On the structure of the pulmonate osphradium. II. Ultrastructure

Structural organization of the nervous system of the mantle cavity wall and organs in prosobranch mollusks

A variety of common histological stains was used, along with the Golgi and Colognier silver impregnation methods and electron microscopy, to perform a comparative study of the morphological characteristics of receptor and nerve cells and their interactions in the nervous system of the wall of the mantle cavity and mantle derivatives - gills, siphon, and osphradia - of the marine prosobranch gastropod mollusks Buccinum undatum and Littorina littorea. The results are discussed along with our own previously obtained data on the organization of the osphradial sensory organs and the afferent elements of the mantle cavity wall in other prosobranch mollusks - the terrestrial Pomatia elegans and the freshwater Viviparus contectus and Pomacea paludosa. Using the nervous system of the complex of mantle organs of prosobranch mollusks as examples, the structural features and evolutionary trends of the afferent part of the visceral nervous system of gastropods are discussed.

Distinct responses of osphradial neurons to chemical stimuli and neurotransmitters in Lymnaea stagnalis L

1. In Lymnaea stagnalis L. (Pulmonata, Basommatophora) the neurons in the osphradium were visualized by staining through the inner right parietal nerve by 5,6-carboxyfluorescein (5,6-CF). Three types of neurons were identified: three large ganglionic cells (GC1-3; 80-100 microns), the small putative sensory neurons (SC; 20 microns) and very small sensory cells (3-5 microns). 2. The ganglionic and putative sensory neurons were investigated by whole cell patch-clamp method in current-clamp condition. The three giant ganglionic neurons (GC1-3) located closely to the root of osphradial nerve, had a membrane potential (MP) between -30 and -70 mV and showed tonic or bursting activities. The small putative sensory cells (SCs) scattered throughout the osphradial ganglion, possessed a MP between -25 and -55 mV and showed an irregular firing pattern with membrane oscillations. At resting MP the GC1-3 cells were depolarized and increased the frequency of their firing, while the SCs were hyperpolarized and inhibited by NaCl (10(-2) M) and L-aspartate (10(-5) M) applied to the osphradium. 3. 5-Hydroxytryptamine (5HT, 10(-6) M), gamma-aminobutyric acid (GABA; 10(-6) M) and the GABAB agonist baclofen (10(-6) M) depolarized the neurons GC1-3 and increased their firing frequency. In contrast, on the GC1-3 neurons, acetylcholine (Ach; 10(-6) M) and FMRFamide (10(-6) M) caused hyperpolarization and cessation of the firing activity. The 5HT effect was blocked by mianserin (10(-6) M) but picrotoxin (10(-5) M) failed to block the GABA-induced effect on the GC1-3 cells. 4. The small putative sensory neurons (SCs) were excited by Ach (10(-6) M) and 5HT (10(-6) M) but were inhibited by GABA (10(-6) M). FMRFamide (10(-6) M) had a biphasic response. The Ach effect was blocked by hexamethonium (10(-6) M) and tetraethylammonium (10(-6) M), indicating the involvement of nicotinic cholinergic receptors. 5. The distinct responses of the two populations of osphradial neurons to chemical stimuli and neurotransmitters suggest that they can differently perceive signals from environment and hemolymph.

GABA-immunoreactive neurones and interactions of GABA with serotonin and FMRFamide in a peripheral sensory ganglion of the pond snail Lymnaea stagnalis

The osphradium is a putative chemosensory organ of aquatic molluscs. Previously, we identified cells with serotonin (5-HT) and FMRFamide (FMRFa)-like immunoreactivity in the osphradial ganglion of Lymnaea stagnalis. The present investigation has established the presence of cells immunoreactive to gamma-aminobutyric acid (GABA). Some of these cells send processes to the sensory epithelium and are thus considered to be primary sensory neurones. Colocalisation of GABA and FMRFamide-like immunoreactivities was found in some of these and other neurones. The responses of the osphradial output electrical activity to the single and combined application of the above neuroactive substances were examined. 5-HT slightly increased and FMRFa decreased the activity. GABA alone was generally ineffective; however, it had a consistent stimulating effect after pretreatment with 5-HT. In its turn, pretreatment with GABA significantly increased the inhibitory action of FMRFa. Primary sensory neurones giving this kind of responses in the nerve were identified electrophysiologically and morphologically in the osphradial ganglion. Our results indicate that GABA takes part in relay of sensory signals into the central nervous system, and transmitter interactions involving GABA, 5-HT, and FMRFa are considerable for the final output pattern of the osphradial sensory network.

Fine structure of olfactory epithelia of gastropod molluscs

Among gastropod molluscs the chemical senses are most important for location of distant objects. They are used in food finding, locating mates, avoiding predators, trail following, and homing. Chemoreceptors are commonly associated with the oral area, the tentacles, and the osphradium, which lies in the mantle cavity. Most chemosensory neurons are primary sensory neurons, although secondary sensory cells have been reported in the osphradium of some prosobranch gastropods. Most chemosensory organs contain sensory cells with ciliated sensory endings that are in contact with the external environment. Some sensory endings have only microvilli or have no surface elaborations. Cilia on sensory endings are commonly of the conventional type, but some species have modified cilia; some lack rootlets, some have an abnormal microtubular content, and some have paddle-shaped endings. The perikarya of sensory neurons may be within the sensory epithelium, below it, or in ganglia near the sensory surface. In some groups of gastropods there are peripheral ganglia in the olfactory pathway; in others chemosensory axons appear to pass directly to the CNS. Olfactory epithelia of terrestrial pulmonates have modified brush borders with long branching plasmatic processes and a spongy layer of cytoplasmic tubules which extend from the epithelial cells. Sensory endings of the olfactory receptors are entirely within this spongy layer. Aquatic pulmonates may have a similar spongy layer in their olfactory epithelia, but the cilia of sensory endings, as well as motile cilia of epithelial cells, extend well beyond the spongy layer.

Axonal mapping of the giant peptidergic neurons VD1 and RPD2 located in the CNS of the pond snail Lymnaea stagnalis, with particular reference to the innervation of the auricle of the heart

VD1 and RPD2 are two giant neuropeptidergic neurons located respectively in the visceral and right parietal ganglion of the central nervous system (CNS) of the pond snail Lymnaea stagnalis. They are the most prominent representatives of a system of neurons expressing a gene that is similar to the gene expressed in R15 of Aplysia californica. Both neuronal systems are involved in the regulation of cardio-respiratory phenomena. In the present study the axonal branches of VD1 and RPD2 were mapped using immunocytochemical and tracer studies. To this end the alpha 1-antiserum (directed to one of the VD1/RPD2 neuropeptides) was used in combination with Lucifer yellow (LY) and Ni-lys tracers. In whole mount preparations of the CNS, immunostained axons of VD1 and RPD2 were observed to run to the pleural, cerebral and pedal ganglia and in several nerves. Upon LY injection of VD1 thin axon branches were observed in the internal right parietal nerve. These run to the skin in the mantle area near the pneumostome and osphradium. The skin of the lips appeared to receive a similar innervation via the lip nerves. Thick LY filled axons of VD1 and RPD2 were observed in the intestinal nerve. They could be traced to the heart region. The pericardial branch of the intestinal nerve innervates the pericardium and heart (Ni-lys tracing). Immunocytochemically, using the alpha 1-antiserum, it was demonstrated that this nerve branch carries the axons of VD1 and RPD2 to the venous side of the auricle, where they enter the pericardial cavity and ramify in the auricle (but not in the ventricle).(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of intracellular free arachidonic acid in Aplysia nervous system

We have studied the regulation of arachidonic acid (AA) uptake, metabolism, and release in Aplysia nervous system. Following uptake of [3H]AA, the distribution of radioactivity in intracellular and extracellular lipid pools was measured as a function of time in the presence or absence of exogenous AA. The greatest amount of AA was esterified into phosphatidylinositol (relative to pool size). We found that the intracellular free AA pool underwent rapid turnover, and that radioactive free AA and eicosanoids were released at a rapid rate into the extracellular medium, both in the presence and absence of exogenous AA. Most of the released radioactivity originated from phosphatidylinositol. Two pharmacological agents were found to modulate AA metabolism in Aplysia ganglia. The phorbol ester, 12-O-tetradecanoylphorbol 13-acetate, stimulated liberation of AA from phosphatidylinositol and phosphatidylcholine. This resulted in an increase in free internal and secreted AA, an increase in conversion of AA to eicosanoids, and an increase in esterification of AA into triacylglycerol. The half maximal dose for TPA-stimulated AA turnover was 15 nM, and the stimulation was dependent on the presence of extracellular calcium. 4-bromophenacylbromide inhibited the redistribution of radioactivity from phospholipid into triacylglycerol, indicating BPB was acting as a phospholipase inhibitor in Aplysia as it does in other systems. These pharmacological agents, in addition to providing information about the regulation of AA metabolism and release, are useful tools for investigating the physiological function of the rapid turnover of AA in Aplysia nervous system.

Ultrastructure of the osphradium of Aplysia californica

The osphradium of Aplysia californica, a sensory organ, is a small yellow-brown epithelial patch located in the mantle cavity immediately anterior to the rostral attachment of the gill. Scanning electron microscopy reveals a round ellipsoid structure of 0.6-1 mm in diameter with a central, occasionally folded, sensory epithelium. The central area is covered with microvilli and surrounded by a densely ciliated epithelium. Transmission electron micrographs show that the columnar supporting cells in the sensory epithelium contain an abundance of apical pigment granules and microvilli. Between the epithelial-supporting cells, the putative sensory elements consist of thin neurites (0.4-1.5 micron in diameter) that reach the sea-water side of the osphradium. The neurites contain many neurotubules, mitochondria, vesicles and cilia in their apices. The nerve endings originate from cell bodies up to 40 microns below the epithelium or in the osphradial ganglion itself, as revealed by electron microscopy and retrograde labeling with Lucifer yellow. There appear to be two populations of putative sensory cells, a large population of heavily stained cell bodies 4-10 microns in diameter and a few scattered cells of large diameter (25-60 microns). Following lanthanum impregnation, septate junctions can be seen between all types of cells in the epithelium, 3-5 microns below the sea-water surface. This study provides new information for further investigation of osmo- and mechanosensation in Aplysia californica.

Observations on the olfactory organ of adult and juvenile Octopus joubini

The olfactory organ has an epithelium containing many sense cells and a large subepithelial mass of receptor cells. The epithelium includes cells with cup-shaped, ciliated endings, and hollow, flask-shaped sense cells with ciliated cavities that open to the surface, through a small pore. Below the epithelium are large hollow cells with ciliated cavities and distal processes that either form patent connections between the ciliated cavity and the surface or have a ciliated ending at the surface. There are many synapses between processes in the olfactory nerve. The possible chemosensory function of the olfactory organ is discussed.