Primary structure and neuronal effects of α-bag cell peptide, a second candidate neurotransmitter encoded by a single gene in bag cell neurons of Aplysia

Abstract

A discharge of impulse activity in a group of neuroendocrine cells, the bag cells, produces several types of prolonged responses in various identified neurons of the abdominal ganglion of Aplysia. Two excitatory responses are almost certainly mediated by egg-laying hormone, but this peptide cannot account for other responses, such as inhibition of left upper quadrant neurons. We report here the isolation from bag cell clusters of three structurally similar peptides, seven, eight, and nine residues long, that are candidate transmitters for mediating bag cell-induced inhibition. They may also serve as autoexcitatory transmitters since the seven-residue peptide produces a slow depolarization of the bag cells similar to that which occurs during bag cell discharge. The amino acid sequence of the largest peptide, termed a-bag cell peptide [1][2][3][4][5][6][7][8][9], is H-Ala-Pro-Arg-Leu-Arg-Phe-Tyr-Ser-Leu-OH. The other two peptides are identical to a-BCP[I-9] except that they lack the COOH-terminal Ser-Leu or leucine residues. The three peptides inhibit left upper quadrant neurons at relative potencies of 10:30:1 (seven-, eight-, and nine-residue peptides, respectively). Recent molecular genetic analysis shows that both a-BCP [1][2][3][4][5][6][7][8][9] and egg-laying hormone are encoded by the same bag cell-specific gene. The multiple neuronal effects of bag cells are therefore likely to be mediated by at least two transmitters that are cleaved from a common precursor molecule.The marine mollusk Aplysia is a convenient experimental system for investigating at the cellular and molecular levels the roles of neuropeptides as neurotransmitters and in the regulation of behavior. Egg laying in Aplysia is accompanied by a stereotyped behavioral pattern that lasts for several hours. The behavior pattern is thought to be initiated and controlled by the bag cells, a group of neuroendocrine cells located in the abdominal ganglion. In intact animals, egg laying is invariably preceded by a repetitive impulse discharge of the bag cells (1). In the isolated abdominal ganglion, where the central actions of the bag cells have been most extensively studied, a bag cell discharge produces at least four types of long-lasting excitatory and inhibitory effects on various identified neurons (2-4).To identify the neurotransmitters mediating these effects, extracts of bag cells have been purified and assayed for activity on various neurons. It has been determined that egg-laying hormone (ELH), a 4,400-dalton peptide that is synthesized and released by bag cells (5-8), is probably the neurotransmitter mediating two types of excitatory neuronal responses, burst augmentation of cell R15 and prolonged excitation of left lower quadrant (LLQ) neurons (9-11). However, when perfused into the abdominal ganglion at physiological concentrations, purified ELH has little or no effect on left upper quadrant (LUQ) neurons, cells L2-L4, L6, which are normally inhibited by bag cell discharge (10, 11). This indicates that ELH does not directly or in...

Proteolytic processing of the Aplysia egg-laying hormone prohormone

“…In addition, the N-terminal peptides from pELH are not at detectable levels (Ͻ5 fmol) and may be lacking. This finding is surprising considering that the application of ␣-BCP effects the membrane properties of L3, as well as other left upper quadrant neurons (17,31,36). Further studies involving the distribution of the bag cell peptides throughout the abdominal ganglion are underway.…”

“…Full-length ␣-BCP has been proposed to be intracellularly processed (17), with bioactivity depending on the relative amounts of the products, as ␣ 1-8 and ␣ 1-7 are 30 and 10 times more potent, respectively, than ␣ 1-9 . The three forms previously were recovered from cellular homogenates (17,31). However, in the presence of peptidase inhibitors, ␣ 1-7 was not observed in releasates within the reported 1-pmol detection limit (28).…”

Post‐translational processing in model neuroendocrine systems: Precursors and products that coordinate reproductive activity in Aplysia and Lymnaea

“…Peptide studies (Nagle et al, 1986;Rothman et al, 1986) suggest that there are two others, which have been designated the A' and B' genes (Nagle et al, 1988~). A cDNA clone representing a transcript of the A' gene has been produced from the central nervous system of ever, peptide A can stimulate activity in both the bag cells (Heller et al, 1980) and the albumen gland (de Jong-Brink et al, in preparation).…”

Nitric oxide induces aspects of egg-laying behavior in Aplysia

“…The direct effects of a variety of bag cell peptides on a number of tissues has been shown, supporting this idea. For example, ELH directly affects the gonads and causes egg release (Choate et al, 1993;Coggeshall, 1970;Rothman et al, 1983b), and directly affects central neurons in the buccal and cerebral ganglia and thereby inhibits feeding (Ram, 1982;Ram, 1983;Teyke et al, 1991), whereas additional peptides cause depolarization of the bag cells as part of a positive feedback loop Kauer et al, 1987;Rothman et al, 1983a). It has also been suggested that egg laying is coordinated by feed-forward and feedback loops (Cobbs and Pinsker, 1982b;Ferguson et al, 1989b;Ter Maat and Ferguson, 1996).…”

“…The neuroendocrine bag cells, which secrete egg-laying hormone (ELH) and a variety of additional peptides (Rothman et al, 1983a;Sigvardt et al, 1986;, are a central component coordinating egg laying. There have been conflicting reports on how different aspects of egg laying are coordinated.…”