The time course of local changes of the extracellular space (ES) was investigated by measuring concentration changes of repeatedly injected tetramethylammonium (TMA+) and choline (Ch+) ions for which cell membranes are largely impermeable. After stimulus-induced extracellular [K+] elevations the delta [TMA+] and delta [Ch+] signals recorded with nominally K+-selective liquid ion-exchanger microelectrodes increased by up to 100%, thus indicating a reduction of the ES down to one half of its initial size. The shrinkage was maximal at sites where the K+ release into the ES was also largest. At very superficial and deep layers, however, considerable increases in extracellular K+ concentration were not accompanied by significant reductions in the ES. These findings can be explained as a consequence of K+ movement through spatially extended cell structures. Calculations based on a model combining the spatial buffer mechanism of Kuffler and Nicholls (1966) to osmolarity changes caused by selective K+ transport through primarily K+ permeable membranes support this concept. Following stimulation additional iontophoretically induced [K+]0 rises were reduced in amplitude by up to 35%, even at sites where maximal decreases of the ES were observed. This emphasizes the importance of active uptake for K+ clearance out of the ES.

The aim of this investigation is to estimate the contribution of spatial glial K+ buffer currents to extracellular K+ homeostasis during enhanced neuronal activity. Neuronal hyperactivity was induced by electrical stimulation of the cortical surface or the ventrobasal thalamic nuclei of cats (5-50 Hz, 0.1-0.2 ms, two to three times threshold stimulation intensity, 5-20 s). The accompanying slow field potential changes were recorded simultaneously across the grey matter with vertical assemblies of eight micropipettes glued 300 microns apart. Using the Poisson equation, the amplitudes of the underlying current sources and sinks were calculated. The current source densities depended on the depth of recording, frequency, strength, and duration of the stimulation. Current sinks, corresponding to a removal of 0.1-0.5 mmoles of monovalent cations per liter of brain tissue and second from the extracellular space, were observed in middle cortical layers, whereas sources appeared at superficial and deeper sites. These sinks and sources might represent K+ moved across glial membranes by spatial buffer currents. The consequences of glial buffer currents of this magnitude were investigated with model calculations. It turned out that measurements of electrolyte and volume changes of the extracellular space (Dietzel et al. Exp. Brain Res. 40:432-439, 1980; Exp. Brain Res. 46:73-84, 1982) could only partially be explained by spatial buffer currents of this magnitude. Comparison of the calculated values with intracellular measurements in neurons and glial cells (Coles et al. Ann. N.Y. Acad. Sci. 481:303-317, 1986; Ballanyi et al. J. Physiol. 382:159-174, 1987) suggests that spatial buffering combines with an approximately equimolar KCl transport and, depending on the preparation, also K+/Na+-exchange across glial membranes.

Interleukin-6 (IL-6) is a multifunctional cytokine mediating inflammatory or immune reactions. Here we investigated the possible role of IL-6 in the intact or lesioned peripheral nervous system using adult IL-6 gene knockout (IL-6(-/-)) mice. Various sensory functions were tested by applying electrophysiological, morphological, biochemical, and behavioral methods. There was a 60% reduction of the compound action potential of the sensory branch of IL-6(-/-) mice as compared with the motor branch in the intact sciatic nerve. Cross sections of L5 DRG of IL-6(-/-) mice showed a shift in the relative size distribution of the neurons. The temperature sensitivity of IL-6(-/-) mice was also significantly reduced. After crush lesion of the sciatic nerve, its functional recovery was delayed in IL-6(-/-) mice as analyzed from a behavioral footprint assay. Measurements of compound action potentials 20 d after crush lesion showed that there was a very low level of recovery of the sensory but not of the motor branch of IL-6(-/-) mice. Similar results of sensory impairments were obtained with mice showing slow Wallerian degeneration (Wlds) and a delayed lesion-induced recruitment of macrophages. However, in contrast to WldS mice, in IL-6(-/-) mice we observed the characteristic lesion-induced invasion of macrophages and the upregulation of low-affinity neurotrophin receptor p75 (p75LNTR) mRNA levels identical to those of IL-6(+/+) mice. Thus, the mechanisms leading to the common sensory deficiencies were different between IL-6(-/-) and WldS mice. Altogether, the results suggest that interleukin-6 is essential to modulate sensory functions in vivo.

Extracellular Na+ - and Cl- -concentrations ([Na+]o, [Cl-]o) were recorded with ion-selective microelectrodes during repetitive stimulation and stimulus-induced self-sustained neuronal afterdischarges (SAD) in the sensorimotor cortex of cats. In all cortical layers [Na+]o initially decreased by 4-7 mM. In depths of more than 600 micrometer below the cortical surface such decreases usually turned into increases of 2-6 mM during the course of the SADs, whereas in superficial layers [Na+]o never rose above its resting level. [Cl-]o always showed an increase in the course of the SADs often preceded by an initial small decrease. The average increase at a depth of 1,000 micrometers was about 7 mM. [Cl-]o reached peak values at about the end of the ictal period, whereas [Na+]o reached its maximum shortly after the end of the SAD, at times when [K+]o was still elevated above the baseline concentration. These data indicate that the extracellular osmolarity can increase during SAD by up to 30 mM. Such an increase in osmolarity can be explained by an increase in the number of intracellular particles, caused by cleavage of larger molecules during enhanced metabolism. This could lead to cell-swelling due to passive water influx from the extracellular space (ES). However, the resulting reduction of the size of the ES is calculated to be less than 10% for an increase in intracellular osmolarity by 30 mOsm. This value is too small as compared to previously measured ES-reductions under similar conditions (i.e., 30% reduction at 1,000 micrometers; Dietzel et al. 1980). Reductions of the size of the ES that accompany the observed changes in the ionic environment, are quantitatively explained on the basis of the extended glial buffering mechanism described in the preceding paper.

SUMMARY1. The release of 5-hydroxytryptamine (5-HT) from presynaptic terminals has been studied by the voltage-clamp technique at synapses made by isolated Retzius and pressure (P) sensory neurones dissected from the leech C.N.s. and maintained in tissue culture. At these synapses facilitation, depression and modulation of release occur with action potentials and with voltage-clamp pulses.2. Depolarization of Retzius cells from a constant holding potential by steps of varying amplitude (5 ms in duration) caused graded release of 5-HT. The steep transfer function for release using these short test pulses resembled that seen at the giant synapse of the squid: synaptic potentials increased markedly with presynaptic depolarizations beyond -25 mV and decreased with large depolarizing pulses beyond +40 mV.3. When the steady holding potential of voltage-clamped Retzius cells was suddenly displaced to a new value within the range of -40 mV to -85 mV, there followed a slow but smaller change of the post-synaptic P-cell membrane potential in the same direction. After an initial delay of about 40 ms, the post-synaptic potential reached its new level with an exponential time course and a time constant of 0 7 s. Since Retzius and P cells are not electrically coupled, these effects can be accounted for by alterations in tonic release of transmitter.4. Changes of presynaptic holding potential to a more depolarized level resulted in an increase in voltage noise recorded in the P cell. Conversely, hyperpolarization from a depolarized level reduced noise. Noise analysis showed that these changes could be accounted for by quantal events with a mean amplitude of about 0-15 mV. This value is similar to that for spontaneous miniature potentials and quantal fluctuations observed at synapses between Retzius and P cells. I. D. DIETZEL, P. DRAPEA U AND J. G. NICHOLLS responses evoked by depolarizations to 0 mV with pulses of 5 ms duration were reduced in amplitude as the holding potential of the Retzius cell was increased from the resting value of -45 to -75 mV. For example, depolarization to 0 mV starting from -45 mV evoked synaptic potentials as much as ten times larger than those evoked by depolarizations to 0 mV starting from -75 mV. With step depolarizations or hyperpolarizations of the holding potential, the effect on release evoked by brief pulses to 0 mV built up over an exponential time course similar to that found for the effect of holding potential on tonic release (r = 0 7 s).6. Frequency-dependent facilitation of transmission occurred with paired stimuli applied to Retzius cells. The time constant of decay of facilitation was similar (approximately 100 ms) with impulses and under voltage-clamp conditions. The time course of the decay of facilitation was independent of the holding potential. It is concluded that two-shock facilitation and modulation of release by changes in the resting potential depend on different presynaptic mechanisms.