An unusually small potassium current that is well-suited to a retinal neuron which is chronically depolarized

Voltage- and calcium-gated ion channels of neurons in the vertebrate retina

In this review, we summarize studies investigating the types and distribution of voltage- and calcium-gated ion channels in the different classes of retinal neurons: rods, cones, horizontal cells, bipolar cells, amacrine cells, interplexiform cells, and ganglion cells. We discuss differences among cell subtypes within these major cell classes, as well as differences among species, and consider how different ion channels shape the responses of different neurons. For example, even though second-order bipolar and horizontal cells do not typically generate fast sodium-dependent action potentials, many of these cells nevertheless possess fast sodium currents that can enhance their kinetic response capabilities. Ca2+ channel activity can also shape response kinetics as well as regulating synaptic release. The L-type Ca2+ channel subtype, CaV1.4, expressed in photoreceptor cells exhibits specific properties matching the particular needs of these cells such as limited inactivation which allows sustained channel activity and maintained synaptic release in darkness. The particular properties of K+ and Cl- channels in different retinal neurons shape resting membrane potentials, response kinetics and spiking behavior. A remaining challenge is to characterize the specific distributions of ion channels in the more than 100 individual cell types that have been identified in the retina and to describe how these particular ion channels sculpt neuronal responses to assist in the processing of visual information by the retina.

Stimulation of sodium pump restores membrane potential to neurons excited by glutamate in zebrafish distal retina

Glutamate either depolarizes or hyperpolarizes retinal neurons. Those are the initial and primary effects. Using a voltage probe (oxonol, DiBaC4 (5)) to study dissociated zebrafish retinal neurons, we find a secondary, longer-term effect: a post-excitatory restoration of membrane potential, termed after-hyperpolarization (AHP). AHP occurs only in neurons that are depolarized by glutamate and typically peaks about 5 min after glutamate application. AHP is seen in dissociated horizontal cells (HCs) and hyperpolarizing, or OFF type, bipolar cells (HBCs). These cells commonly respond with only an AHP component. AHP never occurs in depolarizing, or ON type, bipolar cells (DBCs), which are cell types hyperpolarized by glutamate. AHP is blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). It is evoked by kainate, AMPA and the AMPA-selective agonist (S)-5-fluorowillardiine, but not by NMDA, D-aspartate, the kainate-selective agonist SYM 2081 or by DL-2-amino-4-phosphonobutyric acid (DL-AP4). Cells with exclusively AHP responses are tonically depolarized. Resting potentials can be restored by nifedipine, suggesting a tonic, depolarizing action of L-type Ca2+ channels. However AHP is not blocked by nifedipine and is insensitive to [Cl-]o. AHP is blocked by Li+o substitution for Na+o and by ouabain. A mechanism is proposed in which Na+ entering through ionotropic AMPA channels stimulates Na+,K+-ATPase, which, by electrogenic action, restores membrane potential, generating the AHP response. Patterns of ATPase immunoreactivity support localization in the outer plexiform layer (OPL) as cone pedicles, HCs and BCs were positively labelled. Labelling was weaker in the inner plexiform layer (IPL) than in nuclear layers, though two IPL bands of immunoreactive BC terminals could be discerned, one in sublamina a and the other in sublamina b. Persistent stimulation of distal retina by photoreceptor glutamate may induce increased expression and activity of Na+,K+-ATPase, with a consequent impact on distal glutamate responses.

Membrane properties of an unusual intrinsically oscillating, wide-field teleost retinal amacrine cell

In the retina, amacrine cells modulate the transfer of information from bipolar to ganglion cells. The nature of the modulation depends on the synaptic input and the membrane properties of the cells. In the retina of white bass, we identified a class of bistratified, wide-field amacrine cell characterized by immunopositive labelling for GABA and calmodulin. In isolation, the cells presented resting membrane potentials averaging -69 mV although some cells settled at more depolarized values (-30 mV). Injection of depolarizing current pulses induced oscillatory membrane responses. When elicited from depolarized cells, the oscillations were short-lived (< 40 ms). For the most part, the oscillatory potentials of hyperpolarized cells remained unattenuated throughout the depolarizing pulse. The frequency of the oscillations increased logarithmically with mean membrane potential, ranging from 74 to 140 Hz. Cells exhibiting depolarized membrane potentials oscillated at twice that rate. When the membrane potential of these cells was hyperpolarized to -70 mV, the oscillations became unattenuated and slowed. We found the cells expressed voltage-gated sodium, potassium and calcium currents and calcium-dependent potassium currents. We demonstrate that the oscillatory potentials arose as a result of the interplay between calcium and potassium currents. The cells responded to local application of GABA and glycine, both of which modulate the oscillatory potentials. Glutamate and its analogues depolarized the cell and induced oscillatory potentials. Our results indicate that oscillatory responses of a type of wide-field amacrine cell are an intrinsic feature of the cell and not due to circuit properties.

Glial and neuronal markers in bass retinal horizontal and Müller cells

Retinal horizontal cells (HCs) are second-order neurons that integrate information from photoreceptors over large retinal areas, mediating the lateral spread of visual signals in the distal retina. The 'glial' vs. 'neuronal' nature of the HC has been widely debated. For example, carbonic anhydrase (CA), glutamine synthetase (GS), and glial fibrillary acidic protein (GFAP) are considered 'glial' markers, yet both CA and GFAP have been previously reported in HCs of the teleost retina in species-specific patterns. In contrast, the neurofilament triplet (NFT) proteins are considered 'neuronal' markers; these proteins have been immunolocalized to a mammalian HC, but are absent from teleost HCs. We have studied these cytochemical characteristics in HCs from the white bass, by immunolabeling both cryosections of intact retina and freshly isolated, identified cells attached to coverslips. We found that both HCs (neurons) and Müller cells (MCs; glia) immunolabeled with antisera to CA. Both type 1 (external) HCs and MCs immunolabeled with an antibody to vimentin. Only MCs immunolabeled with antisera to GS and GFAP. Neither HC perikarya (and their major dendrites) nor MCs immunolabeled with an antibody to the 160-kDa subunit of NFT protein. Thus, bass HCs and MCs share the presence of CA and vimentin epitopes and absence of the NFT 160-kDa epitope. Moreover, retinal cell isolation, by itself, does not affect cell-type specific immunolabeling patterns in identified cells, except for what may be lost with the finer processes of the various cells. Isolated cell studies can aid in interpreting immunolabeling patterns observed in the intact retina, especially in retinal layers where several cell types may be present.