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312.
An outline of the present knowledge of neuronal 5-HT in mollwoe is
given. The purpose of the present study was to obtain information on the
structure of 5-HT-eontainiag neurons, on the m^anlsms of transport of 5-® and its precursors into and within neurons, on the nature of the blood supply to the GNS| and m the function of ÿ^fT-containing neurons within the OHS of
Helix oomatia* The follotdng observations were made:
1 . %ers is a giant serotonin-containing neuron (G8G) in each cerebral gang- lio n o f Helix oomatia. Presynaptio endings of the G80# are located in the buccal ganglia and pei^ipheral musculature. Dense-cored vealcles of mean diar meter 100 nm were observed in the perikarya and the awm branthes of the GSGs
within the cerebral ganglia* Evidence is presented # lo h auggeste that these v esicles sequester 5-*®# V esicles with a sim ilar appearance are present in the presumed synaptic endings of the G80# Axonal processes oontaining agran-
ular vesicle# (man diameter $0 nm), or granular v esicles ($0 - 100 xm diameter),
or both, make close contact wiHi the fin e axon brw #es of the GSOs within the cerebral ganglion neuropile* Home o f these axon processes appear to be pre- synaptic endings making synaptic connections with the GHCs*
2 . Baëï paired ganglion of H» wmatia i s supplied by symmetrically arranged brandies from the anterior aorta. C apillaries from these branches open into
a blood space vAiich is adjacent to , and continuous over the surface o f the nervous tissu e. Blood passes from th is opaco throug)i the epineural slieath into the body cavity sinuses. Three tieeuo layers separate tho blood spaces from the neurons of each gm glion. Those are (i) a luminal endotlielium, ( ii) a fibrous eooncotivs tissu e layer lAiicli is mainly collagen, and ( i i i ) g lia l c e lls . Both the luminol endothelium and connective tissu e are freely permeable to
undhargsd particles o f 10 nm or le s s .
3# FoUcwing exposure to tritia to d M ® , electron microscope autoradiography showed that silv er grains, often in very hi^gli conom trations, were located only over oortain fine axon branchea and processes thought to be nerve endings*
133.
The80 proceaaps contained email denoe-oored veoidee, morphologically Idea-
tlo a l to thoso thought to eoquootor 5-® in tW porikarya of the GSGs# It i s
euggeetOd that re-uptOko ie a modiaalem of inaotivatlon o f 5-® in the CN8 o f H elix DOmmtla.
4 . Following oxpoBuro to tritiu to d ollvor grains wore oboorvod over the perikarya of the G8Ge end ether known 5-®-oontaining neurons in H ^ t and oleotrw mioroeoope autoradiograms# There was no indication that the 5^iTP was t^ o n up by nerve mdinge or by nm-nervoue etruoturee# The aoouznulatlon
o f tritia to d tryptophan was loae epeoiflc; a ll the neuron perikaxya é im m â m aoomulotion of radioaotivity after oxpoeure to thin eubetanoe.
5# Eleotrophysiological analyeie ahowod that each GSO isende axon branches to mueolae in the lip e of the animal, Selootive stimulation of the GSG resulted
in an inoreaae of o leetrio a l a ctiv ity reeorded from those muaoloe, but no diange in th eir length* Thl$ e ffe c t was mlmitaied by ^Hff applied to the muscles# I t
ie euggeeted that the GSO has a f acüitK tory effect on the lip muscle potentials# The advantageo of using large rnolluecm neurone for etuchrlng neuronal 541T are dieoueeed# It ie concluded that the 5-® vfithin the OSCe of II. Txmmgt^a eerveo a neurotranm itter ro le.