Modulation of neuromuscular transmission by conventional and peptide transmitters released from excitatory and inhibitory motor neurons in Aplysia

Abstract

The anterior portion of intrinsic buccal muscle (I3a) is innervated by two excitatory motor neurons, B3 and B38, and the newly identified inhibitory motor neuron, B47. We show that B47 is cholinergic while B3 and B38 are not. B3 and B38 have previously been shown to express the neuropeptides FMRFamide and the small cardioactive peptides (SCPs) A and B, respectively. We present evidence here that B47 synthesizes the neuropeptide myomodulin A (Mma). When placed in culture, B3, B38, and B47 continued to synthesize their respective peptides. These peptides were released in a stimulation- and Ca(2+)-dependent manner, suggesting that they are transmitters in these neurons. By using B3-evoked excitatory junction potentials (EJPs) and muscle contractions as assays, we next examined the modulatory effects of superfusion of peptides and stimulation of motor neurons B38 and B47. Superfusing the muscle with low concentrations of the SCPs, FMRFamide, or Mma enhanced B3-evoked EJPs and contractions. Stimulation of B47 simultaneously with B3 reduced the amplitude of B3-evoked contractions. However, when either B47 or B38 was stimulated in extended bursts designed to release their peptide transmitters, subsequent B3-evoked EJPs and contractions were enhanced. We believe that this modulation is due at least in part to the release of peptides from the terminals of B38 and B47. The SCPs potently increase cAMP levels in I3a muscle fibers. Likewise, stimulation of B38 in extended bursts increased cAMP levels in the muscle. This provides independent evidence that the SCPs are released from B38 terminals in the muscle. Therefore, we have described a neuromuscular preparation amenable to the study of both excitatory and inhibitory motor neurons that utilize a variety of conventional and peptide transmitters. Our results suggest that these motor neurons can function in two states. When stimulated in single brief bursts, they primarily release conventional transmitters. When stimulated in a series of prolonged bursts, they release both conventional transmitters and peptide cotransmitters. These dual states are most pronounced in the case of B47, which, depending on the stimulation paradigm, can act selectively to inhibit or enhance the effects of a second motor neuron innervating the same muscle.

Variability of Motor Neuron Spike Timing Maintains and Shapes Contractions of the Accessory Radula Closer Muscle ofAplysia

“…In Aplysia, many other muscles, not only in the buccal mass but also, for example, in the gill musculature, resemble the ARC in the speed and nonlinearity of their neuromuscular transforms (Carew et al, 1974;Evans et al, 1996;Fox and Lloyd, 1997;Yu et al, 1999), the irregular firing of their motor neurons (Byrne et al, 1978;Morton and Chiel, 1993;Wu et al, 1994;Jing et al, 2004), and their complex modulation (Church et al, 1993;Fox and Lloyd, 1997;Evans et al, 1999;Keating and Lloyd, 1999;Hurwitz et al, 2000). With the real, irregular motor neuron spike patterns, the contractions of these muscles have shapes very much like those that we recorded here in Figures 4 -7 (Wu et al, 1994).…”

Fast Synaptic Connections From CBIs to Pattern-Generating Neurons inAplysia: Initiation and Modification of Motor Programs

“…Coincident with the termination of the retraction phase (monitored by the onset of high-frequency firing of B8 or via firing in B64), protraction phase neurons showed a characteristic rapid return to their rest potentials. When CBI-2 was continuously stimulated with pulses, after a short delay, the return to rest was followed by a new cycle of a motor program (see also Church and Lloyd 1994;and Rosen et al 1991). However, if firing in CBI-2 was terminated, BMPs were terminated (Fig.…”

“…The protraction-retraction sequence is variably coupled with radula opening and closing movements. The activity of the buccal muscles is organized by a central pattern generator (CPG) contained in the buccal ganglia (Hurwitz and Susswein 1996;Hurwitz et al 1997;Kirk 1989;Susswein and Byrne 1988;Teyke et al 1993), which drive the motoneurons that innervate the buccal muscles (Church and Lloyd 1994;Church et al 1993;Cohen et al 1978;Hurwitz et al 1994Hurwitz et al , 1996Hurwitz et al , 2000Morton and Chiel 1993a,b).…”

Identification of a New Neuropeptide Precursor Reveals a Novel Source of Extrinsic Modulation in the Feeding System ofAplysia

“…Furthermore, multiple neuropeptides often converge to modulate the very same processes. In this respect, one of the best-studied preparations is the Aplysia feeding system, in which a number of novel neuropeptides have been identified (Cropper et al, 1987a(Cropper et al, ,b, 1988(Cropper et al, , 1994Church and Lloyd, 1991;Church et al, 1993;Brezina et al, 1995Brezina et al, , 1996Vilim et al, 1996Vilim et al, , 2000Evans et al, 1999;Hurwitz et al, 2000;Morgan et al, 2000Morgan et al, , 2002Jing and Weiss, 2001;Sweedler et al, 2002;Koh et al, 2003).…”