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.

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)

Alpha 2-adrenoceptors as a target for formamidine pesticides: in vitro and in vivo studies in mice

While the toxicity in insects of formamidines such as chlordimeform (CDM), its demethylated metabolite DCDM, and amitraz (AMZ) appears to involve activation of an octopamine-sensitive adenylate cyclase, their mechanism of action in mammals remains elusive. There is increasing evidence, however, that alpha 2-adrenoceptors might mediate certain effects of CDM, DCDM, and AMZ. In the present study, we investigated whether formamidines can interact directly with adrenoceptors in mouse forebrain both in vitro and after in vivo administration. Formamidines were potent inhibitors of the binding of [3H]clonidine to alpha 2-adrenoceptors with IC50's of 13 microM, 29 nM, and 130 nM for CDM, DCDM, and AMZ, respectively. Binding of [3H]yohimbine was inhibited with similar potencies. All compounds also inhibited with equal (CDM) or lower potency the binding of [3H]spiperone to dopamine D2 receptors and were weak inhibitors or inactive toward alpha 1- and beta-adrenoceptors, cholinergic muscarinic, GABAA, opiate mu, benzodiazepine, and histamine 1 receptors. Administration of formamidines to mice caused a dose-dependent decrease of [3H]clonidine binding. [3H]Clonidine binding returned to control values within 5 hr following administration of CDM and DCDM, but was still significantly decreased up to 48 hr after AMZ. Among different brain regions, [3H]clonidine binding was decreased to a larger extent in cerebral cortex, hippocampus, and midbrain. In vitro and ex vivo kinetic binding studies indicated that the effect of formamidines on alpha 2-adrenoceptors was due to a decrease in affinity and not to an alteration of the density of [3H]clonidine binding sites. The results of these biochemical studies support the hypothesis that alpha 2-adrenoceptors represent an important target for formamidine neurotoxicity in mammals.

The bradycardic and mydriatic effects of chlordimeform and its demethylated analogs in the rat: antagonism by idazoxan but not by prazosin

Pupillary and cardiac responses to i.v. injections of chlordimeform (CDM, 0.3-10 mg/kg), a formamidine insecticide, and its metabolites demethylchlordimeform (DCDM, 0.03-1 mg/kg) and didemethylchlordimeform (DDCDM, 0.1-3 mg/kg) were studied in rats anesthetized with pentobarbital. Both CDM and DCDM induced a dose-dependent mydriasis and bradycardia and DCDM was 10 times more potent than CDM in causing these effects. In contrast, DDCDM did not induce a mydriasis or bradycardia. The alpha 2-adrenoreceptor antagonist, idazoxan (0.2 mg/kg, i.v.) abolished or reduced CDM- and DCDM-induced mydriasis and bradycardia, whereas the alpha 1-adrenoreceptor antagonist, prazosin (1.5 mg/kg, i.v.) did not change these effects of CDM and DCDM. SKF 525-A (50 mg/kg, i.p.), an inhibitor of enzymatic demethylation, administered 10 min before the first dose of CDM, failed to reduce the effects of CDM. The results suggested: 1) the mydriatic and bradycardic effects of CDM and DCDM are mediated by alpha 2-adrenoreceptors, 2) the monodemethylation of CDM increases its alpha 2-adrenoreceptor agonistic activities, but the didemethylation of CDM abolishes these activities, and 3) CDM can exert alpha 2-adrenoreceptor agonistic activities without undergoing a demethylation process.

Interaction of the pesticide chlordimeform with adrenergic receptors in mouse brain: an in vitro study

Chlordimeform (N'(4-chloro-o-tolyl)-N, N-dimethylformamidine; CDM) is a formamidine insecticide acaricide whose major active metabolite is its N-monomethyl analog, desmethylchlordimeform, (DCDM). While their pesticidal action in invertebrates appears to be related to activation of octopamine receptors, their mechanism of action in mammals has not been established. Because of similarities between octopamine and adrenergic receptors and suggestions of CDM and DCDM action on adrenoceptors, the in vitro interactions of CDM and DCDM with adrenoceptors were studied. In mouse brain membrane preparations CDM inhibited the binding of [3H]-clonidine to alpha 2- adrenoceptors and of [3H]-WB4101 to alpha 1-adrenoceptors with IC50 values of 18.2 and 87 microM, respectively. DCDM was a much more potent inhibitor, with IC50 values toward alpha 2-, and alpha 1-adrenoceptors of 44 nM and 1 microM, respectively. Both compounds were only weak inhibitors of the binding of [3H]-dihydroalprenolol to beta-adrenoceptors and of [3H]-quinuclidinyl benzilate to muscarinic receptors and were inactive toward benzodiazepines and gamma aminobutyric acid (GABAA) receptors. Inhibition of [3H]-clonidine binding by both compounds was competitive, as indicated by a decreased receptor affinity without changes in receptor density. Interaction of CDM and DCDM with [3H]-WB4101 binding, on the other hand, was more complex, and not of the competitive type. These results show that CDM and its metabolite DCDM can interact directly in vitro with alpha-adrenergic receptors, suggesting that these receptors could mediate some of the effects of CDM and DCDM in vivo.

Pharmacological characterization of octopamine-sensitive adenylate cyclase in the flight muscle of Locusta migratoria L

Octopamine stimulates (20-30-fold) the adenylate cyclase activity in a broken cell preparation of locust flight muscle. This stimulation is mediated by one receptor with an apparent affinity of 3.3 X 10(-6) M. The hydroxyl group in the para position on phenylethanolamine was absolutely necessary to obtain an agonist whereas the meta hydroxyl group or the presence of a catechol almost suppressed the activity. Similar pharmacological profiles were obtained on other locust muscles, i.e. mandibular and extensor tibiae muscles. The affinities of a large series of agonists and antagonists for the octopamine receptor coupled with an adenylate cyclase were compared to those reported for octopamine 1, 2A and 2B receptors based on physiological studies in the locust extensor tibiae muscle. It appears that the octopamine receptor coupled with adenylate cyclase is likely to be identical with the octopamine 2A receptor mediating the increase in the amplitude of slow motoneuron twitch tension. The results of the pharmacological experiments are discussed, in particular concerning the possible relationship between the octopamine receptor and alpha 1- and alpha 2-adrenoceptors.

Receptors for 3H-octopamine in the adult firefly light organ

3H-Octopamine binds reversibly and with high affinity to sites on adult firefly light organ membranes. The binding is characterized by multiple affinities. Scatchard analysis supported a two site binding model with a tentative Kd value of about 1 nM for the high affinity component. The more abundant lower affinity site had a Kd value of about 60 nM. Guanyl nucleotides (Gpp(NH)p and GTP) greatly reduced the apparent number of octopamine binding sites. Competition studies with known octopaminergic agonists including the formamidine pesticides chlordimeform (CDM) and N-demethyl chlordimeform (DCDM) showed the following rank order of potencies in displacing octopamine: DCDM greater than octopamine = synephrine greater than naphazoline greater than clonidine greater than CDM. It was also observed that phentolamine was much more active than propranolol in antagonizing OA-binding. These relative activities are similar to the abilities of the same compounds to alter adenylate cyclase activity in light organ homogenates. Together with the effect of GTP on binding, these results suggest that the binding sites are functional octopamine receptors of the light organ.

Caffeine and related methylxanthines: possible naturally occurring pesticides

Natural and synthetic methylxanthines inhibit insect feeding and are pesticidal at concentrations known to occur in plants. These effects are due primarily to inhibition of phosphodiesterase activity and to an increase in intracellular cyclic adenosine monophosphate. At lower concentrations, methylxanthines are potent synergists of other pesticides known to activate adenylate cyclase in insects. These data suggest that methylxanthines may function as natural insecticides and that phosphodiesterase inhibitors, alone or in combination with other compounds, may be useful in insect control.

Effect of formamidines on 5-hydroxytryptamine uptake and biogenic amine levels in rat platelets

Uptake of radioactive 5-hydroxytryptamine (5-HT) by platelets from rats treated intraperitoneally with the insecticide/acaricide chlordimeform (CDF) at 25 mg/kg was not significantly influenced at 1 h; however, uptake of this amine by platelets from rats treated with its N- monodemethyl metabolite (DMC) at the same dosage was significantly inhibited (37%). Two reversed phase high performance liquid chromatographic systems with electrochemical detection were developed with a capacity to separate 11 biogenic amines and related compounds. Only 5-HT, norepinephrine (NE), dopamine (DA), and 3,4-dihydroxyphenylacetic acid (DOPAC) were consistently detected in platelets and plasma samples. At 1 h and/or 24 h postinjection, CDF effected significant decreases in platelet levels of 5-HT, NE, and DA and plasma levels of 5-HT, whereas platelet and plasma levels of DOPAC were significantly increased. DMC effected significant decreases in both platelet and plasma levels of 5-HT. These in vivo studies confirmed previous in vitro experiments which demonstrated that formamidines inhibited uptake of 5-HT by platelets and released endogenous stores of amines from platelets.

Stimulation by octopamine of the sodium efflux in barnacle muscle fibres

The behaviour of the ouabain-insensitive Na efflux in barnacle muscle fibres towards DL-octopamine has been investigated. It is found that these fibres are quite often sensitive to external application of octopamine. A concentration as low as 10(-9) M is effective. The kinetic results indicate that the stimulatory response develops within 5-10 min of exposure of the fibre to octopamine and is transitory in nature. The response to octopamine is greater in size in the presence of 1-isoamyl-3-isobutylxanthine (IAX) than in the presence of 1-propyl-3-methyl-7-(5-hydroxyhexyl)-xanthine (PMX). But neither IAX nor PMX stops the response from decaying. The response to octopamine is highly dependent on the presence of external Ca2+. The provisional conclusion is that the barnacle muscle fibre may be a useful preparation for studying the hormonal function of octopamine.

Interaction of formamidines with octopamine-sensitive adenylate cyclase receptor in the nerve cord of Periplaneta americana L

Chlordimeform (CDM) and demethylchloridimeform (DCDM) mimic the action of octopamine in elevating adenylate cyclase activity in intact nerve cords of the American cockroach, Periplaneta americana. At a concentration of 1 x 10(-5)M, DCDM (13.5x increase within 20 minutes) is a more potent effector of the response than CDM (3x increase within 20 minutes), but both compounds show less efficacy than octopamine (23.5x increase within 15 minutes). DCDM also mimics the stimulatory effect of octopamine on adenylate cyclase activity in nerve cord homogenates whereas CDM has no demonstrable effect on this preparation. The octopamine- and DCDM-induced responses are competitively inhibited by phentolamine (1 x 10(-6)M) and cyproheptadine (1 x 10(-6)M) but not by propranolol (1 x 10(-6)M). DCDM and CDM inhibit the octopamine-induced activation of adenylate cyclase by 33% and 44% respectively. The results are discussed in light of the proposal that DCDM serves as a partial agonist and CDM as an antagonist of the octopamine receptor.

Studies on the mutagenic potential of the pesticide chlordimeforme and its principal metabolites in the mouse heritable translocation assay

Chlordimeforme and its 2 principal metabolites, N-formyl-4-chloro-o-toluidine and 4-chloro-o-toluidine, were studied for induction of heritable translocations. Maximal tolerated doses of the compounds were given daily by gavage for 7 consecutive weeks. After mating with untreated females, about 1000 F1 male offspring per group--including vehicle control and positive control (Tretamine, TEM)--were tested for their reproductive performance by use of a sequential decision procedure on litter sizes to select males with translocation heterozygosity. Partially sterile, sterile and non-classifiable F1 males were examined cytogenetically by scoring meiotic chromosomes for translocation multivalents or analysing mitotic divisions for marker chromosomes. The 3 compounds tested at dose levels showing toxic effects did not induce translocation heterozygosity. Tretamine, the positive control, gave the expected mutagenic response.

The effects of U-40481, demethylchlordimeform, and formetanate on calcium flux in the isolated rabbit aorta

The effects of three formamidine pesticide or pesticide metabolites on calcium flux in the rabbit aorta were examined. U-40481 reduced the rate of both 45Ca uptake and norepinephrine (NE)-stimulated 45Ca uptake into deadventitiated rabbit aorta strips and reduced NE-stimulated efflux from 45Ca-loaded, superfused aorta strips (p less than 0.1). Neither demethylchlordimeform nor formetanate altered either 45Ca-uptake or NE-enhanced 45Ca flux. The action of U-40481 on unstimulated 45Ca uptake may result from uncoupling oxidative phosphorylation, and reduction in NE-induced 45Ca-flux may result from antagonism at NE receptors.

Human health effects of selected pesticides, chloroaniline derivatives

The health effects of two pesticides, chlordimeform and propanil, are discussed. Chlordimeform and 2-methyl-4-chloroaniline, a major metabolite of chlordimeform, cause severe hemorrhagic cystitis in humans. In cats, however, only a mild effect on the bladder was noted. The herbicide propanil has produced chloracne in humans, and along with 3,4-dichloroaniline, hyperkeratosis in rabbits. The contaminants 3,4,3',4'-tetrachloroazobenzene and 3,4,3',4'-tetrachloroazoxybenzene are responsible for the chloracnegenic characteristics of propanil, 3,4-dichloroaniline, and methazole.

Mode of action of formamidine pesticides: an evaluation of monoamine oxidase as the target

The ability of formamidine pesticide, chlordimeform (N'-(4-chloro-o-toyl)-N,N-dimethylformamidine) (CDM), and several of its major metabolites to inhibit monoamine oxidase (MAO) in mouse tissues in vitro and in vivo was examined, and related to the hypothesis that inhibition of MAO is responsible for the lethal effects of CDM. CDM was a readily reversible inhibitor of MAO of medium potency as were most of its metabolites. However, the hydrolysis product, N-formyl-4-chloro-o-toludine (CT) was a significantly more potent reversible inhibitor. A comparison of MAO from brain, liver, and intestine showed no marked variations in their sensitivity to these inhibitors. Greater inhibitory potency was found using Type A substrates (5-hydroxytryptamine) than Type B substrates (beta-phenylethylamine). The activity of MAO in vivo after pretreatment of mice with CDM or its metabolites was assessed in liver and intestine by measuring the amount of [14C] tryptamine which still survived 5 min after an intraperitoneal injection. Established inhibitors of MAO gave appropriate results with this method. CDM also increased tryptamine recoveries but only at does which caused mortality, and then to a lesser extent than MAO inhibitors such as tranylcypromine, pheniprazine, and harmaline at sub lethal doses. For this reason, and in view of the lack of correlation of toxicity to MAO-inhibitory potency among CDM and its metabolites, and because the symptoms of poisoning are inappropriate, it is concluded that MAO inhibition is not an important factor in the acute lethality of CDM.

Chlorodimeform and its effect on monoamine oxidase activity in the cattle tick, Boophilus microplus

The action of the acaricide, chlorodimeform and its metabolite. N-desmethylchlorodimeform, on the activity monoamine oxidase (MAO) from the cattle tick, Boophilus microplus, were studied. Both compounds were found to be potent in vitro and in vivo inhibitors of the enzyme. However the inhibition of MAO does not seem to be related to the toxic action of the acaricide.