David J. Mogul

Differential activation of adenosine receptors decreases N-type but potentiates P-type Ca2+ current in hippocampal CA3 neurons.

Adenosine is released in the brain in significant quantities in response to increased cellular activity. Adenosine has been shown either to decrease synaptic transmission or to produce an excitatory response in hippocampal synapses, resulting in increased glutamate release. Previous reports have shown that adenosine or its analogs reduced Ca2+ current in dorsal root ganglion and hippocampal neurons. Here we show that the selective activation of adenosine receptor subtypes has different effects on Ca2+ channels from acutely isolated pyramidal neurons from the CA3 region of guinea pig hippocampus. Activation of A1 receptors inhibited primarily N-type Ca2+ current. In contrast, activation of A2b receptors resulted in significant potentiation of P-type but not N-type Ca2+ current. This potentiation could be inhibited by blocking the cAMP-dependent protein kinase. Because of the ubiquity of adenosine, the differential effects on Ca2+ channels of adenosine receptor subtype activation may have significant implications for neuronal excitability.

CALCIUM-DEPENDENT AFTERHYPERPOLARIZATION AND LEARNING IN YOUNG AND AGING HIPPOCAMPUS

Hippocampally-dependent trace eyeblink conditioning has been shown to be affected by aging. Aging animals take more trials to acquire the association and are more likely to be unable to learn the task. Hippocampal neurons show decreased post-burst afterhyperpolarizations (AHPs) and less accomodation after conditioning, in a time-dependent fashion which may relate to the role of hippocampus in learning consolidation. CA1 neurons in aging rabbits show increased AHPs and more accomodation, i.e., they are less excitable, and larger calcium action potentials. These age-related changes may underlie the learning deficits in aging rabbits. The lipophylic calcium channel blocker nimodipine reduces the AHP, accomodation and calcium action potential at low concentrations in aging but not young CA1 neurons. Nimodipine also enhances learning rate in a variety of tasks, including eyeblink conditioning, in aging but not young animals and humans. Altered calcium handling by neurons of aging mammals is a striking change, is pharmacologically manipulable, and may be an important factor in altered learning and cognitive abilities in the aging.

Evidence for multiple types of Ca2+ channels in acutely isolated hippocampal CA3 neurones of the guinea-pig

1. Current through Ca2+ channels was studied in acutely isolated guinea‐pig pyramidal neurones from the CA3 region of the hippocampus. Both the whole‐cell and single‐channel patch‐clamp configuration were used. 2. Both whole‐cell and single‐channel currents displayed holding potential sensitivity indicative of two high‐threshold currents similar to L‐ and N‐type Ca2+ currents. 3. A low‐threshold whole‐cell current, similar to T‐type current seen in dorsal root ganglion (DRG) neurones, activated at ‐60 to ‐50 mV and was blocked by nickel (100 microM) and amiloride (500 microM). Exposure to 50 microM‐cadmium left a fraction of the T‐type current intact but blocked N‐ and L‐type current. This T‐like component needed extremely negative holding potentials to be completely reprimed. 4. Whole‐cell N‐type Ca2+ channel current was blocked by omega‐conotoxin (1 microM). From a holding potential of ‐90 mV, omega‐conotoxin decreased the peak whole‐cell current by 33%. 5. A slowly inactivating high‐threshold Ca2+ current (L‐type) that was present at depolarized holding potentials, displayed dihydropyridine sensitivity. From a holding potential of ‐50 mV, addition of the dihydropyridine Ca2+ channel antagonist nimodipine (2 microM) to the bath decreased whole‐cell peak current by 45%. Interestingly, at negative holding potentials nimodipine worked as an agonist. From a holding potential of ‐90 mV, nimodipine (2 microM) increased peak current at test potentials from ‐50 to ‐20 mV and shifted the peak of the current‐voltage relationship in the hyperpolarizing direction similar to the effect of Ca2+ channel agonist Bay K 8644. Exposure to Bay K 8644 (2 microM) increased peak current and single channel open probability independent of holding potential while shifting the peak of the whole‐cell current‐voltage relationship 11 mV in the hyperpolarizing direction. Our experiments suggest that there are approximately the same number of L‐type as omega‐conotoxin sensitive N‐type Ca2+ channels in CA3 neurones. 6. A high‐voltage‐activated whole‐cell current was still present in cells exposed to both nimodipine and omega‐conotoxin (2 and 1 microM, respectively) suggesting the existence of a fourth type of Ca2+ channel in these neurones or that a population of either L‐type or N‐type Ca2+ channels did not respond to dihydropyridine antagonists or omega‐conotoxin, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Optimal spacing of surface electrode arrays for brain–machine interface applications

Brain-machine interfaces (BMIs) use signals recorded directly from the brain to control an external device, such as a computer cursor or a prosthetic limb. These control signals have been recorded from different levels of the brain, from field potentials at the scalp or cortical surface to single neuron action potentials. At present, the more invasive recordings have better signal quality, but also lower stability over time. Recently, subdural field potentials have been proposed as a stable, good quality source of control signals, with the potential for higher spatial and temporal bandwidth than EEG. Here we used finite element modeling in rats and humans and spatial spectral analysis in rats to compare the spatial resolution of signals recorded epidurally (outside the dura), with those recorded from subdural and scalp locations. Resolution of epidural and subdural signals was very similar in rats and somewhat less so in human models. Both were substantially better than signals recorded at the scalp. Resolution of epidural and subdural signals in humans was much more similar when the cerebrospinal fluid layer thickness was reduced. This suggests that the less invasive epidural recordings may yield signals of similar quality to subdural recordings, and hence may be more attractive as a source of control signals for BMIs.

Three types of bovine chromaffin cell Ca2+ channels: facilitation increases the opening probability of a 27 pS channel.

1. Cell‐attached patch recordings from bovine chromaffin cells were performed with 90 mM‐Ba2+ in the patch pipette and with isotonic potassium aspartate in the bathing solution to zero the membrane potential. Three different types of unitary Ca2+ channel activity could be distinguished in these recordings. 2. A 27 pS Ca2+ channel was distinguished by constructing amplitude histograms and measuring slope conductance. This channel activated over a broad range of potentials (depolarizations greater than ‐10 mV). 3. A second Ca2+ channel with a slope conductance of 14 pS could also be detected with amplitude histograms. This channel activated with depolarizations greater than ‐20 mV. 4. An 18 pS Ca2+ channel was observed infrequently indicating that this channel may carry only a small amount of the whole‐cell current. This 18 pS channel was sensitive to changes in holding potential. Depolarizing the patch to +10 mV from a holding potential of ‐80 mV elicited robust unitary activity. Changing the patch holding potential to ‐40 mV while maintaining test depolarizations to +10 mV completely inactivated the 18 pS channel. Neither the 25 pS nor the 14 pS Ca2+ channels were affected by changes in holding potential in the range from ‐80 mV to ‐40 mV, indicating the 18 pS channel was a different type of channel. As the 18 pS channel was observed so infrequently, no detailed studies of it were possible. 5. Chromaffin cell Ca2+ currents exhibited facilitation. Large pre‐depolarizations greatly augmented whole‐cell currents observed in these cells. Whole‐cell currents could double or triple after recruiting facilitation. The application of large pre‐depolarizations altered the gating behaviour of the 27 pS Ca2+ channel manifested as dramatically increased channel opening probabilities measured during subsequent test pulses. Large pre‐depolarizations induced unitary activity in the 27 pS Ca2+ channel similar to the long‐lived openings exhibited by L‐type Ca2+ channels in the presence of Bay K 8644. Large pre‐depolarizations did not change the gating behaviour of the 14 pS Ca2+ channel. 6. Repetitive depolarizations in the physiological range could also induce facilitation. At the single‐channel level facilitation was manifested as a striking increase in opening probability of the 27 pS Ca2+ channel. No effect of repetitive activity was observed on 14 pS channel gating. At the whole‐cell level, repetitive depolarizations dramatically increased the current observed. 7. Facilitation of 27 pS Ca2+ channel activity could be induced by changing the holding potential to a depolarized level (greater than or equal to ‐10 mV).(ABSTRACT TRUNCATED AT 400 WORDS)

Electrical control of epileptic seizures.

Summary: Epilepsy is among the most common neurologic disorders, yet it is estimated that about one third of patients do not respond favorably to currently available drug treatments and up to 50% experience major side effects of these treatments. Although surgical resection of seizure foci can provide reduction or cessation of seizure incidents, a significant fraction of pharmacologically intractable seizure patients are not considered viable candidates for such procedures. Research advances in applying electrical stimulation as an alternative treatment for intractable epilepsy have been reported. The primary focus of these studies has been the search for optimized stimulation protocols by which to electrically suppress, revert or prevent seizures. In this review, the authors discuss some of the promising results that have been achieved. These results are organized in three broad categories based on how such protocols are generated. They focus on how information of the electrical activity in the brain is incorporated in the control schemes, namely: open loop, semiclosed loop, and closed loop protocols. Benefits, potential promises, and challenges of these different control techniques are discussed.

Manipulating epileptiform bursting in the rat hippocampus using chaos control and adaptive techniques

Epilepsy is a relatively common disease, afflicting 1%-2% of the population, yet many epileptic patients are not sufficiently helped by current pharmacological therapies. Recent reports have suggested that chaos control techniques may be useful for electrically manipulating epileptiform bursting behavior in vitro and could possibly lead to an alternative method for preventing seizures. We implemented chaos control of spontaneous bursting in the rat hippocampal slice using robust control techniques: stable manifold placement (SMP) and an adaptive tracking (AT) algorithm designed to overcome nonstationarity. We examined the effect of several factors, including control radius size and synaptic plasticity, on control efficacy. AT improved control efficacy over basic SMP control, but relatively frequent stimulation was still necessary and very tight control was only achieved for brief stretches. A novel technique was developed for validating period-1 orbit detection in noisy systems by forcing the system directly onto the period-1 orbit. This forcing analysis suggested that period-1 orbits were indeed present but that control would be difficult because of high noise levels and nonstationarity. Noise might actually be lower in vivo, where regulatory inputs to the hippocampus are still intact. Thus, it may still be feasible to use chaos control algorithms for preventing epileptic seizures.

Adenosine A3 receptors potentiate hippocampal calcium current by a PKA-dependent/PKC-independent pathway.

The modulation of high-threshold Ca current (I(Ca)) by adenosine receptors was studied using the voltage clamp method on acutely dissociated guinea pig hippocampal CA3 pyramidal neurons. When these neurons were exposed to adenosine in the presence of A1, A2a and A2b receptor antagonists, I(Ca) potentiation occurred at test potentials of -10 mV, but not at -40 mV. Similar potentiation also occurred using the A3 agonist N6-2-(4-aminophenyl)ethyl-adenosine (APNEA), either alone or in the presence of A1 and A2 antagonists. The putative A4 agonist 2-phenylaminoadenosine (CV-1808; Cornfield et al., 1992) did not potentiate I(Ca) at four concentrations tested between 25 nM and 2500 nM. K0.5 for the APNEA-induced potentiation was 25.4 nM, comparable to that determined in binding studies for the cloned receptor (15.5 nM; Zhou et al., 1992). I(Ca) potentiation by APNEA was blocked by intracellular application of WIPTIDE, a PKA inhibitor (p < 0.001), but was not affected by protein kinase C (PKC) inhibitor peptide (19-36). These results indicate that: (1) A3 receptor activation can significantly potentiate I(Ca), and (2) because the A3 receptor has been linked to down-regulation of adenylyl cyclase (Zhou et al., 1992), PKA appears to be negatively coupled to I(Ca).

Deterministic chaos and noise in three in vitro hippocampal models of epilepsy.

AbstractRecent reports have suggested that chaos control techniques may be useful for electrically manipulating epileptiform bursting behavior in neuronal ensembles. Because the dynamics of spontaneous in vitro bursting had not been well determined previously, analysis of this behavior in the rat hippocampus was performed. Epileptiform bursting was induced in transverse rat hippocampal slices using three experimental methods. Slices were bathed in artificial cerebrospinal fluid containing: (1) elevated potassium ([K+]o=10.5 mM), (2) zero magnesium, or (3) the GABAA-receptor antagonists bicuculline (20 μM) and picrotoxin (250 μM). The existence of chaos and determinism was assessed using two different analytical techniques: unstable periodic orbit (UPO) analysis and a new technique for estimating Lyapunov exponents. Significance of these results was assessed by comparing the calculations for each experiment with corresponding randomized surrogate data. UPOs of multiple periods were highly prevalent in experiments from all three epilepsy models: 73% of all experiments contained at least one statistically significant period-1 or period-2 orbit. However, the expansion rate analysis did not provide any evidence of determinism in the data. This suggests that the system may be globally stochastic but contains local pockets of determinism. Thus, manipulation of bursting behavior using chaos control algorithms may yet hold promise for reverting or preventing epileptic seizures.

A role for adenosine A2 receptors in the induction of long-term potentiation in the CA1 region of rat hippocampus

Although reductions in neurotransmission have been reported in response to agonist-mediated adenosine A1 receptor activation, the implications of A2 receptor activation on synaptic transmission have not been well explored. We examined the role adenosine A2 receptors play in the efficacy of neurotransmission between the Schaffer collateral-CA1 pathway in the rat transverse hippocampal slice. A2 receptor blockade in the presence of complete A1 receptor inhibition led to a reversible reduction of the field excitatory post-synaptic potential (EPSP) slope in response to low-frequency test pulses (0.033 Hz) indicating that A2 receptors can enhance synaptic transmission. A2 receptor blockade by the A2 antagonist, DMPX (3,7-dimethyl-1-propargylxanthine) prevented the induction of tetanus-induced long-term potentiation (LTP) of the EPSP. In contrast, no such effect on LTP induction was observed during A1 receptor blockade. We also examined the effects of DMPX on the induction of LTP during continued A1 receptor blockade with CPT. Under this condition, LTP was significantly reduced when compared to LTP induced in the presence of CPT alone. A similar result was found using the highly polar A2 antagonist 8-SPT (8-(p-sulfophenyl)theophylline) suggesting that the effects of DMPX on LTP were not due to a direct action on an intracellular intermediate. DMPX had no effect on LTP expression if applied 45 min following the tetanus indicating that A2 receptors play no significant role in the maintenance phase of LTP. Selective A2a receptor activation did not alter the field EPSP. Similarly, selective blockade of the A2a receptor did not interfere with tetanus-induced LTP. Increases in neuronal firing rates can result in elevations in the concentration of extracellular adenosine. Together, these results suggest that the A2 receptors may play an important role in the induction although not the maintenance of hippocampal LTP and that the effect is likely to be mediated by the A2b receptor.