Intrinsic choroidal neurons in the human eye: projections, targets, and basic electrophysiological data

Area of Science:

• Ophthalmology
• Neuroscience
• Cell Biology

Background:

• Intrinsic choroidal neurons (ICNs) have unclear origins due to overlapping chemical markers with extrinsic nerves.
• Distinguishing intrinsic vs. extrinsic nerve fiber origins in the choroid is challenging.
• Neurobiotin tracing is a valuable technique for mapping neuronal projections and determining innervation sources.

Purpose of the Study:

• To characterize the organization and function of intrinsic choroidal neurons (ICNs) in the human eye.
• To differentiate intrinsic ICN innervation from extrinsic sources using neurobiotin injections.
• To investigate reciprocal contacts between ICNs and their innervation of choroidal smooth muscle.

Main Methods:

• Visualization of ICNs using the fluorescent vital dye 4-Di-2-ASP.
• Intracellular recordings to determine electrophysiological properties of ICNs.
• Iontophoretic filling of impaled neurons with neurobiotin, followed by immunohistochemistry for nNOS, alpha-smooth muscle actin, and CGRP.

Main Results:

• ICN processes were traced up to 2.612 micrometers, showing proximity to other ICNs and smooth muscle fibers.
• Calcitonin gene-related peptide (CGRP)-positive fibers formed boutons associated with ICNs.
• Electrophysiological recordings revealed phasic firing, no spontaneous activity, and specific input resistance and membrane time constants.

Conclusions:

• This study provides the first functional characterization of ICNs.
• Evidence for reciprocal ICN-to-ICN contacts and innervation of choroidal vascular and nonvascular smooth muscle was established.
• The neurobiotin tracing technique offers a promising method for future ICN function research in ocular homeostasis.