An essential step in intricate visual processing is the segregation of visual signals into ON and OFF pathways by retinal bipolar cells (BCs). Glutamate released from photoreceptors modulates the photoresponse of ON BCs via metabotropic glutamate receptor 6 (mGluR6) and G protein (Go) that regulates a cation channel. However, the cation channel has not yet been unequivocally identified. Here, we report a mouse TRPM1 long form (TRPM1-L) as the cation channel. We found that TRPM1-L localization is developmentally restricted to the dendritic tips of ON BCs in colocalization with mGluR6. TRPM1 null mutant mice completely lose the photoresponse of ON BCs but not that of OFF BCs. In the TRPM1-L-expressing cells, TRPM1-L functions as a constitutively active nonselective cation channel and its activity is negatively regulated by Go in the mGluR6 cascade. These results demonstrate that TRPM1-L is a component of the ON BC transduction channel downstream of mGluR6 in ON BCs.egregation of visual signals into ON and OFF pathways originates in BCs, the second-order neurons in the retina (1, 2). ON and OFF BCs express metabotropic glutamate receptors, mGluR6, and ionotropic glutamate receptors (iGluRs), respectively, on their dendrites (3-5). Reduction of glutamate released from photoreceptors by light stimulation depolarizes ON BCs and hyperpolarizes OFF BCs (6-8) mediated through respective glutamate receptors. The mGluR6 couples to a heterotrimeric G protein complex, Go (9, 10). Signals require Goα, which ultimately closes a downstream nonselective cation channel in ON BCs (6, 9, 11-13). However, this transduction cation channel in ON BCs has not been identified, despite intensive investigation.In our screen to identify functionally important molecules in the retina, we found that TRPM1 is predominantly expressed in retinal BCs. Most members of the TRP superfamily, which are found in a variety of sense organs, are non-voltage-gated cation channels (14-16). The founding member of the TRP family was discovered as a key component of the light response in Drosophila photoreceptors (17). TRPM1, also known as melastatin, was the first member of the melanoma-related transient receptor potential (TRPM) subfamily to be discovered (18,19). TRPM1 is alternatively spliced, resulting in the production of a long form (TRPM1-L) and a short N-terminal form devoid of transmembrane segments (TRPM1-S) (18,20). Although mouse TRPM1-S was previously identified as melastatin, mouse TRPM1-L has not been identified (18). The distinct physiological and biological functions of TRPM1 still remain elusive, although some recent evidences including us suggested that TRPM1 might contribute to retinal BC function (21-23). Here, we show that TRPM1-L is the transduction cation channel of retinal ON BCs in the downstream of mGluR6 cascade.

The directional selectivity of retinal ganglion cell responses represents a primitive pattern recognition that operates within a retinal neural circuit. The cellular origin and mechanism of directional selectivity were investigated by selectively eliminating retinal starburst amacrine cells, using immunotoxin-mediated cell targeting techniques. Starburst cell ablation in the adult retina abolished not only directional selectivity of ganglion cell responses but also an optokinetic eye reflex derived by stimulus movement. Starburst cells therefore serve as the key element that discriminates the direction of stimulus movement through integrative synaptic transmission and play a pivotal role in information processing that stabilizes image motion.

Bipolar, amacrine, and ganglion cells of carp retina, stained intracellularly with Procion yellow, can be divided into types a and b, according to the destination of terminals and dendritic trees in the inner plexiform layer (sublamina a and b, respectively). Type a cells showed hyperpolarizing, or off, responses and type b cells depolarizing, or on, responses. Carp thus resembles cat in the basic organization of on and off pathways in the retina.

The ferroelectric BaTiO(3) is a band-gap insulator. Itinerant electrons can be introduced in this material by doping, for example, with oxygen vacancies. Above a critical electron concentration of n(c) approximately 1 x 10(20) cm(-3), BaTiO(3-delta) becomes metallic. This immediately raises a question: Does metallic BaTiO(3-delta) still retain ferroelectricity? One may expect itinerant electrons to destroy ferroelectricity as they screen the long-range Coulomb interactions. We followed the phase transitions in BaTiO(3-delta) as a function of n far into metallic phase. Although their stability range decreases with n, the low-symmetry phases in metallic BaTiO(3-delta) are still retained up to an estimated concentration of n* approximately 1.9 x 10(21) cm(-3). Moreover, it appears that the itinerant electrons partially stabilize the ferroelectric phases in metallic BaTiO(3-delta) by screening strong crystal field perturbations caused by oxygen vacancies.

Exocytosis-mediated glutamate release from ribbon-type synaptic terminals of retinal bipolar cells was studied using AMPA receptors and simultaneous membrane capacitance measurements. Release onset (delay <0.8 ms) and offset were closely tied to Ca2+ channel opening and closing. Asynchronous release was not copious and we estimate that there are approximately 5 Ca2+ channels per docked synaptic vesicle. Depending on Ca2+ current amplitude, release occurred in a single fast bout or in two successive bouts with fast and slow onset kinetics. The second, slower bout may reflect a mobilization rate of reserve vesicles toward fusion sites that is accelerated by increasing Ca2+ influx. Bipolar cell synaptic ribbons thus are remarkably versatile signal transducers, capable of transmitting rapidly changing sensory input, as well as sustained stimuli, due to their large pool of releasable vesicles.

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SUMMARY1. Membrane properties of solitary bipolar cells, mechanically dissociated from the enzyme-treated goldfish retina, were studied under current-and voltage-clamp conditions with 'giga-seal' suction pipettes (pipette solution 138 mM-K).2. The resting potential of solitary bipolar cells was about -30 mV. They responded to depolarizing current pulses with sustained depolarization, and to hyperpolarizing current pulses with an initial hyperpolarizing transient followed by a sag to a less hyperpolarized level.3. The current-voltage relationship determined under voltage-clamp conditions showed strong outward and inward rectification. The membrane currents consisted of four components; Ca current (ICa), voltage-and Ca-dependent K currents (IK(v) and IK(Ca), respectively), and an inward current activated by membrane hyperpolarization (Ih).4. ICa was activated by membrane depolarization beyond -40 mV, was maximum at + 10 mV and became smaller with further depolarization. No polarity reversal was seen. ICa was enhanced by equimolar replacement of Ca with Ba, and was blocked by 4 mM-Co.5. IK(Ca) was observed by membrane depolarization beyond -10 mV, was maximum at about + 40 mV, and became smaller with further depolarization. This current was suppressed by 4 mM-Co, 1-6 mM-Ba, 35 mM-TEA or 30 /SM-quinine. 6. IK(V) was activated by membrane depolarization beyond -60 mV, and had slower kinetics than ICa or IK(Ca). The reversal potential of the tail current was close to the K equilibrium potential (EK), suggesting that this current is carried purely by K ions. IK(V) was inactivated slowly and nearly completely by sustained depolarization. IK(V) was blocked by 35 mM-TEA.7. Ih was activated by membrane hyperpolarization ( < -60 mV). The current showed a time-dependent increase. It was also dependent on the membrane potential, but not on the driving force of K ions. This current seems to be carried by a mixture of Na and K ions, since (1) in low Na solution, Ih became small in amplitude, and (2) the reversal potential of the tail current was between the Na equilibrium potential (ENa) and EK. Ih was blocked by 10 mM-Cs, but was resistant to 0-2 mM-Ba.8. The resting potential and voltage responses of solitary bipolar cells are discussed in reference to the characteristics of each membrane conductance isolated in the present study. 5-2

SUMMARY1. Isolated cones dissociated from the retina of the freshwater turtle were voltage clamped using a single 'patch' pipette electrode. y-Aminobutyric acid (GABA) applied ionophoretically to the axon terminal evoked an inward current in cells held at -66 mV when they were recorded with patch pipettes filled with the 'control' pipette solution containing 120 mM-Cl-. 5. Cones were desensitized to GABA (1) in the presence of GABA ( > 100 nM) in the medium, or (2) by a prolonged ionophoretic application. The maximum reduction in response amplitude was about 70 % in both experiments.6. Muscimol was as potent as GABA, while /J-p-chlorophenyl-GABA (baclofen)was ineffective even at 100 AM. GABA was antagonized by bicuculline competitively, and by picrotoxin non-competitively. These observations suggest that turtle cones have GABAA receptors which associate with chloride channels. 7. The present results suggest that GABA, presumably released continuously from monophasic horizontal cells in the dark, would exert a tonic hyperpolarization in red-sensitive and green-sensitive cones. Suppression by light of tonic GABA release would depolarize these types of cones by disinhibition. Disinhibitory depolarization in cones may contribute to the centre surround antagonism in retinal neurones, and to the biphasic colour responses recorded in a subtype of horizontal cells.