Postnatal development of GABA- and glycine-mediated inhibition of feline retinal ganglion cells in the area centralis

Light-evoked synaptic activity of retinal ganglion and amacrine cells is regulated in developing mouse retina

Recent studies have shown a continued maturation of visual responsiveness and synaptic activity of retina after eye opening, including the size of receptive fields of retinal ganglion cells (RGCs), light-evoked synaptic output of RGCs, bipolar cell spontaneous synaptic inputs to RGCs, and the synaptic connections between RGCs and ON and OFF bipolar cells. Light deprivation retarded some of these age-dependent changes. However, many other functional and morphological features of RGCs are not sensitive to visual experience. To determine whether light-evoked synaptic responses of RGCs undergo developmental change, we directly examined the light-evoked synaptic inputs from ON and OFF synaptic pathways to RGCs in developing retinas, and found that both light-evoked excitatory and inhibitory synaptic currents decreased, but not increased, with age. We also examined the light-evoked synaptic inputs from ON and OFF synaptic pathways to amacrine cells in developing retinas and found that the light-evoked synaptic input of amacrine cells is also downregulated in developing mouse retina. Different from the developmental changes of RGC spontaneous synaptic activity, dark rearing has little effect on the developmental changes of light-evoked synaptic activity of both RGCs and amacrine cells. Therefore, we concluded that the synaptic mechanisms mediating spontaneous and light-evoked synaptic activity of RGCs and amacrine cells are likely to be different.

Visual dysfunction in the spontaneously hypertensive rat

We report two forms of visual defect in the spontaneously hypertensive rat (SHR) compared to its normotensive control strain (WKY). Ten-month-old male SHR and WKY were tested for intensity discrimination ability using a Y-maze and food reward. The SHR took more trials in the acquisition phase of the task using white light. In the test phase, in which the white light intensity difference was reduced, there was no significant difference in performance between SHR and WKY. Also, there was no significant difference between the strains when red light was used. The defect in SHR occurred in the blue range of the spectrum. Using blue light, the performance of SHR declined significantly at an intensity difference of 1.15 lux, whereas the performance of WKY did not decline significantly until the difference reached 0.01 lux. The defect in SHR for intensity discrimination was not related to cataract formation, but we detected a strain difference in the nuclei of the photoreceptor cells. Thus, this visual dysfunction may involve the rod photoreceptors. Further testing revealed a deficit in visual acuity in SHR aged 40-66 days, prior to the development of hypertension. Thus, the visual dysfunction of SHR is not secondary to their development of hypertension, but rather a discrete characteristic of the strain.

GABA responses and their partial occlusion by glycine in cultured rat medullary neurons

Whole-cell current responses to bath application of GABA and glycine were studied in medullary neurons cultured from embryonic rats. Two current components were seen in the responses to bath application of GABA, one component which desensitized and another which did not. These two current components have different dose-response characteristics for GABA, with the nondesensitizing component being activated more effectively and reaching its peak amplitude at lower agonist concentrations than the desensitizing one. The agonist concentrations producing half of the maximum responses are 2.8 +/- 0.3 (+/- S.E.M., n = 9) and 14.7 +/- 2.7 (n = 5) microM for the nondesensitizing and desensitizing components, respectively. The two current components for GABA are differentially affected by the antagonists, picrotoxin and bicuculline. The antagonist concentrations which block 50% of the control desensitizing and nondesensitizing responses to GABA are 33 and 320 microM for picrotoxin, and 3 and 50 microM for bicuculline, respectively. Thus, the characteristics of the GABA responses are analogous to those described previously for glycine in that there are two components which are differentially sensitive to agonist concentration [Lewis et al. (1991) J. Neurophysiol, 40, 1178-1187]. We now find there is occlusion between the responses to GABA and glycine, indicating that they share a population of receptors or channels. The occlusion was incomplete (< 80%) in half of the cells, suggesting that both agonists also activate unique receptors. Furthermore, the current responses to 35 microM GABA are blocked by the glycinergic antagonist, strychnine, with half-maximal blocking concentrations equal to 2 and 30 microM for the desensitizing and nondesensitizing components, respectively. This strychnine sensitivity is less than that for the glycine receptor. At the same time, the current responses to 100 microM glycine are sensitive to the GABAergic antagonists, picrotoxin and bicuculline. The half-maximal blocking concentrations are 36 and 120 microM picrotoxin, and 120 and 500 microM bicuculline, for the desensitizing and nondesensitizing components of the glycine response, respectively. Consequently, these results suggest that these cultured cells have at least three types of inhibitory receptors: glycine receptors, GABA receptors and GABA/glycine receptors, with all three receptors sensitive to block by strychnine, bicuculline and picrotoxin. The GABA/glycine receptor may be an immature form of the inhibitory receptor. Alternatively, some GABA and glycine receptors may have common ionophores.

Actions of baclofen and phaclofen upon ON- and OFF-ganglion cells in the cat retina

The effects of the GABAB (gamma-amino butyric acidB) receptor agonist, baclofen and its antagonist, phaclofen on physiologically identified retinal ganglion cells were studied in the optically intact eye of pentobarbitone-anaesthetized cats. These results were compared with the effects of the GABAA receptor agonist, muscimol and its antagonist, bicuculline. Baclofen inhibited the total visually driven firing of both ON- and OFF-cells more effectively upon OFF- than ON-cells; this action was weaker and slower than that of muscimol. Whilst bicuculline raised the firing level of only ON-cells in the area centralis together with all peripheral cells, phaclofen raised that of all OFF-cells. Paradoxically, in OFF-cells, baclofen enhanced the driven transient component and suppressed the sustained component, whilst phaclofen raised the sustained component. Thus, GABAB receptors activated by tonically released GABA may modulate the sustained and transient excitatory inputs to OFF-cells.

Excitatory amino acid receptors on sustained retinal ganglion cells in the kitten during the critical period of development

Effects of iontophoretically applied excitatory amino acid analogues, kainate, quisqualate and N-methyl-D-aspartate (NMDA) and their receptor antagonists on the sustained class of retinal ganglion cells were studied in the optically intact eye of pentobarbitone-anaesthetized kittens (7-9 weeks of age). These results were compared with the effects obtained in adult cats. All 3 excitatory amino acid agonists had excitatory actions on the majority of On- and Off-sustained ganglion cells in the kitten but at higher current levels than those required for adult cells, suggesting all 3 types of receptors of weaker sensitivity are present on the kitten cells. Whilst the relative potency of kainate, quisqualate and NMDA was 15:3:1 in the adult cells, it was 5:2:1 in the kitten cells. As for other neurones in the CNS, an increase in the potency of kainate receptors and a decrease in that of NMDA receptors appear, therefore, to characterize the postnatal development of the excitatory amino acid receptors on the retinal ganglion cells. In accordance with the agonist results, a broadband receptor antagonist, kynurenate, powerfully antagonised responses of kitten cells as well as those of adult cells. The pure NMDA receptor antagonist, 3((+-)-2-carboxypiperazin-4-yl)propyl-1-phosphonate (CPP), however, only suppressed spontaneous firing of kitten cells. Furthermore, in kitten cells, the visually-driven firing was depressed while the level of firing was raised by these excitatory amino acid analogous, and a long period of inhibition of firing followed the agonist-induced excitation.(ABSTRACT TRUNCATED AT 250 WORDS)

Benzodiazepines and the mammalian retina. II. Actions on retinal ganglion cells

The effects of intravenously and iontophoretically applied benzodiazepines, midazolam and flurazepam and their receptor antagonist, flumazepil (RO 15 1788), on visually evoked and spontaneous activities of the mammalian retinal ganglion cells have been studied. Intravenously applied midazolam and flurazepam suppressed both light-evoked and spontaneous firing of rat optic tract fibres. They reduced both the sensitivity to light and the temporal resolution of the fibres. Flumazepil (RO-15-1788), on the other hand, enhanced both light-evoked and spontaneous firing of the optic tract fibres. It increased their light sensitivity but did not affect their temporal resolution. In the cat, iontophoretically applied benzodiazepines suppressed and flumazepil increased the receptive field centre and surround response as well as the spontaneous firing of ON-type retinal ganglion cells which mainly receive GABAergic inputs. However, these drugs did not affect the activities of the OFF-type cells which mainly receive glycinergic inputs. These results suggest not only that the action of benzodiazepines on the retinal ganglion cells is mediated by benzodiazepine receptors that are linked with GABA receptors, but also that the retinal benzodiazepine receptors receive an endogenous benzodiazepine receptor ligand.

Benzodiazepines and the mammalian retina. III. Postnatal development

Iontophoretic effects of the benzodiazepine receptor agonist, flurazepam, and antagonist, flumazepil (RO-15-1788) on the retinal ganglion cells of kittens (7-9 weeks of age) have been compared with those of adult cats (18-22 weeks of age). In the adult retina, flurazepam decreased and flumazepil increased the visually evoked and spontaneous firing of ON-, but produced no effects on the response of OFF-retinal ganglion cells. However, in the kittens retina, in which ON-cells' selectivity to GABA is not fully developed, both the visually evoked and spontaneous activities of ON- and OFF-cells were inhibited by flurazepam and enhanced by flumazepil. This suggests that postnatal development of benzodiazepine action parallels that of GABA action at the retinal ganglion cells in the cat retina.

Properties of excitatory amino acid receptors on sustained ganglion cells in the cat retina

Iontophoretic effects of N-methyl-D-aspartate, quisqualate and kainate and a variety of excitatory amino acid receptor antagonists, on retinal ganglion cells, were studied in optically intact eyes of barbiturate anesthetized cats. All three agonists raised the spontaneous firing of both ON- and OFF-sustained retinal ganglion cells, with the potency order of kainate much greater than quisqualate greater than N-methyl-D-aspartate. However, the excitatory amino acid analogues readily saturated the receptors and reduced the visually driven firing of cells with high spontaneous firing, but mimicked an increase in endogenous excitatory amino acid release and raised the visually induced response in cells with low spontaneous firing. The quinoxaline compound, 6-cyano-2,3 dihydroxy-7-nitroquinoxaline and 6-7-dinitroquinoxaline-2,3-dione, blocked the visually driven firing and kainate- and quisqualate-induced excitation, whilst 3[+)-2-carboxypiperazin-4-yl)propyl-1-phosphonate, antagonized the N-methyl-D-aspartate-induced excitation, but failed to block visually driven firing of the retinal ganglion cells. The broadband excitatory amino acid receptor antagonists, such as kynurenate, were also effective in antagonizing the visually driven response and also blocked the N-methyl-D-aspartate- as well as kainate- and quisqualate-induced responses. These results suggest that the receptors at the bipolar/ganglion cell synapse are of the non-N-methyl-D-aspartate type, but that N-methyl-D-aspartate receptors are also present on ganglion cells although their physiological role is unclear.

Development of neurochemical separation of ON and OFF channels at retinal ganglion cells

Anatomical and physiological segregation of neurons into ON (brightening detector) and OFF (darkening detector) channels in the retina and subsequent visual system ensure the high sensitivity required for contrast detection and spatial discrimination. This segregation is finest at the visual axis. Neurochemically, ON and OFF ganglion cells at the visual axis seem to be distinguished by different inhibitory transmitters but not excitatory transmitters. Microiontophoretic studies of inhibitory transmitters on the retinal ganglion cells in kittens and adult cats suggest that this neurochemical distinction is poor in immature ganglion cells at the visual axis. Initially both ON and OFF cells seem to be supplied by GABAergic, glycinergic, and catecholaminergic amacrine cells, but in adults, ON cells remain supplied only by GABAergic amacrines, while OFF cells are supplied by glycinergic amacrines. Postnatal elimination of multiple inputs and strengthening of the appropriate inputs, as seen in the central nervous system, also seem to occur at the retinal neurotransmitter synapses during development.

Noradrenaline action on cat retinal ganglion cells is mediated by dopamine (D2) receptors

Effects of iontophoretically applied noradrenaline, dopamine and their receptor antagonists on the retinal ganglion cells, were studied in optically intact eyes of barbiturate-anaesthetized cats. Noradrenaline inhibited visually evoked and spontaneous firing of all classes of retinal ganglion cells: the effect being greater on ON- than on OFF-cells and slightly more potent than dopamine on a given cell. All alpha- and beta-adrenoreceptor blockers tested tended to change spikes, but were generally ineffective in blocking the noradrenaline-induced inhibition, when not affecting spikes. The noradrenaline-induced inhibition was, however, effectively blocked by dopamine D2-receptor antagonists. The alpha- and beta-adrenoreceptor antagonists applied alone had no effect, suggesting the absence of endogenous noradrenergic antagonism, although alpha-type adrenergic antagonism was suggestive on a very small number of cells. These results suggest that: (1) noradrenaline action on cat retinal ganglion cells is mediated via dopamine D2-receptors; (2) noradrenaline is not generally released on them, except there may be physiologically active alpha-receptors on a few cells; and (3) many of the adrenoreceptor blockers affect membrane properties of the retinal ganglion cells, in a similar manner to local anaesthetics.

Evidence for dopaminergic innervation on kitten retinal ganglion cells

Effects of iontophoretically applied dopamine and its receptor antagonists on physiologically identified retinal ganglion cells were studied in the optically intact eye of pentobarbitone anaesthetised kittens (7-9 weeks of age) and the results were compared with the effects obtained from adult cats (18-22 weeks of age). In both the adult and the kitten, dopamine had an inhibitory effect on visually evoked and spontaneous activity of the retinal ganglion cells, irrespective of cell type. However, unlike in the adult, the effects of dopamine in kittens were variable and not dependent on retinal eccentricity. In adult cells, only L-sulpiride (a potent D2 receptor antagonist) reduced the inhibitory effect of exogenous dopamine, whereas in kitten cells, both alpha-flupenthixol (a potent D1 receptor antagonist) and L-sulpiride did so. When applied alone, neither alpha-flupenthixol nor affected the activity of ganglion cells in adults, but in kittens both antagonists produced an excitatory effect in some cells. Physiologically active dopaminergic innervation seemed, therefore, to be present on the immature ganglion cells, but was subsequently 'eliminated' during the course of postnatal development. Furthermore, in immature cells, both D1 and D2 type receptors are present but only D2 receptors remain in adult. Therefore, there is a mismatch between dopamine receptors and dopamine in the adult retina and this mismatch may be the consequence of a developmental event.

The action of inhibitory neurotransmitters, gamma-aminobutyric acid and glycine may distinguish between the area centralis and the peripheral retina in cats

The effects of iontophoretically applied gamma-aminobutyric acid (GABA) and glycine, and of their antagonists, bicuculline and strychnine, were compared between ganglion cells from the central and peripheral retinae of optically intact eyes in barbiturate-anaesthetised cats. The visual response of on-cells was inhibited by GABA and enhanced by bicuculline. The visual response of off-cells was inhibited by glycine and enhanced by strychnine. The sensitivity of cells to the transmitters was lower in the peripheral retina than in the area centralis, whilst the sensitivity to the antagonists was similar in both regions of the retina. Cells from the area centralis were inhibited by either GABA or glycine, but never both. Cells from the periphery were less selective and were inhibited by both transmitters.