No abstract

1. Our experiments were performed to assess the quantitative role of the transient potassium current, IA, in determining the cycle frequency and phasing of neurons in the network generating the pyloric motor rhythm in the stomatogastric ganglion of the spiny lobster, Panulirus interruptus. We used 4-aminopyridine (4-AP) to reduce IA and recorded the effects of this treatment on cell activity. 2. In the intact circuit with an actively cycling pyloric rhythm, 4-AP had three major effects on the rhythm. First, the cycle period was decreased approximately 20%. Second, 4-AP enhanced the activity of all cells, causing increases in spikes/burst and spike frequency within bursts. Third, 4-AP altered the phasing of follower cells relative to the onset of the pacemaker (AB/PD) bursts. The lateral pyloric (LP) and pylorics (PYs) were phase advanced by 4-AP, whereas the ventral dilator (VD) was phase delayed. 3. Voltage-clamp studies indicated that pyloric cells differed in the amount of IA they expressed on or near the soma. IA was largest in pyloric dilator (PD) and PY cells, smaller in the anterior burster (AB), LP, and inferior cardiac (IC) cells, and undetectable in the VD cell. When cells were isolated from synaptic input, however, all were excited by 4-AP, suggesting that all possess functionally significant IA. In VD cells, IA-like currents probably occur primarily in nonsomatic cell regions. 4. We measured postinhibitory rebound by determining the delay to the first spike after a series of 200-ms hyperpolarizing prepulses in the PD, PY, LP, VD, and IC cells. In all five cell types, the delay was progressively increased as the potential of the hyperpolarizing prepulse became more negative. This increased delay reflected the removal of IA inactivation. The delay was greatest in the PY cell and least in the IC. In four cells (the PD, PY, LP, and VD) 4-AP decreased the delay to the first spike at all prepulse potentials. In the IC the delay to the first spike was unaffected by 4-AP, suggesting that IA was not responsible for the relatively short delay after hyperpolarizing prepulses. 5. In all five cell types, 4-AP increased the spike frequency for the duration of a 1-s depolarization. The 4-AP-sensitive current responsible for this behavior appears to have very rapid kinetics and may represent a distinct channel subtype. Functionally, this current may act to dampen cell excitability and to reduce spike frequency during bursts.

Dopamine is widely distributed in the crustacean nervous system and has a diverse array of physiological effects. Immunocytochemical studies of several species have shown that dopamine- and/or tyrosine hydroxylase-containing cells occur in all ganglia of the central nervous system and that processes from some of these cells link ganglia of the ventral nerve cord. This study describes the distribution of tyrosine hydroxylase-containing cells in the central nervous system of a crayfish (Orconectes rusticus) and compares this information to available data from other species. The distribution of tyrosine hydroxylase (an enzyme in the synthetic pathway between tyrosine and dopamine) in O. rusticus is similar to that reported for marine species. However, differences were observed in the number of neurons in some ganglia and in the axonal projections of the L cell, which were more extensive in O. rusticus than in other species studied thus far. We also review the physiological effects of dopamine in crayfish and other crustaceans, focusing on the amine's actions in the endocrine, cardiovascular, and nervous systems, and on behavior when injected into freely-moving animals.

Invertebrate receptors for the neurotransmitter serotonin (5-HT) have been identified in numerous species from diverse phyla, including Arthropoda, Mollusca, Nematoda and Platyhelminthes. For many receptors, cloning and characterization in heterologous systems have contributed data on molecular structure and function across both closely and distantly related species. This article provides an overview of heterologously expressed receptors, and considers evolutionary relationships among them, classification based on these relationships and nomenclature that reflects classification. In addition, transduction pathways and pharmacological profiles are compared across receptor subtypes and species. Previous work has shown that transduction mechanisms are well conserved within receptor subtypes, but responses to drugs are complex. A few ligands display specificity for different receptors within a single species; however, none acts with high specificity in receptors across different species. Two non-selective vertebrate ligands, the agonist 5-methoxytryptamine and antagonist methiothepin, are active in most receptor subtypes in multiple species and hence bind very generally to invertebrate 5-HT receptors. Future challenges for the field include determining how pharmacological profiles are affected by differences in species and receptor subtype, and how function in heterologous receptors can be used to better understand 5-HT activity in intact organisms.

The outer antennular flagella of decapod crustaceans bear chemoreceptive hairs called aesthetascs. In the crayfish Orconectes propinquus these sensilla are located ventrally on the 11–13 most distal segments of the outer flagella. Two clumps of 3–6 aesthetascs occur on each segment, giving a total of approximately 80 aesthetascs per outer flagellum. Aesthetascs are100–150 μm long and about 12 μm in diameter. Each has a single annulation 30 μm from the hair base. The sensilla arise from immovable sockets and are directed distally at a 45° angle to the main body of the antennule. Aesthetascs lack an apical pore. However, the distal portion of each sensillum has thin cuticular walls which are readily penetrated by dye; this is probably the site where chemical stimuli enter. In O. propinquus each aesthetasc is innervated by 40–110 sensory neurons. Each neuron gives rise to a dendrite that branches into two cilia (9 × 2 + 2 structure; 0.15–0.20 μm in diameter). No further branching of outer dendritic segments occurs and thus each aesthetasc contains 80–220 sensory endings. Within the antennule lumen the dendrites are surrounded by two sheath cell layers, an inner layer and an outer layer. The inner sheath cells ascend 50 μm into the aesthetasc lumen; the outer sheath cells terminate at the sensilla bases. The outer dendritic segments gradually taper in diameter and terminate 25 μm from the sensilla tips.