Abstract
Natural variation in neuronal excitability and connectivity has not been extensively studied. In Drosophila melanogaster, a naturally maintained genetic polymorphism at a cGMP-dependent protein kinase (PKG) gene, foraging (for), is associated with alternative food search strategies among the allelic variants Rover (for(R); higher PKG activity) and sitter (for(s); lower PKG activity). We examined physiological and morphological variations in nervous systems of these allelic variants isolated from natural populations. Whole-cell current clamping revealed distinct excitability patterns, with spontaneous activities and excessive evoked firing in cultured sitter, but not Rover, neurons. Voltage-clamp examination demonstrated reduced voltage-dependent K(+) currents in sitter neurons. Focal recordings from synapses at the larval neuromuscular junction demonstrated spontaneous activity and supernumerary discharges with increased transmitter release after nerve stimulation. Immunolabeling showed more diffuse motor axon terminal projections with increased ectopic nerve entry points in sitter larval muscles. The differences between the two natural alleles was enhanced in laboratory-induced mutant alleles of the for gene. The pervasive effects of the for-PKG on neuronal excitability, synaptic transmission, and nerve connectivity illustrate the magnitude of neuronal variability in Drosophila that can be attributed to a single gene. These findings establish the consequences in cellular function for natural variation in an isoform of PKG and suggest a role for natural selection in maintaining variation in neuronal properties.
