Modesto C. Rubio

Area-dependent changes in GABAergic function after acute and chronic cold stress.

(1) The function of the gamma-aminobutyric acid (GABA)ergic system in certain areas of the rat brain was investigated after acute and chronic cold stress. (2) GABA concentration, [3H]GABA uptake and the activity of the synthesis enzyme glutamate decarboxylase (GAD) were measured. (3) Acute stress: (a) reduced GABA concentration in the corpus striatum (29%); (b) decreased GAD activity (under non-saturating substrate concentration) in the olfactory bulbs (24%); (c) diminished neuronal uptake of [3H]GABA in the frontal cerebral cortex (65%), hypothalamus (86%) and olfactory bulbs (82%). (4) Chronic stress: (a) reduced the endogenous levels of GABA in the frontal cerebral cortex (51%), hypothalamus (26%) and olfactory bulbs (15%); (b) decreased GAD activity in the corpus striatum (32%) and olfactory bulbs (34%); (c) decreased neuronal uptake of [3H]GABA in the hypothalamus (83%). (5) These findings suggest that compensatory changes may develop in the GABAergic system after chronic stress.

Chronic treatment with high doses of corticosterone decreases cytoskeletal proteins in the rat hippocampus

Hypercortisolism is a common trait of Cushings disease and depression. These two disorders also share hippocampal volume decrease and cognitive deficits. However, experimentally induced hypercortisolism induces neuronal atrophy, which has been proposed to be the phenomenon underlying the hippocampal shrinkage. We hypothesized that the above‐mentioned atrophy is due to a deleterious effect of high concentrations of glucocorticoids on cytoskeletal proteins. One or two pellets (100u2003mg each) of corticosterone were subcutaneously implanted in adult rats. Twenty‐one days later, light, medium and heavy subunits of intermediate neurofilaments (NFL, NFM and NFH) and the microtubule‐associated protein 2 (MAP2) were quantified by immunohistochemistry in Ammons horn and dentate gyrus. We also evaluated the inu2003vitro glutamate release in hippocampal slices. Both doses of corticosterone induced a decrement of NFL, NFM and NFH in both hippocampal areas but only 200u2003mg decreased MAP2. This dose also diminished the potassium‐stimulated glutamate release. All of these changes seemed not to be due to neuron loss, as no decrement in neuron‐specific nuclear protein‐positive cells was found. With the exception of NFL, the above‐mentioned diminution was not observed in the globus pallidus, one of the brain regions with the lowest glucocorticoid receptor density. These results provide a subcellular insight into the trophic changes found in experimental models of hypercortisolism. The coincidence between decrements in MAP2 and glutamate release suggests possible links between high glucocorticoid levels, dendritic atrophy and the cognitive impairment reported in patients suffering from Cushings disease and depression.

Serotonin in Golden Hamster Testes: Testicular Levels, Immunolocalization and Role during Sexual Development and Photoperiodic Regression-Recrudescence Transition

Serotonin (5-HT) is found in the gonads and accessory reproductive organs of several species. The golden (Syrian) hamster is a seasonal breeder. Exposure of male adult hamsters to short days for 14 weeks results in a severe gonadal regression, while after a photoinhibition period of 22 weeks a spontaneous testicular recrudescence occurs. The aim of this study was to investigate the presence of 5-HT and its major metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the gonads of golden hamsters, its immunolocation and its physiological role in the testis. The influence of age and photoperiod was also analyzed. Hamsters of 23, 36, 46, 60 and 90 days of age were kept in long photoperiod (LP: 14:10 h light/dark), and adult animals were exposed either to LP or to short photoperiod (SP: 6:18 h light/dark) for 14 and 22 weeks. Testicular parenchyma and capsule levels of 5-HT and 5-HIAA increased significantly at ages of 36 and 60–90 days, but decreased markedly during the exposure of adult hamsters to SP for 14 and 22 weeks. Mast cells were found exclusively in the testicular capsule. The testicular number of mast cells increased concomitantly with age, but decreased in adult hamsters exposed to SP. Mast and Leydig cells presented 5-HT-positive immunoreactivity. During sexual maturation as well as during the transfer of adult hamsters from LP to SP, the 5-HIAA/5-HT ratio showed the highest values in active adult animals, indicating that the increase in testicular 5-HT levels in adulthood is accompanied by an augment in 5-HT turnover. In vitro basal and hCG-stimulated testosterone production was significantly inhibited in presence of physiological concentrations of 5-HT. In conclusion, the present studies demonstrate the existence of 5-HT in mast cells and Leydig cells of hamster testes, as well as describe an inhibitory action of this neurotransmitter on gonadal testosterone production. Furthermore, the age-dependent and photoperiodic-related changes detected in testicular 5-HT levels suggest that this neurotransmitter might act as an important local modulator of the action of gonadotropins on steroidogenesis during sexual development and during the photoperiodic regression-recrudescence transition in the golden hamster.

Effects of the noradrenaline metabolites on the adrenergic receptors

SummaryThe effects of the five noradrenaline (NA) metabolites and of the O-methylated metabolite of adrenaline, metanephrine, were studied in a tissue with adrenergic beta-receptors (guinea-pig atria) and in a tissue with predominance of alpha-receptors (cats nictitating membrane). In atria, normetanephrine and metanephrine elicited positive chronotrophic effects which were mediated through the beta-receptors. Both O-methylated metabolites had only 1/1000th of the potency of NA. In the normally innervated nictitating membrane normetanephrine and metanephrine elicited maximal responses of the same magnitude as NA. While normetanephrine had one half of the potency of NA, metanephrine was even more potent than NA. The effects of normetanephrine or metanephrine were mediated through the activation of the alpha-receptors and were not potentiated by either cocaine or denervation. Neither the deaminated nor the deaminated-O-methylated metabolites of NA had activity as agonists on alpha-or beta-receptors in concen-of up to 1×10−4M. These metabolites did not elicit alpha-or beta-receptor block in concentrations of up to 1×10−4M.

Acute effects of S-adenosyl-L-methionine on catecholaminergic central function

Catecholaminergic function was studied in rat brain areas after acute administration of S-adenosyl-L-methionine (SAM: 10 mg/kg i.p.). The noradrenaline (NA) concentration increased in the hippocampus and frontal cortex and decreased in the olfactory bulbs between 1 and 2 h after the SAM injection. NA biosynthesis was stimulated in vivo and the concentration of normetanephrine was increased only in the hippocampus. SAM may indirectly affect NA biosynthesis and metabolism. This effect on noradrenergic function might participate in its putative antidepressive action.

Chronic treatment with fluoxetine decreases seizure threshold in naïve but not in rats exposed to the learned helplessness paradigm: Correlation with the hippocampal glutamate release

The proconvulsive effect of the new generation of antidepressants remains controversial. The authors investigated in naïve rats the effect of chronic treatment with fluoxetine (FLX) on the convulsive threshold and on two parameters of the hippocampal glutamatergic neurotransmission: the in vitro glutamate release and the binding of [3H] MK801 to NMDA receptors. While the acute treatment with FLX provoked no change either in seizure susceptibility or in the glutamate release, the chronic treatment decreased the convulsive threshold in coincidence with an increment in the in vitro glutamate release. No significant effects on the binding of [3H] MK801 to NMDA receptors were found to be attributable to the FLX treatment. We also assessed the effect of the chronic treatment with FLX on the seizure threshold in rats exposed to an experimental model of depression, the learned helplessness paradigm (LH). While a decrease in the K+-stimulated glutamate release was observed in non treated LH animals, when they were chronically injected with FLX, no changes in the epileptic susceptibility and no increments in the glutamate release were found. Our results indicate that chronic treatment with FLX decreases the epileptic threshold in naïve but not in LH rats and that this effect correlates with the levels of the hippocampal glutamate release.

GABAA receptors mediate the changes produced by stress on GABA function and locomotor activity

(1) This study shows the effects of bicuculline pretreatment on the GABAergic system in certain areas of the rat brain after acute ether and cold stress. (2) The spontaneous locomotor activity was diminished by either ether stress or cold stress or bicuculline. (3) Acute ether stress enhanced GABA concentration in the hypothalamus (69%) and in the frontal cerebral cortex (26%). Bicuculline prevented the increase in GABA levels induced by ether stress in both areas. (4) Acute cold stress decreased GABA concentration in the corpus striatum (29%). Bicuculline prevented the decrease in GABA levels induced by cold stress. (5) It is concluded that there is a GABAA receptor involved in stress induced changes on endogenous GABA levels as well as on locomotor activity.

Diazepam fails to potentiate GABA-induced chloride uptake and to produce anxiolytic-like action in aged rats.

The pharmacological response to benzodiazepines has been demonstrated to be different in aged individuals in comparison to adults. We studied the age-dependent changes in some of the in vitro and behavioral effects of diazepam in aged (24 months old) rats, comparing them to adults (3 months old). We evaluated the in vitro gamma-aminobutyric acid (GABA)-induced 36Cl- uptake and the diazepam potentiation of GABA-stimulated 36Cl- uptake in microsacs from cerebral cortex of both groups of animals. We found no differences in the GABA-stimulated 36Cl- uptake between adult and aged animals, and diazepam failed to potentiate GABA-induced 36Cl- flux in the aged cortical microsacs. We also examined the effect of 0.03-10 mg of diazepam on locomotor activity in an open-field test and the anxiolytic-like action of diazepam in doses ranging from 0.03 to 1 in a dark-light transition test. We observed no anxiolytic-like action of the drug in the dark-light transition test in the aged rats, while there was a shift to the left in the diminution of locomotor activity evaluated by the open-field test. We conclude that the pharmacodynamic changes observed in cortical GABA(A) receptors in aged rats could partially explain the lack of anxiolytic-like action but not the oversedation evidenced in this group of animals.

Acute changes in 5-HT metabolism after S-adenosyl-l-methionine administration

1. This study investigates S-adenosyl-L-methionine (SAM) (10 mg/kg, i.p.)acute effects upon 5-HT metabolism in rat brain areas. 2. One hour after SAM injection 5-HT biosynthesis was increased in the corpus striatum (55%), the hippocampus (3-fold) (82% 2 hr after SAM injection) and decreased in the olfactory bulbs (34%). 3. 5-HT levels were: (a) increased in the corpus striatum (39%), the hippocampus (44%) and the frontal cortex (27%), and oppositely reduced in the olfactory bulbs (47%) 1 hr after SAM injection; (b) increased in the hippocampus (39%) and reduced in the olfactory bulbs (35%) 1.5 hr after SAM injection; (c) increased in the hippocampus (25%) 2 hr after SAM injection. 4. 5-HIAA levels were reduced in the olfactory bulbs 27% or 21% after 1 hr or 1.5 hr from SAM injection respectively. 5. These area-related changes in 5-HT biosynthesis and metabolism might suggest a possible neurochemical substrate for the antidepressant properties of SAM.

Lack of tolerance to the anxiolytic effect of diazepam and pentobarbital following chronic administration in perinatally undernourished rats

Adult female rats, undernourished at perinatal age, were evaluated for anxiolytic action in the plus-maze test after acute and chronic administration of diazepam (DZP) and pentobarbital (PTB). Deprived (D) rats chronically treated with vehicle showed an increased anxiety as compared with control (C) animals. A single intraperitoneal (i.p.) administration of DZP (1 mg/kg) or PTB (7.5 mg/kg) produced similar anticonflict effect in both C and D rats. Tolerance to the anxiolytic effect of DZP and PBT developed in C rats after a 15-day administration schedule, whereas no tolerance was observed in D animals. Drug disposition was not altered after chronic treatment either in C or in D rats. Gamma-aminobutyric acid (GABA)-mediated chloride uptake in microsacs of cerebral cortex of naive D rats was decreased as compared with naive C rats. After chronic DZP administration (1 mg/kg/day i.p. for 15 days), GABA-mediated 36Cl- influx in brain cortex microsacs of C rats did not change; however, GABA efficacy was increased in microsacs of D animals. In addition, chronic DZP treatment induced GABA-benzodiazepine uncoupling in brain cortex of C rats, but not in D animals, as assessed by chloride uptake in microsacs. Chronic PTB treatment (7.5 or 30 mg/kg/day i.p. for 15 days) did not modify GABA stimulation or GABA-PTB interaction in cortical microsacs of C or D rats.