SUMMARY1. Double-barrelled ion-sensitive micro-electrodes were used to measure changes in the intracellular activities of K+, Na+ and Cl-(aK, aNa, acl) in glial cells of slices from guinea-pig olfactory cortex during repetitive stimulation of the lateral olfactory tract.2. Base-line levels of aj , aia and ai1 were about 66, 25 and 6 mm, respectively, for cells with resting potentials higher than -80 mV. During stimulation, intraglial aj and ai1 increased, whereas aia decreased. Within about 2 min after stimulation the ion activities returned to their base-line levels.3. The Cl-equilibrium potential was found to be close to the membrane potential (Em). There was also a strong correlation between changes of Em and aC1. These observations indicate a high Cl-conductance of the glial cell membrane. 4. In the presence of Ba2+, the usual depolarizing response of the glial cells to a rise of the extracellular K+ activity (a4) reversed into a membrane hyperpolarization. Furthermore, Ba2+ strongly reduced the stimulus-related rise of intraglial a4. An additional application of ouabain blocked both the membrane hyperpolarization as well as the remaining rise of a.5. In conclusion, our data show that glial cells in guinea-pig olfactory cortex slices possess at least two mechanisms of K+ accumulation. One mechanism is sensitive to the K+ channel blocker Ba2+ and might be a passive KCl influx. The other appears to be the electrogenic Na+/K+ pump, which can be activated by excess extracellular K+.

1. Local stimulus-evoked changes in concentration of extracellular calcium ions, [Ca2+]0, and potassium ions, [K+[0, were measured in the cerebellar cortex of the cat using paired ion-selected micropipettes. 2. Repetitive stimulation of 30 s duration decreased [Ca2+]0 from a base line of 1.2 mM to as low as 0.8 mM and increased [K+]0 from 3 mM to as much as 8 mM. The magnitude of the changes was directly related to stimulus frequency. Laminar analysis showed that the greatest ion changes occurred at the level of maximum parallel fiber-Purkinje cell dendrite stimulation, but that the [Ca2+]0 changes were more localized than the [K+]0 changes. 3. Combining real-time current-source density measurement with [K+]0 determination and local manganese application, showed that the Mn blocked parallel fiber-Purkinje cell synaptic transmission, but that much of the [K+]0 changes persisted. Thus, a large part of the [K+]0 flux most probably originated in the parallel fibers. In contrast, [Ca2+]0 changes were abolished by the Mn, indicating that the decrease in this ion was probably associated with synaptic transmission or dendritic events. 4. In a few cases, spreading depression occurred in the cat cerebellar cortex. This could be accompanied by decreases in [Ca2+]0 to as low as 0.12 mM and increases in [K+]0 in excess of 48 mM. 5. These results show that significant changes in [Ca2+]0 and [K+]0 occur during cerebellar stimulation and indicate possible origins of the ion fluxes in terms of neuronal elements. This work also shows that the cerebellar cortex of the cat can support spreading depression. The present results, together with those of earlier studies on [Ca2+]0 and [K+]0 changes in the presence of aminopyridine in the cat cerebellum, suggest that synaptic or dendritic electroresponsive properties may play a role in the observed [Ca2+]0 and [K+]0 changes.

1. The effect of cholinergic receptor activation on gamma-aminobutyric acid (GABA)-mediated inhibitory synaptic transmission was investigated in voltage-clamped CA1 pyramidal neurons (HPNs) in the guinea pig hippocampal slice preparation. 2. The cholinergic agonist carbachol (1-10 microM) induced a prominent and sustained increase in the frequency and amplitudes of spontaneous inhibitory postsynaptic currents (IPSCs) in Cl(-)-loaded HPNs. The potentiation of spontaneous IPSCs was not dependent on excitatory synaptic transmission but was blocked by atropine (1 microM). 3. Monosynaptically evoked IPSCs were reversibly depressed by carbachol (10 microM). 4. The frequency of miniature IPSCs recorded in the presence of tetrodotoxin (0.6 or 1.2 microM) was reduced by carbachol (10 or 20 microM) in an atropine-sensitive manner. 5. We conclude that, while cholinergic receptor activation directly excites hippocampal GABAergic interneurons, it has, in addition, a suppressant effect on the synaptic release mechanism at GABAergic terminals. This dual modulatory pattern could explain the suppression of evoked IPSCs despite enhanced spontaneous transmission.

Exogenous application of neurotrophic growth factors has emerged as a new and particularly promising approach not only to promote functional recovery after acute brain injury but also to protect neurons against the immediate effect of the injury. Among the various growth factors and cytokines studied so far, the neuroprotective and neurotrophic profile of basic fibroblast growth factor (bFGF) is the best documented. Using an animal model of acute excitotoxic brain injury, we report here that the neuroprotective action of bFGF, which is now being tested in stroke patients, depends on the induction of activin A, a member of the transforming growth factor-beta superfamily. Our evidence for this previously unknown mechanism of action of bFGF is that bFGF strongly enhanced lesion-associated induction of activin A; in the presence of the activin-neutralizing protein follistatin, bFGF was no longer capable of rescuing neurons from excitotoxic death; and recombinant activin A exerted a neuroprotective effect by itself. Our data indicate that the development of substances influencing activin expression or receptor binding should offer new ways to fight neuronal loss in ischemic and traumatic brain injury.

The application of tetanic electrical stimuli to the stratum radiatum fibre pathway in the hippocampus in vitro produces an NMDA (N-methyl-D-aspartate) receptor-dependent enhancement of synaptic efficacy. Repeated application of such stimuli produces a progressive enhancement of synaptic efficacy leading to the genesis of spontaneous and stimulation-evoked epileptiform discharges. We have used this in vitro approach to explore the cellular mechanisms which underlie the kindling model of epilepsy. Kindling of the stratum radiatum fibre pathway in vitro induced a progressive, long-lasting reduction of both spontaneous and stimulation-evoked GABAergic (gamma-aminobutyric acid-mediated) inhibitory postsynaptic potentials (i.p.s.ps). The reduction of i.p.s.ps by kindling was associated with a profound decrease in the sensitivity of CA1 pyramidal neurons to ionophoretically applied GABA and an increase in sensitivity to NMDA. The reduction of i.p.s.ps and GABA sensitivity was prevented by kindling in the presence of the NMDA receptor antagonist D-2-amino-5-phosphonovalerate (D-APV). These results demonstrate that kindling-like stimulus patterns produce a reduction of GABAergic inhibition in the hippocampus resulting from a stimulus-induced postsynaptic activation of NMDA receptors. The modulation of GABAergic inhibition by NMDA receptors may cause the synaptic plasticity which underlies the kindling model of epilepsy.

Changes in extracellular Ca2+ concentration were directly measured in the rat cerebellum, using an ion-selective micropipette. Extracellular K+ was measured simultaneously with a second ion-selective micropipette. The potential reference barrels of the ion electrodes also provided fast field and slow potentials. During The experiments were made on the cerebellum of the rat, because the neuronal circuitry is known in detail (13) and the electrical responses of the mammalian cerebellum are probably better characterized than those of any other brain region (14). This permits defined neuronal circuits to be activated and any consequent ion changes to be related to this activity. In addition, the rat cerebellum has the unusual property of being relatively susceptible (15) to the spreading depression of Leio (16,17 NaCI (19,20). Ca2+-ISMs contained the new neutral carrier (11, 12) ion exchanger and were backfilled with 100 mM CaCl2. K+-ISMs contained Corning 477317 ion exchanger and were backfilled with 100 mM KCI. The electrical outputs of the ISMs were processed to yield ion and reference signals (9) (Fig. 1). Ion and slow potential signals were recorded on a four-channel chart recorder that effectively filtered out high frequency signals. Fast field potentials were photographed from an oscilloscope. The tip diameters of the ISMs were 2-3 jum. A Ca2+-and a K+-ISM were glued together with rapid epoxy so that the inter-tip spacing was 50 4tm or less. Ca2+-ISMs were calibrated in CaCl2 solutions and K+-ISMs in KCI solutions. All calibrating solutions contained 150 mM NaCl to simulate the ionic strength of the extracellular microenvironment and to allow for Na+ interference in the K+-ISM.The selectivity of the Ca2+-ISM for Ca2+ over K+ and Na+ is better than 600:1 and 1000:1, respectively (11). Thus, no 1287

SUMMARY1. Post-synaptic potentials (PSPs) evoked by electrical stimulation of a variety of input systems have been compared in triceps surae motoneurones innervating slow and fast muscle units, the speed of contraction of which was also determined.2. Stimulation of high threshold afferents in both flexor and extensor muscle nerves, and of joint afferents, evoked polysynaptic PSPs which were predominantly hyperpolarizing in both fast and slow twitch motor units.3. Volleys in cutaneous afferents in the sural and saphenous nerves evoked polysynaptic PSPs composed of mixtures of inhibitory and excitatory components. The inhibitory components were predominant in slow twitch motor units, while in fast twitch units there was a trend towards excitatory predominance.4. Repetitive stimulation of the red nucleus caused predominantly inhibitory PSPs in slow twitch units and mixed or predominantly excitatory PSPs in fast twitch units. There was a correlation in the excitatory/ inhibitory balance between PSPs of cutaneous and rubrospinal origin in those motoneurones in which both types of PSPs were studied.5. The amplitudes of group Ia disynaptic inhibitory PSPs were found to be correlated with motor unit twitch type: IPSPs in slow twitch units were larger than those in fast twitch units. Rubrospinal conditioning

1. The neuronal membrane responses to long constant current pulses (essentially current steps) have been studied in cat triceps surae motoneurones identified as to the type of muscle fibres, fast twitch (type F) or slow twitch (type S), innervated by the cell being studied. For each motoneurone the membrane time constant, τM, and input resistance, RN, were determined from the response to a current step. In addition, shorter time constants (‘equalizing time constants’) resulting from current spread into the dendrites were estimated by graphical analysis. 2. The electrotonic length of the combined motoneurone soma and dendritic tree was estimated from the current step data using the neuronal equivalent cylinder model formulated by Rall (Rall, 1969). The mean electrotonic length of the motoneurone equivalent cylinder was approximately 1·5 in both type F and type S motoneurones. The mean membrane time constant of type F cells was 5·6 msec and that of type S motoneurones was 6·7 msec. This difference in mean τM values was of border line statistical significance. 3. The results indicate that the electrotonic length of the combined dendritic trees of both large type F and small type S motoneurones is essentially the same. The implication of this conclusion for interpretation of previous analyses of the monosynaptic EPSP is discussed.