Tolerance in the anxiolytic profile following repeated administration of diazepam but not buspirone is associated with a decrease in the responsiveness of postsynaptic 5-HT-1A receptors.

To understand the role of serotonin (5-hydroxytryptamine; 5-HT)-1A receptors in the treatment of anxiety and the development of tolerance to benzodiazepines the present study was designed to monitor the responsiveness of postsynaptic 5-HT-1A receptors following repeated administration of diazepam and buspirone. Results show that tolerance in the anxiolytic profile is produced following repeated administration (2 weeks) of diazepam (2 mg/kg) but not buspirone (0.5 mg/kg). The behavioral effects of 8-OH-DPAT at a dose of 0.25 mg/kg were monitored 3 days after repeated administration of saline or buspirone or diazepam. The results show that 8-OH-DPAT elicited forepaw treading was smaller in repeated diazepam but not repeated buspirone injected rats, while hyperlocomotive effects of 8-OH-DPAT were smaller in both repeated buspirone and repeated diazepam injected rats. The results suggest that postsynaptic 5-HT-1A receptor-dependent responses were attenuated following long-term administration of diazepam but not buspirone. Role of 5-HT-1A receptors in the development of tolerance to the anxiolytic effects of diazepam but not buspirone is discussed.

Long-term tryptophan administration enhances cognitive performance and increases 5HT metabolism in the hippocampus of female rats.

It has been shown in various studies that increase in serotonergic neurotransmission is associated with increased memory consolidation whereas low brain 5HT impairs memory performance. In the first phase of our study we found that tryptophan (TRP) administration for 6 weeks increased plasma TRP and whole brain TRP, 5HT and 5HIAA levels. Many brain regions are involved in the learning process but particularly the hippocampus is known to have key role in learning and memory. The present study was therefore designed to investigate the effects of TRP loading particularly on hippocampal 5HT metabolism and cognitive performance in rats. TRP-treated rats demonstrated spatial enhancement as evidenced by a significant decrease in time to find the hidden food reward in radial arm maze test (RAM). The important finding of the present study was the greater increase in the 5HT metabolism in hippocampus than in any other brain region of the TRP-treated rats. This increased 5HT metabolism in the hippocampus emphasizes the involvement of this region in memory process.

Brain regional serotonin synthesis following adaptation to repeated restraint.

A single 2 h episode of restraint stress decreased food intake and growth rate of rats. These deficits were not observed after five restraint periods of 2 h a day, suggesting that adaptation occurred. An acute challenge with 2 h restraint increased 5-hydroxytryptamine (5-HT) synthesis rate in the cortex, hypothalamus, midbrain and hindbrain of previously unrestrained rats, but not those adapted to 5 days of 2 h daily restraint. Hippocampal and striatal 5-HT synthesis was not increased significantly by 2 h restraint in previously unrestrained rats but was increased and decreased, respectively, in rats exposed to five 2 h daily restraints, when they were restrained on the sixth day. The findings suggest an important role of 5-HT particularly in the hippocampus, in adaptation to stress.

Food restriction decreases serotonin and its synthesis rate in the hypothalamus.

Rats fed on a restricted feeding (RF) schedule of 4 h day-1 to produce a 15-20% reduction in body weight were killed before (starved) and after (fed) the presentation of food on the sixth day to compare 5-hydroxytryptamine (5-HT; serotonin) metabolism and synthesis rate in the hypothalamus with freely feeding (FF) controls. The RF rats showed lower 5-HT concentration and synthesis rate than FF controls. Restricted feeding did not decrease tryptophan concentration in the hypothalamus. However, RF-fed rats had lower tryptophan concentration than RF starved rats. 5-HIAA concentration was comparable in RF fed rats and FF controls but higher in RF starved rats. Possible implications of the findings in the pathogenesis of the food deprivation/starvation-related disease anorexia nervosa are discussed.

Tolerance to diacetyl morphine antinociception: effects on brain serotonin.

The effects of 5 mg kg-1 diacetyl morphine (DAM) on brain serotonin metabolism of rats were investigated following tolerance to the antinociceptive effects of 2.5 mg kg-1 DAM. Brain levels of tryptophan and 5-hydroxy indoleacetic acid (5-HIAA) were higher in the DAM-tolerant rats killed 24 h after last daily administration of 2.5 mg kg-1 DAM. Administration of 5 mg kg-1 DAM produced less antinociception in DAM-tolerant than DAM-naive rats and increased brain tryptophan concentration in both tolerant and naive rats. 5-HIAA concentrations increased only in naive rats. Combined use of drugs interfering with brain 5-HT turnover along with opiates may be of future benefit for the treatment of chronic pain.

Adaptation of female rats to stress: shift to male pattern by inhibition of corticosterone synthesis.

In a previous study, male rats showed behavioural deficits after a single restraint stress but not after 5 daily restraint periods (i.e. adaptation had developed): female rats although less affected by single restraint failed to adapt over the same time course. This sex difference was associated with the male but not the female rats showing enhanced behavioural responses to the 5-HT agonist 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) after 5 restraint periods. In the present study, the role of the greater increases of plasma corticosterone in stressed females in these sex differences was studied. The corticosterone synthesis inhibitor metyrapone (75 mg/kg i.p.) was given to attenuate the rise of corticosterone to a level typical of stressed males. This resulted in the behavioural deficits of the female rats being shifted in the direction of the male pattern. Thus, their deficits in open field activity and food intake after single and repeated stresses were potentiated and opposed respectively. The latter effect was associated with increased responses to 5-MeODMT. Metyrapone alone was without significant effect. Brain regional 5-HT metabolism was unaffected. The results are consistent with corticosterone facilitating adaptation to single restraint but impairing adaptation to repeated restraint. As failure to adapt to repeated stress is an animal model of depression, results as a whole suggest that increased corticoid levels and decreased 5-HT functional activity may have a role in the development of the illness and its greater incidence in women.

8-OH-DPAT increases corticosterone but not other 5-HT1A receptor-dependent responses more in females.

The 5-HT1A receptor subtype agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) (50-1000 micrograms/kg s.c.) dose dependently increased rat plasma corticosterone. Tube restraint for 30 min also increased plasma corticosterone; this effect was completely blocked by (-)-pindolol (1 mg/kg i.p.). Increases of corticosterone following either 8-OH-DPAT injection or restraint were significantly greater in female animals. The restraint stress-induced changes but not those due to 8-OH-DPAT were decreased by pretreatment with the tranquiliser chlordiazepoxide (10 mg/kg i.p.). In anaesthetized rats, restraint no longer significantly affected corticosterone levels but 8-OH-DPAT caused increases which (though much attenuated) were significantly greater in the females. Dose-dependent increases of plasma corticosterone also resulted on infusing 8-OH-DPAT (500-1500 ng) into the paraventricular nucleus of the hypothalamus; increases were significantly greater in the females. As mentioned in the discussion, these results may be relevant to the greater incidents of depression in women and the possible role of adrenal corticoids in the illness.

Injected tryptophan increases brain but not plasma tryptophan levels more in ethanol treated rats.

In previous studies, long term treatment with ethanol has been shown to enhance brain 5-hydroxytryptamine 5-(HT) metabolism by increasing the activity of the regulatory enzyme tryptophan hydroxylase and or availability of circulating tryptophan secondarily to an inhibition of hepatic tryptophan pyrrolase. In the present study ethanol treatment given for two weeks decreased hepatic apo-tryptophan pyrrolase but not total tryptophan pyrrolase activity in rats. Tryptophan levels in plasma and brain did not increase significantly. But there was a marked increase of 5-HT but not 5-hydroxyindoleacetic acid (5-HIAA) concentration in brain, suggesting a possible increase in the activity of tryptophan hydroxylase. The effect of a tryptophan load on brain 5-HT metabolism was therefore compared in controls and ethanol treated rats. One hour after tryptophan injection (50 mg/kg i.p.) plasma concentrations of total and free tryptophan were identical in controls and ethanol treated rats, but the increases of brain tryptophan 5-HT and 5-HIAA were considerably greater in the latter group. The results are consistent with long term ethanol treatment enhancing brain serotonin metabolism and show that brain uptake/utilization of exogenous tryptophan is increased in ethanol treated rats and may be useful to understand the role and possible mechanism of tryptophan/serotonin involvement in mood regulation.

Function specific supersensitivity of m-chlorophenyl piperazine-induced serotonergic neurotransmission in female compared to male rats.

Administration of various doses of m-chlorophenylpiperazine (m-CPP), a 5-hydroxytryptamine (5-HT) agonist, to rats increased plasma levels of corticosterone and decreased locomotor activity in a dose dependent manner. The increases of plasma corticosterone but not the decreases of locomotor activity were greater in female than male rats. Hypophagic effects of m-CPP not different in freely feeding male and female rats were greater in females when food deprived animals used. The findings along with our previous report show a functional supersensitivity of 5-HT neurotransmission involved in the regulation of hypothalamo-pituitary adrenocortical axis and appetite suppression in female sex, and suggest a possible basis for the greater occurrence of depression and anorexia in women than men.

Repeated corticosterone treatment attenuates behavioural and neuroendocrine responses to 8-hydroxy-2-(di-n-propylamino) tetralin in rats.

The effects of 5 day corticosterone treatment (50 mg/kg s.c.; 2 x daily) are investigated on the behavioural and neuroendocrine responses to a 5-HT-1A selective agonist, 8-hydroxy -2-(di-n-propylamino) tetralin (8-OH-DPAT) in rats. Daily corticosterone treatment decreased body weight and food intake. After 5 day treatment a drug challenge of 0.25 and 0.5 mg/kg 8-OH-DPAT given on the sixth day produced smaller forepaw treading but comparable head waeving, flat body posture and also hypothermia in 5 day corticosterone than 5 day saline injected rats. Hyperphagic effects of only 0.25 mg/kg 8-OH-DPAT were attenuated in 5 day corticosterone injected animals. The effects of 8-OH-DPAT on the increases of plasma corticosterone were markedly attenuated in the 5 day corticosterone injected animals. The findings may help towards an understanding of steroid-induced affective changes and psychosis.

Sex differences in neurochemical and behavioural effects of 8-hydroxy-2-(di-n-propylamino) tetralin.

A number of neurochemical investigations have shown that 5-hydroxytryptamine (5-HT) metabolism and turnover is greater in females than male rats. However increased 5-HT metabolism does not necessarily imply greater 5-HT release at the functional post-synaptic sites. Pharmacological research based on 5-HT receptor stimulation therefore gained attention. Studies of this type are complicated because of the multiplicity of 5-HT receptors in the central nervous system. Chemical ligands may not have sufficient selectivity, to specifically bind to a single receptor population. Moreover, both the density and distribution of 5-HT receptors may follow a different pattern in male and female rats. 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) is a centrally acting 5-HT agonist with a ligand binding profile showing selectivity towards 5-HT-1A receptor sites. The present article integrates research on neurochemical and behavioural effects of 8-OH-DPAT in male and female rats, in order to investigate sex-related differences in 5-HT-1A receptor dependent functions.

Regionally specific effects of diazepam on brain serotonin metabolism in rats: sustained effects following repeated administration.

The effects of single (1mg/kg) and repeated (1mg/kg 2* daily for 4 days) diazepam administration are investigated on brain regional 5-hydroxytryptamine (5-HT; serotonin) and 5-hydroxy indoleacetic acid (5-HIAA) concentration in rats. Daily treatment decreased food intakes but body weights did not decrease. Administration of diazepam (1mg/kg) to 4 day saline injected rats on the 5th day decreased 5-HT levels in the hippocampus and increased it in the hypothalamus. 5-HIAA levels were increased in the striatum and decreased in the hypothalamus 4 day diazepam injected rats injected with saline on the 5th day also exhibited similar changes of 5-HT and 5-HIAA. Cortical levels of 5-HIAA were also smaller in these rats. Administration of diazepam to 4 day diazepam injected rats again decreased 5-HT in the hippocampus and 5-HIAA in the hypothalamus. 5-HT and 5-HIAA were both decreased in the striatum. Regionally specific effects of diazepam on brain serotonin metabolism are discussed in relation to their possible functions.

Inhibition of restraint-induced anorexia by injected tryptophan.

Tryptophan injected at doses of 50mg/kg did not alter 24 h cumulative food intake and growth rate in rats. A single episode of 2 h restraint stress decreased food intake and growth rate of saline and tryptophan injected rats. The decreases of both food intake and growth rate were smaller in tryptophan injected (food intake 23.9% p<0.05; growth rate 2.9% p<0.05) than saline injected (food intake 78.5% p<0.01; growth rate 6.1% p<0.01) rats suggesting that tryptophan administration inhibits restraint-induced anorexia. Following an acute challenge with 2h restraint increases of 5-hydroxytryptamine (5-HT; serotonin) and 5-hydroxyindoleacetic acid (5-HIAA) but not tryptophan were greater in tryptophan injected than saline injected rats. The findings imply that tryptophan-induced increases of brain 5-HT and 5-HIAA have little effect on functional serotoninergic activity under basal conditions but a facilitatory effect on functional response occurs in conditions of increased serotoninergic neuronal activity such as during stress.

Enhancement of hepatic tryptophan pyrrolase activity and decreases of open field locomotion following single and repeated administration of high doses of caffeine in rats.

In view of a possible role of kynurenine in caffeine-induced anxiety syndrome, the effects of single and repeated administration of caffeine on hepatic tryptophan (T)-pyrrolase activity are investigated. Single administration of caffeine at doses of 80 mg/kg decreased open field locomotion and increased hepatic T-pyrrolase activity. Locomotor stimulating effects of 80 mg/kg caffeine, monitored in the home cages of rats, were attenuated following daily administration of caffeine for 5 days. Open field locomotor activity of rats and its caffeine-induced decrement were also attenuated following 5 daily administrations of caffeine on the 6th day. Basal levels of hepatic T-pyrrolase activity increased after 5 daily administrations of caffeine on the 6th day. Acute administration of caffeine did not further elevate hepatic T-pyrrolase activity in 5 day caffeine injected rats. Drug adjuvants decreasing hepatic T-pyrrolase activity may prove valuable for extending the clinical utility of caffeine.

24h withdrawal following repeated administration of caffeine attenuates brain serotonin but not tryptophan in rat brain: implications for caffeine-induced depression.

Caffeine injected at doses of 20, 40 and 80 mg/kg increased brain levels of tryptophan, 5-hydroxytryptamine (5-HT) and 5-hydroxyindole acetic acid (5-HIAA) in rat brain. In view of a possible role of 5-HT in caffeine-induced depression the effects of repeated administration of high doses of caffeine on brain 5-HT metabolism are investigated in rats. Caffeine was injected at doses of 80 mg/kg daily for five days. Control animals were injected with saline daily for five days. On the 6th day caffeine (80 mg/kg) injected to 5 day saline injected rats increased brain levels of tryptophan, 5-HT and 5-HIAA. Plasma total tryptophan levels were not affected and free tryptophan increased. Brain levels of 5-HT and 5-HIAA but not tryptophan decreased in 5 day caffeine injected rats injected with saline on the 6th day. Plasma total and free tryptophan were not altered in these rats. Caffeine-induced increases of brain tryptophan but not 5-HT and 5-HIAA were greater in 5 day caffeine than 5 day saline injected rats. The findings are discussed as repeated caffeine administration producing adaptive changes in the serotonergic neurons to decrease the conversion of tryptophan to 5-HT and this may precipitate depression particularly in conditions of caffeine withdrawal.

Stress and hypertension: role of serum, red cell and tissue electrolytes.

The role of stress in the precipitation of hypertension is often described in clinical studies, although the underlying mechanism remains unknown. The present study concerns the role of electrolytes in stress induced hypertension in rats. Acute immobilization stress of one hour elevated systolic blood pressure (SBP) in rats. Restraint induced blood pressure elevation was associated with increased sodium concentration in the red cells, heart and kidney, and decreased potassium in the red cells. Magnesium concentration increased and calcium concentration decreased in the serum. Increases of calcium and decreases of magnesium were also observed in the heart and kidney tissues. The results may help toward an understanding of the relationship between hypertension and electrolyte homeostasis. A possible role of Na(+)-K(+)-ATPase activity leading to observed changes of electrolytes or vice versa is discussed.

Serotonergic neurotransmission in the regulation of appetite: a receptor approach.

Neurochemical research on brain 5-hydroxytryptamine (5-HT; serotonin) and feeding shows that rat brain serotonin metabolism is increased following ingestion of a carbohydrate rich diet to generate a neurochemical signal for the termination of meal. Increased metabolism may not necessarily enhance postsynaptic function; neuropharmacological studies therefore gained attention. Drugs which mimick 5-HT function at the post synaptic sites have been shown to decrease feeding in experimental animals. Moreover some 5-HTergic drugs are potent anorectic agents. Multiple receptors for 5-HT exist in the central nervous system. Drugs with selectivity towards 5-HT-1B/ 5-HT-1C sites produced hypophagia, while 5-HT-1A selective drugs increased food intake. Studies designed to investigate sensitivity of these receptors following starvation or satiety may prove useful to develop drugs for therapeutic purposes.

Neurochemical and behavioural effects of diazepam: evidences from animal models.

Neurochemical and behavioural research show that benzodiazepines (BZs) are the well-known anxiolytic drugs which are also used for the treatment of epilepsy, hypnosis and insomnia. Administration of BZs to experimental animals produces anxiolytic-like effects in various animal models and decreases exploratory activity. A role of serotonin (5-HT; 5-hydroxytryptamine) in both anxiolytic and anti-exploratory effects of BZs have been suggested. Drugs which mimic 5-HT function at the post synaptic sites have been shown to decrease anxiety in experimental animals. The present study analyses regionally specific effects of BZs on brain serotonin metabolism in relation to the reported behavioural and therapeutic profiles of the drugs.

Increased precursor availability did not increase food intake and 5-HT turnover rate in the hypothalamus of diazepam injected rats.

In view of a possible role of serotonin (5-hydroxytryptamine; 5- HT) in regulation of appetite and anxiety, the effects of 1, 3 and 5 mg/kg doses of diazepam on brain serotonin precursor and effects of single and repeated diazepam (1 mg/kg 2* daily for 4 days) administration on hypothalamic 5-HT, 5-hydroxyindoleacetic acid (5-HIAA and 5-HIAA/5-HT ratio are investigated in rats. Daily diazepam treatment decreased food intakes. Diazepam injected rats exhibited a dose-dependent increase of tryptophan in the hypothalamus. Administration of diazepam (1 mg/kg) to 4 day saline injected rats on the 5th day increased 5-HT and decreased 5-HIAA levels in the hypothalamus. 5-HIAA/5-HT level also decreased. 4-day diazepam injected rats injected with saline on the 5th day also exhibited similar changes of 5-HT. 5-HIAA and 5-HIAA/5-HT ratio. Administration of diazepam to 4 day diazepam injected rats again decreased 5-HIAA concentrations but did not increase 5-HT levels in the hypothalami of rats. Possible mechanism involved in the anorectic effects of diazepam-induced changes of hypothalamic 5-HT turnover rate is discussed.

Behavioral effects of 8-OH-DPAT in single and repeated haloperidol injected rats.

Haloperidol is an antipsychotic drug and shown to be antagonist at D2 receptors and found to cause severe impairment of locomotor performance. The serotonin (5HT1A) receptor agonist 8-OH-DPAT has been reported to attenuate extrapyramidal side effects of haloperidol. The present study was designed to examine the modulatory effect of serotonergic activities on haloperidol induced up-regulation of dopamine D2 receptors. In the acute phase of study, 8-OH-DPAT (0.5 mg/kg/ml) elicited behavioral syndrome was monitored in rats preinjected with haloperidol(5 mg/kg/ml). Results of single haloperidol administration revealed that 8-OH-DPAT induced forepaw treading (p < 0.05) and hyperlocomotion (p < 0.01) were smaller in haloperidol than saline preinjected rats. In repeated phase of study, 8-OH-DPAT (0.5 mg/kg/ml) induced behavioral syndrome was monitored in rats injected with haloperidol for 10 days (x2). The result of repeated haloperidol administration showed that 8-OH-DPAT elicited flat body posture (p < 0.01) was greater in repeated haloperidol injected rats than repeatedly saline injected rats. Relationship between 5HT1A receptors and D2 receptors has been discussed. It is suggested that combining neuroleptics with 5HT1A ligands is thought to improve the preclinical profile of neuroleptics and may be of interest in the development of new compounds that have greater therapeutic potential and/or better tolerated.