Retioal bipolar cells and ganglion cells in Mammalians and Amphibians

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Abstract People gradually pay more and more attention and interested in the type and function of retioal bipolar cells and ganglion cells in Mammalian and Amphibian animals. This essay mainly illustrating the function combining with the two kinds of cell type above. The formation of synapses with amacrince cells, which main function through release of GABA or glycine neurotransmitter, emphatically discussed in this paper to expain the mechanism of amacrine cells release GABA and then respectively unit with GABAA、GABAB and GABAC receptors.

Keywords bipolar cell ganglion cell GABA receptors amacrine cell

The retina is a light-sensitive tissue lining the inner surface of the eye, which create an image of the visual world just to serves much like the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical events , ultimately trigger nerve impulse, sent to variou centers through the fibers of the optic nerve of the brain. And retinal nerve have six types, while will illustrate some types of it.

1.Retinal bipolar

Bipolar cells is the secondary neurous of retina, therefore, reticular layers accepting input from photoreceptor outside while the axon endings of the reticular layers output to amacrine cells and ganglion cells inside. The bipolar cell essentially takes a difference of these two inputs, thus removing the 'dc' component to ensure that the bipolar cell is unaffected by charges in global illumination, since both the center and the surround will by affected equally. In the signal transmission of the retina, bipolar cells play two important functions, one is segregate the visual signals as ON and OFF[1], represent the onset and the offset of a light, through different glutamate receptor on its dendrite, the other one is, through the special way of synaptic transmission with amacrine cells and ganglion cells, the continuity graded potential transmit into instant neural activity[2].

According to its light reflex, divided into OFF or Hyperpolarizing bipolar cells (ones which hyperpolarize with reduction of glutamate) ,which have an off-centre receptive field (since shining a light on its receptive field causes it to hyperpolarize) and they always connect to OFF ganglion cells, and ON or Depolarizing bipolar cells (ones which depolarize with reduction of glutamate), which have an on-centre receptive field (since shining a light on its receptive field causes it to depolarize) and they always connect to always connect to ON ganglion cells[3].

Photoreceptor cells have two types, the rod and the cone. Bipolar cells receiving cone or rod input different depand on animal species. Mammals, bipolar cells can only receive photoreceptor cells of the signal, for rod-biploar cells only have type ON, for cone-biploar cells have both type ON and OFF. But amphibian, like other vertebrates, such as mudpuppy retina also has photoreceptors that synapse on three classes of secondorderneurons: ON bipolars, OFF bipolars, and horizontal cells[4], can accept the input of bipolar cells rod and cone, but usually one in primarily, therefore can be classified into four subtypes, type rod-ON and OFF, type cone -ON and OFF. Because of the type ON bipolar cells and OFF type ON bipolar cells containing different subtype of Glu receptors, which produces the polarity different reaction [3].

2. Ganglion cell

Ganglion cell layer-- this layer of ganglion cells(GC) contain nuclei, optic start from here. During vertebrate embryonic development, retinal ganglion cells in the process RGCs (the) differentiation first. Appear in making an action potential Na+ current represent the mature of the ganglion cells[4].Visual neurons adapt to increases in stimulus contrast by reducing their response sensitivity and decreasing their integration time, a collective process known as 'contrast gain control' [5]. Gain control arises at two stages in retinal ganglion cells: one intrinsic mechanism related to spike generation, and a synaptic mechanism in retinal pathways.

ganglion cells could express all iGluR subunits, when the activity of non-NMDA receptors is blocked, light responses of ganglion cells are generally suppressed, but the suppression appears to be cell type-dependent[6]. the physiological responses of these cells fell into three distinct categories: a sustained ON cell (ON-S), a sustained OFF cell (OFF-S), and a transient OFF cell (OFF-T). The cells had the classic alpha-cell morphology The dendrites of ON-S and OFF-S ganglion cells stratified in the inner and outer borders of the IPL, respectively, on either side of the starburst amacrine cell dendrites, while the dendrites of the OFF-T cell stratified close to the center of the IPL, just below the dendrites of the OFF starburst amacrine cells. Apart from these characteristic differences in the level of stratification, the dendritic morphologies of the ON-S, OFF-S, and OFF-T cells were indistinguishable[7].

3. Interplexiform Cells

Generally thought vertebrate retinas has five kinds of nerve cells, photoreceptor cells, horiziontal cells, bipolar cells, amacrine cells and ganglion cells, but these days more and more evidence indicated that exist in a sixth retinal neurons -- Interplexiform Cells. Such a cell is special, also caused widespread interest. Except for dopaminergic interplexiform cells, Marc use autoradiography reported another glycine interplexiform cells, are different from the dopaminergic interplexiform cells. Their inclusion between inner middle to proximal one third place of the nuclear in the body of the bipolar cells. They receiving from outside of the cell level from outside of reticular layers[8]. Each interplexiform cell collects inputs from several bipolar cells over its large receptive field, and since bipolar cells code spatiotemporal contrast, the IPX cell is sensitive to both high spatial contrast (edges) and high temporal contrast (motion, flickering light). When stimulated under these conditions they will reduce horizontal cell RFs, therefore, in regions of high spatiotemporal contrast the horizontal cell RF is diminished by the IPX cell feedback.

4. The function of GABA recepters between amacrine cells and bipolar cells

More than 20 types amacrine cells have been described by physiological, biochemical and morphological criteria, in which contain many neurotransmitters[9]. Although they may contain a variance of neurotransmitters, all cells receive glutamatergic synaptic input from bipolar cells. The amacrine cells are shown to express GluR2/3 and GluR4 subunits by immunocytochemistry and compose AMPA receptors at the rod bipolar cell dyads[10]. AI amacrine cells receive input from the rod bipolar cell axon terminal through a GABAergic reciprocal synapse, may express KA receptor subunits in the contrast [11-13]. It is a feedback beween amacrine cells and bipolar cells. But there are distinguish in GluR subtypes mediating rod and cone signal from bipolar cells to amacrine cells in the mammalian retina. The NMDA receptor constitute with NR1, NR2A, NR2B while mGluR7 are present on cone-driven amacrine cells, but not expressed at the rod bipolar cell dyads[14,15].

4.1 General introduction of GABA receptors

GABA receptors were categorized in three distinct types traditionally: the ionotropic GABAA receptors, the metabotropic GABAB receptors and a novel GABAC receptors. In the primate retina kainate receptors are expressed on amacrine cell processes[16]. In isolated carp amacrine-like cells, which are quite similar in morphology to the monostratified amacrine cells of tiger salamander[17], current responses to rapid application of glutamate show extremely rapid desensitization (around 3 ms), which is blocked by GYKI 53655[18].

These effects of GABA may reflect a direct input into ganglion cells. In mammalian ganglion cells, large light spots and annuli cause a strong inhibition, which generating the receptive field surround of ganglion cells, is in part mediated by direct inhibitory synapses through the activation of GABAA and probably GABAC receptors, whereas presynaptic surround inhibition occurring in the OPL and IPL could also make contributions, of the excitatory inputs to ganglion cells in the isolated, intact rabbit retina[19]. Alternatively, these effects may be also mediated by GABA receptors expressed on the bipolar cell terminal. Actually, modulation by GABA of light-induced EPSCs in amphibian ganglion cells have been demonstrated to be due to the effect on feedback to bipolar cells mediated by GABAA and GABAC receptors, in addition to GABAB receptors[20].

4.1.1 Comparison between GABAA and GABAC receptors

GABAA receptors are pentameric and consist of several subunits such as α,β,γ,δ. A great number of GABAA receptors could be assembled by combining these subunits with distinct pharmacology. The most common combination is tripletα1 /β2/γ2, which is detected in many cell types including the retina in the CNS[21,22]. Directly coupled with Cl− channels and can be specifically blocked by bicuculline, the GABAA receptor has modulatory binding sites for benzodiazepines, barbiturates, ethanol and neurosteroids[23].While, Structually, GABAC receptors have Cl-pores, which could be blocked by a non-selective chloride channel blocker as picrotoxin, but they are insensitive to bicuculline and baclofen. Besides, GABAC receptors exclusively consist of characteristic component ρ-subunits, which is heterologously expressed and form homoligomeric channels with the characteristic pharmacology of GABAC receptors [24].

In addition to the differences in pharamacology and structure, several lines of evidence indicate that GABAA and GABAC receptor-mediated responses are different in kinetics [25]. Furthermore, the more sensitive the GABAC are than the GABAA recepter. A sequence high degree of homology is found between the GABAC receptor αsubunits of different vertebrate species, and the expression pattern of GABAC receptors in the retinas of different species is quite similar[26].

4.1.2 GABAB receptors

GABAB receptors consist of a single peptide with an amino acid sequence similar to that of mGluRs [27]. They couple to G-proteins through second messenger pathways, which change the activity of post-synaptic K+ channels and/or currents mediated by pre- and post-synaptic voltage-dependent Ca2+ channels[28]. It is activated by baclofen and antagonized by saclofen and phaclofen, while insensitive to bicuculline.

4.1.3 The feedback of Amacrine cells and bipolar cells

Difference in desensitization has led to a speculation thatGABAA receptors mediate transient signals in the retina,whereas GABAC receptors are involved in the processing ofsustained signals[29,30].AII amacrine cells receive input from rod-driven bipolar cells at the synaptic dyads[31,32] and make glycinergic inhibitory synapses with OFF cone-driven bipolar cells[33] and electrically connect with ON cone bipolar cells through large gap junctions[34,35]. The responses of rod bipolar cells were composed primarily of two components, a nonspecific cation current and a chloride current. The chloride current was reduced greatly in axotomized cells and could be suppressed by coapplication of the GABAA antagonist bicuculline and the GABAC antagonist (1,2,5,6-tetrahydropyridine-4-yl) methylphosphinic acid. This suggests that it largely reflects feedback from GABAergic amacrine cells. The response latency of intact rod bipolar cells was shorter than that of the axotomized cells, and the sensitivity curve covered more than twice the dynamic range. Application of the GABA receptor antagonists partially mimicked the effects of axotomy. These findings suggest that functional properties of the axon terminal system-notably synaptic feedback from amacrine cells- play an important role in defining the response properties of mammalian bipolar cells[36].

5. The action as feedforward of Amacine Cells and Ganglin cells through GABA receptors

The effects of GABA on the ganglion cell receptive field organization were first studied in the rabbit retina[37,38]. Amacrine cells are characterized by the abundance of serial conventional GABAergic synapses between the dendrites of these cells[39]. The synapses between dopaminergic cells and AII amacrine cell perikarya may be also GABAergic in both mouse and rat retinas[40]. This GABA-induced current is reversibly inhibited by bicuculline and has a reversal potential close to the chloride equilibrium potential.

In the carp retina GABA selectively abolishes the OFF response to red flash of the ON-OFF transient amacrine cells[41]. This effect is mediated by GABAA receptors. In mudpuppy GABA reduces light-induced responses of transient amacrine cells and decreases greatly the membrane resistance of the cells. GABA-induced currents with significant desensitization from carp amacrine cells are completely blocked by bicuculline[42].

In goldfish retinal slices, all amacrine cells have GABA-induced responses, which are carried by Cl− and sensitive to bicuculline[43]. GABA also shows dramatic effects on light responses of ganglion cells. GABA and GABAA receptor agonists, when ionotrophoretically applied, suppress spontaneous and light-induced responses of all brisk ganglion cell types recorded extracellularly the in vivo cat eye. This action is antagonized by bicuculline[44].

5.1 The GABA recptors bewteen Amacine cells and Ganglin cells

In both mammalian and non-mammalian retinas, GABAA receptor subunits are preferentially expressed on these cells, whole-cell current recordings from amacrine cells show that these cells may exclusively express GABAA receptors consistent with the immunocytochemical evidence. While GABAC receptors are expressed only in a sparse population of amacrine cells and not in ganglion cells at all[45]. Non-radioactive in situ hybridization study, shows that GABAB R1a (one of the two cloned GABAB receptor variants, GABAB R1a and R1b )transcripts are in amacrine and ganglion cells, whereas GABAB R1b transcripts are only present in the ganglion cell layer[46].

6. Amacrine-to-Amacrine Cell Inhibition

Recent studies have elucidated the interactions between bipolar and amacrine cells in the mammalian retina: bipolar cells excite amacrine cells and amacrine cells provide GABAergic and glycinergic feedback inhibition to bipolar cells[47,48]. In other examples, amacrine cells provide feedforward inhibition to ganglion cells[49,50], generating a set of about 12 different ganglion cell response forms[51]. There are more than 27 different morphological types of amacrine cells in the mammalian retina[52,53], addition to inhibiting bipolar and ganglion cells, amacrine cells interact with one another.

But how does the diversity of responses in ganglion cell responses reflect a diversity of amacrine cell activity? In fact, starburst amacrine cells mutually inhibit each other via GABA[54,55] and feed forward to ganglion cells. AII amacrine cells are electrically coupled to each other and to ON bipolar cells and they inhibit OFF cone bipolar cells via glycine[52,57]. Polyaxonal amacrine cells are coupled via gap junctions to other amacrine cells[58]. The most common form of the amacrine cells inhibit each other that wo have measured is "crossover inhibition", where OFF amacrine cells receive ON inhibition or ON amacrine cells receive OFF inhibition[59]. Both of these interactions are mediated by glycine.