Ornithine decarboxylase induction and polyamine synthesis in the kindling of seizures: the effect of alpha-difluoromethylornithine

Polyamine regulation of ion channel assembly and implications for nicotinic acetylcholine receptor pharmacology

Small molecule polyamines are abundant in all life forms and participate in diverse aspects of cell growth and differentiation. Spermidine/spermine acetyltransferase (SAT1) is the rate-limiting enzyme in polyamine catabolism and a primary genetic risk factor for suicidality. Here, using genome-wide screening, we find that SAT1 selectively controls nicotinic acetylcholine receptor (nAChR) biogenesis. SAT1 specifically augments assembly of nAChRs containing α7 or α4β2, but not α6 subunits. Polyamines are classically studied as regulators of ion channel gating that engage the nAChR channel pore. In contrast, we find polyamine effects on assembly involve the nAChR cytosolic loop. Neurological studies link brain polyamines with neurodegenerative conditions. Our pharmacological and transgenic animal studies find that reducing polyamines enhances cortical neuron nAChR expression and augments nicotine-mediated neuroprotection. Taken together, we describe a most unexpected role for polyamines in regulating ion channel assembly, which provides a new avenue for nAChR neuropharmacology.

Small molecule polyamines participate in diverse aspects of cell growth and differentiation and are known to regulate ion channel gating. Here authors reveal that cellular polyamines control nicotinic acetylcholine receptor (nAChR) biogenesis, and either catabolic degradation or inhibition of polyamine production augments nAChR assembly.

Glutamine Synthetase: Role in Neurological Disorders

Glutamine synthetase (GS) is an ATP-dependent enzyme found in most species that synthesizes glutamine from glutamate and ammonia. In brain, GS is exclusively located in astrocytes where it serves to maintain the glutamate-glutamine cycle, as well as nitrogen metabolism. Changes in the activity of GS, as well as its gene expression, along with excitotoxicity, have been identified in a number of neurological conditions. The literature describing alterations in the activation and gene expression of GS, as well as its involvement in different neurological disorders, however, is incomplete. This review summarizes changes in GS gene expression/activity and its potential contribution to the pathogenesis of several neurological disorders, including hepatic encephalopathy, ischemia, epilepsy, Alzheimer's disease, amyotrophic lateral sclerosis, traumatic brain injury, Parkinson's disease, and astroglial neoplasms. This review also explores the possibility of targeting GS in the therapy of these conditions.

Visually driven modulation of glutamatergic synaptic transmission is mediated by the regulation of intracellular polyamines

Ca2+-permeable AMPARs are inwardly rectifying due to block by intracellular polyamines. Neuronal activity regulates polyamine synthesis, yet whether this affects Ca2+-AMPAR-mediated synaptic transmission is unknown. We test whether 4 hr of increased visual stimulation regulates glutamatergic retino-tectal synapses in Xenopus tadpoles. Tectal neurons containing Ca2+-AMPARs form a gradient along the rostro-caudal developmental axis. These neurons had inwardly rectifying AMPAR-mediated EPSCs. Four hours of visual stimulation or addition of intracellular spermine increased rectification in immature neurons. Polyamine synthesis inhibitors blocked the effect of visual stimulation, suggesting that visual activity regulates AMPARs via the polyamine synthesis pathway. This modulation resulted in changes in the integrative properties of tectal neurons. Regulation of polyamine synthesis by physiological stimuli is a novel form of modulation of synaptic transmission important for understanding the short-term effects of enhanced sensory experience during development.

The cellular localization of the L-ornithine decarboxylase/polyamine system in normal and diseased central nervous systems

Natural polyamines, spermidine and spermine, and their precursor putrescine, are of considerable importance for the developing and mature nervous system. They exhibit a number of neurophysiological and metabolic effects in the nervous system, including control of nucleic acid and protein synthesis, modulation of ionic channels and calcium-dependent transmitter release. The polyamine system is also known to be involved in various brain pathologic events (seizures, stroke, Alzheimer's disease and others). While cerebral polyamine concentrations and the activities of polyamine-metabolizing enzymes have been studied in great detail, much less is known about the cells that are responsible for cerebral polyamine synthesis and interconversion. With the present review the attempt is made to show how exact knowledge about the regional distribution and cellular localization of polyamines and the polyamine-synthesizing enzymatic machinery (and especially of L-ornithine decarboxylase) may help to better understand the functional interplay between polyamines and other endogenous agents (transmitters, receptors, growth factors neuroactive drugs etc.). Polyamines have been localized both in neurones and glial cells. However, the main cellular locus of the ODC is the neuron--both in the immature and adult central nervous system. Each period of normal brain development and ageing seems to have its own, characteristic temporo-spatial pattern of neuronal ODC expression. During strong functional activation (kindling, epileptic seizures, neural transplantation) astrocytes and other non-neuronal cells do also express ODC and other polyamine-metabolizing enzymes. Astroglial expression of ODC is accompanied by an increase in glial fibrillary acidic protein in these cells. This shift in the cellular mechanisms of polyamine metabolism is currently far from being understood. In human brain diseases (Alzheimer's disease, schizophrenia) certain neurones show an increased expression of ODC, the first and rate-limiting enzyme of polyamine metabolism. Since polyamines are structurally related to psychoactive drugs (neuroleptics, antidepressants) the polyamine system might be of importance as a putative target for drug intervention in psychiatry.

Kindled seizures do not affect adenosinergic inhibition of DA or ACh release in rat accumbens or PFC

Epileptic seizures are thought to terminate largely as a result of the extracellular accumulation of the purinergic neuromodulator, adenosine, released by discharging neurons. However, the postictal surge in extracellular adenosine and its widespread inhibitory effects are limited in time to only a few minutes and cannot directly account for increased resistance to seizures and the complex behavioural and motivational effects that may persist for hours or days after a seizure. The present study examined whether kindled seizures might alter the sensitivity or efficacy of inhibitory presynaptic adenosine receptors, and thereby induce more enduring changes in downstream transmitter systems. Rats were kindled in the amygdala of the dominant cerebral hemisphere, contralateral to the preferred direction of rotation, and their brains were removed either 2 h or 28 days after completion of kindling. Inhibition of electrically stimulated release of dopamine (DA) and acetylcholine (ACh) by the A1 adenosine-receptor agonist, R-phenylisopropyladenosine (R-PIA) was then measured in the prefrontal cortex (PFC) and nucleus accumbens. R-PIA (1.0 microM) inhibited [1H]DA release from PFC and nucleus accumbens tissue, and [14C]ACh release from nucleus accumbens tissue, but release was unaffected by prior kindling, regardless of the intervening interval. These results do not support suggestions that DA or ACh might mediate the effects of seizure-induced changes in purinergic inhibitory tone so as to cause long-term shifts in seizure threshold and postictal behavior.

Alternative splicing at the C-terminal but not at the N-terminal domain of the NMDA receptor NR1 is altered in the kindled hippocampus

Several lines of evidence suggest that N-methyl-D-aspartate (NMDA) receptors significantly contribute to the development of kindling. In addition, a lasting enhancement of the NMDA receptor function has been suggested to play a significant role in the chronic hyperexcitability occurring in the hippocampus after kindling epileptogenesis. We have investigated whether hippocampal kindling induces changes in the NMDA receptor at the molecular level by assessing the expression of mRNAs of the different spliced variants at the N-terminal (exon 5) and C-terminal (exon 21) position of the NMDA receptor 1 (NR1) gene by means of the reverse transcription-polymerase chain reaction. Alternative splicing at exon 5 confers different sensitivity of the NMDA receptor to polyamines while exon 21 encodes a 37-amino acid insert containing the major phosphorylation sites for protein kinase C. One week after the acquisition of stage 5 of kindling in rats (generalized tonic-clonic seizures), the relative abundance of the two alternatively spliced forms at the C-terminal domain, respectively containing (+) or lacking (-) exon 21, was reversed compared to controls (implanted with electrodes but not stimulated) in the dorsal hippocampus ipsilateral and contralateral to the electrical stimulation. The exon 21+/exon 21- mRNA ratio for controls was 1.3 +/- 0.04 (mean +/- SE); for ipsilaterally kindled rats it was 0.64 +/- 0.05 (P < 0.05), and for contralaterally kindled rats it was 0.48 +/- 0.07 (P < 0.01). Similar bilateral effects were observed in the ventral hippocampus (temporal pole). No changes were found 4 weeks after stage 5 seizures and 1 week after the induction of a single afterdischarge. No significant alterations were induced by kindling in the relative abundance of the spliced variants containing or lacking exon 5. Our findings show selective changes in alternative splicing of the NR1 gene after repeated application of an epileptogenic stimulus. This may generate receptors with different functional properties, which may contribute to the increased sensitivity for the induction of generalized seizures during kindling.

Nitric oxide synthesis, epileptic seizures and kindling

Nitric oxide (NO) has been implicated in synaptic changes underlying long-term potentiation and some forms of learning. It is unclear, however, whether NO contributes to long-term changes associated with the kindling of epileptic seizures. In the present study rats were treated, on the first 6 days of kindling, with L-arginine (L-Arg), the endogenous donor from which NO derives, or with L-nitro-arginine (L-No-Arg), a competitive inhibitor of NO synthesis, or with vehicle. Drugs were given in doses previously shown to affect learning or other behaviour. L-Arg (750 mg/kg IP) did not affect kindling or seizure severity. L-No-Arg (100 mg/kg) prolonged the duration of afterdischarges and convulsions on treatment days but did not advance kindling or affect seizures on subsequent days. A second experiment examined the possible role of NO in the development of resistance to seizures following prior seizures. Six or more stimuli were administered at 10-min intervals to fully-kindled rats after injection of L-No-Arg or vehicle. Vehicle-treated rats became progressively more resistant to afterdischarges and convulsions with successive stimulations but L-No-Arg-treated rats failed to do so. Rats injected with L-NO-Arg also showed an unexpected high mortality in the ensuing 24 h. L-No-Arg appeared to have no direct effect on the course of kindling but impaired the development of postictal resistance, and increased the duration and lethal after-effects of closely repeated seizures. The results do not support suggestions that antagonists of NO might prove clinically useful as anticonvulsants.

Kindled epileptic seizures, postictal refractoriness, status epilepticus, and electrical self-stimulation

A single stimulus applied once daily to the limbic system commonly leads to convulsive seizures yet seizures are relatively infrequent during intracranial self-stimulation (ICSS), a procedure that involves many hundreds of similar stimuli. The present study examined the possible role of electrode site, interstimulus interval, afterdischarge and reinforcement thresholds and postictal refractoriness in accounting for this paradox. Electrode location was an overriding factor: seizures were never seen with hypothalamic implants posterior to the level of the ventromedial nucleus but were elicited by the majority of more rostral reward sites. Frequent repeated stimulation by ICSS did not in itself prevent subsequent kindling or reverse the effects of earlier kindling; on the contrary, seizures induced by ICSS showed a progressive increase in severity similar to the progression produced by conventional kindling. Individual convulsive seizures, as in previous studies, conferred transient protection against further seizures whether from ICSS or from kindling. More prolonged protection occassionally developed after repeated convulsive seizures: protection was accompanied by continuous EEG slow-waves corresponding in presentation to clinical petit mal status. Prolonged resistance to seizures has also been reported after tonic-clonic status epilepticus causing temporal lobe damage. The relative infrequency of seizures during ICSS ordinarily appears to depend on the siting of the electrodes, on distinct short- and long-term postictal refractory states, and on the rat learning to restrict stimulus input to subseizural levels.

Effect of an adenosine A1 agonist injected into substantia nigra on kindling of epileptic seizures and convulsion duration

The substantia nigra pars reticulata (SNr) has been reported to be critically involved in the development and propagation of epileptic seizures, while extracellular adenosine appears to be important for making seizures stop. In the present study, an adenosine A1 receptor agonist [N6-cyclohexyladenosine (CHA); 2.0 nmol/side, or vehicle] was injected bilaterally into the SNr shortly before each of the first five of a series of daily kindling stimuli delivered to the rat amygdala. Injections did not affect the acquisition of kindled afterdischarges or the rate at which seizures developed over subsequent kindling sessions, but convulsions occurring 48-72 h after treatment were significantly shortened. Thus, purinergic mechanisms in the SNr do not appear to be specifically involved in the acquisition of kindled seizures but may contribute to a postictal inhibitory process that shortens the convulsive component.