Brain slice

Kohhng, R., Melani, R., and Koch, U. et al. (2005). Detection of electrophysiological indicators of neurotoxicity in human and rat brain slices by a three dimensional microelectrode array. ATLA 33, 579-589. 

Noraberg, J. (2004). Organotypic brain slice cultures an efficient and reliable method for neu-rotoxicological screening and mechanistic studies. ATLA 32, 329-337. 

Since these studies utilized autoradiographic techniques, it was important to determine the chemical nature of the material measured using radiochemical procedures. Using punch biopsies of the brain slices which were measured autoradiographically, it was shown using thin-layer chromatography that about 30% of the radioactivity was associated with unchanged BCNU (14). It was therefore concluded that the measurements described above accurately reflect brain concentrations of BCNU. 

Because of their strategic localization, astrocytes play a crucial role in maintaining the extracellular ionic homeostasis, provide energetic metabolites to neurons and remove excess of neurotransmitter in schedule with synaptic activity. In addition, the strategic location of astrocytes allows them to carefully monitor and control the level of synaptic activity. Indeed, number of papers during the last 15 years have shown that cultured astrocytes can respond to a variety of neurotransmitters with a variety of different patterns of intracellular calcium increases (Verkhratsky et al. 1998). Later on, studies performed in intact tissue preparations (acute brain slices) further established that the plasma membrane receptors can sense external inputs (such as the spillover of neurotransmitters during intense synaptic activity) and transduce them as intracellular calcium elevations, mostly via release of calcium from internal stores (Dani et al. 1992 Murphy et al. 1993 Porter and McCarthy... 

There is also the interesting possibility that presynaptie inhibition of this form, with or without potential changes, need not be restricted to the effect of the NT on the terminal from which it is released. Numerous studies in which brain slices have been loaded with a labelled NT and its release evoked by high K+ or direct stimulation show... 

Figure 1.9 Comparison of the effects of an endogenously released and exogenously applied neurotransmitter on neuronal activity (identity of action). Recordings are made either of neuronal firing (extracellularly, A) or of membrane potential (intracellularly, B). The proposed transmitter is applied by iontophoresis, although in a brain slice preparation it can be added to the bathing medium. In this instance the applied neurotransmitter produces an inhibition, like that of nerve stimulation, as monitored by both recordings and both are affected similarly by the antagonist. The applied neurotransmitter thus behaves like and is probably identical to that released from the nerve...

These approaches to receptor identification and classification were, of course, pioneered by studies with peripheral systems and isolated tissues. They are more difficult to apply to the CNS, especially in in vivo experiments, where responses depend on a complex set of interacting systems and the actual drug concentration at the receptors of interest is rarely known. However, the development of in vitro preparations (acute brain slices, organotypic brain slice cultures, tissue-cultured neurons and acutely dissociated neuronal and glial cell preparations) has allowed more quantitative pharmacological techniques to be applied to the action of drugs at neurotransmitter receptors while the development of new recording methods such as patch-clamp... 

This can be carried out in vitro (in brain slices, cultured cell preparations) or in vivo and involves penetrating the experimental tissue with a carbon-fibre electrode of 5-30 pm in diameter (Fig. 4.9). This serves as an oxidising electrode and the Faradaic current generated by the oxidation of solutes on the surface of the electrode is proportional to their concentration. Obviously, only neurotransmitters which can be oxidised can be measured in this way so the technique is mainly limited to the study of monoamines and their metabolites. The amplitude of each peak on the ensuing voltammogram is a measure of solute concentration and individual peaks can be identified because different... 

Pali), P and Stamford, JA (1993) Real-time monitoring of endogenous noradrenaline release in rat brain slices using fast cyclic voltammetry. 2. Operational characteristics of the alpha2 autoreceptors in the bed nucleus of the stria terminalis, pars ventralis. Brain Res. 608 134-140. 

A number of studies in fact show clear Di effects. Intracellular recording from striatal neurons in rat brain slices show a cAMP-mediated Di-dependent (blocked by SCH 23390) suppression of a voltage-dependent sodium current which make the cell less responsive. 

Murugaiah, KD and O Donnell, JM (1995) Facilitation of noradrenaline release from rat brain slices by beta-adrenoceptors. Naunyn-Schmiedebergs Arch. Pharmacol. 351 483--490. 

Not a great deal is known about factors that actually activate tryptophan hydroxylase. In particular, the relative contribution of tryptophan supply versus factors that specifically modify enzyme activity under normal dietary conditions is unknown. However, removal of end-product inhibition of tryptophan hydroxylase has been firmly ruled out. Also, it has been established that this enzyme is activated by electrical stimulation of brain slices, even in the absence of any change in tryptophan concentration, and so other mechanisms are clearly involved. 

ATP certainly fulfils the criteria for a NT. It is mostly synthesised by mitochondrial oxidative phosphorylation using glucose taken up by the nerve terminal. Much of that ATP is, of course, required to help maintain Na+/K+ ATPase activity and the resting membrane potential as well as a Ca +ATPase, protein kinases and the vesicular binding and release of various NTs. But that leaves some for release as a NT. This has been shown in many peripheral tissues and organs with sympathetic and parasympathetic innervation as well as in brain slices, synaptosomes and from in vivo studies with microdialysis and the cortical cup. There is also evidence that in sympathetically innervated tissue some extracellular ATP originates from the activated postsynaptic cell. While most of the released ATP comes from vesicles containing other NTs, some... 

There are many studies on the induction and spread of spiking in animals both in vivo and in isolated brain slices, generally initiated by the use of GABA antagonists or removal of Mg + ions in vitro). Unfortunately since neither of these events is likely to occur in or around a human epileptic focus the results do not tell us much about how focal activity arises and spreads in humans. This needs to be achieved by the use of human epileptic tissue even though the procedures found to control experimentally induced spiking may well be applicable to humans. 

Inhibition of glutamate release was thought to be the mode of action of lamotrigine. It reduces MBS and kindling and also glutamate (and to a lesser extent GABA) release induced in brain slices by veratridine, which opens sodium channels. But it now seems likely that the actual block of sodium channels is its primary action (see later). 

Masukawa, LM, Higashima, M, Kim, JH and Spencer, DD (1989) Epileptiform discharges evoked in hippocampal brain slices from epileptic patients. Brain Res. 493 168-174. 

Johnson, M.P. Hoffman, A.J. and Nichols, D.E. Effects of the enantiomers of MDA, MDMA and related analogs on [ H] serotonin and [ Hjdopamine release from superfused rat brain slices. Eur J Pharmacol 132 269-276, 1986. 

Gifford AN, Tang Y, Gatley SJ, Volkow ND, Lan R, Makriyannis A. Effect of the cannabinoid receptor SPECT agent, AM 281, on hippocampal acetylcholine release from rat brain slices. Neurosci Lett 1997 238 84-86. 

Aghajanian, G. K. Lakoski, J. M. (1984). Hyperpolarization of serotoninergic neurons by serotonin and LSD studies in brain slices showing increased K+ conductance. Brain Res. 305, 181-5. 

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