Effects Of APB Research Articles

APB differentially affects the cone contributions to the zebrafish ERG.

APB (DL-2-amino-4-phosphonobutyric acid) has been found to affect the retinal processing of many vertebrate species as evidenced by the suppression of the b-wave component of the electroretinogram (ERG). The present study examined the effects of APB on the cone contributions to the ERG response of adult zebrafish (Danio rerio). ERG responses were obtained from light-adapted adult zebrafish following intravitreal injection of either saline alone or saline with various concentrations of APB ranging from 10 microm to 500 microM. Visual stimuli were 200-ms flashes of various wavelengths and irradiances. Spectral sensitivity functions were calculated from the irradiance versus response amplitude functions of the a-, b-, and d-wave components of the ERG response. Saline had no effects on the ERG response. However, APB had differential effects on the sensitivity of the b- and d-wave components. The effects of APB on the b-wave component were most apparent in the ultraviolet and short-wavelength portions (320-440 nm) of the spectral sensitivity function, although the b-wave was not completely eliminated at these wavelengths. APB-treated subjects were found to possess the same cone mechanisms (L-M and M-S) in the middle- and long-wavelength areas of the spectrum as saline injected subjects, although absolute sensitivity was lower for the APB-injected subjects. Spectral sensitivity based on the d-wave response was affected by APB but only in the short-wavelength region. All results appear to be independent of the APB dose. These results support the notion that glutamate receptors play a specific role in zebrafish visual processing. In addition, the effects of APB support recent anatomical evidence that the zebrafish retina may possess different types of glutamate receptors.

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.

Selective blockade of a slowly inactivating potassium current in striatal neurons by (+/-) 6-chloro-APB hydrobromide (SKF82958).

The ion channels of rat striatal neurons are known to be modulated by stimulation of D1 dopamine receptors. The susceptibility of depolarization-activated K+ currents to be modulated by the D1 agonist, 6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetra-hydro-1H-3-benzaze pine (APB) was investigated using whole-cell voltage-clamp recording techniques from acutely isolated neurons. APB (0.01-100 microM) produced a concentration-dependent reduction in the total K+ current. At intermediate concentrations (ca. 10 microM), APB selectively depressed the slowly inactivating A-current (I(As)). A similar effect was produced by application of the D1 agonist, 7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1-H-2-benzazepine (SKF38393, 10 microM). APB reduced I(As) rapidly, having onset and recovery time constants of 1.2 sec and 1.6 sec, respectively. Unexpectedly, the effect of APB could not be mimicked by application of Sp-adenosine 3',5'-cyclic monophosphothioate triethylamine (Sp-cAMPS, 100-200 microM), a membrane-permeable analog of cyclic AMP (cAMP), or by pretreatment with forskolin (25 microM), an activator of adenylyl cyclase. The reduction in I(As) also was not blocked by pretreatment with the D1 receptor antagonist, R(+)-SCH23390 hydrochloride (SCH23390, 10-20 microM). In addition, intracellular dialysis with guanosine-5'-O-(2-thiodiphosphate (GDP-beta-S, 200 microM) did not preclude the APB-induced inhibition of I(As), nor did dialysis with guanosine-5'-O-(3-thiotriphosphate (GTP-gamma-S, 400 microM) prevent reversal of the effect. The effect of APB was produced by a reduction in the maximal conductance of I(As) without changing the voltage-dependence of the current. Collectively, these results argue that APB does not inhibit I(As) through D1 receptors coupled to stimulation of adenylyl cyclase, but rather by allosterically regulating or blocking the channels giving rise to this current.

Modulation of chromatic difference in receptive field size of H1 horizontal cells in carp retina: dopamine- and APB-sensitive mechanisms

Chromatic aspects of receptive field size in the H1 horizontal cell syncytium of the carp retina were investigated using spectral photostimuli (blue or red) presented in the form of either a pair of a small spot and annulus, or a narrow moving slit. In the light-adapted retina, the receptive field for the blue stimulus was found to be significantly smaller than that for the red, i.e. there was a chromatic difference in the receptive field size. During the course of dark adaptation, the overall receptive field size increased, but the chromatic difference decreased. Immediately after adaptation to bright light, the receptive field sizes were reduced significantly, but the chromatic difference increased, mainly due to a greater reduction in the receptive field for the blue stimulus. Application of dopamine (5 μM) to a dark-adapted retina gradually decreased the receptive field size for both colours, but the chromatic difference became larger, again due to a greater reduction in the receptive field size for the blue stimulus. 2-Amino-4-phosphonobutyrate (APB) applied to light-adapted retinae at a working concentration of 1 mM, greatly expanded the receptive field size and suppressed the chromatic difference due to the effect being greater for the receptive field for the blue stimulus. The effect of APB was slow and cumulative. On the other hand, intracellular injection of cGMP or dibutyryl-cGMP increased the chromatic difference in the receptive field size. It is suggested (i) that the chromatic difference in the receptive field size could be due to a cGMP-coupled, conductance-decreasing receptor mechanism activated by APB; and (ii) that the mechanism is associated with short-wavelength sensitive cone input to the H1 cells and operates in the light-adapted state of the retina.

APB alters photopic spectral sensitivity of the goldfish retina

Because the glutamate analog 2-amino-4-phosphonobutyric acid (APB) alters synaptic transmission at the outer plexiform layer in goldfish we asked whether intraocular injection of APB would alter the spectral sensitivity of the retina. The spectral sensitivity of the ON and OFF components of the optic nerve response (ONR) in goldfish was measured in the presence and absence of APB, under four chromatic adaptation condition. APB decreased absolute sensitivity and altered spectral sensitivity for both ON and OFF responses under each adaptation condition. The spectral sensitivity of the OFF response was altered most at short wavelengths, in a manner consistent with a change in the balance of additive cone inputs. For the ON response, the effects of APB were consistent with a change in spectral antagonism, particularly between M- and L-cones. These results suggest that the activity in the retinal cone pathways in goldfish can be influenced by a mechanism incorporating an APB-sensitive receptor, and that this receptor may be intimately involved with setting the balance of cone inputs to spectrally-opponent neurons.

Effects of 2-amino-4-phosphonobutyric acid on cells in the distal layers of the tiger salamander's retina.

1. We studied the effects of 2-amino-4-phosphonobutyric acid (APB) on the response properties of rods, horizontal cells and bipolar cells in the isolated, perfused retina of the tiger salamander, Ambystoma tigrinum. A concentration of 100 microM was found to be sufficient to elicit maximal effects. 2. Rods hyperpolarized slightly upon exposure to 100 microM-APB and their response amplitudes were slightly reduced. The amplitude of the cone-generated component of the rod's response to 700 nm light was not significantly affected by APB. 3. Horizontal cells hyperpolarized by 2-5 mV upon exposure to 100 microM-APB. The rod-driven component of the horizontal cell response increased in amplitude while the cone-driven component decreased in amplitude. APB thus causes an increase in voltage gain between rods and horizontal cells and a decrease in cone/horizontal cell gain. These findings can be explained in terms of an APB-induced reduction in transmitter release from the cones. 4. APB at a concentration of 100 microM caused an increase in the length constant of the horizontal cell syncytium. Our analysis shows this to be due primarily to a 50% reduction in the coupling impedance between the cells of the syncytium. 5. The effects of APB on off-centre bipolar cells were qualitatively similar to those on horizontal cells. APB increased the amplitudes of rod-driven responses and reduced those of cone-driven responses. The length constants, both of the receptive field centre and of the surround, were increased and the strength of the surround relative to the centre was reduced by about 20%. 6. APB abolished the depolarizing light responses of the receptive field centres of on-centre bipolar cells. A hyperpolarizing response remained whose spatial properties were similar to those of the receptive field surround. We believe this response to reflect a direct (feedforward) input to on-centre bipolar cells from horizontal cells.

2-amino-4-phosphonobutyric acid exerts a light-dependent effect on post-gabaculine levels of retinal gamma-aminobutyric acid (GABA): evidence that ON synaptic pathways regulate retinal GABAergic transmission.

The effects of light, 2-amino-4-phosphonobutyric acid (APB), and kainic acid on rat retinal gamma-aminobutyric acid (GABA)-ergic transmission were studied by measuring levels of retinal GABA following subcutaneous injection of gabaculine, an irreversible inhibitor of GABA-transaminase. Post-gabaculine levels of retinal GABA in light-exposed rats were significantly greater than those in rats held in darkness. The synaptic mechanism of this effect of light was examined by measuring post-gabaculine levels of retinal GABA in rats placed into either lighted or darkened conditions after receiving unilateral intravitreal injections of APB, a glutamate analogue that selectively decreases the activity of ON synaptic pathways in the retina. APB attenuated the post-gabaculine accumulation of GABA in rats held in the light, but not in those placed into darkness. Furthermore, the light-dependent increment in post-gabaculine accumulation of retinal GABA was entirely APB sensitive, and the effect of APB was entirely light dependent. In contrast to APB, kainic acid stimulated the post-gabaculine accumulation of retinal GABA in vivo. Our findings suggest that APB and kainic acid influence GABAergic transmission at different sites in the retina and that some retinal GABAergic neurons are either ON or ON-OFF amacrine cells.

Dopamine and 2-amino-4-phosphonobutyrate differentially modify spectral responses of H1 horizontal cells in carp retina

Cone-driven external H1 horizontal cells (H1 HCs) in the cyprinid fish retina hyperpolarize in response to all visible lights, and their synaptic inputs have been widely believed to be excitatory. Recent experiments indicate, however, that short- and long-lambda (wavelength)-sensitive cone photoreceptors have different types of synaptic mechanisms; a conductance-decreasing, sign-reversing and short-lambda-mediating type, and a more conventional conductance-increasing class of excitatory (sign-conserving) synapse transmitting mainly long-lambda signals to H1 HCs. Here, a new set of evidence is presented for such spectrally segregated synaptic multiplicity, which also supports the notion that H1 HCs are actually color-opponent units where the depolarizing response component due to short-lambda-sensitive cones is normally overshadowed by the dominant hyperpolarizing component ascribed to long-lambda-mediating synaptic inputs. Application of dopamine to the retina preferentially enhanced the H1 HC responses to long-lambda flashes, and also depolarized the resting membrane potential in the dark. The spectral response was also examined after applying APB (2-amino-4-phosphonobutyric acid), in the presence of dopamine included to avoid polysynaptic effects of APB. This treatment enhanced the H1 HC responses to short-lambda stimuli and hyperpolarized the resting potential. These results are consistent with the suggestion that dopamine potentiates the conductance-increasing and long-lambda-mediating excitatory transmitter action, whereas APB acts as an agonist at the receptor involved in the conductance-decreasing and short-lambda-mediating transmitter action.

APB Selectively Reduces Visual Responses in Goldfish to High Spatiotemporal Frequencies

Visual responses of goldfish to rotating square-wave gratings were recorded before and after intraocular injection of 2-amino-4-phosphonobutyric acid (APB). High doses of APB reduced the rate of optokinetic nystagmus (OKN) to a relatively high spatial frequency grating moving at a high temporal frequency. Responses to a low spatial frequency grating were not altered, nor were responses to the higher spatial frequency when it rotated slowly. The effects of APB were transient and lasted no longer than 3 d. We conclude that APB reduces OKN to high spatiotemporal frequencies in goldfish.