L-glutamate-induced responses and cGMP-activated channels in three subtypes of retinal bipolar cells dissociated from the cat

Abstract

Effects of L-glutamate (Glu), the neurotransmitter released by photoreceptors, on isolated cat bipolar cells were examined. Membrane currents of bipolar cells were recorded by the patch-clamp technique in a conventional whole-cell recording configuration using pipettes containing 1 mM cGMP, which has been known to activate a cationic current sensitive to Glu in ON-type bipolar cells. ON-type bipolar cells (depolarized by light in in situ) and OFF-type bipolar cells (hyperpolarized by light) were identified by their response polarity to Glu. When the whole-cell configuration was established, ON-type bipolar cells showed a steady inward current which was suppressed by Glu, consistent with the response polarity observed in in situ recordings. In contrast, OFF-type cells did not show a steady current during the recordings. However, they responded to Glu with an increase in cationic conductance. Among recorded cells, rod-driven bipolar cells were identified by their immunoreactivity to anti-protein kinase C (PKC-IR) antibody. Examination of PKC-IR revealed that ON-type bipolar cells included both rod- and cone-driven bipolar cells, while OFF-type cells were all cone-driven bipolar cells. The cGMP-activated current observed in ON-type cells was accompanied by a change in the current fluctuation due to the opening and closing of underlying channels. Fluctuation analysis gave a unitary conductance value of 13 pS. In half of the cells examined, maximum open probability reached almost 100%. The cGMP- activated channel in bipolar cells seems novel, fundamentally different from those found in photoreceptor cells or olfactory receptor cells.

Modulation by Brain Natriuretic Peptide of GABA Receptors on Rat Retinal ON-Type Bipolar Cells

“…Error bars represent SEM. Villa et al, 1995;Wang and Robinson, 1997;Walters et al, 1998;Shiells and Falk, 2001), although the G-protein G o , rather than transducin, may be involved in postsynaptic mechanisms of these cells (Nawy, 1999). Induction of cGMP-gated currents by BNP is consistent with the fact that NPs elevate intracellular cGMP levels by activating their pGC-coupled receptors (Blute et al, 2000;Herring et al, 2001).…”

Regulation of ON bipolar cell activity

“…Dialysis of salamander ON bipolar cells with cGMP potentiates the standing inward current, but still allows glutamate to suppress this current completely, thus increasing the size of the response to exogenously applied glutamate and to light (Nawy and Jahr, 1990a;Shiells and Falk, 1990;Nawy and Jahr, 1991). Noise analysis of the cation current in dissociated ON bipolar cells from cat retina suggested that dialysis with cGMP dramatically increases the open channel probability (de la Villa et al, 1995). In all of these studies, cGMP was applied under conditions where a fraction of the currently was tonically activated.…”

cGMP-Dependent Kinase Regulates Response Sensitivity of the Mouse On Bipolar Cell

“…Pharmacological interruption of synaptic transmission suggests that these changes most likely occur within the On bipolar cell (Gurevich and Slaughter, 1993), and although the cellular mechanisms involved remain unknown, several research groups have found that mGluR6-mediated responses to glutamate and light are potentiated by intracellular addition of cGMP (Nawy and Jahr, 1990;Falk, 1990, 2002;de la Villa et al, 1995), making it well suited as a modulator of sensitivity.…”