Brain nucleotides in naive and handling-habituated rats: differences in levels and drug-sensitivity

Handling, task complexity, time-of-day, and sleep deprivation as dynamic modulators of recognition memory in mice

Sleep is essential for optimal cognitive functioning. Although we lack a complete understanding of the role of sleep in memory consolidation, we know that various factors that disturb sleep or sleep quality have consequences for cognitive performance. Such factors can be unintended components of behavioral experiments on rodents and other experimental animals that generate differing results from different labs. These experimental variables include habituation to handling, intended or unintended sleep deprivation, task complexity, time of testing, and environmental features. We have examined how these variables impact recognition memory in C57BL/6 mice. Handled mice outperformed their non-handled counterparts across different combinations of delay phase duration and lighting conditions. Results also suggest that simple task recall is more resistant to diurnal variation and the impairing effects of sleep deprivation than is complex task recall. This study underscores the role of protocol and environmental factors in recognition memory and in conflicting results from different laboratories.

The cerebellum under stress

Stress-related psychiatric conditions are one of the main causes of disability in developed countries. They account for a large portion of resource investment in stress-related disorders, become chronic, and remain difficult to treat. Research on the neurobehavioral effects of stress reveals how changes in certain brain areas, mediated by a number of neurochemical messengers, markedly alter behavior. The cerebellum is connected with stress-related brain areas and expresses the machinery required to process stress-related neurochemical mediators. Surprisingly, it is not regarded as a substrate of stress-related behavioral alterations, despite numerous studies that show cerebellar responsivity to stress. Therefore, this review compiles those studies and proposes a hypothesis for cerebellar function in stressful conditions, relating it to stress-induced psychopathologies. It aims to provide a clearer picture of stress-related neural circuitry and stimulate cerebellum-stress research. Consequently, it might contribute to the development of improved treatment strategies for stress-related disorders.

Comparative dopaminergic and muscarinic antagonist activity of clozapine and haloperidol

Clozapine is an atypical antipsychotic drug, and its dopamine and muscarinic antagonist activity has been compared with haloperidol in rodents. Elevation in rat striatal acetylcholine (ACh) and mice cerebellar cGMP has been used as an agonist response for oxotremorine and quinpirole. Pretreatment with clozapine significantly blocked oxotremorine-induced elevation in striatal ACh (p<0.01) and cerebellar cGMP(p<0.05). At the same doses, clozapine had no significant effect on quinpirole-induced increases in ACh and cGMP levels. Pretreatment with haloperidol significantly antagonized quinpirole-induced elevation in striatal ACh (p<0.01) and cerebellar cGMP(p<0.05), and haloperidol had no significant effect on oxotremorine-induced agonist responses. Thus, clozapine is antimuscarinic at a dose level that lacks dopamine antagonist properties, whereas haloperidol is a dopamine antagonist and lacks antimuscarinic activity. The atypical neuroleptic profile of clozapine may be due to its high antimuscarinic and low antidopaminergic activity.

Dietary fat affects plasma prolactin in female F344 rats under conditions of ether stress

The influence of amount and type of dietary fat on circulating concentrations of prolactin and estradiol-17 beta in female F344 rats from which blood was sampled by decapitation under ether anesthesia was compared with that in rats from which blood was collected without anesthesia. The animals were fed isonutrient (adjusted for differences in energy density) semipurified diets containing 5% or 20% (by weight) sunflower seed oil or lard. Blood was sampled by decapitation with or without standardized ether anesthesia during the afternoon of proestrus-estrus or the morning of metestrus-diestrus, as determined by examination of vaginal smears. Plasma hormone concentrations were measured by radioimmunoassay. Prolactin levels were lower during proestrus-estrus in rats fed a low-fat diet than in animals fed a high-fat diet, statistically independent of the type of dietary fat, but only when blood was sampled by decapitation under ether anesthesia [p = 0.0384, 2-way analysis of variance (ANOVA)]. No such difference was found in rats decapitated without anesthesia. This effect of amount of dietary fat on prolactin in proestrus-estrus animals anesthetized with ether was predominantly present in animals fed polyunsaturated fat (p < 0.05, 1-way ANOVA and Tukey's test) and was statistically not significant in rats fed saturated fat diets. During metestrus-diestrus, prolactin levels were significantly lower in animals fed a high-saturated fat diet than in those fed low-saturated fat, low-unsaturated fat, or high-unsaturated fat diets, independent of the blood sampling conditions (p < 0.05, 2-way ANOVA and Tukey's test). No consistent effects on estradiol-17 beta levels were found in type or amount of dietary fat or in presence or absence of ether anesthesia before decapitation. Growth, apparent digestibility of fat, and caloric intake were similar in all four dietary groups, but food consumption was higher and food conversion efficiency was lower in animals fed low-fat diets than in those fed high-fat diets. This study confirms the hypothesis that effects of dietary fat, particularly polyunsaturated fat, on circulating prolactin occur only during (ether) stress. Because stress is a frequent and normal phenomenon, this observation implies that the mammary glands of animals with a high dietary intake of polyunsaturated fat are frequently exposed to higher circulating prolactin concentrations than rats fed a low-fat diet, which may be a major mechanism by which dietary fat enhances rat mammary carcinogenesis.

Stress-induced increase of extracellular levels of cyclic AMP in rat cortex

Previous studies have suggested that stress may increase levels of cyclic AMP (cAMP) in the brain but these findings have been controversial due to the use of stressful procedures to inactivate brain enzymes. The present experiment therefore used a non-stressful technique, microdialysis, to assay extracellular levels of cAMP in the rat cortex after stress. Experiments were conducted 2 days after implantation of probes in the frontal cortex. Significant increases were found after the mild stressors of restraint or intraperitoneal injection of saline suggesting that increased tissue levels of cAMP had occurred. These responses were potentiated by local infusion of the phosphodiesterase (PDE) inhibitor, rolipram. It is concluded that one or more adenylate cyclase-coupled receptors in the cortex is activated by mild stress and that this activation can be detected in vivo by microdialysis.

Gentled and non-handled rats in a stressful open-field situation; differences in performance

Open-field behaviour of individually gentled and non-handled adult Wistar rats in a stressful test situation (loud noise with bright light) was studied. The behaviour on the whole of the two groups differed significantly. While the gentled rats showed mainly signs of stress and explorativity, signs of fear beside stress and, on later trials, explorativity were typical of the non-handled animals. On later trials the behaviour of the non-handled rats neared to that of the gentled ones. In the behaviour of the gentled rats there was but little change as a function of trials. We suggest that individual gentling, beside minimizing fear of human contact, also increases the stability of the rat's reactions.

2-Amino-7-phosphonoheptanoic acid, a selective N-methyl-D-aspartate antagonist, blocks swim-induced elevation of cerebellar cyclic guanosine monophosphate

In order to explore how rapidly locomotor activity induces an elevation in cerebellar cyclic guanosine monophosphate (cGMP) content, Sprague-Dawley rats, pretrained to swim a 2.5-m course, were required to swim from one to 5 laps representing from 7 to 40s of strenuous activity. Immediately after completing the swimming task, each animal was killed by microwave irradiation and the cerebellum was collected for subsequent determination of the cGMP content. There was no difference in the cerebellar cGMP content between rats swimming one lap, i.e. for 7 s, and control rats that did not swim. However, there was a linear increase in the cGMP over control values from 1.8- to 2.4-fold in rats swimming 3 and 5 times, respectively. The first significant elevation of the cerebellar cGMP was seen at 24 s (3 laps). To determine if acidic amino acid pathways were involved in this elevation, a low dosage of a selective NMDA antagonist, 2-amino-7-phosphonoheptanoic acid (APH) was injected intracerebroventricularly 4 min before having rats swim 4 laps. This low dosage of APH, which alone had no effect on the cerebellar cGMP content, completely blocked the swim-induced elevation of this parameter. These data provide the first report of how quickly locomotor activity elevates the cerebellar cGMP content and further suggest that an NMDA receptor-mediated pathway is involved in the activity-induced elevation of this parameter.

Habituation of the local cyclic GMP response during amygdaloid carbachol kindling in the rat

Seizures kindled with amygdaloid carbachol injections are transynaptic, dependent on activation of a specific population of muscarinic receptors, and some components of their expression could be mediated by intracellular second messengers. We measured cyclic GMP and cyclic AMP concentrations in micropunch biopsies of multiple brain regions after microwave fixation during the development and the expression of carbachol-kindled seizures in the rat. In the naive carbachol-injected amygdala, cyclic GMP concentrations rose from 1.03 +/- 0.15 pmol/mg protein to 2.21 +/- 0.46 after 2 min, and significant rises occurred in caudate, hypothalamus and contralateral amygdala. This response did not occur in implanted controls, after injection of mock cerebrospinal fluid, or when carbachol actions were blocked with atropine. The rise in cyclic GMP progressively disappeared upon repeated stimulation (injected amygdala on tenth stimulation: 0.72 +/- 0.23 pmol/mg protein). However, a late rise in both cyclic GMP and cyclic AMP concentrations occurred in many brain regions during convulsive seizures. These data suggest that during the development of kindling, changes in neuronal and synaptic excitability are associated with changes in intracellular second messengers.

The benzodiazepine antagonist Ro 15-1788 reverses the effect of methyl-beta-carboline-3-carboxylate but not of harmaline on cerebellar cGMP and motor performance in mice

The cerebellar cGMP level in mice was decreased in a dose-dependent manner 30 min after diazepam (ED50 = 2 mg/kg p.o.). This effect was reversed by the specific benzodiazepine antagonist Ro 15-1788. Methyl-beta-carboline-3-carboxylate (beta-CCM) and harmaline increased cGMP. Ro 15-1788 dose dependently counteracted the beta-CCM- but not the harmaline-induced increase in cGMP. In the horizontal wire test Ro 15-1788 antagonized the impairment of motor performance induced by beta-CCM, but not that induced by harmaline. These findings further support the view that harmaline in contrast to beta-carboline-3-carboxylates does not act through benzodiazepine receptors, and that Ro 15-1788 antagonizes only those convulsants and stimulants that act through specific benzodiazepine receptors.

Effects of acrylamide on locomotion and central monoamine function in the rat

Male rats receiving acrylamide (ACR) in their drinking water (100 ppm) for a six-week period displayed increased psychomotor stimulation to d-amphetamine (d-A; 1.0 mg/kg SC) under several conditions of handling and drug administration. Following behavioral tests a subset of the animals was sacrificed at 15, 50, 80 and 120 minutes following d-A and the brains removed and dissected for determinations of regional brain levels of several monoamine neurotransmitters and metabolites. ACR rats had elevated levels of 5-hydroxyindoleacetic acid (5-HIAA) in the striatum, septal area, and thalamus. The effect was most pronounced at 15 minutes post-drug with ACR rats not demonstrating a depression in 5-HIAA levels present in controls. Increases in accumben's dopamine and norepinephrine levels, evident after d-A, were of lesser magnitude in ACR-exposed rats. Decreases in dihydroxyphenylacetic acid and homovanillic acid, also evident after d-A, persisted for a longer duration in ACR-exposed rats. Light and electron microscopy of spinal cord, striatum, nucleus accumbens and thalamus did not reveal morphologic abnormalities. Sciatic nerves showed histopathological changes characteristic of multi-focal dying-back peripheral nerve degeneration. It was concluded that acrylamide's effect on the psychomotor stimulant properties of d-A may be related to changes in a serotonergic inhibitory system.

Increase of cyclic GMP in cerebellum by methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM)

The intraperitoneal administration of DMCM (0.5-3 mg/kg) produced a dose-related increase in the content of cyclic GMP in the rat cerebellar cortex. The effect of DMCM on cyclic GMP was abolished by pretreatment with benzodiazepine receptor ligands, diazepam and Ro15-1788 and by the GABA agonist muscimol. The results suggest that DMCM increases cerebellar cyclic GMP content through a direct action on benzodiazepine receptors located in the cerebellar cortex. The interaction between DMCM and the GABAergic system associated with benzodiazepine receptors is discussed. Cerebellar cyclic GMP content can be used as a biochemical index to differentiate agonists and antagonists for benzodiazepine receptors.

Interactions between handling and acrylamide on the striatal dopamine receptor

Rats were either handled for one week or were left undisturbed in their home cages. Twenty-four hours after exposure to 100 mg/kg acrylamide, rats were sacrificed and spiroperidol binding investigated in the striatum. In handling-gentled rats, spiroperidol binding was reduced by acrylamide. In non-handled animals, no overall drug effects were seen, but the effect of acrylamide varied with the time of sacrifice. These results demonstrate the importance of environmental factors in the response to neurotoxicants.

Benzodiazepine antagonist Ro 15-1788: binding characteristics and interaction with drug-induced changes in dopamine turnover and cerebellar cGMP levels

The recently discovered benzodiazepine antagonist Ro 15-1788 was characterized in binding studies, and its potency and selectivity were determined in vivo by interaction with drug-induced changes in dopamine turnover and cerebellar cGMP level. Ro 15-1788 reduced [3H]flunitrazepam binding in the brain in vivo with a potency similar to that of diazepam and effectively inhibited [3H]diazepam binding in vitro (IC50 = 2.3 +/- 0.6 nmol/liter). [3H]Ro 15-1788 bound to tissue fractions of rat cerebral cortex with an apparent dissociation (KD) of 1.0 +/- 0.1 nmol/liter. The in vitro potency of various benzodiazepines in displacing [3H]Ro 15-1788 from its binding site was of the same rank order as found previously in [3H]diazepam binding. Autoradiograms of [3H]Ro 15-1788 binding in sections of rat cerebellum showed the same distribution of radioactivity as with [3H]flunitrazepam. The attenuating effect of diazepam on the chlorpromazine- or stress-induced elevation of homovanillic acid in rat brain was antagonized by Ro 15-1788. Among a series of compounds which either decreased or increased the rat cerebellar cGMP level, only the effect of benzodiazepine receptor ligands (diazepam, zopiclone, CL 218 872) was antagonized by Ro 15-1788. Thus, Ro 15-1788 is a selective benzodiazepine antagonist acting at the level of the benzodiazepine receptor in the central nervous system. Peripheral benzodiazepine binding sites in kidney and schistosomes were not affected by Ro 15-1788.