The pharmacology of molluscan neurons

Acetylcholine activates cerebral interneurons and feeding motor program in Limax maximus

The cellular and network effects of acetylcholine (ACh) on the control system for feeding in Limax maximus were measured by intracellular recordings from feeding command-like interneurons and whole nerve recordings from buccal ganglion motor nerve roots that normally innervate the ingestive feeding muscles. The buccal ganglion motor nerve root discharge pattern that causes rhythmic feeding movements, termed the feeding motor program (FMP), was elicited either by attractive taste solutions applied to the lip chemoreceptors or by ACh applied to the cerebral ganglia. The ability of exogenous ACh applied to the cerebral ganglia to trigger FMP was blocked by the cholinergic antagonists curare and atropine. If the strength of the lip-applied taste stimulus was in the range of 1-2 times threshold, cerebral application of the cholinergic antagonists blocked or greatly decreased the ability of lip-applied taste solutions to trigger FMP (5 of 8 trials). The cerebral feeding interneurons, some of which activate FMP when stimulated intracellularly, are excited by small pulses of ACh applied directly to the cell body from an ACh-filled micropipette. A pulse of ACh that activates several of the feeding interneurons simultaneously triggers FMP. The data suggest that under certain stimulus conditions an obligatory set of cholinergic synapses onto the feedininterneurons must be activated for taste inputs to trigger ingestion. The determination of ACh's action within the feeding control system is necessary for understanding how enhanced cholinergic transmission leads to prolonged associative memory retention (Sahley, et al., 1986).

Acetylcholine-induced responses in the salivary gland cells of Helisoma trivolvis

This study describes the actions of acetylcholine (ACh) on the salivary gland cells of Helisoma. Perfusion of the salivary gland cells with ACh produces a long-lasting depolarization accompanied by an increase in the input conductance of the gland cells. The depolarization is often followed by a long-lasting hyperpolarization. Carbamylcholine, tetramethylammonium, and choline also produce depolarizing responses. Nicotine and pilocarpine produce only a small depolarization in the gland cells. The following cholinergic antagonists are effective in blocking the gland-cell response to ACh: tetraethylammonium, atropine, hexamethonium, d-tubocurarine, and strychnine. A new preparation, the "isolated acinus," was utilized to obtain the reversal potential of the ACh response. The mean reversal potential in 10 preparations was -7 +/- 8 mV. The depolarizing phase of the response is dependent on the presence of both external calcium and external sodium ions. The long-lasting hyperpolarization is produced by the activity of an electrogenic sodium-potassium pump. The properties of the acetylcholine receptors on the salivary gland cells of Helisoma are compared with those described in other gastropod preparations.

Chemical synapses, particle arrays, pseudo-gap junctions and gap junctions of neurons and glia in the buccal ganglion of Helisoma

The nervous system of the snail, Helisoma trivolvis, has been utilized for a wide range of studies of neuronal plasticity; however, the ultrastructural features of this tissue were previously unknown. The present study examined the nature of synaptic interactions of neurons and glia and considered several plasma membrane specializations of these cells. The symmetrical pair of buccal ganglia consisted of a ring of unipolar neurons surrounding a central neuropil. The neurons were separated by two morphologically distinct types of glia: type I were most numerous and possessed an electron-dense homogeneous cytoplasm, whereas type II glia were of lower electron density, possessed a heterogeneous cytoplasm, and appeared to be phagocytic. Gap junctions were abundant between glia and were occasionally found between neuronal processes, including those of neurons 19 injected with horseradish peroxidase (HRP). Comparison of neuron and glial gap junction widths (16.4 and 17.6 nm, respectively) in thin sections and their intramembrane particle diameters (13.1 and 13.7 nm, respectively) by freeze fracture, did not elucidate significant differences. A heterogeneous population of putative chemical synapses, similar to those reported in other molluscs, was also observed between axonal collaterals in the neuropil. Additionally, examination of freeze-fractured neuropil revealed rhombic arrays of particles localized on neuronal membranes; these arrays do not appear to form intercellular junctions but may represent postsynaptic receptor sites. Freeze fracture also revealed small, square arrays consisting of 7-9 nm diameter particles on glial membranes which may correspond to pentalaminar membrane contacts (pseudo-gap junctions) seen in thin sections between glia situated around dilated extracellular spaces (lacunae).

Localization and synthesis of monoamines in regions of Limax CNS controlling feeding behavior

Localization and synthesis of dopamine and serotonin in the cerebral and buccal ganglia of Limax maximus were studied. A combination of fluorescence histochemistry, immunocytochemistry, and microchemical analysis showed that both amines were localized to particular cell groups and fiber tracts within and between the two sets of ganglia. Since these ganglia control feeding behavior, which is readily modified by associative learning, these studies have direct bearing on analysis of both motor control and learning mechanisms.

Uptake and metabolism of [3H]adenosine by Aplysia ganglia and by individual neurons

[3H]Adenosine was taken up and metabolized by isolated ganglia of the marine mollusc Aplysia californica. After 2 h, most of the radioactivity was recovered as metabolites, including ATP, ADP, and AMP, as well as the deaminated products, inosine, hypoxanthine, and uric acid. Little remained in the form of adenosine. These pathways were not uniformly distributed among various tissue elements. In most individual neurons, inosine and its breakdown products were the principal metabolites of [3H]adenosine, whereas ATP and other nucleotides predominated in the connective tissue sheath. Endogenous levels of ATP, ADP, AMP, and adenosine in ganglia, sheath, and individual neurons were also determined using a fluorimetric-HPLC assay. The concentrations of the nucleotides were quite uniform in sheath and among the individual neurons assayed (1-5 pmol/microgram of protein); however, concentrations of adenosine were considerably higher in neurons than in the sheath.