Altered burst responses in hippocampal CA3 neurons injected with EGTA

A tetrodotoxin-insensitive [corrected] sodium current initiates burst firing of neocortical neurons

The ability to generate burst discharges is a widespread phenomenon in central neurons and often has been attributed to Ca2+ currents. The diversity of burst patterns seems to be at variance with a single mechanism. Selected neocortical bursting neurons were examined by intracellular recordings. Selective blockers and pulse protocols were applied to characterize the crucial components of bursting activity. In the presence of tetrodotoxin and Ca2+ channel antagonists a transient depolarization persists, which shares the activation and deinactivation properties with the burst. The sensitivity to Na+ removal and resistance to tetrodotoxin suggests a tetrodotoxin-insensitive Na+ current as the crucial component in neocortical bursting neurons. A tetrodotoxin-insensitive Na+ current has been isolated in neocortical bursting neurons. The biophysical properties of this current allow for burst firing at frequencies up to about 12 Hz. This tetrodotoxin-insensitive Na+ may generate the alpha-rhythm of the electroencephalogram by effectively synchronizing arrays of follower cells. The implications of the intrinsic currents of bursting neurons for the initiation of epileptic discharges are discussed.

Paroxysmal afterpotentials and role of calcium-dependent potassium conductivity in neuronal activity of strychninized neocortex

Reactions of cortical suprasylvian gyrus neurons were investigated intracellularly after supracortical strychnine application in immobilized and anaesthetized cats. It was shown that paroxysmal depolarizing shifts of membrane potential could be accompanied by de- and hyperpolarizing afterpotentials. When passing from epileptiform to normal physiological activity, short afterhyperpolarizations, 300-500 ms in duration, were converted into inhibitory postsynaptic potentials which were also accompanied by a decrease in membrane potential. When the frequency of paroxysmal discharge was less than 1 s, prolonged (1-2 s) afterhyperpolarizations were observed; at a higher frequency their summation led to tonic hyperpolarization of the membrane. The ictal discharges were accompanied by postictal hyperpolarizations of up to 30 s duration. The intracellular injection of EGTA blocking Ca2(+)-dependent potassium conductivity eliminated prolonged after- and postictal hyperpolarizations and produced depolarizing afterpotentials and a gradual depolarization of cell membranes. Our results indicate that the development of short hyperpolarizing afterpotentials could be determined by the inhibitory synaptic effects. The activation of Ca2(+)-dependent potassium conductivity caused by the development of prolonged afterhyperpolarizations and postictal polarizations, as well as maintained tonic hyperpolarization of cell membranes. Obviously, the depolarizing afterpotentials are of a non-synaptic origin and can be induced by inward calcium current.

Spontaneous rhythmic synchronous activity in epileptic human and normal monkey temporal lobe

Intracellular recordings were obtained from neurons in tissue taken from human epileptic temporal lobe and normal monkey hippocampus. Using the in vitro slice preparation, we confirmed that spontaneous rhythmic synchronous events (SRSEs) were predominantly found in cells of mesial temporal lobe. These synaptic-like events appeared to be mediated by a GABAergic mechanism, since they were blocked by bicuculline. An interneuron-like cell type was found which discharged in a burst pattern in parallel with SRSE occurrence in pyramidal neurons. Burst discharges were graded and excitatory postsynaptic potential (EPSP)-triggered; all-or-none paroxysmal depolarizations were extremely rare. These features of SRSE activity suggest that population synchrony in this tissue is largely dependent on local inhibitory interneuronal circuitry. SRSEs were found in normal monkey hippocampus as well as in mesial tissue from human epileptic temporal lobe. This result indicates that SRSEs are not a direct reflection of tissue epileptogenicity. However, the circuitry underlying SRSEs may be important in the determination of tissue seizure susceptibility, since it provides a substrate for cell synchronization.

Adenosine actions on CA1 pyramidal neurones in rat hippocampal slices

Intracellular recordings with a bridge amplifier of CA1 pyramidal neurones in vitro were employed to study the mechanisms of action of exogenously applied adenosine in the hippocampal slice preparation of the rat. Adenosine enhanced the calcium-dependent, long-duration after-hyperpolarization (a.h.p.) at least in part by a reduction in the rate of decay of the a.h.p. Both the reduced rate of decay and that of the control can be described with a single exponential. Antagonism of the calcium-dependent potassium current (and as a result, the a.h.p.) by bath application of CdCl2 or intracellular injection of EGTA (ethyleneglycolbis-(beta-aminoethyl ether)N,N'-tetraacetic acid) did not reduce the adenosine-evoked hyperpolarization or decrease in input resistance. Similarly, TEA (tetraethylammonium), which antagonizes both the voltage- and calcium-sensitive, delayed, outward rectification, had no effect on the adenosine-evoked changes in resting membrane properties. Adenosine did not affect the early, transient, outward rectification. During exposure to 4-aminopyridine (4-AP) in concentrations sufficient to antagonize this early rectification, the changes in resting membrane properties evoked by adenosine were unaffected. We conclude that the enhancement of the a.h.p. and accommodation by adenosine may be mediated by a change in the regulation of intracellular calcium. However, the mechanism responsible for the hyperpolarization and decrease in input resistance evoked by adenosine is both calcium and voltage insensitive. Thus, it appears distinct from that mediating the enhancement of the a.h.p. and accommodation.

Control of the repetitive discharge of rat CA 1 pyramidal neurones in vitro

Experiments using intracellular recording techniques were performed on rat hippocampal neurones in vitro, to study the discharge properties of these cells. When CA 1 pyramidal cells were excited by injecting long depolarizing current pulses (approximately 600-800 ms), they responded with an initial rapid action potential discharge which slowed, or accommodated, and then stopped after 200-300 ms. The train of action potentials was followed by a hyperpolarization which was due primarily to calcium-activated potassium conductance (GK(Ca]. The amplitude of this hyperpolarization increased with an increasing number of action potentials in the initial discharge. Blocking the calcium-activated potassium conductance, by injecting EGTA into the cell, by bathing the cell in cadmium, a calcium channel blocker, or by bathing the cell in calcium-free medium, reduced the after-hyperpolarization (a.h.p.) and accommodation such that the frequency of action potential discharge increased and the duration of this discharge was prolonged. Blocking the calcium-activated potassium conductance had a greater effect on discharge frequency later in the action potential train, as late interspike intervals were shortened more than early ones by the application of cadmium or of calcium-free medium. This was presumably because the calcium-activated potassium conductance was more developed later in the train. Accommodation was not completely abolished in the absence of calcium and presence of cadmium, suggesting that other factors, in addition to calcium-activated potassium conductance, contributed to this process. This remaining accommodation was reduced by low doses of carbachol, suggesting that the M-current also plays a role in accommodation. We conclude that accommodation of the action potential discharge of hippocampal pyramidal cells may be regulated by at least two potassium currents: the calcium-activated potassium current and the M-current. Both of these currents are turned on during excitation of the neurone and act in an inhibitory manner on that neurone to limit further action potential discharge.

A bicuculline-resistant inhibitory post-synaptic potential in rat hippocampal pyramidal cells in vitro

Experiments were performed on rat hippocampal CA1 pyramidal cells in vitro in order to elucidate the origin of the late hyperpolarizing potential, which follows the gamma-aminobutyric acid (GABA)-mediated inhibitory post-synaptic potential (GABA-i.p.s.p.). The late hyperpolarizing potential could be evoked by orthodromic stimulation via stratum radiatum or stratum oriens but not by selective antidromic stimulation. The membrane soluble analogue of cyclic AMP, 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br cyclic AMP), which blocks calcium-activated potassium hyperpolarizations (GK(Ca], did not reduce the late hyperpolarizing potential. The enkephalin analogue, (D-ala2-met5)-enkephalinamide (DALA) reversibly reduced both the GABA-i.p.s.p. and the late hyperpolarizing potential. The late hyperpolarizing potential and GABA-i.p.s.p. were more sensitive to low doses of the calcium antagonist, cadmium, than the excitatory post-synaptic potential (e.p.s.p.). The local application of cadmium to the pyramidal cell layer blocked the antidromic i.p.s.p. but the orthodromically evoked late hyperpolarizing potential was less affected. In contrast to the GABA-i.p.s.p., the late hyperpolarizing potential was not reversed by chloride injection and was enhanced, rather than depressed, by bicuculline. We conclude that the late hyperpolarizing potential is a bicuculline-resistant i.p.s.p. The unidentified transmitter for this i.p.s.p. is released from feed-forward interneurones primarily onto the dendrites of the pyramidal cell and may act by increasing the potassium permeability of the membrane. The epileptiform burst after-hyperpolarization evoked in the presence of GABA antagonists is composed of at least two components, a long-duration hyperpolarization mediated GK(Ca) and an earlier and shorter late hyperpolarizing potential. Blockade of the GK(Ca) by 8-Br cyclic AMP did not alter the duration of epileptiform bursts but did markedly increase the frequency of their occurrence. This suggests that GK(Ca) is involved in controlling the interval between bursts.

Calcium-activated outward current in voltage-clamped hippocampal neurones of the guinea-pig

Slow clamp currents were recorded from CA1 and CA3 pyramidal neurones in slices of guinea-pig hippocampus maintained in vitro, using a single micro-electrode sample-and-hold technique. Depolarizing voltage commands evoked a time- and voltage-dependent outward current which was suppressed by removing external Ca or by adding Cd (0.5 mM) or Mn (5 mM). This Ca-dependent current (Ic) was not reduced by muscarinic agonists (unlike IM) but was greatly reduced by 5-20 mM-tetraethylammonium (TEA). Repolarizing IC tail currents reversed at -73 +/- 5 mV in 3 mM-K solution. The reversal potential became about 30 mV more positive on raising [K]o to 15 mM. No clear change in current amplitude or tail-current reversal potential occurred on adding Cs (2 mM), reducing [Cl]o from 128 to 10 mM, or replacing external Na with Tris. The underlying conductance GC was activated at membrane potentials positive to -45 mV. At -32 mV GC showed an approximately exponential increase with time, with a time constant of approximately 0.6 sec at 26 degrees C. Repolarizing tail currents declined exponentially with time, the time constant becoming shorter with increasing negative post-pulse potentials. When the clamp was switched off at the end of a depolarizing command of sufficient amplitude and duration to activate IC, a membrane hyperpolarization to -73 mV ensued, of similar amplitude and decay time to that following spontaneous action potentials. It is concluded that the clamp current observed in these experiments is probably the Ca-activated K current thought to contribute to the post-activation after-hyperpolarization in hippocampal neurones.