Multilinear models of single cell responses in the medial nucleus of the trapezoid body.

The representation of acoustic stimuli in the brainstem forms the basis for higher auditory processing. While some characteristics of this representation (e.g. tuning curve) are widely accepted, it remains a challenge to predict the firing rate at high temporal resolution in response to complex stimuli. In this study we explore models for in vivo, single cell responses in the medial nucleus of the trapezoid body (MNTB) under complex sound stimulation. We estimate a family of models, the multilinear models, encompassing the classical spectrotemporal receptive field and allowing arbitrary input-nonlinearities and certain multiplicative interactions between sound energy and its short-term auditory context. We compare these to models of more traditional type, and also evaluate their performance under various stimulus representations. Using the context model, 75% of the explainable variance could be predicted based on a cochlear-like, gamma-tone stimulus representation. The presence of multiplicative contextual interactions strongly reduces certain inhibitory/suppressive regions of the linear kernels, suggesting an underlying nonlinear mechanism, e.g. cochlear or synaptic suppression, as the source of the suppression in MNTB neuronal responses. In conclusion, the context model provides a rich and still interpretable extension over many previous phenomenological models for modeling responses in the auditory brainstem at submillisecond resolution.

Glycine-mediated changes of onset reliability at a mammalian central synapse.

Glycine is an inhibitory neurotransmitter activating a chloride conductance in the mammalian CNS. In vitro studies from brain slices revealed a novel presynaptic site of glycine action in the medial nucleus of the trapezoid body (MNTB) which increases the release of the excitatory transmitter glutamate from the calyx of Held. Here, we investigate the action of glycine on action potential firing of single MNTB neurons from the gerbil under acoustic stimulation in vivo. Iontophoretic application of the glycine receptor antagonist strychnine caused a significant decrease in spontaneous and sound-evoked firing rates throughout the neurons' excitatory response areas, with the largest changes at the respective characteristic frequency (CF). The decreased firing rate was accompanied by longer and more variable onset latencies of sound-evoked responses. Outside the neurons' excitatory response areas, firing rates increased during the application of strychnine due to a reduction of inhibitory sidebands, causing a broadening of frequency tuning. These results indicate that glycine enhances the efficacy for on-CF stimuli, while simultaneously suppressing synaptic transmission for off-CF stimuli. These in vivo results provide evidence of multiple excitatory and inhibitory glycine effects on the same neuronal population in the mature mammalian CNS.

The medial nucleus of the trapezoid body: comparative physiology.

Principal cells of the medial nucleus of the trapezoid body (MNTB) receive their excitatory input through large somatic terminals, the calyces of Held, which arise from axons of globular bushy cells located in the contralateral ventral cochlear nucleus. Discharges of MNTB neurons are characterized by high stimulus evoked firing rates, temporally precise onset responses, and a high degree of phase-locking to either pure tones or stimulus envelopes. Since the calyx of Held synapse is accessible to in vitro and to in vivo recordings, it serves as one of the most elaborate models for studying synaptic transmission in the mammalian brain. Although in such studies, the major emphasis is on synaptic physiology, the interpretation of the data will benefit from an understanding of the MNTB's contribution to auditory signal processing, including possible functional differences in different species. This implies the consideration of possible functional differences in different species. Here, we compare single unit recordings from MNTB principal cells in vivo in three different rodent species: gerbil, mouse and rat. Because of their good low-frequency hearing gerbils are often used in in vivo preparations, while mice and rats are predominantly used in slice preparations. We show that MNTB units in all three species exhibit high firing rates and precise onset-timing. Still there are species-specific specializations that might suggest the preferential use of one species over the others, depending on the scope of the respective investigation.

Continuing medical education and career choice among graduates of problem-based and traditional curricula.

A survey was conducted among graduates of two Canadian medical schools who have been in practice for more than 9 years. The purpose of the study was to test the hypothesis that graduates of a problem-based curriculum differ from graduates of a traditional curriculum in their attitude to and participation in continuing medical education (CME) activities. Differences were noted in the rate of participation in certain CME activities (attendance at national and international conferences and meetings) between specialists and family doctors in both groups of alumni. However, the data indicate that the differences in learning-teaching methods employed in the course of the undergraduate medical curriculum do not exert a decisive influence upon the learning habits of the graduates.

Calcium-dependent proteolysis in the myocardium of rats subjected to stress.

Activity of a calcium-dependent neutral protease (calpain II) and its specific endogenous inhibitor was investigated in the myocardium of rats subjected to different stressors: cold, anaesthesia, 24 and 48 h starvation and food restriction for 7 and 14 days. Enzyme and inhibitor activities were determined in the 37,200 g supernatant of homogenates prepared from the free left ventricular wall of the heart. The specific activity of the myocardial calcium-dependent proteinase increased in all rats exposed to stressful stimuli, reaching maximum values in animals starved for 48 hours. Decrease in the specific activity of the inhibitor accompanied the changes in enzyme activity. Differences from normal control values were statistically significant in the starved animals and in animals fed a restricted diet for 7 or 14 days. These observations suggest that interaction between calpain II and its specific inhibitor plays a role in the regulation of the enzyme activity and furthermore, that stressful stimuli lead to increased calcium-dependent proteolysis in the myocardium.

Phosphorylation of bovine cardiac calcium-activated neutral protease by protein kinase-C.

Protein kinase C prepared from rat brain was used to phosphorylate a calcium-activated neutral protease, purified from bovine cardiac muscle. Attempts to phosphorylate the enzyme in the presence of calcium were unsuccessful, unless the protease inhibitor leupeptin was also present. Phosphorylation of the 74K subunit of the protease was completely inhibited in the absence of phosphatidylserine and diolein, indicating that phosphorylation of the enzyme was catalysed by the calcium and phospholipid-dependent protein kinase C.

Calcium-dependent proteolysis and its inhibition in the ischemic rat myocardium.

Activity of calcium-dependent neutral proteinase and its specific inhibition was investigated in the acutely ischemic myocardium after ligation of the left descending coronary artery in anaesthetized open chest rats. In experiments where the mean arterial blood pressure could be maintained above 70 mm Hg the hearts were quickly removed 5 to 30 min after ligation and homogenized in ice-cold buffer. The activity of the calcium-dependent neutral proteinase and of its endogenous inhibitor were determined in the 10,000 g supernatant of ventricular and septal tissues. Hearts from normal anaesthetized and sham operated animals left on the respirator for 10, 20 and 30 min, were used as controls. Only traces of proteinase activity could be found in the supernatants obtained from normal controls, while in the sham-operated animals the specific activity of the Ca2+-dependent proteinase increased with time, reaching significantly elevated values after 30 min on the respirator. In the ischemic groups enzyme activity also increased with increasing duration of ischemia and was substantially elevated in the ventricular myocardium 20 min after ligation. The increased calcium-dependent proteinase activity was accompanied by significantly reduced activity of its endogenous inhibitor. The concomitant changes in the activities of the myocardial calcium-dependent neutral protease and its endogenous inhibitor suggest that interaction between the enzyme and its inhibitor play a role in the regulation of intracellular calcium-dependent proteolysis.

Purification and some characteristics of a Ca2+-activated proteolytic enzyme from beef cardiac muscle.

A calcium-dependent neutral proteinase was purified from beef cardiac muscle. The crude extract prepared from cardiac muscle was subjected to acid precipitation and salt fractionation and then further purified by column chromatography on Sepharose 6B, DE-52, and Sephadex G-200 columns in succession. The final preparation showed an 11 300 fold increase in specific activity of the Ca2+-activated enzyme. Average enzyme protein yield was 2.4 microgram/g fresh tissue. The enzyme was maximally active at pH 7.6 in the presence of 4 mM calcium. Proportionality of enzyme activity in partially purified preparations was retained when activity was measured at 25 degrees C using casein as the substrate. The rate of proteolysis by the purified enzyme was linear for 60 min under similar assay conditions. Fractionation of muscle homogenates showed that 70 to 73% of the total enzyme activity was present in the 24 000 X g and 30 000 X g supernatants. The enzyme was labile in aqueous solutions and storage at 4 degrees C and --20 degrees C resulted in considerable loss of activity, unless glycerol (50% v/v) was added to the solution.

Immunohistochemical localization of vitamin D-dependent calcium-binding protein in duodenum, kidney, uterus and cerebellum of chickens.

Calcium-binding protein (CaBP) has been localized with the immunoperoxidase method using antiserum against purified chick duodenal CaBP. Different preparative procedures were employed to investigate the experimental conditions possibly responsible for the contradictory reports in the literature of the precise cellular localization of CaBP. Freeze substitution, frozen sections followed by fixation and coagulant and non-coagulant fixatives were used with appropriate control sections to demonstrate that the true localization of CaBP in the chick duodenum is in the absorptive cell cytoplasm. The goblet cell localization reported in the literature seems to be a diffusion artifact due to inadequate fixation. CaBP was also localized in several other tissues. In the hen uterus, the tubular glands beneath the surface epithelium showed intense reaction. In the kidney, CaBP was present in the cells of the straight and convoluted segments of distal tubules. The cortex of the chick cerebellum showed the CaBP in Purkinje cells. The entire dendritic trees contained the reaction product. No other neurons in the molecular or the granular layer were stained. In the deep cerebellar nuclei, all neurons were negative and these were outlined by deeply staining axons of the Purkinje cells and their synaptic endings.

Abnormal characteristics of the blood from chick embryos maintained in "shell-less" culture.

Chick embryos were maintained in shell-less culture up to a total age of 15 days. The composition of their blood was analyzed together with the blood from coontrol embryos of comparable degree of differentiation. The blood from cultured embryos had lower hematocrit values; their serum contained a reduced concentration of proteins, phospholipids and total calcium and an increased concentration of inorganic phosphorus. In view of the reduced concentration of proteins and phospholipids, the concomitant hypocalcemia must represent, at least in part, a reduction in the binding capacity of the serum and not only a decrease in the ionic fraction of serum calcium.

Effects produced by the administration of high doses of 1,25-dihydroxycholecalciferol to the chick embryo.

White Leghorn chick embryos were injected on the 15th day of incubation with 70 to 300 pmoles 1,25-(OH)2D3. All doses produced hypercalcemia; with the highest dose, the concentration of calcium in serum started to rise 4 h after the injection, reached a peak 20 h after, and was still high 48 h after. Twenty hours after the injection of the same dose, the concentration of inorganic phosphorus in the serum was significantly lower than in the corresponding controls. The tibias from 17-day-old chick embryos injected with 300 pmoles on day 15 were shorter, lighter, and had a lower ash content than those from controls. Histological signs of resorption appeared to be reduced with respect to controls, but no precise quantitation was conducted. The fact that hypercalcemia was not accompanied by hyperphosphatemia may suggest that the vitamin stimulates resorption of calcium from the shell, which is mainly formed by calcium carbonate rather than from the bone from which calcium and phosphate are usually resorbed together.