A small number (Ͻ2%) of mammalian retinal ganglion cells express the photopigment melanopsin and are intrinsically photosensitive (ipRGCs). Light depolarizes ipRGCs and increases intracellular calcium levels ([Ca 2ϩ] i ) but the signaling cascades underlying these responses have yet to be elucidated. To facilitate physiological studies on these rare photoreceptors, highly enriched ipRGC cultures from neonatal rats were generated using anti-melanopsin-mediated plate adhesion (immunopanning). This novel approach enabled experiments on isolated ipRGCs, eliminating the potential confounding influence of rod/cone-driven input.

Melanopsin, expressed in a subset of retinal ganglion cells, mediates behavioral adaptation to ambient light and other non-image forming photic responses. This has raised the possibility that pharmacological manipulation of melanopsin can modulate several CNS responses including photophobia, sleep, circadian rhythms and neuroendocrine function. Here we describe the identification of a potent synthetic melanopsin antagonist with in vivo activity. Novel sulfonamide compounds inhibiting melanopsin (opsinamides) compete with retinal binding to melanopsin and inhibit its function without affecting rod/cone mediated responses. In vivo administration of opsinamides to mice specifically and reversibly modified melanopsin-dependent light responses including the pupillary light reflex and light aversion. The discovery of opsinamides raises the prospect of therapeutic control of the melanopsin phototransduction system to regulate light-dependent behavior and remediate pathological conditions.

. 5-HT 1B receptor-mediated presynaptic inhibition of GABA release in the suprachiasmatic nucleus. J Neurophysiol 93: 3157-3164, 2005. First published February 16, 2005 doi:10.1152/jn.00770.2004. The suprachiasmatic nucleus (SCN) receives a dense serotonergic innervation that modulates photic input to the SCN via serotonin 1B (5-HT 1B ) presynaptic receptors on retinal glutamatergic terminals. However, the majority of 5-HT 1B binding sites in the SCN are located on nonretinal terminals and most axonal terminals in the SCN are GABAergic. We therefore tested the hypothesis that 5-HT 1B receptors might also be located on SCN GABAergic terminals by examining the effects of the highly selective 5-HT 1B receptor agonist CP-93,129 on SCN miniature inhibitory postsynaptic currents (mIPSCs). Whole cell patch-clamp recordings of mIPSCs were obtained from rat and mouse SCN neurons in hypothalamic slices. Using CsCl-containing microelectrodes with QX314, we isolated mPSCs that were sensitive to the GABA A receptor antagonist, bicuculline. Bath application of CP-93,129 (1 M) decreased the frequency of mIPSCs by an average of 22% (n ϭ 7) in rat SCN neurons and by an average of 30% (n ϭ 8) in mouse SCN neurons with no clear effect on mIPSC amplitude. In mice lacking functional 5-HT 1B receptors, CP-93,129 (1 M) had no clear effect on the frequency or the amplitude of mIPSCs recorded in any of the cells tested (n ϭ 4). The decrease in the frequency of mIPSCs of SCN neurons produced by the selective 5-HT 1B receptor agonist CP-93,129 is consistent with the interpretation that 5-HT 1B receptors are located on GABA terminals in the SCN and that 5-HT inhibits GABA release via a 5-HT 1B presynaptic receptor-mediated mechanism.

Coding a wide range of light intensities in natural scenes poses a challenge for the retina: adaptation to bright light should not compromise sensitivity to dim light. Here we report a novel form of activity-dependent synaptic plasticity, specifically, a "weighted potentiation" that selectively increases output of Mb-type bipolar cells in the goldfish retina in response to weak inputs but leaves the input-output ratio for strong stimuli unaffected. In retinal slice preparation, strong depolarization of bipolar terminals significantly lowered the threshold for calcium spike initiation, which originated from a shift in activation of voltage-gated calcium currents (I Ca ) to more negative potentials. The process depended upon glutamate-evoked retrograde nitric oxide (NO) signaling as it was eliminated by pretreatment with an NO synthase blocker, TRIM. The NO-dependent I Ca modulation was cGMP independent but could be blocked by N-ethylmaleimide (

BackgroundRetinal ganglion cells expressing the photopigment melanopsin are intrinsically photosensitive (ipRGCs). These ganglion cell photoreceptors send axons to several central targets involved in a variety of functions. Within the retina ipRGCs provide excitatory drive to dopaminergic amacrine cells via glutamatergic signals and ipRGCs are coupled to wide-field GABAergic amacrine cells via gap junctions. However, the extent to which ipRGCs are coupled to other retinal neurons in the ganglion cell layer via gap junctions is unclear. Carbenoxolone, a widely employed gap junction inhibitor, greatly reduces the number of retinal neurons exhibiting non-rod, non-cone mediated light-evoked Ca2+ signals suggesting extensive intercellular coupling between ipRGCs and non-ipRGCs in the ganglion cell layer. However, carbenoxolone may directly inhibit light-evoked Ca2+ signals in ipRGCs independent of gap junction blockade.Methodology/Principal FindingsTo test the possibility that carbenoxolone directly inhibits light-evoked Ca2+ responses in ipRGCs, the light-evoked rise in intracellular Ca2+ ([Ca2+]i) was examined using fura-2 imaging in isolated rat ipRGCs maintained in short-term culture in the absence and presence of carbenoxolone. Carbenoxolone at 50 and 100 µM concentrations completely abolished the light-evoked rise in [Ca2+]i in isolated ipRGCs. Recovery from carbenoxolone inhibition was variable.Conclusions/SignificanceWe demonstrate that the light-evoked rise in [Ca2+]i in isolated mammalian ganglion cell photoreceptors is inhibited by carbenoxolone. Since the light-evoked increase in [Ca2+]i in isolated ipRGCs is almost entirely due to Ca2+ entry via L-type voltage-gated calcium channels and carbenoxolone does not inhibit light-evoked action potential firing in ipRGCs in situ, carbenoxolone may block the light-evoked increase in [Ca2+]i in ipRGCs by blocking L-type voltage-gated Ca2+ channels. The ability of carbenoxolone to block evoked Ca2+ responses must be taken into account when interpreting the effects of this pharmacological agent on retinal or other neuronal circuits, particularly if a change in [Ca2+]i is the output being measured.