Some observations on habituation of the flexor reflex in the rat: the influence of strychnine, bicuculline, spinal transection, and decerebration

The involvement of inhibition in habituation of the flexor reflex was investigated in intact, spinal, and decerebrate rats. Strychnine and bicuculline were administered in order to determine the contribution of certain forms of central inhibition to the development of habituation. Both strychnine and bicuculline reduced habituation in the intact rat but did not do so in the spinal preparation. Strychnine, in fact, caused a facilitation of habituation in the spinal rat. The impairment of habituation by strychnine was related to the intensity of stimulation used to elicit the reflex. Thus, the effect of strychnine was only demonstrable when relatively high intensities were used. Flexor reflex habituation was shown to be more pronounced in the decerebrate than in the spinal preparation. This difference could be demonstrated only when stimuli of high intensity were given. It is concluded that, in the absence of supraspinal influences, habituation of the flexor reflex does not require inhibitory mechanisms. However, inhibition may play a role in habituation of the component of the reflex that utilizes descending influences.     

Bendectin and fetal malformations.

Mortality among 172 workers who received workmen''s compensation for asbestosis in Ontario was investigated and the causes of death were compared with those for the general male population in that province. The workers were found to have increased rates of death, relative to the general populations, for nonmalignant respiratory diseases, lung cancer and pleural and peritoneal mesothelioma. In comparison with the general population, the proportion of workers that survived was 69% of that expected 5 years after they were awarded compensation and 53% at 10 years.     

Ionic currents across pancreatic acinar cell membranes and their role in fluid secretion

Fluid and enzyme secretion from a number of mammalian exocrine glands is controlled by the action of neurotransmitters and hormones on acinar cell membranes. Sustained stimulation evoking sustained fluid and enzyme secretion also evokes sustained membrane depolarization and increase in conductance. Mouse and rat pancreatic fluid and enzyme secretion, as well as membrane depolarization and conductance increase evoked by sustained stimulation with acetylcholine or cholecystokinin-gastrin peptides, are acutely dependent on extracellular calcium. However, the initial stimulant-evoked conductance increase and secretion appear to be triggered by calcium released from inside the cells. Direct measurement of membrane current during sustained stimulation in voltage-clamp experiments with resolution of the total current into its Na, Cl and K components has allowed calculations of stimulant-evoked Na and Cl uptake into the acinar cells. The NaCl uptake is quantitatively sufficient to account for the stimulant-evoked fluid secretion. The role of the stimulant-evoked transmembrane ionic current appears to be the supply of salt for the fluid secretion. Calcium derived from intracellular sources in the initial phase of secretion, and from the extracellular fluid in the sustained phase, couples fluid and enzyme secretion to hormone-receptor interaction.     

Kinetic determination of the genome size of the pea

Renaturation of pea (Pisum sativum) DNA has been used to estimate the size of the pea genome and the fraction of pea DNA containing repeated DNA sequences. Pea DNA renaturation and single copy tracer renaturation indicate that the size of the pea genome is 0.5 picograms. More than 70% of pea DNA sequences are repeated from 100 to 5,000 times.     

Activation of human alpha 1-antitrypsin gene in rat hepatoma x human fetal liver cell hybrids depends on presence of human chromosome 14

In order to study the involvement of human chromosomes in the expression of liver-specific functions, we have produced somatic cell hybrids between a rat hepatoma (7777) cell line and human diploid skin fibroblasts (series XIX) or human fetal liver cells (series XXII). Production of human serum proteins was detected by immunoelectrophoretic analyses of concentrated serum-free hybrid culture supernatants. Human alpha 1-antitrypsin (AAT) was secreted by a subset of hybrids but not by the parental cells. The activated human AAT phenotype segregated concordantly with human chromosome 14 in 18 primarily HAT-selected and five azaguanine back-selected series XXII hybrids. All other chromosomes were excluded as playing a role in AAT expression. Therefore, the AAT gene (PI) is assigned to chromosome 14. This quasi-constitutive expression of a liver-specific function was not observed for the other serum proteins studied, nor was it seen in the skin fibroblast-derived hybrids (series XIX) although AAT was produced by some of them.     

Mutational analysis of the transin (rat stromelysin) autoinhibitor region demonstrates a role for residues surrounding the "cysteine switch"

The family of mammalian extracellular matrix metalloproteases (MMPs) are secreted by cells in an inactive (latent) proenzyme form. A highly conserved amino acid sequence, PRCGVPDV, is found near the COOH-terminal end of the pro-domain of these MMPs and believed to act as an "autoinhibitor." Recent studies (Springman, E. B., Angleton, E. L., Birkedal-Hansen, H., and Wart, H. E. V. (1990) Proc. Natl. Acad. Sci. U. S. A. 87, 364-368) indicate the Cys of this sequence ligands to the active-site zinc keeping the proenzyme in an inactive state, and mutational analysis (Sanchez-Lopez, R., Nicholson, R., Gesnel, M. C., Matrisian, L. M., and Breathnach, R. (1988) J. Biol. Chem. 263, 11892-11899) suggests that the conserved residues surrounding this Cys are required for latency. We have constructed 16 new site-directed mutations of the PRCGVPDV autoinhibitor region of the MMP transin (rat stromelysin) and tested whether these mutant enzymes are produced in a latent or activated form. We find that the conserved Arg as well as the Cys are essential for maintaining latency. The Cys cannot be replaced by other zinc-liganding amino acids, and the Arg cannot be replaced by Lys. Residues immediately surrounding the Cys are sensitive to even conservative amino acid substitutions. We show that a synthetic peptide PRCGVPDV is capable of acting as a weak inhibitor of transin and that replacement of the Cys with a Ser abolishes inhibition by the peptide. A review of the current knowledge of MMP substrate specificity in combination with these new results suggests that the PRCGVPDV sequence does not inhibit activity by mimicking the known substrates of the protease.     

Connections of the forewing tegulae in the locust flight system and their modification following partial deafferentation.

The flight motor pattern of the adult locust (Locusta migratoria L.) is able to recover from the loss of the hindwing tegulae. This recovery is due to a functional substitution of the hindwing tegulae by the forewing tegulae (Büschges, Ramirez, and Pearson, 1992). To assess changes in the pathways from the forewing tegulae in the flight system, we investigated the pathways of the forewing tegula in intact locusts and in animals 2 weeks after hindwing tegula removal. The following physiological alterations in these pathways were found to be associated with the recovery: (1) In the intact locusts, the connections of forewing tegula afferents to flight interneurons are variable but this variability did not occur in recovered animals, and (2) larger numbers of forewing tegula afferents connect to interneurons that excite elevator motoneurons (interneurons 566 and 567) and to an interneuron that inhibits depressor motoneurons (interneuron 511). The size of unitary excitatory postsynaptic potentials (EPSPs) evoked by signal forewing tegula afferents was found not to be altered in recovered animals. The changes in connectivity of forewing tegula afferents are correlated with morphological alterations in the structure of the terminal processes of the afferents and with sprouting of some branches of interneurons receiving input from these afferents.