Pre-adult aggression and its long-term behavioural consequences in crickets.

Social experience, particularly aggression, is considered a major determinant of consistent inter-individual behavioural differences between animals of the same species and sex. We investigated the influence of pre-adult aggressive experience on future behaviour in male, last instar nymphs of the cricket Gryllus bimaculatus. We found that aggressive interactions between male nymphs are far less fierce than for adults in terms of duration and escalation. This appears to reflect immaturity of the sensory apparatus for releasing aggression, rather than the motor system controlling it. First, a comparison of the behavioural responses of nymphs and adults to mechanical antennal stimulation using freshly excised, untreated and hexane-washed antennae taken from nymphs and adults, indicate that nymphs neither respond to nor produce sex-specific cuticular semiochemicals important for releasing aggressive behaviour in adults. Second, treatment with the octopamine agonist chlordimeform could at least partially compensate for this deficit. In further contrast to adults, which become hyper-aggressive after victory, but submissive after defeat, such winner and loser effects are not apparent in nymphs. Aggressive competition between nymphs thus appears to have no consequence for future behaviour in crickets. Male nymphs are often attacked by adult males, but not by adult females. Furthermore, observations of nymphs raised in the presence, or absence of adult males, revealed that social subjugation by adult males leads to reduced aggressiveness and depressed exploratory behaviour when the nymphs become adult. We conclude that social subjugation by adults during pre-adult development of nymphs is a major determinant of consistent inter-individual behavioural differences in adult crickets.

Synergistic action of octopamine receptor agonists on the activity of selected novel insecticides for control of dengue vector Aedes aegypti (Diptera: Culicidae) mosquito.

Studying insecticide resistance in mosquitoes has attracted the attention of many scientists to elucidate the pathways of resistance development and to design novel strategies in order to prevent or minimize the spread and evolution of resistance. Here, we tested the synergistic action of piperonyl butoxide (PBO) and two octopamine receptor (OR) agonists, amitraz (AMZ) and chlordimeform (CDM) on selected novel insecticides to increase their lethal action on the fourth instar larvae of Aedes aegypti L. However, chlorfenapyr was the most toxic insecticide (LC50 = 193, 102, and 48 ng/ml, after 24, 48, and 72 h exposure, respectively) tested. Further, PBO synergized all insecticides and the most toxic combinatorial insecticide was nitenpyram even after 48 and 72 h exposure. In addition, OR agonists significantly synergized most of the selected insecticides especially after 48 and 72 h exposure. The results imply that the synergistic effects of amitraz are a promising approach in increasing the potency of certain insecticides in controlling the dengue vector Ae. aegypti mosquito.

Unique biochemical and molecular biological mechanism of synergistic actions of formamidine compounds on selected pyrethroid and neonicotinoid insecticides on the fourth instar larvae of Aedes aegypti (Diptera: Culicidae).

We recently reported that formamidine pesticides such as amitraz and chlordimeform effectively synergize toxic actions of certain pyrethroid and neonicotinoid insecticides in some insect species on the 4th instar larvae of Aedes aegypti. Here we studied the biochemical basis of the synergistic actions of the formamidines in amplifying the toxicity of neonicotinoids and pyrethroids such as dinotefuran and thiamethoxam, as well as deltamethrin-fenvalerate type of pyrethroids. We tested the hypothesis that their synergistic actions are mediated by the octopamine receptor, and that the major consequence of octopamine receptor activation is induction of trehalase to increase glucose levels in the hemolymph. The results show that formamidines cause a significant up-regulation of the octopamine receptor and trehalase mRNA expressions. Furthermore, formamidines significantly elevate levels of free glucose when co-treated with dinotefuran, deltamethrin and fenvalerate, but not with permethrin or fenitrothion, which showed no synergistic toxic effects with formamidines. These results support the conclusion that the main mode of synergism is based on the ability to activate the octopamine receptor, which is particularly effective with insecticides causing hyperexcitation-induced glucose release and consequently leading to quick energy exhaustion.

Octopaminergic modulation of contrast gain adaptation in fly visual motion-sensitive neurons.

Locomotor activity like walking or flying has recently been shown to alter visual processing in several species. In insects, the neuromodulator octopamine is thought to play an important role in mediating state changes during locomotion of the animal [K.D. Longden & H.G. Krapp (2009) J. Neurophysiol., 102, 3606-3618; (2010) Front. Syst. Neurosci., 4, 153; S.N. Jung et al. (2011)J. Neurosci., 31, 9231-9237]. Here, we used the octopamine agonist chlordimeform (CDM) to mimic effects of behavioural state changes on visual motion processing. We recorded from identified motion-sensitive visual interneurons in the lobula plate of the blowfly Calliphora vicina. In these neurons, which are thought to be involved in visual guidance of locomotion, motion adaptation leads to a prominent attenuation of contrast sensitivity. Following CDM application, the neurons maintained high contrast sensitivity in the adapted state. This modulation of contrast gain adaptation was independent of the activity of the recorded neurons, because it was also present after stimulation with visual motion that did not result in deviations from the neurons' resting activity. We conclude that CDM affects presynaptic inputs of the recorded neurons. Accordingly, the effect of CDM was weak when adapting and test stimuli were presented in different parts of the receptive field, stimulating separate populations of local presynaptic neurons. In the peripheral visual system adaptation depends on the temporal frequency of the stimulus pattern and is therefore related to pattern velocity. Contrast gain adaptation could therefore be the basis for a shift in the velocity tuning that was previously suggested to contribute to state-dependent processing of visual motion information in the lobula plate interneurons.

The effect of octopaminergic compounds on the behaviour and transmission of Gyrodactylus.

BACKGROUND: The high transmission potential of species belonging to the monogenean parasite genus Gyrodactylus, coupled with their high fecundity, allows them to rapidly colonise new hosts and to increase in number. One gyrodactylid, Gyrodactylus salaris Malmberg, 1957, has been responsible for devastation of Altantic salmon (Salmo salar L.) populations in a number of Norwegian rivers. Current methods of eradicating G. salaris from river systems centre around the use of non-specific biocides, such as rotenone and aluminium sulphate.Although transmission routes in gyrodactylids have been studied extensively, the behaviour of individual parasites has received little attention. Specimens of Gyrodactylus gasterostei Gläser, 1974 and G. arcuatus Bychowsky, 1933, were collected from the skin of their host, the three-spined stickleback (Gasterosteus aculeatus L.), and permitted to attach to the substrate. The movements of individual parasites were recorded and analysed. RESULTS: The behaviour patterns of the two species were similar and parasites were more active in red light and darkness than in white light. Four octopaminergic compounds were tested and all four inhibited the movements of parasites. Treatment ultimately led to death at low concentrations (0.2 µM), although prolonged exposure was necessary in some instances. CONCLUSIONS: Octopaminergic compounds may affect the parasite's ability to locate and remain on its host and these or related compounds might provide alternative or supplementary treatments for the control of G. salaris infections. With more research there is potential for use of octopaminergic compounds, which have minimal effects on the host or its environment, as parasite-specific treatments against G. salaris infections.

Postembryonic development of centrally generated flight motor patterns in the hawkmoth, Manduca sexta.

This study analyses the maturation of centrally generated flight motor patterns during metamorphosis of Manduca sexta. Bath application of the octopamine agonist chlordimeform to the isolated central nervous system of adult moths reliably induces fictive flight patterns in wing depressor and elevator motoneurons. Pattern maturation is investigated by chlordimeform application at different developmental stages. Chlordimeform also induces motor patterns in larval ganglia, which differ from fictive flight, indicating that in larvae and adults, octopamine affects different networks. First changes in motoneuron activity occur at the pupal stage P10. Rhythmic motor output is induced in depressor, but not in elevator motoneurons at P12. Adult-like fictive flight activity in motoneurons is observed at P16 and increases in speed and precision until emergence 2 days later. Pharmacological block of chloride channels with picrotoxin also induces fictive flight in adults, suggesting that the pattern-generating network can be activated by the removal of inhibition, and that proper network function does not rely on GABA(A) receptors. Our results suggest that the flight pattern-generating network becomes gradually established between P12 and P16, and is further refined until adulthood. These findings are discussed in the context of known physiological and structural CNS development during Manduca metamorphosis.

Differential effects of octopamine and tyramine on the central pattern generator for Manduca flight.

The biogenic amine, octopamine, modulates a variety of aspects of insect motor behavior, including direct action on the flight central pattern generator. A number of recent studies demonstrate that tyramine, the biological precursor of octopamine, also affects invertebrate locomotor behaviors, including insect flight. However, it is not clear whether the central pattern generating networks are directly affected by both amines, octopamine and tyramine. In this study, we tested whether tyramine affected the central pattern generator for flight in the moth, Manduca sexta. Fictive flight was induced in an isolated ventral nerve cord preparation by bath application of the octopamine agonist, chlordimeform, to test potential effects of tyramine on the flight central pattern generator by pharmacological manipulations. The results demonstrate that octopamine but not tyramine is sufficient to induce fictive flight in the isolated ventral nerve cord. During chlordimeform induced fictive flight, bath application of tyramine selectively increases synaptic drive to depressor motoneurons, increases the number of depressor spikes during each cycle and decreases the depressor phase. Conversely, blocking tyramine receptors selectively reduces depressor motoneuron activity, but does not affect cycle by cycle elevator motoneuron spiking. Therefore, octopamine and tyramine exert distinct effects on the flight central pattern generating network.

A subset of octopaminergic neurons are important for Drosophila aggression.

Aggression is an innate behavior that is important for animal survival and evolution. We examined the molecular and cellular mechanisms underlying aggression in Drosophila. Reduction of the neurotransmitter octopamine, the insect equivalent of norepinephrine, decreased aggression in both males and females. Mutants lacking octopamine did not initiate fighting and did not fight other flies, although they still provoked other flies to fight themselves. Mutant males lost to the wild-type males in fighting and in competing for copulation with females. Enhanced octopaminergic signaling increased aggression in socially grouped flies, but not in socially isolated flies. We carried out genetic rescue experiments that revealed the functional importance of neuronal octopamine and identified a small subset of octopaminergic neurons in the suboesophageal ganglion as being important for aggression.

Octopamine partially restores walking in hypokinetic cockroaches stung by the parasitoid wasp Ampulex compressa.

When stung by the parasitoid wasp Ampulex compressa, cockroaches Periplaneta americana enter a hypokinetic state that is characterized by little, if any, spontaneous locomotor activity. In the present study we investigate the effect of an octopamine receptor agonist and an antagonist on the locomotor behavior of stung and control cockroaches. We show that in cockroaches stung by a wasp the octopamine receptor agonist chlordimeform induces a significant increase in spontaneous walking. In good agreement, in control individuals an octopamine receptor antagonist significantly reduces walking activity. Adipokinetic hormone I (AKH-I) promotes spontaneous walking in controls but does not do so in stung individuals, which suggests that the venom effect is most probably not mediated by AKH-I. Dopamine receptor agonists or antagonists had no significant effect on the spontaneous walking of stung or control cockroaches, respectively. The effect of the octopamine receptor agonist was maximal when injected into the brain, suggesting that the wasp venom interferes with octopaminergic modulation of walking initiation in central structures of the cockroach brain.

Octopamine and experience-dependent modulation of aggression in crickets.

Intraspecific aggression is influenced in numerous animal groups by the previous behavioral experiences of the competitors. The underlying mechanisms are, however, mostly obscure. We present evidence that a form of experience-dependent plasticity of aggression in crickets is mediated by octopamine, the invertebrate counterpart of noradrenaline. In a forced-fight paradigm, the experience of flying maximized the aggressiveness of crickets at their first encounter and accelerated the subsequent recovery of aggressiveness of the normally submissive losers, without enhancing general excitability as evaluated from the animals' startle responses to wind stimulation. This effect is transitory and concurrent with the activation of the octopaminergic system that accompanies flight. Hemocoel injections of the octopamine agonist chlordimeform (CDM) had similar effects on aggression but also enhanced startle responses. Serotonin depletion, achieved using alpha-methyl-tryptophan, enhanced startle responses without influencing aggression, indicating that the effect of CDM on aggression is not attributable to increased general excitation. Contrasting this, aggressiveness was depressed, and the effect of flying was essentially abolished, in crickets depleted of octopamine and dopamine using alpha-methyl-p-tyrosine (AMT). CDM restored aggressiveness in AMT-treated crickets, indicating that their depressed aggressiveness is attributable to octopamine depletion rather than to dopamine depletion or nonspecific defects. Finally, the flight effect was blocked in crickets treated with the octopamine receptor antagonist epinastine, or with the alpha-adrenoceptor and octopamine receptor antagonist phentolamine, but not with the beta-adrenoceptor antagonist propranolol. The idea that activity-specific induction of the octopaminergic system underlies other forms of experience-dependent plasticity of aggressive motivation in insects is discussed.

Characterization and comparative evaluation of a novel PAI-1 inhibitor.

Plasminogen activator inhibitor-1 (PAI-1), the primary physiological inhibitor of both tissue-type plasminogen activator and urokinase-type plasminogen activator in plasma, is a well established risk factor in thrombotic diseases. Reduction of active PAI-1 levels may lead to a decreased tendency of thrombosis. Compounds that can suppress pharmacologically active PAI-1 levels are therefore considered as putative drugs. In the present study, we describe the PAI-1 neutralizing properties and mechanism of a newly selected compound (i.e. fendosal, HP129) in comparison to four previously reported compounds (i. e. AR-H029953XX, XR1853, XR5118 and the peptide TVASS) using different assays. The inhibitory effect of these compounds on active PAl-1 was analyzed by a plasmin-coupled chromogenic assay (Coaset t-PA), direct chromogenic assays (t-PA, u-PA) and quantification of complex formation by ELISA, SDS-PAGE and surface plasmon resonance. Comparative evaluation of the obtained IC50 values reveals large differences [i.e. IC50 of 15 microM (HP129) vs. >1000 microM (XR5118) determined at 37 degrees C using SDS-PAGE] between the compounds studied. Importantly, the relative potency of the various compounds is also dependent on the method used (10 to 170-fold differences in IC50 values). Characterization of the PAI-1 forms (i.e. active, non-reactive and substrate) generated upon inactivation reveals that the newly described compound HP129 induces a unique pathway (i.e. active to non-reactive conversion via a substrate-behaving intermediate) of inactivation compared to the other compounds. Taken together, these data strongly suggest that the various compounds act through different mechanisms. In addition, the results stress the necessity for a careful selection of the method used for the evaluation of PAI-1 inhibitors, preferably requiring a panel of screening methods.

Interaction of formamidine pesticides with the presynaptic alpha(2)-adrenoceptor regulating.

The effects of the formamidine pesticides amitraz and chlordimeform on the alpha(2)-adrenergic receptor subtype that mediates the release of [(3)H]noradrenaline by synaptosomes from rat hypothalami were studied. We initially characterized the presynaptic autoreceptor on noradrenergic nerve endings using selective antagonists. Yohimbine (a nonselective alpha(2) antagonist) and BRL 44408 (selective for subtypes alpha(2A)/alpha(2D)) diminished the inhibitory effect of xylazine on K(+)-evoked release of [(3)H]noradrenaline; the K(B) values were 481 and 154 nM, respectively. In contrast, prazosin (a selective alpha(2B)/alpha(2C) antagonist) did not modify the inhibitory effect of xylazine. These results indicate that the release of noradrenaline by noradrenergic nerve endings in the rat hypothalamus is regulated by alpha(2D)-adrenoceptors, a species variation of the human alpha(2A) subtype. We then assessed the effects of the two pesticides on the K(+)-evoked release of [(3)H]noradrenaline. Amitraz reduced release in a dose-dependent manner; the effect observed at the maximal concentration tested (10 microM) was 13.0 +/- 2.0% and it was reversed by yohimbine. Amitraz also diminished the inhibitory effects of the alpha(2)-adrenergic agonists clonidine and xylazine. Chlordimeform displayed no effects, possibly because the true active compound of this insecticide is its demethylated metabolite. Based on these findings we conclude that the formamidine pesticides act as partial agonists of presynaptic alpha(2D)-adrenergic receptors in the rat hypothalamus. This interaction may be responsible for the in vivo alterations in catecholaminergic regulation of cyclic variations in gonadotropin-releasing hormone (GnRH) secretion, which can have grave functional repercussions on the reproductive system of mammals exposed to these xenobiotics.

Octopamine is the synaptic transmitter between identified neurons in the buccal feeding network of the pond snail lymnaea stagnalis.

We report the pharmacological properties of synaptic connections from the three octopamine-containing OC interneurons to identified buccal feeding neurons in the pond snail, Lymnaea stagnalis. Intracellular stimulation of an OC interneuron evokes inhibitory postsynaptic potentials in the B3 motoneurons and N2 (d) interneurons, while the synapse between OC and N3 (phasic) interneurons has two components: an initial electrical excitation followed by chemical inhibition. All synaptic responses persist in a saline with elevated calcium and magnesium suggesting that the connections are monosynaptic. Local perfusion of 10(-4) M octopamine produces the same inhibitory membrane responses from these buccal neurons as OC stimulation. These responses also persist in high Mg(2+)/Ca(2+) saline indicating direct membrane effects. The similarities in reversal potentials for the synaptic hyperpolarization evoked on B3 neurons after OC stimulation (-89.0 mV, S.E.M.=14.1, n=10) and the octopamine response of the B3 neurons (-84.7 mV, S.E.M.=6.6, n=6) indicate that increased K(+)-conductance underlies both responses. Bath application of the octopaminergic drugs phentolamine (10(-6) M), epinastine (10(-6) M) or DCDM (10(-4) M) blocks the inhibitory synapse onto B3 or N2 neurons and the chemical component of the N3 response. They also block the octopamine-evoked inhibition of B3, N2 and N3 neurons. NC-7 (2x10(-5) M) has a hyperpolarizing agonist effect (like octopamine) on these neurons and also blocks their chemical synaptic input from the OC interneurons. These results provide pharmacological evidence that the neurotransmitter between the octopamine-immunopositive OC interneurons and its followers is octopamine. This is the first example of identified octopaminergic synaptic connections within the snail CNS.

The effects of phenelzine and other monoamine oxidase inhibitor antidepressants on brain and liver I2 imidazoline-preferring receptors.

1. The binding of [3H]-idazoxan in the presence of 10(-6) M (-)-adrenaline was used to quantitate I2 imidazoline-preferring receptors in the rat brain and liver after chronic treatment with various irreversible and reversible monoamine oxidase (MAO) inhibitors. 2. Chronic treatment (7-14 days) with the irreversible MAO inhibitors, phenelzine (1-20 mg kg-1, i.p.), isocarboxazid (10 mg kg-1, i.p.), clorgyline (3 mg kg-1, i.p.) and tranylcypromine (10 mg kg-1, i.p.) markedly decreased (21-71%) the density of I2 imidazoline-preferring receptors in the rat brain and liver. In contrast, chronic treatment (7 days) with the reversible MAO-A inhibitors, moclobemide (1 and 10 mg kg-1, i.p.) or chlordimeform (10 mg kg-1, i.p.) or with the reversible MAO-B inhibitor Ro 16-6491 (1 and 10 mg kg-1, i.p.) did not alter the density of I2 imidazoline-preferring receptors in the rat brain and liver; except for the higher dose of Ro 16-6491 which only decreased the density of these putative receptors in the liver (38%). 3. In vitro, phenelzine, clorgyline, 3-phenylpropargylamine, tranylcypromine and chlordimeform displaced the binding of [3H]-idazoxan to brain and liver I2 imidazoline-preferring receptors from two distinct binding sites. Phenelzine, 3-phenylpropargylamine and tranylcypromine displayed moderate affinity (KiH = 0.3-6 microM) for brain and liver I2 imidazoline-preferring receptors; whereas chlordimeform displayed high affinity (KiH = 6 nM) for these receptors in the two tissues studied, Clorgyline displayed very high affinity for rat brain (KiH = 40 pM) but not for rat liver I2 imidazoline-preferring receptors (KiH = 169 nM). 4. Preincubation of cortical or liver membranes with phenelzine (10-4 M for 30 min) did not alter the total density of I2 imidazoline-preferring receptors, indicating that this irreversible MAO inhibitor does not irreversibly bind to I2 imidazoline-preferring receptors. In contrast, preincubation with 10-6 Mclorgyline reduced by 40% the Bmax of [3H]-idazoxan to brain and liver I2 imidazoline-preferring receptors.5. Chronic treatment (7 days) with the inducers of cytochrome P-450 enzymes phenobarbitone (40 or 80 mg kg-1, i.p.), 3-methylcholanthrene (20 mg kg-1, i.p.) or 2-methylimidazole (40 mg kg-1, i.p.) did not alter the binding parameters of [3H]-idazoxan to brain and liver 12 imidazoline-preferring receptors.The compound SKF 525A, a potent inhibitor of cytochrome P-450 enzymes which forms a tight but reversible complex with the haemoprotein, completely displaced with moderate affinity (KiH = 2-10 microM)the specific binding of [3H]-idazoxan to brain and liver 12 imidazoline-preferring receptors. Preincubation of total liver homogenates with 3 x 10-4 M phenelzine in the presence of 10-3 M NADH, a treatment that irreversibly inactivates the haeme group of cytochrome P-450, did not reduce the density of liver I2 imidazoline-preferring receptors. These results discounted a possible interaction of [3H]-idazoxan with the haeme group of cytochrome P-450 enzymes.6. Together the results indicate that the down-regulation of I2 imidazoline-preferring receptors is associated with an irreversible inactivation of MAO (at least in the brain) that is not related either to the affinity of the MAO inhibitors for I2 imidazoline-preferring receptors or to an irreversible binding to these putative receptors. These findings indicate a novel effect of irreversible MAO inhibitors in the brain and suggest a new target for these compounds that could be of relevance in the treatment of depression, a disease in which an increased density of brain I2 imidazoline-preferring receptors has been reported.

Pregnancy alterations following xenobiotic-induced delays in ovulation in the female rat.

Chlordimeform [N'-(4-chloro-o-tolyl)-N,N-dimethylformamidine] has been shown to cause a 1-day delay in the surge of luteinizing hormone (LH) in ovariectomized, steroid-primed female rats, presumably through its ability to block CNS alpha-noradrenergic receptors and consequently CNS regulation of anterior pituitary function. In the present study, we determined whether a chlordimeform-induced delay in the ovulatory surge of LH would alter pregnancy outcome in intact females. Chlordimeform (50 mg/kg) or sodium pentobarbital (35 mg/kg), as a positive control, was administered in order to delay ovulation 24 (1-day delay) or 48 hr (2-day delay). Females were then housed with proven fertile males on the evening of proestrus (0-day delay group), the following evening (1-day delay group), or the evening after that (2-day delay group). The number of receptive females in each group, the mean lordosis quotient, and the number of sperm-positive females in each group were recorded. All females were killed on Gestation Day 20. The number of pregnant females in the 1- or 2-day delay groups was reduced with both chlordimeform and pentobarbital. Also, delaying ovulation for 1 or 2 days with either compound resulted in a significant reduction in the number of live pups present on Gestation Day 20 and a decrease in the number of implantation sites. Litter size was not affected if the females were mated on the same day that treatment was administered (0-day delay).(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of the formamidine pesticide chlordimeform on ovulation in the female hamster: dissociable shifts in the luteinizing hormone surge and oocyte release.

The formamidine pesticide chlordimeform (CDF) has been found to interfere with the hormonal control of ovulation in the rat by a presumptive disruption in the catecholaminergic regulation of the midcycle surge of luteinizing hormone (LH). While the brain hypothalamic mechanisms underlying generation of the hamster surge have not been as well-defined, there is evidence for an adrenergic component. The present experiments were designed to investigate the effects of CDF on ovulation in the golden hamster and whether any observed alterations are associated with differences in the appearance of the surge. Intraperitoneal CDF injections (0, 75, 150, or 200 mg/kg) at 1400 hr on proestrus caused a dose-related reduction in oocytes retrieved at 0700 hr on the day of estrus. Additional experiments demonstrated that this effect was due to a delay in ovulation and not a decrease in the complement of oocytes released. Doses of 150 or 200 mg/kg at 1100 hr or 200 mg/kg at 1600 hr were without a comparable ovulatory effect. Characterization of the LH surge indicated that there was a dose-related delay, but that this effect was not sufficient to account for the delay in ovulation. Animals dosed at 1100 hr (150 or 200 mg/kg) exhibited shifts in the surge identical to those animals injected at 1400 hr, but did not show any such detectable effects on the time of oocyte release. CDF administered at 1100 or 1400 hr also caused alterations in serum estradiol and progesterone, even at a relatively low dose of 37.5 mg/kg. The results indicate that CDF given at a time just prior to the preovulatory rise in LH is able to impede ovulation in the hamster by possibly influencing some ovulatory event(s) triggered by the surge. Also, the ability of CDF to alter the timing of LH release is consistent with catecholaminergic participation in the generation of the gonadotropin signal stimulating the final events leading to ovulation. Under the present conditions, this perturbation can be characterized as a delay in the LH trigger, rather than a blockade.

Calcium entry blockade as a mechanism for chlordimeform-induced inhibition of motor activity in the isolated guinea-pig ileum.

Central and peripheral alpha 2-adrenoceptors, including those of the gastrointestinal tract, have been indicated as a toxicity target of formamidine pesticides in mammals. In this study, the inhibitory effect of chlordimeform on twitch contractions from electrically-stimulated longitudinal muscle-myenteric plexus preparations (LMMPs) of the guinea-pig ileum was found to be resistant to the action of the alpha 2-adrenoceptor antagonist idazoxan. This drug was also ineffective on chlordimeform-induced inhibition of peristalsis recorded in whole ileal segments. As expected, idazoxan antagonized the inhibitory effect of the alpha 2-adrenoceptor agonist clonidine on twitch contractions and peristaltic activity. Chlordimeform reduced the amplitude of direct mechanical responses to a variety of spasmogens such as acetylcholine, histamine and substance P, suggesting a muscular site of action. Moreover, Ca(2+)-free, K(+)-depolarized LMMPs, chlordimeform inhibited submaximal contractions caused by addition of exogenous calcium, through an action apparently similar to that of the Ca2+ entry blocker nifedipine. Both chlordimeform- and nifedipine-induced inhibition of calcium contractions were reversed by the calcium channel activator BAY K 8644. This compound also partially prevented the inhibitory action of chlordimeform on peristaltic activity. On the whole, these results indicate that chlordimeform-induced depression of motor activity in the guinea-pig ileum is, at least in part, related to inhibition of transmembrane Ca2+ fluxes responsible for smooth muscle contraction.

XAMI and DCDM, agonists at cAMP-associated octopamine receptors in cockroach nerve cord, produce centrally mediated antinociception in mice.

The ability of XAMI (2,3-xylylaminomethyl-2'-imidazoline), the most potent agonist of cAMP-associated octopamine-sensitive adenylate cyclase in cockroach (Periplaneta americana) nerve cord yet reported, and DCDM (N-demethylchlordimeform), a partial octopamine agonist in this preparation, to produce centrally mediated antinociception in mice was evaluated. The antinociception produced by these compounds was compared to that previously reported for p-octopamine, a phenylethylamine and endogenous mammalian hydroxyphenolic analog of norepinephrine. Consonant with the reported greater agonistic activity of XAMI on octopamine-sensitive adenylate cyclase, XAMI was more potent than p-octopamine by spinal or supraspinal administration in the abdominal constriction test (E50 = 0.013 micrograms i.t., 1.45 micrograms i.c.v.) and in the 48 degrees C hot-plate test (ED50 = 0.06 micrograms i.t., 0.4 micrograms i.c.v.), but was inactive in the tail-flick test (up to 4.0 micrograms i.c.v. or i.t.). Unlike p-octopamine, both XAMI and DCDM were active by peripheral routes of administration. DCDM was orally active in the mouse acetylcholine-induced abdominal constriction test (ED50 = 9.98 mg/kg p.o.) and was active via the s.c. route in this test (ED50 = 2.36 mg/kg), the 48 degrees C hot-plate test (ED50 = 5.40 mg/kg) and the tail-flick test (ED50 between 15 and 30 mg/kg). It appeared to be a full agonist against these endpoints. XAMI produced dose-related antinociception in the abdominal constriction test (ED50 = 0.10 mg/kg s.c.) and in the 48 degrees C hot-plate test (ED50 = 3.71 mg/kg p.o. and 0.46 mg/kg s.c.), where the antinociceptive response persisted for at least 60 min following subcutaneous or oral administration. Both compounds were less potent via peripheral routes than clonidine (as reference) in these tests. Mechanistically, XAMI-induced antinociception was antagonized by yohimbine and idazoxan, but not the opiate antagonist naloxone.(ABSTRACT TRUNCATED AT 250 WORDS)

Suppression of the luteinizing hormone surge by chlordimeform in ovariectomized, steroid-primed female rats.

The midcycle surge of luteinizing hormone (LH) from the pituitary provides the physiological trigger in the mammalian female for the process of ovulation. Accordingly, any agent that compromises the LH surge could function as a reproductive toxicant. Since ovariectomized (OVX) rats implanted with oestradiol capsules will exhibit daily afternoon surges, such animals can serve as a useful model for the investigation of toxicant-induced alterations in this functional hormonal event. The acaricide chlordimeform (CDF) has previously been found to decrease serum LH, probably by altering the hypothalamic noradrenergic transmitter control of LH secretion. Consequently, the present study focused on the effect of acute CDF administration on the appearance of the induced LH surge. Single intraperitoneal injections of CDF (0, 10, 25, 50 mg/kg) in OVX, oestradiol-implanted female Long-Evans rats approximately 5 hr prior to the expected surge caused a complete suppression at 25 and 50 mg/kg. Ten mg/kg had no effect on surge amplitude, but advanced the LH peak by 2 hr. The observed suppression did not persist beyond the day of CDF administration. Earlier dosing at 11 or 18 hr prior to the surge was without effect. Since CDF has been found to elevate serum corticosterone (CORT), 10 mg CORT/rat were given at different times prior to the surge. Twenty hr after administration only a partial lowering was seen; 5 hr exposure were ineffective. This indicates that an indirect adrenal effect was not the principal route, but may accompany an action of CDF on the hypothalamic mechanisms regulating the surge and becomes evident after more prolonged exposure.

Influence of chlordimeform on alpha-adrenergic receptor-associated mechanisms of hormonal regulation in the rat: pituitary and adrenocortical secretion.

The acaricide chlordimeform (CDF) has been reported to have effects on the central nervous system that appear to involve an interaction with alpha-adrenergic receptor-mediated mechanisms of neurotransmission. The present study examined the effects of CDF on adrenocortical and pituitary prolactin secretion, which are known to involve central adrenergic receptors. Male Long-Evans rats were injected i.p. with 20 or 50 mg/kg CDF and killed after 1, 4, 8 or 24 h. Both noninjected and saline-injected controls were included. Dosing was structured so that trunk blood could be collected during the morning nadir of circulating corticosterone (CORT). Assays for plasma adrenocorticotropic hormone (ACTH), CORT and prolactin (PRL) showed that with 50 mg/kg, all three hormones rose sharply by 1 h. CORT increased in a dose-dependent fashion and declined over the ensuing 8 h. Other rats were treated with the alpha-adrenergic antagonist phenoxybenzamine (PBZ, 20 mg/kg) or the alpha-agonist clonidine (CLON, 0.6 mg/kg) 40 min before and killed 1 h after CDF (25 mg/kg) injection. CLON was found to completely suppress the CDF-induced rise in CORT, while PBZ enhanced the CORT/ACTH response to CDF. CLON also significantly elevated PRL, an alteration not seen in the CLON-pretreated CDF rats. Dexamethasone was able to block the CDF-induced rise in CORT and significantly suppressed PRL levels in both saline- and CDF-treated groups. These effects indicate that CDF is interfering with a regulatory signal mediated by alpha-adrenergic receptor-associated activity.