Specific association of neurotransmitter with somatic lysosomes in an identified serotonergic neuron of Aplysia californica.

Abstract

Injection of [3Hjserotonin directly into the cell body of the giant metacerebral neuron (GCN), an identified serotonergic cell in the Aplysia cerebral ganglion, revealed a striking association of the labeled transmitter with large lysosomes. Most of the [3H] Recycled synaptic membrane, packaged in lysosomes, is thought to return to the perikaryon by retrograde axonal transport. Ultimately these worn constituents are incorporated into the lipofuscin granules, which are plentiful in many nerve cell bodies, both vertebrate and invertebrate (1, 2). Cytological and histochemical studies have provided convincing'evidence that these organelles are lysosomes that are approaching their last developmental stage, and are morphologically similar to terminal lysosomes (residual bodies) found in many other cell types. With age, the number of these bodies increases, as does their content of pigment, lipid, and other materials thought to derive from degraded cellular organelles and from exogenous constituents engulfed by the cell (3). In gastropod molluscs, somatic lysosomes are slightly larger than the lipofuscin granules of vertebrate neurons, reaching 10 gm in diameter (4-8). In pigmented Aplysia neurons they are characteristically situated in a juxtanuclear ring and contain the pigment originally used to classify neurons of the abdominal ganglion'into pigmented and white cells (9).There have been previous indications that somatic lysosomes can retain the ability to carry out activities normally attributed to other cellular organelles. Attention has recently been drawn to Aplysia pigment granules because in some neurons they serve as an intracellular Ca2+ sink (a property assigned to mitochondria) and can release Ca2+ in response to light. Thus Henkart (10) and Brown et al. (11) MATERIALS AND METHODS Animals and Intracellular Injections. Aplysia californica weighing 40-70 g were supplied by Pacific Bio-Marine Supply, Venice, CA. The cerebral ganglion with the lateral lip nerves, cerebrobuccal connectives, and buccal ganglia attached, or the abdominal ganglion, was removed by opening the animal through the foot; nervous tissue was pinned in a chamber filled with an artificial sea water containing amino acids, glucose, and vitamins (18).Before injection, the overlying connective tissue sheath of the cerebral ganglion was removed by dissection. The giant metacerebral neuron (GCN) was injected with [G-3H]serotonin (13.8-14.7 Ci/mmol), [G-3H]

Identified facilitator neurons L29 and L28 are excited by cutaneous stimuli used in dishabituation, sensitization, and classical conditioning of Aplysia

“…Second, the facilitation produced by L29 stimulation is reversibly reduced by the 5-HT antagonist cinanserin (Hawkins et al, 1976). Third, the L29 cells resemble known serotonergic neurons in Aplysia [the metacerebral cells (MCCs)] in the size, electron density, and regional distribution of their vesicles, in making invaginating axosomatic contacts, and in packaging intrasomatically injected )H-5-HT preferentially within end-stage lysosomes (Schwartz et al, 1979;Shkolnik and Schwartz, 1980;Bailey et al, 198 1). Finally, the L29 cells show a specific, highaffinity uptake of 'H-5-HT from the bath (Bailey et al, 1983).…”

Aplysia synaptosomes. II. Release of transmitters

“…These results suggest that synaptosomes are the predominant site of 3H-5-HT accumulation. A relatively minor contribution to uptake (about 10% of the grain clusters) comes from lysosomes, which have been shown previously to be labeled after intracellular injection of 3H-5-HT (Schwartz et al, 1979). The remaining 20% of the grain clusters could not be assigned to identifiable membrane structures.…”

Fast axonal transport of foreign transmitters in an identified serotonergic neurone of Aplysia californica.

“…It has been shown previously that all of the radioactivity in the tissue is restricted to g.c.n. (Goldman et al 1976;Schwartz, Shkolnik & Goldberg, 1979;Shkolnik & Schwartz, 1980). Ganglion and nerves were separately homogenized at 0°C in 0-5 ml 0-2 m-perchloric acid using glass tissue grinders (MicroMetric Instrument Co., Cleveland, Ohio) in order to extract the radioactivity.…”