GABAergic inhibition on dopamine cells of the fish retina: a [3H]dopamine release study with isolated fractions

Inhibitory inputs tune the light response properties of dopaminergic amacrine cells in mouse retina

Dopamine (DA) is a neuromodulator that in the retina adjusts the circuitry for visual processing in dim and bright light conditions. It is synthesized and released from retinal interneurons called dopaminergic amacrine cells (DACs), whose basic physiology is not yet been fully characterized. To investigate their cellular and input properties as well as light responses, DACs were targeted for whole cell recording in isolated retina using two-photon fluorescence microscopy in a mouse line where the dopamine receptor 2 promoter drives green fluorescent protein (GFP) expression. Differences in membrane properties gave rise to cell-to-cell variation in the pattern of resting spontaneous spike activity ranging from silent to rhythmic to periodic burst discharge. All recorded DACs were light sensitive and generated responses that varied with intensity. The threshold response to light onset was a hyperpolarizing potential change initiated by rod photoreceptors that was blocked by strychnine, indicating a glycinergic amacrine input onto DACs at light onset. With increasing light intensity, the ON response acquired an excitatory component that grew to dominate the response to the strongest stimuli. Responses to bright light (photopic) stimuli also included an inhibitory OFF response mediated by GABAergic amacrine cells driven by the cone OFF pathway. DACs expressed GABA (GABA(A)α1 and GABA(A)α3) and glycine (α2) receptor clusters on soma, axon, and dendrites consistent with the light response being shaped by dual inhibitory inputs that may serve to tune spike discharge for optimal DA release.

Pharmacology of GABA(A) receptors of retinal dopaminergic neurons

When the vertebrate retina is stimulated by light, a class of amacrine or interplexiform cells release dopamine, a modulator responsible for neural adaptation to light. In the intact retina, dopamine release can be pharmacologically manipulated with agonists and antagonists at GABA(A) receptors, and dopaminergic (DA) cells receive input from GABAergic amacrines. Because there are only 450 DA cells in each mouse retina and they cannot be distinguished in the living state from other cells on the basis of their morphology, we used transgenic technology to label DA cells with human placental alkaline phosphatase, an enzyme that resides on the outer surface of the cell membrane. We could therefore identify DA cells in vitro after dissociation of the retina and investigate their activity with whole cell voltage clamp. We describe here the pharmacological properties of the GABA(A) receptors of solitary DA cells. GABA application induces a large inward current carried by chloride ions. The receptors are of the GABA(A) type because the GABA-evoked current is blocked by bicuculline. Their affinity for GABA is very high with an EC(50) value of 7.4 microM. Co-application of benzodiazepine receptor ligands causes a strong increase in the peak current induced by GABA (maximal enhancement: CL-218872 220%; flunitrazepam 214%; zolpidem 348%) proving that DA cells express a type I benzodiazepine-receptor (BZ1). GABA-evoked currents are inhibited by Zn(2+) with an IC(50) of 58.9 +/- 8.9 microM. Furthermore, these receptors are strongly potentiated by the modulator alphaxalone with an EC(50) of 340 +/- 4 nM. The allosteric modulator loreclezole increases GABA receptor currents by 43% (1 microM) and by 107% (10 microM). Using outside-out patches, we measured in single-channel recordings a main conductance (29 pS) and two subconductance (20 and 9 pS) states. We have previously shown by single-cell RT-PCR and immunocytochemistry that DA cells express seven different GABA(A) receptor subunits (alpha1, alpha3, alpha4, beta1, beta3, gamma1, gamma2(S), and gamma2(L)) and by immunocytochemistry that all subunits are expressed in the intact retina. We show here that at least alpha1, beta3 and gamma2 subunits are assembled into functional receptors.

Taurine blocks spontaneous cone contraction but not horizontal cell dark suppression in isolated goldfish retina

The objective of this study was to investigate the effects of taurine on cone retinomotor movements and the responses of cone-driven horizontal cells in dark-adapted teleost retina. In isolated goldfish retina preparations maintained in the dark, cones spontaneously contracted, and the responses of horizontal cells were suppressed. Addition of 5 mM taurine to the physiological solution blocked the spontaneous contraction of cones in the dark but did not block the dark-suppression of horizontal cell responses. These results indicate that the mechanism that leads to horizontal cell dark suppression is not sensitive to taurine. Although both cone retinomotor position and horizontal cell responsiveness are known to be modulated by dopamine, the present results do not support the hypothesis that taurine inhibits dopamine release in the dark because only spontaneous cone contraction was affected by taurine. These results also indicate that spontaneous cone contraction in the dark is not the cause of horizontal cell dark suppression because, in the presence of taurine, cones were elongated yet horizontal cell responses were still suppressed. Consequently, these results make it clear that horizontal cell dark suppression is not an artifact produced by incubating isolated teleost retina preparations in taurine-free physiological solution.

Composition of the GABA(A) receptors of retinal dopaminergic neurons

Transgenic technology, single-cell RT-PCR, and immunocytochemistry were combined to investigate the composition of the GABA(A) receptors of dopaminergic (interplexiform) amacrine (DA) cells. A mouse line was used in which these neurons were labeled with human placental alkaline phosphatase and could therefore be identified in vitro after dissociation of the retina. We performed single-cell RT-PCR on the isolated cells and showed that (1) DA cells contained the messages for alpha1, alpha3, alpha4, beta1, beta3, gamma1, gamma2(S), and gamma2(L) subunits; (2) this transcript repertory did not change on dissociation of the retina and throughout the time required for cell harvesting; and (3) all DA cells contained the entire transcript repertory. Immunocytochemistry with subunit-specific antibodies showed that all subunits were expressed and appeared homogeneously distributed throughout the cell membrane at a low concentration. In addition, with the exception of alpha4, the subunits formed clusters at the surface of the dendrites and on the inner pole of the cell body. Because of their size, shape, and topographic coincidence with GABAergic endings, the clusters were interpreted as postsynaptic active zones containing GABA(A) receptors. The composition of the synaptic receptors was not uniform: clusters distributed throughout the dendritic tree contained alpha3, beta3, and, less frequently, beta1 subunits, whereas clusters containing the alpha1 subunit were confined to large dendrites. Therefore, DA cells possess at least two types of GABA(A) receptors localized in different synapses. Furthermore, they exhibit multiple extrasynaptic GABA(A) receptors.

Critical review of GABA-ergic drugs in the treatment of schizophrenia

GABA-ergic medications may have a potential role in the treatment of schizophrenia. Laboratory evidence has generally supported the ability of gamma-aminobutyric acid (GABA) to reduce dopaminergic activity and has suggested that GABA may be effective in combating hypofrontality by acting on mesoprefrontocortical tracts in patients resistant to treatment with antipsychotic drugs. Although the results of clinical trials of several GABA-ergic compounds have been inconclusive because of methodologic limitations and drug toxicity, benzodiazepines and valproate seem to be associated with favorable treatment outcomes, especially when combined with typical antipsychotic agents. This study concludes that further investigation of the use of GABA in schizophrenia is likely to improve the understanding of the psychopathology of this illness and to expand our treatment alternatives. Also provided are suggestions to enhance the design of future studies, improve the potential for favorable treatment outcomes, and assist in predicting patients' responses to GABA-ergic medications.

Gamma hydroxybutyrate is not a GABA agonist

Gamma hydroxybutyrate (GHB) is primarily known and used as a relatively specific inhibitor of central DA release. However, it is also widely assumed to be an agonist or prodrug of gamma-aminobutyric acid (GABA) and its central activity has been attributed to an action exerted at GABA receptors. Nevertheless, there is compelling evidence that: (1) GHB formation may occur independently of GABA; (2) GHB is behaviorally, biochemically and physiologically distinct from GABA in many ways, and does not consistently effect GABAA or GABAB agonist induced responses; (3) GHB has little effect on either GABAA or GABAB receptors at less than millimolar concentrations. Consequently, GHB does not appear to be either a GABA prodrug or a GABA agonist. However, the GHB metabolite gamma butyrolactone (GBL) may possess some limited GABA agonist activity.

Differential changes in neurochemical markers of striatal dopamine nerve terminals in idiopathic Parkinson's disease

To determine the extent that different dopamine (DA) neuronal markers provide similar estimates of striatal (caudate and putamen) DA nerve terminal loss in idiopathic Parkinson's disease (PD), we compared, in postmortem striatum of 12 patients with PD and 10 matched controls, levels of five different DA neuronal markers. These markers included DA itself, three different estimates of the density of the DA transporter (DAT) ([3H])GBR 12,935 and [3H]WIN 35,428 binding; DAT protein immunoreactivity), and one estimate of the vesicular monoamine transporter (VMAT2; [3H]DTBZ binding). Striatal levels of all examined DA markers in PD were significantly intercorrelated. However, the magnitude of loss relative to controls was unequal (DAT protein = DA > [3H]WIN 35,428 > [3H]DTBZ > [3H]GBR 12, 935), with the differences more marked in the severely affected putamen. The less severe reduction of binding of the DAT/VMAT2 radioligands relative to DA and DAT protein could be explained by differential regulation/degeneration of different DA nerve terminal components or lack of specificity of the radioligands for the DA neuron. These postmortem data may help in interpretation of in vivo neuroimaging studies in PD in which only one radioligand is routinely employed.

Regulation of endogenous dopamine release in amphibian retina by gamma-aminobutyric acid and glycine

Endogenous dopamine release in the retina of the African clawed frog (Xenopus laevis) increases in light and decreases in darkness. The roles of the inhibitory amino acid transmitters gamma-aminobutyric acid (GABA) and glycine in regulating this light/dark difference in dopamine release were explored in the present study. Exogenous GABA, the GABA-A receptor agonist muscimol, the GABA-B receptor agonist baclofen, and the GABA-C receptor agonist cis-aminocrotonic acid (CACA) suppressed light-evoked dopamine overflow from eyecups. The effects of GABA-A and -B receptor agonists were selectively reversed by their respective receptor-specific antagonists, whereas the effect of CACA was reversed by the competitive GABA-A receptor antagonist bicuculline. The benzodiazepine diazepam enhanced the effect of muscimol on light-evoked dopamine release. Both GABA-A and -B receptor antagonists stimulated dopamine release in light or darkness. Bicuculline was more potent in light than in darkness. These data suggest that retinal dopaminergic neurons are inhibited by GABA-A and -B receptor activation in both light and darkness but that GABA-mediated inhibitory tone may be greater in darkness than in light. Exogenous glycine inhibited light-stimulated dopamine release in a concentration-dependent and strychnine-sensitive manner. However, strychnine alone did not increase dopamine release in light or darkness, nor did it augment bicuculline-stimulated release in darkness. Additionally, both strychnine and 7-chlorokynurenate, an antagonist of the strychnine-insensitive glycine-binding site of the N-methyl-D-aspartate subtype of glutamate receptor, suppressed light-evoked dopamine release. Thus, the role of endogenous glycine in the regulation of dopamine release remains unclear.

Inhibitory effect of prostaglandins on dopamine release from the retina

1. Prostaglandins have been shown to modulate transmitter release from both central and peripheral neuroeffector junctions. In the present study, we examined the effect of prostaglandins on [3H]-dopamine release from isolated, superfused rabbit retina. 2. Both naturally occurring and synthetic prostaglandins produced concentration-dependent reduction of electrically evoked [3H]-dopamine overflow without affecting basal tracer efflux. The rank order of potencies of the agonists was: sulprostone > 16,16-dimethyl PGE2 > PGE2 >> 11-deoxy-PGE1 > PGF2 alpha. 3. The PGE2-mediated inhibition of field stimulated [3H]-dopamine release was not blocked by the selective EP1-receptor antagonist, AH6809 (5-30 microM). 4. The cyclooxygenase inhibitor, flurbiprofen (3 microM) had no effect on basal or evoked [3H]-dopamine overflow nor did it affect the inhibition caused by PGE2 suggesting that endogenous prostaglandins are not involved in the regulation of dopamine release in the retina. 5. The inhibition of [3H]-dopamine release produced by submaximal concentrations of PGE2, apomorphine and melatonin were not additive indicating that presynaptic PGE2, D2- and melatonin receptors coexist at sites for neurotransmitter release and may share a common mechanism for regulation of dopamine release. 6. We conclude that prostaglandin-induced inhibition of electrically evoked [3H]-dopamine release from the rabbit retina may be mediated by specific prostaglandin receptors of the EP3 subtype.

Modulation of endogenous dopamine release in the turtle retina: effects of light, calcium, and neurotransmitters

In the turtle retina, dopamine has been observed in a small population of amacrine cells. Whereas the effect of dopamine has been intensively studied, knowledge about the release of this transmitter and the neuronal control of its release are still poorly understood. We therefore decided to study the release of endogenous dopamine. Isolated retinas were superfused with Ringer's solutions and stimulated with increased potassium, light, or drugs which interfere with neurotransmitter systems. Dopamine was analyzed by using aluminum-oxide extraction and high-pressure liquid chromatography (HPLC) with electrochemical detection. Increased potassium (25 mM) caused a five-fold increase in the basal release. When calcium was replaced by cobalt, no increase was induced by 25 mM potassium. Flickering light increased the basal release of endogenous dopamine by a factor of three. The effect of flickering light was greater in the presence of additional steady background illumination. Kainate (10 microM), an agonist for excitatory amino acids, doubled the basal dopamine release. Bicuculline (10 microM), a gamma-amino butyric acid (GABA) antagonist, increased the release to about six times the basal level. Naloxone (10 microM), an opiate antagonist, increased the release to eight times the basal level. These findings suggest that dopamine is released from amacrine cells in the turtle retina in a calcium-dependent manner, which is most likely a vesicular release. Dopamine release is induced by flickering light vs. darkness and vs. steady background illumination. A moderate background illumination alone does not significantly increase basal dopamine release.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of melatonin and dopamine biosynthesis in chick retina: the role of GABA

Melatonin biosynthesis in chick retina occurs as a circadian rhythm. Biosynthesis of the neurohormone is highest at night in darkness, and is suppressed by light. The role of gamma-aminobutyric acid (GABA) in the nocturnal regulation of melatonin synthesis was examined. Systemic or intravitreal administration of muscimol, a GABA-A receptor agonist, to light-exposed chicks at the beginning of the dark phase of the light/dark cycle increased retinal melatonin levels and the activity of serotonin N-acetyltransferase (NAT), a key regulatory enzyme of the melatonin biosynthetic pathway. Baclofen, a GABA-B receptor agonist, also increased NAT activity of light-exposed retinas, but muscimol was approximately 40-fold more potent than baclofen. Effects of both muscimol and baclofen on NAT activity were inhibited by GABA-A antagonists, bicuculline and picrotoxin, and the effect of baclofen was unaffected by the GABA-B selective antagonist, CGP 35348. Thus, activation of GABA-A receptors appears to be associated with increased melatonin biosynthesis. The GABA-uptake inhibitor, nipecotic acid, and the GABA-transaminase inhibitor, aminooxyacetic acid, also increased NAT activity of light-exposed retinas. The high levels of NAT activity associated with exposure to darkness were unaffected by either muscimol or baclofen, but picrotoxin and bicuculline significantly inhibited retinal NAT activity in darkness. The rate of dopamine synthesis, estimated from in situ tyrosine hydroxylase activity, was higher in light-exposed retinas than in darkness. Muscimol inhibited dopamine synthesis in light, and picrotoxin stimulated dopamine synthesis in darkness. The stimulation of melatonin synthesis by muscimol in light-exposed retinas appears to be related to inhibition of retinal dopamine neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuromodulation of pigment movement in the RPE of normal and 6-OHDA-lesioned goldfish retinas

The role of dopamine as the endogenous signal-initiating light-dependent changes in the distribution of pigment granules in goldfish retinal pigment epithelium was investigated. In normal retinas, light adaptation resulted in the dispersion of pigment granules. This effect of light was mimicked by the intraocular injection of dopamine or serotonin, which is thought to increase endogenous dopamine release, into dark-adapted eyes. The effect of light, dopamine, or serotonin on dark-adapted retinas was blocked by the dopamine receptor antagonists haloperidol and sulpiride. However, lesioning the endogenous source of retinal dopamine, by prior intraocular injection of 6-hydroxydopamine (6-OHDA), did not block the dispersion of pigment granules in light-adapted retinas. No significant differences in pigment dispersion were noted between unlesioned and lesioned light- or dark-adapted retinas. However, the effect of light on pigment dispersion was no longer blocked by haloperidol or sulpiride in 6-OHDA lesioned animals. Dopamine and serotonin mimicked the effect of light when injected into lesioned dark-adapted eyes, but their effects were also not blocked by haloperidol or sulpiride. These results suggest that dopamine, acting on D2 receptors, is sufficient to induce pigment migration in unlesioned animals. In 6-OHDA-lesioned animals, however, pigment migration is mediated by a receptor mechanism other than D2.

Glutamate antagonists that block hyperpolarizing bipolar cells increase the release of dopamine from turtle retina

Some neurochemical features of the neuronal circuitry regulating dopamine release were examined in the retina of the turtle, Pseudemys scripta elegans. Glutamate antagonists that block hyperpolarizing bipolar cells, such as 2,3 piperidine dicarboxylic acid (PDA), produced dose-dependent dopamine release. In contrast, the glutamate agonist 2-amino-4-phosphonobutyric acid (APB), which blocks depolarizing bipolar cell responses with high specificity, had no effect on the release of dopamine. The gamma-aminobutyric acid (GABA) antagonist, bicuculline, also produced potent dose-dependent release of dopamine. The release of dopamine produced by PDA was blocked by exogenous GABA and muscimol, suggesting that the PDA-mediated release process was polysynaptic and involved a GABAergic synapse interposed between the bipolar and dopaminergic amacrine cells. The only other agents that produced dopamine release were chloride-free media and high extracellular K+; in particular, kainic acid and glutamate itself were ineffective. These results suggest that the primary neuronal chain mediating dopamine release in the turtle retina is: cone----hyperpolarizing bipolar cell----GABAergic amacrine cell----dopaminergic amacrine cell.

Binding of the benzodiazepine ligand [3H]-RO 15-1788 to membrane preparations of the rabbit and turtle retina

1. We have studied the binding of [3H]-RO 15-1788 to membrane preparations of the retina of rabbit (Lepus cunicula) and turtle (Pseudemys scripta elegans). 2. In both species, [3H]-RO 15-1788 binding was maximal at 0 degrees C and decreased with increasing temperature. It was saturable, protein concentration-dependent and specific. Flunitrazepam, unlabelled RO 15-1788 and ethyl-beta-carboline were the most effective displacers, whereas RO 5,4864 was ineffective. 3. In both turtle and rabbit retina, Scatchard analysis indicated the presence of a single binding site for [3H]-RO 15-1788. The KD was 0.75 nM in both turtle and rabbit, while the Bmax were 520 and 250 fmol/mg protein in turtle and rabbit respectively. A study of the association rate of [3H]-RO 15-1788 binding revealed faster kinetics in turtle, as compared to rabbit.

Localization and function of dopamine in the adult vertebrate retina

Dopamine (DA) has satisfied many of the criteria for being a major neurochemical in vertebrate retinae. It is synthesized in amacrine and/or interplexiform cells (depending on species) and released upon membrane depolarization in a calcium-dependent way. Strong evidence suggests that it is normally released within the retina during light adaptation, although flickering and not so much steady light stimuli have been found to be most effective in inducing endogenous dopamine release. DA action is not restricted to those neurones which appear to be in "direct" contact with pre-synaptic dopaminergic terminals. Neurones that are several microns away from such terminals can also be affected, presumably by short diffusion of the chemical. DA thus affects the activity of many cell types in the retina. In photoreceptors, it induces retinomotor movements, but inhibits disc shedding acting via D2 receptors, without significantly altering their electrophysiological responses. DA has two main effects upon horizontal cells: it uncouples their gap junctions and, independently, enhances the efficacy of their photoreceptor inputs, both effects involving D1 receptors. In the amphibian retina, where horizontal cells receive mixed rod and cone inputs, DA alters their balance in favour of the cone input, thus mimicking light adaptation. Light-evoked DA release also appears to be responsible for potentiating the horizontal cell-->cone negative feed-back pathway responsible for generation of multi-phasic, chromatic S-potentials. However, there is little information concerning action of DA upon bipolar and amacrine cells. DA effects upon ganglion cells have been investigated in mammalian (cat and rabbit) retinae. The results suggest that there are both synaptic and non-synaptic D1 and D2 receptors on all physiological types of ganglion cell tested. Although the available data cannot readily be integrated, the balance of evidence suggests that dopaminergic neurones are involved in the light/dark adaptation process in the mammalian retina. Studies of the DA system in vertebrate retinae have contributed greatly to our understanding of its role in vision as well as DA neurobiology generally in the central nervous system. For example, the effect of DA in uncoupling horizontal cells is one of the earliest demonstrations of the uncoupling of electrotonic junctions by a neurally released chemical. The many other, diverse actions of DA in the retina reviewed here are also likely to become model modes of neurochemical action in the nervous system.(ABSTRACT TRUNCATED AT 400 WORDS)

Rhythmic regulation of retinal melatonin: metabolic pathways, neurochemical mechanisms, and the ocular circadian clock

1. Current knowledge of the mechanisms of circadian and photic regulation of retinal melatonin in vertebrates is reviewed, with a focus on recent progress and unanswered questions. 2. Retinal melatonin synthesis is elevated at night, as a result of acute suppression by light and rhythmic regulation by a circadian oscillator, or clock, which has been localized to the eye in some species. 3. The development of suitable in vitro retinal preparations, particularly the eyecup from the African clawed frog, Xenopus laevis, has enabled identification of neural, cellular, and molecular mechanisms of retinal melatonin regulation. 4. Recent findings indicate that retinal melatonin levels can be regulated at multiple points in indoleamine metabolic pathways, including synthesis and availability of the precursor serotonin, activity of the enzyme serotonin N-acetyltransferase, and a novel pathway for degradation of melatonin within the retina. 5. Retinal dopamine appears to act through D2 receptors as a signal for light in this system, both in the acute suppression of melatonin synthesis and in the entrainment of the ocular circadian oscillator. 6. A recently developed in vitro system that enables high-resolution measurement of retinal circadian rhythmicity for mechanistic analysis of the circadian oscillator is described, along with preliminary results that suggest its potential for elucidating general circadian mechanisms. 7. A model describing hypothesized interactions among circadian, neurochemical, and cellular mechanisms in regulation of retinal melatonin is presented.

GABA and tyrosine hydroxylase immunocytochemistry reveal different patterns of colocalization in retinal neurons of various vertebrates

Colocalization of GABA- and tyrosine hydroxylase-like immunoreactivity was studied in the retinae of various vertebrate species in order to ascertain whether the presumed coexistence of GABA and dopamine, reported earlier for mammals (Kosaka et al.: Exp. Brain Res. 66:191-210, '87: Wässle and Chun: J. Neurosci. 8:3383-3394,'88) is a common phenomenon. GABA-immunopositive cells constituted a separate population from tyrosine hydroxylase-positive cells in fish and amphibians, whilst in higher--i.e., amniote--vertebrates, such as reptiles, birds, and mammals, all dopaminergic cells contained GABA-like immunoreactivity. No clear correlation was found between the type of dopaminergic cell (amacrine/interplexiform) and the presence or absence of colocalization.

Dopamine release via protein kinase C activation in the fish retina

Calcium-dependent phospholipid-sensitive protein kinase [protein kinase C (PKC)] was partially purified from the carp (Cyprinus carpio) retina through DE 52 ion exchange and Cellulofine gel filtration chromatography. The phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) activated PKC in the nanomolar range. A major 38-kDa protein in the retinal supernatants (105,000 g) was phosphorylated in vitro by PKC during a short period (3 min). Other phosphoproteins also appeared during a further prolonged period (greater than 15 min). Rod-bipolar and dopamine (DA) interplexiform cells in the fish retina were immunoreactive to a monoclonal antibody to PKC (alpha/beta-subtype). The PKC antibody recognized a 78-kDa native PKC enzyme by means of an immunoblotting method. Subsequently, the effects of two kinds of PKC activators were investigated on [3H]DA release from retinal cell fractions containing DA cells that had been preloaded with [3H]DA. A phorbol ester (TPA) induced a calcium- and dose-dependent [3H]DA release during a short period (2 min), with the minimal effective dose being approximately 1 nM. Other phorbols having no tumor-promoting activity, such as 4 beta-phorbol and 4 alpha-phorbol 12,13-didecanoate, were ineffective on [3H]DA release. A synthetic diacylglycerol [1-oleoyl-2-acetylglycerol (OAG)], which is an endogenous PKC activator, was also able to induce a significant release of [3H]DA. Furthermore, TPA was found to release endogenous DA from isolated fish retina by a highly sensitive HPLC with electrochemical detection method. The OAG- or TPA-induced [3H]DA or DA release was completely blocked by inhibitors of PKC, such as 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H7) and staurosporine.(ABSTRACT TRUNCATED AT 250 WORDS)

Release pattern of endogenous dopamine in teleost retinae during light adaptation and pharmacological stimulation

The release of endogenous dopamine from teleost retinae was studied using high-performance-liquid-chromatography and electrochemical detection. Dopamine was measured in superfusates of isolated retinae after stimulation with flickering light as well as in the presence of GABA, L-glutamate, kainate or taurine. The effect of the receptor antagonists bicuculline, picrotoxin and kynurenic acid was also tested in retinae kept in the dark. We report a low level, basal release of dopamine in the dark (20 pg x 10 min-1/retina), which is transiently increased by stimulation with flickering light. This light evoked release of dopamine is inhibited by GABA and L-glutamate, whilst antagonists of these retinal transmitters stimulate release in the dark.

Dopamine cells and rod bipolar cells contain protein kinase C-like immunoreactivity in some vertebrate retinas

The localization of cells immunoreactive to a monoclonal antibody against protein kinase C (PKC) and to polyclonal antibodies against tyrosine hydroxylase (TH) was investigated in the retina of fish (carp, goldfish, dace and catfish), frog, turtle, chick and some mammalians (guinea pig, rat, cat and rabbit) by means of fluorescence microscopy. PKC-like immunoreactivity was found in dopamine (DA) or TH-like immunoreactive (IR) cells in all the species examined and also in rod bipolar cells in the fish (except for catfish), and in presumed rod bipolar cells in the other animals (except for frog and turtle). In the catfish, frog and turtle retinas, no PKC-like IR bipolar cells were found. In the rat retina, some other amacrine cells in addition to TH-like IR amacrine cells were reactive to the anti-PKC antibody. It is of interest that PKC-like immunoreactivity is commonly found in DA cells and probably in rod bipolar cells in most animal species, although the functional significance is unknown at present.