to cAMP are consistent with the possibility that small single cones and rods may possess different Ca2+ channel subtypes than large single cones. The results with dopamine and quinpirole showing inhibition of ICa in large single cones and enhancement of rod ICa were unexpected because previous studies have shown that dopamine suppresses rod inputs and enhances cone inputs into second-order neurons. The present results therefore indicate that the dopaminergic enhancement of cone inputs does not arise from modulation of photoreceptor ICa.

Intrarenal dopamine: a key signal in the interactive regulation of sodium metabolism

The kidney regulates sodium metabolism with extraordinary precision and sensitivity. This is accomplished by an intricate interaction between signals from extrarenal and intrarenal sources and between anti-natriuretic and natriuretic factors. Dopamine, produced in renal proximal tubule cells, plays a central role in this interactive network. Natriuretic hormones that are released from extrarenal sources, such as atrial natriuretic peptide, mediate some of their effects via renal dopamine receptors. On the level of the tubules, dopamine acts by opposing the effects of anti-natriuretic factors, such as angiotensin II and alpha-adrenergic receptors. Sodium retention leads to an increase in renal dopamine tonus, and the natriuretic effects of dopamine are more prominent under this condition. Inhibition or down-regulation of dopamine receptors significantly attenuates the natriuretic response to salt loading. Renal dopamine is modulated by the supply of filtered L-DOPA and the metabolism of dopamine via catechol-O-methyldopamine. The importance of dopamine as a natriuretic hormone is reflected by its capacity to inhibit the majority of renal tubule sodium transporters. Notably, the activity of Na+, K+ ATPase is inhibited in most tubule segments by dopamine. Recent studies have elucidated many of the signaling pathways for renal dopamine receptors. Novel principles for homologous and heterologous sensitization of dopamine receptors have been detected that may explain some of the interaction between dopamine and other first messengers that modulate renal tubule sodium transport. A broad understanding of the renal dopamine system has become increasingly important, since there is now strong evidence from both clinical and experimental studies that dysregulation of the renal dopamine system plays a role in many forms of multigenetic hypertension.

Mechanisms of sodium pump regulation

The Na(+)-K(+)-ATPase, or sodium pump, is the membrane-bound enzyme that maintains the Na(+) and K(+) gradients across the plasma membrane of animal cells. Because of its importance in many basic and specialized cellular functions, this enzyme must be able to adapt to changing cellular and physiological stimuli. This review presents an overview of the many mechanisms in place to regulate sodium pump activity in a tissue-specific manner. These mechanisms include regulation by substrates, membrane-associated components such as cytoskeletal elements and the gamma-subunit, and circulating endogenous inhibitors as well as a variety of hormones, including corticosteroids, peptide hormones, and catecholamines. In addition, the review considers the effects of a range of specific intracellular signaling pathways involved in the regulation of pump activity and subcellular distribution, with particular consideration given to the effects of protein kinases and phosphatases.

Diurnal metabolism of dopamine in the mouse retina

Dopamine is an important retinal neurotransmitter and neuromodulator that regulates key diurnal cellular and physiological functions. In the present study we carried out a comprehensive analysis of dopamine metabolism during the light phase of the diurnal cycle and evaluated the presence of diurnal and circadian rhythms of dopaminergic activity in the mouse retina. Steady-state levels of dopamine did not change significantly between the dark phase (night) and the light phase (day) of the diurnal cycle, nor did they change between early and late points in the day. Dopamine synthesis and utilization, however, revealed significant alterations between the night and day and between early and late time points in the day. A spike in synthesis and utilization was measured immediately after light onset at the end of the night. Subsequently, dopamine synthesis and utilization partially declined and remained stable throughout the remainder of the day at a level that was significantly higher than that at night. The burst of dopamine synthesis and utilization at the beginning of the day is entirely light evoked and not driven by a circadian clock. Similarly, there was no circadian rhythm in dopamine synthesis and utilization in mice kept in constant darkness. This daily pattern of dopaminergic activity may impact upon a variety of temporally regulated retinal events. Moreover, these data will provide a basis for evaluating the role of dopamine in retinal pathology in mouse models of retinal degeneration where mutations affect light perception.

Lead and calcium produce rod photoreceptor cell apoptosis by opening the mitochondrial permeability transition pore

Calcium overload is suggested to play a fundamental role in the process of rod apoptosis in chemical-induced and inherited retinal degenerations. However, this hypothesis has not been tested directly. We developed an in vitro model utilizing isolated rat retinas to determine the mechanisms underlying Ca(2+)- and/or Pb(2+)-induced retinal degeneration. Confocal microscopy, histological, and biochemical studies established that the elevated [Ca(2+)] and/or [Pb(2+)] were localized to photoreceptors and produced rod-selective apoptosis. Ca(2+) and/or Pb(2+) induced mitochondrial depolarization, swelling, and cytochrome c release. Subsequently caspase-9 and caspase-3 were sequentially activated. Caspase-7 and caspase-8 were not activated. The effects of Ca(2+) and Pb(2+) were additive and blocked completely by the mitochondrial permeability transition pore (PTP) inhibitor cyclosporin A, whereas the calcineurin inhibitor FK506 had no effect. The caspase inhibitors carbobenzoxy-Leu-Glu-His-Asp-CH(2)F and carbobenzoxy-Asp-Glu-Val-Asp-CH(2)F, but not carbobenzoxy-Ile-Glu-Thr-Asp-CH(2)F, differentially blocked post-mitochondrial events. The levels of reduced and oxidized glutathione and pyridine nucleotides in rods were unchanged. Our results demonstrate that rod mitochondria are the target site for Ca(2+) and Pb(2+). Moreover, they suggest that Ca(2+) and Pb(2+) bind to the internal metal (Me(2+)) binding site of the PTP and subsequently open the PTP, which initiates the cytochrome c-caspase cascade of apoptosis in rods.

A circadian clock regulates the pH of the fish retina

Although it is generally accepted that the acid/base ratio of tissue, as represented by the pH, is strictly regulated to maintain normal function, recent studies in the nervous system have shown that neuronal activity can result in significant shifts in pH. In the vertebrate retina, many cellular phenomena, including neuronal activity, are regulated by a circadian clock. We thus investigated whether a circadian clock regulates the pH of the retina. pH-sensitive microelectrodes were used to measure the extracellular pH of the in vitro goldfish retina superfused with a bicarbonate-based Ringer solution in the subjective day and night; that is, under conditions of constant darkness. These measurements demonstrated that a circadian clock regulates the pH of the vertebrate retina so that the pH is lower at night compared to the day. This day-night difference in retinal pH was observed at two different values of Ringer solution pH, indicating that the circadian phenomenon is independent of the superfusion conditions. The circadian-induced shift in pH was several times greater than light-induced pH changes and large enough to influence synaptic transmission between retinal neurons. These findings indicate that a circadian clock regulates the pH of the vertebrate retina. Thus, an intrinsic oscillator in neural tissue may modulate metabolic activity and pH as part of normal daily function.

Modulation of the light response by cAMP in Drosophila photoreceptors

Phototransduction in Drosophila is mediated by a G-protein-coupled phospholipase C transduction cascade in which each absorbed photon generates a discrete electrical event, the quantum bump. In whole-cell voltage-clamp recordings, cAMP, as well as its nonhydrolyzable and membrane-permeant analogs 8-bromo-cAMP (8-Br-cAMP) and dibutyryl-cAMP, slowed down the macroscopic light response by increasing quantum bump latency, without changes in bump amplitude or duration. In contrast, cGMP or 8-Br-cGMP had no effect on light response amplitude or kinetics. None of the cyclic nucleotides activated any channels in the plasma membrane. The effects of cAMP were mimicked by application of the non-specific phosphodiesterase inhibitor IBMX and the adenylyl cyclase activator forskolin; zaprinast, a specific cGMP-phosphodiesterase inhibitor, was ineffective. Bump latency was also increased by targeted expression of either an activated G(s) alpha subunit, which increased endogenous adenylyl cyclase activity, or an activated catalytic protein kinase A (PKA) subunit. The action of IBMX was blocked by pretreatment with the PKA inhibitor H-89. The effects of cAMP were abolished in mutants of the ninaC gene, suggesting this nonconventional myosin as a possible target for PKA-mediated phosphorylation. Dopamine (10 microM) and octopamine (100 microM) mimicked the effects of cAMP. These results indicate the existence of a G-protein-coupled adenylyl cyclase pathway in Drosophila photoreceptors, which modulates the phospholipase C-based phototransduction cascade.

Modulation of the light response by cAMP in Drosophila photoreceptors

Phototransduction in Drosophila is mediated by a G-protein-coupled phospholipase C transduction cascade in which each absorbed photon generates a discrete electrical event, the quantum bump. In whole-cell voltage-clamp recordings, cAMP, as well as its nonhydrolyzable and membrane-permeant analogs 8-bromo-cAMP (8-Br-cAMP) and dibutyryl-cAMP, slowed down the macroscopic light response by increasing quantum bump latency, without changes in bump amplitude or duration. In contrast, cGMP or 8-Br-cGMP had no effect on light response amplitude or kinetics. None of the cyclic nucleotides activated any channels in the plasma membrane. The effects of cAMP were mimicked by application of the non-specific phosphodiesterase inhibitor IBMX and the adenylyl cyclase activator forskolin; zaprinast, a specific cGMP-phosphodiesterase inhibitor, was ineffective. Bump latency was also increased by targeted expression of either an activated G(s) alpha subunit, which increased endogenous adenylyl cyclase activity, or an activated catalytic protein kinase A (PKA) subunit. The action of IBMX was blocked by pretreatment with the PKA inhibitor H-89. The effects of cAMP were abolished in mutants of the ninaC gene, suggesting this nonconventional myosin as a possible target for PKA-mediated phosphorylation. Dopamine (10 microM) and octopamine (100 microM) mimicked the effects of cAMP. These results indicate the existence of a G-protein-coupled adenylyl cyclase pathway in Drosophila photoreceptors, which modulates the phospholipase C-based phototransduction cascade.

Regulation of Na+, K+-ATPase isoforms in rat neostriatum by dopamine and protein kinase C

Our previous studies showed that dopamine inhibits Na+,K+-ATPase activity in acutely dissociated neurons from striatum. In the present study, we have found that in this preparation, dopamine inhibited significantly (by approximately 25%) the activity of the alpha3 and/or alpha2 isoforms, but not the alpha1 isoform, of Na+,K+-ATPase. Dopamine, via D1 receptors, activates cyclic AMP-dependent protein kinase (PKA) in striatal neurons. Dopamine is also known to activate the calcium- and phospholipid-dependent protein kinase (PKC) in a number of different cell types. The PKC activator phorbol 12,13-dibutyrate reduced the activity of Na+,K+-ATPase alpha3 and/or alpha2 isoforms (by approximately 30%) as well as the alpha1 isoform (by approximately 15%). However, dopamine-mediated inhibition of Na+,K+-ATPase activity was unaffected by calphostin C, a PKC inhibitor. Dopamine did not affect the phosphorylation of Na+,K+-ATPase isoforms at the PKA-dependent phosphorylation site. Phorbol ester treatment did not alter the phosphorylation of alpha2 or alpha3 isoforms of Na+,K+-ATPase in neostriatal neurons but did increase the phosphorylation of the alpha1 isoform. Thus, in rat neostriatal neurons, treatment with either dopamine or PKC activators results in inhibition of the activity of specific (alpha3 and/or alpha2) isoforms of Na+,K+-ATPase, but this is not apparently mediated through direct phosphorylation of the enzyme. In addition, PKC is unlikely to mediate inhibition of rat Na+,K+-ATPase activity by dopamine in neostriatal neurons.

Dopamine receptor localization in the mammalian retina

After a short history of dopamine receptor discovery in the retina and a survey on dopamine receptor types and subtypes, the distribution of dopamine receptors in the retinal cells is described and correlated with their possible role in cell and retinal physiology. All the retinal cells probably bear dopamine receptors. For example, the recently discovered D1B receptor has a possible role in modulating phagocytosis by the pigment epithelium and a D4 receptor is likely to be involved in the inhibition of melatonin synthesis in photoreceptors. Dopamine uncouples horizontal and amacrine cell-gap junctions through D1-like receptors. Dopamine modulates the release of other transmitters by subpopulations of amacrine cells, including that of dopamine through a D2 autoreceptor. Ganglion cells express dopamine receptors, the role of which is still uncertain. Müller cells also are affected by dopamine. A puzzling action of dopamine is observed in the ciliary retina, in which D1- and D2-like receptors are likely to be involved in the cyclic regulation of intraocular pressure. Most of the dopaminergic actions appear to be extrasynaptic and the signaling pathways remain uncertain. Further studies are needed to better understand the multiple actions of dopamine in the retina, especially those that implicate rhythmic regulations.

The dopamine D2 receptor subfamily in rat retina: ultrastructural immunogold and in situ hybridization studies

Dopamine, a major neurotransmitter in the vertebrate retina, is released from interplexiform cells and a restricted subset of amacrine cells. Dopamine effects vary between different retinal cell types, most likely due to differences in cell-specific receptor subtype expression. Identification of cells expressing receptors of the D2-subfamily (D2R, D3R, D4R) on a light microscopical level has rendered equivocal results, and no information is as yet available concerning the subcellular distribution of receptor protein. In the present study, D2R and D2/3R subtype-specific antisera, and D2R-, D3R- and D4R-specific oligonucleotide probes were used for ultrastructural and in situ hybridization analyses of the receptor subtype distribution in the rat retina. Light and electron microscopy showed that in addition to the known localization of intense D2R-immunoreactivity in all dopaminergic cells immunoreactive for tyrosine hydroxylase (TH), homogeneous, less intense D2R-immunoreactivity was also seen throughout the inner plexiform layer (IPL). Ultrastructurally, many additional amacrine cell processes devoid of TH-immunoreactivity at all levels of the inner plexiform layer were immunoreactive. D2R-immunoreactivity was found mainly on intracellular vesicles, and immunoreactivity associated with the plasma membrane was always extrasynaptic. No D2R-immunoreactivity was found in amacrine cell somata postsynaptic to the so-called dopaminergic 'ring endings'. Many D2R-mRNA reactive cells were observed throughout the inner nuclear layer. Morphologically, labelled cells resemble amacrines and bipolars but not horizontal cells. Reactivity with splice variant-specific oligonucleotide probes suggested that the D2LR variant is the predominant if not the only D2R isoform in the rat retina. D2R-mRNA reactivity was not observed in other retinal layers, in particular not in photoreceptor inner segments, which displayed D4R-mRNA reactivity. D3R-mRNA reactivity was not detected. The results indicate that D2-like responses are mediated through the D2R subtype, by an autoreceptor mechanism in dopaminergic cells, and by volume transmission in non-dopaminergic cells of the inner retina. D2-like responses in photoreceptors probably represent D4R activation.

Phospholipase C beta 4 is involved in modulating the visual response in mice

Expression of G protein-regulated phospholipase C (PLC) beta 4 in the retina, lateral geniculate nucleus, and superior colliculus implies that PLC beta 4 may play a role in the mammalian visual process. A mouse line that lacks PLC beta 4 was generated and the physiological significance of PLC beta 4 in murine visual function was investigated. Behavioral tests using a shuttle box demonstrated that the mice lacking PLC beta 4 were impaired in their visual processing abilities, whereas they showed no deficit in their auditory abilities. In addition, the PLC beta 4-null mice showed 4-fold reduction in the maximal amplitude of the rod a- and b-wave components of their electroretinograms relative to their littermate controls. However, recording from single rod photoreceptors did not reveal any significant differences between the PLC beta 4-null and wild-type littermates, nor were there any apparent differences in retinas examined with light microscopy. While the behavioral and electroretinographic results indicate that PLC beta 4 plays a significant role in mammalian visual signal processing, isolated rod recording shows little or no apparent deficit, suggesting that the effect of PLC beta 4 deficiency on the rod signaling pathway occurs at some stage after the initial phototransduction cascade and may require cell-cell interactions between rods and other retinal cells.