Reacciones químicas en sistemas lácteos de humedad reducida y su relación con transformaciones fí... more Reacciones químicas en sistemas lácteos de humedad reducida y su relación con transformaciones físicas del medio" El objetivo fue investigar la influencia de las propiedades físicas de las matrices (grado de compactación, transición vítrea, cristalinidad), con cambios a nivel molecular, evaluados a través de la determinación del grado de avance de reacciones químicas: pardeo no enzimático (PNE), actividad enzimática, resistencia térmica enzimática y su relación con la composición, los cambios de pH y Ia humedad. Los sistemas empleados consistieron en matrices de baja humedad con componentes de suero lácteos. Tanto el PNEcomo la inactivación enzimática tuvieron lugar en muestras en estado vítreo. La velocidad a la que ocurrieron las reacciones químicas (PNE, actividad enzimática, resistencia térmica enzimática) en matrices amorfas estuvo afectada por modificaciones en las matrices que no se manifestaron en cambios en el valor de la temperatura de transición vítrea de los sistemas. En sistemas vítreos, las muestras comprimidas desarrollaron mayor velocidad de PNEque las muestras en polvo, para todas las composiciones estudiadas. La cristalización (parcial o total) de las matrices provoca un incremento de la velocidad de reacciones químicas debido a la concentración de reactivos en las regiones amorfas excluidas del cristal. Estas observaciones llevan a distinguir entre fenómenos que ocurren a un nivel supramolecular, e involucra aspectos cooperativos entre las moléculas (como lo es la transición vítrea) de fenómenos que ocurren a un nivel molecular, como las reacciones de PNE, actividad enzimática o resistencia térmica enzimática. Es importante notar que ambos tipos de fenómenos se desarrollan en diferentes escalas de espacio y tiempo, y esto lleva a que las reacciones químicas puedan no estar gobernadas por las propiedades macroscópicas del medio. Palabras clave: transición vítrea, estabilidad térmica enzimática, pardeamiento no enzimático, cristalización, formulaciones lácteas, porosidad. "Chemicalreactions in reduced moisture dairy systems as related to changes ¡n the physical state" The objective of the present work was to investigate the influence of physical properties of the system (porosity degree, glass transition, crystalline state) with molecular changes evaluated through measurements of chemical reactions: non-enzymatic browning (NEB),enzyme activity and enzyme thermal resistance, and its relationship with the system composition, changes on pH and moisture content. The systems consisted in low moisture matrices composed mainly of dairy components. NEBand enzyme inactivation were observed to occur in the glassy state. The rate at which the chemical reactions (NEB, enzyme activity and enzyme thermal resistance) were performed in amorphous matrices were affected by matrices modifications, not manifested as changes in the value of the glass transition temperature. Glassy compressed samples developed higher NEB rates than porous samples at all studied compositions. System crystallization (partial or total) promoted an increase of chemical reactions due to the reactants concentration at the amorphous regions, excluded from the crystal. These observations able us to distinguish between supramolecular phenomena, which involucrate cooperative mechanisms between molecules (like the glass transition), from a molecular level, as NEBreactions, enzyme activity or enzyme thermal resistance. It's important to note that both phenomena developed at different time/space scales, accordingly, chemical reactions may not be governed by the macroscopic properties of the system.
The water activity (aw) and pH of acidified (vinegar) bottled vegetables and meat with vegetables... more The water activity (aw) and pH of acidified (vinegar) bottled vegetables and meat with vegetables--mostly home-canned--was examined in relation to the potential growth of Clostridium botulinum. Most products (vegetables or meat with vegetables) had water activity above the "per se" inhibitory limit (aw < 0.95) for growth of C. botulinum. Regarding pH, 96% of canned vegetables had a pH lower than 4.6, but 81% of the canned meat with vegetables had a pH above 4.6. This was attributed to the well known buffer effect of food proteins, which makes it difficult to lower food pH during acidification with vinegar. It is concluded that most bottled meat with vegetables constitute a potential hazard since these foods are marketed at room temperature, and botulism toxin may be produced if spores are present.
The aim of this study was to investigate the stability to hydrolysis and nonenzymatic browning at... more The aim of this study was to investigate the stability to hydrolysis and nonenzymatic browning at 453C of low-moisture trehalose, sucrose and raznose in model systems containing amino compounds in relation to their potential use as protectants of biomaterials. Sugar hydrolysis was negligible in trehalose systems and very low in raznose. Hydrolysis was noticeable in sucrose systems, however, leading to browning development. Sucrose systems showed, by far, the fastest colour development, particularly in the presence of lysine and glycine as sources of amino groups. The results conxrm the extraordinary stability of trehalose. Raznose was also found to be quite resistant to hydrolysis, indicating that this molecule has low tendency to participate in Maillard reactions.
Retention of b-galactosidase activity was analyzed in whey model systems in relation to browning ... more Retention of b-galactosidase activity was analyzed in whey model systems in relation to browning development and physical characteristics of the matrix (degree of matrix compression, crystallinity, glass transition). Amorphous systems were obtained by freeze-drying solutions containing b-galactosidase and reduced lactose milk whey, with or without maltodextrin. Samples (porous or previously compressed) were exposed to atmospheres of relative humidities of 0%, 22% or 44% and stored at 70 C. Remaining enzymatic activity and color development were determined at selected time intervals. The decrease of enzyme activity was simultaneous to browning development, which was in turn accelerated in the low porosity samples and affected by the glass transition of the systems and lactose crystallization. The addition of maltodextrin improved retention of enzyme activity and delayed both browning development and lactose crystallization.
The causes of darkening in honey have been attributed to Maillard reaction, fructose caramelizati... more The causes of darkening in honey have been attributed to Maillard reaction, fructose caramelization and reactions of polyphenols, however, no systematic studies exist on this subject. The in¯uence of composition and initial color on the rate of darkening of several Argentine honeys submitted to storage at 37 C during 90 days was evaluated through spectrocolorimetric measurements. The most suitable color functions to evaluate darkening of honeys [lightness (L ab *), browning index (BI), metric chroma (C ab *), metric hue (H ab *) and 1/Z] increased linearly as storage time increased, after an initial induction period of very low browning development. The slope of the linear browning development zone with time was an index of browning rate, and it was analyzed in relation to the initial color and the composition of honeys (moisture content, total nitrogen, total lipids and polyunsaturated fatty acids, fructose and glucose content). Of the analyzed variables, the initial color was the parameter which better de®ned the rate of darkening of honeys.
The objective was to study the remaining activity of the enzyme β-galactosidase in dehydrated dai... more The objective was to study the remaining activity of the enzyme β-galactosidase in dehydrated dairy systems and its relationship with simultaneous chemical (i.e. non-enzymatic browning, NEB) and physical changes (structural collapse) at temperatures from 70 to 105 °C. The presence of milk proteins had a structure-stabilizing effect, which was not reflected by enzymic stabilization or NEB prevention. Although the remaining enzyme
Enzymatic activity observed in low moisture systems has been analyzed from several standpoints. T... more Enzymatic activity observed in low moisture systems has been analyzed from several standpoints. The rates of enzymatic reactions depend on many factors, the molecular mobility of the reactants and the properties of the matrices in which they are embedded (pH, viscosity, ionic strength) being often controlling aspects. The objective of present work was to investigate -galactosidase activity as affected by pH-moisture dependence and by physical properties of the matrix in reduced moisture systems of whey (W) and dextranes (Dx). -galactosidase from 3 different sources, Escherichia coli, Aspergillus oryzae, and Saccharomyces lactis, were tested. -galactosidase activity from S. lactis was reduced in W systems. pH-moisture dependence was proposed as the main cause of this low activity. The decrease of apparent pH upon water removal and the molecular mobility of the polymeric matrix, and of the enzyme, which is related to their size, appear to be more significant factors than the actual value of the glass transition temperature (T g) of the matrix on galactosidase activity. Therefore, all properties concerning physico-chemical aspects have to be considered to analyze enzyme activity in low moisture biomaterials.
The thermal stability of enzymes lactase and invertase in dried, amorphous matrices of sugars (tr... more The thermal stability of enzymes lactase and invertase in dried, amorphous matrices of sugars (trehalose, maltose, lactose, sucrose, raffinose) and some other selected systems (casein, PVP, milk) was studied. The glass transition temperature (T g) was limited as a threshold parameter for predicting enzyme inactivation because (a) enzyme inactivation was observed in glassy matrices, (b) a specific effect of enzyme stabilization by certain matrices particularly trehalose was observed, and (c) enzyme stability appeared to depend on heating temperature (T) "per se" rather than (T-T g). For these reasons, a protective mechanism by sugars related to the maintenance of the tertiary structure of the enzyme was favored. A rapid loss of enzyme (lactase) activity was observed in heated sucrose systems at T > T g , and this was attributed to sucrose crystallization since it is known that upon crystallization the protective effect of sugars is lost. Thus, the stabilizing effect could be indirectly affected by the T g of the matrix, since crystallization of sugars only occurs above T g. Trehalose model systems (with added invertase) showed an exceptional stability toward "darkening" (e.g., nonenzymatic browning) when heated in the dried state to elevated temperatures and for long periods of time.
Whey powders have attracted attention for use in the food industry. The Maillard reaction is a ma... more Whey powders have attracted attention for use in the food industry. The Maillard reaction is a major deteriorative factor in the storage of these and other dairy food products.The objective of the present work was to further study the Maillard reaction as related to the physical structure of the matrix, either porous or mechanically compressed, or to storage above the T g of anhydrous whey systems. Sweet whey (W), reduced minerals whey (WRM), whey protein isolate (WPI), and whey protein concentrate (WPC) were stored in ovens at selected temperatures. Colorimetric measurements were performed with a spectrocolorimeter, thermal analyses (TGA) by means of a thermobalance, and glass transition temperature studies by DSC. The browning order in the vials and in the compressed systems followed the order W > WRM> WPC > WPI. k w2 , the slope of the second linear segment of the TGA curve, was related to the loss of water due to nonenzymatic browning (NEB). Browning development was in good relationship with this loss of weight. In the glassy state, the compressed systems developed higher rates of browning and weight loss (assigned to NEB reactions) than the porous systems. Reaction rates in both (porous and compressed) systems became similar as (T-T g) increased.
Acta physiologica, pharmacologica et therapeutica latinoamericana : órgano de la Asociación Latinoamericana de Ciencias Fisiológicas y [de] la Asociación Latinoamericana de Farmacología, 1992
This study describes the effects of two stressors, swimming for 15 min or i.p. beta-carboline (be... more This study describes the effects of two stressors, swimming for 15 min or i.p. beta-carboline (beta-CCE) injection, on rat pineal melatonin levels when applied either at the end of the light period of daily photoperiod (1800 h) or at the expected melatonin maximum (at 2200 h). Neither procedure affected melatonin levels at 1800 h. In contrast, both stressors were effective to augment pineal melatonin content when applied at 2200 h. When applied at light-dark transition, only beta-CCE injection increased pineal melatonin levels at the middle of the night. Neither procedure was effective to counteract the inhibition of pineal melatonin content caused by prolonging the light phase for 4 h. Deprivation for two days of water, but not of food, augmented pineal melatonin levels in rats killed at the end of the light period, up to levels that were undistinguishable from those found at 2200 h. No differences were found between the two stressors and controls when tested at 2200 h. The present...
The aim of this study was to determine whether melatonin-induced analgesia in mice exhibits the t... more The aim of this study was to determine whether melatonin-induced analgesia in mice exhibits the time dependency known to occur for several other effects of the hormone, and to analyze to what extent the activity of melatonin can be inhibited by the opiate antagonist naloxone or the central-type benzodiazepine (BZP) antagonist Ro 15-1788. Analgesia was assessed with the hot plate procedure. There was a significant diurnal variation in the pain threshold, with an increase in latency during the dark phase of the daily photo period. Melatonin (20-40 mg/kg i.p.) exhibited maximal analgesic effects at late evening (20:00 h). The administration of naloxone or Ro 15-1788 at 20:00 h, although unable by themselves to modify pain threshold, blunted the analgesic response to melatonin. Significant increases in the latency of the hot plate response were found after diazepam injection, an effect blocked by Ro 15-1788 or naloxone. These results indicate that time-dependent melatonin analgesia is sensitive to opioid or central-type BZP antagonism.
The objective of the present study was to assess whether the anticonvulsant activity of melatonin... more The objective of the present study was to assess whether the anticonvulsant activity of melatonin displays diurnal variability in hamsters. Convulsions were induced by administering 3-mercaptopropionic acid (3-MP). There was a significant diurnal variation in 3-MP-induced convulsions, hamsters being more prone to exhibit seizures during the night than during the day. Melatonin (50 mg/kg i.p.) had a maximal anticonvulsive effect in the early evening (20:00 h). The administration at 20:00 h of the central-type benzodiazepine antagonist, Ro 15-1788, although unable by itself to modify seizure threshold, blunted the anticonvulsant response to melatonin. The results indicate that the time-dependent anticonvulsant activity of melatonin is sensitive to central-type benzodiazepine antagonism.
We endeavored to determine whether three behavioral effects of melatonin in rodents, i.e., depres... more We endeavored to determine whether three behavioral effects of melatonin in rodents, i.e., depression of locomotor activity in hamsters, analgesia in mice, and impairment of 3-mercaptopropionic acid (3-MP) convulsions, exhibited the time dependency known to occur for several neuroendocrine effects of the hormone. Activity was monitored and registered by means of an optical actometer, and analgesia was assessed by the hot-plate procedure. Locomotor activity, analgesia, and seizure susceptibility were maximal at the beginning of the scotophase and minimal at noon. The effects of melatonin on the three parameters peaked at early night. The administration of the benzodiazepine antagonist flumazenil, although unable by itself to modify locomotor activity, pain, or seizure threshold, blunted the activity of melatonin. These results suggest that the time-dependent effects of melatonin on specific rodent behaviors may be mediated by central synapses employing gamma-aminobutyric acid (GABA) as an inhibitory transmitter.
Reacciones químicas en sistemas lácteos de humedad reducida y su relación con transformaciones fí... more Reacciones químicas en sistemas lácteos de humedad reducida y su relación con transformaciones físicas del medio" El objetivo fue investigar la influencia de las propiedades físicas de las matrices (grado de compactación, transición vítrea, cristalinidad), con cambios a nivel molecular, evaluados a través de la determinación del grado de avance de reacciones químicas: pardeo no enzimático (PNE), actividad enzimática, resistencia térmica enzimática y su relación con la composición, los cambios de pH y Ia humedad. Los sistemas empleados consistieron en matrices de baja humedad con componentes de suero lácteos. Tanto el PNEcomo la inactivación enzimática tuvieron lugar en muestras en estado vítreo. La velocidad a la que ocurrieron las reacciones químicas (PNE, actividad enzimática, resistencia térmica enzimática) en matrices amorfas estuvo afectada por modificaciones en las matrices que no se manifestaron en cambios en el valor de la temperatura de transición vítrea de los sistemas. En sistemas vítreos, las muestras comprimidas desarrollaron mayor velocidad de PNEque las muestras en polvo, para todas las composiciones estudiadas. La cristalización (parcial o total) de las matrices provoca un incremento de la velocidad de reacciones químicas debido a la concentración de reactivos en las regiones amorfas excluidas del cristal. Estas observaciones llevan a distinguir entre fenómenos que ocurren a un nivel supramolecular, e involucra aspectos cooperativos entre las moléculas (como lo es la transición vítrea) de fenómenos que ocurren a un nivel molecular, como las reacciones de PNE, actividad enzimática o resistencia térmica enzimática. Es importante notar que ambos tipos de fenómenos se desarrollan en diferentes escalas de espacio y tiempo, y esto lleva a que las reacciones químicas puedan no estar gobernadas por las propiedades macroscópicas del medio. Palabras clave: transición vítrea, estabilidad térmica enzimática, pardeamiento no enzimático, cristalización, formulaciones lácteas, porosidad. "Chemicalreactions in reduced moisture dairy systems as related to changes ¡n the physical state" The objective of the present work was to investigate the influence of physical properties of the system (porosity degree, glass transition, crystalline state) with molecular changes evaluated through measurements of chemical reactions: non-enzymatic browning (NEB),enzyme activity and enzyme thermal resistance, and its relationship with the system composition, changes on pH and moisture content. The systems consisted in low moisture matrices composed mainly of dairy components. NEBand enzyme inactivation were observed to occur in the glassy state. The rate at which the chemical reactions (NEB, enzyme activity and enzyme thermal resistance) were performed in amorphous matrices were affected by matrices modifications, not manifested as changes in the value of the glass transition temperature. Glassy compressed samples developed higher NEB rates than porous samples at all studied compositions. System crystallization (partial or total) promoted an increase of chemical reactions due to the reactants concentration at the amorphous regions, excluded from the crystal. These observations able us to distinguish between supramolecular phenomena, which involucrate cooperative mechanisms between molecules (like the glass transition), from a molecular level, as NEBreactions, enzyme activity or enzyme thermal resistance. It's important to note that both phenomena developed at different time/space scales, accordingly, chemical reactions may not be governed by the macroscopic properties of the system.
The water activity (aw) and pH of acidified (vinegar) bottled vegetables and meat with vegetables... more The water activity (aw) and pH of acidified (vinegar) bottled vegetables and meat with vegetables--mostly home-canned--was examined in relation to the potential growth of Clostridium botulinum. Most products (vegetables or meat with vegetables) had water activity above the "per se" inhibitory limit (aw < 0.95) for growth of C. botulinum. Regarding pH, 96% of canned vegetables had a pH lower than 4.6, but 81% of the canned meat with vegetables had a pH above 4.6. This was attributed to the well known buffer effect of food proteins, which makes it difficult to lower food pH during acidification with vinegar. It is concluded that most bottled meat with vegetables constitute a potential hazard since these foods are marketed at room temperature, and botulism toxin may be produced if spores are present.
The aim of this study was to investigate the stability to hydrolysis and nonenzymatic browning at... more The aim of this study was to investigate the stability to hydrolysis and nonenzymatic browning at 453C of low-moisture trehalose, sucrose and raznose in model systems containing amino compounds in relation to their potential use as protectants of biomaterials. Sugar hydrolysis was negligible in trehalose systems and very low in raznose. Hydrolysis was noticeable in sucrose systems, however, leading to browning development. Sucrose systems showed, by far, the fastest colour development, particularly in the presence of lysine and glycine as sources of amino groups. The results conxrm the extraordinary stability of trehalose. Raznose was also found to be quite resistant to hydrolysis, indicating that this molecule has low tendency to participate in Maillard reactions.
Retention of b-galactosidase activity was analyzed in whey model systems in relation to browning ... more Retention of b-galactosidase activity was analyzed in whey model systems in relation to browning development and physical characteristics of the matrix (degree of matrix compression, crystallinity, glass transition). Amorphous systems were obtained by freeze-drying solutions containing b-galactosidase and reduced lactose milk whey, with or without maltodextrin. Samples (porous or previously compressed) were exposed to atmospheres of relative humidities of 0%, 22% or 44% and stored at 70 C. Remaining enzymatic activity and color development were determined at selected time intervals. The decrease of enzyme activity was simultaneous to browning development, which was in turn accelerated in the low porosity samples and affected by the glass transition of the systems and lactose crystallization. The addition of maltodextrin improved retention of enzyme activity and delayed both browning development and lactose crystallization.
The causes of darkening in honey have been attributed to Maillard reaction, fructose caramelizati... more The causes of darkening in honey have been attributed to Maillard reaction, fructose caramelization and reactions of polyphenols, however, no systematic studies exist on this subject. The in¯uence of composition and initial color on the rate of darkening of several Argentine honeys submitted to storage at 37 C during 90 days was evaluated through spectrocolorimetric measurements. The most suitable color functions to evaluate darkening of honeys [lightness (L ab *), browning index (BI), metric chroma (C ab *), metric hue (H ab *) and 1/Z] increased linearly as storage time increased, after an initial induction period of very low browning development. The slope of the linear browning development zone with time was an index of browning rate, and it was analyzed in relation to the initial color and the composition of honeys (moisture content, total nitrogen, total lipids and polyunsaturated fatty acids, fructose and glucose content). Of the analyzed variables, the initial color was the parameter which better de®ned the rate of darkening of honeys.
The objective was to study the remaining activity of the enzyme β-galactosidase in dehydrated dai... more The objective was to study the remaining activity of the enzyme β-galactosidase in dehydrated dairy systems and its relationship with simultaneous chemical (i.e. non-enzymatic browning, NEB) and physical changes (structural collapse) at temperatures from 70 to 105 °C. The presence of milk proteins had a structure-stabilizing effect, which was not reflected by enzymic stabilization or NEB prevention. Although the remaining enzyme
Enzymatic activity observed in low moisture systems has been analyzed from several standpoints. T... more Enzymatic activity observed in low moisture systems has been analyzed from several standpoints. The rates of enzymatic reactions depend on many factors, the molecular mobility of the reactants and the properties of the matrices in which they are embedded (pH, viscosity, ionic strength) being often controlling aspects. The objective of present work was to investigate -galactosidase activity as affected by pH-moisture dependence and by physical properties of the matrix in reduced moisture systems of whey (W) and dextranes (Dx). -galactosidase from 3 different sources, Escherichia coli, Aspergillus oryzae, and Saccharomyces lactis, were tested. -galactosidase activity from S. lactis was reduced in W systems. pH-moisture dependence was proposed as the main cause of this low activity. The decrease of apparent pH upon water removal and the molecular mobility of the polymeric matrix, and of the enzyme, which is related to their size, appear to be more significant factors than the actual value of the glass transition temperature (T g) of the matrix on galactosidase activity. Therefore, all properties concerning physico-chemical aspects have to be considered to analyze enzyme activity in low moisture biomaterials.
The thermal stability of enzymes lactase and invertase in dried, amorphous matrices of sugars (tr... more The thermal stability of enzymes lactase and invertase in dried, amorphous matrices of sugars (trehalose, maltose, lactose, sucrose, raffinose) and some other selected systems (casein, PVP, milk) was studied. The glass transition temperature (T g) was limited as a threshold parameter for predicting enzyme inactivation because (a) enzyme inactivation was observed in glassy matrices, (b) a specific effect of enzyme stabilization by certain matrices particularly trehalose was observed, and (c) enzyme stability appeared to depend on heating temperature (T) "per se" rather than (T-T g). For these reasons, a protective mechanism by sugars related to the maintenance of the tertiary structure of the enzyme was favored. A rapid loss of enzyme (lactase) activity was observed in heated sucrose systems at T > T g , and this was attributed to sucrose crystallization since it is known that upon crystallization the protective effect of sugars is lost. Thus, the stabilizing effect could be indirectly affected by the T g of the matrix, since crystallization of sugars only occurs above T g. Trehalose model systems (with added invertase) showed an exceptional stability toward "darkening" (e.g., nonenzymatic browning) when heated in the dried state to elevated temperatures and for long periods of time.
Whey powders have attracted attention for use in the food industry. The Maillard reaction is a ma... more Whey powders have attracted attention for use in the food industry. The Maillard reaction is a major deteriorative factor in the storage of these and other dairy food products.The objective of the present work was to further study the Maillard reaction as related to the physical structure of the matrix, either porous or mechanically compressed, or to storage above the T g of anhydrous whey systems. Sweet whey (W), reduced minerals whey (WRM), whey protein isolate (WPI), and whey protein concentrate (WPC) were stored in ovens at selected temperatures. Colorimetric measurements were performed with a spectrocolorimeter, thermal analyses (TGA) by means of a thermobalance, and glass transition temperature studies by DSC. The browning order in the vials and in the compressed systems followed the order W > WRM> WPC > WPI. k w2 , the slope of the second linear segment of the TGA curve, was related to the loss of water due to nonenzymatic browning (NEB). Browning development was in good relationship with this loss of weight. In the glassy state, the compressed systems developed higher rates of browning and weight loss (assigned to NEB reactions) than the porous systems. Reaction rates in both (porous and compressed) systems became similar as (T-T g) increased.
Acta physiologica, pharmacologica et therapeutica latinoamericana : órgano de la Asociación Latinoamericana de Ciencias Fisiológicas y [de] la Asociación Latinoamericana de Farmacología, 1992
This study describes the effects of two stressors, swimming for 15 min or i.p. beta-carboline (be... more This study describes the effects of two stressors, swimming for 15 min or i.p. beta-carboline (beta-CCE) injection, on rat pineal melatonin levels when applied either at the end of the light period of daily photoperiod (1800 h) or at the expected melatonin maximum (at 2200 h). Neither procedure affected melatonin levels at 1800 h. In contrast, both stressors were effective to augment pineal melatonin content when applied at 2200 h. When applied at light-dark transition, only beta-CCE injection increased pineal melatonin levels at the middle of the night. Neither procedure was effective to counteract the inhibition of pineal melatonin content caused by prolonging the light phase for 4 h. Deprivation for two days of water, but not of food, augmented pineal melatonin levels in rats killed at the end of the light period, up to levels that were undistinguishable from those found at 2200 h. No differences were found between the two stressors and controls when tested at 2200 h. The present...
The aim of this study was to determine whether melatonin-induced analgesia in mice exhibits the t... more The aim of this study was to determine whether melatonin-induced analgesia in mice exhibits the time dependency known to occur for several other effects of the hormone, and to analyze to what extent the activity of melatonin can be inhibited by the opiate antagonist naloxone or the central-type benzodiazepine (BZP) antagonist Ro 15-1788. Analgesia was assessed with the hot plate procedure. There was a significant diurnal variation in the pain threshold, with an increase in latency during the dark phase of the daily photo period. Melatonin (20-40 mg/kg i.p.) exhibited maximal analgesic effects at late evening (20:00 h). The administration of naloxone or Ro 15-1788 at 20:00 h, although unable by themselves to modify pain threshold, blunted the analgesic response to melatonin. Significant increases in the latency of the hot plate response were found after diazepam injection, an effect blocked by Ro 15-1788 or naloxone. These results indicate that time-dependent melatonin analgesia is sensitive to opioid or central-type BZP antagonism.
The objective of the present study was to assess whether the anticonvulsant activity of melatonin... more The objective of the present study was to assess whether the anticonvulsant activity of melatonin displays diurnal variability in hamsters. Convulsions were induced by administering 3-mercaptopropionic acid (3-MP). There was a significant diurnal variation in 3-MP-induced convulsions, hamsters being more prone to exhibit seizures during the night than during the day. Melatonin (50 mg/kg i.p.) had a maximal anticonvulsive effect in the early evening (20:00 h). The administration at 20:00 h of the central-type benzodiazepine antagonist, Ro 15-1788, although unable by itself to modify seizure threshold, blunted the anticonvulsant response to melatonin. The results indicate that the time-dependent anticonvulsant activity of melatonin is sensitive to central-type benzodiazepine antagonism.
We endeavored to determine whether three behavioral effects of melatonin in rodents, i.e., depres... more We endeavored to determine whether three behavioral effects of melatonin in rodents, i.e., depression of locomotor activity in hamsters, analgesia in mice, and impairment of 3-mercaptopropionic acid (3-MP) convulsions, exhibited the time dependency known to occur for several neuroendocrine effects of the hormone. Activity was monitored and registered by means of an optical actometer, and analgesia was assessed by the hot-plate procedure. Locomotor activity, analgesia, and seizure susceptibility were maximal at the beginning of the scotophase and minimal at noon. The effects of melatonin on the three parameters peaked at early night. The administration of the benzodiazepine antagonist flumazenil, although unable by itself to modify locomotor activity, pain, or seizure threshold, blunted the activity of melatonin. These results suggest that the time-dependent effects of melatonin on specific rodent behaviors may be mediated by central synapses employing gamma-aminobutyric acid (GABA) as an inhibitory transmitter.
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