Regulation of intracellular free arachidonic acid inAplysia nervous system

Summary We have studied the regulation of arachidonic acid (AA) uptake, metabolism, and release inAplysia nervous system. Following uptake of [³H]AA, the distribution of radioactivity in intracellular and extracellular lipid pools was measured as a function of time in the presence or absence of exogenous AA. The greatest amount of AA was esterified into phosphatidylinositol (relative to pool size). We found that the intracellular free AA pool underwent rapid turnover, and that radioactive free AA and eicosanoids were released at a rapid rate into the extracellular medium, both in the presence and absence of exogenous AA. Most of the released radioactivity originated from phosphatidylinositol.Two pharmacological agents were found to modulate AA metabolism inAplysia ganglia. The phorbol ester, 12-O-tetradecanoylphorbol 13-acetate, stimulated liberation of AA from phosphatidylinositol and phosphatidylcholine. This resulted in an increase in free internal and secreted AA, an increase in conversion of AA to eicosanoids, and an increase in esterification of AA into triacylglycerol. The half maximal dose for TPA-stimulated AA turnover was 15nm, and the stimulation was dependent on the presence of extracellular calcium. 4-bromophenacylbromide inhibited the redistribution of radioactivity from phospholipid into triacylglycerol, indicating BPB was acting as a phospholipase inhibitor inAplysia as it does in other systems. These pharmacological agents, in addition to providing information about the regulation of AA metabolism and release, are useful tools for investigating the physiological function of the rapid turnover of AA inAplysia nervous system.     

Phospholipase A2 has a role in proliferation but not in differentiation of HL-60 cells

The role of phospholipase A₂ (PLA₂) and its metabolite arachidonic acid (AA) in the proliferation and differentiation of HL-60 cells was investigated. Addition of either 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) or retinoic acid (RA) to HL-60 cells for 2 h inhibited PMA-stimulated PLA₂ activity measured by [³H]AA release. The inhibitor of PLA₂ activity, p-bromophenacyl bromide (BPB), significantly inhibited the proliferation of HL-60 cells and of fibroblast L929 and Swiss 3T3 cells in a dose-dependent manner. The effect of BPB on proliferation is probably through its inhibitory effect on PLA₂ activity, since the same doses of BPB which inhibited proliferation also inhibited PLA₂ activity determined by [³H]AA release. The importance of PLA₂ activity for cell growth was further supported by the effect of two other PLA₂ inhibitors, AACOCF₃ and scalaradial, which inhibited HL-60 proliferation in a dose-dependent manner. BPB, AACOCF₃ and scalaradial significantly increased the doubling time to 32.4 h, 34.0 h and 31.8 h, respectively, compared with 24.6 h in the control. The inhibitory effect of BPB on HL-60 proliferation was reversed by addition of exogenous free AA to HL-60 cells, indicating the importance of this metabolite for the proliferation process. This reversible effect is specific for AA since it was not achieved by other fatty acids like linolenic acid (LA) or oleic acid (OA). Addition of free AA to HL-60 cells did not induce differentiation, as expected. Although BPB, AACOCF₃, or scalaradial inhibited proliferation, they did not induce differentiation nor affect the differentiation induced by 1,25(OH)₂D₃ or RA. These results implicate that PLA₂ activity has no regulatory role in differentiation of HL-60 cells. The differential effect of PLA₂ inhibitors on proliferation and differentiation of HL-60 cells suggests that these two processes function under different regulatory mechanisms.     

Inhibition of the calcitonin-induced outward current in identifiedAplysia neurons by interleukin-1 and interleukin-2

Summary 1. The effects of bath-applied recombinant human interleukin-1 (rhIL-1) and interleukin-2 (rhIL-2) on the calcitonin (CT)-induced outward current recorded from identified neurons (R9–R12) ofAplysia kurodai were investigated with conventional voltage-clamp and pressure ejection techniques.2. Micropressure ejection of CT onto the soma of the neuron induced a slow outward current [I _o(CT); 4–6 nA in amplitude, 30–40 sec in duration] associated with a decrease in input membrane conductance.3.I _o(CT) was increased by hyperpolarization.4. The extrapolated reversal potential was +10 mV. Additionally,I _o(CT) was sensitive to changes in (Na⁺)_o but not to changes in (K⁺)_o, (Ca²⁺)_o, and (Cl^–)_o.5. Micropressure-ejected forskolin produced a slow outward current similar to that induced by CT.6. Bath-applied rhIL-1 and rhIL-2 (10–40 U/ml) reduced the CT-induced current in identifiedAplysia neurons without affecting the resting membrane conductance or the holding current.7. The inhibitory effects of both cytokines on the current were completely reversible. Heat-inactivated rhIL-1 and rhIL-2 were without effect.8. These results suggest that the immunomodulators, IL-1 and IL-2, can modulate the CT-induced outward current associated with a decrease in Na⁺ conductance in the nervous system ofAplysia. Therefore, the study suggests that these cytokines may also serve as neuromodulators.     

Aplysia hemolymph promotes neurite outgrowth and synaptogenesis of identifiedHelix neurons in cell culture

Hemolymph of adultAplysia californica significantly affects neurite outgrowth of identified neurons of the land snailHelix pomatia. The metacerebral giant cell (MGC) and the motoneuron C3 from the cerebral ganglion and the neuron B2 from the buccal ganglion ofH. pomatia were isolated by enzymatic and mechanical dissociation and plated onto poly-l-lysine-coated dishes either containing culture medium conditioned byHelix ganglia, or pre-treated withAplysia hemolymph. To determine the extent of neuronal growth we measured the neurite elongation and the neuritic field of cultured neurons at different time points.Aplysia hemolymph enhances the extent and rate of linear outgrowth and the branching domain ofHelix neurons. With the hemolymph treatment the MGC neuron more consistently forms specific chemical synapses with its follower cell B2, and these connections are more effective than those established in the presence of the conditioned medium.     

Opioid peptides in the nervous system ofAplysia: A combined biochemical,immunocytochemical, and electrophysiological study

Summary 1. We have used biochemical, immunocytochemical, and electrophysiological techniques to evaluate the role of opioid peptides in the central nervous system of the marine mollusc,Aplysia california.2. Binding studies using³H-d-Ala², met-enkephalinamide (³H-DAMA) showed a single class of high-affinity binding sites with aK _d of 1.3 nM and a binding density of 45 pmol/g.3. HPLC extracts of ganglia revealed multiple peaks with immunoreactivity for either leu (LEU-IR)- or met-enkephalin (MET-IR), but the amounts were not uniformly distributed in all ganglia.4. LEU-IR and MET-IR neurons were demonstrated immunocytochemically in all ganglia, but MET-IR neurons were more frequent and were concentrated in pedal and pleural ganglia. While absorption control studies abolished MET-IR, LEU-IR was only partially abolished in the neuropil.5. In electrophysiological studies, both depolarizing and hyperpolarizing responses were found tod-Ala²-leu-enkephalin (DALEU) andd-Ala²-met enkephalin (DAMET) on some and different neurons.6. HPLC fractions from regions with retention times corresponding to authentic leu- or met-enkephalin showed physiologic responses similar to those of DALEU and DAMET, respectively.7. These studies suggest that a variety of endogeneous opioid peptides play physiologically important roles in the nervous system ofAplysia, including but not necessarily limited to leu- and met-enkephalin.     

Metabolism of exogenous arachidonic acid by murine macrophage-like tumor cell lines

Murine macrophage-like cell lines, J774.2, P388D1, RAW264.7 and PU-5-1R, were incubated with exogenous arachidonic acid (AA). The major metabolites were identified by comigration with known standards in TLC and HPLC and by characteristic behavior following reduction. During a 30 min incubation J774.2 cells metabolized exogenous ¹⁴C-AA (10 μM) to PGE₂ (14.8%), 12-hydroxy-5,8,10-heptadecatrienoic acid (HTT)_ (13.0%), thromboxane B₂ (TXB₂) (7.4%), PGD₂ (4.4%) and PGF_2α (3.0%). The remainder was incorporated into phospholipids (39.0%), triglycerides (6.1%), and as yet unidentified metabolites (8.2%). No PGF_1α was found. Metabolism of exogenous AA was rapid, being >90% completed at 3.5 min. Metabolism of exogenous AA is not increased by the simultaneous addition of macrophage stimuli including the cation ionophore A-23187, particulate phagocytic stimuli and endotoxin. The synthesis of cyclooxygenase products was inhibited by low doses of indomethacin (ID₅₀=0.6 μM) while the synthesis of TXB₂ and HHT was selectively inhibited by benzylimidazole (ID₅₀=9.5 μM). Identification of a probable lipoxygenase product is being pursued. The synthesis of this product is not inhibited by indomethacin and migrates with an R_f value close to 5,12-diHETE in TLC. P388D1 and RAW264.7 cells metabolize exogenous AA to the same products as J774.2, in different proportions, while PU-5-1R does not produce cylooxygenase metabolites to any appreciable extent.     

Biosynthesis of acyclic methyl branched polyunsaturated hydrocarbons inPseudomonas maltophilia

Summary The hydrocarbon composition ofPseudomonas maltophilia was determined by gas chromatography-mass spectrometry. Mono-, di- and tri-unsaturated alkenes were identified with a predominance of polyunsaturated components. The carbon chains of the alkenes contained single methyl branches iniso andanteiso position and double methyl branches in theiso-iso andanteiso-anteiso configurations. The composition of the hydrocarbons from cells grown in synthetic media enriched with amino acids or volatile fatty acids demonstrated that the probable precursors incorporated into individual hydrocarbons were branched and normal fatty acid chains in the range from C₃ to C₁₆. The probable fatty acid precursors which were connected together to form the major triunsaturated hydrocarbon chains were two monounsaturated chains, whereas the major liunsaturated chains resulted from condensation of saturated and monounsaturated chains. The probable precursors for the major monounsaturated hydrocarbons were C₁₄ (C₁₅) and C₁₆ (C₁₅) fatty acids. The accumulation of hydrocarbons was not detected until the cells were in the late exponential phase of growth; the maximal levels were reached at the mid-stationary phase of growth.     

THE CHARACTERIZATION OF HISTIDINE DECARBOXYLASE AND ITS DISTRIBUTION IN NERVES, GANGLIA AND IN SINGLE NEURONAL CELL BODIES FROM THE CNS OF APLYSIA CALIFORNICA

Histamine (HA) is present in substantial quantities in all ganglia of Aplysia californica. Within the cerebral ganglia this amine is known to be concentrated in at least two identified neurons designated C-2 neurons. In this study a combination of chemical and enzymatic analyses was employed to provide evidence for the existence of a biochemical pathway for HA synthesis in ganglia and individual neurons of this marine mollusk. Examination of extracts of individual neurons dissected from ganglia organ-cultured in the presence of [³H]histidine showed that every neuron accumulated labelled histidine, but only the HA-containing C-2 neurons synthesized and stored labelled HA suggesting that the formation of HA in Aplysia could be catalyzed by the enzyme histidine decarboxylase (HDC). HDC activity was studied with a new microradiometric assay. Many of the properties of the molluscan HDC studied were found to correspond to the vertebrate enzyme. Enzyme activity was inhibited by α-hydrazino-histidine but unaffected by concentrations of α-methyldopa or by 5-(3,4-dihydroxycinnamoyl) salicylic acid which produced nearly complete inhibition of aromatic amino acid decarboxylase activity. HDC was measurable in nervous but not other Aplysia tissues assayed. All 5 major ganglia contained HDC activity which spanned a 15-fold range between the least and most active ganglia. Only 4 of the 13 nerve trunks assayed yielded measurable enzymic activity; these active nerves were associated with the cerebral ganglia which has the highest HDC activity of all measured ganglia. Of the numerous individual neurons assayed for HDC, only the C-2 cells showed measurable enzyme activity, about 25 pmol/cell/h or 70 μmol/g protein/h. Since the activity of HDC in the HA-containing neurons was at least three orders of magnitude larger than all other neurons assayed in the cerebral and other ganglia, these data appear to provide a direct metabolic basis for the selective presence of HA in these cells, and they indicate that the cellular presence of HDC provides a useful biochemical marker for the location of HA-rich neurons in Aplysia.     

Activity-dependent induction of facilitation,depression, and post-tetanic potentiation at an insect central synapse

Summary In Manduca sexta larvae, sensory neurons innervating planta hairs on the tips of the prolegs make monosynaptic excitatory connections with motoneurons innervating proleg retractor muscles. Tactile stimulation of the hairs evokes reflex retraction of the proleg. In this study we examined activity-dependent changes in the amplitude of the excitatory postsynaptic potentials (EPSPs) evoked in a proleg motoneuron by stimulation of individual planta hair sensory neurons. Deflection of a planta hair caused a phasic-tonic response in the sensory neuron, with a mean peak instantaneous firing frequency of >300 Hz, and a tonic firing rate of 10–20 Hz. Direct electrical stimulation was used to activate individual sensory neurons to fire at a range of frequencies including those observed during natural stimulation of the hair. At relatively low firing rates (e.g., 1 Hz), EPSP amplitude was stable indefinitely. At higher instantaneous firing frequencies (>10 Hz), EPSPs were initially facilitated, but continuous stimulation led rapidly to synaptic depression. High-frequency activation of a sensory neuron could also produce post-tetanic potentiation, in which EPSP amplitude remained elevated for several min following a stimulus train. Facilitation, depression, and post-tetanic potentiation all appeared to be presynaptic phenomena. These activity-dependent changes in sensory transmission may contribute to the behavioral plasticity of the proleg withdrawal reflex observed in intact insects.Abbreviations ACh acetylcholine - AChE acetylcholine esterase - CNS central nervous system - EPSP excitatory postsynaptic potential - I _h injected hyperpolarizing current - LTP long-term potentiation - PPR principal planta retractor motoneuron - PTP post-tetanic potentiation - R _in input resistance - V _h hyperpolarized potential - V _m membrane potential - VN ventral nerve - VNA anterior branch of the ventral nerve - V _r resting potential.     

Lipids of isolated neurons

1. Lipids were extracted from neurons isolated from the lateral vestibular nucleus of ox (Bos taurus L.) and the ganglia of Aplysia punctata Cuvier. 2. Thin-layer chromatography of ox-neuron lipid revealed three major fractions corresponding to neutral lipid, phosphatidylethanolamine and phosphatidylserine. Part of the phosphatidylethanolamine was present as the plasmalogen. 3. Aplysia-neuron lipid contained neutral lipid, phosphatidylethanolamine and phosphatidylserine. Both phospholipids appeared to be present predominantly as the plasmalogen form. 4. The fatty acids of alkali-labile lipids of ox neurons were examined by gas–liquid chromatography. The major fatty acids were oleic acid, stearic acid and palmitic acid.     

Carbohydrate and lipid components of hyphae and conidia of human pathogenFonsecaea pedrosoi

The carbohydrate and lipid components of mycelium and conidia ofFonsecaea pedrosoi (Brumpt) were analysed by paper, thin-layer and gas-chromatography, mass spectrometry and ultraviolet spectroscopy. Glucose, mannose, galactofuranose, rhamnose and glucosamine were polysaccharide components identified inF. pedrosoi. Significant changes in the carbohydrate pattern occurred during the conversion of mycelium into conidia. Rhamnose was predominant in conidia whereas galactose was prominent in mycelium. Palmitic, stearic, oleic, linoleic, and arachidonic acids were the fatty acids identified in the total lipid fraction. Palmitic and oleic acids were major fatty acids. Marked alterations in the fatty acid constituents were observed between the cell types ofF. pedrosoi. Arachidonic acid was detected only in conidia and linoleic acid was preferentially identified in mycelium. Differences in the sterol composition was also associated with morphogenesis inF. pedrosoi. Two main sterols, ergosterol and another less polar sterol, not fully characterized, were found in mycelium whereas in conidia only the latter sterol was present.     

Age-dependent behavioral changes and physiological changes in identified neurons in Aplysia californica

The gill withdrawal reflex (GWR) to direct gill stimulation was studied in sexually mature Aplysia and in those older by at least two months. The GWR threshold in old Aplysia was five- to sevenfold higher than that in mature animals. In the habituation paradigm, the GWR amplitude decremented rapidly to zero in old animals whereas in mature animals it persisted for at least ten trials. The GWR could not be dishabituated in old animals. The GWR is an age-dependent behavior in that parieto-visceral ganglion suppression of the GWR appears to increase with age. Also the electrophysiological properties of two neurons in the parieto-visceral ganglion were compared in the two age groups: L₇ a neuron which dishabituates the GWR in mature and not in old animals; and R₂ which manifests cytological changes with age. In old animals L₇′s input resistance was lower, the time constant was increased, and the size of the psp evoked by gill stimulation was smaller than those of mature L₇s. Similar membrane changes with age were measured in R₂. Soma size of L₇ was approximately the same in the two age groups as was that of R₂. The physiological parameters of neurons of known function continue to change during postmetamorphic life of Aplysia.     

Effect of monoterpenes on lipid oxidation in maize

The monoterpenes 1,8-cineole, thymol, geraniol, menthol and camphor strongly inhibited the root growth of Zea mays L. seedlings. They induced an oxidative stress as measured by the production of malondialdehyde, conjugated dienes and peroxides. This oxidative stress depended on the length of the exposure and on the monoterpene applied. The total fatty acid content was measured and fatty acid composition was analyzed. Unsaturated fatty acids increased in the treated samples. The alcoholic and non-alcoholic monoterpenes appeared to have different modes of action.Abbreviations MDA Malondialdehyde - TFA Total fatty acid content - FA Fatty acid - IC₈₀ Concentration causing 80% inhibition     

Age-dependent anatomical changes in an identified neuron in the cns of Aplysia californica

Neurons of Aplysia californica are naturally pigmented and the pigment accumulates with age. In the present study the pigment was examined in the same neuron from Aplysia of three postmetamorphic ages: young, sexually mature, and old. The large central neuron, R₂, was examined by light and electron microscopy to determine if the pigment possessed properties similar to lipofuscin pigment seen in aging mammalian neurons. We used the same microscopic techniques that demonstrate the presence of lipofuscin in mammalian neurons. Light microscopic studies demonstrated a regional correlation between autofluorescence, staining with Sudan Black, and the naturally occurring pigment in old R₂s. Electron microscopic studies revealed the presence of large vacuolated and lamellated membrane-bound bodies in the peripheral cytoplasm of old R₂s, similar to those found in mammalian neurons. The bodies were located in the same region in which autofluorescence and Sudan Black staining were observed. Although the naturally occurring pigment accumulates with age, it acquires characteristics of lipofuscin pigment in the neurons of older sexually mature animals. The presence of these pigment characteristics can be used as an index of aging in Aplysia neurons as they are in mammalian neurons.     

Indomethacin but not aspirin inhibits basal and stimulated lipolysis in rabbit kidney

The concurrent effect of indomethacin or aspirin on prostaglandins (PGs) biosynthesis and on cellular fatty acid efflux were compared. Studies with rabbit kidney medulla slices and with isolated perfused rabbit kidney showed a marked difference between the two non-steroidal anti-inflammatory drugs, with regard to their effects on fatty acid efflux from kidney tissue. While aspirin effect was limited to inhibition of PGs biosynthesis, indomethacin also reduced the release of free fatty acids. In medullary slices, indomethacin inhibited the Ca²⁺ stimulation of phospholipase A₂ activity and the resulting release of arachidonic and linoleic fatty acids. In the isolated perfused rabbit kidney, indomethacin inhibited the basal efflux of all fatty acids as well as the angiotensin II — induced selective release of arachidonate. Indomethacin also blunted the angiotensin II — induced temporal changes in the efflux of all other fatty acids. Neither indomethacin nor aspirin affected significantly the uptake and incorporation of exogenous (¹⁴C)-arachidonic acid into kidney total lipid fraction.Our tentative conclusion is that indomethacin inhibits basal as well as Ca²⁺ or hormone stimulated activity of kidney lipolytic enzymes. This action of indomethacin reduces the pool size of free arachidonate available for conversion to oxygenated products (both prostaglandin and non-prostaglandin types). The non-steroidal anti-inflammatory drugs can therefore be divided into two groups: a) $\text{aspirin-type compounds}$ which inhibit PGs formation only by interacting with the prostaglandin endoperoxide synthetase and b) $\text{indomethacin-type compounds}$ which inhibit PG generation by both reduction in the amount of available arachidonate and direct interaction with the enzyme.     

Dependence of sodium-calcium exchange current through the membrane of salivary gland cells ofChironomus on pH of the extracellular solution

The dependence of sodium-calcium exchange current (I _Na(Ca)) through the membrane of isolated secretory cells ofChironomus larva on pH of the extracellular solution was studied with the voltage-clamp technique with intracellular perfusion.I _Na(Ca) evoked by hyperpolarization of the membrane from –20 to –60 mV was recorded within physiological values of Na⁺ and Ca²⁺ gradients. It was established that acidification of extracellular solution from pH 7.2 to 4.0 gradually decreased the amplitude ofI _Na(Ca) with pK' — 3.72. In all cases at pH 3.0 an outward current of considerable amplitude emerged in response to membrane hyperpolarization. The reversal of the current occurred at pH around 3.25. A decrease inI _Na(Ca) was due to protonation of acid ionogenic groups (quite possibly, of the residues of aspartic or glutamic amino acids), which had been involved in binding of cations. Alkalization of extracellular solution from pH 7.2 to 10.0 produced a gradual increase in theI _Na(Ca) amplitude; pK' was in the pH range between 9 and 10. The increase inI _Na(Ca) in alkaline medium was probably due to the appearance of negatively charged cations at binding sites, which could be carried by deprotonated thiosulfate groups of cysteine residues. This was indicated by the possibility of initial decrease inI _Na(Ca) under the action of Hg²⁺ ions.Neirofiziologiya/Neurophysiology, Vol. 28, No. 4/5, pp. 193–196, July–October, 1996.     

Sodium-coupled amino acid and sugar transport byNecturus small intestine

Summary Necturus small intestine actively absorbs sugars and amino acids by Na-coupled mechanisms that result in increases in the transepithelial electrical potential difference ( ^ms ) and the short-circuit current (I _sc) which can be attributed entirely to an increase in the rate of active Na absorption. Studies employing conventional microelectrodes indicate that the addition of alanine or galactose to the mucosal solution is followed by a biphasic response. Initially, there is a rapid depolarization of the electrical potential difference across the apical membrane ( ^ms ) which reverses polarity (i.e. cell interior becomes positive with respect to the mucosal solution) and a marked decrease in the ratio of the effective resistance of the mucosal membrane to that of the serosal membrane (R ^m /R ^s ); these events do not appear to be dependent on the availability of metabolic energy. These initial, rapid events are followed by a slow increase in (R ^m /R ^s ) toward control values which is paralleled by a repolarization of ^ms and increases in ^ms andI _sc; this slow series of events is dependent upon the availability of metabolic energy.The results of these studies indicate that: (i) the Na-coupled mechanisms that mediate the entry of sugars and amino acids across the apical membrane are rheogenic (conductive) and result in a decrease inR ^m and a depolarization of ^ms ; and (ii) the subsequent increase in (R ^m /R ^s ) and repolarization of ^ms are the results of a decrease inR ^s which is associated with an increase in the activity of the Na pump at the basolateral membrane.The physiologic implications of these findings are discussed and an equivalent electrical circuit model for rheogenic Na-coupled solute transport processes is analyzed.     

Gramicidin toxicity in NG108-15 cells: protective effects of acetamidine and guanidine

Studies were conducted using a novel in vitro approach to investigate the efficacy of acetamidine hydrochloride (ACE) and guanidine hydrochloride (GUAN), previously shown to block gramicidin D (GRAM) channels in artificial membranes, in preventing the toxic effects of GRAM in NG108-15 (neuroblastoma×glioma hybrid) cells. Specifically, intracellular microelectrode techniques were employed to examine changes in membrane resting potential (V _m) and input resistance (R _in). At 1 mol/L, ACE significantly reduced loss of V _m induced by 1 or 10 g/ml GRAM, although higher concentrations of ACE did not afford enhanced antagonism. GUAN, in contrast, produced a concentration-dependent antagonism of GRAM-induced V _m and R _in loss, with high concentrations (10 or 100 mol/L) completely preventing diminutions in both V _m and R _in. In control cells superfused without GRAM, ACE produced a direct, concentration-dependent reduction in V _m and R _in, whereas GUAN hyperpolarized NG108-15 cells but did not alter R _in. These data represent the initial demonstration of the reversal of GRAM toxicity in an intact cell system.     

Qualitative study of a dynamical system for metrazol-induced paroxysmal depolarization shifts

The convulsant pentylenetetrazol (PTZ) was used to trigger spike bursts and paroxysmal discharges inAplysia neurons. Voltage clamp experiments showed that PTZ induced a slow voltage-dependent potassium current and a persistent inward current. These currents are incorporated into a membrane model together with modified spike-generating Hodgkin-Huxley equations. From these data a metaphoric model is constructed and represented by a slow-fast dynamical system defined inR ⁴. With some values of the main physiological parameters, the system might have limit cycles for the fast dynamic. A qualitative study of the system shows that it satisfactorily reproduces the various observed patterns produced by PTZ.