Neuromuscular modulation in Limulus by both octopamine and proctolin

Both octopamine and proctolin potentiate nerve-evoked skeletal muscle contractions in the horseshoe crab, Limulus. The threshold concentration for octopamine was 10^?9 to 10^?8M, while for proctolin it was 3 × 10^?9M. Norepinephrine and dopamine produced effects similar to octopamine but at higher thresholds; tyramine and serotonin were ineffective. Octopamine caused significant increases in amplitudes of excitatory postsynaptic potentials (epsps) of muscle fibers, but had little effect on muscle fiber input resistance or membrane potential. Also, octopamine did not affect depolarization of muscle fibers and subsequent contraction due to the direct action of exogenously applied glutamate. These results suggest that octopamine potentiates nerve-evoked contractions primarily by facilitating release of neuromuscular transmitter. At concentrations above 10^?7M, however, octopamine sometimes caused muscle spikes in response to motoneuron stimulation, a finding that suggests that octopamine may also have some postsynaptic action. Proctolin potentiated the muscle contractions evoked by glutamate but had little effect on glutamate-evoked muscle fiber depolarization, muscle fiber input resistance, or membrane potential. Thus, proctolin appears to act directly on skeletal muscle to enhance contractility. The proctolin-induced potentiations of contraction were sometimes accompanied by modest increases in epsp amplitude, so that unlike lobster skeletal and Limulus cardiac neuromuscular preparations, proctolin may have a secondary direct synaptic effect. Both octopamine and proctolin have been found in Limulus cardiac ganglion. This potential access to the hemolymph and the relatively low threshold concentrations needed for physiological action suggest that octopamine and proctolin could function as hormonal modulators of neuromuscular function in Limulus.     

Influence of aminergic and peptidergic substances on heart beat frequency in the stick insect Carausius morosus (Insecta,Phasmatodea)

The dorsal heart of the Indian stick insect, Carausius morosus, is responsible for the anterograde flow of hemolymph to the aorta and into the body cavity. The contraction frequency of the insect heart is known to be influenced by several substances of neural source. Here, a semi‐exposed heart assay was employed to study the effect of an aminergic substance (octopamine) and three neuropeptides (C. morosus hypertrehalosemic hormone [Carmo‐HrTH], crustacean cardioactive peptide [CCAP], and proctolin) on heart contraction. The contraction frequency was measured as beats per minute in adults ligated between the head and the prothorax. All three investigated neuropeptides had a stimulatory effect on heart contraction that lasted approximately 6 min, after which the normal heart beat rate was restored. Proctolin and CCAP stimulated the rate of heart beat also in unligated stick insects, whereas Carmo‐HrTH was active only in ligated insects. The latter could suggest that when the stick insect is not ligated, a competing substance may be released from the head of C. morosus; the competing substance is, apparently, not physiologically active but it binds or blocks access to the receptor of Carmo‐HrTH‐II, thereby rendering the HrTH peptide “not active.” In ligated stick insects, 6.7 × 10^?8 M Carmo‐HrTH‐II significantly increased the heart beat rate; higher doses resulted in no further increase, suggesting the saturation of the HrTH receptor. Octopamine inhibited the rate at which the heart contracted in a dose‐dependent manner; inhibition was achieved with 10^?4 M of octopamine.     

Octopamine--a single modulator with double action on the heart of two insect species (Apis mellifera macedonica and Bactrocera oleae): Acceleration vs. inhibition

The effects of octopamine, the main cardioacceleratory transmitter in insects, were investigated, in the isolated hearts of the honeybee, Apis mellifera macedonica, and the olive fruit fly, Bactrocera oleae. Octopamine induced a biphasic effect on the frequency and force of cardiac contractions acting as an agonist, with a strong acceleratory effect, at concentrations higher than 10⁻¹² M for the honeybee and higher than 50 × 10⁻⁹ M for the olive fruit fly. The heart of the honeybee is far more sensitive than the heart of olive fruit fly. This unusual sensitivity is extended to the blockers of octopaminergic receptors, where phentolamine at 10⁻⁵ M stopped the spontaneous contractions of the honeybee heart completely and permanently, while the same blocker at the same concentration caused only 50% inhibition in the heart of the olive fruit fly. Phentolamine and mianserin at low concentrations of 10⁻⁷ M also blocked the heart octopaminergic receptors, but for a short period of time, of less than 15.0 min, while a partial recovery in heart contraction started in spite of the presence of the antagonist. The unusual response of the honeybee heart in the presence of phentolamine and/or mianserin suggests excitatory effects of octopamine via two different receptor subtypes. At lower concentrations, 10⁻¹⁴ M, the agonist octopamine was converted to an antagonist, inducing a hyperpolarization in the membrane potential of the honeybee cardiac pacemaker cells and inhibiting the firing rate of the heart. The inhibitory effects of octopamine on certain parameters of the rhythmic bursts of the heart of the honeybee, were similar to those of mianserin and phentolamine, typical blockers of octopaminergic receptors. The heart of the olive fruit fly was 10⁵ times less sensitive to octopamine, since a persistent inhibition of heart contractions occurred at 10⁻⁹ M. In conclusion, the acceleration of the insect heart is achieved by increasing the levels of octopamine, while there is a passive but also an active decrease in heart activity due to the minimization of octopamine.     

Octopamine modulates the sensitivity of silkmoth pheromone receptor neurons

Effects of octopamine and its antagonist epinastine on electrophysiological responses of receptor neurons of Antheraea polyphemus specialised to the pheromone components (E,Z)-6,11-hexadecadienyl acetate and (E,Z)-6,11-hexadecadienal were investigated. Injections of octopamine and epinastine into the moths had no effect on the transepithelial potential of the antennal-branch preparation nor on the spontaneous nerve impulse frequency in either type of receptor neuron. However, in the presence of continuous low-intensity pheromone stimulation, octopamine significantly increased the nerve impulse frequency in the acetate receptor neuron, but not in the aldehyde receptor neuron. Octopamine and epinastine had no significant effect on the receptor potential amplitudes elicited in both receptor neuron types by pheromone stimulation. However, the peak nerve impulse frequency in the response of both receptor neuron types to pheromone was significantly affected: decreased by epinastine and increased by octopamine over a broad range of pheromone concentrations. In control experiments, injection of physiological saline did not significantly alter the peak nerve impulse frequency. The effect of octopamine was established within 1 h after injection and persisted for about 4 h. The possibility of a direct action of octopamine on the nerve impulse generation by the receptor neurons is discussed. Accepted: 8 January 2000     

Octopaminergic modulation of locust motor neurones

The effect of octopamine on the fast extensor and the flexor tibiae motor neurones in the locust (Schistocerca gregaria) metathoracic ganglion, and also on synaptic transmission from the fast extensor to the flexor motor neurones, was examined. Bath application or ionophoresis of octopamine depolarized and increased the excitability of the flexor tibiae motor neurones. 1 mM octopamine reduced the amplitude of the fast extensor-evoked EPSP in the slow but not the fast flexor motor neurones, whereas 10 mM octopamine could reduce the EPSP amplitude in both. Octopamine broadened the fast extensor action potential and reduced the amplitude of the afterhyperpolarization, the modulation requiring feedback resulting from movement of the tibia. Octopamine also increased the frequency of synaptic inputs onto the tibial motor neurones, and could cause rhythmic activity in the flexor motor neurones, and reciprocal activity in flexor and extensor motor neurones. Octopamine also increased the frequency of spontaneous spiking in the octopaminergic dorsal unpaired median neurones. Repetitive stimulation of unidentified dorsal unpaired median neurones could mimic some of the effects of octopamine. However, no synaptic connections were found between dorsal unpaired median neurones and the tibial motor neurones. The diverse effects of octopamine support its role in mediating arousal.     

Metabolic integration in locust flight: the effect of octopamine on fructose 2,6-bisphosphate content of flight muscle in vivo

The biogenic amine octopamine was injected into the haemolymph of 20-days old male locusts,Locusta migratoria, and the content of fructose 2,6-bisphosphate, a potent activator of glycolysis, was measured in the flight muscle after various time. Octopamine brought about a transient increase in fructose 2,6-bisphosphate. After the injection of 10 l of 10 mmol·l^-1 d, l-octopamine fructose 2,6-bisphosphate was increased by 61% within 2 min. Ten minutes after the injection fructose 2,6-bisphosphate was increased to 6.71±0.89 nmol·g^-1 flight muscle, almost 300% over the control value. Flight caused fructose 2,6-bisphosphate in flight muscle to decrease, but this decrease was counteracted by octopamine injected into the haemolymph of flying locusts. Octopamine and fructose 2,6-bisphosphate may act as signals to stimulate the oxidation of carbohydrate and to integrate muscle performance and metabolism. This mechanism appears particularly significant in the initial stage of flight when carbohydrates are the main fuel.Abbreviations F2,6P₂ fructose 2,6-bisphosphate - F6P fructose 6-phosphate - PFK1 6-phosphofructokinase (EC 2.7.1.11) - P _i inorganic phosphate - PP _i -PFK pyrophosphate dependent fructose 6-phosphate phosphotransferase (EC 2.7.1.90)     

Structure and function of spiny lobster ligamental nerve plexuses: Evidence for synthesis,storage, and secretion of biogenic amines

Three pairs of ligaments support not only the heart of spiny lobsters but also ligamental nerve plexuses, the complex terminal aborizations of segmental nerves. Segmental nerves 1–4 project from the thoracic ganglia into the pericardial cavity and ultimately ramify along the strands of the anterior, medial, and posterior ligaments. In each branch, a core of large axons sends fibers to terminate in a surrounding cortex of fine and varicose secretory processes. Electron micrographs reveal at least five distinct populations of granule-filled neuronal profiles, many with vesicles clustered at membrane thickenings adjacent to the epineural sheath. The ligamental nerve plexuses synthesize and accumulate octopamine, dopamine, 5-HT, and acetylcholine. Octopamine and 5-HT are predominant, comprising 33% and 65%, respectively, of the synthetic activity devoted to the four amines. Thus, the anatomy, ultrastructure, and neurochemistry of the ligamental nerve plexuses establishes their homology with the pericardial organs of other Crustacea. Octopamine and 5-HT are released by a Ca⁺⁺-dependent mechanism upon electrical stimulation of preterminal nerve trunks, and, in vivo, would be swept immediately through ostia into the heart. These observations, when considered with known effects of octopamine and 5-HT on crustacean cardiac activity, neuromuscular transmission, muscle tension, and cyclic AMP metabolism provide a strong case for hormonal actions at target sites throughout the animal. Segmental nerve processes in the dorsal nerve trunk ramify into a plexus around the dorsal nerve apparatus, a small muscular bulb that lies recessed in the cardiac surface. The dorsal nerve, carrying excitatory and inhibitory input to the cardiac ganglion directly through the bulb's hollow interior. The apparatus synthesizes and contains acetylcholine and the three amines mentioned above. In situ, it may beat rhythmically out of phase with the heart.     

Myocardial oxygen consumption and mechanical efficiency of a perfused dogfish heart preparation

Summary Oxygen consumption of an in-pericardium heart preparation from the spiny dogfish (Squalus acanthias) was linearly related to cardiac power output. Basal oxygen consumption, predicted from the regression, was 0.127 l · s^-1 · g ventricle mass^-1 and increased by 0.189 l · s^-1 · g ventricle mass^-1 per milliwatt of power generated. From the relationship between cardiac power output and mechanical efficiency, mechanical efficiency was predicted to increase with cardiac power output to a maximum of 21 %. Mechanical efficiency was measured during volume loading and pressure loading at two power outputs (50% and 72% of maximum power output). At 50% of maximum power output, mechanical efficiency increased significantly by 2.87%, from 11.9±0.3% to 14.8±0.5% (n=7), when flow was halved and output pressure doubled to achieve the same power output. Similarly, at 72% of maximum power output, mechanical efficiency increased from 14.74±0.92% to 17.61±0.84% (n=6) when flow was halved and output pressure doubled to generate the same higher level of power output. The increased mechanical efficiency at higher output pressures is believed to result from cardiac myocytes working within a length range where they are able to generate the most tension during contraction and are most efficient. We speculate that the loss of mechanical efficiency associated with large changes in sarcomere length, when stroke volume is large, is a driving force behind the use of frequency as the principal means of increasing cardiac output as observed in more active fishes, birds and mammals.Abbreviations BM body mass - CO cardiac output - HR heart rate - P _i mean cardiac input pressure - P _o mean cardiac output pressure - PO partial pressure of oxygen - SV stroke volume of heart - VM ventricle mass     

The effects of octopamine on juvenile hormone biosynthesis,electrophysiology, and cAMP content of the corpora allata of the cockroach Diploptera punctata

Summary Juvenile hormone production by the corpora allata of the adult female cockroach, Diploptera punctata, can be modulated by treatment with the biogenic amine, octopamine. Endogenous octopamine has been identified within the CA, using HPLC and electrochemical detection. Treatment with octopamine results in a sinusoidal, dose-dependent inhibition of JH biosynthesis by CA from day 2 virgin females, with maximal inhibition occurring at 10^-10 M and 10^-4 M. In day 4 and day 8 mated female corpora allata octopamine inhibited JH biosynthesis at 5·10^-5 M. Although the elevation of either cAMP or cGMP within the CA is known to be associated with an inhibition of JH biosynthesis, treatment with high concentrations of octopamine results in an increase in the level of cAMP but not cGMP. This effect is both dose- and time-dependent.Octopamine treatment also initiates changes in the passive membrane responses of the CA. Superfusion of CA with octopamine results in a pronounced hyperpolarization of CA cells and an increase in the electrotonic potential (indicative of the degree of electrical coupling between CA cells). This effect could be blocked by the octopamine receptor blocker phentolamine. Treatment with octopamine or phentolamine also blocked the hyperpolarization of CA cells normally associated with electrical stimulation of the axon tracts innervating the CA.We hypothesize that octopamine may be a natural neuromodulator of JH production by CA, regulating ion channels in CA cells themselves as well as release of the inhibitory neuropeptide, allatostatin, from the terminals within the CA.Abbreviations 4-AP 4-aminopyridine - CA corpora allata - CC corpora cardiaca - cAMP cyclic adenosine monophosphate - cGMP cyclic guanosine monophosphate - EDTA ethylenediamine tetraacetic acid - HEPES N-2-hydroxyethylpiperazine-N2-ethanesulfonic acid - HPLC high pressure liquid chromatography - IBMX 3-isobutyl-1-methylxanthine - JH juvenile hormone - ms millisecond - nA nanoampere - NCA I nervi corporis allati I - OCT octopamine - TEA tetraethyl ammonium     

Neuromodulation of flight initiation by octopamine in the cockroach Periplaneta americana

We have tested the effect of a known insect neuromodulator, octopamine, on flight initiation in the cockroach. Using minimally dissected animals, we found that octopamine lowered the threshold for windevoked initiation of flight when applied to either of two major synaptic sites in the flight circuitry: 1) the last abdominal ganglion, where wind-sensitive neurons from the cerci excite dorsal giant interneurons, or 2) the metathoracic ganglion, where the dorsal giant interneurons activate interneurons and motoneurons which are involved in producing the rhythmic flight motor pattern in the flight muscles (Fig. 2).Correlated with this change in flight initiation threshold, we found that octopamine applied to the last abdominal ganglion increased the number of action potentials produced by individual dorsal giant interneurons when recruiting the cereal wind-sensitive neurons with wind puffs (Figs. 3, 4, 5) or with extracellular stimulation of their axons (Fig. 6). Octopamine increases the excitability of the giant interneurons (Figs. 7, 8). Also, when we stimulated individual dorsal giant interneurons intracellularly, the number of action potentials needed to initiate flight was reduced when octopamine was applied to the metathoracic ganglion (Fig. 9).Abbreviations EMG electromyogram - dGIs dorsal giant interneurons - GI giant interneuron - A6 sixth abdominal ganglion - T3 third thoracic ganglion - EPSP excitatory postsynaptic potential     

Effects of octopamine on fluid secretion by isolated salivary glands of a feeding ixodid tick

Octopamine elicited a dose-related secretory response by salivary glands isolated from the feeding female tick Amblyomma americanum. Half-maximal stimulation occurred at about 60 μM. Phentolamine (10 μM) failed to inhibit the octopamine-mediated response; however, thioridazine (50 μM) inhibited both octopamine (1,000 μM) and dopamine-stimulated (0.1 μM) secretion. Maximal stimulation by dopamine (1.0 μM) showed no further increase in the rate of secretion after adding octopamine (1,000 or 0.1 μM). Glands responded to octopamine (100 μM) with rates significantly lower than controls following exposure to amphetamine (1,000 μM). Octopamine receptors do not appear to mediate the secretory response, and octopamine may stimulate secretion by releasing catecholamines from presynaptic neurons. These results support the hypothesis that dopamine is the natural transmitter mediating fluid secretion in the feeding tick salivary gland.     

Possible Regulation of Feeding Behavior in Cockroach Nymphs by the Neurotransmitter Octopamine

Insects, including cockroaches, have the ability to select a proper diet from different nutrient choices. We have showed previously that various neurotransmitters and neuromodulators appear to regulate certain aspects of feeding in the cockroach, Rhyparobia madera. In the current study, we examined the role of octopamine in feeding behavior of cockroach nymphs. By either injection or direct incorporation into the diet blocks, an octopamine agonist (octopamine or synephrine) or antagonist (phentolamine) was effective in altering feeding in R. madera nymphs. Both octopamine and synephrine increased feeding slightly, while phentolamine decreased feeding dramatically. Phentolamine was able to decrease feeding, but not motor activity, when injected directly into the nymphs. Octopamine appears to cause increased feeding in the cockroach.     

Cardiac function of the leopard shark,Triakis semifasciata

Summary The pressure difference between the cardinal sinus and the pericardium, and the transmural ventricular diastolic pressure at rest and during swimming in the leopard shark, Triakis semifasciata, was measured to characterize the mechanism of cardiac filling in chronically-instrumented fish and to evaluate cardiac responses to swimming. Echo-Doppler and radiographic imaging were also used to fully describe the cardiac cycle. Swimming induces an increase in preload as manifested by a large increment of cardinal sinus pressure (0.26/0.20 [systolic/diastolic] to 0.49/0.32 kPa) which always exceeds pericardial pressure. Increases in both mean ventricular diastolic transmural pressure (0.30–0.77 kPa) and cardinal sinus pressure during swimming suggest increased cardiac filling by vis a tergo as the mechanism for augmenting cardiac output. In contrast to mammals, the fluid-filled pericardial space of elasmobranchs is considerably larger and the pericardium itself does not move in concert with the heart throughout the cardiac cycle. Also, modest increases in heart rate drastically curtail the duration of diastole, which becomes much less than that of systole, a phenomenon not found in mammals. In the absence of tachycardia (<40 bpm), ventricular filling is characterized by a period of early rapid filling, and a late period of filling owing to atrial systole, separated by a period of diastasis. Ventricular filling in elasmobranchs is thus biphasic and is not solely dependent on atrial systole. Atrial diastole is characterized by three filling periods associated with atrial relaxation, ventricular ejection, and sinus venosus contraction. The estimated ventricular ejection fraction of Triakis (80%) exceeds that of the mammalian left ventricle.     

Characterization of cardiac innervation in the nudibranch,Archidoris montereyensis

Summary The heart of the nudibranch mollusc Archidoris montereyensis is regulated by a small number of powerful effector neurons located in the right pleural and visceral ganglia. Two identifiable neurons in the pleural ganglion, a heart excitor (pl_HE) and a heart inhibitor (Pl_HI), are especially important regulators of cardiac function in that low levels of spontaneous activity in either cell significantly alters the amplitude and rate of heart contractions. These neurons have extensive dendritic arbors within the right pleural ganglion and branching axonal processes within the visceral ganglion. The visceral ganglion also contains a heart excitor neuron (V_HE) and at least two heart inhibitor neurons (V_HI cells), but their influence on cardiac activity is weaker than that of the pleural ganglion cells. All of these heart effector cells appear to be motor neurons with axons that terminate predominately in the atrio-ventricular valve region of the heart via the pericardial nerve. The simplicity and strength of these neuronal connections to the heart of Archidoris make this a favorable preparation for studies of cardiac regulation.Abbreviations Pl _HE pleural ganglion heart excitor neuron - Pl _HI pleural heart inhibitor neuron - V _HE visceral ganglion heart excitor neuron - V _HI cells, visceral heart inhibitor neurons - V _K visceral kidney excitor neuron - V _G visceral gill excitor neuron     

Interaction between octopamine and proctolin on the oviducts of Locusta migratoria

The biogenic amine octopamine and the pentapeptide proctolin are two important neuroactive chemicals that control contraction of the oviducts of the African locust Locusta migratoria. The physiological responses and signal transduction pathways used by octopamine and proctolin have been well characterized in the locust oviducts and this therefore provides the opportunity to examine the interaction between these two pathways. Octopamine, via the intracellular messenger adenosine 3',5'-cyclic monophosphate (cyclic AMP), inhibits contraction of the oviducts, while proctolin, via the phosphoinositol pathway, stimulates contraction. We have examined the physiological response of the oviducts to combinations of octopamine and proctolin and also looked at how combinations of these affect one of the main intracellular mediators of the octopamine response, namely cyclic AMP. It was found that application of octopamine to the oviducts led to a dose-dependent reduction in tonus of the muscle and also a decrease in the amplitude and frequency of spontaneous phasic contractions. Octopamine-induced relaxation was enhanced in the presence of the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX). Octopamine was also able to inhibit proctolin-induced contractions of the oviducts in a dose-dependent manner. A 10(-9) M proctolin-induced contraction was inhibited by 83% in the presence of 10(-5) M octopamine, and was completely inhibited in the presence of 10(-5) M octopamine plus 5x10(-4) M IBMX. Octopamine led to a dose-dependent increase in cyclic AMP content as measured by radioimmunoassay. In the presence of 10(-9) M proctolin, this octopamine-induced increase in cyclic AMP was reduced by as much as 60%. Proctolin also caused a dose-dependent decrease in the cyclic AMP elevation produced by 5x10(-6) M octopamine. These results indicate that octopamine and proctolin can antagonize each other's physiological response when added in combination, and that proctolin is able to modulate the response of the oviducts to octopamine by influencing cyclic AMP levels.     

Insulin‐like receptor substrate gene chico regulates octopamine metabolism in Drosophila melanogaster

Octopamine, one of the main insect biogenic amines, plays an important role in the control of fitness in Drosophila melanogaster Meigen. The present study examines the effects of a null mutation of the gene of the insulin‐like receptor substrate (chico), in the heterozygous state, on octopamine metabolism, heat stress resistance and fecundity of D. melanogaster. A rise in the activity of one of the key enzymes of octopamine synthesis, tyrosine decarboxylase, as well as that of an enzyme of its degradation, octopamine‐dependent N‐acetyl transferase, is observed in chico^1/+ females. It is also found that the resistance to heat stress is decreased and fecundity is reduced dramatically in chico^1/+ flies. Such changes in these parameters in D. melanogaster females result from a rise in octopamine titre, which suggests that chico affects the octopamine level by regulating the activity of tyrosine decarboxylase.     

Performance of the isolated and perfused working heart of the teleostConger conger: study of the inotropic effect of prostacyclin

Summary An in vitro preparation of the heart of the teleostConger conger, isolated without the pericardium, was set up. The procedure allowed subambient pressures to develop in the perfusion chamber during contraction, mimicking the in vivo situation with the pericardium intact. The ventricle produced a cardiac output of about 15 ml·min^-1·kg wet body weight^-1 at subambient input pressure, and was able to double the stroke work with an increase of preload up to about 0.2 kPa. Using this preparation it was found that prostacyclin has a positive inotropic effect on the atrium and ventricle, but it does not affect the heart rate. Semilogarithmic doseresponse curves of prostacyclin on the atrium are reported, showing a threshold concentration of about 10^-9 M. The isolated and perfusedConger conger heart provides a useful model for a detailed analysis of the action of prostacyclin on myocardial contractility.     

Octopamine receptor subtypes and their modes of action

Octopamine receptor subclasses were first proposed to explain differences in the pharmacological profiles of a range of physiological responses to octopamine obtained in the extensor-tibiae neuromuscular preparation of the locust. Thus, OCTOPAMINE₁ receptors which inhibit an endogenous myogenic rhythm, increase intracellular calcium levels. Also OCTOPAMINE₂ receptors which modulate neuromuscular transmission in this preparation, increase the level of adenylate cyclase activity. The current status of this classification is reviewed by examining the pharmacology of responses to octopamine in a range of preparations. It is concluded that the distinction between OCTOPAMINE₁ and OCTOPAMINE₂ receptor types is still valid, but that OCTOPAMINE₂ receptors exhibit some tissue specific variations. Studies on a clonedDrosophila octopamine/tyramine (phenolamine) receptor are discussed and illustrate many of the difficulties presently encountered in making a definitive classification of octopamine receptors. These include the possibilities that single receptors may activate multiple second messenger systems and that different agonists may differentially couple the same receptor to different second messenger systems.     

Characterization of insect neuronal octopamine receptors (OA3 receptors)

Octopamine receptors in the nervous tissue of insects were investigated using a ligand-receptor assay with [³H]NC-5Z or [³H]octopamine as the radioligands. Both ligands recognized a homogenous class of binding sites with the properties of an octopamine receptor. This receptor has been characterized pharmacologically. Both high-affinity agonists (e.g. NC 7, K₁=0.3 nM) and antagonists (e.g. maroxepine, K₁=1.02 nM) were investigated. The neuronal octopamine receptor belongs to a receptor class that can easily be distinguished from peripheral octopamine receptors. Initial investigations of the localization of octopamine receptors within the insect nervous tissue show the greatest receptor density in the optic lobes.     

Distribution of dopamine and octopamine in the central nervous system and ovary during the ovarian maturation cycle of the giant freshwater prawn, Macrobrachium rosenbergii

Dopamine (DA), octopamine (OA) and serotonin (5-HT) are the key neurotransmitters that control gonadal development in decapod crustaceans. 5-HT stimulates, while DA and OA delay gonadal development in Macrobrachium rosenbergii. In the present study, we have further investigated the distribution patterns of DA and OA in the central nervous system (CNS) and ovary during various stages of the ovarian maturation cycle of this giant freshwater prawn. DA- and OA-immunoreactive neurons and fibers were distributed extensively in several regions of the brain, subesophageal ganglion (SEG), thoracic ganglia and abdominal ganglia. In the brain, the two neurotransmitters were present in neurons of clusters 6, 7, 11, 17, and nearby neuropil regions. In the SEG, thoracic ganglia and abdominal ganglia, immunoreactive neurons and fibers were found along the midline and in several neuronal clusters around each neuropil region. Staining for DA and OA was more intense in the thoracic ganglia than in other parts of the CNS. In the ovary, DA- and OA-immunoreactivities were present at high intensity in early oocytes. The presence of DA- and OA-immunoreactivities in neural ganglia as well as ovary suggests that DA and OA may also be involved in the reproductive process, particularly ovarian development and differentiation of oocytes in this species.