Tyramine as a possible neurotransmitter/neuromodulator at the spermatheca of the African migratory locust, Locusta migratoria

Tyramine-like immunoreactivity was identified in neurons of the VIIIth abdominal ganglion and in axons projecting to the spermatheca of adult females of Locusta migratoria. Tyramine-like immunoreactive processes were also found throughout all regions of the spermatheca and tyramine-like immunoreactive bipolar or multipolar neurons were present on the spermathecal sac. HPLC coupled with electrochemical detection revealed more tyramine than octopamine present in spermathecal tissue. Electrical stimulation of the ventral ovipositor nerve resulted in a significant increase in calcium-dependent release of tyramine from the spermatheca. Both tyramine and octopamine increase the frequency and basal tonus of spermathecal contractions in a dose-dependent manner, with octopamine having a lower threshold. When tyramine is applied along with a half maximal octopamine dose, there is an additive effect on contractions of the spermatheca with slight synergistic effects at lower doses of tyramine. High concentrations of tyramine (10(-4)M) stimulated increases in cyclic AMP levels of the spermatheca; an effect blocked by phentolamine. Phentolamine has a higher affinity (and thus a lower IC(50) value congruent with5.6x10(-8)M) than yohimbine (IC(50) congruent with1.1x10(-4)M) in reducing tyramine-induced spermathecal contractions. Taken together, these results suggest that tyramine may be a co-transmitter with octopamine at the spermatheca, with both neuroactive chemicals acting on an octopamine receptor.     

The neural control of spermathecal contractions in the locust,Locusta migratoria

The innervation of the spermatheca and demonstration of neural control of spermathecal contractions in Locusta migratoria was illustrated using anterograde and retrograde fills, combined with electrophysiological stimulation and recording. The anterior portion of the spermatheca receives innervation via the receptaculum seminis nerve (N2B2) from two large ventral neurons and one dorsal neuron. All were bilaterally paired and situated in the VIIIth abdominal ganglion. Three ventral bilaterally paired neurons situated in the VIIIth abdominal ganglion also provide innervation to the posterior portion of the spermatheca via the ductus seminalis aperture nerve (N2B3). Six DUM neurons, located in the VIIIth abdominal ganglion, in addition to two centroposteriorly situated DUM neurons in the VIIth abdominal ganglion, are also associated with these two nerves. N2B4 also provides innervation to the posterior portion of the spermatheca. N2B6b is associated with sensory cells identified in the anterior lateral regions of the genital chamber. The spermatheca contracts spontaneously, with peristaltic contractions beginning at the spermathecal sac and continuing along the length of the spermathecal duct. However electrical stimulation of the ventral ovipositor nerve (VON or N2B), receptaculum seminis nerve (N2B2) and the ductus seminalis aperture nerve (N2B3) indicates that contractions are also under neural control. In particular contractions of the spermathecal sac, coil duct and anterior straight duct are initiated via motor projections from the receptaculum seminis nerve (N2B2) and posterior straight duct contractions are controlled by motor input from the ductus seminalis aperture nerve (N2B3). The results suggest that spermathecal contractions of the anterior and posterior portions of the spermatheca are under separate neural control.     

Octopamine action on the spontaneous contractions of the isolated nerve cord of Lumbricus terrestris

Octopamine and synephrine were observed to effect the spontaneous rhythmic contractions displayed by the isolated ventral nerve cord of the earthworm, Lumbricus terrestris. octopamine and synephrine produced dose-dependent significant changes in the frequency, amplitude and basal tonus of the spontaneous contractions. Application of adrenergic receptor antagonists suggested the octopamine receptors to have some similarity to vertebrate alpha 1-adrenergic receptors. The spontaneous contractions were not abolished by tetrodotoxin (TTX) which suggested a myogenic origin for the contraction of the ventral nerve cord sheath muscles. Octopamine, in the presence of TTX, increased the basal tonus and maximum force of the spontaneous contractions.     

The association of crustacean cardioactive peptide with the spermatheca of the African migratory locust, Locusta migratoria

Crustacean cardioactive peptide (CCAP)-like immunoreactivity was identified in neurons of the VIIIth abdominal ganglion and in axons in the nerves that project to the spermatheca of 3-4 week old adult female locusts. In addition, lightly stained CCAP-like immunoreactive processes were localized over the spermathecae. The amount of CCAP in the spermathecal tissue was quantified using an enzyme-linked immunosorbent assay (ELISA) performed on extracts of the whole spermatheca, and on its constituent parts, namely the sperm sac, coiled duct and straight duct. The spermatheca contains 920+/-273 fmol (mean+/-SE) of CCAP equivalents, with the majority localized in the coiled duct. There are age-related differences in the amount of CCAP present in the spermathecae with less content in spermathecae from 1 to 5 day old and greater content in spermathecae from 3 to 4 week old adults. There was also no difference in CCAP content of spermathecae in mated and virgin 3 to 4 week old adults. Reversed phase-high performance liquid chromatography (RP-HPLC) followed by ELISA further confirmed the presence of CCAP-like material in extracts of locust spermathecae. Physiological assays demonstrated that CCAP increased the basal tonus and frequency of spontaneous contractions of the spermatheca, with thresholds between 10(-10) and 10(-9)M and maxima at 10(-7)M CCAP. CCAP also increases the amplitude of neurally evoked contractions with a threshold less than 10(-11)M and a maximum of 10(-7)M CCAP. The present study suggests that CCAP acts as a neuromodulator/neurotransmitter at the spermathecal visceral tissue of female Locusta migratoria.     

Pharmacological profile of octopamine and 5HT receptors on the lateral oviducts of the cockroach,Periplaneta americana

The effects of the amines 5HT and octopamine on the myogenic activity of Periplaneta americana (L.) oviducts and the pharmacological profile of octopamine and 5HT receptors on the lateral oviducts have been determined. Application of 5HT to the oviducts resulted in a dose-dependent increase in basal tonus and amplitude of contractions. Antagonist studies revealed that the 5HT receptor on the cockroach oviduct most resembles the mammalian 5HT₂ receptor. Application of octopamine resulted in a decrease in basal tonus and had a biphasic effect on the amplitude of contractions, being stimulatory at low doses and inhibitory at higher ones. The inhibitory effects of octopamine appear to be mediated via cAMP and are blocked by antagonists which indicate that the octopamine receptor is of the octopamine-2 type. © 1995 Wiley-Liss, Inc.     

Evidence for the association of FMRFamide-related peptides with the spermatheca of Locusta migratoria

The association of FMRFamide-related peptides (FaRPs) with the spermatheca of Locusta migratoria was demonstrated using radioimmunoassay and immunohistochemical techniques. The physiological effects of various FaRPs on the neurally evoked contractions of the spermatheca were also examined. FMRFamide-like immunoreactivity (FLI) was demonstrated in processes and cell bodies situated in the VIIIth (terminal) abdominal ganglion. These included an anterior, central and posterior pair of ventral cell bodies positioned near the midline of the ganglion, in addition to two bilaterally paired dorsal cell bodies in the posterior region of the VIIIth abdominal ganglion. Two axons displaying FLI proceed down the ventral ovipositor nerve (VON) and into the receptaculum seminis nerve which innervates the anterior regions of the spermatheca. FLI was also noted in processes on the spermathecal muscle with the highest density occurring on the spermathecal sac and coil duct. FaRPs applied to the spermathecal muscle included GQERNFLRFamide, NFIRFamide, ADDRNFIRFamide, YGGFMRFamide, FMRFamide, ADVGHVFLRFamide and SchistoFLRFamide (PDVDHVFLRFamide). Dose-dependent physiological effects were only noted for FMRFamide, ADVGHVFLRFamide and SchistoFLRFamide. FMRFamide led to a dose-dependent increase in the amplitude of neurally evoked contractions with a threshold of approximately 5 x 10(-7) M. SchistoFLRFamide, and ADVGHVFLRFamide, had an inhibitory effect, decreasing the amplitude of neurally evoked spermathecal contractions.     

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.     

Mechanism for amine modulation of the neurogenic Limulus heart: evidence for involvement of cAMP

The role of cyclic nucleotides as intracellular second messengers mediating the excitatory chronotropic and inotropic actions of octopamine (OCT) and dopamine (DA) on the neurogenic Limulus heart was investigated. Tissue levels of cAMP, but not cGMP, were significantly increased in isolated cardiac ganglia and cardiac muscle following 10 min exposure to 10(-5) M OCT or 10(-5) M DA. In both tissues, OCT elicited larger increases in cAMP than did DA. Amine-induced cAMP accumulation in the cardiac ganglion and in the cardiac muscle was prevented by the alpha-adrenergic blocker phentolamine. The adenylate cyclase activator forskolin and the phosphodiesterase inhibitor IBMX produced amine-like chronotropic and inotropic effects when applied to the isolated heart preparation. However, the kinetics of the responses differed for the two agents. Additional pharmacological agents (RO-20-1724, papaverine, SQ 20,009, and 8-parachloro-phenylthio cAMP) also had amine-like effects but to a lesser extent. The chronotropic, but not inotropic, effects of OCT and DA were potentiated in the presence of IBMX. These data suggest that a cAMP-dependent mechanism underlies the excitatory effects of the neuromodulators OCT and DA on the Limulus heart.     

IP3-mediated octopamine-induced synaptic enhancement of crayfish LG neurons

The biogenic amines, octopamine and serotonin, modulate the synaptic activity of the lateral giant interneuron (LG) circuitry of the crayfish escape behavior. Bath application of both octopamine and serotonin enhances the synaptic responses of LG to sensory stimulation. We have shown previously (Araki et al. J Neurophysiol 94:2644-2652, 2005) that a serotonin-induced enhancement of the LG response was mediated by an increase in cAMP levels following activation of adenylate cyclase; however, octopamine acts independently. Here, we clarify how octopamine enhances the LG response during sensory stimulation using physiological and pharmacological analyses. When phospholipase C inhibitor U-73122 was directly injected into the LG before biogenic amine application, it abolished the enhancing effect of octopamine on direct sensory input to the LG, but did not block indirect input via sensory interneurons or the effect of serotonin. Direct injection of IP(3), and its analogue adenophostin A, into the LG increased the synaptic response of the LG to sensory stimulation. Thus, IP(3) mediates octopamine-induced synaptic enhancement of the LG, but serotonin acts independently. These results indicate that both octopamine and serotonin enhance the synaptic responses of the LG to sensory stimulation, but that they activate two different signaling cascades in the LG.     

Stimulatory effects of male accessory-gland extracts on the myogenicity and the adenylate cyclase activity of the oviduct of Locusta migratoria

The effects of male accessory-gland extracts on the myogenic contractions and the adenylate cyclase activity of the oviduct of Locusta migratoria have been examined. The extracts stimulated first the frequency and the amplitude, then the tonus of the oviduct contractions in dose-dependent and reversible ways. They also stimulated the adenylate cyclase activity of oviduct disrupted-cell preparations. Extracts of opalescent glands (one of the 15 male accessory glands) gave similar results with only a quantitative difference. The tonus response is probably independent of the adenylate cyclase activity because octopamine and forskolin did not mimic this effect, and also because phentolamine was unable to inhibit the effect.Frequency and mainly amplitude responses can be induced through an adenylate cyclase-dependent receptor as shown by the similitude of actions with octopamine and forskolin. However, since the effects on the adenylate cyclase activity of octopamine and the accessory-gland extracts were cumulative, we concluded that these compounds are acting on two discrete types of receptors. All these results suggest that male accessory-gland secretions directly act upon the oviduct, in one case through adenylate cyclase-dependent receptors.     

Pharmacological evidence for the existence of presynaptic alpha-like receptors in Mytilus muscle

The effects of alpha-adrenoceptor agonists and antagonists on contractions of the ABRM of M. edulis were examined. Naphazoline (10(-11)-10(-7)M) as well as octopamine potentiated both contractions in response to ACh and to repetitive electrical stimulation with brief pulses, while clonidine (10(-12)-10(-6)M), imidazole (10(-4)-10(-3)M) and tolazoline (10(-6)-10(-4)M) potentiated only the contraction in response to repetitive electrical stimulation. Potentiating actions of octopamine and naphazoline on ACh contraction were blocked by tolazoline but were little affected by clonidine and imidazole. Potentiating after-effect of repetitive electrical stimulation on subsequent ACh-contraction was not affected by clonidine and imidazole but was blocked by tolazoline. These results suggest that in the ABRM there are probably at least two classes of alpha-like receptors for octopamine; one may be on the muscle fibres and the other may be on the excitatory nerve terminals. Activation of the latter class of receptors may result in enhancement of ACh release from the terminals.     

Actions of substance P on central neurones and peripheral tissues,and co-existence of 5-hydroxytryptamine and a substance p-like peptide in the heart of the snail Helix aspersa

The actions of substance P (SP) were tested on central neurones of the snail Helix aspersa, and on the in vitro heart and pharyngeal retractor muscle (PRM). SP was found to show very limited effects in the CNS, but did produce consistent, though weak, inhibition of a group of cells in the right pedal-cerebral connective. On the isolated beating heart 5-hydroxytryptamine (5-HT) produced short latency cardioexcitation, whereas long latency excitation was produced by SP. Electrically-induced contractions of the PRM were found to be inhibited by SP and augmented by the molluscan SP analogue eledoisin, suggesting the presence of separate receptor types in this tissue.Double immunohistochemical staining of sections through the central ganglia for 5-HT and SP revealed that the 2 compounds usually occur in separate neurones, although a few examples showing coexistence were also observed. In the heart, however, all the nerve endings visible were labelled by both antibodies suggesting that two cardioexcitatory compounds, 5-HT and a SP-like peptide, occur together in the same nerve endings in the Helix heart. An assay to measure cAMP levels in homogenized tissue showed that 5-HT and dopamine stimulated cAMP production, whereas production was inhibited by SP, indicating that the actions of cAMP may be mediated via an adenylate cyclase/cAMP system.     

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.     

Neurogenic and myogenic motor patterns of rabbit proximal, mid, and distal colon

The rabbit colon consists of four distinct regions. The motility of each region is controlled by myogenic and neurogenic mechanisms. Associating these mechanisms with specific motor patterns throughout all regions of the colon has not previously been achieved. Three sections of the colon (the proximal, mid, and distal colon) were removed from euthanized rabbits. The proximal colon consists of a triply teniated region and a single tenia region. Spatio-temporal maps were constructed from video recordings of colonic wall diameter, with associated intraluminal pressure recorded from the aboral end. Hexamethonium (100 μM) and tetrodotoxin (TTX; 0.6 μM) were used to inhibit neural activity. Four distinct patterns of motility were detected: 1 myogenic and 3 neurogenic. The myogenic activity consisted of circular muscle (CM) contractions (ripples) that occurred throughout the colon and propagated in both antegrade (anal) and retrograde (oral) directions. The neural activity of the proximal colon consisted of slowly (0.1 mm/s) propagating colonic migrating motor complexes, which were abolished by hexamethonium. These complexes were observed in the region of the proximal colon with a single band of tenia. In the distal colon, tetrodotoxin-sensitive, thus neurally mediated, but hexamethonium-resistant, peristaltic (anal) and antiperistaltic (oral) contractions were identified. The distinct patterns of neurogenic and myogenic motor activity recorded from isolated rabbit colon are specific to each anatomically distinct region. The regional specificity motor pattern is likely to facilitate orderly transit of colonic content from semi-liquid to solid composition of feces.     

Aminergic modulation of the myogenic heart in the branchiopod crustacean Triops longicaudatus

Although crustaceans typically have a neurogenic heart, the primitive crustacean Triops longicaudatus has a myogenic heart with the heartbeat arising from the endogenous rhythmic activity of the myocardium. In the present investigation, the effects of six biogenic amines, epinephrine, norepinephrine, dopamine, octopamine, serotonin and histamine, on the myogenic heart of T. longicaudatus were examined. Epinephrine, norepinephrine, dopamine and octopamine accelerated the heartbeat, increasing both the frequency and amplitude of the action potential of the myocardium in a concentration dependent manner. The ability of epinephrine and norepinephrine to produce the acceleratory effects was more potent than that of dopamine and octopamine; the threshold concentrations of epinephrine and norepinephrine were approximately 10(-10) M and those of dopamine and octopamine approximately 10(-7) M. Serotonin weakly inhibited the heartbeat, decreasing both the frequency and amplitude of the myocardial action potential in a concentration dependent manner with a threshold concentration of approximately 10(-6) M. Histamine exhibited no effect on the heartbeat. The results provide the first evidence for direct effects of amines on the crustacean myocardium and suggest neurohormonal regulation of the myogenic heart in T. longicaudatus.     

A novel role for cholecystokinin: regulation of mesenteric vascular resistance

The aim of this work was to characterize the vasoactive effect of cholecystokinin on mesenteric vasculature. The mesenteric vascular bed of 3-month-old Sprague-Dawley rats was isolated and perfused at constant flow and changes in perfusion pressure monitored. CCK peptides lacked any direct contractile or relaxing effect on the mesenteric smooth muscle. Transmural nerve stimulation (TNS, 200 mA, 0.2 ms, 8 and 16 Hz) elicited an increase in perfusion pressure reflecting contraction of the bed and CCK inhibited neurogenic contractions elicited by 8 and 16 Hz TNS. The inhibition of neurogenic contractions was blocked by the CCK2 receptor (CCK2R) antagonist, L-365,260 (10 and 100 nM), but not by the CCK1R antagonist, SR-27897. The inhibition of neurogenic contractions was reversed by the non-specific NOS inhibitor, L-NAME as well as by the specific nNOS inhibitor, S-methyl-L-thiocitrulline. In whole-mount segments of mesenteric arteries, CCK2R was detected in the adventitia, in nerve terminals, where it co-localized with synaptophysin and nNOS. CCK-8 immunoreactive fibers were also detected. These results suggest that CCK mediates vasodilatation of the mesenteric vascular bed through the release of NO via its presynaptic CCK2R. Our findings provide, for the first time, a neural mechanism by which CCK may increase mesenteric blood flow.     

Evidence of a neural loop involved in controlling spermathecal contractions in Locusta migratoria

The control of spermathecal contractions in Locusta migratoria via a neural loop was demonstrated using mechanical stimulation and electrophysiological recordings. Extracellular electrophysiological recordings from the receptaculum seminis nerve (N2B2), which innervates the spermathecal sac, were conducted during mechanical stimulation of the genital chamber sensory cells. Activation of the genital chamber sensory cells, using a glass probe approximating the shape and size of an egg, was found to increase the action potential frequency and initiate bursts of action potentials if a tonic frequency of action potentials was present prior to stimulation. If the motor pattern initially consisted of bursts of action potentials, then mechanical stimulation of the genital chamber sensory cells resulted in an increase in firing frequency, in most preparations, with the bursting remaining. Removal of the probe from the genital chamber always returned the motor activity to that noted prior to sensory cell stimulation. Simultaneous electrophysiological recordings from both the left and right receptaculum seminis nerves (N2B2) revealed that the bursts of action potentials were coordinated, although individual action potentials were not coupled one to one. Activation of the genital chamber sensory cells also resulted in increases in spermathecal contraction frequency, an effect which was coordinated with the changes in motor activity. It is proposed that an egg in the genital chamber activates the sensory cells resulting in increases in spermathecal contraction frequency and the subsequent release of spermatozoa onto the micropyle of the egg for fertilization.     

Passive smoking impairs endothelium-dependent relaxation of isolated rabbit arteries

The aim of the present study was to examine the effect of prolonged passive smoking (lasting 3 weeks) on plasma catecholamine levels and reactivity of isolated rabbit arteries. Plasma noradrenaline, adrenaline and dopamine levels were determined radioenzymatically. Isolated rings of the thoracic aorta and carotid artery were suspended in organ chambers and connected to a force transducer for the recording of isometric tension. Plasma noradrenaline levels were found to be significantly elevated in rabbits subjected to passive smoking for 3 weeks. Plasma adrenaline and dopamine levels were not changed. Transmural nerve stimulation of arterial rings evoked frequency-dependent contractions. Prolonged passive smoking did not affect neurogenic contractions of the arteries tested. On the other hand, endothelium-dependent relaxations of phenylephrine-precontracted arteries were significantly impaired. Furthermore, hypertrophy of the left ventricle was observed. In conclusion, passive smoking impairs endothelium-dependent relaxations but not neurogenic contractions of systemic arteries. The impaired relaxations of arteries may be, at least in part, mediated through the degradation of released nitric oxide by superoxide anions derived from cigarette smoke.     

Interactions of neurogenic responses of longitudinal and circular muscle in the guinea-pig ileum

The relationship between neurogenic responses of longitudinal and circular muscle was studied by measuring contractions and EMG or nonadrenergic, non-cholinergic (NANC) relaxations and NANC inhibitory junction potentials in different preparations of the guinea-pig ileum. NANC relaxation of longitudinal muscle was observed also without any preceding or concomitant circular muscle contraction ruling out the possibility that the latter might be the cause of the NANC relaxation. Circular muscle twitches or powerful contractions were absent if there was no preceding neurogenic or myogenic excitation of longitudinal muscle; in preparations with myenteric plexus-longitudinal muscle layers removed only small residual responses were seen although still under neurogenic influences. Thus excitation of longitudinal muscle seemed a prerequisite for synchronized and powerful contractions of circular muscle to occur. Cholinergic contraction and NANC relaxation of longitudinal muscle evoked by field stimulation were partly inhibited if the submucous plexus was also present suggesting the involvement of a more complex neuronal circuitry in these responses.     

Evidence for a possible neurotransmitter/neuromodulator role of tyramine on the locust oviducts

Visualization of the tyraminergic innervation of the oviducts was demonstrated by immunohistochemistry, and the presence of tyramine was confirmed using high-performance liquid chromatography coupled to electrochemical detection. Oviducts incubated in high-potassium saline released tyramine in a calcium-dependent manner. Stimulation of the oviducal nerves also resulted in tyramine release, suggesting that tyramine might function as a neurotransmitter/neuromodulator at the locust oviducts. Tyramine decreased the basal tension, and also attenuated proctolin-induced contractions in a dose-dependent manner over a range of doses between 10(-7) and 10(-4) M. Low concentrations of tyramine attenuated forskolin-stimulated cyclic AMP levels in a dose-dependent manner. This effect was not blocked by yohimbine. High concentrations of tyramine increased basal cyclic AMP levels of locust oviducts in a dose-dependent manner; however, the increases in cyclic AMP were only evident at the highest concentrations tested, 5 x 10(-5) and 10(-4) M tyramine. The tyramine-induced increase in cyclic AMP shared a similar pharmacological profile with the octopamine-induced increase in cyclic AMP. Tyramine increased the amplitude of excitatory junction potentials at low concentrations while hyperpolarizing the membrane potential by 2-5 mV. A further increase in the amplitude of the excitatory junction potentials and the occurrence of an active response was seen upon washing tyramine from the preparation. These results suggest that tyramine can activate at least three different endogenous receptors on the locust oviducts a putative tyramine receptor at low concentrations, a different tyramine receptor to inhibit muscle contraction, and an octopamine receptor at high concentrations.