Promotion of sleep by gonococcal peptidoglycan fragments. Structural requirements for the somnogenic activity

We exploited an extensive inventory of gonococcal peptidoglycan (PG) fragments to define the essential structural determinants of PG-mediated sleep-promoting activity in a rabbit sleep model. PG fragments, purified using reverse phase HPLC and structurally defined by fast atom bombardment mass-spectrometry, were administered intracerebroventricularly and the duration of specific sleep stages was determined electroencephalographically. Of the compounds tested, the principal naturally occurring sleep factor isolated from sleep-deprived animals (N-acetylglucosaminyl-[NAG]-1,6-anhydro-N-acetylmuramyl [anh.NAM]-alanyl-glutamyl-diaminopimelyl-alanine), the structurally identical PG monomer derived from gonococci, and individual analogs of the gonococcal compound which lacked the NAG residue or contained an additional alanine at the C-terminus possessed maximal potency; as little as 1 pmol of these anh.NAM-containing monomers induced excess slow wave sleep (p less than .05). In fact, each of five different anh.NAM-containing disaccharide peptides tested was somnogenic at 10 pmol or less, but none of a matched set of analogous PG monomers, differing only in replacement of anh.NAM by a hydrated NAM residue, was somnogenic at this dose. Together, these data suggested that the anh.NAM end, but not the NAG moiety, is a crucial structural determinant of gonococcal PG-mediated somnogenic activity. The somnogenic activity of anh.NAM-containing fragments was also modulated (albeit to a lesser extent) by the length and composition of the peptide side chain. On a much broader basis, the data also help raise the intriguing hypothesis that bacterial products may serve as natural regulators of nervous system function in higher animals.