Isolation and partial characterization of neuropeptides that mimic prolonged inhibition produced by bag cell neurons in Aplysia

Complex behavior induced by egg-laying hormone in Aplysia

Previous studies have described a pattern of complex behavior that occurs in the marine mollusc Aplysia during egg laying. Egg laying and the behavior are initiated by a burst of impulse activity in the neuroendocrine bag cells of the abdominal ganglion or by injection of bag cell extract. To more precisely identify the factors responsible for inducing the behavior we injected animals with egg laying hormone (ELH), one of the neuropeptides secreted by the bag cells. We found that ELH causes a behavior pattern similar to what occurs during spontaneous egg laying. This includes a temporal pattern of head movements consisting of waves and undulations, followed near the beginning of egg deposition by a transition to head weaves and tamps and inhibition of locomotion. There was also a small decrease in respiratory pumping. Except for respiratory pumping, a similar pattern occurred in a second group of animals injected with atrial gland homogenate, which is presumed to induce bag cell activity, but not in controls. These results further implicate ELH in regulation of the behavior. We discuss possible sites of action of ELH and the neural mechanisms by which the behavior is controlled.

Molecular cloning of myomodulin cDNA, a neuropeptide precursor gene expressed in neuron L10 of Aplysia californica

Screening of an abdominal ganglion cDNA library of Aplysia californica with a synthetic oligonucleotide based on the myomodulin peptide sequence has been used to identify and characterize a cDNA clone expressed in interneuron L10. The complete sequence of 1,534 nucleotides in length shows an open reading frame of 370 amino acids that encodes a 42-kD pre-propeptide. Proteolytic cleavage of the precursor potentially gives rise to 10 copies of myomodulin A, single copies of 6 myomodulin-related peptides, and other unrelated sequences. Southern blot analysis of sperm DNA shows that the haploid Aplysia genome contains only one copy of the gene. RNA blot analyses of central nervous system (CNS) mRNA show that the myomodulin gene is expressed in all major ganglia and that a single transcript of around 1,600 nucleotides can be detected in pooled CNS mRNA, suggesting that the same sequence is transcribed in each ganglia. Nucleotide sequences of partial cDNA clones isolated from a cerebral ganglion and a CNS cDNA library are identical to the abdominal ganglion sequence, further suggesting that the same myomodulin gene is expressed in other ganglia.

Identification and molecular cloning of a neuropeptide Y homolog that produces prolonged inhibition in Aplysia neurons

The neuroendocrine bag cell neurons of the marine mollusk Aplysia produce prolonged inhibition that lasts for more than 2 hr. We purified a peptide from the abdominal ganglion that mimics this inhibition. Mass spectrometry and microsequence analysis indicate that the peptide is 40 aa long and is amidated at its carboxyl terminus. It is highly homologous to vertebrate neuropeptide Y (NPY) and other members of the pancreatic polypeptide family. As determined from cloned cDNA, the gene coding for the precursor protein shares a common structural organization with genes encoding precursors of the vertebrate family. The peptides may therefore have arisen from a common ancestral gene. Bag cell neurons are immunoreactive for Aplysia NPY, and Northern blot analysis indicates that as with its vertebrate counterparts, the peptide is abundantly expressed in the CNS. This suggests that peptides related to NPY may have important functions in the nervous system of Aplysia as well as in other invertebrates.

Structure-activity relationship of the neurotransmitter alpha-bag cell peptide onAplysia LUQ neurons: Implications regarding its inactivation in the extracellular space

Alpha-bag cell peptide [alpha-BCP (Ala-Pro-Arg-Leu-Arg-Phe-Tyr-Ser-Leu)] is a neurotransmitter that mediates bag cell-induced inhibition of left-upper-quadrant (LUQ) neurons L2, L3, L4, and L6 in the abdominal ganglion of Aplysia. Our recent biochemical studies have shown that alpha-BCP[1-9] is cleaved into alpha-BCP[1-2], [3-9], [1-5], [6-9], and [7-9] by a combination of three distinct peptidase activities located within the extracellular spaces of the CNS: A diaminopeptidase-IV (DAP-IV)-like enzyme cleaves alpha-BCP[1-9] at the 2-3 peptide bond; a neutral metalloendopeptidase (NEP)-like enzyme cleaves either alpha-BCP[1-9] or alpha-BCP[3-9] at the 5-6 bond; an aminopeptidase M-II (APM-II)-like enzyme cleaves alpha-BCP[6-9] at the 6-7 bond, but cleaves neither alpha-BCP[1-9], nor the other ganglionic peptidase products. To further understand the manner in which alpha-BCP is inactivated after release, that is loses its electrophysiological activity, we studied its structure-activity relationship by recording intracellularly from LUQ neurons in isolated abdominal ganglia that were arterially perfused with peptides dissolved in artificial sea water. The effects of alpha-BCP[1-9] and 15 of its fragments ([1-8], [1-7], [1-6], [1-5], [2-9], [3-9], [3-8], [6-9], [7-9], [8-9], [6-7], [6-8], [1-2], Phe, Tyr) indicated that the sequence Phe6-Tyr7 was both necessary and sufficient to produce LUQ inhibitory activity.(ABSTRACT TRUNCATED AT 250 WORDS)