Attila Baranyi

Long-lasting potentiation of synaptic transmission requires postnaptic modifications in the neocortex

The mechanisms of associative long-lasting potentiation (LLP) of excitatory postsynaptic potentials (EPSPs) were studied in the motor cortex of anesthetized cats. Mono- and oligosynaptic EPSPs were evoked by stimulations of thalamic VL nucleus, pyramidal tract, callosal and somatosensory system and paired with orthodromic, antidromic or current-induced action potentials. EPSP-spike stimulus pairs with 0.1-0.2 Hz frequency and 0-200 ms interstimulus intervals induced increases in the amplitudes and durations of EPSPs for 40-60 min or longer after 20-50 pairings. The LLP was prevented when postsynaptic firing was blocked by intracellular current injection or by juxtasomatic application of gamma-aminobutyric acid. LLP was also prevented when the level of intracellular free calcium was lowered by the intracellular injection of the calcium chelator EGTA or when neuronal transport was blocked by the intracellular injection of colchicine. Neither EGTA nor colchicine blocked postsynaptic firing. Thus, these findings show that LLP in the neocortex is a postsynaptic phenomenon which requires conjunctive pre- and postsynaptic activity, adequate levels of intracellular free calcium, and functional intracellular transport.

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)

Mechanism of aminopyridine-induced ictal seizure activity in the cat neocortex

Intracellular recordings were obtained from neurons in the motor cortex of anesthetized cats in order to examine membrane and synaptic processes involved in aminopyridine (AP)-induced ictal seizure activity. Depolarizing and hyperpolarizing membrane potential sequences which behaved as large, synchronized excitatory and inhibitory postsynaptic potentials, were found to accompany the ictal seizure potentials. After several repetitions of the seizure attack, partial responses, bursts and depolarizing plateaus with spike inactivation occurred. In layers IV and V we found non-pyramidal tract neurons showing endogenous bursting ability activated by AP. These neurons seemed to be the initiators of the rhythmic synchronous activity of the epileptic neuron population. Our results suggest that AP-induced epileptogenesis represents an adequate model of ictal events in the neocortex.

Effect of inhibitory amino acid antagonists on IPSPs induced in lumbar motoneurons upon stimulation of the nucleus reticularis gigantocellularis during active sleep

The present study was performed to generate data implicating glycine or gamma-aminobutyric acid as neurotransmitter candidates mediating the IPSPs which are recorded in lumbar motoneurons following electrical stimulation of the nucleus reticularis gigantocellularis (NRGc) during the atonia of active sleep. Accordingly, intracellular records were obtained from lumbar motoneurons in unanesthetized, normally respiring cats during naturally occurring states of active sleep, while inhibitory amino acid antagonists were microiontophoretically released next to the recorded cell. Electrical stimuli, applied to the NRGc during active sleep under drug-free conditions, evoked inhibitory postsynaptic potentials (IPSPs) in all of the lumbar motoneurons which were examined. These NRGc-induced IPSPs exhibited an average latency-to-onset of 26.6 +/- 1.3 ms, a latency-to-peak of 42.5 +/- 1.3 ms, an average amplitude of 3.9 +/- 0.4 mV and a duration of 34.4 +/- 2.1 ms. Strychnine, when applied microiontophoretically, abolished or markedly suppressed these NRGc-induced IPSPs. In contrast, the microiontophoretic application of picrotoxin or bicuculline methiodide failed to block these IPSPs. To the extent that strychnine may be considered to be a specific antagonist of glycine, the present results suggest that glycine (or a structurally related amino acid) participates in the generation of NRGc-induced IPSPs during the atonia of active sleep.

Activation of protein kinase C induces long-term changes of postsynaptic currents in neocortical neurons ☆

Intracellularly injected phorbol 12,13-dibutyrate (PdiB), a phorbol ester that activates protein kinase C (PKC), altered the postsynaptic responses of neurons of the motor cortex of cats. PdiB increased the amplitudes and durations of EPSPs and decreased the amplitudes and durations of IPSPs elicited by stimulation of the ventrolateral (VL) thalamus or the pyramidal tract (PT). The changes lasted for 50 min or longer. Corresponding changes in peak excitatory and inhibitory postsynaptic currents (EPSCs, IPSCs) were measured directly with the single electrode voltage clamp technique. Quantitative analysis of EPSCs in response to VL thalamic stimulation and IPSCs in response to PT stimulation made in a subgroup of fast PT cells suggested that PdiB acted within the injected neuron rather than presynaptically to alter the synaptic currents. No consistent changes in resting membrane parameters that would account for these modifications were found. Control injections of a phorbol ester that did not activate PKC failed to induce changes in synaptic responses or resting membrane properties. These observations suggest that activation of PKC, in vivo, can induce long-lasting changes in synaptic responses of neocortical neurons by direct modification of postsynaptic ion channel conductivities.

Intracellular injection of phorbol ester increases the excitability of neurons of the motor cortex of awake cats

The electrophysiological effects of two intracellularly injected phorbol esters (PhEs) which activate protein kinase C, phorbol 12,13-dibutyrate and phorbol 12-myristate 13-acetate, were investigated in neurons of the motor cortex of awake cats. The major finding was that intracellularly injected PhEs increased the excitability of the neurons. This was indicated by (1) an increase in spontaneous firing and depolarizing current-induced spike activity, accompanied by a decrease in the latency and threshold of current-induced spike discharges, (2) a reduction in slow afterhyperpolarizations following action potentials and depolarizing pulses, and (3) the development of bursting activity. Neither increases in input resistance nor depolarization of the resting potential sufficient to account for these excitability changes were found. Increases in the amplitudes of action potentials and their fast afterhyperpolarizations were also observed. All changes occurred within 2-8 min after injection and lasted for 50 min or longer. Control injections of 4 alpha-phorbol 12,13-didecanoate, which does not activate protein kinase C, failed to induce changes in neuronal excitability or in any of the above parameters. We conclude that the excitability of neurons of the motor cortex of the awake cats can be increased by phorbol esters that translocate and activate protein kinase C.

Intracellular injection of apamin reduces a slow potassium current mediating afterhyperpolarizations and IPSPs in neocortical neurons of cats

Electrophysiologic effects of intracellularly injected apamin, a Ca2+-dependent K+ channel blocker, were investigated in neurons of the motor cortex of awake cats. Single-electrode voltage clamp techniques were used to measure changes in membrane currents including those that were synaptically activated. All changes occurred within 2-4 min after pressure injection of apamin with partial recovery observed within 8-15 min. Apamin selectively abolished an outward current that mediated a slow afterhyperpolarization (AHP) following intracellular depolarizing current pulses and action potentials without influencing the time course of the action potentials or an associated fast AHP component. In addition apamin increased the number and frequency of spike discharges evoked by the depolarizing current pulses and produced a small increase in the rate of background firing activity. The baseline resting potential and input resistance were essentially unchanged by apamin. Apamin also diminished a late, slowly decaying component of inhibitory postsynaptic potentials (IPSPs) and currents (IPSCs) elicited by stimulation of the ventrolateral thalamus or the pyramidal tract. The apamin-induced changes were concomitant with a decrease of the decay time constant of both IPSPs and IPSCs and a positive shift in their reversal potential. The results suggest that the late, slowly decaying component of these inhibitory postsynaptic responses is generated by an apamin-sensitive Ca2+-dependent K+ conductance which is also responsible for the slow AHP.

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.

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

Studies were performed in cortical neurons to determine if voltage- and time-dependent membrane currents could be recognized and characterized in the dynamic, in vivo state. Intracellular measurements made in neurons of the precruciate cortex of awake cats with single-electrode voltage-clamp (SEVC) techniques disclosed an early outward current to depolarizing command steps in 124 of 137 cells studied. The voltage-dependent properties of the early outward current closely resembled those of A-currents studied in vitro in vertebrate and invertebrate neurons. The current was activated rapidly at onset latencies of less than two ms, fell to flat plateau levels within 60-120 ms during sustained depolarization, and was reduced or eliminated in 22 of 23 cells following intracellular administration of 3- or 4-aminopyridine. The magnitude of outward current in response to depolarizing commands was increased by preceding steady hyperpolarization and reduced by preceding steady depolarization. (The steady potentials were of 9.8 s duration and +/- 40 mV apart from the holding potentials.) Since return to the holding potentials occurred 80 ms before the onset of the command steps, the changes in membrane properties that were induced lasted beyond cessation of the steady polarizing stimuli themselves. Spiking did not prevent recognition of the early outward current as judged from its appearance before and after intracellular application of QX-314 to reduce spike activity. Apart from fast inward currents associated with spike potentials, the early outward current was the most conspicuous and characteristic membrane current noted in these recordings. An additional current component that was noted but not characterized in these studies was a slow, depolarization-induced inward current that could be reduced by intracellular injection of QX-314.

Selective facilitation of synapses in the neocortex by heterosynaptic activation.

Heterosynaptic facilitation (HF) of different excitatory postsynaptic potentials (EPSPs) can be recorded in the motor cortex os anesthetized cats following repetitively applied EPSP-spike stimulus pairs. HF turned out to be synapse-specific in many cases, because not all of the stimulated inputs in the same neuron could produce it. Furthermore, membrane depolarization, increase in membrane resistance and firing activity, can appear with or without HF of a test EPSP.