Nonsynaptic NMDA receptors mediate activity-dependent plasticity of gap junctional coupling in the AII amacrine cell network.

Many neurons are coupled by electrical synapses into networks that have emergent properties. In the retina, coupling in these networks is dynamically regulated by changes in background illumination, optimizing signal integration for the visual environment. However, the mechanisms that control this plasticity are poorly understood. We have investigated these mechanisms in the rabbit AII amacrine cell, a multifunctional retinal neuron that forms an electrically coupled network via connexin 36 (Cx36) gap junctions. We find that presynaptic activity of glutamatergic ON bipolar cells drives increased phosphorylation of Cx36, indicative of increased coupling in the AII network. The phosphorylation is dependent on activation of nonsynaptic NMDA receptors that colocalize with Cx36 on AII amacrine cells, and is mediated by CaMKII. This activity-dependent increase in Cx36 phosphorylation works in opposition to dopamine-driven reduction of phosphorylation, establishing a local dynamic regulatory mechanism, and accounting for the nonlinear control of AII coupling by background illumination.

Nonlinearity and noise at the rod-rod bipolar cell synapse.

In the retina, rod bipolar (RBP) cells synapse with many rods, and suppression of rod outer segment and synaptic noise is necessary for their detection of rod single-photon responses (SPRs). Depending on the rods' signal-to-noise ratio (SNR), the suppression mechanism will likely eliminate some SPRs as well, resulting in decreased quantum efficiency. We examined this synapse in rabbit, where 100 rods converge onto each RBP. Suction electrode recordings showed that rabbit rod SPRs were difficult to distinguish from noise (independent SNR estimates were 2.3 and 2.8). Nonlinear transmission from rods to RBPs improved response detection (SNR = 8.7), but a large portion of the rod SPRs was discarded. For the dimmest flashes, the loss approached 90%. Despite the high rejection ratio, noise of two distinct types was apparent in the RBP traces: low-amplitude rumblings and discrete events that resembled the SPR. The SPR-like event frequency suggests that they result from thermal isomerizations of rhodopsin, which occurred at the rate 0.033/s/rod. The presence of low-amplitude noise is explained by a sigmoidal input-output relationship at the rod-RBP synapse and the input of noisy rods. The rabbit rod SNR and RBP quantum efficiency are the lowest yet reported, suggesting that the quantum efficiency of the rod-RBP synapse may depend on the SNR in rods. These results point to the possibility that fewer photoisomerizations are discarded for species such as primate, which has a higher rod SNR.

Note: Investigation of a null measurement of optical absorbance using a pulse width modulated light-emitting diode.

We examined the possibility of making a null measurement of optical absorbance by using pulse-width modulation to control the intensity of a light-emitting diode (LED). This type of measurement is intriguing because the instrumental sources of noise are different from those in the traditional absorbance measurement. Our experiment employed phototransistor detectors and a RC integrator to measure the amount of light being transmitted. However, instead of measuring the ratio of the detector voltages for a blank and sample, we measured the ratio of the LED duty cycles required to give the same detector voltage for a blank and a sample. Using this method, it is presumed that the same amount of light reaches the detector during measurement of the blank and sample.

Differential output of the high-sensitivity rod photoreceptor: AII amacrine pathway.

In the mammalian retina, the scotopic threshold of ganglion cells is in part dependent on how rod inputs are summed by their presynaptic cone bipolar cells. For ON cone bipolar cells, there are two anatomical routes for rod signals: 1) cone photoreceptors receive inputs via gap junctions with the surrounding, more numerous rods; and 2) ON cone bipolar cells receive highly convergent input via gap junctions with AII amacrine cells, which each receive input from hundreds of rods. Rod-cone coupling is thought to be utilized at higher photon fluxes relative to the AII-ON cone bipolar pathway due to the impedance mismatch of a single small rod driving a larger cone. Furthermore, it is widely held that the convergence of high-gain chemical synapses onto AIIs confers the highest sensitivity to ON cone bipolar cells and ganglion cells. A lack of coupling between one or more types of ON cone bipolar cells and AIIs would obviate this high-sensitivity pathway and explain the existence of ganglion cells with elevated scotopic thresholds. To investigate this possibility, we examined Neurobiotin and glycine diffusion from AIIs to bipolar cells and found that approximately one-fifth of ON cone bipolar cells are not coupled to AIIs. Unlike AII-AII coupling, which changes with ambient background intensity, the fraction of noncoupled ON cone bipolar cells was unaltered by dark or light adaptation. These data suggest that one of five morphologically distinct ON cone bipolar cell types is not coupled to AIIs and suggest that AII-ON cone bipolar coupling is modulated differently from AII-AII coupling.

Micro Photo Detector Fabricated of Ferroelectric-Metal Heterostructure.

The anomalous photovoltaic effect (APE) in ferroelectric thin films has been utilized for the development of an optical micro-detector active in the visible range (from 350 to 800 nm). La-doped Pb(Zr,Ti)O(3) (PLZT) ferroelectric films epitaxially grown on Pt(001)/Mg(001) substrate were fabricated into micro-detector arrays and characterized as to their optical response. The Au/PLZT/Pt/MgO device was self-polarized in the as-deposited form with the polarization vector perpendicular to film surface. The heterostructure photovoltage response ranged from 100 to 200 mV, and the photocurrent was ~30 nA/cm(2) for devices of ~250 µm diameter under illumination of 100 mW/cm(2) at wavelengths from 400 to 580 nm. Such micro-detectors can be used for optical sensors in MEMS devices as well as for electrical stimulators of biological cells.

Simultaneous contribution of two rod pathways to AII amacrine and cone bipolar cell light responses.

Rod signals traverse several synapses en route to cone bipolar cells. In one pathway, rods communicate directly with cones via gap junctions. In a second pathway, signals flow rods-rod bipolars-AII amacrines-cone bipolars. The relative contribution of each pathway to retinal function is not well understood. Here we have examined this question from the perspective of the AII amacrine. AIIs form bidirectional electrical synapses with on cone bipolars. Consequently, as on cone bipolars are activated by outer plexiform inputs, they too should contribute to the AII response. Rod bipolar inputs to AIIs were blocked by AMPA receptor antagonists, revealing a smaller, non-AMPA component of the light response. This small residual response did not reverse between -70 and +70 mV and was blocked by carbenoxolone, suggesting that the current arose in on cone bipolars and was transmitted to AIIs via gap junctions. The residual component was evident for stimuli 2 log units below cone threshold and was prolonged for bright stimuli, demonstrating that it was rod driven. Because the rod bipolar-AII pathway was blocked, the rod-driven residual current likely was generated via the rod-cone pathway activation of on cone bipolars. Thus for a large range of intensities, rod signals reach the inner retina by both rod bipolar-AII and rod-cone coupling pathways.

Multiple neuronal connexins in the mammalian retina.

Gap junctions are abundant in the mammalian retina and many neuronal types form neural networks. Several different neuronal connexins have now been identified in the mammalian retina. Cx36 supports coupling in the AII amacrine cell network and is essential for processing rod signals. Cx36 is probably also responsible for photoreceptor coupling. Horizontal cells appear to be extensively coupled by either Cx50 or Cx57. These results indicate that multiple neuronal connexins are expressed in the mammalian retina and that different cell types express different connexins.

Confocal analysis of reciprocal feedback at rod bipolar terminals in the rabbit retina.

Amacrine cells in the mammalian retina are famously diverse in shape and function. Here, we show that two wide-field GABA amacrine cells, S1 and S2, have stereotyped synaptic contacts with the appropriate morphology and distribution to perform specific functions. S1 and S2 both supply negative feedback to rod bipolar terminals and thus provide a substrate for lateral inhibition in the rod pathway. Synapses are specialized structures, and the presynaptic compartment is normally characterized by a swelling or varicosity. Each S1 amacrine cell has approximately 280 varicosities, whereas an S2 cell has even more, approximately 500 per cell. Confocal analysis shows that essentially all varicosities aggregate around rod bipolar terminals where they are apposed by postsynaptic GABA receptors. Each rod bipolar terminal is contacted by varicosities from approximately 25 different S1 and 50 different S2 amacrine cells. In fact, rod bipolar cells are the only synaptic target for S1 and S2 amacrine cells: all of the output from these two wide-field GABA amacrine cells goes to rod bipolar terminals. It has long been a puzzle why two amacrine cells, apparently with the same connections, are required. However, an analysis of the distribution of varicosities suggests that S1 and S2 amacrine cells provide different signals. S2 amacrine cells dominate within 200 mu from a rod bipolar terminal and can provide an inhibitory input with spatial characteristics that match the size of the surround signal recorded from AII amacrine cells in the rod pathway. In contrast, the larger, better-coupled S1 amacrine cells may provide a more distant network signal.

Glutamate receptors at rod bipolar ribbon synapses in the rabbit retina.

In the mammalian retina, maximum sensitivity is achieved in the rod pathway, which serves dark-adapted vision. Rod bipolar cells carry the highly convergent rod input and make ribbon synapses with two postsynaptic elements in the inner retina. One postsynaptic neuron is the AII amacrine cell, which feeds the rod signal into the cone pathways. The other postsynaptic element is either an S1 or S2 amacrine cell. These two wide-field GABA amacrine cells both make reciprocal synapses with rod bipolar terminals but their individual roles are unknown. AII and S1/S2 dendrites come in close together and form a dyad opposing the presynaptic ribbon, which is the site of glutamate release. Therefore, two postsynaptic neurons sense the very same neurotransmitter yet serve different functions in the rod pathway. This functional diversity could be derived partly from the expression of different glutamate receptors on each postsynaptic element. In this study, we labeled all pre- and postsynaptic combinations and a signal-averaging method was developed to locate glutamate receptor subunits. In summary, GluR2/3 and GluR4 are expressed by AII amacrine cells but not by S1/S2 amacrine cells. In contrast, the orphan subunit delta1/2 is exclusively located on S1 varicosities but not on AII or S2 amacrine cells. These results confirm the prediction of divergence mediated by different glutamate receptors at the rod bipolar dyad. Each different amacrine cell type appears to express specific glutamate receptors. Finally, the differential expression of glutamate receptors by S1 and S2 may partly explain the need for two wide-field GABA amacrine cells with the same feedback connections to rod bipolar terminals.