Push-pull cannula for localized application of drugs and sampling of medium, combined with electrophysiological recordings in an interface slice chamber

Efficient measurement of endogenous neurotransmitters in small localized regions of central nervous systems in vitro with HPLC

High performance liquid chromatography (HPLC) is widely used to determine neurotransmitter concentrations in the central nervous system (CNS). Finding the optimal methods to sample from CNS tissue poses a challenge for neuroscientists. Here, we describe a method that allows assay of neurotransmitters (or other chemicals) in small regions (down to 180mum in diameter) in in vitro preparations concurrently with electrophysiological recordings. The efficiency for measuring small amounts of chemicals is enhanced by a sample collecting pipette with filter paper at the tip that makes close contact with the target region in CNS tissue. With a wire plunger in the calibrated pipette controlled by a microsyringe pump, there is virtually no dead volume. Samples in a volume of 10muL (taken, e.g., at 2muL/min over 5min) can be injected into a HPLC machine with microbore columns. We demonstrate the effectiveness of this method by measuring acetylcholine (ACh) in the ventral horn and its surrounding areas of the spinal cord in en bloc brainstem-spinal cord preparations. In control conditions, endogenous ACh levels in these regions were detectable. Application of neostigmine (an inhibitor of acetylcholinesterases (AChEs)) increased ACh concentrations, and at the same time, induced tonic/seizure-like activity in efferent motor output recorded from cervical ventral nerve roots. Higher ACh concentrations in the ventral horn were differentiated from nearby regions: the lateral and midline aspects of the ventral spinal cord. In addition, ACh in the preBötzinger Complex (preBötC) and the hypoglossal nucleus in medullary slice preparations can also be measured. Our results indicate that the method proposed in this study can be used to measure neurotransmitters in small and localized CNS regions. Correlation between changes in neurotransmitters in target regions and the neuronal activities can be revealed in vitro. Our data also suggest that there is endogenous ACh release in spinal ventral motor columns at fourth cervical (C4) level that regulates the respiratory-related motor activity.

Simultaneous comparison of cerebral dialysis and push-pull perfusion in the brain of rats: a critical review

Over the last 30 years, studies of the in vivo activity of neurotransmitters and other endogenous factors in the brain have comprised a major effort in the neurosciences. Historically, the technology of push-pull perfusion was utilized as a major approach to investigations in this field. In the last 10 years, cerebral dialysis has been used as an alternative method essentially for the same scientific purpose, since the perfusion technique was viewed as difficult and excessively damaging to tissue. This review considers the representative literature in which both systems have been used to study local neurochemical responses to a drug or other chemical factor, a physiological condition or other situation. In addition, new experiments have been undertaken to compare, in the same animal and at the same time, the utility and properties inherent in the techniques of push-pull perfusion and cerebral dialysis in terms of the profile of a neurotransmitter activity and their local histopathological effects. A miniaturized 33/26 ga push-pull needle and a 24 ga dialysis probe were implanted simultaneously in the left and right caudate nuclei, respectively, in the anesthetized rat. An artificial cerebrospinal fluid (CSF) was perfused simultaneously through both devices at a rate of 10 microliters/min in the push-pull cannula and at 1.0 or 2.0 microliters/min in the dialysis probe. Within a series of 8-10 successive perfusions, excess K+ ions in a concentration of either 30 or 60 mM were incorporated in the CSF and delivered simultaneously to both the push-pull cannula and dialysis probe. Samples of perfusate and dialysate were assayed chromatographically by coulometric HPLC detector and quantitated in terms of the pg/min efflux of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA). The results showed that the resting level of DA was almost undetectable in dialysate samples from either structure; in push-pull perfusates the recovery of DA ranged between 7.0 to 10.0 pg/min, which was increased threefold by excess K+ ions. The recovery of DA and the three metabolites in samples of push-pull perfusate was two to four times that in samples of dialysate during the condition of excess K+ ions. Post-mortem histological analysis of the sites of perfusion and dialysis revealed little or no differences in the cytological damage induced by either the perfusion needle or dialysis probe. Finally, the advantages and limitations of each of these two experimental approaches to in vivo analysis of neurotransmitter efflux are reviewed in relation to the selection of an open or closed system for the on-line study of in vivo neurochemical events.

Enhanced propagation of epileptiform activity through the kindled dentate gyrus

Extracellular recordings were performed in combined hippocampal-entorhinal cortex (HC-EC) slices obtained from control and commissural kindled rats to investigate the propagation of epileptiform activity from the entorhinal cortex (EC) to the hippocampus (HC) after chronic epilepsy. Lowering extracellular Mg2+ concentration in control slices induced epileptiform activity consisting of spontaneous epileptiform bursts in area CA3 and of electrographic seizures in the EC. In contrast, the CA3 region of HC-EC slices obtained from kindled rats displayed significantly longer lasting epileptiform bursts and electrographic seizures. The electrographic seizures that were absent in controls propagated from the EC because disconnecting the HC from the EC stopped their occurrence in the CA3, whereas epileptiform bursts persisted with an unaltered pattern and frequency. Thus the area CA3 is affected by kindling and contributes to the spread of epileptiform activity within the EC-HC complex. We developed a method to induce focal epileptiform activity in the EC by locally perfusing the gamma-aminobutyric acid-A (GABA) antagonist bicuculline (50 mM) in 10 mM KCl containing artificial cerebrospinal fluid. This method enabled us to investigate the propagation of epileptiform discharges from the disinhibited EC to the DG without affecting the DG with the epileptogenic medium. We show here that kindling facilitates the propagation of epileptiform activity through the DG. These data are consistent with the normal function of the DG as a filter limiting the spread of epileptiform activity within the HC-EC complex. This gating mechanism breaks down after chronic epilepsy induced by kindling.

Microdialysis monitoring of extracellular glutamate combined with the simultaneous recording of evoked field potentials in hippocampal organotypic slice cultures

These experiments combined extracellular electrophysiological multirecordings from hippocampal organotypic slice cultures with application of drugs to and sampling of extracellular fluid from a restricted region of the slice using a microdialysis probe. Glutamate (Glu) concentrations were monitored in 0.5 or 2 min microdialysis samples, while evoked field potentials responses (EvFPR) in the CA1 region of the hippocampus (stimulation in the CA3 area) were simultaneously recorded using a multi-electrodes array (Physiocard). Glu was assayed by capillary electrophoresis with laser-induced fluorescence detection combined with a continuous flow derivatization of dialysates. The performance of this combined approach was demonstrated by monitoring extracellular Glu concentrations and EvFPR after K+ induced depolarisation, Glu uptake blockade by trans-pyrrolidine-2,4-dicarboxylic acid (PDC), and electrical stimulation. Such an approach allows a global monitoring of the neuronal functioning with a fine time resolution (up to 30 s) on a simple in vitro brain slice model, to be used as a complement to conventional in vivo microdialysis studies.

Patch sensor detection of glutamate release evoked by a single electrical shock

We tried to detect minimal stimulation-induced glutamate overflow from the surface of a hippocampal slice using an outside-out patch electrode excised from pyramidal cell membranes. The amplitude of the stimulation-induced patch current was dependent on the distance between the slice surface and the tip of patch sensor. The current-voltage relations of the stimulation-induced patch current were similar to those of the current evoked puff by application of L-glutamate to the patch. This indicates that the stimulation-induced patch current was produced by glutamate released from presynaptic terminals, and thus this technique may be useful in the study of transmitter release evoked by minimal electrical stimulation in brain slices.