Brain Na+/K(+)-ATPase regulation by serotonin and norepinephrine in normal and kindled rats

Monoaminergic Control of Cellular Glucose Utilization by Glycogenolysis in Neocortex and Hippocampus

Brainstem nuclei are the principal sites of monoamine (MA) innervation to major forebrain structures. In the cortical grey matter, increased secretion of MA neuromodulators occurs in response to a wealth of environmental and homeostatic challenges, whose onset is associated with rapid, preparatory changes in neural activity as well as with increases in energy metabolism. Blood-borne glucose is the main substrate for energy production in the brain. Once entered the tissue, interstitial glucose is equally accessible to neurons and astrocytes, the two cell types accounting for most of cellular volume and energy metabolism in neocortex and hippocampus. Astrocytes also store substantial amounts of glycogen, but non-stimulated glycogen turnover is very small. The rate of cellular glucose utilization in the brain is largely determined by hexokinase, which under basal conditions is more than 90 % inhibited by its product glucose-6-phosphate (Glc-6-P). During rapid increases in energy demand, glycogen is a primary candidate in modulating the intracellular level of Glc-6-P, which can occur only in astrocytes. Glycogenolysis can produce Glc-6-P at a rate higher than uptake and phosphorylation of glucose. MA neurotransmitter are released extrasinaptically by brainstem neurons projecting to neocortex and hippocampus, thus activating MA receptors located on both neuronal and astrocytic plasma membrane. Importantly, MAs are glycogenolytic agents and thus they are exquisitely suitable for regulation of astrocytic Glc-6-P concentration, upstream substrate flow through hexokinase and hence cellular glucose uptake. Conforming to such mechanism, Gerald A. Dienel and Nancy F. Cruz recently suggested that activation of noradrenergic locus coeruleus might reversibly block astrocytic glucose uptake by stimulating glycogenolysis in these cells, thereby anticipating the rise in glucose need by active neurons. In this paper, we further develop the idea that the whole monoaminergic system modulates both function and metabolism of forebrain regions in a manner mediated by glycogen mobilization in astrocytes.

Differential effects of Na+-K+ ATPase blockade on cortical layer V neurons

Sodium-potassium ATPase ('Na(+)-K(+) ATPase') contributes to the maintenance of the resting membrane potential and the transmembrane gradients for Na(+) and K(+) in neurons. Activation of Na(+)-K(+) ATPase may be important in controlling increases in intracellular sodium during periods of increased neuronal activity. Down-regulation of Na(+)-K(+) ATPase activity is implicated in numerous CNS disorders, including epilepsy. Although Na(+)-K(+) ATPase is present in all neurons, little is known about its activity in different subclasses of neocortical cells. We assessed the physiological properties of Na(+)-K(+) ATPase in fast-spiking (FS) interneurons and pyramidal (PYR) cells to test the hypothesis that Na(+)-K(+) ATPase activity would be relatively greater in neurons that generated high frequency action potentials (the FS cells). Whole-cell patch clamp recordings were made from FS and PYR neurons in layer V of rat sensorimotor cortical slices maintained in vitro using standard techniques. Bath perfusion of Na(+)-K(+) ATPase antagonists (ouabain or dihydro-ouabain) induced either a membrane depolarization in current clamp, or inward current under voltage clamp in both cell types. PYR neurons were divided into two subpopulations based on the amplitude of the voltage or current shift in response to Na(+)-K(+) ATPase blockade. The two PYR cell groups did not differ significantly in electrophysiological properties including resting membrane potential, firing pattern, input resistance and capacitance. Membrane voltage responses of FS cells to Na(+)-K(+) ATPase blockade were intermediate between the two PYR cell groups (P < 0.05). The resting Na(+)-K(+) ATPase current density in FS interneurons, assessed by application of blockers, was 3- to 7-fold larger than in either group of PYR neurons. Na(+)-K(+) ATPase activity was increased either through direct Na(+) loading via the patch pipette or by focal application of glutamate (20 mM puffs). Under these conditions FS interneurons exhibited the largest increase in Na(+)-K(+) ATPase activity. We conclude that resting Na(+)-K(+) ATPase activity and sensitivity to changes in internal Na(+) concentration vary between and within classes of cortical neurons. These differences may have important consequences in pathophysiological disorders associated with down-regulation of Na(+)-K(+) ATPase and hyperexcitability within cortical networks.

Methylphenidate treatment increases Na(+), K (+)-ATPase activity in the cerebrum of young and adult rats

Methylphenidate is a central nervous system stimulant used for the treatment of attention-deficit hyperactivity disorder. Na(+), K(+)-ATPase is a membrane-bound enzyme necessary to maintain neuronal excitability. Considering that methylphenidate effects on central nervous system metabolism are poorly known and that Na(+), K(+)-ATPase is essential to normal brain function, the purpose of this study was to evaluate the effect of this drug on Na(+), K(+)-ATPase activity in the cerebrum of young and adult rats. For acute administration, a single injection of methylphenidate (1.0, 2.0, or 10.0 mg/Kg) or saline was given to rats on postnatal day 25 or postnatal day 60, in the young and adult groups, respectively. For chronic administration, methylphenidate (1.0, 2.0, or 10.0 mg/Kg) or saline injections were given to young rats starting at postnatal day 25 once daily for 28 days. In adult rats, the same regimen was performed starting at postnatal day 60. Our results showed that acute methylphenidate administration increased Na(+), K(+)-ATPase activity in hippocampus, prefrontal cortex, and striatum of young and adult rats. In young rats, chronic administration of methylphenidate also enhanced Na(+), K(+)-ATPase activity in hippocampus and prefrontal cortex, but not in striatum. When tested in adult rats, Na(+), K(+)-ATPase activity was increased in all cerebral structures studied. The present findings suggest that increased Na(+), K(+)-ATPase activity may be associated with neuronal excitability caused by methylphenidate.

Effects of cypermethrin on the electroencephalographic activity of the rat: a model of chemically induced seizures

Cypermethrin is a potent representative member of the type II pyrethroid insecticides. This pyrethroid is used worldwide and has become a part of our environment. Until the present study, little information about its toxic effects in the central nervous system (CNS) was available. The aim of this study was, then, to determine the effects of repeated exposure to cypermethrin by means of assessing the electroencephalographic (EEG) activity in the rat. Cypermethrin was administered daily in a 300 mg/kg i.p. dose, below the LD50 value. After daily administration, the EEG activity was recorded and evaluated for 30 min. Paroxysmal epileptic activity appeared after the first and second days of cypermethrin administration. Frequency and numbers of bursts of epileptic activity also increased throughout the days of exposure to cypermethrin. Some of the paroxysmal events were present with behavioral anomalies, such as generalized tonic-clonic seizures. These effects are important because they could be related to the incidence of epileptic activity in humans chronically exposed to cypermethrin.

Serotonin modulation of low-affinity ouabain binding in rat brain determined by quantitative autoradiography

Previous results showed that Na+/K+-ATPase may have a functional relationship with the neurotransmitter serotonin which activates the glial sodium pump in the rat brain. Both the reaction rate (V) of Na+/K+-ATPase activity and [3H]ouabain binding were significantly increased in the presence of serotonin. It is not known, however, which alpha isoform is involved in the Na+/K+-ATPase response to serotonin and its regional distribution. Quantitative autoradiography of [3H]ouabain binding to rat brain slices was employed at different [3H]ouabain concentrations in order to gain information on both the distribution and the possible isoform involved. The results showed that 1500 nM [3H]ouabain binding was sensitive to serotonin 10(-3) M and significantly increased in the following brain regions: frontal cortex, areas CA1, CA2, and CA3 of the hippocampus, presubiculum, zona incerta, caudate putamen and the amygdaloid area, confirming and extending previous results. An effect of serotonin on brain but not kidney tissue at high, 1500 nM, and the lack of effect at low, 50 nM [3H]ouabain concentrations, strongly suggests the participation of the alpha2 isoform in the response of the pump to the neurotransmitter. Glial cells showed stimulation of ouabain binding by serotonin at ouabain concentrations above 350 nM. The present results open interesting questions related to the brain regions involved and the K+ handling by the glial alpha2 isoform of the pump.

Comparison of the effects of serotonin in the hippocampus and the entorhinal cortex

Among the molecular, cellular, and systemic events that have been proposed to modulate the function of the hippocampus and the entorhinal cortex (EC), one of the most frequently cited possibilities is the activation of the serotonergic system. Neurons in the hippocampus and in the EC receive a strong serotonergic projection from the raphe nuclei and express serotonin (5-HT) receptors at high density. Here we review the various effects of 5-HT on intrinsic and synaptic properties of neurons in the hippocampus and the EC. Although similar membrane-potential changes following 5-HT application have been reported for neurons of the entorhinal cortex and the hippocampus, the effects of serotonin on synaptic transmission are contrary in both areas. Serotonin mainly depresses fast and slow inhibition of the principal output cells of the hippocampus, whereas it selectively suppresses the excitation in the entorhinal cortex. On the basis of these data, we discuss the possible role of serotonin under physiological and pathophysiological circumstances.

Serotonin blocks different patterns of low Mg2+-induced epileptiform activity in rat entorhinal cortex, but not hippocampus

Low Mg2+-induced epileptiform activity in the entorhinal cortex is characterized by an initial expression of seizure-like events followed by late recurrent discharges. Both these forms of activity as well as the transition between them were blocked by serotonin. In contrast, serotonin had little effect upon the epileptiform activity in areas CA3 and CA1 of the hippocampus. Both forms of epileptiform activity in the entorhinal cortex are sensitive to N-methyl-D-aspartate receptor antagonists and it is shown here that serotonin blocked both types of epileptiform activity through an effective concentration-dependent reduction of N-methyl-D-aspartate receptor-mediated excitatory postsynaptic potentials in deep layer entorhinal cortex cells. Serotonin also prolonged or even prevented the transition between the two types of epileptiform activity and we suggest that this may be through activation of the Na+/K+-ATPase. The resistance of epileptiform activity in CA1 and CA3 to serotonin was most likely related to the inability of serotonin to reduce Schaffer collateral-evoked excitatory postsynaptic potentials. Given the strong serotonergic inputs to both the hippocampus and entorhinal cortex, the differential sensitivity of the two regions to serotonin suggests functional differences. In addition since the late recurrent discharges in the entorhinal cortex are resistant to all clinically used anticonvulsants, serotonin may open new avenues for the development of novel anticonvulsant compounds.

Acute and chronic effects of potassium and noradrenaline on Na+, K+-ATPase activity in cultured mouse neurons and astrocytes

Acute and chronic effects of elevated extracellular concentrations of potassium ions ([K+]0) and/or noradrenaline were studied in homogenates of primary cultures of mouse astrocytes, from the cerebral cortex or the spinal cord, and of primary cultures of mouse cerebral cortical neurons. NA+, K+-ATPase activity in cerebral cortical astrocytes showed a Km value of 1.9 mM with confidence limits of 1.3-2.9 mM and a Vmax of 5.4 mumol/h/mg protein with confidence limits of 3.3-8.1 mumol/h/mg protein. Due to the high Km value, the activity of the enzyme was significantly increased by an increase in [K+]0 in the interval 5-12 mM. In cerebral cortical neurons, Vmax was lower (1.77 +/- 0.06 mumol/h/mg protein) but the affinity was higher (Km 0.43 +/- 0.8 mM). With these kinetics, there is no stimulation of enzyme activity when [K+]0 is increased beyond control levels. In spinal cord astrocytes, the relative effect of increasing [K+]0 above 6 mM was larger than in cerebral astrocytes but the absolute activity of the enzyme was lower. Na+, K+-ATPase activity in both types of astrocyte was stimulated by noradrenaline and its beta-adrenergic subtype agonist isoproterenol but mainly or exclusively at 6 mM [K+]0. Noradrenaline also caused a stimulation in cortical neurons, but at non-physiological K+ concentrations this stimulation was converted to an inhibition, and isoproterenol had no stimulatory effect. Chronic exposure of cerebral cortical astrocytes to elevated [K+]0 caused a decrease in Na+, K+-ATPase activity when enzyme activity in the cells was subsequently measured at normal [K+]0. During exposure to 30 mM [K+]0 this "down-regulation" took place within 10 min. Conversely, chronic exposure to reduced [K+]0 led to an increase in Na+, K+-ATPase activity. Chronic exposure to noradrenaline had no significant effect but there was a tendency towards an increase.

Correlation between oxytocin neuronal sensitivity and oxytocin-binding sites in the amygdala of the rat: electrophysiological and histoautoradiographic study

Central nucleus (Ce), basomedial and medial nuclei of the amygdala (AMG), and some parts of the striato-pallidal system, present high densities of oxytocin (OT)-binding sites. In order to examine whether these OT-binding sites are functional receptors, the OT neuronal sensitivity and the presence of OT-binding sites were investigated using electrophysiological and autoradiographical techniques. To identify the AMG cells, electrical stimulation of the oval subnucleus of the bed nucleus of the stria terminalis (Ov) and of the parabrachial nucleus (Pb) were performed. Somatic and auditory sensory stimulations were also tested. OT was applied by iontophoresis during extracellular single unit recordings of cells which were localized in frontal brain sections subsequently used for histoautoradiographic detection of OT-binding sites. Cells responding to Ov nucleus stimulation were located in the AMG, mainly in the Ce nucleus, whereas those responding to Pb nucleus stimulation were distributed in the Ce nucleus and in the postero lateral part of the caudate putamen. Iontophoretic OT application excited 45% of the recorded cells (43/96) among which OT alone activated spontaneous firing rate of 30 and potentiated the L-Glutamate (GLU)-induced activation on 13. These OT-sensitive neurons were located mainly in the AMG and caudate putamen areas containing OT-binding sites. These results strongly suggest that OT-binding sites found in the AMG are functional receptors upon which OT could act as a neurotransmitter and as a neuromodulator to regulate autonomic functions.