Temperature dependence of T-type calcium channel gating.

T-type calcium channel isoforms are expressed in a multitude of tissues and have a key role in a variety of physiological processes. To fully appreciate the physiological role of distinct channel isoforms it is essential to determine their kinetic properties under physiologically relevant conditions. We therefore characterized the gating behavior of expressed rat voltage-dependent calcium channels (Ca(v)) 3.1, Ca(v)3.2, and Ca(v)3.3, as well as human Ca(v)3.3 at 21 degrees C and 37 degrees C in saline that approximates physiological conditions. Exposure to 37 degrees C caused significant increases in the rates of activation, inactivation, and recovery from inactivation, increased the current amplitudes, and induced a hyperpolarizing shift of half-activation for Ca(v)3.1 and Ca(v)3.2. At 37 degrees C the half-inactivation showed a hyperpolarizing shift for Ca(v)3.1 and Ca(v)3.2 and human Ca(v)3.3, but not rat Ca(v)3.3. The observed changes in the kinetics were significant but not identical for the three isoforms, showing that the ability of T-type channels to conduct calcium varies with both channel isoform and temperature.

Exposure to a theta-burst patterned magnetic field impairs memory acquisition and consolidation for contextual but not discrete conditioned fear in rats.

Preceding or immediately following fear-conditioning rats were exposed for 30 min to either a sham field, one of two symmetrical (sine-wave 7, 20 Hz) magnetic fields or to one of two complex magnetic fields whose waveforms were modeled after salient electrophysiological patterns within either the hippocampal formation (theta-burst) or the amygdaloid complex (burst-firing). The magnetic fields were presented in successive 2s intervals through each of the three spatial planes and then simultaneously within all three planes. Field strengths ranged between 0.5 and 1 microTesla. Only the group exposed after the conditioning to the theta-burst (hippocampal) magnetic fields displayed evidence of forgetting, as inferred by their marked attenuation of freezing behavior, during contextual extinction 24h later. This powerful treatment explained 75% of the variance in the extinction scores. Behavioral responses to the discrete conditioned stimulus were not affected. These findings are consistent with the involvement of the hippocampus in learned fear to contextual stimuli but not to discrete auditory stimuli and suggest that physiologically relevant stimuli may be delivered to the brain by weak, complex magnetic fields whose intensities are ubiquitous within modern environments.

Combined effects of complex magnetic fields and agmatine for contextual fear learning deficits in rats.

Acute post-training exposures to weak intensity theta-burst stimulation (TBS) patterned complex magnetic fields attenuated the magnitude of conditioned fear learning for contextual stimuli. A similar learning impairment was evoked in a linear and dose-dependent manner by pre-conditioning injections of the polyamine agmatine. The present study examined the hypothesis that whole-body applications of the TBS complex magnetic field pattern when co-administered with systemic agmatine treatment may combine to evoke impairments in contextual fear learning. Within minutes of 4 mg/kg agmatine injections, male Wistar rats were fear conditioned to contextual stimuli and immediately exposed for 30 min to the TBS patterned complex magnetic field or to sham conditions. TBS patterned complex magnetic field treatment was found to linearly summate with the contextual fear learning impairment evoked by agmatine treatment alone. Furthermore, we report for sham-treated rats, but not rats exposed to the synthetic magnetic field pattern, that the magnitude of learned fear decreased and the amount of variability in learning increased, as the K-index (a measure of change in intensity of the time-varying ambient geomagnetic field) increased during the 3-hr intervals over which conditioning and testing sessions were conducted.

Normal spatial and contextual learning for ketamine-treated rats in the pilocarpine epilepsy model.

Cognitive impairments frequently accompany epileptic disorders. Here, we examine two neuroprotective agents, the noncompetitive NMDA antagonist ketamine and the dopaminergic antagonist acepromazine, for their efficacy in attenuating cognitive impairments in the lithium-pilocarpine (LI-PILO) model of rat limbic epilepsy. Declarative-like cognitive behaviors were assessed in a Morris water maze task that consisted successively of spatial and nonspatial (cued platform) training. Whereas the ketamine-treated (Ket) LI-PILO rats performed equally in all respects to nonseized control rats for the spatial and nonspatial components of the water maze task, the acepromazine-treated (Ace) LI-PILO rats failed to demonstrate learning in either the hidden or cued platform variants of the task and did not demonstrate any place learning in the platform-removed probe trials. We further assessed nondeclarative (associative) cognitive behaviors with a standard contextual fear-conditioning protocol. LI-PILO rats treated with acepromazine failed to learn the Pavlovian relationship; Ket LI-PILO rats performed equivalently to nonseized controls. Cumulatively, these data suggest robust cognitive sparing for LI-PILO rats with pharmacological NMDA receptor antagonism following induction of status epilepticus (SE). This cognitive sparing occurs despite earlier findings that the mean amount of total brain damage with LI-PILO is equivalent for Ket and Ace rats.

Learning and memory in agmatine-treated rats.

Agmatine, a noncompetitive N-methyl-D-aspartate (NMDA) antagonist, was examined for its role in water maze place learning, contextual and auditory-cued (discrete) fear learning and conditioned taste aversion learning, when administered systemically. Male Wistar rats were given saline or 1, 5, 10 or 50 mg/kg agmatine ip 20 min prior to or 30 min following daily training sessions in a hidden-platform (place learning) water maze task. Agmatine did not affect latencies to find the hidden platform or preference for the training quadrant during probe trials. When administered 20 min prior to contextual or auditory-cued fear-conditioning sessions, these doses of agmatine evoked a linear dose-dependent impairment in the magnitude of learned fear to the contextual stimuli when assessed during extinction trials 24 h later, but had no effect on the magnitude of learned fear to the auditory stimulus. Inferences of baseline motor activity and ability to respond to the presentation of footshock stimuli were not affected by the treatment. Injections of 50 mg/kg agmatine concurrently with a malaise-evoking agent following presentations to a novel sucrose solution abolished learned taste aversions; this agent did not evoke conditioned taste aversions alone. These studies indicate that systemically administered agmatine selectively impairs behavioral inferences of specific types of learning and memory.

Conditioned taste aversion is not disrupted in rats exposed to weak, complex magnetic fields during the CS-UCS interval.

40 normal male Wistar rats were trained for 8 successive days to consume water ad libitum during once-daily 20-min. sessions. On the following day (training day) the rats were presented with a novel solution of 10% sucrose for 20 min. followed by a single exposure for 2 hr. to one of two weak (200 to 500 nanoTesla) complex magnetic fields or to sham-field conditions. The patterns of the two magnetic fields and the durations of their repeated presentations (interstimulus interval) were designed to be resonant with the intrinsic firing of hippocampal pyramidal and solitary neurons, respectively. Immediately after the applications of the fields one-half the number of rats were injected with lithium to evoke gastrointestinal malaise. Although on the test day, three days later, rats previously injected with the lithium exhibited the usual robust reduction in the consumption of sucrose compared to the training day, there were no statistically significant differences between field-exposed and sham-field groups for these ratios. We conclude that a 2-hr. exposure to weak magnetic fields designed to simulate the pattern of two structures likely involved with conditioned taste aversion between the conditioned stimulus and the unconditioned stimulus did not affect this behavior.

Geophysical variables and behavior: LXXXVII. Effects of synthetic and natural geomagnetic patterns on maze learning.

12 normal male albino rats were exposed or not exposed in their home cages for 5 min. and 50 sec. once every hour 8 times per night to a 7-Hz square-wave magnetic field whose amplitudes were shifted by about 50 nT approximately every 10 sec. Although there were no statistically significant differences between the two groups for numbers of working errors, numbers of reference errors, or speed during the acquisition of an Olton (8-arm) maze, the strength of the group differences (F ratios) for daily working errors was reduced (rho = .70) if there had been enhanced geomagnetic activity during the time of the night when the experimental fields were present.

Postnatal anoxia from CO2 for one minute produces sex-specific changes in contextual fear conditioning in adult rats.

In a split-litter design, equal numbers of male and female rats were exposed either to carbon dioxide for 1 min, or to the control condition on either postnatal Days 9-10 or 17-18. At 120 days of age half the numbers of rats in each condition were given subclinical dosages of lithium (1.5 mEq/kg) and pilocarpine (15 mg/kg) or saline. 40 days later all rats were tested in a conditioned fear paradigm. A four-way analysis of variance as a function of the variables anoxia, age at the time of anoxia sex, and subclinical seizure induction indicated a significant interaction between sex and anoxic history. Post hoc analysis showed that performance of male rats exposed to the anoxic condition did not differ from normal or anoxic female rats. The results suggest that only 1 min. of preweaning anoxia may produce permanent changes in contextual fear conditioning for male but not female rats.

Geophysical variables and behavior: XCIX. Reductions in numbers of neurons within the parasolitary nucleus in rats exposed perinatally to a magnetic pattern designed to imitate geomagnetic continuous pulsations: implications for sudden infant death.

Correlational analyses have shown a moderate strength association between the occurrence of continuous pulsations, a type of geomagnetic activity within the 0.2-Hz to 5-Hz range, and the occurrence of Sudden Infant Deaths. In the present study, rats were exposed continuously from two days before birth to seven days after birth to 0.5-Hz pulsed-square wave magnetic fields whose intensities were within either the nanoTesla or microTesla range. The magnetic fields were generated in either an east-west (E-W) or north-south (N-S) direction. At 21 days of age, the area of the parasolitary nucleus (but not the solitary nucleus) was significantly smaller, and the numbers of neurons were significantly less in rats that had been exposed to the nanoT fields generated in the east-west direction or to the microTesla fields generated within either E-W or N-S direction relative to those exposed to the N-S nanoTesla fields. These results suggest nanoTesla magnetic fields, when applied in a specific direction, might interact with the local geomagnetic field to affect cell migration in structures within the brain stem that modulate vestibular-related arousal and respiratory or cardiovascular stability.

Weak, physiologically patterned magnetic fields do not affect maze performance in normal rats, but disrupt seized rats normalized with ketamine: possible support for a neuromatrix concept?

The concept of a neuromatrix as a determinant of behavior proposes that complex neuroelectromagnetic patterns supported by specific spatial configurations of neurons underlie the generation of behaviors. When the pattern of neuronal connectivity is changed, as occurs during limbic epilepsy, neuroelectromagnetic patterns change in parallel to sustain behavioral output. Thus, a testable prediction of the neuromatrix concept is that the "normal" behaviors of animals with markedly reorganized neuroelectromagnetic patterns are vulnerable to specific stimuli that are ineffective when applied to a normal population. Because rats treated with ketamine after being induced to seize with pilocarpine exhibit behaviors indistinguishable from those of control populations despite marked changes in brain structure, they represent an ideal population in which to examine this hypothesis. Ketamine-treated pilocarpine-seized rats and normal rats were exposed continuously either to a complex sequence magnetic field or to control conditions during the acquisition of a radial arm maze task for 8 consecutive days. After 14 days of subsequent exposure to a frequency-modulated field (7-500 nT), during which time there was no training, the rats that had been induced to seize and had been exposed continuously to this magnetic configuration exhibited conspicuously slower response durations per arm than rats that had been induced to seize and exposed to control conditions or normal rats that had been exposed to either magnetic fields or control conditions. Thus, the behaviors of rats who have sustained multiple, discrete injuries throughout the brain may be seriously disrupted by the appropriate pattern of exogenous weak magnetic fields. Our results represent the first empirical support for the concept of the neuromatrix.

Complex magnetic fields enable static magnetic field cue use for rats in radial maze tasks.

Male Wistar rats were trained in an eight-arm radial maze task (two sessions per day, delayed-non-matching-to-sample) that included an intramaze static magnetic field "cue" (185 microT) specific to the entrance point of one of the arms. Rats were exposed daily for 60 min to a complex magnetic field waveform (theta-burst pattern, 200-500 nT), presented with several different interstimulus intervals (ISIs), either immediately following training sessions or immediately preceding testing sessions. Application of the theta-burst stimulus with a 4000 ms ISI significantly improved the rats' memory for the arm of the radial maze whose position was indicated by the presence of a static magnetic field cue. Reference memory errors were homogeneously distributed among all eight arms of the maze for sham-exposed rats, and among the other seven arms of the maze for complex magnetic field-treated rats. These results suggest that static magnetic field cues may be salient orienting cues even in a microenvironment such as a radial maze, but their use as a cue during maze learning in rats is dependent on whole-body application of a specific time-varying complex magnetic field.

Sudden death in epileptic rats exposed to nocturnal magnetic fields that simulate the shape and the intensity of sudden changes in geomagnetic activity: an experiment in response to Schnabel, Beblo and May.

To test the hypothesis that sudden unexplained death (SUD) in some epileptic patients is related to geomagnetic activity we exposed rats in which limbic epilepsy had been induced to experimentally produced magnetic fields designed to simulate sudden storm commencements (SSCs). Prior studies with rats had shown that sudden death in groups of rats in which epilepsy had been induced months earlier was associated with the occurrence of SSCs and increased geomagnetic activity during the previous night. Schnabel et al. [(2000) Neurology 54:903-908] found no relationship between SUD in human patients and geomagnetic activity. A total of 96 rats were exposed to either 500, 50, 10-40 nT or sham (less than 10 nT) magnetic fields for 6 min every hour between midnight and 0800 hours (local time) for three successive nights. The shape of the complex, amplitude-modulated magnetic fields simulated the shape and structure of an average SSC. The rats were then seized with lithium and pilocarpine and the mortality was monitored. Whereas 10% of the rats that had been exposed to the sham field died within 24 h, 60% of the rats that had been exposed to the experimental magnetic fields simulating natural geomagnetic activity died (P<.001) during this period. These results suggest that correlational analyses between SUD in epileptic patients and increased geomagnetic activity can be simulated experimentally in epileptic rats and that potential mechanisms might be testable directly.

Lithium ion "cyclotron resonance" magnetic fields decrease seizure onset times in lithium-pilocarpine seized rats.

The cyclotron resonance equation predicts that the frequency of an applied magnetic field that might optimally interact with a single ion species may be computed as a function of the charge-to-mass ratio of the ion and the strength of the background static magnetic field. The present study was undertaken to discern the applicability of this equation for optimizing lithium ion utilization in the rat, as inferred by the predicted magnetic "ion resonance "field-induced shift of lithium's dose-dependent curve for seizure onset times (SOTs) when combined with the cholinergic agent pilocarpine. Groups of rats were administered 1.5 thru 3 mEq/kg lithium chloride (in 0.5 mEq/kg increments) and exposed to reference conditions or to one of three intensities (70 nanoTesla, 0.8 microTesla, or 25 microTesla) of a 85 Hz magnetic field calculated to resonate with lithium ions given the background static geomagnetic field of approximately 38,000 nanoTesla (0.38 Gauss). A statistically significant quadratic relationship for SOT as a function of magnetic field intensity (irrespective of lithium dose) was noted: this U-shaped function was characterized by equal SOTs for the reference and 25 microTesla groups, with a trend toward shorter SOTs for the 70 nanoTesla and 0.8 microTesla groups. Although not predicted by the equations, this report extends other findings suggestive of discrete intensity windows for which magnetic field frequencies derived from the cyclotron ion resonance equation may affect ion activity.

Kv3 K+ channels enable burst output in rat cerebellar Purkinje cells.

The ability of cells to generate an appropriate spike output depends on a balance between membrane depolarizations and the repolarizing actions of K(+) currents. The high-voltage-activated Kv3 class of K(+) channels repolarizes Na(+) spikes to maintain high frequencies of discharge. However, little is known of the ability for these K(+) channels to shape Ca(2+) spike discharge or their ability to regulate Ca(2+) spike-dependent burst output. Here we identify the role of Kv3 K(+) channels in the regulation of Na(+) and Ca(2+) spike discharge, as well as burst output, using somatic and dendritic recordings in rat cerebellar Purkinje cells. Kv3 currents pharmacologically isolated in outside-out somatic membrane patches accounted for approximately 40% of the total K(+) current, were very fast and high voltage activating, and required more than 1 s to fully inactivate. Kv3 currents were differentiated from other tetraethylammonium-sensitive currents to establish their role in Purkinje cells under physiological conditions with current-clamp recordings. Dual somatic-dendritic recordings indicated that Kv3 channels repolarize Na(+) and Ca(2+) spikes, enabling high-frequency discharge for both types of cell output. We further show that during burst output Kv3 channels act together with large-conductance Ca(2+)-activated K(+) channels to ensure an effective coupling between Ca(2+) and Na(+) spike discharge by preventing Na(+) spike inactivation. By contributing significantly to the repolarization of Na(+) and especially Ca(2+) spikes, our data reveal a novel function for Kv3 K(+) channels in the maintenance of high-frequency burst output for cerebellar Purkinje cells.

Acquisition deficit and time-dependent retrograde amnesia for contextual fear conditioning in agmatine-treated rats.

Cumulative evidence indicates that the hippocampus plays a time-limited role in contextual learning paradigms. Pharmacological studies have indicated that acquisition of background contextual cues during Pavlovian fear conditioning is dependent upon hippocampal function, whereas early inactivation of the hippocampus after training produces retrograde amnesia. When administered prior to contextual fear conditioning, agmatine (5 and 10 mg/kg, i.p.), an endogenous polyamine and N-methyl-D-aspartate (NMDA) receptor ligand found at excitatory synapses in the hippocampus, impaired the acquisition of contextual fear (measured as defensive freezing 26 hours later) without a reduction in baseline motor activity during training. Furthermore, ascending doses of agmatine were found not to exert analgesic effects on response thresholds to peripheral shock. This negated the possibility that the observed learning deficit resulted from a difference in perceived shock intensity. Post-training agmatine treatment produced a time-dependent impairment of consolidation, with subjects approaching a level of fear equivalent to that of a reference group as the delay of treatment increased (up to 6 hours). Since physiologically high levels of agmatine are able to inhibit NMDA receptor activity, these results suggest that polyamine modulation of NMDA receptors, most likely within the hippocampus, is required for the acquisition and consolidation of contextual fear stimuli.

Application timing of complex magnetic fields delineates windows of posttraining-pretesting vulnerability for spatial and motivational behaviors in rats.

Male Wistar rats were trained in a two-session-per-day, non-matching-to-sample radial maze task. During the 2 hr interval between training sessions and testing sessions subjects were exposed for 60 min to either sham fields or to a low intensity (200 to 500 nT) electromagnetic (EM) complex pattern whose electrical (current) equivalent has been shown to elicit long-term potentiation (LTP) in hippocampal slices. This pattern was applied either immediately following the training sessions or immediately antecedent to the testing sessions. Exposures to the experimental field immediately following the training session were associated with an impairment in spatial memory as inferred by increased commissions of reference errors. Exposures immediately prior to the testing session were associated with decreased motivation as inferred by a reduction in speed of responding. These results suggest that the timing of the presentation of patterned magnetic fields may differentially affect the representations of habit strength and drive within the consolidating memory trace.

Behavioral effects of combined perinatal L-NAME and 0.5 Hz magnetic field treatments.

The behavioral effects of the nitric oxide synthase inhibitor N-nitro-L-arginine methyl ester (L-NAME), when perinatally (2 d prenatal-14 d postnatal) co-administered with extremely low frequency magnetic fields, were examined in weanling and adult rats. Litters of rat pups and their dams were exposed continuously to biphasic pulsed fields presented once every 2 s. The magnetic fields were amplitude modulated in successively increasing and decreasing steps (each 30 min) between 0 and 1.8 microT or between 0 to 13 nanoT (reference field) during 4-h periods (6 periods per day). These two treatments were subdivided into dams that received tap water and dams that received 1.0 g/L L-NAME in tap water. The behavioral sequelae to these treatments for 242 progeny from 41 litters were followed from weaning (1 wk after termination of treatment) into adulthood. Compared to exposures to water and nanoT magnetic fields, perinatal exposures to the microT magnetic fields or to L-NAME in the maternal water supply were associated with increased activity levels when the rats were tested as weanling, but decreased activity levels when the rats were tested as adults. However, the activity of rats that received the combination of L-NAME and microT magnetic fields did not differ significantly from the activity of the rats that had received water and the nanoT fields. Long-term (adulthood) effects of these perinatal treatments on associative learning, as inferred by learned fear to contextual stimuli, were not evident. These results indicate that L-NAME and this particular pattern of magnetic field antagonized one another when co-administered during the perinatal period.

Radial maze proficiency of adult Wistar rats given prenatal complex magnetic field treatments.

Exposure to sinusoidal (power-frequency) magnetic fields during prenatal development is implicated in adulthood behavioral impairments. However, the effects of prenatal exposure to weak-intensity, nonsinusoidal complex magnetic fields (CMFs), an increasingly common feature of the modern environment, have not been rigorously examined. In the present study, male and female Wistar-strain rats were exposed continually during prenatal development to one of three extremely low-frequency CMFs or a sham condition. As adults, rats were trained in an acquisition/reversal radial maze task. All rats exposed to the prenatal CMFs increased their commission of reference memory errors, but differences in working memory and motivation to complete the maze task were specific to the type of prenatal CMF. These results provide the first evidence that prenatal exposures to specific shapes of CMFs impair complex learning behaviors into adulthood.