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T H Brown

Publications and source records attributed to T H Brown.

At least 37 records · Page 2Linked to original sources
PubMed1996

Amygdala stimulation enhances the rat eyeblink reflex through a short-latency mechanism.

Amygdala stimulation was shown to enhance the trisynaptic (fast, R1) component of the electromyogram recorded in the rat orbicularis oculi (oo) muscle, which is responsible for the active force generating eyelid closure. The eyeblink was elicited via direct electrical stimulation of the supraorbital branch of the trigeminal nerve. Possible mechanisms responsible for the effect of amygdala stimulation on the eyeblink reflex were evaluated by measuring the amount of R1 enhancement as a function of the interstimulus interval (ISI) between the onset of amygdala and trigeminal nerve stimulation. Amygdala stimulation produced significant R1 enhancement at ISIs that imply short-latency excitation of the eyeblink circuit by way of a fast-acting neurotransmitter.

Amygdala↗
PubMed1996

Electrotonic architecture of hippocampal CA1 pyramidal neurons based on three-dimensional reconstructions.

1. The spread of electrical signals in pyramidal neurons from the CA1 field of rat hippocampus was investigated through multicompartmental modeling based on three-dimensional morphometric reconstructions of four of these cells. These models were used to dissect the electrotonic architecture of these neurons, and to evaluate the equivalent cylinder approach that this laboratory and others have previously applied to them. Robustness of results was verified by the use of wide ranges of values of specific membrane resistance (Rm) and cytoplasmic resistivity. 2. The anatomy exhibited extreme departures from a key assumption of the equivalent cylinder model, the so-called "3/2 power law." 3. The compartmental models showed that the frequency distribution of steady-state electrotonic distances between the soma and the dendritic terminations was multimodal, with a large range and a sizeable coefficient of variation. This violated another central assumption of the equivalent cylinder model, namely, that all terminations are electronically equidistant from the soma. This finding, which was observed both for "centrifugal" (away from the soma) and "centripetal" (toward the soma) spread of electrical signals, indicates that the concept of an equivalent electrotonic length for the whole dendritic tree is not appropriate for these neurons. 4. The multiple peaks in the electrotonic distance distributions, whether for centrifugal or centripetal voltage transfer, were clearly related to the laminar organization of synaptic afferents in the CA1 region. 5. The results in the three preceding paragraphs reveal how little of the electrotonic architecture of these neurons is captured by a simple equivalent cylinder model. The multicompartmental model is more appropriate for exploring synaptic signaling and transient events in CA1 pyramidal neurons. 6. There was significant attenuation of synaptic potential, current, and charge as they spread from the dendritic tree to the soma. Charge suffered the least and voltage suffered the most attenuation. Attenuation depended weakly on Rm and strongly on synaptic location. Delay of time to peak was more distorted for voltage than for current and was more affected by Rm. 7. Adequate space clamp is not possible for most of the synapses on these cells. Application of a somatic voltage clamp had no significant effect on voltage transients in the subsynaptic membrane. 8. The possible existence of steep voltage gradients within the dendritic tree is consistent with the idea that there can be some degree of local processing and that different regions of the neuron may function semiautonomously. These spatial gradients are potentially relevant to synaptic plasticity in the hippocampus, and they also suggest caution in interpreting some neurophysiological results.

Animals↗
PubMed1995

Reversible inhibitors of the gastric (H+/K+)-ATPase. 4. Identification of an inhibitor with an intermediate duration of action.

3-Acyl-4-(arylamino)quinolines were previously identified as gastric (H+/K+)-ATPase inhibitors, and clinical efficacy has been demonstrated for compound 3 (SK&F 96067). In the present study the further structure-activity relationship of this series is developed. Only a limited range of substituents are tolerated on the N-aryl ring or the 6- and 7-positions of the quinoline, and although hydroxylated derivatives were identified possessing markedly greater affinity for the enzyme, none of these proved to have adequate potency after oral dosing. In contrast, the 8-position of the quinoline ring proved suitable for a wide variety of substituents, allowing modification of physicochemical properties while retaining primary activity. This led to the identification of compound 4 (SK&F 97574), which combines good oral potency with a somewhat longer duration of action than 3 (though much shorter than covalent inhibitors such as omeprazole). This compound was selected for further development and evaluation in man.

Magnetic Resonance Spectroscopy↗
PubMed1995

Reversible inhibitors of the gastric (H+/K+)-ATPase. 5. Substituted 2,4-diaminoquinazolines and thienopyrimidines.

Quinazolines bearing a secondary 4-(arylamino) substituent demonstrate an SAR for inhibition of the gastric (H+/K+)-ATPase different from the previously described 3-acylquinolines, suggesting that, although these compounds are also K(+)-competitive, they probably bind to the enzyme in a different orientation. Compounds bearing a tertiary 4-(arylamino) substituent, however, in particular 4-(N-methylarylamino), appear to possess an SAR quite similar to the 3-acylquinolines. We show that this arises from the effect of the N-methylation, which is to orientate the 4-(arylamino) substituent syn to C5, analogous to the 3-acylquinolines. Compounds possessing both a 4-(N-methylarylamino) substituent and a 2-(arylamino) substituent proved to be very potent, K(+)-competitive inhibitors of K(+)-stimulated ATPase activity with Ki values down to 12 nM. Some compounds also proved to be effective inhibitors of stimulated acid secretion in both the rat and dog when dosed intravenously. However, although a number of these demonstrated activity after oral administration in the dog, the level and variability precluded further evaluation.

Animals↗
PubMed1994

Confocal imaging of dendritic Ca2+ transients in hippocampal brain slices during simultaneous current- and voltage-clamp recording.

Changes in the intracellular Ca2+ concentration ([Ca2+]i) within CA1 hippocampal pyramidal neurons were imaged using confocal laser scanning microscopy in conjunction with Ca(2+)-sensitive fluorescent indicators. The imaging was performed in thick hippocampal brain slices while simultaneously measuring or controlling electrical activity with sharp microelectrodes or whole-cell patch-clamp electrodes. The combination of imaging and electrophysiology was essential for interpreting the changes in [Ca2+]i. We compared the increases in [Ca2+]i produced by either of two methods--direct depolarization of the cell via the somatic electrode or high-frequency stimulations of synaptic inputs. The increases in [Ca2+]i in the soma and proximal dendrites caused by both methods were of comparable magnitude and they always decayed within seconds in healthy cells. However, the spatial patterns of distal Ca2+ increases were different. Separate sets of synaptic inputs to the same cell resulted in different spatial patterns of [Ca2+]i transients. We isolated and observed what appeared to be a voltage-independent component of the synaptically mediated [Ca2+]i transients. This work demonstrates that the combination of neurophysiology and simultaneous confocal microscopy is well suited for visualizing and analyzing [Ca2+]i changes within highly localized regions of neurons in thick brain slices. The approach should allow further analysis of the relative contribution of voltage- and agonist-dependent influences on [Ca2+]i within neurons throughout the CNS and it raises the possibility of routinely relating subcellular [Ca2+]i changes to structural and functional modifications.

Aging↗
PubMed1994

Confocal laser scanning microscopy reveals voltage-gated calcium signals within hippocampal dendritic spines.

The induction of long-term potentiation (LTP) is generally assumed to be triggered by Ca2+ entry into dendritic spines via NMDA receptor-gated channels. A previous computational model proposed that spines serve several functions in this process. First, they compartmentalize and amplify increase in [Ca2+]i. Second, they augment the nonlinear relationship between synaptic strength and the probability or magnitude of LTP induction. Third, they isolate the metabolic machinery responsible for LTP induction from increases in [Ca2+]i produced by voltage-gated Ca2+ channels in the dendritic shaft. Here we examine this last prediction of the model using methods that combine confocal microscopy with simultaneous neurophysiological recordings in hippocampal brain slices. Either of two Ca(2+)-sensitive dyes were injected into CA1 pyramidal neurons. Direct depolarization of the neurons via the somatic electrode produced clear increases in Ca2+ signals within the dendritic spines, a result that was not predicted by the previous spine model. Our new spine model suggests that some of this signal could theoretically result from Ca(2+)-bound dye diffusing from the dendritic shaft into the spine. Dye diffusion alone cannot, however, explain the numerous cases in which the Ca2+ signal in the spine was considerably larger than that in the adjacent dendritic shaft. The latter observations raise the possibility of voltage-gated Ca2+ entry directly into the spine or else perhaps via Ca(2+)-dependent Ca2+ release. The new spine model accommodates these observations as well as several other recent experimental results.

Calcium↗
PubMed1994

Three-dimensional quantification of mossy-fiber presynaptic boutons in living hippocampal slices using confocal microscopy.

Confocal laser scanning microscopy (CLSM) was used to visualize presynaptic elements of mossy-fiber synapses in living rat hippocampal slices. Mossy fiber (mf) axons and their boutons were labeled in transverse hippocampal slices by injecting one of three fluorescent dyes (diI, diA, or fast diI) into stratum granulosum of the dentate gyrus and/or stratum lucidum of CA3. Three-dimensional (3D) images of the mf boutons were obtained from serial optical sections in stratum lucidum. The 3D data were used to quantify and characterize the size and shape of the boutons based on their volumes (V) and surface areas (A), and the latter were compared with conventional 2D analyses. Various geometric models were fitted to the V-A relationship, one of which provided a reasonable approximation to the data. The results demonstrate that this approach is useful for quantifying and characterizing the size and shape of mf expansions and raise the possibility of detecting and analyzing in real time activity-dependent modifications in presynaptic structure.

Animals↗
PubMed1994

Quantal mechanism of long-term potentiation in hippocampal mossy-fiber synapses.

1. The quantal mechanism underlying the expression of long-term potentiation (LTP) was studied in the mossy-fiber (mf) synapses of the rat hippocampus. Whole-cell recordings were used to measure the excitatory postsynaptic currents (EPSCs) before and after LTP induction in brain slices maintained at 31 +/- 1 degrees C. 2. Evoked EPSCs were recorded from 473 CA3 pyramidal neurons. The mf synapses were stimulated using paired pulses (40-ms interpulse interval) repeated every 2-10 s. At least 400 pairs of mf responses were obtained before and during the expression of LTP, which was produced by high-frequency (100 Hz) mf stimulation. Sufficiently stationary data were obtained from five neurons that exhibited LTP and that also satisfied strict criteria and procedures that are necessary for eliciting and identifying unitary mf responses. 3. Three independent lines of evidence implicated a presynaptic component to the mechanism underlying mf LTP. The first was based on a graphical version of the classical method of variance. The graphical variance (GV) method was evaluated by clamping the cell at two different holding potentials during paired-pulse facilitation (PPF). The results indicated that the GV method can distinguish changes in mean quantal content m and mean quantal size q in rat mf synapses. The same analysis, when applied to PPF before and after LTP induction, indicated that both result from an increase in m. 4. The second line of evidence was based on the classical method of failures. Consistent with the inference that mf LTP is due to an increase in m, there was a statistically significant reduction in the number of quantal release failures.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1994

Metabotropic glutamate receptor activation induces calcium waves within hippocampal dendrites.

1. We investigated the effects of metabotropic glutamate (mGlu) receptor activation on intracellular Ca2+ concentration ([Ca2+]i) in the soma and dendrites of hippocampal CA1 pyramidal neurons. Changes in [Ca2+]i were measured using confocal imaging simultaneously with whole-cell recording techniques. Differences in [Ca2+]i were visualized as changes in the fluorescence of the Ca(2+)-sensitive dye Fluo-3. 2. Brief application of the specific mGlu receptor agonist (1S,3R)-ACPD to either the apical or basal dendrites produced initially localized increases in [Ca2+]i that subsequently propagated as waves throughout much of the neuron. These Ca2+ waves, which propagated at approximately 40 microns/s, were shown not to reflect intracellular Ca2+ diffusion or extracellular diffusion of ACPD and were always accompanied by small outward membrane currents. 3. Repetitive application of ACPD failed to trigger further Ca2+ release. We found that a threshold level of voltage-gated Ca2+ entry during trains of action potentials was needed to prime further mGlu-stimulated Ca2+ release. In contrast, the passage of time alone did not cause the mGlu-release system to reactivate--restoration of ACPD-stimulated Ca2+ release. The spike-mediated Ca2+ signal was unaffected by mGlu-stimulated depletion of intracellular stores. 4. These experiments demonstrate that specific mGlu receptor activation can mobilize Ca2+ in dendrites of CA1 neurons and trigger waves of Ca(2+)-induced Ca(2+)-release throughout the cell. A use-dependent relationship between voltage-gated Ca2+ entry during trains of action potentials and mGlu-stimulated Ca2+ release is suggested.

Animals↗
PubMed1993

Hippocampal circuitry complicates analysis of long-term potentiation in mossy fiber synapses.

Much of the current interest in the hippocampus concerns a rapid and persistent form of synaptic plasticity, called long-term potentiation (LTP), that is a candidate substrate for some of the mnemonic functions of this structure. There are at least two kinds of LTP in the hippocampus. One form is found at the synapse between the mossy fibers of the granule cells and the pyramidal neurons of the CA3 region. Attempts to examine the mechanism underlying this form of LTP have yielded contradictory conclusions. The authors show how the complex circuitry of the dentate gyrus and adjacent hippocampus may have caused the contradictions. To overcome problems introduced by the circuitry, a specific set of procedures and criteria for evoking and identifying mossy fiber responses is proposed. Use of these or similar procedures and criteria will improve the design and interpretation of experiments on mossy fiber LTP and allow more informative comparisons among species and brain regions and across laboratories.

Animals↗
PubMed1993

SB 201823-A, a neuronal Ca2+ antagonist is neuroprotective in two models of cerebral ischaemia.

We have characterised the Ca2+ channel blocking properties of a new non-peptide Ca2+ channel antagonist, SB 201823-A, in cultures of rat sensory neurones. The IC50 for SB 201823-A against total Ca2+ current in sensory neurones was 4.9 microM. SB 201823-A showed little selectivity for sub-types of neuronal Ca2+ channel but was selective for Ca2+ channels over Na+ and K+ channels. Efficacy against other types of cation channel such as agonist gated channels was not assessed. SB 201823-A was neuroprotective in vivo when administered post-ischaemia in one focal and one global model of neuronal ischaemia. In the rat photothrombotic focal lesion model, SB 201823-A administered i.p. 10 min post-ischaemia resulted in a dramatic reduction in lesion volume. In the gerbil bilateral carotid artery occlusion global model, SB 201823-A dosed i.p. 30 min post-occlusion resulted in both histological and functional improvements when compared to vehicle treated animals. These data suggest that such novel neuronal Ca2+ channel antagonists may have potential in ameliorating both the pathological and functional consequences of stroke in man.

Animals↗
PubMed1992

Reversible inhibitors of the gastric (H+/K+)-ATPase. 3. 3-substituted-4-(phenylamino)quinolines.

Previously, gastric (H+/K+)-ATPase inhibitors such as 2 have been prepared as analogues of 1a on the presumption that the 3-carbethoxy substituent plays a key role in establishing the orientation of the 4-arylamino group. In this paper we explore further the contribution made to activity by the quinoline 3-substituent. We show that, for compounds bearing such a substituent, only a particular combination of properties provides high activity, both in vitro and as inhibitors of gastric acid secretion in vivo. The ability of the substituent to affect activity by restricting rotation about the Cquin-N bond through a combination of both a pi-electron withdrawal and hydrogen bonding is supported by the current study. However, high activity is only achieved if the effect of this group on the quinoline pK(a) is kept to a minimum. 3-Acyl substituents provide an optimum combination of electronic properties. From this series, compound 17c (SK&F 96067) was shown to be a potent inhibitor of histamine-stimulated gastric acid secretion after oral dosing in the Heidenhain pouch dog and was selected for further development and evaluation in man.

Adenosine Triphosphatases↗
PubMed1992

Reversible inhibitors of the gastric (H+/K+)-ATPase. 2. 1-Arylpyrrolo[3,2-c]quinolines: effect of the 4-substituent.

Further work on compounds 1 has identified the 4-position as a site where substantial modifications are tolerated, leading to analogues which are more potent and less toxic than those described previously. The best compound in the series is 13a (SK&F 96356), which is a potent inhibitor of gastric acid secretion in both the pentagastrin-stimulated rat and the histamine-stimulated dog. This compound shows reversible, K(+)-competitive binding to the enzyme. Because of its fluorescent properties, it is also proving useful in vitro as a probe of the structure and function of the (H+/K+)-ATPase.

Adenosine Triphosphatases↗
PubMed1992

Synchronous aortic and gastrointestinal surgery.

The synchronous performance of aortic and gastrointestinal surgery may hold an increased risk of aortic graft infection. The experience of one surgeon who performed 32 such synchronous operations over a period of 11 years is reviewed. No graft infection occurred and it is concluded that such combined operations may be safer to perform than staged surgery in elderly patients.

Aged↗