Lobster Cardiac Ganglion Research Articles

Differential neuropeptide modulation of premotor and motor neurons in the lobster cardiac ganglion.

The American lobster, Homarus americanus, cardiac neuromuscular system is controlled by the cardiac ganglion (CG), a central pattern generator consisting of four premotor and five motor neurons. Here, we show that the premotor and motor neurons can establish independent bursting patterns when decoupled by a physical ligature. We also show that mRNA encoding myosuppressin, a cardioactive neuropeptide, is produced within the CG. We thus asked whether myosuppressin modulates the decoupled premotor and motor neurons, and if so, how this modulation might underlie the role(s) that these neurons play in myosuppressin’s effects on ganglionic output. Although myosuppressin exerted dose-dependent effects on burst frequency and duration in both premotor and motor neurons in the intact CG, its effects on the ligatured ganglion were more complex, with different effects and thresholds on the two types of neurons. These data suggest that the motor neurons are more important in determining the changes in frequency of the CG elicited by low concentrations of myosuppressin, whereas the premotor neurons have a greater impact on changes elicited in burst duration. A single putative myosuppressin receptor (MSR-I) was previously described from the Homarus nervous system. We identified four additional putative MSRs (MSR-II–V) and investigated their individual distributions in the CG premotor and motor neurons using RT-PCR. Transcripts for only three receptors (MSR-II–IV) were amplified from the CG. Potential differential distributions of the receptors were observed between the premotor and motor neurons; these differences may contribute to the distinct physiological responses of the two neuron types to myosuppressin.NEW & NOTEWORTHY Premotor and motor neurons of the Homarus americanus cardiac ganglion (CG) are normally electrically and chemically coupled, and generate rhythmic bursting that drives cardiac contractions; we show that they can establish independent bursting patterns when physically decoupled by a ligature. The neuropeptide myosuppressin modulates different aspects of the bursting pattern in these neuron types to determine the overall modulation of the intact CG. Differential distribution of myosuppressin receptors may underlie the observed responses to myosuppressin.

The mixed-mode oscillations in Av-Ron-Parnas-Segel model

Mixed-mode oscillations (MMOs) as complex firing patterns with both relaxation oscillations and sub-threshold oscillations have been found in many neural models such as the stellate neuron model, HH model, and so on. Based on the work, we discuss mixed-mode oscillations in the Av-Ron-Parnas-Segel model which can govern the behavior of the neuron in the lobster cardiac ganglion. By using the geometric singular perturbation theory we first explain why the MMOs exist in the reduced Av-Ron-Parnas-Segel model. Then the mixed-mode oscillatory phenomenon and aperiodic mixed-mode behaviors in the model have been analyzed numerically. Finally, we illustrate the influence of certain parameters on the model.

Monoamine pharmacology of the lobster cardiac ganglion

Monoamine agonists and antagonists were applied to the lobster cardiac ganglion in an attempt to clarify the different actions of 5-hydroxytryptamine (5HT) and dopamine (DA) on this rhythmic pattern generator. Experiments were designed to determine whether the similar responses to 5HT and DA applied to the anterior region of the ganglion could be separated by pharmacological approaches, and whether the different responses to 5HT applied to the anterior and posterior regions of the ganglion could be attributed to mediation by different receptors. A small number of the 5HT agonists which were tested mimic the effects of 5HT, in that they increase the frequency of bursting and decrease burst duration when applied to the whole ganglion, but decrease burst frequency and increase burst duration when applied only to the posterior half. Other 5HT agonists decrease frequency and prolong bursts when applied to the whole ganglion. Of the DA agonists tested, none acts as DA itself does. Rather, they mimic the effects of 5HT applied to the posterior ganglion, by slowing bursting and prolonging bursts. The actions of agonists do not correspond in any clear way to the receptor specificities as defined in vertebrates. Most antagonists tested do not show similar specificities to their effects in vertebrates. In particular, most of the DA antagonists tested are more effective in blocking exogenous 5HT than DA. One monoamine agonist directly alters the properties of endogenous burst-organizing potentials (driver potentials) in the motorneurons of the ganglion.

Relationship between burst properties and sensitivity to input: A theoretical analysis

This paper examines the sensitivity of endogenous bursters to a brief input pulse. The interneurons of the lobster cardiac ganglion were selected as a case study. Using a mathematical model specifically developed for the neurons in the cardiac ganglion of the lobster (Av-Ron et al., 1993), we show a tight link between burst characteristics and certain other parameters. We show that cells with different burst properties differ in their sensitivity to an input of a brief pulse. Irrespective of these differences, all cells display a bimodal response to a brief pulse applied during the quiescent period. During the first three-quarters of the quiescent period, they respond by producing a single spike at most. During the remaining one-quarter, the brief pulse can initiate the cells' intrinsic burst. Our predictions fit experimental results obtained by Tazaki and Cooke (1979). The results obtained herein are discussed with respect to fault tolerance considerations.

MODELING A SMALL NEURONAL NETWORK: THE LOBSTER CARDIAC GANGLION

The lobster cardiac ganglion is a nine cell network that stimulates the lobster heart. The individual neurons are modeled by a simple biophysical bursting model. The neurons are then connected to form the network. Theoretical experiments examine the role of individual cells and network connectivity, in bringing about the network behavior.

Heterogeneity of motorneuron driver potential properties along the anterior-posterior axis of the lobster cardiac ganglion

Endogenous long-duration burst-organizing potentials (driver potentials, DPs) generated by neurons in the lobster cardiac ganglion play a critical role in the ability of the system to generate rhythmic bursts of nerve impulses. The DPs are normally terminated by a voltage-dependent potassium current, but when this is suppressed by tetraethylammonium ion (TEA), the five motorneurons in the system show heterogeneity in properties. Perfusion with TEA increases DP amplitude in all cells in a similar fashion, but it increases DP duration disproportionately in the most anterior motorneurons. Substitution of barium ions for calcium also prolongs DPs in all of the motorneurons, but the effect of this treatment is not different along the anterior-posterior axis of the ganglion. The results suggest that the different behavior of the neurons reflects a difference in either the extent of calcium inactivation of the calcium current responsible for the DP, or in the kinetics or magnitude of a calcium-activated potassium conductance which contributes to membrane repolarization. In contrast to previously reported results, reduction in extracellular sodium ion concentration decreases driver potential amplitude and/or duration. This effect is not differentially expressed in different motorneurons.

The effects of glutamate agonists on voltage-clamped motoneurons of the lobster cardiac ganglion.

The effects of L-glutamate and its analogues were studied in voltage-clamped motoneurons of the lobster cardiac ganglion. These excitatory amino acids caused a dose-dependent increase in membrane conductance and an inward current at the resting membrane potential. The EC50 for L-glutamate was 150 mumol 1(-1). The rank order of potencies of the various agonists was quisqualate greater than L-glutamate = L-aspartate greater than kainate greater than cysteine. Kainate, unlike the other agonists, showed no desensitization. Of various antagonists studied, only the quinoxalinediones inhibited the response to glutamate. These antagonists also reduced the amplitude and duration of the pacemaker-driven burst potential, suggesting that glutamate may be released by some of the endogenous synapses within the ganglion. The reversal potential of the glutamate-induced current was -15 mV. When Na+ was replaced with K+, the glutamate-induced current still reversed between 0 and -20 mV. When Na+ was replaced with the impermeant ion N-methyl-D-glucamine, the current was inhibited. The amplitude of responses evoked by glutamate and its analogues was reduced in salines containing either high or low concentrations of Ca2+. These results of pharmacological and of reversal potential and ion substitution experiments indicate that glutamate acts on receptors of the non-NMDA (N-methyl-D-aspartate), quisqualate/kainate type to open a channel permeable to both Na+ and K+.

Histamine activates chloride conductance in motor neurons of the lobster cardiac ganglion.

1. Individual motor neurons of the lobster cardiac ganglion were voltage clamped with two microelectrodes. Superfusion of histamine evoked a concentration-dependent membrane current. The mean effective concentration (EC50) for the concentration-effect relationship was 28 microM. 2. The amplitude and polarity of the histamine-activated current depended on intracellular and extracellular Cl- concentration. The membrane potential at which the current polarity reversed was a function of the Cl- equilibrium potential. 3. The histamine-activated Cl- conductance was voltage dependent, increasing with depolarization. As a consequence, the histamine-evoked current showed outward rectification. 4. We conclude that histamine activates a Cl- conductance with biophysical properties similar to the crustacean Cl- conductance activated by gamma-aminobutyric acid (GABA) and to the histamine responses described in lobster olfactory and stomatogastric neurons. 5. The response to histamine was competitively inhibited (IC50 = 7 microM) by cimetidine, an H2 subtype inhibitor in mammals. Ranitidine, pyrilamine, chlorpheniramine, diphenhydramine, and cyproheptadine were 50-100 times less potent than cimetidine. Tubocurarine, a Cl- channel blocker, blocked with an IC50 of 20 microM, but picrotoxin did not begin to inhibit the histamine response until concentrations exceeded 0.1 mM. 6. These results suggest that the response cannot easily be classified with the use of the pharmacological categories developed in mammals. Like the Cl(-)-dependent responses to various neurotransmitters in a number of invertebrates, the histamine response in the lobster cardiac ganglion was inhibited by tubocurarine. 7. Both GABA and histamine had similar effects on the motor neurons, but only GABA inhibited pacemaker bursts. In this respect, GABA more resembles the endogenous inhibitory postsynaptic potential.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of γ-aminobutyric acid on voltage-clamped motoneurons of the lobster cardiac ganglion

1. We examined the electrophysiological and pharmacological effects of GABA on voltage-clamped motoneurons of the lobster cardiac ganglion. 2. GABA caused a dose-dependent current ( ec 50 = 0.7 mM) , which reversed at the estimated Cl − equilibrium potential. 3. The conductance activated by GABA was voltage-dependent, increasing as a non-linear function of depolarization. 4. A Na +-dependent GABA uptake mechanism was only weakly sensitive to nipecotic acid. 5.Picrotoxin inhibited the GABA response, but bicuculline had no effect. 6. We conclude that the effect of GABA in the lobster cardiac ganglion is similar to its effect on other crustacean neuromuscular tissues and on vertebrate GABA A receptors. 7. There appear to be differences among species with respect to the physiology and pharmacology of the Na +-dependent GABA transporter. 8. The effect of GABA is also similar to the ionic mechanism underlying the action of histamine in the cardiac ganglion.

The effect of subtype-selective muscarinic receptor antagonists on the cholinergic current in motoneurons of the lobster cardiac ganglion

Muscarinic agonists evoke a voltage-dependent inward current in motoneurons of the lobster cardiac ganglion. In this study, a number of drugs, known to show muscarinic receptor subtype selectivity in mammals, were used to determine the pharmacological profile of the muscarinic receptor on lobster motoneurons. The neurons were held under voltage-clamp, and various concentrations of the antagonists were applied in the presence of 1 mM methacholine. From competition curves plotting agonist-induced current against antagonist concentration, the inhibitor affinity constant and the slope factor were determined. The rank order of potencies of antagonists having an effect was: atropine > pirenzepine > 4-DAMP > methoctramine > HHSiD = (R)-HHD > (S)-HHD. Neither AF-DX 116 nor gallamine were effective at concentrations as high as 10 mM. The M 1-selective agonist McN-A-343 had no effect. Although this crustacean muscarinic receptor resembles the mammalian M 1 muscarinic receptor because of its relatively high affinity for pirenzepine, the rank order of other subtype-specific antagonist does not otherwise resembles that of any of the pharmacologically defined muscarinic receptors in mammals. It may be preferable, therefore, to use a term such as ‘pirenzepine-sensitive’ muscarinic receptor rather than M 1 or ‘M 1-like’ for invertebrte muscarinic receptors with pharmacological characteristics like those reported here.

Cholinergic activation of the lobster cardiac ganglion

The frequency of rhythmic burst activity of the isolated lobster cardiac ganglion is increased by exogenously applied acetylcholine and muscarinic agonists. Responses of individual motor neurons isolated from the ganglion by transection consist of a slow depolarization and repetitive bursting. The pharmacological profile of the receptors mediating this response is similar to that of vertebrate neuronal muscarinic receptors. Isolated ganglia incubated in the presence of [3H]-choline (18-19 h) exhibited radiolabelled acetylcholine accumulation. It is suggested that ganglionic excitation may be accomplished by extrinsic or intrinsic activation of muscarinic receptors on the motor neurons.

Characterization of Ca current underlying burst formation in lobster cardiac ganglion motorneurons

1. The anterior motorneurons of the cardiac ganglion of Homarus americanus were ligated less than 300 microns from the soma. This removes impulse-generating membrane and sites of synaptic input while preserving the ability of the soma to generate the burst-forming potentials termed "driver potentials" regenerative, slow (250-ms duration) depolarizations (to -20 mV) in response to brief, depolarizing stimuli. At stimulus intervals corresponding to rates of bursting observed in spontaneously active, intact ganglia (0.3-1.2/s), driver potential amplitude increases with increasing stimulus interval. 2. A two-electrode voltage clamp was used to characterize inward current observable from the ligated neurons in tetrodotoxin (TTX)-tetraethylammonium (TEA)-containing salines. The amplitude of inward current shows a hyperbolic relation to [Ca]o that is well fitted by a form of the Michaelis-Menten equation. Inward current is maintained but not augmented when Ca2+ is replaced by Ba2+ or Sr2+. It is concluded that the inward current, to be referred to as ICa, is mediated by voltage-dependent Ca channels. 3. Contamination of ICa by early outward current (IA) was evaluated by addition of 4-aminopyridine (4-AP, 4 mM). In the presence of 4-AP, the net inward current is increased and the potential at which maximum ICa occurs is shifted 10 mV more positive. 4. Subtraction of outward currents recorded in Mn2(+)-containing saline from overall currents in the absence of Mn2+ provided another means to separate inward from outward current. I-V curves from such "Mn-subtracted" records show ICa approaches a saturating value for steps to -5 mV and more depolarized. The time to peak ICa is voltage dependent. The largest inward currents (up to 240 nA) and minimal time to peak (4 ms) are observed for steps from holding potentials of -50 to -60 mV. 5. Decline of ICa during depolarized steps observed in Mn-subtracted records represents inactivation rather than development of competing outward current. Inactivation is slow and incomplete; the rate and fractional amount of inactivation are not directly voltage dependent. Nonsubtracted responses to 500-ms depolarizations to potentials evoking little outward current show that an initial rapid decline of ICa (tau approximately 40 ms) is followed at approximately 80 ms by a slower phase of decline (tau approximately 180 ms). With repetitive clamps, the early phase proved labile.(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane current underlying muscarinic cholinergic excitation of motoneurons in lobster cardiac ganglion.

1. We studied the effect of cholinergic agonists on motoneurons of the lobster cardiac ganglion under voltage clamp. 2. In unclamped neurons, acetylcholine (ACh) caused a depolarization and increase in burst potential frequency. By the use of nicotinic and muscarinic agonists, we determined that both types of receptors are present on the neurons. We therefore used specific muscarinic agonists to further study ionic mechanisms underlying the muscarinic cholinergic current (Imch). 3. Muscarinic agonists produced detectable inward current at doses above 10(-6) M, and maximum effect was seen at doses above 10(-3) M. 4. Imch was voltage-dependent. When the membrane holding potential was shifted to levels negative to the resting potential, the response declined, nulling but not reversing at -80 to -100 mV. The response enlarged with membrane depolarization, reaching a maximum at between -30 and -10 mV. With further depolarization, the response declined and then reversed at potentials around +20 mV. 5. The muscarinic response varied as a function of extracellular Na+ concentration and was completely blocked in Na+-free solutions. The relationship between response amplitude and external Na+ was well described by the electrodiffusion equation for Na+ driving force. 6. Imch amplitude also varied as a function of extracellular potassium concentration, becoming larger with low external K+ and smaller at higher concentrations. Shifting the Cl- equilibrium potential did not affect the properties of the Imch. 7. Tetrodotoxin (TTX) had no effect on Imch. In concentrations of 1-10 mM, such K+-channel blocking agents as Ba2+, Cs+, 4-aminopyridine (4-AP), or tetraethylammonium (TEA), and such Ca2+-channel blockers as Co2+ or Mn2+, when applied externally, did not suppress Imch. Above 30 mM, TEA did inhibit the response, and combinations of K+-channel blocking agents, each at concentrations insufficient alone to block the current, also inhibited Imch. 8. Current-voltage (I-V) curves obtained during muscarinic agonist perfusion consistently crossed the control I-V curves at a mean membrane potential of +24 mV. The reversal potential shifted to a more negative value in low extracellular Na+. 9. Although no reversal of Imch was seen when agonists were applied to cells clamped at negative holding potentials, the averaged curve of Imch, obtained by subtracting control ramp I-V curves from those obtained in the presence of agonist, did show a small net outward current at membrane potentials negative to -100 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Feedback from Motor Neurones to Pacemaker Neurones in Lobster Cardiac Ganglion Contributes to Regulation of Burst Frequency

ABSTRACT The crustacean cardiac ganglion has traditionally been viewed as a two-layered system in which pacemaking is a function of the four small cells which trigger the five follower motor neurones via chemical and electrotonic synaptic excitation. The work reported here shows that there is strong feedback from motor neurones to small cells, by which endogenous burst-organizing potentials (driver potentials or DPs) and their hyperpolarizing afterpotentials contribute to regulation of bursting frequency. Isolated cardiac ganglia were placed in a two-chamber perfusion system which allowed independent treatment of small cells and motor neurones. When the motor neurones were silenced with tetrodotoxin (TTX), the small cells continued organizing bursts of activity which recurred at an average frequency 41 % higher than bursting by normal ganglia in saline. The average burst duration was not altered. Driver potentials were evoked in TTX-treated motor neurones by electrical stimulation, by ionic alteration of the medium, or by treatment with the cardioexcitor peptide proctolin. DPs which occurred synchronously with small-cell bursts prolonged and intensified the bursts (more spikes per burst). When DPs were evoked in motor neurones during the interburst interval, they triggered small-cell bursts even at very short intervals after a spontaneous burst had occurred. All small-cell bursts which were associated with motor neurone DPs were followed by interburst intervals of longer than normal duration. The decrease in instantaneous burst frequency (increase in total burst cycle duration) caused by motor neurone DPs whs similar in magnitude to the drop in burst frequency observed when the ganglion recovered normal activity after T1X washout.

Endogenous burst-organizing potentials in two classes of neurons in the lobster cardiac ganglion respond differently to alterations in divalent ion concentration

1. Crustacean cardiac ganglion neurons generate endogenous burst-organizing potentials termed ‘driver potentials’. In the five motorneurons (‘large cells’) in the system these are slow local potentials with the depolarizing current carried by calcium ions. The existence of similar endogenous potentials in the second class of neurons in the ganglion, the four small ‘pacemaker’ cells, has been postulated primarily on the basis of indirect evidence. The results presented here show that burst-generating mechanisms in the pacemaker neurons and motorneurons respond differently to alterations in the ionic content of the medium, and suggest that driver potentials have different ionic bases in the two cell types. 2. Bursting in isolated lobster (Homarus americanus) cardiac ganglia persists in altered form in salines containing high levels of manganese (Fig. 1) or reduced calcium (Figs. 3, 4). Both of these ionic alterations suppress driver potentials generated by the motorneurons (Fig. 2). Bursts generated in low-calcium salines were of very long duration. 3. In low-calcium salines the long-duration bursts resulted from prolonged and intensified discharge of the small pacemaker neurons; motorneuron activity was suppressed as would be predicted from blockade of the large cell driver potentials. These conclusions derive from the use of ligatured preparations, in which a large cell group was functionally isolated from the rest of the ganglion (Fig. 5); from two-pool preparations in which the two classes of cells were perfused independently in different ionic media (Fig. 6); and from ganglia in which one or the other cell type was silenced by selective application of tetrodotoxin (Figs. 7, 8). 4. These experiments support the idea that driver potentials in the motorneurons are primarily based on inflow of calcium ions, but that a significant proportion of the inward current responsible for the driver potentials in the small pacemaker neurons is carried by an ion other than calcium. 5. The results support current models for the roles of endogenous properties and network interactions in determining the pattern of impulse production by the motorneurons; however, the synaptic mechanisms by which the pacemakers influence the motorneurons, which has previously been thought to be mainly chemical in lobsters, appears to contain a significant electrical component.

The cardio-regulator nerves of the hermit crabs: anatomical and electrophysiological identification of their distribution inside the heart

1. The cardiac ganglion and the routes of the cardio-regulator nerves inside the heart were investigated anatomically and electrophysiologically in the hermit crabs (Aniculus aniculus andDardanus crassimanus). It was proved that the dorsal cardiac nerve containing three regulator axons on each side, one inhibitor and two accelerators, pierced the heart wall and connected with the cardiac ganglion. 2. The cardiac ganglion consisted of five large neurons and four small neurons. Configuration of the cardiac ganglion was analogous to the crab (Cancer orPortunus) cardiac ganglion in having a pair of circular trunks. However, arrangement of the large cells and the small cells was analogous to the lobster (Homarus, Panulirus orPalinurus) cardiac ganglion in alignment of the ganglionic cells. 3. The two acceleratory nerves had almost identical innervating routes. The route of the inhibitory nerve was significantly different from the routes of the acceleratory nerves inside the heart. 4. The inhibitory axon had the route only along the main trunk of the cardiac ganglion. IPSPs were recorded from large ganglionic cell bodies, one-toone to inhibitory impulses. 5. The acceleratory axons were carried not only in the main trunk but also in the circular trunk. There were acceleratory branches extending toward the periphery away from both the trunks. 6. EPSPs were recorded from large cell bodies in a one-to-one relationship to acceleratory stimuli. This provided direct evidence for ordinary synapses of the accelerator on the large ganglionic cells.

Coupling ratio of the Na-K pump in the lobster cardiac ganglion.

The electrogenic Na-K pump coupling ratio in the large neurons of the lobster cardiac ganglion was determined by two different electrophysiological techniques. A graphical analysis plotting exp(EmF/RT) vs. [K]o after the pump was blocked by ouabain was used to determine values for [K]i, PNa/PK, and the pump coupling ratio. These measurements were made 4-8 h after the cells were penetrated with microelectrodes, and thus represent non-Na-loaded steady state values. The value obtained for the pump coupling ratio under these conditions was 1.44 +/- 0.06 (n = 9) or close to 3 Na for 2 K. The second technique used to measure the coupling ratio was to iontophoretically inject Na ions into the neuron. Neurons were penetrated with three microelectrodes, two of which were filled with 2 M Na-citrate; the third electrode contained either 2 M K-citrate or 3 M KCl. By passing current between the Na salt-containing electrodes, Na was injected into the cell soma. The injection system was calibrated by injecting 24Na-citrate into counting vials from representative microelectrodes (calculated 24Na transport = 0.92). By knowing the Na load injected into the cells, and by measuring the time-current area produced by the Na activation of the Na-K pump, the coupling ratio was calculated to be 1.54 +/- 0.05 (n = 19), which is not significantly different from the value obtained by the first method. This value represents a Na-loaded experimental situation. When Na was removed from the external bathing solution, the coupling ratio shifted to 2 Na to 1 K (2.0 +/- 0.07, n = 4). These results suggest that the pump normally operates with a 3:2 ratio both in steady state and under Na load but that in the absence of external Na, it can operate with less than a full complement (2) of K on the external surface of the pump.

The identification and localization of a catecholamine in the motor neurons of the lobster cardiac ganglion.

The cardiac ganglion from Homarus americanus was investigated for the purpose of providing biochemical and histochemical information as to the identity of the neurotransmitter(s) utilized by this system. Three techniques were employed in this study: (1) the glyoxylic acid histofluorescence staining technique (GA), which showed fluorescence characteristic of catecholamines localized in the five motor neurons; (2) high-voltage electrophoresis (HVE) in one dimension followed by ascending chromatography in the second dimension, which indicated incorporation of label from tritiated tyrosine into norepinephrine (NE) and small amounts of dopamine (DA); (3) high-pressure liquid chromatography with electrochemical detection (HPLC/EC), which indicated the presence of endogenous norepinephrine.

Some effects of proctolin on the cardiac ganglion of the Maine Lobster, Homarus americanus (Milne Edwards).

The neuropeptide proctolin has excitatory effects on the isolated lobster cardiac ganglion. Selective application to the anterior cell body region produces a dose-dependent (10(-8)--10(-5) M) prolonged depolarization of large anterior cells as well as marked increases in burst frequency and/or duration. In ganglia which have been silenced with tetrodotoxin, proctolin application to anterior cells elicits long-lasting depolarizing responses which are accompanied by a 10-30% increase of the apparent membrane input resistance. Higher proctolin concentrations produce high-frequency trains of driver potentials. It is proposed that a proctolin like peptide may serve a neurohumoral role in the lobster cardiac ganglion and that the anterior motor neurons exhibit endogenous rhythmicity in its presence.

Cyclic Adenosine Monophosphate Mediation of Peptide Neurohormone Effects on the Lobster Cardiac Ganglion

ABSTRACT Two cardioexcitors which can be extracted or released from crustacean neurohaemal structures, the pericardial organs, alter the endogenous bursting activity of isolated lobster cardiac ganglia in different ways. A small peptide increases the burst frequency of the ganglion, the duration of each burst, and the number of spikes per burst, while 5-hydroxytryptamine (5-HT) increases burst frequency, but decreases the burst duration and number of spikes per burst. Four lines of evidence are presented that cyclic adenosine monophosphate (cAMP) mediates the effects of peptide on the ganglion, but does not play a significant role in 5-HT action: (1) exogenous cAMP mimics the effects of peptide but not of 5-HT on burst parameters; (2) inhibition of the enzyme phosphodiesterase, which increases endogenous levels of cAMP, mimics the effects of peptide and potentiates peptide action but not that of 5-HT; (3) putative blockers of adenylate cyclase block the action of peptide but not of 5-HT; (4) peptide causes a large increase in cAMP content of the ganglion, while 5-HT application results in a much smaller increase.