Novel hybrid action of GABA mediates inhibitory feedback in the mammalian retina.

The stream of visual information sent from photoreceptors to second-order bipolar cells is intercepted by laterally interacting horizontal cells that generate feedback to optimize and improve the efficiency of signal transmission. The mechanisms underlying the regulation of graded photoreceptor synaptic output in this nonspiking network have remained elusive. Here, we analyze with patch clamp recording the novel mechanisms by which horizontal cells control pH in the synaptic cleft to modulate photoreceptor neurotransmitter release. First, we show that mammalian horizontal cells respond to their own GABA release and that the results of this autaptic action affect cone voltage-gated Ca2+ channel (CaV channel) gating through changes in pH. As a proof-of-principle, we demonstrate that chemogenetic manipulation of horizontal cells with exogenous anion channel expression mimics GABA-mediated cone CaV channel inhibition. Activation of these GABA receptor anion channels can depolarize horizontal cells and increase cleft acidity via Na+/H+ exchanger (NHE) proton extrusion, which results in inhibition of cone CaV channels. This action is effectively counteracted when horizontal cells are sufficiently hyperpolarized by increased GABA receptor (GABAR)-mediated HCO3- efflux, alkalinizing the cleft and disinhibiting cone CaV channels. This demonstrates how hybrid actions of GABA operate in parallel to effect voltage-dependent pH changes, a novel mechanism for regulating synaptic output.

Quantitative analysis of enteric neurons containing choline acetyltransferase and nitric oxide synthase immunoreactivities in the submucosal and myenteric plexuses of the porcine colon.

The enteric nervous system (ENS) controls gastrointestinal functions. In large mammals' intestine, it comprises an inner (ISP) and outer (OSP) submucous plexus and a myenteric plexus (MP). This study quantifies enteric neurons in the ISP, OSP, and MP of the pig ascending (AC) and descending colon (DC) using the HuC/D, choline acetyltransferase (ChAT), and neuronal nitric oxide synthase (nNOS) neuronal markers in whole mount preparations with multiple labeling immunofluorescence. We established that the ISP contains the highest number of HuC/D neurons/mm2, which were more abundant in AC vs. DC, followed by OSP and MP with similar density in AC and DC. In the ISP, the density of ChAT immunoreactive (IR) neurons was very similar in AC and DC (31% and 35%), nNOS-IR neurons were less abundant in AC than DC (15% vs. 42%, P < 0.001), and ChAT/nNOS-IR neurons were 5% and 10%, respectively. In the OSP, 39-44% of neurons were ChAT-IR in AC and DC, while 45% and 38% were nNOS-IR and 10-12% were ChAT/nNOS-IR (AC vs. DC P < 0.05). In the MP, ChAT-IR neurons were 44% in AC and 54% in DC (P < 0.05), nNOS-IR neurons were 50% in both, and ChAT/nNOS-IR neurons were 12 and 18%, respectively. The ENS architecture with multilayered submucosal plexuses and the distribution of functionally distinct groups of neurons in the pig colon are similar to humans, supporting the suitability of the pig as a model and providing the platform for investigating the mechanisms underlying human colonic diseases.

Prox1 Is a Marker for AII Amacrine Cells in the Mouse Retina.

The transcription factor Prox1 is expressed in multiple cells in the retina during eye development. This study has focused on neuronal Prox1 expression in the inner nuclear layer (INL) of the adult mouse retina. Prox1 immunostaining was evaluated in vertical retinal sections and whole mount preparations using a specific antibody directed to the C-terminus of Prox1. Strong immunostaining was observed in numerous amacrine cell bodies and in all horizontal cell bodies in the proximal and distal INL, respectively. Some bipolar cells were also weakly immunostained. Prox1-immunoreactive amacrine cells expressed glycine, and they formed 35 ± 3% of all glycinergic amacrine cells. Intracellular Neurobiotin injections into AII amacrine cells showed that all gap junction-coupled AII amacrine cells express Prox1, and no other Prox1-immunostained amacrine cells were in the immediate area surrounding the injected AII amacrine cell. Prox1-immunoreactive amacrine cell bodies were distributed across the retina, with their highest density (3887 ± 160 cells/mm2) in the central retina, 0.5 mm from the optic nerve head, and their lowest density (3133 ± 350 cells/mm2) in the mid-peripheral retina, 2 mm from the optic nerve head. Prox1-immunoreactive amacrine cell bodies comprised ~9.8% of the total amacrine cell population, and they formed a non-random mosaic with a regularity index (RI) of 3.4, similar to AII amacrine cells in the retinas of other mammals. Together, these findings indicate that AII amacrine cells are the predominant and likely only amacrine cell type strongly expressing Prox1 in the adult mouse retina, and establish Prox1 as a marker of AII amacrine cells.