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W D Knowles

Publications and source records attributed to W D Knowles.

At least 19 recordsLinked to original sources
PubMed1994

Morphology and projections of neurobiotin-labeled nucleus tractus solitarii neurons recorded in vitro.

The suitability of the anterograde tracer neurobiotin to provide information about the morphology and projections of extracellularly or intracellularly recorded medial nucleus tractus solitarii (nTS) neurons was evaluated in horizontally oriented rat dorsal medulla in vitro slices. After responsiveness to angiotensin (Ang) II, substance P (SP), and L-glutamate was evaluated, neurons were labeled by electrophoresis of neurobiotin at the recording site. Extracellular application (2 microA for 2 min) produced discrete injection sites (40-70 microns) with a small group of labeled neurons. Ejections into the solitary tract documented that the tracer was not taken up by axons traversing the injection site. Neuronal perikarya, primary and secondary dendrites, and axons exhibited a dense Golgi-like appearance, with well-defined dendritic spines and axonal varicosities. Dendritic or axonal processes could be followed for more than 1 mm from the cell soma in a 50 microns thick section, documenting the horizontal architecture of the medial nTS. Intracellular electrophoresis filled the soma, primary and secondary dendrites, and axons of neurons characterized for responsiveness to peptides, L-glutamate and solitary tract stimulation. The location within the nTS and axonal projections of neurons responsive to Ang II and SP appeared to differ from those of cells responsive to Ang II and L-glutamate. Thus, either extracellular or intracellular application of neurobiotin in the in vitro slice can reveal differences in axonal or dendritic targets of neuronal subgroups responsive to different neurotransmitters or peptides and provide evidence for the likely autonomic significance of the neurons.

Afferent Pathways↗
PubMed1994

Pharmacodynamic interactions between phenytoin and valproate: individual and combined antiepileptic and neurotoxic actions in mice.

Although the current trend is to use monotherapy in the treatment of epilepsy, combination therapy is still employed in patients who have failed to respond to monotherapy. There is little clinical or experimental documentation of evidence against or in favor of anticonvulsant combination therapy. In this context, anticonvulsant and neurotoxic pharmacodynamic interactions between phenytoin (PHT) and valproate (VPA) were assessed in an experimental model in mice. All results were expressed in terms of brain drug concentrations for eliminating any pharmacokinetic interaction from the analysis. Both the median neurotoxic and the median anticonvulsant brain concentrations were determined for each drug used alone and for the combination. The interaction for the combination of PHT and VPA was shown to be supraadditive for the anticonvulsant activity, indicating an antiepileptic potentiation, whereas neurotoxicity was simply additive. These results suggest a potential benefit in terms of overall efficacy versus toxicity for the combination of PHT and VPA, as compared with PHT or VPA used alone.

Animals↗
PubMed1992

Normal anatomy and neurophysiology of the hippocampal formation.

This article reviews the anatomy and neurophysiology of the normal hippocampal formation, with emphasis on the human hippocampus. The hippocampus receives inputs from numerous limbic, cortical, and subcortical areas, primarily via the entorhinal cortex and subiculum. The primary pathway of neural activity entering the hippocampus is from entorhinal cortex via the perforant path to the dentate granule cells, with collaterals to CA1 and CA3 pyramidal cells. Mossy fibers from granule cells excite CA3 pyramidal cells and hilar interneurons. CA3 pyramidal cells excite CA1 pyramidal cells, with local and commissural excitatory collaterals exciting other CA3 pyramidal cells and septum. CA1 pyramidal cells send efferent fibers to subiculum, entorhinal cortex, and several subcortical areas. The principal excitatory synapses are glutamatergic, with two important postsynaptic receptor types, alpha-amino-3-hydroxy-5-methyl-isoxazolepropionic acid and N-methyl-D-aspartate. The primary inhibitory transmitter is gamma-aminobutyric acid (GABA), with two postsynaptic receptor types, GABAA and GABAB. A number of modulatory transmitters and neuropeptides are also present. Inhibitory local synaptic networks in the hippocampus are described. Membrane ion channels in hippocampal neurons, particularly Ca2+ channels and K+ channels, are responsible for the regulation and patterning of neural activity. Long-term potentiation and axon sprouting are two experimental paradigms of neural plasticity presumably involved in hippocampal memory function.

Animals↗
PubMed1990

Angiotensin II and angiotensin (1-7) excite neurons in the canine medulla in vitro.

Our group showed previously that the heptapeptide angiotensin (1-7) [Ang-(1-7)] is a bioactive product of the renin-angiotensin system, and produces dose-dependent cardiovascular effects similar to those evoked by Ang II when microinjected into the nucleus tractus solitarii (nTS) of the rat. The effects of Ang II were compared with those of Ang-(1-7) on single neuron activity recorded from the medial nTS or dorsal motor nucleus of the vagus (dmnX) in perifused horizontal slices of the canine dorsomedial medulla. Ang II excited 48% of 31 medial nTS neurons, but only activated 14% of 22 dmnX cells. Ang-(1-7) also excited half of the medial nTS cells and 14% of the dmnXl neurons. Although most medial nTS neurons excited by Ang II were also activated by Ang-(1-7), two cells were excited by Ang II but not by Ang-(1-7), and one cell was excited by Ang-(1-7) but not by Ang II. Because Ang-(1-7) lacks direct vasoconstrictor effects, neurons in the dorsomedial medulla may have different receptor characteristics than peripheral tissues. The observation of a few medial nTS neurons excited by only one Ang peptide suggests that there may be a separate Ang-(1-7) receptor that participates in the physiological effects of Ang peptides mediated by the brain.

Action Potentials↗
PubMed1989

Fetal substantia nigra grafts. Effect on dopamine receptors in the rat corpus striatium.

Effects of fetal substantia nigra grafts on the dopamine receptors in the corpus striatum in rats were investigated after the destruction of the nigrostriatal dopaminergic pathways with intraventricular 6-hydroxydopamine injections. The expected dopamine receptor denervation supersensitivity was demonstrated by a 53.7% increase of [3H]spiroperidol binding in rats with sham grafts compared with normal control-group rats. In contrast, rats with grafts showed a significant reduction of supersensitivity, with a 25% decrease in binding to the graft-bearing caudate when compared with the sham-graft group. A nonsignificant 15% decrease in binding on the nongrafted side was also observed. The fetal substantia nigra grafts thus reduced the denervation supersensitivity toward a normal level.

Animals↗
PubMed1988

Simultaneous measurement of magnetic and electric responses of in vitro hippocampal slices.

Simultaneous measurement of magnetic and electrical activity in in vitro hippocampal slices has been performed. A magnetic signal of 300-400 fT was observed coincident with extracellular electrical potential activity. Comparison is made with a current dipole model similar to that used for the analysis of magnetoencephalographic data, but with additional constraints obtained from the electrical signal. Limitations of this model for the hippocampal slice are discussed.

Animals↗
PubMed1988

Neuronal responses to angiotensin II in the in vitro slice from the canine medulla.

The present studies utilized the in vitro slice preparation of the canine dorsomedial medulla, which we have recently developed, to obtain direct evidence for the effects of angiotensin II (Ang II) on the activity of single neurons in this region. Horizontally oriented slices (300 micron) containing the area postrema, nucleus tractus solitarii (NTS), and dorsal motor nucleus of the vagus were perifused with oxygenated artificial cerebrospinal fluid. The effects of microdrop application of Ang II and its antagonist [Sar1,Thr8]Ang II on spontaneous firing rate were determined in 27 extracellularly recorded neurons. Ang II substantially increased the firing rate of 13 neurons located in the medial NTS, but it did not alter the spontaneous activity of the remaining 14 neurons. In most cases Ang II elicited a slowly developing, prolonged excitatory response. The effects of both Ang II and [Sar1,Thr8]Ang II were tested in 13 neurons. [Sar1,Thr8]Ang II produced a short latency, brief excitation in three neurons, marked inhibition of spontaneous firing in two cells, and no effect on the other eight neurons. Administration of [Sar1,Thr8]Ang II blocked the excitatory effects of subsequent administration of Ang II in three neurons. To our knowledge, these observations provide the first evidence for direct actions of both Ang II and [Sar1,Thr8]Ang II on neurons in the canine NTS and for the specificity of the neuronal effects of Ang II as documented by blockade of the excitatory response to Ang II by [Sar1,Thr8]Ang II.

Action Potentials↗
PubMed1987

The initiation and spread of epileptiform bursts in the in vitro hippocampal slice.

We recorded spontaneous synchronized epileptiform bursts from hippocampal slices from guinea pig using an array of 16 extracellular electrodes placed over the stratum pyramidale of CA2 and CA3. The slices were made epileptogenic with the GABA antagonist picrotoxin (or occasionally penicillin). We found that spontaneous bursts always originate at a discrete focus at or near CA2. These bursts spread smoothly and uniformly across CA3 at an average velocity of 0.13 m/s. This velocity is slower than the conduction velocity of the Schaffer collaterals or mossy fibers. Picrotoxin produced afterdischarges following the initial primary burst, and these afterdischarges were found to originate and spread in a fashion nearly identical to the primary burst. These results indicate that CA2 is a unique region which must possess unusual cellular and/or synaptic connectivity properties which result in a decreased threshold for initiation of epileptiform activity. We consider several hypothetical patterns of local synaptic connectivity in the light of these results, and we discuss the possible role of residual inhibition in limiting the spread of synchronized discharges.

Animals↗
PubMed1987

Models of the cellular mechanism underlying propagation of epileptiform activity in the CA2-CA3 region of the hippocampal slice.

We have shown experimentally in the previous paper that spontaneous epileptiform activity, as recorded by extracellular field potentials, propagates smoothly across the CA2-CA3 region of the convulsant-treated hippocampal slice of the guinea pig at velocities of about 0.1 m/s. In the present paper, we used computer simulations of either 500 or 1000 cell arrays of model neurons to examine possible mechanisms underlying this propagation. We show that propagation of epileptiform field potentials can be explained plausibly by slow conduction along axons interconnecting CA2-CA3 neurons, provided that there are sufficiently many interconnections. This propagation can take place even if the interconnections occur randomly. The number of interconnections required decreases as the number of synchronously activated cells initiating a population burst increases. Axonal propagation at 0.1 m/s appears to be a plausible assumption, since conduction velocities along Schaffer collaterals have been experimentally estimated to be as slow as 0.2 m/s, and small recurrent collaterals are likely to conduct more slowly than the main axonal branches. If spontaneous synchronized population bursts are initiated by activity in four or fewer cells, then our model requires, for smooth field potential propagation, more interconnections than are believed to occur on the basis of dual intracellular recording.

Animals↗
PubMed1985

A portable in vitro brain slice chamber.

A portable chamber is described which is suitable for maintaining live slices of brain tissue while they are being transported by automobile. The chamber supplies nutrient medium, oxygen, temperature control, and shock isolation.

Brain↗
PubMed1985

Computer simulations indicate that electrical field effects contribute to the shape of the epileptiform field potential.

In the presence of convulsant drugs such as picrotoxin, neurons in the hippocampal-slice preparation generate synchronized depolarizing bursts. This synchrony occurs on a time scale of tens of milliseconds and is produced by excitatory synaptic interactions between neurons. The synaptic interactions themselves occur on a time scale of tens of milliseconds. The "epileptiform" local-field potential during such synchronized bursts is comb-shaped ("ringing"), whereas the field potential expected if action potentials in neighboring neurons were uncorrelated is noisy and not comb-shaped. This suggests that individual action potentials are locally synchronized on a time scale of 1 ms. We have previously shown, using computer simulations, that electrical interactions--mediated by currents flowing in the extracellular medium--can plausibly explain action-potential synchronization in experiments where chemical synapses are blocked. The present simulations demonstrate that electrical interactions can also account for action-potential synchronization--and thus the "ringing" shape of the field potential--during epileptiform bursts, where excitatory synapses are functional. The field potential is thus a modulating influence on, as well as a reflection of, underlying neuronal transmembrane events.

Action Potentials↗
PubMed1985

Simulation of hippocampal afterdischarges synchronized by electrical interactions.

Recent experiments have shown that hippocampal pyramidal cells can generate synchronized action potentials even when chemical synapses are blocked. The computer simulations reported here showed that communication between cells by extracellular currents could cause this synchrony, provided that (1) individual neurons were sufficiently excitable and that (2) the resistivity of the extracellular medium was sufficiently high. Synchronization was enhanced if electronic junctions were also present.

Action Potentials↗
PubMed1985

Neuronal interactions during epileptic events in vitro.

Epileptic events can be produced in in vitro brain slices after perfusion with convulsant agents such as penicillin or picrotoxin. These events consist of one or more synchronized neuronal bursts. In this experimental system, epileptic events occur because of blockade of synaptic inhibition by the convulsant agent. A sparse network of excitatory synaptic interconnections in the hippocampus serves to synchronize a population of neurons, each of which is capable of bursting after appropriate stimulation.

Action Potentials↗
PubMed1984

Intracellular study of human epileptic cortex: in vitro maintenance of epileptiform activity?

Intracellular recordings were obtained in the vitro slice preparation from neurons of lateral and mesial temporal cortex removed from human epileptics suffering from intractable temporal lobe seizures. Spontaneous rhythmic synaptic events, which were capable of triggering action potential discharge, were observed in many neurons, particularly in mesial tissue slices. Such activity may reflect the epileptogenic capacity of this human cortex.

Action Potentials↗
PubMed1984

Hyperpolarizing potentials in guinea pig hippocampal CA3 neurons.

There is a bewildering variety of hyperpolarizing potentials which control activity in hippocampal pyramidal cells. These include an inhibitory postsynaptic potential (IPSP) with early and late components, voltage- and calcium-dependent potassium conductances, a voltage-dependent potassium conductance modulated by muscarinic agents (the M-current), and a complex and poorly understood afterhyperpolarization following epileptiform bursts. In hippocampal CA3 pyramidal cells, mossy fiber stimulation elicits an IPSP which is made up of two readily separable components. Using the in vitro slice preparation, we investigated the underlying ionic basis of these IPSP components and compared them to other hyperpolarizing potentials characteristic of the CA3 neurons. Intracellular recordings were obtained and then tissue was exposed to bathing medium low in chloride concentration or high in potassium concentration; the ion "blockers" EGTA (intracellular); tetraethylammonium (TEA) (intra- and extracellular), and barium and cobalt (extracellular); and the gamma-aminobutyric acid (GABA)/chloride antagonists penicillin, bicuculline and picrotoxin.

Animals↗
PubMed1984

Synchronized afterdischarges in the hippocampus: simulation studies of the cellular mechanism.

Synchronized multiple bursts represent an epileptic neuronal behavior transitional between synchronized single bursts (interictal spikes) and self-sustained seizures. As described in the previous paper, synchronized multiple bursts occur in hippocampal slices treated with picrotoxin. Multiple bursts consist of an initial prolonged depolarizing burst followed by a rhythmical series of afterdischarges. Both the initial burst and the afterdischarges are synaptically elicited. Our previously described model of the interictal spike illustrates that the generation of a single synchronized burst requires a neuronal network possessing the following properties: intrinsic bursting capability of individual neurons, the presence of recurrent excitatory connections between principal neurons and the blockade of synaptic inhibition. The model demonstrates that the generation of single synchronized bursts involves the initial excitation of one or more neurons, and the subsequent sequential spread of excitation through a population of neurons via recurrent excitatory synapses. In the present study, we examined whether this same mechanism assumed in the previous model could also allow for the generation of synchronized afterdischarges in a population of neurons. We tested the effects of manipulating three network factors: synaptic strength, synaptic density and the refractoriness in the population members following a period of excitation. We discovered that the refractory period following prolonged excitation assumed in our previous model was insufficient to allow for afterdischarge generation. Once sufficient refractoriness was introduced, afterdischarges appeared in our network of neurons. In the present study, the required refractoriness was attributed to the properties of pyramidal cell axons. In principle, such refractoriness might be located elsewhere in the network. The possible contribution of axonal properties is emphasized because of the known intermittent conduction in other axons. Our present model also reproduced other experimental data. Thus, if the network was too small or if synaptic strength was too small, then only a single synchronized burst occurred. The basic assumptions of this model are both biologically plausible and experimentally testable.

Action Potentials↗
PubMed1983

Studies of human and monkey "epileptic" neocortex in the in vitro slice preparation.

The in vitro slice technique was used to study neuronal activity in human cortical tissue removed during neurosurgical procedures for intractable epilepsy and in monkey neocortex rendered epileptogenic by injection of alumina gel. In both cases, biopsies were guided by electrocorticographic signs of epileptiform activity. Intracellular recordings were made from 167 neurons in human tissue and from 73 neurons in monkey tissue samples. There was little spontaneous activity in these biopsies and no indication of spontaneous cellular bursting. Stimulation at the pial surface or in white matter evoked synaptically driven activity that was primarily excitatory. Graded bursts of activity could be elicited from some cells, but no all-or-none paroxysmal depolarization shifts were recorded. Inhibitory postsynaptic potentials were relatively rare. Intracellular injections of dye in a small number of neurons revealed no obvious differences between bursting and nonbursting neurons. These in vitro studies of chronic epileptic cortex have thus far provided few clear insights into the basic mechanisms of epilepsy.

Action Potentials↗