Similar effects of carbachol and dopamine on neurons in the distal retina of the tiger salamander

Altered spatial summation optimizes visual function in axial myopia

This study demonstrates significant differences between the area of complete spatial summation (Ricco's area, RA) in eyes with and without non-pathological, axial myopia. Contrast thresholds were measured for six stimuli (0.01-2.07 deg²) presented at 10º eccentricity in 24 myopic subjects and 20 age-similar non-myopic controls, with RA estimated using iterative two-phase regression analysis. To explore the effects of axial length-induced variations in retinal image size (RIS) on the measurement of RA, refractive error was separately corrected with (i) trial lenses at the anterior focal point (near constant inter-participant RIS in mm), and (ii) contact lenses (RIS changed with axial length). For spectacle corrected measurements, RA was significantly larger in the myopic group, with a significant positive correlation also being observed between RA and measures of co-localised peripheral ocular length. With contact lens correction, there was no significant difference in RA between the groups and no relationship with peripheral ocular length. The results suggest RA changes with axial elongation in myopia to compensate for reduced retinal ganglion cell density. Furthermore, as these changes are only observed when axial length induced variations in RIS are accounted for, they may reflect a functional adaptation of the axially-myopic visual system to an enlarged RIS.

Studies on retinal mechanisms possibly related to myopia inhibition by atropine in the chicken

PURPOSE

While low-dose atropine eye drops are currently widely used to inhibit myopia development in children, the underlying mechanisms are poorly understood. Therefore, we studied possible retinal mechanisms and receptors that are potentially involved in myopia inhibition by atropine.

METHODS

A total of 250 μg atropine were intravitreally injected into one eye of 19 chickens, while the fellow eyes received saline and served as controls. After 1 h, 1.5 h, 2 h, 3 h, and 4 h, eyes were prepared for vitreal dopamine (DA) measurements, using high-pressure liquid chromatography with electrochemical detection. Twenty-four animals were kept either in bright light (8500 lx) or standard light (500 lx) after atropine injection for 1.5 h before DA was measured. In 10 chickens, the α_2A-adrenoreceptor (α_2A-ADR) agonists brimonidine and clonidine were intravitreally injected into one eye, the fellow eye served as control, and vitreal DA content was measured after 1.5 h. In 6 chickens, immunohistochemical analyses were performed 1.5 h after atropine injection.

RESULTS

Vitreal DA levels increased after a single intravitreal atropine injection, with a peak difference between both eyes after 1.97 h. DA was also enhanced in fellow eyes, suggesting a systemic action of intravitreally administered atropine. Bright light and atropine (which both inhibit myopia) had additive effects on DA release. Quantitative immunolabelling showed that atropine heavily stimulated retinal activity markers ZENK and c-Fos in cells of the inner nuclear layer. Since atropine was recently found to also bind to α_2A-ADRs at doses where it can inhibit myopia, their retinal localization was studied. In amacrine cells, α_2A-ADRs were colocalized with tyrosine hydroxylase (TH), glucagon, and nitric oxide synthase, peptides known to play a role in myopia development in chickens. Intravitreal atropine injection reduced the number of neurons that were double-labelled for TH and α_2A-ADR. α_2A-ADR agonists clonidine and brimonidine (which were also found by other authors to inhibit myopia) severely reduced vitreal DA content in both injected and fellow eyes, compared to eyes of untreated chicks.

CONCLUSIONS

Merging our results with published data, it can be concluded that both muscarinic and α_2A-adrenergic receptors are expressed on dopaminergic neurons and both atropine and α_2A-ADR antagonists stimulate DA release whereas α_2A-ADR agonists strongly suppress its release. Stimulation of DA by atropine was enhanced by bright light. Results are in line with the hypothesis that inhibition of deprivation myopia is correlated with DA stimulation, as long as no toxicity is involved.

Somatic and neuritic spines on tyrosine hydroxylase-immunopositive cells of rat retina

Dopamine- and tyrosine hydroxylase-immunopositive cells (TH cells) modulate visually driven signals as they flow through retinal photoreceptor, bipolar, and ganglion cells. Previous studies suggested that TH cells release dopamine from varicose axons arborizing in the inner and outer plexiform layers after glutamatergic synapses depolarize TH cell dendrites in the inner plexiform layer and these depolarizations propagate to the varicosities. Although it has been proposed that these excitatory synapses are formed onto appendages resembling dendritic spines, spines have not been found on TH cells of most species examined to date or on TH cell somata that release dopamine when exposed to glutamate receptor agonists. By use of protocols that preserve proximal retinal neuron morphology, we have examined the shape, distribution, and synapse-related immunoreactivity of adult rat TH cells. We report here that TH cell somata, tapering and varicose inner plexiform layer neurites, and varicose outer plexiform layer neurites all bear spines, that some of these spines are immunopositive for glutamate receptor and postsynaptic density proteins (viz., GluR1, GluR4, NR1, PSD-95, and PSD-93), that TH cell somata and tapering neurites are also immunopositive for a γ-aminobutyric acid (GABA) receptor subunit (GABA_A R_α1 ), and that a synaptic ribbon-specific protein (RIBEYE) is found adjacent to some colocalizations of GluR1 and TH in the inner plexiform layer. These results identify previously undescribed sites at which glutamatergic and GABAergic inputs may stimulate and inhibit dopamine release, especially at somata and along varicose neurites that emerge from these somata and arborize in various levels of the retina. J. Comp. Neurol. 525:1707-1730, 2017. © 2016 Wiley Periodicals, Inc.

Role of dopamine in distal retina

Dopamine is the most abundant catecholamine in the vertebrate retina. Despite the description of retinal dopaminergic cells three decades ago, many aspects of their function in the retina remain unclear. There is no consensus among the authors about the stimulus conditions for dopamine release (darkness, steady or flickering light) as well as about its action upon the various types of retinal cells. Many contradictory results exist concerning the dopamine effect on the gross electrical activity of the retina [reflected in electroretinogram (ERG)] and the receptors involved in its action. This review summarized current knowledge about the types of the dopaminergic neurons and receptors in the retina as well as the effects of dopamine receptor agonists and antagonists on the light responses of photoreceptors, horizontal and bipolar cells in both nonmammalian and mammalian retina. Special focus of interest concerns their effects upon the diffuse ERG as a useful tool for assessment of the overall function of the distal retina. An attempt is made to reveal some differences between the dopamine actions upon the activity of the ON versus OFF channel in the distal retina. The author has included her own results demonstrating such differences.

Effects of dopamine receptor blockade on the intensity-response function of ERG b- and d-waves in dark adapted eyes

The effects of dopamine receptor blockade by sulpiride (D2-class antagonist) and sulpiride plus SCH 23390 (D1-class antagonist) on the V - log I function of the ERG b- and d-waves were investigated in dark adapted frog eyes. We observed that sulpiride enhanced the amplitude of the suprathreshold b- and d-waves in the lower intensity range, where the responses were mediated by rods, but diminished it in the higher intensity range, where the responses were mediated by cones. A similar effect on the b-, but not d-wave amplitude was seen during the perfusion with sulpiride plus SCH 23390. The d-wave amplitude was enhanced over the whole intensity range with the exception of the highest intensities during the combined D1 and D2 receptor blockade. The results obtained indicate that the endogenous dopamine has an overall inhibitory action on the suprathreshold rod-mediated ON and OFF responses, while its action on the cone-mediated responses shows clear ON-OFF asymmetry. It is excitatory upon the ON responses, but inhibitory upon the OFF responses except for those in the highest intensity range. Participation of different types of dopamine receptors (predominantly D2 for the ON versus D1 for the OFF response) is probably responsible for this difference.

Regulation of synaptic transmission at the photoreceptor terminal: a novel role for the cation-chloride co-transporter NKCC1

The Na(+)-K(+)-2Cl(-) co-transporter type 1 (NKCC1) is localized primarily throughout the outer plexiform layer (OPL) of the distal retina, a synaptic lamina that is comprised of the axon terminals of photoreceptors and the dendrites of horizontal and bipolar cells. Although known to play a key role in development, signal transmission and the gating of sensory signals in other regions of the retina and in the CNS, the contribution of NKCC1 to synaptic transmission within the OPL is largely unknown. In the present study, we investigated the function of NKCC1 at the photoreceptor-horizontal cell synapse by recording the electrical responses of photoreceptors and horizontal cells before and after blocking the activity of the transporter with bumetanide (BMN). Because NKCC1 co-transports 1 Na(+), 1 K(+) and 2 Cl(-), it is electroneutral and its activation had little effect on membrane conductance. However, recordings from postsynaptic horizontal cells revealed that inhibiting NKCC1 with BMN greatly increased glutamate release from both rod and cone terminals. In addition, we found that NKCC1 directly regulates Ca(2+)-dependent exocytosis at the photoreceptor synapse, raising the possibility that NKCC1 serves to suppress bulk release of glutamate vesicles from photoreceptor terminals in the dark and at light offset. Interestingly, NKCC1 gene and protein expressions were upregulated by light, which we attribute to the light-induced release of dopamine acting on D1-like receptors. In sum, our study reveals a new role for NKCC1 in the regulation of synaptic transmission in photoreceptors.

Topical mydriatics affect light-evoked retinal responses in anesthetized mice

PURPOSE

To characterize effects of the muscarinic antagonist atropine (A) and the alpha-adrenergic agonist, phenylephrine (P), on mydriasis and light-evoked signaling in mice anesthetized by ketamine and xylazine (K+X).

METHODS

Pupillary areas of anesthetized C57BL/6 mice were measured, with or without topical application of A or A+P. Dark-adapted ERGs were recorded from 2- to 4-month-old C57BL/6 and 7.5-month-old albino hrhoG/hrhoG mice after application of A or P singly or in combination, before or after induction of K+X anesthesia. Effects of GABA were tested in the hrhoG/hrhoG mice.

RESULTS

K+X anesthesia resulted in maximum mydriasis that was not enhanced by A or A+P. Dark-adapted b-wave amplitudes (-1.3 log sc td s) after K+X anesthesia were similar with or without A or P. A+P in the presence of K+X produced a slow growth in b-wave amplitude, reaching a plateau of twofold enhancement in 1 hour. Recordings of responses to varying flash energies revealed that the effects of A+P were on the maximum amplitude of the a- and b-waves and not on their sensitivity. Scotopic threshold responses were augmented as well. In photoreceptor-degenerated mice (hrhoG/hrhoG), an electronegative ERG wave recorded with K+X+A, was converted to a gamma-aminobutyric acid (GABA)-sensitive response with two electropositive components with A+P after K+X.

CONCLUSIONS

Topical administration of A and P together, but not separately, in the presence of K+X, leads to a slow, dramatic enhancement of a- and b-waves by an unknown mechanism independent of pupil dilation.

Role of acetylcholine in nitric oxide production in the salamander retina

Although acetylcholine is one of the most widely studied neurotransmitters in the retina, many questions remain about its downstream signaling mechanisms. In this study we initially characterized the cholinergic neurotransmitter system in the salamander retina by localizing a variety of cholinergic markers. We then examined the link between both muscarinic and nicotinic receptor activation and nitric oxide production by using immunocytochemistry for cyclic guanosine monophosphate (cGMP) as an indicator. We found a large increase in cGMP-like immunoreactivity (cGMP-LI) in the inner retina in response to muscarinic (but not nicotinic) receptor activation. Based on the amplification of mRNA transcripts, receptor immunocytochemistry, and the use of selective antagonists, we identified these receptors as M2 muscarinic receptors. Using double-labeling techniques, we established that these increases in cGMP-LI were seen in GABAergic but not cholinergic amacrine cells, and that the increases were blocked by inhibitors of nitric oxide production. The creation of nitric oxide in response to cholinergic receptor activation may provide a mechanism for modulating the well-known mutual interactions of acetylcholine-glycine-GABA in the inner retina. As GABA and glycine are the primary inhibitory neurotransmitters in the retina, signaling pathways that modulate their levels or release will have major implications for the processing of complex stimuli by the retina.

Natural images and contrast encoding in bipolar cells in the retina of the land- and aquatic-phase tiger salamander

Intracellular recordings were obtained from 57 cone-driven bipolar cells in the light-adapted retina of the land-phase (adult) tiger salamander (Ambystoma tigrinum). Responses to flashes of negative and positive contrast for centered spots of optimum spatial dimensions were analyzed as a function of contrast magnitude. On average, the contrast/response curves of depolarizing and hyperpolarizing bipolar cells in the land-phase animals were remarkably similar to those of aquatic-phase animals. Thus, the primary retinal mechanisms mediating contrast coding in the outer retina are conserved as the salamander evolves from the aquatic to the land phase. To evaluate contrast encoding in the context of natural environments, the distribution of contrasts in natural images was measured for 65 scenes. The results, in general agreement with other reports, show that the vast majority of contrasts in nature are very small. The efficient coding hypothesis of Laughlin was examined by comparing the average contrast/response curves of bipolar cells with the cumulative probability distribution of contrasts in natural images. Efficient coding was found at 20 cd/m2 but at lower levels of light adaptation, the contrast/response curves were much too shallow. Further experiments show that two fundamental physiological factors-light adaptation and the nonlinear transfer across the cone-bipolar synapse are essential for the emergence of efficient contrast coding. For both land- and aquatic-based animals, the extent and symmetry of the dynamic range of the contrast/response curves of both classes of bipolar cells varied greatly from cell to cell. This apparent substrate for distributed encoding is established at the bipolar cell level, since it is not found in cones. As a result, the dynamic range of the bipolar cell population brackets the distribution of contrasts found in natural images.

Calcium channels at the photoreceptor synapse

Presynaptic Ca2+ channels mediate early stages of visual information processing in photoreceptors by facilitating the release of neurotransmitter and by receiving modulatory input that alters transmission. Two types of L-type Ca2+ channels, composed of alpha1F and alpha1D subunits and having similar biophysical andpharmacological properties, appear to form the principle voltage-dependent Ca2+ influx pathways in rods and cones, respectively. The role played by these channels in neurotransmitter release at these graded potential, non-spiking synapses, has been well described. The channels mediate sustained glutamate release in darkness where the cells rest at potentials near -40 mV, and signal increases in light intensity as the cells hyperpolarize negative to this value. Synaptic modulation and integration mediated by these channels has not yet been as fully described but appears to involve GABA, nitric oxide (NO), glutamate, and dopamine. Ca2+ permeable cyclic nucleotide gated (CNG) channels appear to have supporting roles at the photoreceptor output synapse and may transduce NO signals from other cells by either directly permitting Ca2+ influx or by providing depolarizing influences that gate voltage dependent Ca2+ channels.

Effects of APB, PDA, and TTX on ERG responses recorded using both multifocal and conventional methods in monkey. Effects of APB, PDA, and TTX on monkey ERG responses

Results from studies of human subjects suggest that the multifocal ERG technique developed by Erich Sutter and colleagues has considerable potential for assessment of retinal function in both the clinic and laboratory. While the utility of this measure depends to a large extent upon an understanding of the physiological origin for the different response components, relatively little is known in this regard. For the experiments described in this report, we made ERG recordings using both multifocal and conventional methods. Intravitreal injections of APB, PDA, and TTX were used to identify contributions from activity in ON pathway, OFF pathway, and third order retinal neurons, respectively. The results show that photoreceptor activity makes a small direct contribution to 1st and 2nd order multifocal photopic luminance responses. TTX-sensitive activity in third order retinal neurons contributes to both 1st and 2nd order responses with relatively greater contribution to the 2nd order response. Blockade of TTX-sensitive activity in third order cells produces effects on the 2nd order response which are very similar to changes observed in eyes suffering selective loss of retinal ganglion cells resulting from experimental glaucoma. Effects of these intravitreally injected test agents were also determined, in the same recording session, for flash, 30 Hz flicker, and oscillatory potential responses.

A dopamine- and protein kinase A-dependent mechanism for network adaptation in retinal ganglion cells

Vertebrates can detect light intensity changes in vastly different photic environments, in part, because postreceptoral neurons undergo "network adaptation." Previous data implicated dopaminergic, cAMP-dependent inhibition of retinal ganglion cells in this process yet left unclear how this occurs and whether this occurs in darkness versus light. To test for light- and dopamine-dependent changes in ganglion cell cAMP levels in situ, we immunostained dark- and light-adapted retinas with anti-cAMP antisera in the presence and absence of various dopamine receptor ligands. To test for direct effects of dopamine receptor ligands and membrane-permeable protein kinase ligands on ganglion cell excitability, we recorded spikes from isolated ganglion cells in perforated-patch whole-cell mode before and during application of these agents by microperfusion. Our immunostainings show that light, endogenous dopamine, and exogenous dopamine elevate ganglion cell cAMP levels in situ by activating D1-type dopamine receptors. Our spike recordings show that D1-type agonists and 8-bromo cAMP reduce spike frequency and curtail sustained spike firing and that these effects entail protein kinase A activation. These effects resemble those of background light on ganglion cell responses to light flashes. Network adaptation could thus be produced, to some extent, by dopaminergic modulation of ganglion cell spike generation, a mechanism distinct from modulation of transmitter release onto ganglion cells or of transmitter-gated currents in ganglion cells. Combining these observations with results obtained in studies of photoreceptor, bipolar, and horizontal cells indicates that all three layers of neurons in the retina are equipped with mechanisms for adaptation to ambient light intensity.

The minimal local-asperity hypothesis of early retinal lateral inhibition

Recently we found that the theories related to information theory existent in the literature cannot explain the behavior of the extent of the lateral inhibition mediated by retinal horizontal cells as a function of background light intensity. These theories can explain the fall of the extent from intermediate to high intensities, but not its rise from dim to intermediate intensities. We propose an alternate hypothesis that accounts for the extent's bell-shape behavior. This hypothesis proposes that the lateral-inhibition adaptation in the early retina is part of a system to extract several image attributes, such as occlusion borders and contrast. To do so, this system would use prior probabilistic knowledge about the biological processing and relevant statistics in natural images. A key novel statistic used here is the probability of the presence of an occlusion border as a function of local contrast. Using this probabilistic knowledge, the retina would optimize the spatial profile of lateral inhibition to minimize attribute-extraction error. The two significant errors that this minimization process must reduce are due to the quantal noise in photoreceptors and the straddling of occlusion borders by lateral inhibition.

Influence of light and neural circuitry on tyrosine hydroxylase phosphorylation in the rat retina

Light has been shown to increase dopamine synthesis and release in vertebrate retinas, but the retinal circuits mediating the light signal are unknown. We utilized three antibodies which recognize phosphorylated forms of tyrosine hydroxylase (TH) at serines 19, 31, and 40 to study the effects of light and neuroactive drugs on TH phosphorylation in the rat retina. Phosphorylated TH and total TH immunoreactivities were co-localized exclusively in retinal neurons whose shape and location are characteristic of dopaminergic interplexiform cells. Phosphorylated TH was weak to absent in darkness, but light strongly stimulated phosphorylation in all the three serine residues. Light-induced phosphorylation of TH induction by light was uniformly blocked by a combination of NMDA and AMPA glutamate receptor antagonists. In darkness, the combination of NMDA+AMPA induced phosphosphorylation of TH at serines 19 and 40 but it was weak at serine 31. A GABA(A) antagonist had the same effect. An agonist of depolarizing (ON) bipolar cells, L-(+)-2-amino-4-phosphonobutyric acid, did not prevent light-induced phosphorylated TH formation. Carbachol, a non-specific cholinergic agonist, selectively induced phosphorylation of TH at serine 31 in darkness, an effect which was blocked by the nicotinic antagonist, d-tubocurarine. These results show that retinal circuits involving glutamatergic, GABAergic and cholinergic synapses influence phospho-TH formation at different serine residues in this enzyme. Gamma amino butyric acid (GABA) and glutamate influence TH phosphorylation at serines 19 and 40, whereas cholinergic inputs affect its phosphorylation at serine 31.

Gain of rod to horizontal cell synaptic transfer: relation to glutamate release and a dihydropyridine-sensitive calcium current

We related rod to horizontal cell synaptic transfer to glutamate release by rods. Simultaneous intracellular records were obtained from dark-adapted rod-horizontal cell pairs. Steady-state synaptic gain (defined as the ratio of horizontal cell voltage to rod voltage evoked by the same light stimulus) was 3.35 +/- 0.60 for dim flashes and 1.50 +/- 0.03 for bright flashes. Under conditions of maintained illumination, there was a measurable increment of horizontal cell hyperpolarization for each light-induced increment of rod hyperpolarization over the full range of rod voltages. In separate experiments we studied glutamate release from an intact, light-responsive photoreceptor layer, from which inner retinal layers were removed. Steady light reduced glutamate release as a monotonic function of intensity; spectral sensitivity measures indicated that we monitored glutamate release from rods. The dependence of glutamate release on rod voltage was well fit by the activation function for a high-voltage-activated, dihydropyridine-sensitive L-type calcium current, suggesting a linear dependence of glutamate release on [Ca]i in the synaptic terminal. A simple model incorporating this assumption accounts for the steady-state gain of the rod to horizontal cell synapse.

Dynamic processes shape spatiotemporal properties of retinal waves

In the developing mammalian retina, spontaneous waves of action potentials are present in the ganglion cell layer weeks before vision. These waves are known to be generated by a synaptically connected network of amacrine cells and retinal ganglion cells, and exhibit complex spatiotemporal patterns, characterized by shifting domains of coactivation. Here, we present a novel dynamical model consisting of two coupled populations of cells that quantitatively reproduces the experimentally observed domain sizes, interwave intervals, and wavefront velocity profiles. Model and experiment together show that the highly correlated activity generated by retinal waves can be explained by a combination of random spontaneous activation of cells and the past history of local retinal activity.

Effects of horizontal cell network architecture on signal spread in the turtle outer retina. Experiments and simulations

In the Pseudemys turtle retina five functionally distinct, electrically coupled networks of horizontal cells distribute signals in the outer plexiform layer. These networks differ significantly in their architecture, as determined by intracellular labeling with Neurobiotin after physiological recording and identification. The density of H1 horizontal cells is highest, ranging around 1800 cells/mm2 at approximately 2.3 mm eccentricity. H1 horizontal cell somata are connected via 6-10 thin, short dendrites. The H1 horizontal cell axon terminal network is composed of thick axon terminals, forming a three-dimensional, sheath-like structure. Networks of coupled H2 and H3 horizontal cells have cell densities of around 210 cells/mm2 and 350 cells/mm2, respectively, at the same eccentricity of 2.3 mm. Cell bodies are connected with 6-12 long, thin dendrites. Here we report for the first time H4 horizontal cell networks. Cell density is approximately 970 cells/mm2 at 2 mm eccentricity, and cell bodies are connected with 6-10 thin, short dendrites. General properties of passive voltage spread were compared for three of these horizontal cell networks using NeuronC. Realistic network architectures were obtained by digitizing the intracellularly labeled networks, respectively. One network obtained from coupled H1 horizontal cell bodies, one from coupled H1 horizontal cell axon terminals, and one from H2 horizontal cells were simulated. These three realistic networks were compared with an artificial, electrically coupled regular triangular network. Passive signal spread in these networks strongly depended on the exact network architecture using otherwise identical parameters. Changes in coupling strength affected signal spread in these networks differently. As in the experimental situation, changes in synaptic conductance influenced signal spread. Some principal effects of extensively coupled horizontal cells on photoreceptor signal processing were simulated with one type of photoreceptor connected by telodendria, synapsing onto an underlying triangular network and receiving feedback synapses. Under certain conditions, spatial information is coded in single photoreceptors. This was also the case in the experimental situation. In the simulation, spatial filter adjustment for optimal spatial coding in photoreceptors can be achieved by changing coupling strength in the horizontal cell network.

Cholinergic modulation of dopamine release and horizontal cell coupling in mudpuppy retina

The effects of cholinergic agonists and antagonists on electrical coupling between horizontal cells were studied in dark-adapted mudpuppy retinas. Carbachol and the nicotinic agonist 1,1-dimethyl-4-phenylpiperazinium (DMPP) uncoupled horizontal cells, but the muscarinic agonist oxotremorine did not. The uncoupling effects of carbachol and DMPP were blocked by the nicotinic antagonist D-tubocurarine and by the dopamine antagonist fluphenazine, indicating that carbachol uncoupled horizontal cells by stimulating dopamine release via nicotinic receptors. Carbachol also caused an increase in release of [3H]dopamine from retinas. D-Tubocurarine increased horizontal cell coupling, indicating that tonic cholinergic input was present in dark-adapted retinas. D-Tubocurarine did not reduce light-evoked uncoupling of horizontal cells, suggesting that cholinergic neurons are not an essential part of the direct pathway by which light causes an immediate increase in dopamine release.