Neurohormones and lobsters: biochemistry to behavior

Temperature and acid-base balance in the American lobster Homarus americanus

Lobsters (Homarus americanus) in the wild inhabit ocean waters where temperature can vary over a broad range (0-25 degrees C). To examine how environmental thermal variability might affect lobster physiology, we examine the effects of temperature and thermal change on the acid-base status of the lobster hemolymph. Total CO(2), pH, P(CO)2 and HCO(-)(3) were measured in hemolymph sampled from lobsters acclimated to temperature in the laboratory as well as from lobsters acclimated to seasonal temperatures in the wild. Our results demonstrate that the change in hemolymph pH as a function of temperature follows the rule of constant relative alkalinity in lobsters acclimated to temperature over a period of weeks. However, thermal change can alter lobster acid-base status over a time course of minutes. Acute increases in temperature trigger a respiratory compensated metabolic acidosis of the hemolymph. Both the strength and frequency of the lobster heartbeat in vitro are modulated by changes in pH within the physiological range measured in vivo. These observations suggest that changes in acid-base status triggered by thermal variations in the environment might modulate lobster cardiac performance in vivo.

Serotonin activates a Ca(2+)-dependent K(+) current in identified peptidergic neurons from the crayfish

The effects of 5-hydroxytryptamine (5-HT) were investigated in red pigment concentrating hormone (RPCH)-containing neurons isolated from the X-organ of the crayfish (Procambarus clarkii). Under current-clamp conditions and using the gramicidin-perforated-patch configuration, 5-HT elicited a prolonged hyperpolarization that suppressed neuronal firing concomitant with an increase in membrane conductance. Under voltage-clamp conditions, 5-HT evoked an outward current at a holding potential of −50 mV. This current reversed at an E(K) of −90 mV, which shifted by 30 mV when the extracellular K(+) concentration was increased from 5.4 to 19 mmol l(−1). The effect of 5-HT was dose-dependent within the range 1–100 micromol l(−1) and followed simple Michaelis-Menten kinetics, with a half-maximal response being elicited at 10 micromol l(−1). Preincubation with charybdotoxin (100 nmol l(−1)), tetraethylammonium (500 micromol l(−1)) or methysergide (100 micromol l(−1)) was effective in blocking the response to 5-HT. These results suggest that 5-HT is an inhibitory mediator of the release of red pigment concentrating hormone by acting on a Ca(2+)-dependent K(+) current.

Peripheral proprioceptive modulation in crayfish walking leg by serotonin

In vitro serotoninergic modulation of intracellularly recorded sensory responses was examined in primary afferent terminals of a crayfish leg proprioceptor, the coxo-basal chordotonal organ (CB CO). The effects of different concentrations of serotonin (5-HT) on static and dynamic sensory responses were analysed following bath or pressure applications of the monoamine directly on the strand of the mechanoreceptor. Consequently, the reported effects result from the direct peripheral action of 5-HT on the sensory organ itself. Serotonin modulates the sensory activity by modifying the sensory discharge frequency. The firing discharge of the primary afferents is increased in a dose-dependent manner. The maximal effect is obtained with a concentration of 10(-6) M. Higher concentrations are less effective, and for 20% of the recorded cells, 10(-4) M 5-HT induces a decrease of the sensory discharge, i.e. has an inhibitory effect. Alteration in the pattern of sensory firing, resulting in bursting discharge, was observed in some units. All the recorded sensory units were responsive to the neuromodulator whatever their functional properties. The effects of 5-HT lasted as long as the amine was applied and were reversible after wash. The results suggest that 5-HT could exert a modulatory action on the proprioceptive feedback, by peripheral action on the sensory organ. The natural modalities of 5-HT action are discussed on the basis of immunohistochemistry data suggesting: (i) connections between CB CO and central serotoninergic cells, (ii) 5-HT content in sensory cells of the CB CO. Since the CB CO is involved in the control of leg movement and position, the modulation of its primary afferents might influence the organization of the locomotor pattern. The functional significance of this peripheral sensory neuromodulation was approached by the analysis of the motor reflex activity.

Neurotensin terminals form synapses primarily with neurons lacking detectable tyrosine hydroxylase immunoreactivity in the rat substantia nigra and ventral tegmental area

A light and electron microscopic double antigen localization technique was employed to examine the fine structural relationship between neurotensin-containing axon terminals and dopaminergic neurons in the substantia nigra and ventral tegmental area of the rat. At the light microscopic level, neurotensin-immunoreactive terminals were densely distributed throughout the substantia nigra pars compacta and ventral tegmental area in close proximity to tyrosine hydroxylase-immunoreactive somata and dendrites. On electron microscopic examination, direct synaptic connections were identified between neurotensin-immunoreactive axon terminals and tyrosine hydroxylase-immunopositive perikarya and dendrites. However, only 8.2% and 8.8% of the neurotensin-immunoreactive axonal profiles detected in the substantia nigra and ventral tegmental area, respectively, were found in direct apposition with tyrosine hydroxylase-immunostained elements. In turn, only 9.3% and 10.0% of tyrosine hydroxylase immunoreactive dendrites sampled from the substantia nigra and ventral tegmental area, respectively, were seen in contact with neurotensin immunopositive axon terminals. However, neurotensin-immunoreactive and tyrosine hydroxylase-immunolabelled elements were frequently identified in close anatomical proximity (less than 5 microns) to one another. These results are interpreted in light of the selective association of neurotensin receptors with dopaminergic neurons in the substantia nigra and ventral tegmental area to suggest a predominantly parasynaptic mechanism of action for neurotensin in the ventral midbrain.

Liquid chromatographic procedures for the analysis of compounds in the serotonergic and octopamine pathways of lobster hemolymph

High-performance liquid chromatography, with serial electrochemical and ultraviolet detectors, was used with a reduced activity catecholamine C18 column to separate and quantify compounds important in the serotonergic and octopamine pathways in lobster hemolymph. The chromatographic mobile phase was composed of potassium dihydrogenphosphate buffer, trichloroacetic acid, sodium dodecyl sulfate, the sodium salt of ethylenedinitrilotetraacetic acid and the organic solvents, acetonitrile and methanol. The compounds serotonin, 5-hydroxyindoleacetic acid, tryptophan, 5-hydroxytryptophan, tryptamine, melatonin, octopamine and tyrosine were well resolved within 13 min. Good electrode maintenance, the use of a silica gel precolumn and careful sample preparation were necessary to give a stable baseline, high resolution of these compounds and reproducibility of retention times and peak heights. The electrochemical detector extended the range of detection to the picogram level. Because of the instability of the solutes and of the chromatographic baseline, sample preparation procedures were investigated. Deproteinization with ammonium sulfate gave the best recovery of the compounds of interest and the most stable baseline with the electrochemical detector. Peaks in the hemolymph were characterized by addition of standards, dual detection (electrochemical and ultraviolet) and the enzyme peak shift technique. With this methodology, important endogenous neurohormones in the hemolymph of lobsters can be quantitatively determined with respect to the molt cycle.

Distribution of opiate receptor subtypes and enkephalin and dynorphin immunoreactivity in the hippocampus of squirrel, guinea pig, rat, and hamster

The distribution of enkephalin and dynorphin immunoreactivity in the hippocampus of four rodent species (gray squirrel, guinea pig, rat, and hamster) is compared with the pattern of opiate receptor subtypes (mu, delta, and kappa). The distribution of opioid peptides is fairly consistent in the anterior hippocampus of these four species. Intense immunoreactivity for dynorphin and enkephalin is found in the hilus of the dentate gyrus and in the mossy fiber system. Occasional immunoreactive processes are seen in the dentate molecular layer and scattered throughout the CA1 and CA3 fields. In the rat and hamster, an additional plexus of enkephalinergic fibers straddles both sides of the hippocampal fissure. Cells immunoreactive for both opioid peptides are located in and just superficial to the dentate granule cell layer. Opiate receptors are variably distributed in these rodent species. In the squirrel, guinea pig, and hamster, mu and kappa binding is dense in the stratum lucidum of CA3 and the molecular layer of the dentate gyrus. In the rat, dense mu and kappa binding is localized within and adjacent to the pyramidal and granule cell layers. Delta receptor patterns show additional species differences. In the rat, the delta distribution is similar to the mu and kappa patterns. In the other species, the delta binding pattern is generally the inverse of the mu/kappa pattern: most areas of the hippocampus are enriched in delta sites, whereas the stratum lucidum and the pyramidal cell layer are receptor-sparse. Thus, the stratum lucidum--site of dense terminations of mossy fibers containing opioid peptides--is characterized by selectively sparse delta receptors in four species and by selectively dense kappa receptors in three species. The three receptor subtypes, taken either individually or together and compared to the peptides, are more variably and more widely distributed throughout the hippocampus and fail to show a correspondence with opioid-peptide-containing terminals. The mismatches suggest that receptor locations and densities are organized without relation to the sites of relevant transmitter release.

Hormones and the barnacle muscle fiber as a preparation

This minireview discusses the use of single barnacle muscle fibers as a model system for studying hormonal actions. The response of barnacle muscle fibers to serotonin, proctolin, octopamine, aldosterone and insulin is described. Recent data relating to the actions of these hormones on other invertebrate and vertebrate preparations is touched upon. The use of the barnacle muscle fiber as a preparation to investigate hormone-stimulated protein phosphorylation is emphasized.

Stimulation by proctolin of the ouabain-insensitive sodium efflux in single barnacle muscle fibers

A study has been made of the action of the neuropeptide proctolin on the radiosodium efflux from single barnacle muscle fibers. Proctolin (10(-8) M) when applied externally causes stimulation of the Na efflux in unpoisoned and ouabain-poisoned fibers. The response is prompt in onset, reaches a peak in 15 min and decays slowly. The response of the ouabain-insensitive Na efflux to external proctolin is dose-dependent, the concentration threshold being less than 10(-10) M. The response to proctolin is dependent on external Ca2+ but not on Na+. (i) The response to proctolin is abolished by high external Mg2+, as well as by verapamil, Co2+, Cd2+ and WB-4101. (ii) The response is also abolished by preinjecting 0.5 M MgCl2 or 0.1 M EGTA. The calmodulin antagonists trifluoperazine and imipramine are without effect on the response to proctolin. (i) Adenylate cyclase agonists, e.g. forskolin, fail to augment the response to proctolin. (ii) Prior injection of the phosphodiesterase inhibitors 1-propyl-3-methyl-7-(5-hydroxyhexyl)-xanthine (PMX) or 1-isoamyl-3-isobutylxanthine (IAX) fails to augment the response to proctolin. (iii) Prior injection of protein kinase inhibitor is ineffective. The response to proctolin is significantly reduced in the presence of tyramine. Taken together, these results support the view that proctolin stimulates the ouabain-insensitive Na efflux by activating Ca2+ channels and that the cAMP-protein kinase system is not involved in this response.

Molecular regulators of brain function: a new view

A working hypothesis is proposed based on two mutually dependent concepts: neurons may be functionally regulated not only by presently known neurotransmitters but by many kinds of informational substances. Known informational substances are considered in categories corresponding to major regulators of the central nervous system, including transmitters, peptides, hormones, "factors" and various proteins. Many new informational substances are being discovered by the application of DNA technology. Alongside neuronal circuitry that forms the basis for conventional neuroanatomy and neurophysiology, and that operates through conventional synaptic junctions, is a system here called parasynaptic, i.e. in "parallel with" synapse-linked circuitry. In the parasynaptic system, informational substances reach specific target cell receptors by diffusion from release points through extracellular fluids. The parasynaptic system has the same degree of selectivity as synaptic circuitry but possesses, in addition, a domain of versatility and plasticity lacking in "hardwired" circuitry; the latter is, however, also influenced by highly potent informational substances in the ambient extracellular fluid.