Biomedical subjects

Robert E Baker

Publications and source records attributed to Robert E Baker.

4 recordsLinked to original sources

PubMed2006

Homeostatically regulated spontaneous neuronal discharges protect developing cerebral cortex networks from becoming hyperactive following prolonged blockade of excitatory synaptic receptors.

In order to further examine the role of spontaneous action potential (SAP) discharges in neocortical development, amino-acid-mediated synaptic transmission was selectively blocked in an improved organotypic neocortex culture preparation. Contralateral occipital cortex slices from neonatal rats were co-cultured for several weeks in a ventricle-to-ventricle orientation known to greatly enhance cyto-morphological and electrophysiological maturation. Such preparations are highly resistant to attempts to suppress neuronal firing by blocking ionotropic glutamate receptors: not only can kainate receptors partly substitute for NMDA- and AMPA-mediated neurotransmission when these receptors are pharmacologically blocked, but (muscarinic) cholinergic receptors also begin to drive SAP activity when the kainate receptors, too, are chronically blocked. Only tetrodotoxin proved able to eliminate SAPs altogether in these co-cultures, while GABAergic receptor blockade (using bicucculine) led to persistent epileptiform discharges. Treatment effects were assayed upon transfer to control medium by means of a quantitative analysis of spontaneously occurring polyneuronal spike trains. Total suppression of action potentials for several weeks (by tetrodotoxin treatment) led, as in earlier experiments, to strongly intensified burst firing upon transfer to control medium. Chronic glutamate receptor blocked cultures, on the other hand, showed only minor deviations from control firing levels and patterns when assayed in normal medium. Protection against the development of hyperactivity despite partial blockade of synaptic transmission was roughly proportional to the degree to which spontaneous firing during the treatment period approximated normal SAP levels. This homeostatic response to chronically reduced excitatory drive thus differs from earlier results using isolated organotypic cortex cultures, in which the restoration of SAP activity failed to prevent the development of network hyperactivity. Chronic bicucculine treatment, in contrast, had little or no homeostatic effect on SAP firing patterns; on the contrary, opposite to earlier findings using isolated occipital cortex explants, paroxysmal discharges persisted even after transfer to control medium.

Action Potentials↗

PubMed2006

Spontaneous neuronal discharge patterns in developing organotypic mega-co-cultures of neonatal rat cerebral cortex.

Sagittal slices of neonatal rat neocortex, extending from the prefrontal to the occipital area, were cultured separately or in pairs, oriented in such a way that axons projecting from the ventricular surface of each explant could innervate the other one. Functional connections were made between as well as within the explants, and spontaneous field potentials and associated action potentials occurred in variable bursts, and with varying degrees of synchrony. Spike-train analysis revealed that the activity patterns seen in these 'mega' co-cultures closely mimic 'tracé alternant' patterns, consisting of trains of burst discharges recurring several times per minute, which are characteristic for the immature intact cerebral cortex during slow-wave sleep. The prefrontal region was consistently less active than the occipital area but the two were qualitatively similar with respect to their patterns of neuronal firing. Isolated mega-cultures, on the other hand, despite their large size, exhibited only intermittent brief bursts that closely resembled those observed both in occipital cortex tissue fragments and in dissociated cell cultures. The mega-co-culture preparation thus appears to give the best currently available approximation to intrinsic cerebral discharge patterns in vivo.

Action Potentials↗

PubMed2005

Compensatory physiological responses to chronic blockade of amino acid receptors during early development in spontaneously active organotypic cerebral cortex explants cultured in vitro.

Paired organotypic explants from rat occipital cortex were cultured for up to three weeks in the presence of selective blockers of amino acid receptor blockers, during which period spontaneous action potential generation was monitored electrophysiologically. In contrast to isolated explants (Corner, M.A., van Pelt, J., Wolters, P.S., Baker, R.E.and Nuytinck, R.H. (2002) Physiological e.ects of sustained blockade of excitatory synaptic transmission on spontaneously active developing neuronal networks--an inquiry into the reciprocal linkage between intrinsic biorhythms and neuroplasticity in early ontogeny. Neurosci. Biobehav. Rev., 26: 127-185), which upregulated their initially depressed spontaneous bursting activity only under conditions of N-methyl D-aspartate (NMDA) receptor blockade, cross-innervated co-cultures showed a large degree of functional recovery even when combined NMDA and AMPA receptor blockade was carried out. This compensatory activity could be eliminated by acute addition of a selective kainate receptor blocker to the medium. When kainate along with AMPA and NMDA receptor mediated activity was chronically suppressed, however, considerable functional recovery--in the form of recurrent burst discharges--took place gradually over a period of three weeks in vitro. These spontaneous bursts disappeared rapidly upon treatment with the muscarinic receptor blocker, atropine, but continuous low-level firing emerged at the same time. Similar "tonic" background activity was induced in control cultures as well, but without any noticeable reduction in burst discharges. Co-cultured neocortex explants, in which cyto-morphological maturation proceeds to a far greater degree than in isolated explants (Baker, R.E.and van Pelt, J. (1997) Co-cultured but not isolated cortical explants display normal dendritic development: a longterm quantitative study. Dev. Brain Res., 98: 21-27) are evidently capable of an astonishing degree of functional compensation for loss of excitatory synaptic drive during development. It could be shown, furthermore, that such homeostatic responses are not mediated largely by a weakening of inhibitory mechanisms in the absence of spontaneous firing. Chronic inhibitory synaptic blockade, on the other hand, led to intensified bursting activity which gradually normalized over a 3-week culture period. The cellular basis for this reversal of the disinhibited state, as well as for the residual neuronal firing even after cholinergic mechanisms have been largely eliminated, is at present unknown. The degree to which immature cortical networks attempt to compensate for altered levels of physiological activity, as documented in the present report, is another indication of how important such activity can be for normal development (see Corner, M.A., van Pelt, J., Wolters, P.S., Baker, R.E. and Nuytinck, R.H. (2002) Physiological e.ects of sustained blockade of excitatory synaptic transmission on spontaneously active developing neuronal networks-an inquiry into the reciprocal linkage between intrinsic biorhythms and neuroplasticity in early ontogeny. Neurosci. Biobehav. Rev., 26: 127-185).. At the same time, the large variations in overall firing levels and "macro-scale" temporal patterns from culture to culture within a given series, despite all attempts at identical preparation of the explants, can only mean that the "set-points" for such regulation are themselves subject to unknown ontogenetic factors which, apparently, are nonuniformly distributed even within a restricted region of the neocortex. On the other hand, it was striking to note that, regardless of age or treatment, an unexpected degree of consistency in temporal patterning existed at "mini-" and "micro-" time-scales (viz., EEG delta and beta frequency ranges, respectively) even when network bursting tendencies became greatly reduced in favor of tonic firing.

Adaptation, Physiological↗

PubMed2002

Cells in human postmortem brain tissue slices remain alive for several weeks in culture.

Animal models for human neurological and psychiatric diseases only partially mimic the underlying pathogenic processes. Therefore, we investigated the potential use of cultured postmortem brain tissue from adult neurological patients and controls. The present study shows that human brain tissue slices obtained by autopsy within 8 h after death can be maintained in vitro for extended periods (up to 78 days) and can be manipulated experimentally. We report for the first time that 1) neurons and glia in such cultures could be induced to express the reporter gene LacZ after transduction with adeno-associated viral vectors and 2) cytochrome oxidase activity could be enhanced by the addition of pyruvate to the medium. These slice cultures offer new opportunities to study the cellular and molecular mechanisms of neurological and psychiatric diseases and new therapeutic strategies.

Aged↗