Antisense oligonucleotides to the a3 subunit were used by Listerud et al. (1991) to illustrate that functional nAChRs in chick sympathetic neurons are largely composed of a3-containing receptors. Removal of surface receptors using bromoacetylcholine, followed by incubation with a3 antisense for 48 hours, led to a >90% reduction in channel activity compared with control cells. Antisense to a4 had no affect on nAChR channel conductance or kinetics. Removal of the a3 subunit with antisense also led to a compensatory expression of the a7 subunit.
Interestingly, a5 antisense caused the selective loss of a 52 pS conductance channel in single-channel recordings from chick lumbar sympathetic neurons, indicating that a5 subunits make a functional contribution to nAChRs in these cells (Ramirez-Latorre et al., 1996). Deletion of a5 from chick sympathetic neurons using antisense modified the agonist potency profile, but also led to compensatory changes in a7 subunit expression (Yu and Role, 1998a). Antisense deletion of a7 subunits from chick sympathetic neurons removed the ACh-activated, whole-cell current components sensitive to a7 receptor antagonists, a-BnTx, and methyllycaconitine. In single channel recordings, three different conductance states were found to be sensitive to a7 antagonists and/or a7 antisense, suggesting a heterogeneous population of a7-containing nAChRs in chick sympathetic ganglion neurons (Yu and Role, 1998b).
An a3 null mouse (homozygous a3 -/-) has been developed by an exon replacement that deletes three of the transmembrane domains in the a3 subunit (Xu et al., 1999a). The mice survive to birth, but have impaired growth, increased postnatal mortality, and bladder defects, although no other significant peripheral or CNS defects are apparent. In dissociated SCG neurons from the a3 null mice, half the patches tested lacked ACh-activated channels, compared with all patches being responsive in wildtype mice. Patches from the SCG neurons in the a3 null mice responsive to ACh showed low opening probabilities and fewer conductance classes than wildtype mice. These data support the results from antisense deletion experiments, suggesting a3 subunits make a substantial contribution to the functional nAChRs expressed in sympathetic ganglion neurons.
The development of mice lacking specific P subunits (P2 -/-; P4 -/-; P2-/-P4-/-) enabled the contribution of the P subunits to functional nAChRs to be investigated (Xu et al., 1999b). Mice with a single mutation for either P2 or P4 grow to adulthood with no obvious phenotypic abnormalities, while the P2-/-P4-/- double mutant shows similar problems to the a3 null mouse: increased postnatal mortality and bladder defects. Dissociated SCG neurons from the double mutant mice show no responsiveness to nicotine in whole-cell recordings; responses to nicotine are substantially reduced in SCG neurons from the P4 -/- mutant, while in the P2 -/- mutant, responses are similar to those seen in wildtype mice. These findings suggest that the main functional nAChR in sympathetic neurons is composed of a3P4 subunits, although P2 subunits might contribute to function, as the double mutant shows more marked attenuation of responses than the single P4 null mouse.
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