The effects of intraperitoneal injection of 6-hydroxydopamine on the turnover and the levels of the brain catecholamines and the levels of plasma corticosterone in rats

1 . The effects of intraperitoneal injection of 6-hydroxydopamine (6-OHDA) on the levels and the turnover of brain catecholamines and the levels of plasma corticosterone were studied in rats. 2. Two weeks after intraperitoneal injection of 6-OHDA (150 mg/kg) a virtually complete disappearance of cardiac noradrenaline was observed. 3. An increment and an accelerated turnover of noradrenaline in the hypothalamus was observed 2 weeks after peripheral administration of 6-OHDA (150 mg/kg). 4. There was no change in the levels and the turnover of noradrenaline in the cortex of the rats so treated. 5. There was no change in the levels and the turnover of dopamine in either the hypothalamus or the cortex of the 6-OHDA-treated rats. 6. An increment and an accelerated turnover of hypothalamic noradrenaline were not associated with any change in plasma corticosterone.     

Strain-dependency in motor activity and in concentration and turnover of catecholamines in synchronized rats

Circadian rhythm in motor activity was studied with an Animex motimeter in six strains of rats (ACI, BH, BS, DA, LEW, TNO) synchronized by a 12 hr light: 12 hr dark cycle. ANOVA revealed significant interstrain differences in motor activity as well as in the concentration and turnover of central noradrenaline and dopamine. Strain-dependent differences were also found with regard to tyrosine hydroxylase inhibition on motor activity. However, no significant interstrain correlations were found between endogenous concentration and/or turnover rates of the catecholamines and motor activity in normal and drug-treated rats.     

A comparison of the effects of chemical sympathectomy by 6-hydroxydopamine in newborn and adult rats

1. The effects of chemical sympathectomy with 6-hydroxydopamine (6-OHDA) on the cardiovascular system of the rat were compared in, (a) 10-week-old rats treated during the first 14 days after birth with 150 mug/g subcutaneously, and (b) adult rats injected intravenously with 2 x 50 mg/kg on day 1 and 2 x 100 mg/kg on day 7 and the experiments performed on day 8.2. Intravenous administration of 6-OHDA to adult rats almost completely abolished the pressor responses to stimulation of the entire sympathetic outflow in the pithed rat, the contractions of the lower eyelid to stimulation of the cervical sympathetic trunk and the vasconstrictor responses produced by periarterial nerve stimulation of the isolated renal artery preparation. Pressor responses to physostigmine and to tyramine were markedly reduced or abolished in anaesthetized and pithed rat preparations, respectively.3. In corresponding experiments, 10-week-old rats treated as newborns with 6-OHDA showed a marked reduction in the stimulation-induced pressor responses and contractions of the lower eyelid, but completely normal vasoconstrictor responses to periarterial nerve stimulation of the isolated perfused renal artery were obtained. The pressor responses to physostigmine were slightly reduced but the tyramine responses were unchanged.4. Treatment with 6-OHDA at birth caused an almost complete and long-lasting noradrenaline depletion in the heart, spleen, salivary glands and ileum but only a partial depletion in the mesentery from 10-week-old rats. These low noradrenaline levels showed no recovery in rats up to an age of 4 months. The tyrosine hydroxylase activity in both the cervical and stellate ganglia from 10-week-old rats was markedly reduced by treatment with 6-OHDA after birth.5. Injections of 6-OHDA after birth produce an almost complete and permanent sympathectomy of various adrenergically innervated organs in the rat. The vascular system represents a major exception, exhibiting a surprisingly high resistance to this type of chemical adrenergic denervation.     

Action of 6-hydroxydopamine on lamb sympathetic ganglia, vas deferens and adrenal medulla: A combined histochemical, ultrastructural and biochemical comparison with the effects of reserpine

1. The effects of a single dose of 6-hydroxydopamine (6-OHDA) compared with those of chronic reserpine treatment were studied in lamb sympathetic neurones and adrenal medulla by a combination of fluorescence histochemistry, electron microscopy and radiochemical assay.2. In sympathetic ganglia, 6-OHDA produced a rise in noradrenaline concentration within 24 h, and falls in tyrosine hydroxylase and monoamine oxidase activities, whereas reserpine caused a fall in noradrenaline, a rise in tyrosine hydroxylase activity and no change in monoamine oxidase activity. The fluorescence of intra- and postganglionic axons increased greatly within 24 h of 6-OHDA, and there was a corresponding accumulation of large dense-core vesicles within many axons whose neurotubules were disrupted. The changes were almost reversed after 3 weeks.3. In the vas deferens, the concentration of noradrenaline and tyrosine hydroxylase and monoamine oxidase activities had all fallen 24 h after 6-OHDA treatment and had started to recover 3 weeks later. In the adrenal medulla, 6-OHDA did not alter NA concentrations but increased tyrosine hydroxylase activity whereas reserpine depleted noradrenaline and increased tyrosine hydroxylase activity.4. The changes produced in sympathetic ganglia by 6-OHDA may be due both to a direct action on the axoplasmic transport of noradrenaline containing vesicles and indirectly to the reaction of the neurones to loss of the integrity of their axons.     

Behaviour of rats and biogenic amine level in brain after 6-hydroxy-dopamine

Male, Wistar rats were injected into the right lateral ventricle of the brain with 250 g of 6-hydroxy-dopamine (6-OHDA). The behaviour of animals and the level of noradrenaline (NA), dopamine (DA) and 5-hydroxytryptamine (5-HT) were measured after different periods of time. During the first hour after 6-OHDA injection the behaviour of rats was similar to that observed after reserpine or a benzoquinolizine derivative; 6 h after the drug injection signs of sedation occurred. 24 and 48 h after 6-OHDA treatment locomotor activity of rats was similar to the activity of untreated animals. 68 h after 6-OHDA application a decrease of exploratory activity was noted. 7 h after 6-OHDA treatment a decrease of NA level was observed in the cortex, mesencephalon, thalamus and hypothalamus but an increase of NA content in the striatum. 3 days after 6-OHDA injection a decrease of the NA content in the cortex, thalamus and hypothalamus was seen. The DA level increased in most of the examined areas 7 h after 6-OHDA application; 3 days after the drug injection the DA level decreased in hypothalamus, thalamus and striatum and did not change in other brain areas. The 5-HT level increased in the pons and medulla oblongata and decreased in the mesencephalon 7 h after 6-OHDA injection and was unchanged in all examined structures 3 days after 6-OHDA application. These biochemical changes in the brain were not correlated with the behavioural changes of rats.     

An assessment of sympathetic function in isolated tissues from mice given nerve-growth-factor antiserum and 6-hydroxydopamine

1. Experiments were done on isolated tissues from mice injected with 0.9% w/v NaCl solution (saline), 6-hydroxydopamine (6-OHDA), nerve-growth-factor antiserum (NGF-As) or a combination of these agents (NGF-As+6-OHDA).2. Fluorescence histochemistry of vasa deferentia showed clear differences between each of the treatments but no such distinction was possible in cardiac ventricle or intestine.3. Compared with controls, the chronotropic responses of atria to field stimulation were reduced by all three treatments in the order NGF-As<6-OHDA相似文献   

Different mechanisms of the rabbit pulmonary arterial contraction caused by 5- or 6-hydroxydopamine

The contractile responses of rabbit arterial strips to 5-hydroxydopamine (5-OHDA) or 6-hydroxydopamine (6-OHDA) were assessed in vitro. The 3H efflux from the pulmonary artery after preloading with [3H]noradrenaline was markedly enhanced by 100 microM 5-OHDA or 6-OHDA. In non-superfused strips, 6-OHDA produced a biphasic contraction, an initial small transient and a subsequent large sustained contraction, whereas 5-OHDA elicited a large monophasic contraction. The 6-OHDA-evoked second large contraction was followed by a marked tachyphylaxis after repeated application of the drug. This contraction was greatly diminished by prazosin or cocaine and by pretreatment with reserpine, indicating an indirect action via noradrenaline release. In contrast, the initial contraction caused by 6-OHDA as well as the contraction caused by 5-OHDA was not inhibited by these agents, except prazosin, implying a direct action at the postsynaptic alpha 1-adrenoceptors. In addition, the concentration-response curve for noradrenaline was significantly shifted to the right by pre-addition of 5-OHDA whereas the curve for 5-HT was virtually unaffected. The results thus suggest that 5-OHDA acts as a postsynaptic alpha 1-adrenoceptor agonist.     

The depletion and recovery of noradrenaline in the brain and some sympathetically innervated mammalian tissues after tetrabenazine

The depleting effect of tetrabenazine on the monoamine levels appears generally to be less in peripheral tissues than in the central nervous system. The noradrenaline levels in brain and some sympathetically innervated tissues such as heart, submandibular glands, and skeletal muscle were examined in the rat after administration of the drug. The levels of 5-hydroxytryptamine, dopamine and noradrenaline were also estimated in the rabbit brain after tetrabenazine and compared with the levels in the rabbit heart. In both the brain and peripheral tissues the monoamine levels were strongly reduced 4 hr after tetrabenazine and increased thereafter, reaching normal levels after about 36 to 48 hr. The site of action of tetrabenazine is briefly discussed and compared to the site of action of reserpine.     

Opposite changes in turnover of noradrenaline and dopamine in the CNS of ethanol-dependent mice

Turnover rates of noradrenaline and of dopamine were examined in regions of the CNS (brainstem, frontal cortex, hippocampus, striatum) and in cardiac tissue of mice that had ingested ethanol for 2 or 7 days and during withdrawal of ethanol. Turnover of neurotransmitters was assessed from the accumulation of dihydroxyphenylalanine and from the depletion in levels of noradrenaline, dopamine and their deaminated metabolites in tissue, after irreversible inhibition of aromatic L-amino acid decarboxylase with DL-alpha-monofluoromethyldopa. There were no consistent changes in the turnover of noradrenaline or dopamine after 2 or 7 days of continuous ingestion of ethanol, but at the time that animals exhibited peak ethanol-withdrawal symptoms (8 hr after ethanol consumption was stopped), the turnover of dopamine in the striatum was markedly decreased to less than a third of that observed in control animals, whereas the turnover of noradrenaline in the brainstem was increased by a half. The changes in turnover of neurotransmitters in specific regions of the brain may reflect alterations in neuronal activity that result from withdrawal of ethanol, or may be determinants of particular ethanol-withdrawal symptoms.     

The effect of 5,7-dihydroxytryptamine on peripheral adrenergic nerves in the mouse

Summary Injections of 5,7-dihydroxytryptamine (60 mg/kg, i.p.) caused a significant reduction in the noradrenaline content of the mouse heart, large intestine and seminal vesicle, 2 and 6 days after drug application. The long-lasting depletion of noradrenaline was found to be due to a degeneration of adrenergic nerve terminals as verified by fluorescence and electron microscopical observations.The results show that 5,7-dihydroxytryptamine is either as potent as (e.g. in the heart) or less potent than 6-hydroxydopamine (e.g. in the vas deferens) in depleting noradrenaline. The pronounced effects of 5,7-dihydroxytryptamine on the adrenergic nerves of the seminal vesicle (supplied by the same short adrenergic neurones as the vas deferens) suggest that 5,7-dihydroxytryptamine may be used as a tool for the induction of a chemical sympathectomy in certain peripheral organs of laboratory animals, supplementary to 6-hydroxydopamine.Supported by grants from the Deutsche Forschungsgemeinschaft.     

Neonatal chemical sympathectomy: Functional control of denervation of the vascular system and tissue noradrenaline level in the rat after 6-hydroxydopamine

Summary In adult rats, the degree of vascular sympathectomy following neonatal administration of 6-hydroxydopamine (6-OHDA) was assessed. Doses of 6-OHDA were 100 g/g on days 1 (day of birth) and 2, and 250 g/g on days 8 and 15. In treated rats, anaesthetized with pentobarbital, mean arterial blood pressure, heart rate and pressor response to tyramine were reduced, while pressor responses to noradrenaline, adrenaline, angiotensine II and arginine-vasopressin were markedly increased. The change of both the noradrenaline and tyramine response was also present in rats treated on days 1, 2 and 8, but absent in rats treated on days 1 and 2 only. Pressor responses to electrical stimulation of the sympathetic outflow of the spinal cord and of the posterior hypothalamus of treated rats were diminished and were practically absent after adrenalectomy. The 6-OHDA treatment caused gross depletion of NA from the spinal cord, and from peripheral organs and tissues except the adrenals. A reduced NA level in brain cortex was associated with an increase of that of the brain stem.The 6-OHDA treatment schedule used in the present experiments results in a high degree of vascular sympathectomy. The use of a functional test appears to be a better control than measurement of tissue NA levels alone to determine the degree of vascular sympathectomy.     

On the mechanism of central action of amphetamine: the role of catecholamines

The effects of amphetamine were studied on the spontaneous motor activity and other behaviour in aggregated mice using a modified jiggle cage. Amphetamine alone (2.5, 7.5 and 10 mg/kg) produced a fairly dose-dependent increase in spontaneous motor activity, excitatory behaviours and mortality. The effects were almost totally abolished after pretreatment with α-methyl tyrosine (α-MT) but could be restored temporarily by l-DOPA-treatment preceding amphetamine. Multiple-dose pretreatments with reserpine and α-methyl-meta-tyrosine (α-MMT), singly or in combination, somewhat reduced but did not abolish the effects of amphetamine (at 8 hr) and invariably augmented these at longer intervals (16 hr), especially after combined pretreatments. Adrenergic blockers were used to block these augmented amphetamine effects. Chlorpromazine totally suppressed all amphetamine effects, whereas phenoxybenzamine blocked the hyperexcitability, aggressiveness and mortality but not the spontaneous motor activity-effects significantly. These results are discussed with relevance to the possible roles of noradrenaline and dopamine in mediating amphetamine-induced behaviour as observed in these experiments designed for depletion, repletion and blockade of endogenous catecholamines.     

The effects of 6-hydroxydopamine and guanethidine on peripheral adrenergic nerves in the guinea pig

Chemical sympathectomy was achieved in guinea pig trachea 24 hr after a single dose of 50 mg/kg (i.v. or i.p.) 6-hydroxydopamine (6-OHDA). This was shown by (a) the dissapearance of fluorescent nerve terminals (b) decreased uptake-with-retention of radioactivity following incubation in (?)-³H-noradrenaline (50 nM) and (c) the supersensitive response of the isolated tracheal chain preparation to noradrenaline. The susceptibility of the adrenergic nerve terminals in the lung of 6-OHDA resembled that of trachea, in that complete dissapearance of fluorescent nerve terminals was achieved with the above doses, whereas doses higher than 100 mg/kg were required to produce this in heart and iris. Chronic treatment of guinea pigs with guanethidine (25 or 30 mg/kg daily by i.p. injection for up to 6 weeks) did not produce destruction of adrenergic nerves in peripheral tissues.     

Cardiovascular and metabolic responses to thyroid hormones in animals after sympathectomy or treatment with nerve growth factor

The cardiovascular and metabolic effects of thyroxine (T₄) and triiodothyronine (T₃) were examined in rats and mice with decreased or permanently enhanced adrenergic innervations to the heart. 6-Hydroxydopamine (6-OHDA) and the antiserum to the nerve growth factor (AS) were administered to damage sympathetic fibers; the density of the cardiac adrenergic plexus was increased through the use of the nerve growth factor (NGF). Some animals were additionally adrenal demedullated or treated with N,N-diisopropyl- N′-isoamyl-N′-diethylaminoethylurea (P-286) to reduce circulating catecholamine levels. The results of these experiments showed that the rise in heart phosphorylase a activity produced by thyroid hormones was absent after adrenal demedullation and significantly inhibited after chemical sympathectomy with 6-OHDA. Interference with adrenal medullary function by P-286 in combination with chemical or immunological sympathectomy similarly prevented the hormone-induced activation of myocardial phosphorylase a. While an increase in the density of the peripheral sympathetic fibres to the heart accentuated the tachycardia characteristic of hyperthyroidism, the high metabolic rate which accompanies thyrotoxicosis was unaltered either by proliferation of adrenergic fibres or sympathectomy.     

Differential effects of reserpine and 6-hydroxydopamine on neuropeptide Y (NPY) and noradrenaline in peripheral neurons

The effects of 6-hydroxydopamine (6-OHDA) and reserpine pretreatment on peripheral neuropeptide Y (NPY)- and noradrenaline (NA)-containing neurons were studied in guinea-pigs. Ten days after 6-OHDA pretreatment, a 60-80% reduction of the NA content was observed in the right atrium of the heart, stellate ganglion and spleen. The content of NPY-like immunoreactivity (LI) was reduced by about 50% in the heart, not changed in the spleen while it increased to 200% of control in the stellate ganglion. Immunohistochemistry showed a pronounced loss of NPY- and tyrosinehydroxylase (TH)-immunoreactive (IR) nerves in the heart but not in the spleen. Increased NPY-IR was seen in axons and cell bodies of the stellate ganglion. Reserpine pretreatment (thereshold dose 0.5 mg X kg-1) caused a dose- and time-dependent reduction of the content of NPY-LI in the heart. A maximal depletion of NPY-LI (about 80%) was observed 5 days after reserpine. Reserpine pretreatment also reduced the content of NPY-LI in the spleen, while no significant change was observed in the adrenal gland or vas deferens. The levels of NPY-LI increased in the stellate ganglion to about 180% of control 5 days after reserpine. Immunohistochemical analysis revealed an almost total loss of NPY-IR nerve fibres in the heart as well as around blood vessels in the lung and skeletal muscle. No detectable changes were observed in perivascular NPY-IR nerves in the spleen, vas deferens or kidney. TH-IR nerves remained unchanged after reserpine, thus indicating that the observed loss of NPY-IR nerves was due to a depletion of NPY and not a degeneration. No change in the levels of substance P-LI was observed in the right atrium 5 days after reserpine. NA was, in contrast to NPY, markedly depleted in all tissues investigated after reserpine treatment. The depletion of NA was more extensive, and occurred more rapidly and at much lower doses as compared to the effects on NPY-LI. Ligations of the sciatic nerve revealed that NPY-LI was transported axonally with a rapid rate (3 mm/h). Reserpine pretreatment significantly increased the amount of accumulated NPY-IR above the ligation, suggesting an increase in axonal transport. High performance liquid chromatography revealed that the NPY-LI consisted of two major peaks in the stellate ganglia, while only one peak closely corresponding to porcine NPY was seen in the right atrium. In conclusion, 6-OHDA pretreatment depletes NPY-LI in certain terminal regions and increases NPY-LI in ganglia.(ABSTRACT TRUNCATED AT 400 WORDS)     

Long-lasting peripheral and central effects of 6-hydroxydopamine in rats

1. In newborn rats treated with 6-hydroxydopamine hydrobromide (6-OHDA) (50-150 mg/kg on 5-7 days) a widespread and long-lasting dose related sympathectomy was demonstrated.2. When rats given 6-hydroxydopamine (100 mg/kg, seven treatments) in the neonatal period were killed at 10 weeks the concentration of noradrenaline (NA) in the heart, mesentery and vas deferens was significantly reduced. There was no alteration in the catecholamine content of the adrenal glands.3. The amplitude of the responses of perfused mesenteric arteries from 6-hydroxydopamine treated rats to intra-arterial noradrenaline was not increased, compared with controls, but the duration of responses was increased.4. 6-Hydroxydopamine given to newborn rats caused a long-lasting depletion of noradrenaline in three of the five regions of the central nervous system (cortex, cerebellum and spinal cord) studied. The concentration of noradrenaline in the pons-medulla was increased, but in the thalamic region was unchanged. The treated rats showed significantly lower exploratory activity.5. Treatment of newborn rats with 6-hydroxydopamine thus has striking and long-lasting effects on peripheral and central adrenergic systems.     

A study of the role of noradrenaline in behavioural changes produced in the rat by psychotomimetic drugs

1. LSD-25, psilocybin and JB-329 reduced the noradrenaline content of the rat hypothalamus.2. All three drugs affected the acquisition of a conditioned avoidance response, LSD-25 and psilocybin retarding and JB-329 enhancing the acquisition. With the exception of JB-329, doses affecting the acquisition of a conditioned avoidance response were lower than those required to decide hypothalamic noradrenaline concentrations. The time of peak drug effect on the acquisition of a conditioned avoidance response occurred approximately 1.5 hr after injection as opposed to 3 hr in the case of noradrenaline content.3. The amount of LSD-25, psilocybin and JB-329 necessary to elicit gross behavioural excitation was similar to the dose producing noradrenaline depletion. Here also the peak behavioural effect was detected earlier.4. Pretreatment with reserpine and alpha-MT had no effect on the intensity of gross behavioural excitation induced by LSD-25 and psilocybin but shortened the duration of the response. The excitation induced by JB-329 was abolished by reserpine pretreatment and was markedly reduced both in intensity and duration by the prior injection of alpha-MT.     

The effect of pithing and of nerve stimulation on the depletion of noradrenaline by reserpine in the rat anococcygeus muscle and vas deferens

1 The depletion of noradrenaline (NA) in the rat anococcygeus muscle and vas deferens by reserpine and the effect on this of the abolition of nerve activity by pithing and reinforcement of nerve activity by stimulation of the spinal cord outflows has been studied.2 NA depletion of the anococcygeus and vas deferens measured 24 h after reserpine was similar and was related to dose. The heart was depleted faster than the two smooth muscle tissues.3 In the absence of reserpine neither abolition of nerve activity by pithing nor its reinforcement by nerve stimulation had a detectable influence on NA content of the anococcygeus or vas deferens.4 In rats given reserpine (200 mug/kg), increasing nerve activity by spinal stimulation significantly increased NA depletion in both the anococcygeus and the vas deferens when compared with animals pithed but not stimulated. These results confirm that nerve impulse traffic can be an important factor in determining the rate of depletion of NA by reserpine.5 The mechanical response to nerve stimulation in both the vas deferens and anococcygeus was resistant to quite severe depletion of their NA content, with the exception of the initial fast component of the response in the vas. The implications of these results for motor transmission in the vas deferens are discussed.     

Time course of the effects of 6-hydroxydopamine on catecholamine-containing neurones in rat hypothalamus and striatum

1. The effects of intraventricular injection of 6-hydroxydopamine (6-OHDA) on tyrosine hydroxylase activity, uptake of ³H-noradrenaline and endogenous catecholamine concentration in rat hypothalamus and striatum were investigated at various times after the injection of 6-OHDA.

2. In the hypothalamus after the injection of 250 μg of 6-OHDA there was a rapid decrease in tyrosine hydroxylase activity, ³H-noradrenaline uptake and noradrenaline content, which was essentially complete within 2 hours.

3. In the striatum after this dose of 6-OHDA there was a much slower reduction in tyrosine hydroxylase activity and ³H-noradrenaline uptake during the first 48 h after drug injection. For the first 24 h the dopamine concentration in this brain area was increased significantly above control values, but had fallen below control values by 48 hours.

4. After the injection of a smaller dose of 6-OHDA (25 μg) the only detectable change in the striatum was a rapid increase in the dopamine concentration. In the hypothalamus this dose induced a rapid depletion of noradrenaline, not accompanied initially by any significant reduction in tyrosine hydroxylase activity.

5. These results are consistent with the hypothesis that 6-OHDA causes a rapid degeneration of catecholamine-containing nerve terminals in the central nervous system (CNS). These degenerative changes, indicated by the loss of tyrosine hydroxylase and noradrenaline uptake sites, did not appear to be preceded by an initial displacement of endogenous catecholamines by 6-OHDA, except possibly at early times after the administration of small doses of the drug.

     

Time of day and access to food alter water intake in rats after water deprivation

1. Drinking behaviour after water deprivation is one of the standard tests used to study thirst in humans and animals. Diurnal cycle and food availability are known to influence water intake, but have not been considered in previous studies of thirst after water deprivation. In the present study, we examined the effects of diurnal variation and food availability on water intake after 24 h water deprivation in rats. 2. All rats cycled through four treatments in varying order. These treatments were: (i) 24 h water deprivation with free access to food from 1900 h one day to 1900 h the next day, followed by free access to both food and water (Night-with-Food); (ii) 24 h water deprivation with free access to food from from 1900 h one day to 1900 h the next day, followed by free access to water but not food (Night-without-Food); (iii) 24 h water deprivation with free access to food from 0700 h one day to 0700 h the next day, followed by free access to both food and water (Day-with-Food); or (iv) 24 h water deprivation with free access to food from 0700 h one day to 0700 h the next day, followed by free access to water but not food (Day-without-Food). The amount of water consumed during the first 6 h, post-24 h water deprivation, was examined under each condition. 3. There was a significant diurnal effect (P < 0.001) and a significant food availability effect (P = 0.007) on the water consumed in the 6 h period after water deprivation. Most water was consumed by the Night-with-Food group and the least amount of water was consumed by the Day-without-Food group. These effects persisted after correction for water intake during 6 h periods from 0700 and 1900 h with and without food but without previous water deprivation. The diurnal and food availability effects on water consumption were independent (P = 0.5). 4. The coefficient of variability for each group suggests that the most sensitive measurements of water intake are obtained during the day in the absence of food. 5. We conclude that both the time of day and access to food independently alter water intake in rats subjected to a previous 24 h water deprivation. Our study also supports the validity of performing water intake measurements in thirst studies in rats during the day.