Clonidine and Brain Mitochondrial Energy Metabolism: Pharmacodynamic Insights Beyond Receptorial Effects.

Clonidine is an anti-hypertensive drug that inhibits the release of norepinephrine from pre-synaptic terminals binding to pre-synaptic α2-adrenoreceptors. Some studies suggest that this drug decreases brain energy expenditure, particularly in hypoxic-ischemic injury. However, data about clonidine effects on the functional parameters regulating brain energy metabolism are lacking. In this study, the effects of acute clonidine treatment (5 µg×kg-1 i.p., 30 min) were evaluated on the catalytic activity of regulatory energy-linked enzymes of Krebs' cycle, Electron Transport Chain and glutamate metabolism of temporal cerebral cortex of 3-month-old male Sprague-Dawley rats. Enzyme activities were assayed on non-synaptic "free" mitochondria (FM) of neuronal perikaryon and partly of glial cells, and on intra-synaptic "light" (LM) and "heavy" mitochondria (HM), localized within synaptic terminals. This subcellular analysis differentiates clonidine effects on post-synaptic and pre-synaptic neuronal compartments. The results showed that clonidine increased citrate synthase, cytochrome oxidase and glutamate-oxaloacetate transaminase activities of FM. In LM, citrate synthase activity was decreased, while cytochrome oxidase and glutamate-oxaloacetate transaminase activities were increased; on the contrary, citrate synthase, cytochrome oxidase and glutamate dehydrogenase were all decreased in HM. Therefore, clonidine exerted different effects with respect to brain mitochondria, coherently with the in vivo energy requirements of each synaptic compartment: the drug increased energy-linked enzyme activities in post-synaptic compartment, while the metabolic variations were complex in the pre-synaptic one, being enzyme activities heterogeneously modified in LM and decreased in HM. This study highlights the relationships existing between the clonidine-induced neuroreceptorial effects and the energy metabolism in pre- and post- synaptic bioenergetics.

Glutamate metabolism in cerebral mitochondria after ischemia and post-ischemic recovery during aging: relationships with brain energy metabolism.

Glutamate is involved in cerebral ischemic injury, but its role has not been completely clarified and studies are required to understand how to minimize its detrimental effects, contemporarily boosting the positive ones. In fact, glutamate is not only a neurotransmitter, but primarily a key metabolite for brain bioenergetics. Thus, we investigated the relationships between glutamate and brain energy metabolism in an in vivo model of complete cerebral ischemia of 15 min and during post-ischemic recovery after 1, 24, 48, 72, and 96 h in 1-year-old adult and 2-year-old aged rats. The maximum rates (Vmax ) of glutamate dehydrogenase (GlDH), glutamate-oxaloacetate transaminase, and glutamate-pyruvate transaminase were assayed in somatic mitochondria (FM) and in intra-synaptic 'Light' mitochondria and intra-synaptic 'Heavy' mitochondria ones purified from cerebral cortex, distinguishing post- and pre-synaptic compartments. During ischemia, none of the enzymes were modified in adult animals. In aged ones, glutamate-oxaloacetate transaminase was increased in FM and GlDH in intra-synaptic 'Heavy' mitochondria, stimulating glutamate catabolism. During post-ischemic recovery, FM did not show modifications at both ages while, in intra-synaptic mitochondria of adult animals, glutamate catabolism was increased after 1 h of recirculation and decreased after 48 and 72 h, whereas it remained decreased up to 96 h in aged rats. These results, with those previously published about Krebs' cycle and Electron Transport Chain (Villa et al., [2013] Neurochem. Int. 63, 765-781), demonstrate that: (i) Vmax of energy-linked enzymes are different in the various cerebral mitochondria, which (ii) respond differently to ischemia and post-ischemic recovery, also (iii) with respect to aging.

Energy metabolism of synaptosomes from different neuronal systems of rat cerebellum during aging: a functional proteomic characterization.

Functional proteomics was used to characterize age-related changes in energy metabolism of different neuronal pathways within the cerebellar cortex of Wistar rats aged 2, 6, 12, 18, and 24 months. The "large" synaptosomes, derived from the glutamatergic mossy fibre endings which make synaptic contact with the granule cells of the granular layer, and the "small" synaptosomes, derived from the pre-synaptic terminals of granule cells making synaptic contact with the dendrites of Purkinje cells, were isolated by a combined differential/gradient centrifugation technique. Because most brain disorders are associated with bioenergetic changes, the maximum rate (Vmax) of selected enzymes of glycolysis, Krebs' cycle, glutamate and amino acids metabolism, and acetylcholine catabolism were evaluated. The results show that "large" and "small" synaptosomes possess specific and independent metabolic features. This study represents a reliable model to study in vivo (1) the physiopathological molecular mechanisms of some brain diseases dependent on energy metabolism, (2) the responsiveness to noxious stimuli, and (3) the effects of drugs, discriminating their action sites at subcellular level on specific neuronal pathways.

Effects of Chronic Hypertension on the Energy Metabolism of Cerebral Cortex Mitochondria in Normotensive and in Spontaneously Hypertensive Rats During Aging.

In this study the subcellular modifications undergone by cerebral cortex mitochondrial metabolism in chronic hypertension during aging were evaluated. The catalytic properties of regulatory energy-linked enzymes of Tricarboxylic Acid Cycle (TCA), Electron Transport Chain (ETC) and glutamate metabolism were assayed on non-synaptic mitochondria (FM, located in post-synaptic compartment) and on intra-synaptic mitochondria of pre-synaptic compartment, furtherly divided in "light" (LM) and "heavy" (HM) mitochondria, purified form cerebral cortex of normotensive Wistar Kyoto Rats (WKY) versus Spontaneously Hypertensive Rats (SHR) at 6, 12 and 18 months. During physiological aging, the metabolic machinery was differently expressed in pre- and post-synaptic compartments: LM and above all HM were more affected by aging, displaying lower ETC activities. In SHR at 6 months, FM and LM showed an uncoupling between TCA and ETC, likely as initial adaptive response to hypertension. During pathological aging, HM were particularly affected at 12 months in SHR, as if the adaptive modifications in FM and LM at 6 months granted a mitochondrial functional balance, while at 18 months all the neuronal mitochondria displayed decreased metabolic fluxes versus WKY. This study describes the effects of chronic hypertension on cerebral mitochondrial energy metabolism during aging through functional proteomics of enzymes at subcellular levels, i.e. in neuronal soma and synapses. In addition, this represents the starting point to envisage an experimental physiopathological model which could be useful also for pharmacological studies, to assess drug actions during the development of age-related pathologies that could coexist and/or are provoked by chronic hypertension.

Functional proteomics related to energy metabolism of synaptosomes from different neuronal systems of rat hippocampus during aging.

The effect of aging on hippocampus is often confounded by diseases that commonly occur in the elderly. In this research, functional proteomics was used to characterize age-related changes in energy metabolism of different neuronal pathways within the hippocampus of Wistar rats aged 2, 6, 12, 18, and 24 months. The "large" synaptosomes, derived from glutamatergic mossy fiber endings connecting granule cells of dentate gyrus with apical dendrites of CA3 pyramidal cells, and the "small" synaptosomes, derived from the cholinergic small nerve endings of septo-hippocampal fibers, whose projections reach CA1 pyramidal cells, were isolated. Because most brain disorders are associated with bioenergetic changes, the maximum rate (V(max)) of selected enzymes of glycolysis, Krebs cycle, glutamate and amino acids metabolism, and acetylcholine catabolism were evaluated. The results show that "large" and "small" synaptosomes possess specific and independent metabolic features coherently with the selective vulnerability of the respective hippocampal subfields to Alzheimer's disease and cerebral ischemia. This study represents a reliable model to study in vivo (i) the physiopathological molecular mechanisms of some brain diseases dependent on energy metabolism, (ii) the responsiveness to noxious stimuli, and (iii) the effects of drugs, discriminating their action sites at subcellular level.

Effect of in vivo L-acetylcarnitine administration on ATP-ases enzyme systems of synaptic plasma membranes from rat cerebral cortex.

The maximum rates (V (max)) of some enzymatic activities related to energy consumption (ATP-ases) were evaluated in two types of synaptic plasma membranes (SPM) isolated from cerebral cortex of rats subjected to in vivo treatment with L: -acetylcarnitine at two different doses (30 and 60 mg kg(-1) i.p., 28 days, 5 days/week). The following enzyme activities were evaluated: acetylcholinesterase (AChE); Na(+), K(+), Mg(2+)-ATP-ase; ouabain insensitive Mg(2+)-ATP-ase; Na(+), K(+)-ATP-ase; direct Mg(2+)-ATP-ase; Ca(2+), Mg(2+)-ATP-ase; Low- and High-affinity Ca(2+)-ATP-ase. Sub-chronic treatment with L: -acetylcarnitine increased Na(+), K(+)-ATP-ase activity on SPM 2 and Ca(2+), Mg(2+)-ATP-ase activity on both SPM fractions. These results suggest (1) that the sensitivity to drug treatment is different between the two populations of SPM, confirming the micro-heterogeneity of these sub-fractions, probably originating from different types of synapses, (2) the specificity of the molecular site of action of the drug on SPM and (3) its interference on ion homeostasis at synaptic level.

Effect of ageing and ischemia on enzymatic activities linked to Krebs' cycle, electron transfer chain, glutamate and aminoacids metabolism of free and intrasynaptic mitochondria of cerebral cortex.

The effect of ageing and the relationships between the catalytic properties of enzymes linked to Krebs' cycle, electron transfer chain, glutamate and aminoacid metabolism of cerebral cortex, a functional area very sensitive to both age and ischemia, were studied on mitochondria of adult and aged rats, after complete ischemia of 15 minutes duration. The maximum rate (Vmax) of the following enzyme activities: citrate synthase, malate dehydrogenase, succinate dehydrogenase for Krebs' cycle; NADH-cytochrome c reductase as total (integrated activity of Complex I-III), rotenone sensitive (Complex I) and cytochrome oxidase (Complex IV) for electron transfer chain; glutamate dehydrogenase, glutamate-oxaloacetate-and glutamate-pyruvate transaminases for glutamate metabolism were assayed in non-synaptic, perikaryal mitochondria and in two populations of intra-synaptic mitochondria, i.e., the light and heavy mitochondrial fraction. The results indicate that in normal, steady-state cerebral cortex, the value of the same enzyme activity markedly differs according (a) to the different populations of mitochondria, i.e., non-synaptic or intra-synaptic light and heavy, (b) and respect to ageing. After 15 min of complete ischemia, the enzyme activities of mitochondria located near the nucleus (perikaryal mitochondria) and in synaptic structures (intra-synaptic mitochondria) of the cerebral tissue were substantially modified by ischemia. Non-synaptic mitochondria seem to be more affected by ischemia in adult and particularly in aged animals than the intra-synaptic light and heavy mitochondria. The observed modifications in enzyme activities reflect the metabolic state of the tissue at each specific experimental condition, as shown by comparative evaluation with respect to the content of energy-linked metabolites and substrates. The derangements in enzyme activities due to ischemia is greater in aged than in adult animals and especially the non-synaptic and the intra-synaptic light mitochondria seems to be more affected in aged animals. These data allow the hypothesis that the observed modifications of catalytic activities in non-synaptic and intra-synaptic mitochondrial enzyme systems linked to energy metabolism, amino acids and glutamate metabolism are primary responsible for the physiopathological responses of cerebral tissue to complete cerebral ischemia for 15 min duration during ageing.

Synaptic ATPases system of rat frontal cerebral cortex during aging.

Energy metabolism is fundamental to maintain Central Nervous System homeostasis because of high requirement of adenosine triphosphate (ATP), that is necessary to sustain neuronal events. During aging, changes in brain bioenergetics may influence the recovery of cerebral tissue in coping with pathophysiological conditions and pharmacological treatments. For this reason, we have previously studied enzyme catalytic activities related to energy-yielding systems. In the present study, the maximum rates (Vmax) of some enzymatic activities related to energy consumption (ATPases) were evaluated on synaptic plasma membranes (S.P.M.) isolated from frontal cerebral cortex of male Wistar rats aged 2, 6, 12, 18 and 24 months, because of the key role of these enzymes in modulating presynaptic nerve ending homeostasis. The following enzyme activities were evaluated: Na+, K+, Mg2+-ATPase; ouabain-insensitive Mg2+-ATPase; Na+, K+-ATPase; specific Mg2+-ATPase; Ca2+, Mg2+-ATPase; acetylcholinesterase (AChE). The present results show that both the activities of (i) ATPases and (ii) AChE were significantly decreased during aging. Comparing these observations with those previously done on rat striatum on the same functional parameters and in the same experimental settings, ATPases activities were influenced by the age factor in different ways, suggesting that the frontal cerebral cortex independently adapt to the different age-dependent biochemical situations at each single age. Overall, this experimental approach is therefore important to add pieces of information for the understanding of the correlation between aging and brain energy metabolism, and could be a suitable model to assess also drug effects, differentiating between different cerebral areas.

Mitochondrial energy metabolism of rat hippocampus after treatment with the antidepressants desipramine and fluoxetine.

Alterations in mitochondrial functions have been hypothesized to participate in the pathogenesis of depression, because brain bioenergetic abnormalities have been detected in depressed patients by neuroimaging in vivo studies. However, this hypothesis is not clearly demonstrated in experimental studies: some suggest that antidepressants are inhibitors of mitochondrial metabolism, while others observe the opposite. In this study, the effects of 21-day treatment with desipramine (15 mg/kg) and fluoxetine (10 mg/kg) were examined on the energy metabolism of rat hippocampus, evaluating the catalytic activity of regulatory enzymes of mitochondrial energy-yielding metabolic pathways. Because of the micro-heterogeneity of brain mitochondria, we have distinguished between (a) non-synaptic mitochondria (FM) of neuronal perikaryon (post-synaptic compartment) and (b) intra-synaptic light (LM) and heavy (HM) mitochondria (pre-synaptic compartment). Desipramine and fluoxetine changed the catalytic activity of specific enzymes in the different types of mitochondria: (a) in FM, both drugs enhanced cytochrome oxidase and glutamate dehydrogenase, (b) in LM, the overall bioenergetics was unaffected and (c) in HM only desipramine increased malate dehydrogenase and decreased the activities of Electron Transport Chain Complexes. These results integrate the pharmacodynamic features of desipramine and fluoxetine at subcellular level, overcoming the previous conflicting data about the effects of antidepressants on brain energy metabolism, mainly referred to whole brain homogenates or to bulk of cerebral mitochondria. With the differentiation in non-synaptic and intra-synaptic mitochondria, this study demonstrates that desipramine and fluoxetine lead to adjustments in the mitochondrial bioenergetics respect to the energy requirements of pre- and post-synaptic compartments.

Effect of desipramine and fluoxetine on energy metabolism of cerebral mitochondria.

Brain bioenergetic abnormalities in mood disorders were detected by neuroimaging in vivo studies in humans. Because of the increasing importance of mitochondrial pathogenetic hypothesis of Depression, in this study the effects of sub-chronic treatment (21days) with desipramine (15mg/kg) and fluoxetine (10mg/kg) were evaluated on brain energy metabolism. On mitochondria in vivo located in neuronal soma (somatic) and on mitochondria of synapses (synaptic), the catalytic activities of regulatory enzymes of mitochondrial energy-yielding metabolic pathways were assayed. Antidepressants in vivo treatment modified the activities of selected enzymes of different mitochondria, leading to metabolic modifications in the energy metabolism of brain cortex: (a) the enhancement of cytochrome oxidase activity on somatic mitochondria; (b) the decrease of malate, succinate dehydrogenase and glutamate-pyruvate transaminase activities of synaptic mitochondria; (c) the selective effect of fluoxetine on enzymes related to glutamate metabolism. These results overcome the conflicting data so far obtained with antidepressants on brain energy metabolism, because the enzymatic analyses were made on mitochondria with diversified neuronal in vivo localization, i.e. on somatic and synaptic. This research is the first investigation on the pharmacodynamics of antidepressants studied at subcellular level, in the perspective of (i) assessing the role of energy metabolism of cerebral mitochondria in animal models of mood disorders, and (ii) highlighting new therapeutical strategies for antidepressants targeting brain bioenergetics.

Functional proteomics of synaptic plasma membrane ATP-ases of rat hippocampus: effect of l-acetylcarnitine and relationships with Dementia and Depression pathophysiology.

Synaptic energy state and mitochondrial dysfunction are crucial factors in many brain pathologies. l-acetylcarnitine, a natural derivative of carnitine, improves brain energy metabolism, and has been proposed for the Therapy of many neurological and psychiatric diseases. The effects of the drug on the maximum rate (Vmax) of enzymatic activities related to hippocampal synaptic energy utilization were evaluated, in the perspective of its employment for Dementias and Depression Therapy. Two types of synaptic plasma membranes (SPM1 and SPM2) were isolated from the hippocampus of rats treated with l-acetylcarnitine (30 and 60mg/kg i.p., 28 days, 5 days/week). Acetylcholinesterase (AChE); Na(+), K(+), Mg(2+)-ATP-ase; ouabain-insensitive Mg(2+)-ATP-ase; Na(+), K(+)-ATP-ase; Ca(2+), Mg(2+)-ATP-ase activities were evaluated. In control animals, enzymatic activities were differently expressed in SPM1 , being the evaluated enzymatic activities higher in SPM2. Subchronic treatment with l-acetylcarnitine (i) did not modify AChE on both SPMs; (ii) increased Na(+), K(+), Mg(2+)-ATP-ase, ouabain-insensitive Mg(2+)-ATP-ase and Na(+), K(+)-ATP-ase at the dose of 30 and 60mg/kg on SPM1 and SPM2; (iii) increased Ca(2+), Mg(2+)-ATP-ase activity on both SPMs at the dose of 60mg/kg. These results have been discussed considering the pathophysiology and treatment of Dementias and Depression because, although referred to normal healthy animals, they support the notion that l-acetylcarnitine may have positive effects in these pathologies.

ATPases enzyme activities during ageing in different types of somatic and synaptic plasma membranes from rat frontal cerebral cortex.

The catalytic properties of energy-utilizing ATPases enzyme systems related to ions homeostasis were evaluated in different types of synaptic plasma membranes (SPM) and in somatic plasma membranes (SM) from cerebral cortex of rats aged 5, 10, and 22 months. The following enzymes were evaluated: Na+, K+-ATPase, Ca2+, Mg2+-ATPase, Mg2+-ATPase and the activity of acetylcholine esterase (AChE) was also evaluated. The ATPases located on SM and SPM and synaptic vesicles are involved in the regulation of presynaptic nerve ending homeostasis and postsynaptic activities. Different types of SM and SPM (three types) were obtained by combinations of differential and density gradient ultracentrifugation techniques in sucrose-Ficoll media: the first was obtained by purification of the sediment of mitochondrial supernate and the second after synaptosomal lysis and purification on density gradient. In the cerebral cortex of 5-month-old rats, the catalytic properties of ATPases systems markedly differ according to the different types of SPM and SM, thus indicating that the metabolic role of each ATPase is determined by their subcellular in vivo localization. As regards ageing: (i) ATPase enzyme catalytic activities tend to decrease during ageing in a complex way; (ii) ageing induced specific modifications in individual ATPases according to their subsynaptic localization; and (iii) these effects are probably due to specific biochemical situations that take place at each age, reflecting the bioenergetic state of the cerebral tissue with respect to the energy demand. The cerebral concentration and content of SM proteins were increased by ageing suggesting that many defective noncatalytic proteins may be formed during ageing, as shown by immunoblotting techniques.

Structural damage induced by peroxidation may account for functional impairment of heavy synaptic mitochondria.

Coenzyme Q distribution, as well as respiratory chain features, in rat brain mitochondria depend on mitochondrial subpopulation, brain region and age. Heavy mitochondria (HM) usually display the lowest content of respiratory components and the lowest enzymatic activities and it has been suggested that they represent the oldest mitochondrial population. In this study, we confirmed that HM are considerably compromised in their structure. In fact, HM showed to have the highest hydroperoxide content and the most consistent modifications in their fatty acid pattern with wide loss of fatty acids (or part of them) in the phospholipid moiety. Such situation could explain the typical impairment of HM and could support the hypothesis that they represent an old mitochondrial population.

Energy metabolism of cerebral mitochondria during aging, ischemia and post-ischemic recovery assessed by functional proteomics of enzymes.

Stroke is a leading cause of death and disability, but most of the therapeutic approaches failed in clinical trials. The energy metabolism alterations, due to marked ATP decline, are strongly related to stroke and, at present, their physiopathological roles are not fully understood. Thus, the aim of this study was to evaluate the effects of aging on ischemia-induced changes in energy mitochondrial transduction and the consequences on overall brain energy metabolism in an in vivo experimental model of complete cerebral ischemia of 15min duration and during post-ischemic recirculation after 1, 24, 48, 72 and 96h, in 1year "adult" and 2year-old "aged" rats. The maximum rate (Vmax) of citrate synthase, malate dehydrogenase, succinate dehydrogenase for Krebs' cycle; NADH-cytochrome c reductase and cytochrome oxidase for electron transfer chain (ETC) were assayed in non-synaptic "free" mitochondria and in two populations of intra-synaptic mitochondria, i.e., "light" and "heavy" mitochondria. The catalytic activities of enzymes markedly differ according to: (a) mitochondrial type (non-synaptic, intra-synaptic), (b) age, (c) acute effects of ischemia and (d) post-ischemic recirculation at different times. Enzyme activities changes are injury maturation events and strictly reflect the bioenergetic state of the tissue in each specific experimental condition respect to the energy demand, as shown by the comparative evaluation of the energy-linked metabolites and substrates content. Remarkably, recovery of mitochondrial function was more difficult for intra-synaptic mitochondria in "aged" rats, but enzyme activities of energy metabolism tended to normalize in all mitochondrial populations after 96h of recirculation. This observation is relevant for Therapy, indicating that mitochondrial enzymes may be important metabolic factors for the responsiveness of ischemic penumbra towards the restore of cerebral functions.

Effect of CDP-choline on age-dependent modifications of energy- and glutamate-linked enzyme activities in synaptic and non-synaptic mitochondria from rat cerebral cortex.

The effect of aging and CDP-choline treatment (20 mg kg⁻¹ body weight i.p. for 28 days) on the maximal rates (V(max)) of representative mitochondrial enzyme activities related to Krebs' cycle (citrate synthase, α-ketoglutarate dehydrogenase, malate dehydrogenase), glutamate and related amino acid metabolism (glutamate dehydrogenase, glutamate-oxaloacetate- and glutamate-pyruvate transaminases) were evaluated in non-synaptic and intra-synaptic "light" and "heavy" mitochondria from frontal cerebral cortex of male Wistar rats aged 4, 12, 18 and 24 months. During aging, enzyme activities vary in a complex way respect to the type of mitochondria, i.e. non-synaptic and intra-synaptic. This micro-heterogeneity is an important factor, because energy-related mitochondrial enzyme catalytic properties cause metabolic modifications of physiopathological significance in cerebral tissue in vivo, also discriminating pre- and post-synaptic sites of action for drugs and affecting tissue responsiveness to noxious stimuli. Results show that CDP-choline in vivo treatment enhances cerebral energy metabolism selectively at 18 months, specifically modifying enzyme catalytic activities in non-synaptic and intra-synaptic "light" mitochondrial sub-populations. This confirms that the observed changes in enzyme catalytic activities during aging reflect the bioenergetic state at each single age and the corresponding energy requirements, further proving that in vivo drug treatment is able to interfere with the neuronal energy metabolism.

Functional proteomics of adenosine triphosphatase system in the rat striatum during aging.

The maximum rates of adenosine triphosphatase (ATPase) systems related to energy consumption were systematically evaluated in synaptic plasma membranes isolated from the striata of male Wistar rats aged 2, 6, 12, 18, and 24 months, because of their key role in presynaptic nerve ending homeostasis. The following enzyme activities were evaluated: sodium-potassium-magnesium adenosine triphosphatase (Na(+), K(+), Mg(2+)-ATPase); ouabain-insensitive magnesium adenosine triphosphatase (Mg(2+)-ATPase); sodium-potassium adenosine triphosphatase (Na(+), K(+)-ATPase); direct magnesium adenosine triphosphatase (Mg(2+)-ATPase); calcium-magnesium adenosine triphosphatase (Ca(2+), Mg(2+)-ATPase); and acetylcholinesterase. The results showed that Na(+), K(+)-ATPase decreased at 18 and 24 months, Ca(2+), Mg(2+)-ATPase and acetylcholinesterase decreased from 6 months, while Mg(2+)-ATPase was unmodified. Therefore, ATPases vary independently during aging, suggesting that the ATPase enzyme systems are of neuropathological and pharmacological importance. This could be considered as an experimental model to study regeneration processes, because of the age-dependent modifications of specific synaptic plasma membranes. ATPases cause selective changes in some cerebral functions, especially bioenergetic systems. This could be of physiopathological significance, particularly in many central nervous system diseases, where, during regenerative processes, energy availability is essential.