Developmental disinhibition: turning off inhibition turns on breathing in vertebrates

Development requires age-dependent changes in essential behaviors. While the mechanisms determining the developmental expression of such behavior in vertebrates remain largely unknown, a few studies have identified permissive mechanisms in which the appearance of promoting signals activates pre-established networks. Here we report a different developmental process. Specifically, we show that the neuronal substrate that produces putative lung breathing in tadpoles is formed early in development, but remains more or less inactive until metamorphosis because of suppression mediated by a GABA(B) receptor-dependent mechanism. Blocking this suppression using 2-hydroxy-saclofen, a GABA(B) receptor antagonist, results in the precocious production of the putative lung breathing motor pattern. This blocker failed to augment putative lung breaths after metamorphosis. Thus, our results suggest that loss of an inhibitory signal during development (i.e., developmental disinhibition) is responsible for the developmental expression of air breathing.

Respiratory activity in neonatal rats

In neonatal animals in vitro preparations have been employed widely to study the central control of respiration. These preparations have limitations in that reflex afferent inputs and kinesiological studies cannot be performed. Here, we describe an alternative in situ experimental model for studying both peripheral and central control of the respiratory system in neonatal rats. Using technology based on adult mammals, we introduce an intra-arterially perfused working heart-brainstem preparation (WHBP) that permits studies on eupnoeic respiration in neonatal rats from within a few hours of birth. Using this preparation we demonstrate a three-phase respiratory rhythm as revealed by the activity in phrenic and recurrent laryngeal motor nerves, the respiratory modulation of laryngeal resistance and the firing patterns of respiratory neurones recorded from the ventrolateral medulla. We conclude that the neonatal rat WHBP is an in situ preparation because it produces a respiratory rhythm similar to that of adult in vivo mammal preparations but distinct from in vitro preparations.

Baclofen eliminates cluster lung breathing of the tadpole brainstem, in vitro

Intermittent lung ventilation is a respiratory pattern wherein breaths occur in clusters. Intermittent lung ventilation is common in amphibians and can occur in mammals. Isolated brainstems from postmetamorphic tadpoles exhibiting episodic lung ventilatory bursts were superfused with baclofen, a potent gamma-aminobutyric acid (GABA)(B) receptor agonist. At moderate concentrations (0.125 to 0.5 microM), the number of lung bursts per episode decreased but their overall frequency was unchanged. At 0.5 microM, only 1.22+/-0.24 lung bursts occurred per episode, indicating virtually no clustering. Only at higher concentrations was overall breathing frequency decreased. Therefore, at moderate concentrations of baclofen continuous ventilation replaced episodic ventilation, suggesting that a GABA(B) receptor-dependent pathway may regulate the clustering of lung breaths.

Chemical and thermal stimuli have short-lived effects on the retzius cell in the medicinal leech

During the appetitive phase of feeding, hungry leeches detect a prey by the integration of signals perceived by different sensory systems. Earlier reports suggested that chemical or thermal sensory stimulation of the lip was associated with increased afferent activity in cephalic nerves connecting the lip to the central nervous system. These authors further suggested that this activity was relayed to Retzius cells in segmental ganglia, which then released serotonin to initiate and control all aspects of feeding behavior. In this study, we show that chemosensory or thermal activation of the lip lasting for at least 5 min produces a distinct signal in the cephalic nerves consisting of action potentials of low amplitude. These small amplitude signals are clearly distinguishable from the large action potentials evoked by mechanosensory stimuli applied to the same area of the lip. Both types of sensory stimuli also evoke an increase in the firing frequency of the Retzius cells in segmental ganglia. However, the response recorded in the nerves and the Retzius cells during a maintained stimulus is not constant but decreases with an exponential time course. These results agree with our earlier observations on a semi-intact feeding preparation in which we showed that the firing frequency of the Retzius cell decreased as soon as the leech began to ingest its meal. Therefore, our data provide further evidence suggesting that it is unlikely that heat or chemical cues maintain the Retzius cell in an active state throughout the consummatory phase of feeding.

Segmental control of midbody peristalsis during the consummatory phase of feeding in the medicinal leech, Hirudo medicinalis

The ingestive behavior of the medicinal leech includes peristalsis-like movements of the midbody that are organized into both rostral-to-caudal and caudal-to-rostral waves (C. M. Lent, K. H. Fliegner, E. Freedman, & M. H. Dickenson, 1988). The neuronal control of this behavior is unknown. Using surgical manipulations and electromyograms, the authors show that (a) the head and tail ganglia are not necessary for this behavior; (b) the circuit is distributed, with components reiterated along the length of the leech; (c) excitatory signals transmitted from rostral segments via the nerve cord can initiate peristalsis in "empty" caudal segments; (d) inhibitory signals from caudal segments limit the frequency of peristalsis; and (e) stretch of the gut and/or body wall is sufficient to produce peristalsis in the absence of heat or chemical cues. These results are compared with peristalsis in the digestive tract of mammals. The leech may be a good model for studying peristalsis-like behaviors at the cellular level.

Actinobacillus suis infection in pigs in Australia

This is the first report of the isolation of Actinobacillus suis in association with significant disease among preweaned pigs in Australia. Sudden deaths occurred in preweaned piglets at one facility and enlarged joints, particularly the stifles and tarsi, occurred at another. Isolates with the biochemical phenotype and apx genotype profile of A suis were cultured from affected piglets. Both facilities were of high health status and one had undergone a recent depopulation, disinfection and sow repopulation process. Reviewing initial reports of A suis disease overseas, it is apparent that outbreaks occurred sporadically in various locations, with disease occurring among the herds involved for some months only, before development of immunity. A suis disease is now considered an emerging disease on high health status farms.

Impact of a plastid-bearing endocytobiont on apicomplexan genomes

Both the chromosomal and extrachromosomal components of the apicomplexan genome have been supplemented by genes from a plastid-bearing endocytobiont: probably an algal cell. The sequence of the apicomplexan plastid's vestigial genome indicates that a large number (>100) of genes of endocytobiotic origin must have transferred laterally to the host cell nucleus where they control maintenance of the plastid organelle and supply its functional components by means of post-translational protein trafficking. Should the nuclear genes prove to be less divergent phylogenetically than those left on the plastid genome, they might give better clues than we have at present to the origin of the plastid-bearing endocytobiont. Most of these nuclear genes still await discovery, but the on-going genome sequencing project will reveal the function of the organelle, as well as many "housekeeping" processes of interest on a wider front. The plastid's own protein synthetic machinery, being cyanobacterial in origin, offers conventional targets for antibiotic intervention, and this is discussed here using a structural model of elongation factor Tu. Uncovering the vital function(s) of the plastid organelle will provide new drug targets.

Antibiotic inhibitors of organellar protein synthesis in Plasmodium falciparum

Elongation factor Tu (EF-Tu) is encoded by the tuf gene of the plastid organelle of the malaria parasite Plasmodium falciparum. A range of structurally unrelated inhibitors of this GTP-dependent translation factor was shown to have antimalarial activity in blood cultures. The most active was the cyclic thiazolyl peptide amythiamicin A with an IC50 = 0.01 microM. Demonstrable complexes were formed in vitro between a recombinant version of P. falciparum EF-Tu(pl) and inhibitors that bind to different sites on EF-Tu; these included the antibiotics kirromycin, GE2270A and enacyloxin IIa.

Protein synthesis in the plastid of Plasmodium falciparum

The plastid organelle of malarial and other apicomplexan parasites contains ribosome-like particles as well as a genome dedicated largely to specifying components of a protein expression system. We have identified plastid ribosomes using hybridization studies and show that in erythrocytic stages of Plasmodium falciparum a subset of polysomes carries plastid-specified rRNAs and mRNA, supporting the idea that protein synthesis is active in the plastid.

Efficacy of a low volume recirculating superfusion chamber for long term administration of expensive drugs and dyes

We have characterized the efficacy of a low volume (2 ml) two-compartment experimental chamber in which a gas inflow equilibrates and recirculates the bathing fluid. This type of chamber is suitable for experiments employing en bloc preparations that require the administration of expensive molecular probes. The fluid in the chamber is pumped from a compartment holding the preparation to an elevated reservoir compartment using gas bubbles. The fluid returns via gravity along a different path. The flow rate of superfusate in the chamber was 30 ml min(-1). To determine the effectiveness of the chamber in dissolving gas, we filled the chamber with bicarbonate-buffered physiological saline and measured pH and P(O(2)) with ion-selective and Clark-style microelectrodes. Steady state values of pH and P(O(2)) in the chamber were almost identical to those in an external tonometer bubbled vigorously with the same gas mixture. When CO(2) was increased from 2 to 4.4%, the chamber pH fell with a time constant of 56 s (about twice that of the tonometer). To determine the effectiveness of gas exchange between a brain preparation and the fluid in the chamber we measured pH and P(O(2)) depth profiles of the in vitro tadpole brainstem. We found virtually no unstirred layer owing to excellent mixing and the high flow created by the recirculating mechanism. We demonstrate that despite the high flow rates, preparations are mechanically stable allowing intracellular electrophysiological recordings.

Emotional congruence in sexual offenders against children

The tendency of some sexual offenders against children to display an exaggerated cognitive and emotional affiliation with childhood has been labeled "emotional congruence" by Finkelhor (1984) and as overidentification with childhood by others. This study consisted of an evaluation of emotional congruence in 194 men--homosexual and heterosexual pedophiles (N's = 41 and 31), heterosexual nonpedophilic incest offenders (N = 62), sexual offenders against women (N = 27), and nonsexual offender criminals (N = 33). A revised version of the Child Identification Scale (CIS-R) was used in combination with a semistructured interview (SSI) also intended to measure the construct. Maximum-likelihood factor analysis of the CIS-R generated eight factors. All subjects, particularly the nonsexual offenders, scored high on a factor indicating a wish to restart their lives. With respect to emotional congruence, results showed that only the homosexual pedophiles' scores indicated a preference for interacting with children on the child's level, while the incest offenders seemed to prefer to elevate their victims to adult status rather than fixating on the child role themselves. The group of heterosexual pedophiles, however, seemed to be motivated more by sexual gratification than by an emotional or relationship interest in their victims or children.

Acute anxiety--not always a psychiatrist's problem

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Actomyosin motor in the merozoite of the malaria parasite, Plasmodium falciparum: implications for red cell invasion

The genome of the malaria parasite, Plasmodium falciparum, contains a myosin gene sequence, which bears a close homology to one of the myosin genes found in another apicomplexan parasite, Toxoplasma gondii. A polyclonal antibody was generated against an expressed polypeptide of molecular mass 27,000, based on part of the deduced sequence of this myosin. The antibody reacted with the cognate antigen and with a component of the total parasite protein on immunoblots, but not with vertebrate striated or smooth muscle myosins. It did, however, recognise two components in the cellular protein of Toxoplasma gondii. The antibody was used to investigate stage-specificity of expression of the myosin (here designated Pf-myo1) in P. falciparum. The results showed that the protein is synthesised in mature schizonts and is present in merozoites, but vanishes after the parasite enters the red cell. Pf-myo1 was found to be largely, though not entirely, associated with the particulate parasite cell fraction and is thus presumably mainly membrane bound. It was not solubilised by media that would be expected to dissociate actomyosin or myosin filaments, or by non-ionic detergent. Immunofluorescence revealed that in the merozoite and mature schizont Pf-myo1 is predominantly located around the periphery of the cell. Immuno-gold electron microscopy also showed the presence of the myosin around almost the entire parasite periphery, and especially in the region surrounding the apical prominence. Labelling was concentrated under the plasma membrane but was not seen in the apical prominence itself. This suggests that Pf-myo1 is associated with the plasma membrane or with the outer membrane of the subplasmalemmal cisterna, which forms a lining to the plasma membrane, with a gap at the apical prominence. The results lead to a conjectural model of the invasion mechanism.

Outcome of out-of-hospital cardiorespiratory arrest in south Glamorgan

During 138 weeks an emergency medical service (EMS) of mixed skill-level attempted to resuscitate 954 patients from prehospital cardiac arrest (883 attempts per million population per year); 75% of the arrests were of cardiac cause. This paper is one of the first analyses from europe to use the 'Utstein template' to report outcomes of such arrests. In cases where an arrest rhythm could be recorded, 38.4% were ventricular fibrillation (VF), 45.5% were asystolic, and the remainder were either electromechanical dissociation or respiratory arrests. Using univariate analysis factors associated with a greater likelihood of survival include the presence of a witness, bystander-initiated cardiopulmonary resuscitation (CPR), early CPR and VF as the arrest rhythm. Twenty of 155 cases (13%) survived where VF arrest was witnessed by non-EMS personnel.

A plastid of probable green algal origin in Apicomplexan parasites

Protozoan parasites of the phylum Apicomplexa contain three genetic elements: the nuclear and mitochondrial genomes characteristic of virtually all eukaryotic cells and a 35-kilobase circular extrachromosomal DNA. In situ hybridization techniques were used to localize the 35-kilobase DNA of Toxoplasma gondii to a discrete organelle surrounded by four membranes. Phylogenetic analysis of the tufA gene encoded by the 35-kilobase genomes of coccidians T. gondii and Eimeria tenella and the malaria parasite Plasmodium falciparum grouped this organellar genome with cyanobacteria and plastids, showing consistent clustering with green algal plastids. Taken together, these observations indicate that the Apicomplexa acquired a plastid by secondary endosymbiosis, probably from a green alga.

Extrachromosomal DNA in the Apicomplexa

Malaria and related apicomplexan parasites have two highly conserved organellar genomes: one is of plastid (pl) origin, and the other is mitochondrial (mt). The organization of both organellar DNA molecules from the human malaria parasite Plasmodium falciparum has been determined, and they have been shown to be tightly packed with genes. The 35-kb circular DNA is the smallest known vestigial plastid genome and is presumed to be functional. All but two of its recognized genes are involved with genetic expression: one of the two encodes a member of the clp family of molecular chaperones, and the other encodes a conserved protein of unknown function found both in algal plastids and in eubacterial genomes. The possible evolutionary source and intracellular location of the plDNA are discussed. The 6-kb tandemly repeated mt genome is the smallest known and codes for only three proteins (cytochrome b and two subunits of cytochrome oxidase) as well as two bizarrely fragmented rRNAs. The organization of the mt genome differs somewhat among genera. The mtDNA sequence provides information not otherwise available about the structure of apicomplexan cytochrome b as well as the unusually fragmented rRNAs. The malarial mtDNA has a phage-like replication mechanism and undergoes extensive recombination like the mtDNA of some other lower eukaryotes.

Predicting survival from out-of-hospital cardiac arrest: a multivariate analysis

From 954 attempts to resuscitate patients from out-of-hospital cardiac arrest two datasets were derived, namely 861 cases of cardiac arrest and 906 cases of either cardiac or primary respiratory arrest. For each dataset, multivariate analysis was performed by fitting a number of explanatory variables with respect to the outcomes of admission to hospital and discharge home in logistic regression models. There were numerous interactions between these variables. Being conscious at the time of the arrival of the ambulance crew and subsequently having cardiac arrest strongly predicted survival, as did both the presence of a witness to the arrest and the initiation of cardiopulmonary resuscitation (CPR) by a bystander; this latter effect was a marker for early CPR. The strongest predictor of a poor outcome was delay to CPR or delay to advanced cardiac life support.

A model of the hydrostatic skeleton of the leech

A mathematical model of the hydrostatic skeleton of the leech has been developed to predict the shape of and internal pressure within the animal in response to a given pattern of motor neuron activity in different behaviors. The model incorporates experimental data on: the dimensions of the animal at behavioral extremes, the passive properties of the tissues, the active length-tension behavior of the muscles in response to neural activation, the relations between firing frequencies and forces developed by the muscles. The model is based on three general assumptions: (i) the cross-sectional geometry of each segment is elliptical, (ii) the volume of each segment remains constant during movement, (iii) the shape of the animal reflects dimensions that minimize the total potential energy. Presently the model is implemented to simulate the vermiform elongation of the leech, predicting the shape and the pressure changes during behavior. The results are in good agreement with the experimental measurements. The pattern of motor neuronal activity was determined by the known intersegmental travel time and estimated delay time between relaxation of the longitudinal muscles and the activation of the circular muscles. The anesthetized state of the leech was taken as the reference state for the model in which the active and passive stresses are zero.

Complete gene map of the plastid-like DNA of the malaria parasite Plasmodium falciparum

Malaria parasites, and other parasitic protists of the Phylum Apicomplexa, carry a plastid-like genome with greatly reduced sequence complexity. This 35 kb DNA circle resembles the plastid DNA of non-photosynthetic plants, encoding almost exclusively components involved in gene expression. The complete gene map described here includes genes for duplicated large and small subunit rRNAs, 25 species of tRNA, three subunits of a eubacterial RNA polymerase, 17 ribosomal proteins, and a translation elongation factor. In addition, it codes for an unusual member of the Clp family of chaperones, as well as an open reading frame of unknown function found in red algal plastids. Transcription is polycistronic. This plastid-like DNA molecule is conserved in several genera of apicomplexans and is conjectured to have been acquired by an early progenitor of the Phylum by secondary endosymbiosis. The function of the organelle (plastid) carrying this DNA remains obscure, but appears to be specified by genes transferred to the nucleus.

An increase in activity of serotonergic Retzius neurones may not be necessary for the consummatory phase of feeding in the leech Hirudo medicinalis

During the consummatory phase of feeding, in which blood is ingested, medicinal leeches display a characteristic set of behaviours: they extend their jaws, are less responsive to sensory input, produce mucus, relax the body wall and exhibit waves of peristalsis that can run the length of the body. Earlier reports suggested that this pattern of behaviour is orchestrated by serotonin released from Retzius cells in response to the appropriate sensory stimulation of the lip. We have developed a semi-intact preparation in which only the nervous system in the posterior half of the leech was exposed. The front half of the leech was free to explore, bite through and feed until satiated from a blood-filled sausage casing while continuous intracellular and extracellular recordings were made from identified cells and the nerve roots of the exposed segments. Prior to attachment of the animal to the feeding device, the firing frequency of the Retzius cell increased transiently during spontaneous movements or tactile stimuli to its front or posterior end. In contrast, Retzius cell activity decreased after the anterior sucker attached to the membrane of the feeding device at about the time when ingestion was initiated. Increased activity of Leydig cells, which are known to modulate several circuits in the leech, was also associated with exploration. However, unlike that of Retzius cells, the activity of Leydig cells increased significantly following the onset of consumption. These results suggest that increased activity of Retzius cells in midbody ganglia is not a prerequisite for the consummatory phase of feeding and raises questions regarding the role of serotonin in regulating this behaviour.

Mapping motor neurone activity to overt behaviour in the leech: internal pressures produced during locomotion

Several behaviour patterns have been studied in the leech at both the kinematic and neuronal levels. However, very little is known about how patterns of motor neurone activity map to actual movements. Internal pressure is an essential biomechanical property in this process, being responsible for producing the rigidity and posture that allow the directed delivery of forces produced by muscle contraction. To obtain a better understanding of the biomechanical processes involved in movement of the leech, we have measured the internal pressure of the animal by placing catheters through the body wall and into the gut of intact animals showing normal patterns of behaviour. Each type of behaviour had a characteristic pressure waveform. The elongation phase of crawling produced a rapid increase in pressure that peaked when midbody segments were maximally elongated. The pressure produced during the contraction phase of crawling depended on the type of crawl, only inchworm crawling producing a second peak. Whole-body shortening in response to a head poke also produced a pressure peak, but it had a faster rise time. Swimming produced the largest pressure, which was marked by a large sustained increase that fluctuated phasically with undulations of the body. Dual pressure recordings using two catheters demonstrated that pressure was not uniform along the length of the leech, indicating that the body cavity is functionally compartmentalised. Injecting fluid into the gut via a recording catheter allowed us to determine the effects of increasing internal volume on pressure. In line with previous predictions made using an abstract biomechanical model of the leech hydroskeleton, we found that an increase in the volume caused a reduction in the pressure. We are in the process of constructing a more realistic biomechanical model of the leech, based on actual data reported elsewhere. The results in this paper will provide key tests for refining these models.