Crystallization and preliminary X-ray analysis of restriction endonuclease FokI bound to DNA

FokI is a type IIs restriction endonuclease which recognizes an asymmetric DNA sequence and cleaves DNA a short distance away from the sequence. The enzyme is bipartite in nature with its DNA recognition and cleavage functions located on distinct domains. We report here cocrystals of the complete FokI enzyme (579 amino acids) bound to a 20-bp DNA fragment containing its recognition sequence. The complex is amongst the largest protein-DNA complexes to be crystallized, and required macroseeding techniques for optimal crystal growth. The cocrystals diffract to at least 2.8 A in resolution and belong to space group P2(1) with unit cell dimensions of a=67.9 A, b=119.8 A, c=69.1 A, beta = 96.6 degrees. Using specific amino acid analysis we show that asymmetric unit contains a single FokI molecule bound to the 20-bp DNA fragment. This paper reports the first cocrystals of a type IIs restriction endonuclease.

Clinical assessment of MR of the brain in nonsurgical inpatients

PURPOSE

To evaluate the effect of MR imaging of the brain on four domains of patient care: diagnosis, diagnostic workup, therapy, and prognosis.

METHODS

Pre- and post-MR written questionnaires and oral interviews were administered to the referring clinicians of 103 medical and neurologic inpatients at a tertiary care institution. Additional information was obtained from radiologic reports and records.

RESULTS

The study population had a diverse array of signs and symptoms and of presumptive clinical diagnoses, reflecting the breadth of disease seen at our institution. The vast majority of physicians (89%) reported that MR imaging added significant diagnostic information, playing an important role in guiding diagnostic workup (24%), planning treatment (34%), and estimating prognosis (47%). MR imaging was significantly more likely to decrease than to increase confidence in the presumptive clinical diagnosis. Thus, MR imaging may be most useful in the setting of diagnostic uncertainty.

CONCLUSION

Our results show that MR imaging of the brain has important effects on each of the four domains of care for medical inpatients.

Residual pressor effects of chronic alcohol in detoxified alcoholics

Although research in population studies has indicated that recent alcohol intake is positively correlated with blood pressure, there is a need to study the relationship of blood pressure to measures of lifetime alcohol intake in alcoholics. To this end, we assessed systolic and diastolic pressures and lifetime alcohol intake through structured interviews with 253 normotensive recovering alcoholics. Blood pressures were first corrected with multiple linear regression for the influence of confounding or modifying variables and then were regressed against alcohol consumption measures. Systolic pressure was significantly correlated (positively) with only a few measures of recent alcohol intake, and the correlations were not high (r2 = .05 to .11, P < .05). Diastolic pressure was found to be highly and positively correlated with the duration of the drinking career, but more so in blacks than in whites. The total lifetime dose of alcohol was found to be positively correlated with diastolic but not systolic pressure, but only in black male alcoholics. The steeper slope of the regression of blood pressure versus lifetime total alcohol or duration of the drinking career in black alcoholics suggests greater cardiovascular susceptibility to alcohol toxicity as lifetime doses increase and as the drinking career lengthens.

MR spectroscopy in the evaluation of enhancing lesions in the brain in multiple sclerosis

PURPOSE

To compare proton MR spectroscopic alterations with the degree of contrast enhancement in multiple sclerosis (MS) lesions.

METHODS

Thirty-five patients with clinically diagnosed MS were studied with MR spectroscopy. A total of 47 lesions were examined. Solvent-suppressed proton spectra were acquired with an echo time of 16 milliseconds using the point-resolved spectroscopic localization method from 1.5 cm3 voxels. Marker peaks/creatine (Cr) ratios obtained from these spectra were plotted against the relative enhancement of the lesion seen after administration of contrast material. The relative enhancement was defined as S(contrast) -S0/S0, where S0 is the signal intensity of a T1-weighted image and S(contrast) is the signal intensity after administration of gadopentetate dimeglumine.

RESULTS

We found a positive linear relationship between the marker peaks/Cr ratio and the degree of enhancement of the lesion. That is, the marker peaks/Cr ratio was higher in the lesions that showed the greatest enhancement.

CONCLUSION

The MR/Cr ratio obtained with the use of short-echo-time proton MR spectroscopy correlates with the degree of contrast enhancement. This ratio may therefore serve as a means for evaluating and quantifying the level of inflammatory activity within the plaques of patients with MS.

Structure of the even-skipped homeodomain complexed to AT-rich DNA: new perspectives on homeodomain specificity

even-skipped is a homeobox gene important in controlling segment patterning in the embryonic fruit fly. Its homeobox encodes a DNA binding domain which binds with similar affinities to two DNA consensus sequences, one AT-rich, the other GC-rich. We describe a crystallographic analysis of the Even-skipped homeodomain complexed to an AT-rich oligonucleotide at 2.0 A resolution. The structure reveals a novel arrangement of two homeodomains bound to one 10 bp DNA sequence in a tandem fashion. This arrangement suggests a mechanism for the homeoproteins' regulatory specificity. In addition, the functionally important residue Gln50 is observed in multiple conformations making direct and water-mediated hydrogen bonds with the DNA bases.

Visually evoked calcium action potentials in cat striate cortex

Early intracellular studies of cerebral cortical neurons indicated that synaptic input evokes dendritic action potentials that convey information towards the soma. Subsequent work in vitro established that neocortical neurons produce dendritic Ca2+ action potentials. To determine whether natural stimuli elicit Ca2+ spikes, we combined the techniques of whole-cell recording, pharmacology and quantitative receptive field mapping. Our findings show that visual stimulation routinely evoked Ca2+ spikes in distinct functional and anatomical classes of cells in different layers of the cat striate cortex. Hence regenerative Ca2+ potentials appear to play a role in both the initial and later stages of cortical sensory processing.

Application of bioelectric impedance methodology and prediction equations to determine the volume of distribution for ethanol

In large-scale epidemiologic studies of drinking behavior there is a need for simple and reliable estimates of the body water compartment of subjects. This, in turn, provides an estimate of the volume of distribution of ingested ethanol and a better estimate of tissue exposure levels than the use of total body weight as the volume of distribution for alcohol. The volume of distribution for ethanol (total body water, TBW) was estimated in a racially mixed group of 276 alcoholics and 166 nonalcoholics (aged 20-59 years) by means of bioelectric impedance methodology (BIA) and by means of prediction equations based upon age, body weight, and height. Estimations of mean TBW from BIA were found to be only slightly higher (1-4%) than those provided by the prediction equations. TBW values generated from both prediction equations were also highly correlated with TBW values obtained by impedance methodology, with the highest correlations observed in females (particularly black) and in alcoholics (particularly female).

Drinking patterns and health status in smoking and nonsmoking alcoholics

Alcoholics who smoked also reported that they drank more frequently and consumed more alcohol on drinking occasions than alcoholics who did not smoke, a practice that resulted in a substantially greater lifetime alcohol consumption in the smokers. Smoking alcoholics also consumed more cigarettes and reported more smoking-related physical symptoms than social drinkers who smoked. The heart rates (HRs) of smoking and nonsmoking alcoholics were similar and both exceeded the HRs for the smoking social drinkers by approximately 13 beats/min (bpm) in males and by approximately 7 bpm in females. Surprisingly, correlations between HR and lifetime alcohol consumption were higher and slopes were steeper in controls than in alcoholics. HRs in a subset of the male alcoholics fell only approximately 3 bpm after 24 weeks of abstinence, but changed no further over an additional 24-week period. Taken together, the findings suggest that HRs may have been higher in this group of alcoholics before the onset of alcohol abuse and that alcohol intake contributed only slightly to the high HR.

Depression of an inhibitory reflex, the masseteric silent period, in recovering alcoholics

Because inhibitory deficits may contribute to motor incoordination of alcoholics, we proposed that the duration of the masseteric silent period (SP), an inhibitory reflex, might be shorter in alcoholics (ALs) than in nonalcoholics (NAs). To test this hypothesis, we measured the SP in a racially mixed group of 12 ALs and 12 NAs matched for age (31-49 years) and sex. All subjects were normotensive, had full dentition, and reported no major medical problems. Sensory and motor conduction velocities of the anterior tibialis, ulnar, and medial nerves were measured for each subject, and were not different in ALs and NAs. Jaw jerk and SP were evoked by tapping the chin manually with a rubber hammer containing a circuit to trigger the oscilloscope display of the masseter EMGs. Averaged mean latencies and durations of the SPs were not significantly different between the right and left masseters. SP latencies were significantly (0.7 msec) shorter in ALs than NAs (p < 0.01). The mean SP duration for ALs (13.9 +/- 1.6 msec) was also significantly shorter than that of the NAs (24.4 +/- 2.4 msec; p < 0.001). Results support the hypothesis that chronic alcoholism may interfere with inhibitory mechanisms in the CNS.

Purification, crystallization, and preliminary X-ray diffraction analysis of even-skipped homeodomain complexed to DNA

Embryonic development in metazoa, to a significant extent, is directed by genes which contain a conserved sequence motif named the homeobox. This sequence encodes a polypeptide called the homeodomain which has sequence specific DNA-binding activity. We report the purification, crystallization, and preliminary diffraction analysis of the Drosophila Even-skipped homeodomain (Eve HD) bound to two different oligonucleotides. Crystals of Eve HD complexed with an AT-rich sequence belong to space group P2(1), a = 34.06, b = 61.61, c = 39.99 Angstrom, beta = 90.0 degrees. These crystals diffract to at least 2.0 Angstrom and both native and derivative data sets have been collected. Crystals of Eve HD complexed with a GC-rich sequence belong to space group P6(3), a = b = 124.52, c = 66.78 Angstrom and diffract to 3.5 Angstrom resolution. A native data set has been collected.

Synaptic integration in layer IV of the ferret striate cortex

1. Whole-cell patch recording were made with dye-filled electrodes from layer IV in slices of the ferret striate cortex. Projections from the thalamus and layer VI provide most of the extralaminar input to layer IV. Interactions between these two pathways are thought to play a role in the generation of suppressive non-linearities such as end-inhibition. Thus, synaptic responses evoked by stimulating each pathway individually were compared with those produced by activating both projections together. 2. Spiny stellate cells are the majority population in layer IV and were the most frequently patched neurons (n = 23); all fired adapting trains of large, fast action potentials. About half of those tested (n = 13) were progressively inhibited by strengthening the stimulus to layer VI, while the rest became more excited. For the former, the response evoked by stimulating both pathways in coincidence was often more hyperpolarizing than would have been predicted by summing the responses to either projection alone (n = 4). Hence, the inputs from the thalamus and layer VI are integrated by circuits that can produce strong and non-linear inhibition. 3. Recordings from various basket cells, which are inhibitory, have provided a first view of the suppressive circuits in layer IV (n = 5). Two cells were excited by stimulation of both pathways. The remaining three cells were mainly excited by activation of thalamic afferents but were largely inhibited by stimulation of layer VI. Thus, inhibition seen at the level of the spiny stellate cells could result from two mechanisms operating via presynaptic smooth cells: convergent excitation provided by both ascending pathways on the one hand, and a push-pull relationship between pathways on the other.

Small-cell carcinoma of the esophagus with regression after combination chemotherapy and radiation therapy

The authors present an unusual case of small-cell carcinoma of the esophagus, which manifested on double-contrast esophagography as an ulcerated submucosal mass. The lesion underwent dramatic regression after combination chemotherapy and radiation therapy, which has occasionally been used as an alternative to surgery in patients with this rare but aggressive esophageal neoplasm.

Control of breathing patterns and abdominal muscles during graded loads and tilt

Tilting from supine to upright purportedly enhances both segmental and pulmonary proprioceptive feedback, whereas an expiratory threshold load (ETL) preferentially enhances pulmonary feedback. To test this we studied 13 adults when supine and 60 degrees and 90 degrees head up. We measured tidal volume, inspiratory duration (TI), and expiratory duration (TE) from flow; estimated end-expiratory lung volume from inspiratory capacity; and determined burst amplitudes and durations from abdominal electromyograms (EMGs). ETLs were incremented from 0 (control) to 25 cmH2O in 5-cmH2O steps. Tidal volume was significantly increased by ETL but was unaffected by body position. Every load prolonged TE, whereas TI remained unchanged. When subjects were supine, abdominal EMGs were silent but became tonically active when subjects were upright. During ETL, abdominal activity became rhythmical and phase locked to expiration. Bursts amplitudes were enhanced with each increment in ETL, but burst durations did not change even though TE was prolonged. The altered breathing pattern and active expiration augmented inspiratory flow and decreased end-tidal PCO2. Responses were greatest when subjects were 90 degrees head up. The load-related increments in abdominal recruitment, with no change in burst durations, fit the concept of two central pattern generators: one controlling pattern and the other controlling rhythm of the central respiratory drive.

Recovery of respiratory sinus arrhythmia in detoxified alcoholic subjects

Although most alcoholic subjects show little autonomic dysfunction, severe alcoholic subjects may have pathological changes in autonomic nerves. We asked if respiratory sinus arrhythmia amplitude (RSA), an index of vagal cardiac control, is decreased in alcoholism and, if so, whether the decrease is reversed with abstinence. RSA was assessed in 17 normotensive alcoholic subjects (A) at 1, 4, 12, and 24 wk of abstinence after detoxification and at similar intervals in 17 controls (C) matched for age, race, and gender. Subjects were studied in both supine and seated positions while breathing in a prescribed deep (> 50% vital capacity) and slow (5-7/min) pattern. Mean heart rate (HR) was determined over 30 s from the electrocardiogram; RSA (the difference between maximum and minimum instantaneous HRs after inspiratory onset) was determined from 10 consecutive breaths. In C, both HR (supine: 61.5 +/- 2.2 beats/min; seated: 71.3 +/- 1.7 beats/min; P < 0.002) and RSA (supine: 22.5 +/- 1.0 beats/min; seated: 28.4 +/- 1.4 beats/min; P < 0.003) were higher when seated than when supine, but neither HR nor RSA varied over 24 wk. At week 1 of abstinence, HRs for A were higher than those for C (supine: 74.2 +/- 2.3 beats/min, P < 0.001; seated: 83.2 +/- 2.7 beats/min, P < 0.003), but by week 24, both seated and supine values returned to control levels. RSA in A at week 1, was only one-half that of C (supine: 11.1 +/- 1.4 beats/min, P < 0.001; seated: 14.7 +/- 1.9 beats/min, P < 0.001) and independent of body position.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term changes in synaptic strength along specific intrinsic pathways in the cat visual cortex

1. The dense system of horizontal connections that arise and course within the striate cortex are thought to inform single cells about stimuli arising in disparate points in visual space and to modulate responses evoked from within the receptive field. To learn whether or not the strength of the horizontal connections could vary over the long term, and if such changes could affect the integration of vertical, interlaminar inputs, we have recorded intracellularly from the superficial layers in slices of the adult cat's visual cortex. 2. The monosynaptic EPSP evoked by stimulating horizontal fibres showed long-term facilitation in twelve of the twenty cells that were conditioned by repetitively pairing synaptic responses with depolarizing pulses of current; the maximum increase observed was 200%. Strong inhibition present in the postsynaptic response usually indicated that facilitation would not occur. 3. In instances where horizontal input evoked both mono- and polysynaptic EPSPs, both early and late events showed facilitation, with the most dramatic enhancement contributed by the polysynaptic components. 4. For the twenty-eight cells whose responses to stimulation of interlaminar as well as horizontal pathways were assessed, all were found to receive non-overlapping inputs from each source. Conditioning produced long-term changes in the strength of the interlaminar inputs. 5. Changes in synaptic strength were usually confined to the conditioned pathway, though in four out of twenty-six times we observed heterosynaptic facilitation of polysynaptic EPSPs. 6. The conditioning protocol led to lasting depression rather than facilitation in three out of eleven instances; the reduction was only observed in the multisynaptic components. 7. We suggest that the synaptic changes observed here may be related to certain dynamic changes in receptive field properties that have been characterized in vivo.

Respiratory and abdominal muscle responses to expiratory threshold loading in cystic fibrosis

We hypothesized that the hyperinflation and pulmonary dysfunction of cystic fibrosis (CF) would distort feedback and therefore alter the abdominal muscle response to graded expiratory threshold loads (ETLs). We compared the respiratory and abdominal muscle responses with graded ETLs of seven CF patients with severe lung dysfunction with those of matched healthy control subjects in the supine and 60 degrees head-up positions. Breathing frequency, tidal volume, and ventilatory timing were determined from inspiratory flow recordings. Abdominal electromyograms (EMGs) were detected with surface electrodes placed unilaterally over the external and internal oblique and the rectus abdominis muscles. Thresholds, times of onset, and durations of phasic abdominal activity were determined from raw EMGs; peak amplitudes were determined from integrated EMGs. Graded ETLs were imposed by submerging a tube from the expiratory port of the breathing valve into a column of water at depths of 0-25 cmH2O. We found that breathing frequency, tidal volume, and expired minute ventilation were higher in CF patients than in control subjects during low ETLs; a change in body position did not alter these ventilatory responses in the CF patients but did in the control subjects. All CF patients, but none of the control subjects, had tonic abdominal activity while supine. CF patients recruited abdominal muscles at lower loads, earlier in the respiratory cycle, and to a higher recruitment level in both positions than the control subjects, but burst duration of phasic activity was not different between groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Synaptic physiology of horizontal connections in the cat's visual cortex

Horizontal connections are a principal component of intrinsic cortical circuitry. They arise mainly from pyramidal cells and course parallel to the brain's surface for distances as long as 8 mm, linking columns with shared orientation preference and allowing cells to integrate visual information from outside their receptive fields. We examined the synaptic physiology of the horizontal pathway in slices of the cat's striate cortex and found that activating lateral fibers produced both excitation and inhibition. We recorded the postsynaptic responses of identified pyramidal cells in layer 2 + 3 of area 17 to electrical shocks applied at three sites: in the home column of the impaled neuron either in layer 2 + 3 or 4, or at a lateral distance of 0.9-3 mm in layer 2 + 3. Within the home column, suprathreshold stimuli produced compound EPSPs with action potentials, followed by fast, GABAAergic IPSPs and a slower, GABABergic IPSP. For the distant stimulating site, the threshold response was an EPSP. Stronger shocks frequently evoked a disynaptic, GABAAergic IPSP that truncated the EPSP and could dominate the postsynaptic response. At the resting potential, the horizontally evoked EPSP was too small to elicit spikes. With depolarization of the membrane, however, it grew several hundred-fold. This amplification was blocked by N-(2,6-dimethylphenylcarbamoylmethyl)triethylammonium bromide (QX-314), but not by 2-amino-5-phosphonovalerate (APV), indicating that it was mediated by Na+ channels, rather than by NMDA receptors. We propose that the horizontal connections provide the means for stimuli outside the receptive field to modulate activity elicited within its confines. The voltage-dependent enhancement of the laterally evoked EPSP may explain why stimulating the surround by itself fails to drive cells but can facilitate their response to stimuli within the receptive field. The ability to initiate disynaptic inhibition from lateral sites shows that recruiting appropriate groups of horizontal fibers can also have a suppressive effect. Thus, the effect of horizontal input is state dependent, with the size and sign of the laterally evoked response changing according to the balance of converging inputs.

Origin and selection of peripheral CD4-CD8- T cells bearing alpha/beta T cell antigen receptors in autoimmune gld mice

We have analyzed the origin and development of unusual CD4-CD8- alpha/beta T cell receptor-positive peripheral T cells produced in large numbers by mice homozygous for the gld mutation (C3H-gld/gld). These mice may be an important model for investigating processes controlling T cell development. Bone marrow transfers demonstrated that the gld defect was intrinsic to bone marrow-derived cells. Clonal deletion of potentially autoreactive cells was observed in peripheral gld CD4-CD8-, CD4+CD8-, and CD4-CD8+ T cells, as well as mature thymocytes. This suggests that gld CD4-CD8- T cells have passed through the thymus in ontogeny and that gld autoimmunity does not result from a general defect in elimination of self-reactive thymocytes. These observations, combined with demethylation of the CD8 gene in the CD4-CD8- population, support prior expression of CD4 and/or CD8 in gld CD4-CD8- T cell ontogeny, perhaps at a CD4+CD8+ stage. Steroid sensitivity of gld thymocytes and CD4-CD8- T cells was normal. Therefore, we found no gross abnormalities in two major mechanisms of inducible cell death in the gld thymus, the clonal deletion process associated with tolerance and the steroid-inducible endogenous endonuclease thought to be involved in apoptosis of unselected thymocytes. The data suggest that if gld CD4-CD8- T cells arise via escape from normal elimination in the thymus, they must do so by a novel defect in thymic selection (perhaps related to aberrant positive signals) and/or are expanded by an extrathymic process which allows clonal deletion to occur.

Lateral interactions in visual cortex

The findings presented in these studies have brought out different ideas concerning the mechanisms of processing in primary visual cortex than were held at the outset. Rather than thinking of receptive fields as being restricted in their extent, with the process of integration of the components of an image occurring at a much later stage along the visual pathway, we have shown that the integrative process is a progressive one, beginning in the primary visual cortex (or perhaps even earlier) and building up in a cascading series of converging and diverging connections. Rather than thinking of the filter characteristics of a cell as being fixed, it is apparent that they are dynamic and can be modified by the context in which features are presented. Finally, rather than a cortex with a functional architecture that is fixed after a critical period ending in infancy, we find that perturbing the system can lead to long-term topographical reorganization. Other examples of contextual interactions have been demonstrated in the submodalities of motion, where a cell's directional selectivity is modulated by the presence of movement in the surround (Allman et al. 1985; Tanaka et al. 1986; Gulyas et al. 1987; Orban et al. 1987). In the domain of color, the phenomenon of color constancy, reported for cells in visual area V4 (Zeki 1983), also requires lateral interactions in visual space, comparing the wavelength distribution of light coming from surfaces in different parts of the visual field. The influences presented in these studies, as in our own work in the domain of orientation, are modulatory. The long-term changes in cortical topography following removal of somatosensory input (Merzenich et al. 1984, 1988) or by retinal lesions suggest that with the appropriate manipulations the lateral interactions can be enhanced to the point of activating the postsynaptic cells. Although retinal lesions clearly represent an abnormal disruption of sensory input, they may nevertheless be representative of long-term reorganizations of neural networks occurring under normal circumstances, such as those required for memory.

Resting oxygen consumption and ventilation in cystic fibrosis

Resting oxygen consumption (VO2) and minute ventilation (VE) were measured on nine control subjects and 13 patients with cystic fibrosis (CF). In patients with CF, VO2 was 20% higher (P less than 0.01) when expressed per m2 and 47% higher (P less than 0.001) when expressed per kg body weight, and VE was 58% higher when expressed per m2 and 94% higher when expressed per kg body weight (P less than 0.001) than in control subjects. Repeated measures of VO2 and VE were highly reproducible over 2 hours (within-subject coefficients of variation: in controls, VO2 = 5.5%, VE = 7.4%; in CF, VO2 = 3.1%, VE = 5.1%). Since the increases in VE were greater than those in VO2, the ventilatory equivalent (VE/VO2) was 32% higher in CF than in controls. We conclude that size-corrected total energy expenditures and VE at rest are higher in patients with CF than in control subjects. Ventilation appears mechanically inefficient but necessary to keep arterial PCO2 from rising and oxygen saturation from falling at rest. Our results are consistent with observations that these patients have an abnormally large physiological dead space (Featherby et al.: Ann Rev Respir Dis 1969; 102:737).

Intrinsic properties of neurones in the dorsal cochlear nucleus of mice, in vitro

1. Intracellular recordings were made from the dorsal cochlear nucleus (DCN) in slices of the cochlear nuclear complex. Probably the larger and most frequent cells were impaled. 2. The steady-state current-voltage (I-V) properties of all cells impaled were nonlinear. The I-V curve was steepest in the voltage range depolarized from the resting potential and most shallow when the cell was hyperpolarized from rest by more than about 10 mV. Thus, the inwardly rectifying I-V characteristics of cells in the DCN distinguish them from those of ventral cochlear nuclear neurones (Oertel, 1983). 3. When depolarized with current, most cells fired trains of large, all-or-none action potentials. The undershoot after single spikes comprised an initial, fast component followed by a second, slower wave. A few cells (15%) generated bursts of smaller, graded spikes in addition to the large ones. 4. Repetitive firing evoked by depolarizing pulses of current was followed by an after-hyperpolarization whose magnitude depended on the strength and duration of the preceding current pulse. 5. Blocking the large action potentials with tetrodotoxin (TTX) revealed Ca2+-dependent spikes in all cells examined. 6. The steady-state I-V relationship became linear in the presence of TTX, suggesting that a persistent Na+ conductance probably mediates the inward rectification seen above the resting potential. 7. Muscarine at micromolar concentrations excited cells and increased their input resistance.

Synaptic connections in the dorsal cochlear nucleus of mice, in vitro

1. Intracellular recordings were made from the dorsal cochlear nucleus (DCN) in slices that contained the root of the auditory nerve and parts of the dorsal and ventral cochlear nuclei. Probably the largest and most common cells were impaled. 2. Weak shocks to the nerve usually evoked an excitatory postsynaptic potential (EPSP) that lasted about 90 ms and whose latency was often less than 1.2 ms, indicating monosynaptic input. 3. Stronger shocks elicited a larger EPSP and a later train of inhibitory postsynaptic potentials (IPSPs). Increasing the stimulus voltage shortened the latency of the train of IPSPs and increased its efficacy so that at large stimulus strengths inhibition dominated the synaptic response. 4. To determine whether any of the neuronal circuitry which generated the synaptic responses involved the ventral cochlear nucleus, recordings were made from slices containing only the dorsal nucleus. Synaptic responses to stimulation of the pial surface of the isolated DCN resembled those driven from the nerve root. That is, weak shocks evoked long-lasting, monosynaptic EPSPs and stronger stimuli elicited a larger EPSP followed by trains of IPSPs. The DCN, therefore, contains intrinsic inhibitory interneurones. 5. The parallel fibres of the DCN course superficially, near the stimulating electrodes, whereas the axons of the auditory nerve terminate in deeper areas. Thus, the monosynaptic EPSPs evoked from the pial surface are probably generated by parallel fibres. Apparently the inhibitory interneurones are also excited by a circuit including parallel fibres. 6. The putative neurotransmitter of parallel fibres, glutamate, excited all neurones tested. 7. Cells were sensitive both to glycine and to gamma-aminobutyric acid (GABA). Only strychnine, however, not picrotoxin or bicuculline, blocked IPSPs.

Responses of neurons in the cat's superior colliculus to acoustic stimuli. II. A model of interaural intensity sensitivity

Most neurons in the deep and intermediate layers of the superior colliculus (SC) that respond to acoustic stimuli are sensitive to interaural intensity disparities (IIDs). We examine a model for the generation of sensitivity to IIDs that depends upon temporal coincidence of the inputs from each ear at a given binaural neuron. Because the neural response latency decreases with increasing stimulus intensity, IIDs affect the relative timing of arrival of the inputs. If this model were true, the neurons sensitive to IIDs should also respond to interaural time differences (ITDs) of isointensive stimuli, provided that the magnitude of the delays reflect the neural latency-intensity relationship. For both major classes of binaural cells in the SC, namely those that exhibit binaural inhibition (BI) and binaural facilitation (BF), our results support the model in that the detection of IIDs is largely due to their sensitivity to the temporal overlap of inputs from each ear. The shapes of the IID and ITD functions for each class are similar. The summation of inputs includes inhibitory as well as facilitatory interactions. Estimates of the durations of the subliminal excitatory events in BF cells using the model indicate that they are relatively short (1-4 ms), whereas the durations of the inhibitory processes in BI cells are much longer. The model specifies a common neuronal mechanism for comparison of interaural disparities of time and intensity and does not separate the processing of IIDs and ITDs, as the classic duplex theory suggests. The model provides a physiological explanation for certain features of the psychophysical phenomenon of time-intensity trading. It is also consistent with recent experiments that have shown that the auditory system is sensitive to behaviorally significant ITDs of high-frequency complex signals. The model applies only to the processing of transient stimuli and does not address neural sensitivity to IIDs of continuous high-frequency tones.

Responses of neurons in the cat's superior colliculus to acoustic stimuli. I. Monaural and binaural response properties

Using extracellular electrodes we studied acoustic responses in the superior colliculus (SC) of the barbiturate-anesthetized cat. Pure tonal stimuli were delivered through sealed and calibrated earphones and were presented either monaurally or binaurally with interaural intensity differences (IIDs) and interaural time differences (ITDs). Acoustically sensitive cells were found in the intermediate and deep layers of the SC throughout its rostrocaudal and mediolateral extent. Most cells (80%) discharged only at stimulus onset; the rest had more complex firing patterns. For 88% of our sample the mean first-spike latency measured at 20 dB above threshold ranged between 6 and 16 ms. The sharpness and threshold intensity of the frequency tuning curves varied widely. In the SC, the average characteristic frequency and threshold intensity were higher than in other auditory brain stem nuclei. Neurons whose characteristic frequency was low were never sharply tuned. The probability of response decreased when the repetition rate at which the stimuli were delivered increased. The mean stimulus interval at which spike count reached 50% of maximum was 360 ms. Most (83%) of the cells discharged only to monaural stimulation of the contralateral ear, 7% responded to tones applied to either ear and only 1% to only ipsilateral input. The remaining cells responded only to stimulation of both ears. With binaural stimuli, most neurons (80%) could be shown to receive input from both ears. Seventy percent of the binaural cells showed predominant binaural inhibition (BI), 25% binaural facilitation (BF), and 5% a more complex mixture. Because the majority of SC neurons had high characteristic frequencies, we examined their responses to IIDs. The spike count vs. IID functions of BI cells were monotonic and sigmoidal, those of BF cells were nonmonotonic and bell-shaped. The slopes and horizontal positions of the curves varied among neurons. IIDs favoring the contralateral ear were the most effective. For a given cell, increasing the mean binaural level extended the range of IIDs that evoked maximal discharge. A small number of cells was sensitive to physiologically significant interaural time differences of low-frequency tones or the envelopes of amplitude-modulated, high-frequency tones.

Structure-activity relationship between guanidine alkyl derivatives and norepinephrine release: site(s) and mechanism(s) of action

The effect of guanidine alkyl derivatives on the evoked release of [3H]norepinephrine [( 3H]NE) from spleen strips was examined. Guanidine, methyl guanidine and N,N-dimethyl guanidine all enhanced the field-stimulated release of [3H]NE 2- to 3-fold, whereas N,N'-dimethyl guanidine and propyl guanidine were without effect. The latter compound blocked the stimulatory effect of an equimolor concentration (4 mM) of guanidine. Guanidine enhanced moderately the field-stimulated release of [3H]NE from spleen strips pretreated with phenoxybenzamine. The efflux of [3H]NE from spleen slices induced by calcium ionophore A-23187 was not altered by guanidine incubation. The effect of guanidine on intracellular calcium movement was also tested by monitoring the effect of the drug on evoked secretion of ATP from human platelets. Guanidine did not modify this release. It is concluded that guanidine and its active structural derivatives augment [3H]NE release by increasing the influx of calcium through the voltage-sensitive calcium channels, but not by the mobilization of intracellular calcium pools. The biochemical basis for the action of the guanidinium cation is discussed.