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The development of ACH- and GABA-activated currents in normal and target-deprived embryonic chick ciliary ganglia
1Department of Anatomy & Neurobiology, Washington University School of Medicine, St. Louis, Missouri 63110.
Insights
Acetylcholine (ACh) and GABA receptor expression increases significantly in embryonic chick ciliary ganglion neurons during development. This developmental increase in functional ACh receptors is independent of target innervation.
Area of Science:
- Neuroscience
- Developmental Biology
- Neurophysiology
Background:
- Ciliary ganglion neurons play a crucial role in innervating ocular muscles and the iris.
- The expression and function of neurotransmitter receptors are critical for neuronal development and function.
- Understanding receptor dynamics during embryonic development provides insights into neuronal maturation.
Purpose of the Study:
- To investigate the developmental expression of functional acetylcholine (ACh) and gamma-aminobutyric acid (GABA) receptors in embryonic chick ciliary ganglion neurons.
- To determine the relationship between receptor expression, cell size, and target innervation during neuronal development.
Main Methods:
- Whole-cell patch-clamp electrophysiology was used to measure currents activated by ACh and GABA.
- Neuronal cell size was estimated using membrane capacitance measurements.
- Target deprivation was achieved by removing the primordial eye in early embryonic stages.
Main Results:
- Peak inward current activated by ACh (IACh) increased sevenfold between embryonic stages 29 and 44, occurring in two distinct phases.
- Inward current activated by GABA (IGABA) showed a twofold gradual increase without distinct phases.
- IACh increase was largely independent of target innervation, with a 10-fold increase observed even in neurons deprived of their eye.
Conclusions:
- Functional expression of ACh and GABA receptors undergoes significant developmental regulation in chick ciliary ganglion neurons.
- The dramatic increase in ACh receptor function during development is an intrinsic process, not solely dependent on target-derived signals.
- While target innervation may have a minor influence, it is not the primary driver for the developmental upregulation of ACh receptors.
Abstract:
We have examined the expression of functional ACh and GABA receptors on embryonic chick ciliary ganglion neurons between Stages (St) 29 and 44 (Embryonic Day 6 to Embryonic Day 18). Whole-cell currents activated by ACh or GABA were measured in neurons 3-6 hr after dissociation to estimate the level of functional receptors in vivo. The mean peak IACh increased sevenfold between St 29 (321 pA) and St 44 (2345 pA) in two steps, separated by a plateau between St 35 and St 38 (E9 to E12). Cell size, estimated from measurements of membrane capacitance, increased only threefold over the same interval. Moreover, IACh and cell size were not well correlated at any stage examined. IGABA increased twofold between St 29 and St 38; the change was gradual and without any indication of two phases. The increase in IACh during development was not dependent on innervation of target cells within the eye. We removed the primordial eye between St 11 and St 13 (E2) and allowed the embryos to mature to various stages. Despite a small (20-50%) reduction in IACh at every stage examined, IACh still increased dramatically (about 10-fold) between St 29 and St 44 in target-deprived neurons. IACh was not uniquely affected by early target removal; IGABA and capacitance were also slightly reduced in target-deprived neurons.