Signaling of Pigment-Dispersing Factor (PDF) in the Madeira Cockroach Rhyparobia maderae

The insect neuropeptide pigment-dispersing factor (PDF) is a functional ortholog of vasoactive intestinal polypeptide, the coupling factor of the mammalian circadian pacemaker. Despite of PDF''s importance for synchronized circadian locomotor activity rhythms its signaling is not well understood. We studied PDF signaling in primary cell cultures of the accessory medulla, the circadian pacemaker of the Madeira cockroach. In Ca²⁺ imaging studies four types of PDF-responses were distinguished. In regularly bursting type 1 pacemakers PDF application resulted in dose-dependent long-lasting increases in Ca²⁺ baseline concentration and frequency of oscillating Ca²⁺ transients. Adenylyl cyclase antagonists prevented PDF-responses in type 1 cells, indicating that PDF signaled via elevation of intracellular cAMP levels. In contrast, in type 2 pacemakers PDF transiently raised intracellular Ca²⁺ levels even after blocking adenylyl cyclase activity. In patch clamp experiments the previously characterized types 1–4 could not be identified. Instead, PDF-responses were categorized according to ion channels affected. Application of PDF inhibited outward potassium or inward sodium currents, sometimes in the same neuron. In a comparison of Ca²⁺ imaging and patch clamp experiments we hypothesized that in type 1 cells PDF-dependent rises in cAMP concentrations block primarily outward K⁺ currents. Possibly, this PDF-dependent depolarization underlies PDF-dependent phase advances of pacemakers. Finally, we propose that PDF-dependent concomitant modulation of K⁺ and Na⁺ channels in coupled pacemakers causes ultradian membrane potential oscillations as prerequisite to efficient synchronization via resonance.     

Using embryonic stem cells to form a biological pacemaker via tissue engineering technology

Biological pacemakers can be achieved by various gene‐based and cell‐based approaches. Embryonic stem cells (ESCs)‐derived pacemaker cells might be the most promising way to form biological pacemakers, but there are challenges as to how to control the differentiation of ESCs and to overcome the neoplasia, proarrhythmia, or immunogenicity resulting from the use of ESCs. As a potential approach to solve these difficult problems, tissue‐engineering techniques may provide a precise control on the different cell components of multicellular aggregates and the forming of a construct with‐defined architectures and functional properties. The combined interactions between ESC‐derived pacemaker cells, supporting cells, and matrices may completely reproduce pacemaker properties and result in a steady functional unit to induce rhythmic electrical and contractile activities. As ESCs have a high capability for self‐renewal, proliferation, and potential differentiation, we hypothesize that ESCs can be used as a source of pacemaker cells for tissue‐engineering applications and the ambitious goal of biological cardiac pacemakers may ultimately be achieved with ESCs via tissue‐engineering technology.     

Redox control of cardiac rhythm

The rhythm of cardiac beats is generated by pacemaker cells differing from other cardiomyocytes by the presence of slow diastolic depolarization. Consistently activated transmembrane ionic currents provide cyclic excitation of pacemakers, forming the original “membrane clocks”. A new concept has been forwarded in the last decade according to which periodic fluctuations in myoplasmic Ca²⁺ level (“calcium clocks”) not only influence a course of “membrane clocks”, but they also can serve as independent generators of the rhythm. Transport of Ca²⁺ in cells is under constant influence of active forms of oxygen and nitrogen. Both superoxide and NO in moderate doses facilitate Ca²⁺ output from the sarcoplasmic reticulum, accelerating the course of “calcium clocks”, but in higher doses they have opposite effect that may be neutralized mainly by reduced glutathione. The control of cardiac rhythm by active forms of oxygen and nitrogen represents a feedback mechanism by which mitochondria and NO-synthases support Ca²⁺ homeostasis in cells that can be temporarily disturbed under mechanical loads or hypoxia.     

The vicissitudes of the pacemaker current <Emphasis Type="Italic">I</Emphasis><Subscript>Kdd</Subscript> of cardiac purkinje fibers

Summary The mechanisms underlying the pacemaker current in cardiac tissues is not agreed upon. The pacemaker potential in Purkinje fibers has been attributed to the decay of the potassium current I _Kdd. An alternative proposal is that the hyperpolarization-activated current I _f underlies the pacemaker potential in all cardiac pacemakers. The aim of this review is to retrace the experimental development related to the pacemaker mechanism in Purkinje fibers with reference to findings about the pacemaker mechanism in the SAN as warranted. Experimental data and their interpretation are critically reviewed. Major findings were attributed to K⁺ depletion in narrow extracellular spaces which would result in a time dependent decay of the inward rectifier current I _K1. In turn, this decay would be responsible for a “fake” reversal of the pacemaker current. In order to avoid such a postulated depletion, Ba²⁺ was used to block the decay of I _K1. In the presence of Ba²⁺ the time-dependent current no longer reversed and instead increased with time and more so at potentials as negative as −120 mV. In this regard, the distinct possibility needs to be considered that Ba²⁺ had blocked I _Kdd (and not only I _K1). That indeed this was the case was demonstrated by studying single Purkinje cells in the absence and in the presence of Ba²⁺. In the absence of Ba²⁺, I _Kdd was present in the pacemaker potential range and reversed at E _K. In the presence of Ba²⁺, I _Kdd was blocked and I _f appeared at potentials negative to the pacemaker range. The pacemaker potential behaves in a manner consistent with the underlying I _Kdd but not with I _f. The fact that I _f is activated on hyperpolarization at potential negative to the pacemaker range makes it suitable as a safety factor to prevent the inhibitory action of more negative potentials on pacemaker discharge. It is concluded that the large body of evidence reviewed proves the pacemaker role of I _Kdd (but not of I _f) in Purkinje fibers.     

Sick Sinus Syndrome: Experience of a Cardiac Pacemaker Clinic

Out of a pacemaker clinic population of 182 patients 21 (11·5%) were found to have the sick sinus syndrome. Their ages ranged from 30 to 80 years and averaged 62. Ischaemic heart disease was more commonly an aetiological factor than in patients with chronic atrioventricular heart block. Other aetiologies were familial cardiomyopathy, postcardiac surgery, and dystrophia myotonica.Cardioversion before pacemaker insertion was a hazardous procedure. After insertion the occurrence of tachycardias and the need for drug therapy were reduced. One patient no longer required a pacemaker once atrial fibrillation became established.A high incidence of cerebral embolization was observed and the use of anticoagulant drugs therefore merits serious consideration.Failure of inhibition of demand-type pacemakers occurred in two patients. Two patients who exhibited competition with fixed-rate pacemakers died. Two patients were treated with electrodes surgically implanted on the right atrium. It is suggested that fixed-rate pacemakers may be contra-indicated and that sequential atrioventricular demand pacing is theoretically ideal.     

Differences between Pacemaker and Nonpacemaker Neurons of Aplysia on Voltage Clamping

The responses of pacemaker and nonpacemaker Aplysia neurons to voltage clamp commands of less than 200 msec duration are essentially identical. For moderate depolarizing commands there is an early inward transient current followed by a late outward current and an outward tail current when the membrane is clamped back to resting potential. On long (1–2 sec) commands in pacemakers there is a marked sag in the late current and an inward tail current. E_tail, the potential of the membrane at which there is no net current flow under the conditions prevailing at the end of the clamp, shifts from about -9.0 mv on short commands to +5.0 mv on long commands. In contrast there is no marked sag of the late current or inward tail current on long commands in nonpacemakers, and E_tail is near -9.0 mv for both short and long commands. The current sag and shift in E_tail can be ascribed to a decreased conductance (presumably to K⁺) at the end of the long as compared to the short command in half of the pacemaker neurons. In the remaining cells the essential difference from nonpacemakers appears to be either a greater restricted extracellular space or a more active potential-dependent electrogenic Na⁺ pump in pacemakers.     

Sex differences in selection of pacemakers: retrospective observational study

Objective: To evaluate the effect of patients’ sex on selection of pacemakers. Design: Retrospective univariate and multivariate analysis of a large database. Setting: German central pacemaker register. Subjects: Records collected at the register for 1992 and 1993 (n=31 913), covering 64% of all implantations in Germany. Main outcome measure: Probability of receiving a single chamber, dual chamber, or rate responsive pacemaker in relation to sex. Results: Univariate analysis showed that women were more likely to receive single chamber pacemakers and less likely to receive dual chamber or rate responsive systems than men. After demographic and clinical variables were controlled for, women were still more likely to receive a single chamber system (atrial pacing: odds ratio 0.89, 95% confidence interval 0.74 to 1.07; ventricular pacing: 0.85, 0.80 to 0.92) and less likely to receive a dual chamber (1.20, 1.12 to 1.30) or a rate responsive system (1.26, 1.17 to 1.37) than men. Conclusions: The data suggest sex differences in the selection of a pacemaker system which cannot be explained by the underlying cardiac disorder. Further research is needed to evaluate why guidelines for implanting pacemakers are not better adhered to.

Key messages

• Use of pacemakers varies despite guidelines, and the reasons for this are unclear
• In this study women were more likely to receive single chamber pacemakers and less likely to receive dual chamber and rate responsive pacemakers than men
• Demographic and clinical variables cannot fully explain these differences
• Prospective studies are needed to evaluate the effect of sex and other non-medical variables on the selection of pacemakers

     

Electromagnetic interference on pacemakers

External sources, either within or outside the hospital environment, may interfere with the appropriate function of pacemakers which are being implanted all around the world in current medical practice. The patient and the physician who is responsible for follow-up of the pacing systems may be confronted with some specific problems regarding the various types of electromagnetic interference (EMI). To avoid these unwanted EMI effects one must be aware of this potential problem and need to take some precautions. The effects of EMI on pacemaker function and precautions to overcome some specific problems were discussed in this review article. There are many sources of EMI interacting with pacemakers. Magnetic resonance imaging creates real problem and should be avoided in pacemaker patients. Cellular phones might be responsible for EMI when they were held on the same side with the pacemaker. Otherwise they don't cause any specific type of interaction with pacemakers. Sale security systems are not a problem if one walks through it without lingering in or near it. Patients having unipolar pacemaker systems are prone to develop EMI because of pectoral muscle artifacts during vigorous active physical exercise.     

Gender Differences in Selection of Pacemakers: A Single-Center Study

Background: Previous studies have reported gender differences in pacemaker selection.Objective: This study aimed to assess gender-related differences in pacemaker mode selection in patients undergoing their first implantation.Methods: A retrospective analysis was undertaken from a single-center database of pacemaker implants during the years 2001 to 2003. Univariate and multivariate analyses were used to compare pacemaker mode selection adjusted for any significant difference between the sexes.Results: A total of 274 pacemakers were implanted during the study period, 259 of which formed the basis of this study. Of the patients receiving pacemakers, 132 were male and 127 were female. The majority of patients (144 [55.6%]) had sick sinus syndrome as their indication for receiving a pacemaker, followed by complete heart block (75 [29.0%]), and second-degree or high-grade atrio-ventricular block (36 [13.9%]). Four (1.5%) patients had hypersensitive carotid sinus syndrome. The mean (SD) age of patients was 61.35 (15) years. Most (155 [59.8%]) patients were younger than age 65, and the women were significantly older than the men (P = 0.004). Atrial fibrillation (AF) was present in 53 (20.5%) patients. Dual-chamber (DDD) pacemakers were implanted in 196 (75.7%) patients, and single-chamber ventricular pacemakers in 63 (24.3%) patients. Significantly more DDD pacemakers were implanted in patients aged <65 years compared with those aged ≥65 years (P < 0.01). This difference was, however, primarily due to the higher rate of AF in the older patients versus patients aged <65 years. Although the rate of DDD implantations was observed to be higher among women (101/127 [79.5%]) compared with men (95/132 [72.0%]), the rate difference was nonsignificant, even when adjusted for the significant age difference between the sexes. Furthermore, DDD selection was independent of patients' health insurance status.Conclusions: We found no significant difference in pacemaker mode selection between male and female patients. However, we did find that patients aged <65 years were more likely to have DDD pacemakers implanted compared with older patients. This age-dependent difference was primarily due to the higher prevalence of AF in the older age group versus the younger patients.Key words:pacemaker implantgender differencegender and pacemaker selection     

Organization of the Circadian System in Insects

The circadian systems of different insect groups are summarized and compared. Emphasis is placed on the anatomical identification and characterization of circadian pacemakers, as well as on their entrainment, coupling, and output pathways. Cockroaches, crickets, beetles, and flies possess bilaterally organized pacemakers in the optic lobes that appear to be located in the accessory medulla, a small neuropil between the medulla and the lobula. Neurons that are immunoreactive for the peptide pigment-dispersing hormone (PDH) arborize in the accessory medulla and appear to be important components of the optic lobe pacemakers. The neuronal architecture of the accessory medulla with associated PDH-immunoreactive neurons is best characterized in cockroaches, while the molecular machinery of rhythm generation is best understood in fruit flies. One essential component of the circadian clock is the period protein (PER), which colocalizes with PDH in about half of the fruit fly's presumptive pacemaker neurons. PER is also found in the presumptive pacemaker neurons of beetles and moths, but appears to have different functions in these insects. In moths, the pacemakers are situated in the central brain and are closely associated with neuroendocrine functions. In the other insects, neurons associated with neuroendocrine functions also appear to be closely coupled to the optic lobe pacemakers. Some crickets and flies seem to possess central brain pacemakers in addition to their optic lobe pacemakers. With respect to neuronal organization, the circadian systems of insects show striking similarities to the vertebrate circadian system. (Chronobiology International, 15(6), 567-594, 1998)     

All-or-none control of the bursting properties of the pacemaker neurons of the labster pyloric pattern generator

In Crustacea the central pattern generator for the pyloric motor rhythm (filtration to the midgut) is known to be located within the stomatogastric ganglion (STG); its cycling activity is known to be organized by three endogenous burster neurons acting as pacemakers and driving 11 follower neurons. In Homarus, recordings from the isolated stomatogastric nervous system (Fig. 1) indicate that (1) the pyloric output can be generated only when the STG is afferented (i.e., connected to the more rostral oesophageal and commissural ganglia) (Fig. 2) and (2) the deafferntation of the STG results in a complete loss of the bursting properties of the pacemaker neurons (Fig. 4). Manipulation of the STG inputs responsible for unmasking the properties of the pacemakers strongly suggests that (1) they are not phasic inputs (Fig. 5) and (2) they are long-term acting inputs (Fig. 6). These results provide evidence for a neural all-or-none control of the bursting properties of the pacemaker neurons of a motor pattern generator.     

Effects of transient receptor potential channel blockers on pacemaker activity in interstitial cells of Cajal from mouse small intestine

The interstitial cells of Cajal (ICCs) are pacemakers in the gastrointestinal tract and transient receptor potential melastatin type 7 (TRPM7) is a candidate for pacemaker channels. The effect of the 5-lipoxygenase (5-LOX) inhibitors NDGA, AA861, MK886 and zileuton on pacemaking activity of ICCs was examined using the whole cell patch clamp technique. NDGA and AA861 decreased the amplitude of pacemaker potentials in ICC clusters, but the resting membrane potentials displayed little change, respectively. Also, perfusing NDGA and AA861 into the bath reduced both inward current and outward current in TRPM7-like current in single ICC, respectively. But, they had no effects on Ca²⁺ activated Cl⁻ currents. The 5-LOX inhibitors MK886 and zileuton were, however, ineffective in pacemaker potentials in ICC clusters and in TRPM7-like current in single ICC, respectively. A specific TRPC3 inhibitor, pyrazole compound (Pyr3), and a specific TRPM4 inhibitor, 9-phenanthrol, had no effects in pacemaker potentials in ICC clusters and in TRPM7-like current in single ICC. These results suggest that, among the tested 5-LOX inhibitors, NDGA and AA861 modulate the pacemaker activities of the ICCs, and that the TRPM7 channel can affect intestinal motility.     

Sphingosine and FTY720 modulate pacemaking activity in interstitial cells of Cajal from mouse small intestine

Interstitial cells of Cajal (ICCs) are the pacemakers of the gastrointestinal tract, and transient receptor potential melastatin type 7 (TRPM7) and Ca²⁺ activated Cl⁻ channels (ANO1) are candidate the generators of pacemaker potentials in ICCs. The effects of D-erythro-sphingosine (SPH) and structural analogues of SPH, that is, N,N-dimethyl-Derythro-sphingosine (N,N-DMS), FTY720, and FTY720-P on the pacemaking activities of ICCs were examined using the whole cell patch clamp technique. SPH, N,N-DMS, and FTY720 decreased the amplitudes of pacemaker potentials in ICC clusters, but resting membrane potentials displayed little change. Also, perfusing SPH, N,N-DMS, or FTY720 in the bath reduced both inward and outward TRPM7-like currents in single ICCs, and inhibited ANO1 currents. The another structural analogue of SPH, FTY720-P was ineffective at the pacemaker potentials in ICC clusters and the TRPM7-like currents in single ICCs. Furthermore, FTY720- P had no effect on ANO1. These results suggest that SPH, N,N-DMS, and FTY720 modulate the pacemaker activities of ICCs, and that TRPM7 and ANO1 channels affect intestinal motility.     

Reuse Of Pacemakers In Ghana And Nigeria: Medical,Legal, Cultural And Ethical Perspectives

According to the World Health Organization (WHO) cardiovascular disease (CVD) is the leading cause of death globally. Over 80% of CVD deaths take place in low‐ and middle‐income countries (LMICs). It is estimated that 1 million to 2 million people worldwide die each year due to lack of access to an implantable cardiac defibrillator (ICD) or a pacemaker. Despite the medical, legal, cultural and ethical controversies surrounding the pacemaker reutilization, studies done so far on the reuse of postmortem pacemakers show it to be safe and effective with an infection rate of 1.97% and device malfunction rate of 0.68%. Pacemaker reutilization can be effectively and safely done and does not pose significant additional risk to the recipient. Heart patients with reused pacemakers have an improved quality of life compared to those without pacemakers. The thesis of this paper is that pacemaker reutilization is a life‐saving initiative in LMICs of Nigeria and Ghana. It is cost effective; consistent with the principles of beneficence, nonmaleficence, and justice with a commitment to stewardship of resources and the Common Good. Used pacemakers with adequate battery life can be properly sterilized for use by patients in LMICs who cannot afford the cost of a new pacemaker.     

Reciprocal interaction between IK1 and If in biological pacemakers: A simulation study

Pacemaking dysfunction (PD) may result in heart rhythm disorders, syncope or even death. Current treatment of PD using implanted electronic pacemakers has some limitations, such as finite battery life and the risk of repeated surgery. As such, the biological pacemaker has been proposed as a potential alternative to the electronic pacemaker for PD treatment. Experimentally and computationally, it has been shown that bio-engineered pacemaker cells can be generated from non-rhythmic ventricular myocytes (VMs) by knocking out genes related to the inward rectifier potassium channel current (I_K1) or by overexpressing hyperpolarization-activated cyclic nucleotide gated channel genes responsible for the “funny” current (I_f). However, it is unclear if a bio-engineered pacemaker based on the modification of I_K1- and I_f-related channels simultaneously would enhance the ability and stability of bio-engineered pacemaking action potentials. In this study, the possible mechanism(s) responsible for VMs to generate spontaneous pacemaking activity by regulating I_K1 and I_f density were investigated by a computational approach. Our results showed that there was a reciprocal interaction between I_K1 and I_f in ventricular pacemaker model. The effect of I_K1 depression on generating ventricular pacemaker was mono-phasic while that of I_f augmentation was bi-phasic. A moderate increase of I_f promoted pacemaking activity but excessive increase of I_f resulted in a slowdown in the pacemaking rate and even an unstable pacemaking state. The dedicated interplay between I_K1 and I_f in generating stable pacemaking and dysrhythmias was evaluated. Finally, a theoretical analysis in the I_K1/I_f parameter space for generating pacemaking action potentials in different states was provided. In conclusion, to the best of our knowledge, this study provides a wide theoretical insight into understandings for generating stable and robust pacemaker cells from non-pacemaking VMs by the interplay of I_K1 and I_f, which may be helpful in designing engineered biological pacemakers for application purposes.     

Efficacy and safety of leadless pacemaker: A systematic review,pooled analysis and meta-analysis

BackgroundLeadless pacemakers have been designed as an alternative to transvenous systems which avoid some of the complications associated with transvenous devices. We aim to perform a systematic review of the literature to report the safety and efficacy findings of leadless pacemakers.MethodsWe searched MEDLINE and EMBASE to identify studies reporting the safety, efficacy and outcomes of patients implanted with a leadless pacemaker. The pooled rate of adverse events was determined and random-effects meta-analysis was performed to compare rates of adverse outcomes for leadless compared to transvenous pacemakers.ResultsA total of 18 studies were included with 2496 patients implanted with a leadless pacemaker and success rates range between 95.5 and 100%. The device or procedure related death rate was 0.3% while any complication and pericardial tamponade occurred in 3.1% and 1.4% of patients, respectively. Other complications such as pericardial effusion, device dislodgement, device revision, device malfunction, access site complications and infection occurred in less than 1% of patients. Meta-analysis of four studies suggests that there was no difference in hematoma (RR 0.67 95%CI 0.21–2.18, 3 studies), pericardial effusion (RR 0.59 95%CI 0.15–2.25, 3 studies), device dislocation (RR 0.33 95%CI 0.06–1.74, 3 studies), any complication (RR 0.44 95%CI 0.17–1.09, 4 studies) and death (RR 0.45 95%CI 0.15–1.35, 2 studies) comparing patients who received leadless and transvenous pacemakers.ConclusionLeadless pacemakers are safe and effective for patients who have an indication for single chamber ventricular pacing and the findings appear to be comparable to transvenous pacemakers.     

心室细胞生成的生物起搏器的仿真研究

植入电子起搏器可以治疗因窦房结功能失常引起的猝死等心脏疾病.但是,电子起搏器存在很多弊端,比如电池寿命有限、容易感染等.因此,生物起搏器被期待能够取代电子起搏器.为了探讨在心室内诱导心室细胞生成生物起搏器的可行性,我们首先抑制内向整流钾电流(I_(K1)),使心室肌细胞产生起搏行为,然后基于理想心室组织和真实人体心室切片数据,构建2D生物起搏器模型.基于该模型,我们研究细胞间的电偶联和起搏电流I_f对起搏器功能的影响.发现起搏电流I_f对起搏器功能有增强作用,但细胞间的弱电偶联对起搏器的起搏有更为关键的影响.     

Low frequency magnetic emissions and resulting induced voltages in a pacemaker by iPod portable music players

Background

Recently, malfunctioning of a cardiac pacemaker electromagnetic, caused by electromagnetic interference (EMI) by fields emitted by personal portable music players was highly publicized around the world. A clinical study of one patient was performed and two types of interference were observed when the clinicians placed a pacemaker programming head and an iPod were placed adjacent to the patient's implanted pacemaker. The authors concluded that "Warning labels may be needed to avoid close contact between pacemakers and iPods". We performed an in-vitro study to evaluate these claims of EMI and present our findings of no-effects" in this paper.

Methods

We performed in-vitro evaluations of the low frequency magnetic field emissions from various models of the Apple Inc. iPod music player. We measured magnetic field emissions with a 3-coil sensor (diameter of 3.5 cm) placed within 1 cm of the surface of the player. Highly localized fields were observed (only existing in a one square cm area). We also measured the voltages induced inside an 'instrumented-can' pacemaker with two standard unipolar leads. Each iPod was placed in the air, 2.7 cm above the pacemaker case. The pacemaker case and leads were placed in a saline filled torso simulator per pacemaker electromagnetic compatibility standard ANSI/AAMI PC69:2000. Voltages inside the can were measured.

Results

Emissions were strongest (≈ 0.2 μT pp) near a few localized points on the cases of the two iPods with hard drives. Emissions consisted of 100 kHz sinusoidal signal with lower frequency (20 msec wide) pulsed amplitude modulation. Voltages induced in the iPods were below the noise level of our instruments (0.5 mV pp in the 0 - 1 kHz band or 2 mV pp in the 0 - 5 MHz bandwidth.

Conclusion

Our measurements of the magnitude and the spatial distribution of low frequency magnetic flux density emissions by 4 different models of iPod portable music players. Levels of less than 0.2 μT exist very close (1 cm) from the case. The measured voltages induced inside an 'instrumented-can' pacemaker were below the noise level of our instruments. Based on the observations of our in-vitro study we conclude that no interference effects can occur in pacemakers exposed to the iPod devices we tested.     

The amplitude of circadian oscillations: temperature dependence, latitudinal clines, and the photoperiodic time measurement.

This paper develops several propositions concerning the lability of the amplitude of Drosophila circadian pacemakers. The first is that the amplitude of the pacemaker's motion, unlike its period, is markedly temperature-dependent. The second is that latitudinal variation in pacemaker amplitude (higher in the north) is responsible for two very different sets of observations on Drosophila circadian systems at successively higher latitudes. One of these is a cline in D. auraria's phase-shifting response to light, which steadily weakens in a succession of more northerly strains. The other, concerning D. littoralis in the very far north, is a cline in the rate at which eclosion activity becomes arrhythmic (the circadian rhythm damps out) in constant darkness; damping is faster in the north. The third proposition concerns a plausible selection pressure for the cline in pacemaker amplitude that we propose underlies the two directly observed clines. Two points are emphasized: (1) The amplitude of the pacemaker's daily oscillation declines as the duration of the entraining light pulse (photoperiod) is increased; and (2) the duration of the daily photoperiods throughout the breeding season is steadily increased as one moves toward the poles. Selection for conservation of pacemaker amplitude (during the breeding season) would produce the latitudinal cline we propose. The fourth, and final proposition is that since the amplitude of the pacemaker's daily motion responds systematically to change in photoperiod, amplitude is clearly one way--and a temperature-dependent way--in which insect circadian systems may sense seasonal change. These propositions concerning the temperature and latitude dependence of pacemaker amplitude may be relevant to a wider array of circadian pacemakers than Drosophila.     

The circadian neuropeptide PDF signals preferentially through a specific adenylate cyclase isoform AC3 in M pacemakers of Drosophila

The neuropeptide Pigment Dispersing Factor (PDF) is essential for normal circadian function in Drosophila. It synchronizes the phases of M pacemakers, while in E pacemakers it decelerates their cycling and supports their amplitude. The PDF receptor (PDF-R) is present in both M and subsets of E cells. Activation of PDF-R stimulates cAMP increases in vitro and in M cells in vivo. The present study asks: What is the identity of downstream signaling components that are associated with PDF receptor in specific circadian pacemaker neurons? Using live imaging of intact fly brains and transgenic RNAi, we show that adenylate cyclase AC3 underlies PDF signaling in M cells. Genetic disruptions of AC3 specifically disrupt PDF responses: they do not affect other Gs-coupled GPCR signaling in M cells, they can be rescued, and they do not represent developmental alterations. Knockdown of the Drosophila AKAP-like scaffolding protein Nervy also reduces PDF responses. Flies with AC3 alterations show behavioral syndromes consistent with known roles of M pacemakers as mediated by PDF. Surprisingly, disruption of AC3 does not alter PDF responses in E cells—the PDF-R(+) LNd. Within M pacemakers, PDF-R couples preferentially to a single AC, but PDF-R association with a different AC(s) is needed to explain PDF signaling in the E pacemakers. Thus critical pathways of circadian synchronization are mediated by highly specific second messenger components. These findings support a hypothesis that PDF signaling components within target cells are sequestered into “circadian signalosomes,” whose compositions differ between E and M pacemaker cell types.