The giant serotonergic neuron of Aplysia: a multi-targeted nerve cell

Abstract

We have examined the various classes of cells that can be innervated by the giant cerebral neuron (GCN), an identified serotonergic cell that functions in arousal and maintenance of feeding behavior. We have found that this single neuron innervates a remarkable variety of postsynaptic targets by means of varicosities bearing active zones. The neuron's presynaptic terminals were identified by electron microscopic radioautography after intrasomatic injection of a tritiated amino sugar precursor of membrane glycoproteins; these are moved to nerve endings by fast axonal transport. In addition to endings on buccal muscle, we have found that GCN forms appositions with the morphological characteristics of synapses on axonal processes and cell bodies of neurons in the buccal ganglion and, unexpectedly, it forms appositions most often with glial cells which form the lining of intraganglionic hemal sinuses. Thus, GCN, through contacts on a variety of postsynaptic targets, has the potential of mediating several different functions, each of which is usually associated with a specific specialized type of neuron. In random electron micrographs, approximately 14% of GCN's varicosities had membrane specializations presumed to be the sites where transmitter is released. In these sections, GCN's active zones were quite small, 0.25 micrometer or approximately five vesicle diameters long. One of GCN's terminals was reconstructed completely from a series of thin sections. It had a single, flat ovoid active zone with an area of 17 micrometers2. We suggest that active zones often are overlooked in random sections of monoaminergic terminals because they are small.

Structural and functional analysis of synaptic transmission between identified leech neurones in culture.

“…2A have not yet been observed. These junctions between Retzius and P cells are similar to those described for 5-HT synapses in molluscs (Pentreath & Berry, 1978;Schwartz & Shkolnik, 1981) where again the complete spectrum of widened cleft, thickened straight membranes, vesicle clustering and intercellular material have not so far been found. Whether or not they represent normal synapses, features such as those observed in P1.…”

Colonic Migrating Motor Complexes, High Amplitude Propagating Contractions, Neural Reflexes and the Importance of Neuronal and Mucosal Serotonin

“…For instance, the giant cerebral serotonergic neuron in Aplysia (sea slug) synapses with many different post synaptic targets including glial cells, other neurons, muscle, gut and glands to mediate a number of behaviors including arousal and feeding. 49 In fact, serotonergic neurons coordinate or modulate firing in neural networks that mediate motor behaviors in many invertebrates. [50][51][52] These include swimming in the leech, escape reflexes in crustaceans, as well as pyloric motor patterns generated by gastric mill neurons in crabs.…”

Metamodulation of the Crayfish Escape Circuit

“…These same synaptic, paracrine and endocrine mechanisms of serotonin release occur in Aplysia [Schwartz and Shkolnik, 1981;Kistler, Jr. et al, 1985;Longley and Longley, 1986]. Although serotonin has unitary, concentration-dependent effects in many systems, its facilitatory effect on central synapses in Aplysia has recently been shown to depend on the temporal pattern of exposure [Mauelshagen et al, 1998].…”