L-5-hydroxytryptophan-induced drinking in rats: Possible mechanisms for induction

Administration of L-5-hydroxytryptophan (25 mg/kg body weight, SC) to female rats resulted in copious drinking. The dipsogenic response to administration of L-5-hydroxytryptophan (5-HTP) was blocked by propranolol (6 mg/kg body weight, IP), a β-adrenergic antagonist, and captopril (35 mg/kg body weight, IP), an angiotensin converting enzyme inhibitor. In addition, clonidine (12.5 and 25 μg/kg body weight, IP), a central α-adrenergic agonist known to inhibit renin release, attenuated drinking during 1, 2 and 3 hours after 5-HTP was administered. These results suggest that 5-HTP-induced drinking is mediated by way of the renin-angiotensin system. Haloperidol (150 μg/kg body weight, IP), a dopaminergic antagonist, also attenuated the dipsogenic response to administration of 5-HTP. In addition, incremental reductions in 5-HTP-induced drinking with increasing doses of spiperone (37.5 to 150 μg/kg body weight, IP), a more potent dopaminergic antagonist, were demonstrated. Thus, the dipsogenic response to administration of 5-HTP to rats is dependent on both the renin-angiotensin system and an intact dopaminergic pathway.     

Teratogenic effects of carboplatin, an oncostatic drug, administered during the early organogenetic period in rats

In the preceding study, no teratological effects against rat fetuses were observed when carboplatin, an oncostatic platinum coordination complex, was dosed to their dams from days 7 to 17 of gestation at a dose level of 4 mg/kg/day. However, there are a few reports which show the teratogenic action of carboplatin injected from days 6 to 15 of pregnancy at a dose level of 6 mg/kg/day. In the present study, carboplatin was administered intravenously to pregnant female Crj: CD (Sprague-Dawley) rats from days 6 to 9 or 7 to 10 of gestation at a dose level of 6 mg/kg/day in order to know the teratogenicity of this drug. Carboplatin were highly embryolethal in dams when dosed from days 6 to 9 of gestation, but not in animals when injected from days 7 to 10 of pregnancy. Carboplatin also produced external, internal and skeletal anomalies in fetuses such as gastroschisis, dilatation of cerebral ventricles, cleft sternum, fused ribs, malformed thoracic vertebra when administered from days 6 to 9 of gestation, but not in conceptuses when dosed from days 7 to 10 of pregnancy. However, the delayed ossification which was ascribed to the fetal growth retardation was observed in rats treated with this drug during both administration periods. These results suggest that carboplatin is embryotoxic, inducing intrauterine death and congenital malformations in rats, when injected during the early stages of gestation including day 6 of pregnancy.     

Acute immobilization stress reduces (±DOI)-induced 5-HT2A receptor-mediated head shakes in rats

Acute immobilization stress induced by taping four limbs, applying tail pinch stress and electric foot shock stress immediately reduced the frequency of head shakes induced by 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane ((±)DOI), a 5-HT_2A/C agonist in rats. Immobilization stress due to the use of cylinder restraint and forced swimming did not affect 5-HT_2A-mediated behavior. Acute immobilization stress did not affect [³H]ketanserin binding to the 5HT_2A receptor in the prefrontal cortex and hippocampus. Presynaptic serotonergic lesions with 5,7-dihydroxytryptamine (5,7-DHT) did not affect the reduction in 5-HT_2A-mediated behavior after acute immobilization stress. The decreases in head shake frequency after acute immobilization stress by taping were attenuated by pretreatment with diazepam (2.5 mg/kg IP): This attenuation was reversed by pretreatment with flumazenil (10 mg/kg IP). The reduction in (±)DOI- induced 5-HT_2A-mediated behavior caused by stress may be related to a change in agonist affinity to the receptor or changes in other neurotransmitter systems or the effect of PI turnover.     

Pharmacokinetics of intravenously administered L-5-hydroxytryptophan in man.

The pharmacokinetics of 5-hydroxytryptophan were investigated in relation to intravenous administration of single doses of the amino acids (0.2 mg/kg b.wt.) to five patients pretreated for one week with carbidopa, an L-aromatic amino acid decarboxylase inhibitor. The plasma concentration/time lapse of 5-hydroxytryptophan exhibited bi-exponential disposition characteristics and the data obtained could be closely fitted to an open two compartment pharmacokinetic model with elimination taking place from the central compartment. The apparent composite 1. order rate constants alpha and beta were 1.433 hr-1 +/- 0.177 S.D. and 0.113 hr-1 +/- 0.012 hr-1 S.D., respectively. The biological half-life of 5-hydroxytryptophan under the experimental conditions was thus about 6 hours. The apparent volume of the central compartment, Vc, was found to be 0.336 1 kg-1 +/- 0.059 S.D. and the plasma clearance 0.105 1 hr-1 kg-1 +/- 0.015 S.D. The derived pharmacokinetic constants were used for doses regimen calculations and predetermination of the plasma concentration/time lapse, during continuous intravenous infusion therapy. Measured plasma concentrations of 5-hydroxytryptophan during infusion deviated by less than 10 per cent from the predicted values. However, the infusions had to be stopped in four of the experiments because the patients became nauseated and vomited after total doses of only 36--128 mg.     

Changes in brain serotonin turnover,body and head shakes in kainic acid-treated rats

Kainic acid induces seizures and neurotoxicity in rats, produces changes in brain serotonin (5-HT), dopamine and noradrenaline metabolites among other changes in neurotransmitters. In this work, we investigated the changes in 5-HT turnover in brain regions from 84 rats intraperitoneally injected with kainic acid and a specific behavioural change, the body and head shakes, exerted by this neurotoxin in the presence of 5-HT receptor antagonists. Kainic acid produced an increase in 5-hydroxyindoleacetic acid levels in frontal cortex (212%; 180%), striatum (177%; 116%), amygdala (202%; 337%) and hippocampus (43%; 70 %) at 2 and 24 hr as compared with controls, respectively. Serotonin turnover was increased in amygdala (157%) and frontal cortex (169%) at 2 hr; whereas 24 hr after kainic acid administration, increases were observed in amygdala (207%), and frontal cortex (178%). Kainic acid also produced an increase in the frequency of head and body shakes when administered alone or together with pargyline, a monoamine oxidase inhibitor; whereas the administration of 5-HT receptor antagonists such as ketanserin and methiothepin, decreased this behaviour 54% and 50% as compared with kainic acid alone, respectively. These results suggest an active participation of 5-HT neurotransmission on the excitotoxic action of kainic acid in the brain.     

Oral dosage study of miporamicin administered during the period of fetal organogenesis in rats

Experiments were conducted to assess the effects of miporamicin (MPM) on prenatal and postnatal development of fetuses and offsprings of rats receiving the compound at oral dosages of 40, 200, or 1,000 mg/kg/day during the organogenesis stage of gestation. The drug treatment had no appreciable effect on maternal body weight during pregnancy or lactation period. The rats showed decreased food intake and increased water intake during gestation period in the groups given greater than or equal to 200 mg/kg/day, and increased food and water intakes during lactation period in the group given 1,000 mg/kg/day. There was no macroscopic evidence of changes indicative of any effect of the treatment in the viscera of rat dams at terminal necropsy. Observation of the fetuses did not reveal any effect of the treatment with MPM with respect to the number of implantations, the number of living fetuses, the death rate of fetuses or incidence of external, visceral, or skeletal anomalies. Male fetal weights were low in the groups given greater than or equal to 200 mg/kg/day. Observation of the offspring post partum failed to disclose any abnormalities indicative of adverse effects of the treatment with respect to birth index, viability index, weaning index, postnatal external differentiation, body weight changes, external morphology, skeleton, viscera, organ weight, functional and behavioral tests, emotion, learning ability, or reproductive performance, or in respect of prenatal development of their fetuses (F2). It is concluded from the results that no effect dose levels of MPM was 40 mg/kg/day in rat dams, 40 mg/kg/day also for their fetuses, and 1,000 mg/kg/day for postnatal development of the offspring under the experimental conditions described.     

Effects of triiodothyronine on the 5-hydroxytryptophan-induced head twitch and its potentiation by antidepressants in mice

Daily injection of triiodothyronine (T3) for 3 consecutive days dose dependently enhanced 1-5-HTP (4 mg/kg)-induced head twitches in mice. This effect was blocked by 1-penbutolol. Pretreatment with a sub-effective dose of T3 (0.06 mg/kg) markedly enhanced the ability of clenbuterol but not indalpine to potentiate the response to 1-5-HTP. Likewise, the effects of T3 were more pronounced on the desipramine- or maprotiline-induced potentiation of head-twitches than on the action of citalopram or clomipramine. These data suggest that the increased responsiveness to 1-5-HTP caused by T3 involves an indirect (noradrenalin-mediated) rather than a direct effect on serotonergic processes.     

Harmine-, LON-954- and 5-hydroxytryptophan-induced tremors in rats withdrawn from ethanol

The tremors induced by harmine, LON-954 (N-carbamoyl-2-(2,6-dichlorophenyl)acetamidine hydrochloride) and 5-hydroxytryptophan (5-HTP) were studied in control rats and in rats withdrawn for 16-48 hrs from 6 to 9 days' ethanol administration. The frequencies and the intensities of the tremors were determined electronically. In control rats the frequency spectra of the tremors induced by harmine (20 mg/kg) and LON-954 (10 mg/kg) showed a narrow peak frequency at about 10 Hz. Atropine (1.2 mg/kg) altered neither the frequency nor the intensity of these tremors. 5-HTP (50 mg/kg) when given 3.5 hrs after iproniazid (100 mg/kg) induced tremor with peak frequencies at 6-7 Hz and 12 Hz. In ethanol-withdrawn rats treated with harmine or LON-954 the frequency analysis of tremor revealed a narrow peak frequency at about 12 Hz, which was neither the characteristic frequency of ethanol withdrawal tremor (6 Hz) nor that of harmine or LON-954 (10 Hz). The intensity of both harmine- and LON-954-induced tremor was significantly increased in ethanol-withdrawn rats. The ethanol-withdrawn rats were markedly sensitized to the effect of iproniazid+ 5-HTP, shown by deaths. The peak frequencies of this tremor were the same as those in control rats. The results suggest that harmine-induced tremor involves a dopaminergic-5-HT'ergic imbalance and the tremor induced by LON-954 a dopaminergic-cholinergic imbalance in the brain. The tremor in ethanol-withdrawn rats seems to be mediated by alterations in the activity of the cerebral 5-HT'ergic system.     

A strain dependent sex difference in ouabain-induced cardiotoxicity in rats

Male rats of the BDIX strain were about twice as sensitive to the lethal effects of intravenously infused ouabain than were their female counterparts. This sex difference in ouabain sensitivity was not exhibited by rats of the Wistar strain. In bile duct-cannulated animals, ouabain biliary excretion was much faster in female BDIX rats. This finding could partly explain the greater ouabain resistance, although certain aspects of the cardiac/plasma ouabain concentration data did suggest that there may have been a component of the sex-related resistance that was not explicable on a pharmacokinetic basis. The faster ouabain biliary excretion in female rats could also partly explain the fact that for similar rates of infusion, the plasma ouabain concentration during infusion was similar in both sexes, despite the body size of the female rats being about one-third smaller.     

Production of wet-dog shakes in rats and down-regulation of spinal 5-HT2 receptors

5-HT2 receptors, labelled with [3H]ketanserin, were shown to occur in a low concentration in the spinal cord of rats. The receptor numbers were reduced by 37% within 3 h of giving paroxetine to phenelzine-treated rats and remained low for at least 24 h. Paroxetine also evoked wet-dog shakes. These were of similar frequency in spinal (Cl transection) and intact rats and lessened within 3 h, perhaps because of receptor down-regulation.     

Role of central 5-HT2 receptors in mediating head bobs and body shakes in the rabbit

Systemic administration of the 5-HT(2A/2C) agonist DOI [(1(2,5-dimethoxy-4-iodophenyl)-2-aminopropane)hydrochloride] in rabbits elicits head bobs and body shakes, which are mediated by 5-HT(2A) and 5-HT(2C) receptors, respectively. This study was designed to determine whether the receptors mediating these behaviors are primarily located in the brain or in the periphery. Systemic administration of the peripheral 5-HT(2A/2C) antagonist xylamidine 30 min before systemic DOI challenge attenuated DOI-elicited body shakes by 50% without an effect on head bobs, suggesting a central origin for head bobs and a partial peripheral and a partial central origin for body shakes. Central administration of DOI into the lateral ventricle (ICV) elicited head bobs but not body shakes, demonstrating that the receptors mediating head bobs are centrally located. Pretreatment with ICV xylamidine blocked head bobs elicited by ICV DOI, indicating that the lack of inhibition, when systemically administered, is due to xylamidine's failure to reach central receptors. ICV pretreatment with the 5-HT(2A) receptor antagonist ketanserin inhibited ICV DOI-elicited head bobs establishing that 5-HT(2A) receptors activation elicits head bobs. In conclusion, 5-HT(2A) receptors mediating head movements are located in the brain whereas 5-HT(2C) receptors mediating the body movements appear to be located at different central sites as well as in the periphery.     

Plasma accumulation of metabolism of orally administered single dose L-5-hydroxytryptophan in man

Single oral doses of L-5-hydroxytryptophan (5-HTP) were administered in combination with L-aromatic amino acid decarboxylase inhibitors. The time courses of plasma concentrations of 5-HTP, 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) and the concentrations of 5-HT in blood platelets were measured. Carbidopa enhanced the rise in plasma concentrations of 5-HTP 5-15 fold and counteracted the increase in plasma 5-HIAA levels induced by 5-HTP alone. A single dose of the decarboxylase inhibitor was equipotent to 14 days' pretreatment. Plasma or platelet concentrations of 5-HT failed to reflect the metabolism of 5-HTP. The ratio of 5-HTP to carbidopa influenced the systemic bioavailability of single dose administered 5-HTP indicating dose dependent absorption kinetics. Co-administration of L-dopa with 5-HTP and decarboxylase inhibitors had no effect on gastrointestinal absorption of 5-HTP in six parkinsonian patients.     

Reproductive and developmental toxicity study of T-3762 in rats administered intravenously during the period of organogenesis

A teratogenicity study of T-3762, an injectable new quinolone antibacterial agent, was conducted in Sprague-Dawley rats to determine the effects on dams and next generations. T-3762 was administered intravenously to pregnant rats at the dose levels of 26, 78 and 156 mg/kg/day from day 7 to day 17 of gestation, during the organogenesis. 1. In the dams, there were no effects on general condition, food intake, water intake and body weight in the T-3762 treated groups. There were no abnormal findings on the autopsy at the end of gestation and lactation periods in the T-3762 treated groups. 2. In the fetuses, there were no effects on the number of dead and live fetuses, sex ratio and body weight in the T-3762 treated groups. No external, visceral and skeletal abnormalities attributed to T-3762 were observed. 3. In the offspring, there were no effects on birth rate, viability, differentiation of external development, body weight, sensory function, emotionality, learning ability and reproductive performance in the T-3762 treated groups. From these results, no-toxic dose levels of T-3762 are considered to be 156 mg/kg for the general toxicity and the reproductive toxicity of parents and for the development of next generation, respectively.     

Effects of antidepressant drug combinations on cortical 5-HT2 receptors and wet-dog shakes in rats

Rats pretreated with the monoamine oxidase inhibitor, phenelzine 18 h (46.8 mg/kg) and 90 min (11.7 mg/kg) previously or only 90 min (46.8 mg/kg) previously developed a 5-HT dependent syndrome (including wet-dog shakes, WDS) when given the 5-HT uptake inhibitor, paroxetine (11.6 mg/kg). After 2 h, but only in rats pretreated with 2 injections of phenelzine, there was a gradual reduction in the number of cortical 5-HT2 receptors, determined in vitro with [3H]ketanserin, and this was temporally related to a reduction in the frequency of WDS. Both effects (down-regulation and WDS) were prevented by the 5-HT2 receptor antagonist, pirenperone. A second injection of paroxetine at 3 h evoked additional WDS in rats pretreated with 1 injection of phenelzine but not in rats pretreated with 2 injections, suggesting that spinal 5-HT2 receptors might also have been down-regulated at the same time. Similar results were obtained when rats were pretreated instead with the selective MAO A inhibitor, clorgyline or when given either citalopram or fenfluramine instead of paroxetine. 5-HTP also evoked WDS in phenelzine-treated rats and markedly increased brain 5-HT concentration but only slowly down-regulated 5-HT2 receptors; in carbidopa-treated animals, 5-HTP was without effect on receptor numbers despite production of frequent WDS. It thus appears that drugs which increase synaptic 5-HT (as indicated by production of WDS) by interference with the release or reuptake of 5-HT more readily down-regulate 5-HT2 receptors than 5-HTP which does not directly affect these mechanisms.     

Inhibition by antiepileptics of carbachol but not lithium- or 5-methoxytryptamine-induced wet dog shakes in rats

The influence of antiepileptic drugs on the wet dog shakes (WDS) induced by intracerebroventricular injections of carbachol (30 micrograms icv) was investigated in rats. Diphenylhydantoin (DPH, 8 and 4 mg/kg), diazepam (0.4, 0.2 and 0.1 mg/kg), phenobarbital (12.5, 6.25 and 3.12 mg/kg), sodium valproate (Depakine, 200, 100 and 50 mg/kg) and trimethadione (200, 100 and 50 mg/kg) given ip inhibited the WDS in a dose-dependent manner. These drugs at the same doses did not change the intensity of shaking behavior induced by lithium chloride or 5-hydroxytryptamine. As the antiepileptic drugs tested in these experiments did not have anticholinergic activity and at used doses were not able to prevent electrical convulsions or pentetrazol-induced seizures, it appears that carbachol-induced WDS could be connected with convulsive activity and could be the initial stage of seizures.     

Kinetics of an intratracheally administered chromium catalyst in rats

Chromium-based catalysts are used for the synthesis of polyethylene, but little is known about the hazard and biomonitoring possibilities of this type of chromium for workers who may be occupationally exposed to such compounds. Therefore, the bioavailability and toxicokinetics of chromium were studied in male Wistar rats after a single intratracheal instillation (2 ml/kg body weight) of various doses (1, 5, or 25 mg/kg body weight) of the catalyst (approximately 1% chromium bound to an amorphous silica matrix), either before (CAT-Cr[III]) or after (CAT-Cr[VI]) heat treatment. The results were compared with those of equivalent amounts of two chromium salts (CrCl(3) and K(2) Cr(2) O (7). Each dose group was composed of three rats. The concentration of chromium was determined by atomic absorption spectrometry in urine (collected daily for 7 d) and in plasma, erythrocytes, lung, and liver tissue obtained 2 d (only highest concentration) and 7 d after dosing. On d 2, a significant increase in lung weight was found in the animals treated with the highest dose of the hexavalent Cr products. On d 7, on the basis of body weights, lung weights, and lung histology, there was no overt toxicity, except after the highest dose of CAT-Cr(VI). The elimination of all forms of chromium was apparently monoexponential, with calculated half-life elimination times in urine of 4-11 h for Cr(III) (CAT-Cr[III] and CrCl3 ) and 8-21 h for Cr(VI) (CAT-Cr[VI] and K(2) Cr(2) O(7). On d 2, the erythro-cytes Cr concentrations were significantly higher for the hexavalent Cr products than for the trivalent Cr products. After 7 d, the erythrocytes Cr concentrations were significantly increased above control values (3 microg/L) only in rats treated with the 2 highest doses of Cr( VI) compounds (12 and 64 microg/L for K(2) Cr(2) O(7), and 14 and 79 microg/L for CAT-Cr[VI]). The present study shows that intratracheally instilled Cr(VI) and Cr(III) have different toxicokinetic profiles and that the Cr(VI) catalyst has the same bioavailability and excretion kinetics as a water-soluble Cr(VI) salt. Exposure to chromium compounds could be monitored by measuring Cr concen-trations in urine (shortly after exposure) and in erythrocytes (also at later time points after high Cr[VI] exposure).     

Peripheral conversion of L-5-Hydroxytryptophan to serotonin induces drinking in rats

Female rats administered serotonin (0.25 to 4.0 mg/kg, s.c.) showed a dose-dependent increase in water intake. The dipsogenic response was nearly maximal when 2 mg/kg was administered s.c. and plateaued by 2 hr after treatment. l-5Hydroxytryptophan (5-HTP), the precursor of serotonin, is also a potent dipsogen which induces drinking by way of the renin-angiotensin system. The possibility that the dipsogenic activity of 5-HTP is dependent on decarboxylation to serotonin was the objective of these studies. Either benserazide (30 mg/kg. s.c.), a central and peripheral decarboxylase inhibitor, or carbidopa (6.5 mg/kg, s.c.), a peripheral decarboxylase inhibitor, was administered 15 min prior to the dipsogen. Both decarboxylase inhibitors attenuated the dipsogenic response to 5-HTP (25 mg/kg, s.c.) but not to serotonin (2 mg/kg, s.c.). The peripheral serotonergic receptor antagonist, methysergide (3 mg/kg, i.p.), blocked the dipsogenic responses to both 5-HTP (25 mg/kg, s.c.) and serotonin (2 mg/kg, s.c.). There was no interaction between 5-HTP (18 mg/kg, s.c.) and serotonin (1 mg/kg, s.c.) when administered simultaneously with respect to their dipsogenic effects. Thus, the drinking response accompanying administration of 5-HTP occurs following peripheral conversion to serotonin which, in turn, activates peripheral serotonergic receptors. The mechanism(s) by which activation of peripheral serotonergic receptors increases water intake is not known, but appears to involve release of renin from the kidney.     

Effects of monoamine reuptake inhibitors on wet-dog shakes mediated by 5-HT2A receptors in ACTH-treated rats

We examined the influence of imipramine, a serotonin (5-HT) and noradrenaline (NA) reuptake inhibitor, desipramine, a NA reuptake inhibitor, bupropion, a dopamine reuptake inhibitor, fluvoxamine, a selective 5-HT reuptake inhibitor, and mazindol, a catecholamine reuptake inhibitor, on a 5-HT2A receptor-mediated behavior, (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI)-induced wet-dog shakes, in naive and adrenocorticotropic hormone (ACTH)-treated rats. Chronic administration of imipramine, desipramine and mazindol suppressed the number of wet-dog shakes in naive rats. Chronic ACTH (100 microg/rat, s.c.) treatment increased the number. Chronic administration of imipramine did not decrease the number of wet-dog shakes in ACTH-treated rats. On the other hand, desipramine and mazindol inhibited the increase in wet-dog shakes in ACTH-treated rats. Fluvoxamine and bupropion did not have any effect on the (+/-)-DOI-induced response in naive and ACTH-treated rats. NA reuptake inhibitors may improve the hyperfunction of 5-HT2A receptors induced by chronic ACTH treatment.     

Dose-response relationships between systemically administered L-tryptophan or L-5-hydroxytryptophan and raphe unit activity in the rat.

In contrast to previous reports that l-5-hydroxytryptophan (l-5-HTP) is without effect on raphe unit activity, systemic administration of l-tryptophan or l-5-HTP produced similar dose-dependent decreases in raphe unit activity, ranging from 0–15% at 10 mg/kg to 60–90% at 100 mg/kg doses. The dose-response curve forms a plateau for doses above 100 mg/kg of l-5-HTP or l-tryptophan. Neurochemical studies revealed that administration of varying doses of l-tryptophan or l-5-HTP (10–100 mg/kg, i.p.) under conditions simulating those of the unit recording procedure, produced dosedependent increases in forebrain serotonin of up to 42% and 353% for a 100 mg/kg dose of l-tryptophan or l-5-HTP, respectively. These data suggest that exogeneously administered l-5-HTP serves as an effective serotonin precursor in central serotonergic neurones. Furthermore, increases in forebrain serotonin levels within the range of normal diurnal variations dramatically depress raphe unit activity, indicating that diurnal changes in brain serotonin metabolism may significantly affect the activity of serotonin-containing neurones.     

Adrenal function affects morphine-induced feeding during dark period, but not during light period in rats

The present study was undertaken to investigate the relationships between morphine-induced feeding and the adrenal functions. Morphine (5 mg/kg) was intraperitoneally administered at 10:45 (light period) or 18:45 (dark period). The orectic effects of morphine during the light period in normal rats were not influenced by adrenalectomy; however, the anorectic effects during the dark period in normal rats were attenuated by both adrenalectomy and adrenodemedullation. Corticosterone (10 mg/kg) itself had no effects on feeding during the light and dark period. Morphine did not alter blood insulin levels during the light period, but markedly decreased it during the dark period independently of feeding. These results show that morphine has two different effects on feeding by administration time, and they suggest that the adrenal affects morphine-induced feeding only during the dark period (hungry state), presumably through insulin release, but not during the light period (satiated state).