MUTUALLY EXCLUSIVE EXPRESSION OF ALTERNATIVELY SPLICED FMRFAMIDE TRANSCRIPTS IN IDENTIFIED NEURONAL SYSTEMS OF THE SNAIL LYMNAEA

The FMRFamide gene of the snail Lymnaea encodes tetrapeptides (FMRFami de/FLRFamide) and heptapeptides (GDPFLRFamide/SDPFLRFamide) on separat e exons. In situ hybridization probes specific to these exons were use d to map the expression of the two exons in identified neuronal system s of the CNS. Analysis of more than 200 preparations showed that cytop lasmic expression of mRNA was exclusively of one type, with individual neurons expressing either the tetrapeptide or heptapeptide exon. Of t he approximately 340 neurons expressing the two exons, the majority (8 0%) expressed the tetrapeptide exon. The tetrapeptide exon was more wi de-spread, occurring in neurons from all 11 ganglia of the CNS. The he ptapeptide was mainly confined to two ganglia (visceral and right pari etal), with a small number of cells in three other ganglia. Mapping st udies combined with dye marking of identified neurons showed the prese nce of the tetrapeptide exon in several behaviorally important network s: heart motoneurons, whole body withdrawal response motoneurons, and probably penis motoneurons as well as giant identified neurons (LP1, R PD1). The heptapeptides were prominent in two main clusters of cells ( Bgp and Fgp) together with a smaller number of tetrapeptide-expressing cells.