Enhanced ACh sensitivity is accompanied by changes in ACh receptor channel properties and segregation of ACh receptor subtypes on sympathetic neurons during innervation in vivo

Abstract

Although presynaptic input can influence the number and distribution of ACh receptors (AChRs) on muscle, the role of cellular interactions in the development of transmitter sensitivity in neurons is less clear. To determine whether presynaptic input modifies neuronal AChR channel function and distribution, we must first ascertain the profile of changes in receptor properties relative to the timing of synapse formation. We have examined the temporal aspects of synaptogenesis in the lumbar sympathetic ganglia of the embryonic chick in anatomical experiments with anterograde 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate labeling of presynaptic inputs and cytochrome oxidase histochemistry. Biophysical studies of sympathetic neurons, within hours of removal from animals at different stages relative to synapse formation, show that both the properties and distribution of AChR channels are modified concurrent with a significant increase in presynaptic input to the neurons. The most striking change in AChR channel distribution is revealed by patching multiple sites on the surface of individual neurons. Following innervation in vivo, many neurons express only one of the four AChR channel subtypes and the AChRs are clustered in discrete, high-activity patches. Furthermore, when neurons at this stage express more than one AChR channel subtype, the different classes are often spatially segregated from one another on the cell surface. This contrasts with patches from neurons removed earlier on, which have lower overall activity, often comprised of multiple channel subtypes. Comparison of the AChR properties of acutely dispersed neurons to those of neurons maintained in vitro indicates that most features of AChR channels are conserved despite their removal from presynaptic and other in vivo influences. These findings are consistent with inductive interactions between pre- and postsynaptic neurons playing an important regulatory role in transmitter receptor expression.

Functional expression of A‐currents in embryonic chick sympathetic neurones during development in situ and in vitro.

“…Similar results were obtained when E9 sympathetic neurones were cultured for 4 days in the presence of aortic smooth muscle cells or aortic smooth muscle-conditioned medium (Fig. 8) (Hrushak, Friedrich & Giacobini, 1982;Moss & Role, 1993). Interactions with the afferent preganglionic innervation are known to regulate the expression of acetylcholine receptors on embryonic chick sympathetic neurones developing in vivo (Moss & Role, 1993) and in vitro (Role, 1988;Gardette et al 1991).…”

“…8) (Hrushak, Friedrich & Giacobini, 1982;Moss & Role, 1993). Interactions with the afferent preganglionic innervation are known to regulate the expression of acetylcholine receptors on embryonic chick sympathetic neurones developing in vivo (Moss & Role, 1993) and in vitro (Role, 1988;Gardette et al 1991). It is possible that the afferent innervation also controls the expression of other developmentally regulated ionic channels, such as IA.…”

The long internal loop of the α3 subunit targets nAChRs to subdomains within individual synapses on neurons in vivo

“…Because the biophysical and pharmacological properties of neuronal nicotinic cholinergic receptors are exquisitely sensitive to subunit composition 6 , the spatial segregation of receptor subtypes is likely to result in the formation of functionally specialized synaptic microdomains, as suggested by microiontophoretic electrophysiological analyses of innervated sympathetic neurons 30 . In comparison to nAChRs, Bgt-nAChRs have faster kinetics of activation and desensitization, lower affinity for ACh and higher permeability to calcium 31,32 .…”

Regulatory mechanisms that govern nicotinic synapse formation in neurons

“…In particular, the ␣-bgt-sensitive receptors have higher calcium permeability, and in some neuron populations faster kinetics of activation and desensitization, but a slowly desensitizing response in others (Seguela et al, 1993;Ullian et al, 1997;Chang and Berg, 1999;Cuevas et al, 2000). Within one neuron, the diverse nAChR subtypes are spatially segregated relative to one another and target to discrete synapse-associated sites: the presynaptic terminal, the specialized postsynaptic membrane, and the perisynaptic dendritic surface membrane (Jacob and Berg, 1983;Jacob et al, 1984Jacob et al, , 1986Loring et al, 1985;Loring and Zigmond, 1987;Moss and Role, 1993;Horch and Sargent, 1995;McGehee and Role, 1995;Gray et al, 1996;Shoop et al, 1999). The spatial segregation and distinct biophysical properties of the diverse nAChR subtypes are likely to create functionally specialized synapse-associated microregions and establish distinct spatial and temporal patterns of calcium influx that locally target different downstream signaling events (see review by D.K.…”

“…nAChR subunit transcript and protein levels increase significantly during the period of pre-and postganglionic synapse formation (Jacob, 1991;Corriveau and Berg, 1993;Devay et al, 1994;Mandelzys et al, 1994;Levey et al, 1995). Functional nAChRs at the neuronal cell surface also change during this period, with developmental increases in ACh current density and maturational changes in the relative abundance and kinetics of nAChR conductance classes (Margiotta and Gurantz, 1989;Moss et al, 1989;Engisch and Fischbach, 1990;Moss and Role, 1993). These changes in nAChR expression are likely to increase the efficacy of synaptic transmission.…”