Hans-Christian Pape

Nitric oxide controls oscillatory activity in thalamocortical neurons.

Nitric oxide (NO) is considered a diffusible messenger involved in neuronal communication, although the post-synaptic target cells of NO action and the associated biological function in the CNS are still a matter of controversy. Within the discrete pattern of NO-synthesizing neurons in the brain, NO synthase is specifically colocalized with the cholinergic brain stem-thalamic system, which is thought to regulate the state-dependent activity of the thalamocortical circuit. Here we report evidence indicating that the release of NO onto thalamocortical neurons results in an alteration in voltage dependence of the hyperpolarization-activated cation conductance, probably mediated via the cGMP system. NO selectively dampens oscillatory neuronal activity, indicating a rapidly diffusing signaling mechanism that controls the functional state of the thalamocortical network.

Prevention of Ca(2+)-mediated action potentials in GABAergic local circuit neurones of rat thalamus by a transient K+ current.

1. Neurones enzymatically dissociated from the rat dorsal lateral geniculate nucleus (LGN) were identified as GABAergic local circuit interneurones and geniculocortical relay cells, based upon quantitative analysis of soma profiles, immunohistochemical detection of GABA or glutamic acid decarboxylase, and basic electrogenic behaviour. 2. During whole‐cell current‐clamp recording, isolated LGN neurones generated firing patterns resembling those in intact tissue, with the most striking difference relating to the presence in relay cells of a Ca2+ action potential with a low threshold of activation, capable of triggering fast spikes, and the absence of a regenerative Ca2+ response with a low threshold of activation in local circuit cells. 3. Whole‐cell voltage‐clamp experiments demonstrated that both classes of LGN neurones possess at least two voltage‐dependent membrane currents which operate in a range of membrane potentials negative to the threshold for generation of Na(+)‐K(+)‐mediated spikes: the T‐type Ca2+ current (IT) and an A‐type K+ current (IA). Taking into account the differences in membrane surface area, the average size of IT was similar in the two types of neurones, and interneurones possessed a slightly larger A‐conductance. 4. In local circuit neurones, the ranges of steady‐state inactivation and activation of IT and IA were largely overlapping (VH = 81.1 vs. ‐82.8 mV), both currents activated at around ‐70 mV, and they rapidly increased in amplitude with further depolarization. In relay cells, the inactivation curve of IT was negatively shifted along the voltage axis by about 20 mV compared with that of IA (Vh = ‐86.1 vs. ‐69.2 mV), and the activation threshold for IT (at ‐80 mV) was 20 mV more negative than that for IA. In interneurones, the activation range of IT was shifted to values more positive than that in relay cells (Vh = ‐54.9 vs. ‐64.5 mV), whereas the activation range of IA was more negative (Vh = ‐25.2 vs. ‐14.5 mV). 5. Under whole‐cell voltage‐clamp conditions that allowed the combined activation of Ca2+ and K+ currents, depolarizing voltage steps from ‐110 mV evoked inward currents resembling IT in relay cells and small outward currents indicative of IA in local circuit neurones. After blockade of IA with 4‐aminopyridine (4‐AP), the same pulse protocol produced IT in both types of neurones. Under current clamp, 4‐AP unmasked a regenerative membrane depolarization with a low threshold of activation capable of triggering fast spikes in local circuit neurones.(ABSTRACT TRUNCATED AT 400 WORDS)

Different Types of Potassium Outward Current in Relay Neurons Acutely Isolated from the Rat Lateral Geniculate Nucleus

Different classes of potassium (K+) outward current activated by depolarization were characterized in relay neurons acutely isolated from the rat lateral geniculate nucleus (LGN), using the whole‐cell version of the patch‐clamp technique. A fast‐transient current (IA), activated at around –70 mV, declined rapidly with a voltage‐dependent time constant (τ= 6 ms at + 45 mV), was 50% steady‐state inactivated at –70 mV, and rapidly recovered from inactivation with a monoexponential time course (τ= 21 ms). IA was blocked by 4‐aminopyridine (4‐AP, 2–8 mM) and was relatively insensitive to tetraethylammonium (TEA, 2–10 mM). After elimination of IA by a conditioning prepulse (30 ms to –50 mV), a slow‐transient K+ current could be studied in isolation, and was separated into three components, IKm, IKs and a calcium (Ca2+)‐dependent current, IK[Ca]. The slow‐transient current was not consistently affected by 4‐AP (up to 8 mM), while TEA (2–10 mM) predominantly blocked IKs and IK[Ca]. The component IKm persisted in a solution containing TEA and 4‐AP, activated at around –55 mV, declined monoexponentially during maintained depolarization (τ= 98 ms at +45 mV), was 50% inactivated at –39 mV, and recovered with τ= 128 ms from inactivation. IKs activated at a similar threshold, but declined much slower with τ= 2662 ms at +45 mV. Steady‐state inactivation of IKs was half‐maximal at –49 mV, and recovery from inactivation occurred relatively fast with τ= 116 ms. From these data and additional current‐clamp recordings it is concluded that the K+ currents, due to their wide range of kinetics and dependence on membrane voltage or internal Ca2+ concentration, are capable of cooperatively controlling the firing threshold and of shaping the different states of electrophysiological behaviour in LGN relay cells.

Noradrenergic modulation of retinogeniculate transmission in the cat.

1. Relay neurones were extracellularly recorded from the A‐layers of the dorsal lateral geniculate nucleus (dLGN) of the anaesthetized cat. The noradrenergic influence on retinogeniculate transmission was investigated through microiontopheretic techniques in a total of 140 dLGN relay cells using three experimental approaches: (i) the effects of agonists for alpha 1‐, alpha 2‐ and beta‐adrenoceptors were separately analysed; (ii) the noradrenergic influence was related to the global state of activity of the relay neurones, which was associated with discrete patterns of the electroencephalogram (EEG); (iii) distinct phases of visual responses evoked from the area of the retinal receptive field, and of binocular and lateral inhibitory responses, were evaluated before, during and after the action of noradrenergic agonists. 2. The spontaneous generation of high‐frequency bursts of spikes in dLGN relay neurones, associated with periods of highly synchronized, delta‐like patterns of the EEG, was selectively suppressed by the beta‐adrenoceptor agonist isoprenaline or the alpha 1‐ adrenoceptor agonist phenylephrine. Single action potentials, occurring at a low frequency between bursts, were significantly less affected. Depending upon the ejection level of the adrenoceptor agonists, burst activity was suppressed by 23‐73%, compared with a reduction in single spike firing in the range 7‐24%. The suppression of burst firing occurred in all functional types of dLGN relay neurones (X, Y; on, off), enhanced burst activity was observed in less than 1% of the cells. 3. On‐going tonic sequences of action potentials (around 15 Hz), occurring during periods of EEG activity characterized by lower amplitudes and higher frequencies, were separately affected by adrenoceptor agonists. Isoprenaline had no significant effect, phenylephrine induced a global reduction of spike firing with no obvious relation to the ejection level, and the alpha 2‐adrenoceptor agonist clonidine inhibited action potential generation in a near dose‐dependent manner. 4. Visual response properties were investigated during periods of less synchronized states of EEG activity. Responses to visual stimulation of the retinal receptive field centre were not significantly influenced by isoprenaline, while phenylephrine or clonidine attenuated the phasic and the tonic response component in all functional types of relay neurones and independent of the stimulus contrast being used. At low ejection levels, slight facilitatory effects were observed with isoprenaline (65% of neurones that were tested) or phenylephrine (15%). The inhibitory influence of the antagonistic surround area of the receptive field appeared unaltered during action of isoprenaline or phenylephrine.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrical properties and membrane currents in the ciliateDidinium

Summary1.The electrical properties of the ciliateDidinium nasutum, aParamecium predator, have been investigated using techniques of constant current injection, voltage-clamp, exchange of solutions, and non-lethal deciliation. The electrical observations have been related to structural characteristics (Fig. 1).2.The resting potential was primarily K-dependent. K concentrations beyond 1 mmol/1 depolarized the cell and reduced the input resistance. The resting conductance for Ca was low. Partial substitution of Ca by Mg changed the membrane potential very little and had no effect on the input resistance. Chloride did not influence the membrane properties.3.The late V/I-relationship was characterized by pronounced outward and minimal inward rectification. It was very little modified by Ca concentrations ranging between 0.125 mmol/1 and 8 mmol/1. Raising the K concentration between 0.063 and 16 mmol/1 did not affect membrane rectification.4.Membrane depolarization triggered a stimulus-graded action potential, which was Ca-dependent. The action potential rose with rates of up to 6 V/s; its overall duration was 100 to 500 ms. Repolarization occurred along a fast and a slow (shoulder) component and was followed by pronounced after-hyperpolarization.5.Complete deciliation ofDidinium eliminated the regenerative depolarization of the membrane. The resting potential, input resistance and late current/voltage relationship were not modified by the removal of ciliary membranes. Under voltage-clamp deciliation removed the early inward current and the hump component of the late outward current.6.Depolarizing voltage-clamp pulses of up to + 20 mV revealed a residual inward current following the early transient. The transient included two decay time constants (3.3 ms; 31.8 ms). An 0.8 nA persistent inward current was isolated using the difference current of ciliated and deciliated cells. A late outward current rose with depolarizations beyond 20 mV including an upward inflection (hump) about 50 ms after step onset.7.A decreasing Ca/Ba mole-fraction increased the frequency and duration of spontaneous action potentials, and depressed the amplitudes of both, early inward and late outward current under voltage-clamp. In Ca-free Ba solution the unclamped membrane fired repetitively; the depolarizations included shoulders of 10 s or more. Inactivation of inward Ba currents was slow under depolarizing voltage-clamp; the late outward current was strongly depressed. In deciliated cells an early inward current was missing in Ca-free Ba solution, and the late I/V relation resembled that of deciliated cells in Ba-free Ca solution.8.Tail currents following the late outward current of ciliated cells in Ca-solution and of deciliated cells in Ca- or Ba-solutions decayed with similar time constants.9.In conclusion, the voltage-activated Ca conductance of the ciliary membrane ofDidinium compares well with data from ciliates so far studied. Unique is (i) a persistent inward current isolated using minor step depolarization or the difference current of ciliated and deciliated cells, (ii) a slow activating voltage-dependent K conductance, and (iii) a late, potentially Ca-dependent, outward current, which may be related to spatial separation of the cilia from the majority of the soma membrane. Ba interferes with the Ca channel which favours a previous two-binding site model. Ba can inhibit the K conductance from inside the cell after passing the ciliary Ca channel.

Lateral excitation in the cat lateral geniculate nucleus

SummaryVisual responses were elicited by global phase reversal stimuli in cells of the cat dorsal lateral geniculate nucleus (dLGN) after small retinal lesions had been centered on each receptive field. After acute lesions of different sizes exclusively lateral inhibition was found. When GABAergic inhibition was blocked by continuous microiontophoretic application of bicuculline lateral excitation emerged in dLGN cells partially deafferented by small and medium size acute retinal lesions, but not in those affected by large lesions. This indicates the presence of excitatory retinal inputs at the periphery of the dLGN cell dendrites which are normally suppressed by strong, long-ranging lateral inhibitory processes. After chronic deafferentation, the remaining excitatory inputs increase in effectiveness and lateral excitation is seen without blockade of inhibition. The maximal lateral spread of excitation (300 μm) in the dLGN is distinctly smaller than the extent of lateral inhibition (1000 μm).

8 Atmung und Säure-Basen-Haushalt (I)

Das Ziel der Atmung ist es, den Körper mit Sauerstoff (O2) zu versorgen und im Gegenzug Kohlendioxid (CO2) abzugeben. Dazu muss die Lunge ventiliert (belüftet) und perfundiert (durchblutet) werden, sodass O2 aus den Alveolen ins Blut und CO2 aus dem Blut in die Alveolen diffundieren können. Das Blut transportiert O2 von der Lunge ins Gewebe und CO2 von den Geweben in die Lunge. Die Atmung muss den Erfordernissen des Körpers angepasst werden können. Quasi nebenbei sorgt die Lunge auch noch für einen ausgeglichenen Säure-Basen-Haushalt. Diese Prozesse sollen im Folgenden näher betrachtet werden.