Serotonergic disturbances in autistic disorder: L-5-hydroxytryptophan administration to autistic youngsters increases the blood concentrations of serotonin in patients but not in controls

Some studies have suggested that disorders in the peripheral and central metabolism of serotonin (5-HT) may play a role in the pathophysiology of autistic disorder. This study examines the whole blood concentrations of 5-HT and 5-hydroxy-indoleacetic acid (5-HIAA) in baseline conditions and during a challenge with L-5-OH-tryptophane (5-HTP; 4 mg/kg in non enteric-coated tablets), the precursor of 5-HT, in a study group of 18 male, post-pubertal, Caucasian autistic patients (age 13-19 y.; I.Q.>55) and 20 matched healthy volunteers. In baseline conditions, no significant differences in 5-HT or 5-HIAA levels could be found between autistic youngsters and normal controls. 5-HTP administration significantly increased the levels of 5-HT in autistic youngsters but not in normal controls. Following 5-HTP challenge the 5-HT levels were significantly higher in autistic patients than in healthy volunteers. After challenge with 5-HTP, no significant differences were found in the concentrations of 5-HIAA or the test substance between autistic youngsters and normal controls. Differences in the peripheral metabolism of 5-HT which may not be observed in baseline conditions but which became clear after loading with 5-HTP, suggest that an increased synthesis of 5-HT from its precursor 5-HTP might be a one factor responsible for differences in the serotonergic system between autistic post-pubertal youngsters and normal controls.     

Mechanism of prolactin release by 5-hydroxytryptophan

Male Wistar rats were intraperitoneally administered 300 mg/kg b.w. of α-methyl-p-tyrosine methyl ester(α-MT). These α-MT pretreated rats were anesthetized with urethane and then 5% glucose or dopamine (1 μg/kg b.w./min) was infused for 45 min. At 1 min before or 15 min after dopamine infusion, 10 or 50 mg/kg of 5-hydroxytryptophan (5-HTP) was injected intraperitoneally, and blood samples were taken from the jugular vein for prolactin determination. In rats treated with α-MT, the administration of 5-HTP increases the serum prolactin level in a dose-related manner. Dopamine infusion caused a marked decrease in serum prolactin level. The concomitant administration of dopamine and 5-HTP prevented the dopamine-induced decrease of serum prolactin in α-MT treated rats. These results indicate that the serotonergic stimulus enhanced prolactin release, not by inhibiting the dopaminergic activity, but by stimulating a prolactin-releasing factor or by activating other neurotransmitter systems.     

Tramadol reduces the 5-HTP-induced head-twitch response in mice via the activation of mu and kappa opioid receptors

Tramadol, an atypical opioid analgesic, stimulates both opiatergic and serotonergic systems. Here we have investigated the effect of tramadol in mice on 5-hydroxyptrytophan (5-HTP)-induced head twitch response (HTR), which is an animal model for the activation of the CNS 5-HT(2A) receptors in mice. Tramadol attenuated 5-HTP-induced HTR in a dose-dependent manner as morphine. Furthermore, the nonselective opioid receptor antagonists, naloxone and diprenorphine (M5050), reversed the effect of tramadol on 5-HTP-induced HTR dose-dependently. Interestingly, in contrast to the selective delta opioid receptor antagonist NTI, beta-FNA, a selective mu receptor antagonist, and nor-BNI, a selective kappa opioid receptor antagonist, antagonized the attenuation of 5-HTP-induced HTR by tramadol. In conclusion, administration of tramadol systemically inhibits 5-HTP-induced HTR in mice by activating opiatergic system in the CNS. Our findings show that mu and kappa opioid receptors, but not delta opioid receptor, play an important role in the regulation of serotonergic function in the CNS.     

Evidence that serotonin neurons stimulate secretion of prolaction releasing factor.

The purpose of the present study was to determine if serotonin was stimulatory to prolactin release by inhibition of the dopaminergic system or by stimulating release of a prolactin releasing factor (PRF). We measured the amount of prolactin secreted after administration of 30 mg/kg of 5-hydroxytryptophan (5-HTP) to male rats pretreated with fluoxetine (10 mg/kg) and compared it with the amount of prolactin released in male rats treated with αmethyl-p-tyrosine methyl ester (αMT) or various dopamine receptor blocking agents. In every experiment the serotonergic stimulus provided by 5-HTP in fluoxetine-pretreated rats released considerably more prolactin than did treatment with αMT or dopaminergic blockers. We conclude that serotonin releases prolactin not by inhibiting dopaminergic neurons but rather by stimulating the release of PRF.     

Enhanced serum cortisol response to 5-hydroxytryptophan in depression and mania

The serum cortisol responses to D, L-5-hydroxytryptophan (5-HTP), 200 mg per oral, in unmedicated depressed and manic patients, were both significantly greater than that of normal controls. The cortisol response to 5-HTP in depressed patients was significantly correlated with ratings of specific symptoms of depression. It was also greater in non-psychotic than in psychotic depressed patients as well as in those manic or depressed patients who attempted suicide compared to those who had not. In view of evidence for decreased brain serotonergic activity in depression and perhaps mania, the results suggest at least some serotonin receptors may be supersensitive in some patients with affective disorders.     

Inhibitton of prolactin release by stimulation of presynaptic serotonin autoreceptors

The effects of 5-methoxy-N, N-dimethyltryptamine (5-MeODMT), a serotonin agonist with a preferential action on presynaptic autoreceptors, on prolactin release in male rats was determined. Basal serum prolactin levels were not altered after administration of 1.0, 2.0, 5.0, 10.0 or 20.0 mg/kg of 5-MeODMT.Pretreatment with 5-MeODMT reduced prolactin release by agents that depend on serotonergic neurotransmission for part of their prolactin release stimulation. Prolactin release in response to L-5-hydroxytryptophan (5-HTP) or morphine was significantly reduced by pretreatment of the rats with 5-MeODMT.The results of this experiment indicate that 5-MeODMT act as a presynaptic serotonin autoreceptor stimulant and not as a postsynaptic serotonin agonist on the neuronal systems that control prolactin release.     

Hyperleptinemia elicited by the 5-HT precursor, 5-hydroxytryptophan in mice: involvement of insulin

Mechanisms for hyperleptinemia elicited by a serotonin (5-hydroxytryptamine, 5-HT) precursor, 5-hydroxytryptophan (5-HTP), were investigated. 5-HTP elicited apparent increases in serum leptin levels of mice. Administration of 5-HTP did not alter expression of leptin mRNA in white adipose tissues. Furthermore, neither 5-HTP nor 5-HT increased leptin secretion from isolated fat pads of mice. Since insulin is known to enhance leptin release, involvement of insulin in 5-HTP-induced hyperleptinemia was examined. 5-HTP significantly elevated serum insulin levels. In mice treated with streptozotocin, which depletes insulin, 5-HTP did not increase serum leptin levels. These results suggest that hyperinsulinemia participates the elevation of serum leptin levels elicited by 5-HTP.     

Serotonin regulation of aldosterone secretion

The circulating levels of aldosterone (A), cortisol (F), prolactin, ACTH and potassium and the PRA were studied in 8 (6 males and 2 females) healthy normotensive subjects after 5-hydroxy-tryptophan (5OHT), or pizotifen (Piz) or placebo oral administration. In the same subjects 5OHT was administered twice: after placebo and after dexamethasone pretreatment. The results showed a significant increase of A, ACTH and F after 5OHT plus placebo administration without any change of PRA, potassium or prolactin levels; dexamethasone pretreatment suppressed ACTH and F but was uneffective on the response of A to 5OHT. Only A levels showed a significant decrease after Piz administration, the other studied parameters were unaffected by the blockade of the 5HT2 receptors by Piz. The administration of placebo induced a slight but not significant decrease of the studied parameters. Our results suggest the existence of a physiologic serotonergic control of A secretion, a pituitary factor could be one of the putative links between the central serotonergic activation and the adrenal secretory response.     

Serotonergic stimulation of pituitary-adrenocortical activity in rats: evidence for multiple sites of action

5-Hydroxytryptophan (5-HTP) elevated serum corticosterone concentrations when administered either intraperitoneally (i.p.) or intraventricularly. Inhibition of aromatic L-amino acid decarboxylase outside of the blood-brain barrier antagonized the corticosterone response, but only when the 5-HTP was given i.p. Stimulation of the pituitary-adrenocortical system by fenfluramine was not affected by 5,7-dihydroxytryptamine pretreatment, whereas the stimulation produced by quipazine administration was blocked by lesions of the basomedial hypothalamus. These results suggest that serotonergic drugs can act at multiple sites (i.e., both central and peripheral) to evoke a pituitary-adrenocortical response.     

Reverse tolerance to amphetamine evokes reverse tolerance to 5-hydroxytryptophan

Repeated intermittent administration of amphetamine in mice caused reverse tolerance to 5-hydroxy-L-tryptophan (5-HTP)-induced head twitch, as well as to amphetamine-induced stereotypy. The repeated administration of 5-HTP alone also resulted in reverse tolerance in the head-twitch test. Daily pretreatment with haloperidol prior to amphetamine administration blocked the development of both reverse tolerance to amphetamine and to 5-HTP, whereas daily pretreatment with cyproheptadine prior to amphetamine blocked only the reverse tolerance to 5-HTP. On the other hand, 5-HTP-induced reverse tolerance was blocked by daily pretreatment with cyproheptadine, but not with haloperidol. There appears to be no difference in the persistence of the reverse tolerance to 5-HTP, whether induced by amphetamine or by 5-HTP; in both instances, the persistence does not correlate with the persistence of reverse tolerance to amphetamine. The data suggest that the reverse tolerance to amphetamine and the associated reverse tolerance to 5-HTP are independent events, both of which are mediated by dopaminergic mechanisms.     

5-Hydroxytryptophan-induced respiratory recovery after cervical spinal cord hemisection in rats.

The present study investigates the role of serotonin in respiratory recovery after spinal cord injury. Experiments were conducted on C(2) spinal cord hemisected, anesthetized, vagotomized, paralyzed, and artificially ventilated rats in which end-tidal CO(2) was monitored and maintained. Before drug administration, the phrenic nerve ipsilateral to hemisection showed no respiratory-related activity due to the disruption of the descending bulbospinal respiratory pathways by spinal cord hemisection. 5-Hydroxytryptophan (5-HTP), a serotonin precursor, was administrated intravenously. 5-HTP induced time- and dose-dependent increases in respiratory recovery in the phrenic nerve ipsilateral to hemisection. Although the 5-HTP-induced recovery was initially accompanied by an increase in activity in the contralateral phrenic nerve, suggesting an increase in descending respiratory drive, the recovery persisted well after activity in the contralateral nerve returned to predrug levels. 5-HTP-induced effects were reversed by a serotonin receptor antagonist, methysergide. Because experiments were conducted on animals subjected to C(2) spinal cord hemisection, the recovery was most likely mediated by the activation of a latent respiratory pathway spared by the spinal cord injury. The results suggest that serotonin is an important neuromodulator in the unmasking of the latent respiratory pathway after spinal cord injury. In addition, the results also suggest that the maintenance of 5-HTP-induced respiratory recovery may not require a continuous enhancement of central respiratory drive.     

Effects of naloxone on the secretion of prolactin and corticosterone induced by 5-hydroxytryptophan and a serotonergic agonist, mCPP

The effects of naloxone, an opiate “pure” receptor antagonist, on the release of prolactin and corticosterone in the rat were studied following the administration of the serotonin precursor 5-hydroxytryptophan or the serotonin receptor agonist (?) -$\text{m}$-chloropnehylpiperazine. Naloxone clearly antagonizes the release of prolactin induced by 5-hydroxytryptophan administered alone at a dosage of 50 mg/Kg/b.wt. or at dosage of 30 mg/Kg/b.wt. preceded 60 minutes before injection by the administration of the serotonin uptake blocker fluoxetine. The opiate antagonist does not modify the increase in blood level of prolactin induced by (?) ?$\text{m}$-chlorohenylpiperazine. Naloxone itself does not reduce the increase in plasma level of corticosterone induced by 5-hydroxytryptophan, 5-hydroxytryptophan +fluoxetine or (?)?$\text{m}$-chlorophenylpiperazine.The results suggest that endogenous opioids may be involved in the increase in serum level of prolactin induced by 5-hydroxytryptophan and also indicate the existence of different serotonergic neurotransmitter circuits capable of modulating the release of prolactin and corticosterone. A mutual interplay between serotonergic and opiate neurons may be involved in controlling the release of prolactin, but such an interplay does not seem to occur in the secretion of corticotrophin-releasing hormone.     

Effect of hypothalamic surgery on prolactin release induced by 5-hydroxytryptophan (5-HTP) in rats.

Intravenous injections of varying doses of 5-HTP (1, 3 and 5 mg/100 g body wt), a precursor of serotonin, caused a significant and dose-related increase in plasma prolactin concentrations in urethane-anesthetized rats. Increases in plasma prolactin concentrations caused by 5-HTP (1 mg/100 g body wt iv) were abolished by the concomitant administration of L-DOPA (2 mg/100 g body wt iv). Plasma prolactin levels were also significantly elevated following the injection of 5-HTP in rats with complete hypothalamic deafferentation, whereas 5-HTP had no significant effect on plasma prolactin levels in rats with extensive hypothalamic ablation. These results suggest that 5-HTP causes prolactin secretion by stimulating the serotoninergic mechanism in the hypothalamus.     

Higher environmental temperature-induced increase in body temperature: Involvement of serotonin in GABA mediated interaction of opioidergic system

Exposure (2 h) of adult male albino rats to higher environmental temperature (HET, 40°C) significantly increased body temperature (BT). Administration of (a) 5-HTP (5 mg/kg, i.p.) or morphine (1 mg/kg, i.p.) or physostigmine (0.2 mg/kg, i.p.) alone significantly increased and (b) methysergide (1 mg/kg, i.p.) or naloxone (1 mg/kg, i.p.) or atropine (5 mg/kg, i.p.) reduced the BT of both normal and HET exposed rats. Further, it was observed that morphine prevented the methysergide-induced hypothermia and 5-HTP potentiated the morphine-induced hyperthermia in both normal and HET exposed conditions. Biochemical study also indicates that serotonin metabolism was increased but GABA utilization was reduced following exposure to HET. 5-HTP or bicuculline-induced hyperthermia in control and HET exposed rat was potentiated with the coadministration of bicuculline and 5-HTP. The cotreatment of bicuculline with methysergide prevented the methysergide-induced attenuation of BT of heat exposed rat, rather BT was significantly enhanced indicating that inhibition of GABA system under heat exposed condition may activate the serotonergic activity. Further (a) enhancement of (i) morphine-induced hyperthermia with physostigmine (ii) physostigmine- or morphine + physostigmine-induced increase of BT with 5-HTP and (b) reduction of (i) morphine- or morphine + 5-HTP-induced hyperthermia with atropine and (ii) atropine-induced hypothermia with 5-HTP in both normal and HET exposed conditions suggest that HET exposure activates the cholinergic system through the activation of opioidergic and serotonergic system and hence increased the BT. Thus, it may be concluded that there is an involvement of serotonergic regulation in the opioidergic-cholinergic interaction via GABA system in HET-induced increase in BT.     

Selective serotonin reuptake inhibitors and neuroendocrine function.

Selective serotonin (5-HT) reuptake inhibitors (SSRIs) are effective drugs for the treatment of several neuropsychiatric disorders associated with reduced serotonergic function. Serotonergic neurons play an important role in the regulation of neuroendocrine function. This review will discuss the acute and chronic effects of SSRIs on neuroendocrine function. Acute administration of SSRIs increases the secretion of several hormones, but chronic treatment with SSRIs does not alter basal blood levels of hormones. However, adaptive changes are induced by long-term treatment with SSRIs in serotonergic, noradrenergic and peptidergic neural function. These adaptive changes, particularly in the function of specific post-synaptic receptor systems, can be examined from altered adrenocorticotrophic hormone (ACTH), cortisol, oxytocin, vasopressin, prolactin, growth hormone (GH) and renin responses to challenges with specific agonists. Neuroendocrine challenge tests both in experimental animals and in humans indicate that chronic SSRIs produce an increase in serotonergic terminal function, accompanied by desensitization of post-synaptic 5-HT1A receptor-mediated ACTH, cortisol, GH and oxytocin responses, and by supersensitivity of post-synaptic 5-HT2A (and/or 5-HT2C) receptor-mediated secretion of hormones. Chronic exposure to SSRIs does not alter the neuroendocrine stress-response and produces inconsistent changes in alpha2 adrenoceptor-mediated GH secretion. Overall, the effects of SSRIs on neuroendocrine function are dependent on adaptive changes in specific neurotransmitter systems that regulate the secretion of specific hormones.     

Naloxone stimulation of luteinizing hormone release in prepubertal female rats; role of serotonergic system

The purpose of this study was to determine whether naloxone stimulated LH release via a serotonergic mechanism. Injection of naloxone hydrochloride (2 mg/kg B.W.) into 25-day old female prepubertal rats resulted in a significant elevation in serum LH 30 min later. Injection of this dose of naloxone together with morphine sulfate (2 or 5 mg/kg B.E.) resulted in inhibition of naloxone-induced LH release. When rats were first injected with 5-hydroxytryptophan (5-HTP) to increase hypothalamic serotonin content, naloxone failed to increase serum LH levels. On the other hand, when parachlorophenylalanine (PCPA) was given first to reduce hypothalamic serotonin content, naloxone-induced LH release was potentiated. Morphine failed to inhibit the naloxone-induced rise in serum LH when PCPA was first administered. Neither 5-HTP nor PCPA, when injected alone, altered serum LH values. These results suggest that naloxone promotes LH release by reducing hypothalamic serotonergic activity, and morphine inhibits LH release by increasing hypothalamic serotonergic activity. This does not exclude possible involvement of other neurotransmitters.     

Approach to a social stranger is associated with low central nervous system serotonergic responsivity in female cynomolgus monkeys (Macaca fascicularis)

It is widely hypothesized that individual differences in central nervous system (CNS) serotonergic activity underlie dimensional variation in "impulsive" vs. "inhibited" social behavior in both humans and nonhuman primates. To assess relative impulsivity in a social context, a behavioral challenge involving animals' exposure to a social stranger (termed the "Intruder Challenge") was recently validated in adolescent and adult male vervet monkeys (Cercopithecus aethiops sabaeus). Among these animals, monkeys that quickly approached the intruder were found to have lower cerebrospinal fluid (CSF) concentrations of the serotonin (5-HT) metabolite, 5-hydroxyindoleacetic acid, than less impulsive animals. In the present study we extended these observations to determine whether approach to a social stranger, as operationalized by the Intruder Challenge, is similarly associated with diminished CNS serotonergic function in female cynomolgus monkeys (Macaca fascicularis). Study animals were 25 adult monkeys that had been housed for 2 years in stable social groups. In each animal, the rise in plasma prolactin concentration induced by acute administration of the 5-HT agonist, fenfluramine, was used to assess "net" central serotonergic responsivity. When exposed later to an unfamiliar female of the same species in a catch-cage placed for 20 min within the subjects' home enclosure, monkeys that approached to within 1 m of the intruder (median latency to approach=3 min) were found to have significantly smaller prolactin responses to fenfluramine (diminished serotonergic responsivity) compared to "inhibited" animals that failed to approach the intruder (t=2.9, df=23, P<0.009; rpb=-0.51). Neither approach behavior nor the animals' fenfluramine-induced prolactin responses covaried significantly with nondirected expressions of arousal (or anxiety) or with aggressive behaviors exhibited during testing. We conclude that in female cynomolgus monkeys, social impulsivity (vs. inhibition) correlates inversely with individual differences in CNS serotonergic activity, as assessed by neuroendocrine challenge.     

Social status,behavior, and central serotonergic responsivity in female cynomolgus monkeys

The relationship between social status, behavioral characteristics, and central serotonergic function was examined in eight adult female cynomolgus monkeys (Macaca fascicularis). The subjects were housed in four-member social groups for 6 months. Social behavior was observed three times a week. The early follicular phase of the menstrual cycle was determined based on the occurrence of menses, and the prolactin response to acute administration of fenfluramine HCl during the early follicular phase was used as a measure of net brain serotonergic responsivity. Delta prolactin responses (the prolactin response to fenfluramine minus baseline concentrations) were lower in dominant than in subordinate females. Dominant females were more aggressive and less submissive than subordinates. Likewise, females with the lowest delta prolactin concentrations were more aggressive and less submissive than females that had relatively large increases in prolactin. Delta prolactin responses also correlated negatively with percent time eating and investigating in close proximity to penmates, and correlated positively with percent time scanning anxiously while alone. The same pattern of correlation was noted between social status and the aforementioned affiliative behaviors. It was concluded that high central serotonergic responsivity may be associated with low rates of aggression, high rates of submission, and subordinate social status in female cynomolgus monkeys. © 1995 Wiley-Liss, Inc.     

Antagonism of corticotropin-releasing hormone alters serotonergic-induced changes in brain temperature, but not sleep, of rats

Serotonin is involved in many physiological processes, including the regulation of sleep and body temperature. Administration into rats of low doses (25, 50 mg/kg) of the 5-HT precursor l-5-hydroxytryptophan (5-HTP) at the beginning of the dark period of the 12:12-h light-dark cycle initially increases wakefulness. Higher doses (75, 100 mg/kg) increase nonrapid eye movement (NREM) sleep. The initial enhancement of wakefulness after low-dose 5-HTP administration may be a direct action of 5-HT in brain or due to 5-HT-induced activation of other arousal-promoting systems. One candidate arousal-promoting system is corticotropin-releasing hormone (CRH) and the hypothalamic-pituitary-adrenal axis. Serotonergic activation by 5-HTP at the beginning of the dark period also induces hypothermia. Because sleep and body temperature are influenced by circadian factors, one aim of this study was to determine responses to 5-HTP when administered at a different circadian time, the beginning of the light period. Results obtained show that all doses of 5-HTP (25-100 mg/kg) administered at light onset initially increase wakefulness; NREM sleep increases only after a long delay, during the subsequent dark period. Serotonergic activation by 5-HTP at light onset induces hypothermia, the time course of which is biphasic after higher doses (75, 100 mg/kg). Intracerebroventricular pretreatment with the CRH receptor antagonist alpha-helical CRH does not alter the impact of 5-HTP on sleep-wake behavior but potentiates the hypothermic response to 50 mg/kg 5-HTP. These data suggest that serotonergic activation by peripheral administration of 5-HTP may modulate sleep-wake behavior by mechanisms in addition to direct actions in brain and that circadian systems are important determinants of the impact of serotonergic activation on sleep and body temperature.     

Alteration of 5-hydroxytryptophan-induced hypothermia by serotonergic agents in the adult domestic fowl (Gallus domesticus)

• 1.1.|5-Hydroxytryptophan (5-HTP) induced a dose-dependent hypothermia in adult fowls.
• 2.2.|The hypothermic effect of 5-HTP was potentiated by carbidopa, citalopram, additive with (±), (−) and (+) propanolol and antagonised by methysergide and metitepine.
• 3.3.|Cyproheptadine, xylamidine and ketanserin did not antagonised 5-HTP-induced hypothermia.
• 4.4.|The results suggest that the hypothermic effect of 5-HTP in fowls may be mediated mainly via activation of central 5-HT receptors, probably 5-HT₁ receptors.