An aminopyridine-sensitive, early outward current recorded in vivo in neurons of the precruciate cortex of cats using single-electrode voltage-clamp techniques

Discharge profiles of abducens, accessory abducens, and orbicularis oculi motoneurons during reflex and conditioned blinks in alert cats

The discharge profiles of identified abducens, accessory abducens, and orbicularis oculi motoneurons have been recorded extra- and intracellularly in alert behaving cats during spontaneous, reflexively evoked, and classically conditioned eyelid responses. The movement of the upper lid and the electromyographic activity of the orbicularis oculi muscle also were recorded. Animals were conditioned by short, weak air puffs or 350-ms tones as conditioned stimuli (CS) and long, strong air puffs as unconditioned stimulus (US) using both trace and delayed conditioning paradigms. Motoneurons were identified by antidromic activation from their respective cranial nerves. Orbicularis oculi and accessory abducens motoneurons fired an early, double burst of action potentials (at 4-6 and 10-16 ms) in response to air puffs or to the electrical stimulation of the supraorbital nerve. Orbicularis oculi, but not accessory abducens, motoneurons fired in response to flash and tone presentations. Only 10-15% of recorded abducens motoneurons fired a late, weak burst after air puff, supraorbital nerve, and flash stimulations. Spontaneous fasciculations of the orbicularis oculi muscle and the activity of single orbicularis oculi motoneurons that generated them also were recorded. The activation of orbicularis oculi motoneurons during the acquisition of classically conditioned eyelid responses happened in a gradual, sequential manner. Initially, some putative excitatory synaptic potentials were observed in the time window corresponding to the CS-US interval; by the second to the fourth conditioning session, some isolated action potentials appeared that increased in number until some small movements were noticed in eyelid position traces. No accessory abducens motoneuron fired and no abducens motoneuron modified their discharge rate for conditioned eyelid responses. The firing of orbicularis oculi motoneurons was related linearly to lid velocity during reflex blinks but to lid position during conditioned responses, a fact indicating the different neural origin and coding of both types of motor commands. The power spectra of both reflex and conditioned lid responses showed a dominant peak at approximately 20 Hz. The wavy appearance of both reflex and conditioned eyelid responses was clearly the result of the high phasic activity of orbicularis oculi motor units. Orbicularis oculi motoneuron membrane potentials oscillated at approximately 20 Hz after supraorbital nerve stimulation and during other reflex and conditioned eyelid movements. The oscillation seemed to be the result of both intrinsic (spike afterhyperpolarization lasting approximately 50 ms, and late depolarizations) and extrinsic properties of the motoneuronal pool and of the circuits involved in eye blinks.

Stimulation of substantia nigra pars reticulata suppresses neocortical seizures

The effects of unilateral electrical stimulation of the substantia nigra pars reticulata (SNpr) on the electrocorticographic (ECoG) manifestations of seizures were studied in anesthetized rats. Epileptiform activity was provoked in the primary focus (Pf) by unilateral, local application of 3-aminopyridine which induced secondary focus in the homologous area of the contralateral cortex (mirror focus, Mf). The position of the electrode for stimulation of SNpr was contralateral to the Pf. The results showed a strong suppressive nigral effect on cortical seizure propagation and on seizure susceptibility in both hemispheres. Stimulation of the SNpr prevented the manifestation of sustained epileptiform events, decreased the rate of seizure appearance in the Mf, delayed the onset of paroxysmal activity and markedly reduced the amplitude and duration of ictal episodes at both foci. Seizure potentials of lower frequencies disappeared, while the relative proportion of those of higher frequency increased in SNpr-stimulated animals. SNpr stimulation had no significant effect on fully developed seizures. Our observations support the idea that SNpr might be involved in the control of cortical seizure susceptibility, regulating other structures which are possibly involved in the generation and propagation of seizure.

Two transient potassium currents in layer V pyramidal neurones from cat sensorimotor cortex

1. Two transient outward currents were identified in large pyramidal neurones from layer V of cat sensorimotor cortex ('Betz cells') using an in vitro brain slice preparation and single-microelectrode voltage clamp. Properties of the currents deduced from voltage-clamp measurements were reflected in neuronal responses during constant current stimulation. 2. Both transient outward currents rose rapidly after a step depolarization, but their subsequent time course differed greatly. The fast-transient current decayed within 20 ms, while the slow-transient current took greater than 10 s to decay. Raised extracellular potassium reduced current amplitude. Both currents were present in cadmium-containing or calcium-free perfusate. 3. Tetraethylammonium had little effect on the slow-transient current at a concentration of 1 mM, but the fast-transient current was reduced by 60%. 4-Aminopyridine had little effect on the fast-transient current over the range 20 microM-2 mM, but these concentrations reduced the slow-transient current and altered its time course. 4. Both transient currents were evoked by depolarizations below action potential threshold. The fast-transient current was evoked by a 7 mV smaller depolarization than the slow-transient current, but its chord conductance increased less steeply with depolarization. 5. Voltage-dependent inactivation of the fast-transient was steeper than that of the slow-transient current (4 vs. 7 mV per e-fold change), and half-inactivation occurred at a less negative potential (-59 vs. -65 mV). The activation and inactivation characteristics of each current overlapped, however, implying the existence of a steady 'window current' extending over a range of approximately 14 mV beginning negative to action potential threshold. 6. The fast-transient current displayed a clear voltage dependence of both its activation and inactivation kinetics, whereas the slow-transient current did not. Recovery of either current from inactivation took about 1 s near -70 mV. The recovery of the slow-transient current became faster with hyperpolarization. 7. The contribution of each transient current to repolarization of the action potential was assessed from pharmacological responses. Blockade of calcium influx had little or no effect on the rate of action potential repolarization, whereas the selective reduction of either transient current caused significant slowing of repolarization. 8. We conclude that Betz cells possess at least two transient potassium currents, each a member of the rapidly expanding family of voltage-gated potassium currents that have been identified in various cell types.(ABSTRACT TRUNCATED AT 400 WORDS)

Changes in membrane currents during Pavlovian conditioning of single cortical neurons

Single electrode voltage clamp recordings were made during Pavlovian conditioning of single units of the motor cortex of cats. Units that developed a conditioned spike discharge in response to a click conditioned stimulus (CS) after pairing the click with glabella tap and local ionophoretic application of glutamate showed increases in input resistance and reductions of an early outward current induced by depolarizing commands and by return to holding potentials after hyperpolarizing commands. Changes in later currents were also found in some cells. Units that failed to develop a conditioned response did not show these changes. The decreases in membrane currents could contribute to an increased spike discharge in response to the CS as could the increased input resistance observed after conditioning. Conductance changes of this type may serve as engrams by which some forms of memory and learning are expressed across both vertebrate and invertebrate species.

Properties of associative long-lasting potentiation induced by cellular conditioning in the motor cortex of conscious cats

Mechanisms of long-lasting potentiation of synaptic responses induced in the thalamocortical and recurrent collateral pathways of the pyramidal tract were studied in intracellular recordings from the motor cortex of unanesthetized, chronically implanted cats. The observations provide the first description of long-lasting potentiation in the unanesthetized neocortex in vivo. Monosynaptic excitatory postsynaptic potentials of 2-5 mV in amplitude were evoked as test responses by stimulation of the pyramidal tract and thalamic ventrolateral nucleus at 0.1-0.5 Hz frequency. Pressure microinjections of drugs and ions were also performed during intracellular recordings. In the first series of experiments, test synaptic responses were paired with intracellular current injection-induced action potentials at an interstimulus interval set between 0-200 ms and 0.1-0.5 Hz frequency. Pairings (30-100 x) induced long-lasting potentiation of the test responses in 58% of cells. The increased synaptic responses typically initiated action potentials and their potentiation usually lasted over the period of recordings. Increases in amplitude of synaptic responses were not correlated with statistically significant changes in electrical membrane properties (resting potential, input resistance, time constant, spike threshold) or parameters of action potentials and their afterpotentials. The failure to induce increases in synaptic efficacy by unpaired stimuli (pseudoconditioning) demonstrated the associative property of the long-lasting potentiation. In a second series of experiments, differential cell conditioning was employed. This paradigm induced long-lasting potentiation of the explicitly paired synaptic response without noticeable modification of unpaired or pseudorandomly paired synaptic responses tested conjointly in the same neuron. These observations demonstrated the input-specificity of long-lasting potentiation. In a third series of experiments, subthreshold depolarizing current pulses were summated with synaptic responses to induce firing in the recorded neuron during pairing. Long-lasting potentiation occurred in 55% of the summated synaptic inputs. Pseudoconditioning did not induce synaptic potentiation in these cells. In a fourth series of experiments, conditioning was employed in neurons in which firing activity was suppressed by an intracellularly injected lidocaine derivative. Long-lasting potentiation was induced in 50% of the attempts when synaptic responses were paired with current-induced depolarizations greater than 30 mV. These results suggest that postsynaptic induction of long-lasting synaptic potentiation can be successful in the absence of postsynaptic sodium spikes in neurons of the motor cortex in vivo. In a fifth series of experiments, homosynaptic high-frequency tetanization (80-200 Hz for 5-15 s) was applied to the thalamocortical and recurrent pyramidal afferents.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of protein kinase C inhibitor H-7 on membrane properties and synaptic responses of neocortical neurons of awake cats

Electrophysiological effects of intracellularly pressure-injected H-7, an inhibitor of protein kinase C, were investigated in neocortical neurons of awake cats. H-7 reduced spontaneous and depolarizing current-induced firing activity and increased the latency and apparent threshold of action potentials elicited by depolarizing currents. Slow afterhyperpolarizations following action potentials and depolarizing pulses increased after injection of H-7, without detectable changes in the time course of the fast components of the action potentials. H-7 induced increases in IPSPs evoked by stimulation of the ventrolateral thalamus (VL) or the pyramidal tract (PT). Besides slight increases in the amplitude of IPSPs measured at peak, H-7 induced pronounced increases in the amplitude measured 50-100 ms after stimulation and in the total duration of IPSPs. EPSPs evoked by VL or PT stimulation did not show measurable alterations after injection of H-7. The effects occurred 2-15 min after injection of H-7 and lasted at least 90 min without essential changes in the baseline values of resting membrane potential or input resistance. The results suggest that in addition to playing a role in regulating membrane excitability, protein kinase C influences the inhibitory synaptic mechanisms of neocortical neurons.

Properties of depolarizing plateau potentials in aminopyridine-induced ictal seizure foci of cat motor cortex

The mechanisms of generation of self-sustained depolarizing plateau potentials (DPs) were studied in intracellular recordings in aminopyridine-induced ictal seizure foci in the motor cortex of the cat. In some experiments single-electrode voltage clamp techniques were used and intracellular pressure injection of aminopyridine (Ap), phorbol esters (PhEs) and tetraethylammonium (TEA) was carried out. After several ictal episodes, DPs with bursts of action potentials or with spike inactivation developed gradually in the clonic and interictal phases, without synchronism with surface ictal seizure potentials. In many cases DPs were followed by hyperpolarizing afterpotentials and neuronal inhibition. In bursting neurons DPs originated from the augmented depolarizing envelope of bursts of action potentials. In non-bursting neurons DPs were initiated from summated depolarizing afterpotentials and slow spikes with high threshold, resembling Ca-spikes. In a few neurons DPs were triggered by enlarged excitatory postsynaptic potentials. It was possible to evoke DPs by injections of depolarizing current pulses into single neurons of the Ap-focus, or by intracellular injection of AP, PhEs or TEA. We conclude that DPs are not causal cellular bases of the ictal paroxysmal discharges, rather they occur as consequences of abnormal neuronal activity. It is suggested that DPs are intrinsic regenerative membrane events induced by a transient dominance of voltage-dependent inward currents (carried primarily by calcium ions although sodium ions may contribute) by simultaneous decreases in concurrent outward potassium currents.