The existence of a high-affinity choline transporter in rat brain was first reported almost 30 years ago. Transport activity is sodium- and chloride-dependent, and is associated with cholinergic nerve terminals. This transporter was first cloned (from C. elegans) by Okuda et al., in 2000. The amino acid sequence of the choline transporter suggests 12 or 13 transmembrane domains, and has some similarity to a class of sodium-dependent transporters for glucose (SGLT1) and inositol (SMIT1). We obtained a deletion allele of the choline transporter gene
cho-1 (C48D1.3) from the C. elegans Gene Knockout Consortium. The
tm373 mutation is associated with a precise 1695-nucleotide deletion and eliminates more than half of the
cho-1 coding sequences; it is almost certainly a null allele. Animals homozygous for
tm373 are viable, and their growth and development appear to be normal. However, unlike wild-type animals,
cho-1 mutants are able to grow and reproduce in the presence of the AChE inhibitor aldicarb such drug resistance is usually associated with decreased ACh release (Miller et al., 1996). On agar plates, the
cho-1 mutants moved a bit more slowly than wild-type, but seemed to be normally coordinated. However, when placed in liquid, the
cho-1 animals were clearly less vigorous than wild-type animals, and thrashed at approximately 60% of the wild-type rate. In addition, the
cho-1 animals were unable to maintain this swimming behavior for very long: within 5 minutes, almost all of the
cho-1 animals had slowed down significantly, and after 30 minutes, 80% of them had stopped swimming and were completely immobile (wild-type animals don't slow down even after 30 minutes). We interpret these results to mean that there is a sufficient pool of endogenous choline in the
cho-1 mutant nerve terminals to support a significant level of ACh synthesis even without any reuptake activity, and that this level of ACh synthesis was sufficient for locomotion in a calm environment. However, this level of ACh was not adequate to support the increased demands of vigorous swimming, and the
cho-1 motor neurons just ran out of transmitter. We believe that this demonstration of "functional fatigue" provides the first genetic-based evidence for a functional coupling of choline transport and ACh synthesis, and supports previous models based on other methods (Bussiere et al., 2001;Collier, 1988;Jope and Jenden, 1980).