Concomitant development of [3H]-dopamine and [3H]-5-hydroxytryptamine uptake systems in rat brain regions

Adolescent male rats are less sensitive than adults to the anxiogenic and serotonin-releasing effects of fenfluramine

Risk taking behavior increases during adolescence, which is also a critical period for the onset of drug abuse. The central serotonergic system matures during the adolescent period, and its immaturity during early adolescence may contribute to adolescent risk taking, as deficits in central serotonergic function have been associated with impulsivity, aggression, and risk taking. We investigated serotonergic modulation of behavior and presynaptic serotonergic function in adult (67-74 days old) and adolescent (28-34 days old) male rats. Fenfluramine (2 mg/kg, i.p.) produced greater anxiogenic effects in adult rats in both the light/dark and elevated plus maze tests for anxiety-like behavior, and stimulated greater increases in extracellular serotonin in the adult medial prefrontal cortex (mPFC) (1, 2.5, and 10 mg/kg, i.p.). Local infusion of 100 mM potassium chloride into the mPFC also stimulated greater serotonin efflux in adult rats. Adult rats had higher tissue serotonin content than adolescents in the prefrontal cortex, amygdala, and hippocampus, but the rate of serotonin synthesis was similar between age groups. Serotonin transporter (SERT) immunoreactivity and SERT radioligand binding were comparable between age groups in all three brain regions. These data suggest that lower tissue serotonin stores in adolescents limit fenfluramine-stimulated serotonin release and so contribute to the lesser anxiogenic effects of fenfluramine.

The emergence of gonadal hormone influences on dopaminergic function during puberty

Adolescence is the developmental epoch during which children become adults-intellectually, physically, hormonally and socially. Brain development in critical areas is ongoing. Adolescents are risk-taking and novelty-seeking and they weigh positive experiences more heavily and negative experiences less than adults. This inherent behavioral bias can lead to risky behaviors like drug taking. Most drug addictions start during adolescence and early drug-taking is associated with an increased rate of drug abuse and dependence. The hormonal changes of puberty contribute to physical, emotional, intellectual and social changes during adolescence. These hormonal events do not just cause maturation of reproductive function and the emergence of secondary sex characteristics. They contribute to the appearance of sex differences in non-reproductive behaviors as well. Sex differences in drug use behaviors are among the latter. The male predominance in overall drug use appears by the end of adolescence, while girls develop the rapid progression from first use to dependence (telescoping) that represent a female-biased vulnerability. Sex differences in many behaviors including drug use have been attributed to social and cultural factors. A narrowing gap in drug use between adolescent boys and girls supports this thesis. However, some sex differences in addiction vulnerability reflect biologic differences in brain circuits involved in addiction. The purpose of this review is to summarize the contribution of sex differences in the function of ascending dopamine systems that are critical to reinforcement, to briefly summarize the behavioral, neurochemical and anatomical changes in brain dopaminergic functions related to addiction that occur during adolescence and to present new findings about the emergence of sex differences in dopaminergic function during adolescence.

Structural effects and neurofunctional sequelae of developmental exposure to psychotherapeutic drugs: experimental and clinical aspects

The advent of psychotherapeutic drugs has enabled management of mental illness and other neurological problems such as epilepsy in the general population, without requiring hospitalization. The success of these drugs in controlling symptoms has led to their widespread use in the vulnerable population of pregnant women as well, where the potential embryotoxicity of the drugs has to be weighed against the potential problems of the maternal neurological state. This review focuses on the developmental toxicity and neurotoxicity of five broad categories of widely available psychotherapeutic drugs: the neuroleptics, the antiepileptics, the antidepressants, the anxiolytics and mood stabilizers, and a newly emerging class of nonprescription drugs, the herbal remedies. A brief review of nervous system development during gestation and following parturition in mammals is provided, with a description of the development of neurochemical pathways that may be involved in the action of the psychotherapeutic agents. A thorough discussion of animal research and human clinical studies is used to determine the risk associated with the use of each drug category. The potential risks to the fetus, as demonstrated in well described neurotoxicity studies in animals, are contrasted with the often negative findings in the still limited human studies. The potential risk fo the human fetus in the continued use of these chemicals without more adequate research is also addressed. The direction of future research using psychotherapeutic drugs should more closely parallel the methodology developed in the animal laboratories, especially since these models have already been used extremely successfully in specific instances in the investigation of neurotoxic agents.

Activation of metabotropic glutamate receptor 5 is associated with effect of amphetamine on brain neurons

The role of metabotropic glutamate receptor 5 (mGluR5) was explored in mechanisms underlying the action of amphetamine (AMPH). The activity of mGluR5 was monitored by measuring the level of [3H]inositol monophosphates in brain neurons, in response to stimulation of 2-choloro-5-hydroxyphenylglycine (CHPG), a selective agonist of mGluR5. Treatment with 1 microM of AMPH for 1 h or 7 days increased the CHPG (1 mM, 30 min)-evoked phosphoinositide turnover by 46% or 92% and 26% or 84% in cultured cortical and hippocampal neurons, respectively, from that of CHPG-only treated cells. When AMPH was present during CHPG application post-1 h or 7 day AMPH incubation, the rate of phosphoinositide hydrolysis in cortical neurons became 121% or 142% higher than that treated with CHPG only. The postnatal day (P) 21 (juvenile) and P60 (adult) rats received three intraperitoneal injections of 5 mg/kg of AMPH or saline daily for 6 days. They were challenged on the eighth day with one dosage and sacrificed 3 h later. Reversible 3H-glutamate binding detected increases of 22-89% in the binding levels of cortex and hippocampus of both ages following the AMPH injections. Increases of 13-18% in the levels of mGluR5 mRNA were seen in the juvenile pyramidal neurons of hippocampal CA1-4, granular cells of dentate gyrus, and ventral thalamic nuclei, as shown by in situ hybridization. The AMPH-induced altered activity of mGluR5 is probably associated with changes in the expression of the glutamate receptors, including mGluR5. AMPH may modify the sensitivity of mGluR5 or interact with the receptor itself.

Speed kills: cellular and molecular bases of methamphetamine‐induced nerve terminal degeneration and neuronal apoptosis

Methamphetamine (METH) is a drug of abuse that has long been known to damage monoaminergic systems in the mammalian brain. Recent reports have provided conclusive evidence that METH can cause neuropathological changes in the rodent brain via apoptotic mechanisms akin to those reported in various models of neuronal death. The purpose of this review is to provide an interim account for a role of oxygen-based radicals and the participation of transcription factors and the involvement of cell death genes in METH-induced neurodegeneration. We discuss data suggesting the participation of endoplasmic reticulum and mitochondria-mediated activation of caspase-dependent and -independent cascades in the manifestation of METH-induced apoptosis. Studies that use more comprehensive approaches to gene expression profiling should allow us to draw more instructive molecular portraits of the complex plastic and degenerative effects of this drug.

Safety and mechanism of appetite suppression by a novel hydroxycitric acid extract (HCA-SX)

A growing body of evidence demonstrates the efficacy of Garcinia cambogia-derived natural (-)-hydroxycitric acid (HCA) in weight management by curbing appetite and inhibiting body fat biosynthesis. However, the exact mechanism of action of this novel phytopharmaceutical has yet to be fully understood. In a previous study, we showed that in the rat brain cortex a novel HCA extract (HCA-SX, Super CitriMax) increases the release/availability of radiolabeled 5-hydroxytryptamine or serotonin ([3H]-5-HT), a neurotransmitter implicated in the regulation of eating behavior and appetite control. The aim of the present study was 2-fold: (a) to determine the effect of HCA-SX on 5-HT uptake in rat brain cortex in vitro; and (b) to evaluate the safety of HCA-SX in vivo. Isolated rat brain cortex slices were incubated in oxygenated Krebs solution for 20 min and transferred to buffer solutions containing [3H]-5-HT for different time intervals. In some experiments, tissues were exposed to HCA-SX (10 microM - 1 mM) and the serotonin receptor reuptake inhibitors (SRRI) fluoxetine (100 microM) plus clomipramine (10 microM). Uptake of [3H]-5-HT was expressed as d.p.m./mg wet weight. A time-dependent uptake of [3H]-5-HT occurred in cortical slices reaching a maximum at 60 min. HCA-SX, and fluoxetine plus clomipramine inhibited the time-dependent uptake of [3H]-5-HT. At 90 min, HCA-SX (300 microM) caused a 20% decrease, whereas fluoxetine plus clomipramine inhibited [3H]-5-HT uptake by 30%. In safety studies, acute oral toxicity, acute dermal toxicity, primary dermal irritation and primary eye irritation, were conducted in animals using various doses of HCA-SX. Results indicate that the LD50 of HCA-SX is greater than 5,000 mg/kg when administered once orally via gastric intubation to fasted male and female Albino rats. No gross toxicological findings were observed under the experimental conditions. Taken together, these in vivo toxicological studies demonstrate that HCA-SX is a safe, natural supplement under the conditions it was tested. Furthermore, HCA-SX can inhibit [3H]-5-HT uptake (and also increase 5-HT availability) in isolated rat brain cortical slices in a manner similar to that of SRRIs, and thus may prove beneficial in controlling appetite, as well as treatment of depression, insomnia, migraine headaches and other serotonin-deficient conditions.

Sex, steroids, and stimulant sensitivity

The current study investigated ovarian modulation of the locomotor response to cocaine in rats. Ovariectomy in females lowered the response to cocaine (10 mg/kg i.p.), whereas castration did not change that of males. The locomotor responses of prepubertal males and females to cocaine were similar. However, the postpubertal sex difference resulted from a fall in cocaine-stimulated locomotion in males rather than a rise in females. Neonatal testosterone treatment of female rat pups decreased the response to cocaine in adulthood. These findings suggest that both the activational and organizational effects of gonadal steroids contribute to the greater response of females to cocaine.

Chlorpyrifos releases norepinephrine from adult and neonatal rat brain synaptosomes

Exposure of developing animals to apparently subtoxic doses of chlorpyrifos (CPF) during a critical period of synaptogenesis has been shown to affect catecholaminergic synaptic development and neuronal activity separably from its inhibition of cholinesterase. We used rat brain synaptosome preparations to examine whether CPF has a direct effect on the release of norepinephrine (NE). Synaptosomes were preloaded with [3H]NE in the absence of CPF and were then exposed to the compound during subsequent neurotransmitter release. There was a robust increase in release at 50 microg/ml of CPF. The effect was not mediated through cholinergic receptors, as neither atropine nor mecamylamine interfered with the actions of CPF. Enhanced NE release was seen in synaptosomes derived from neonatal rat brain as well as adult rat brain, albeit with a smaller effect in neonates. Our results suggest that CPF interacts directly with presynaptic nerve terminals to influence neurotransmitter release; in the context of the immature brain, these effects can alter synaptic development through the trophic actions of catecholamines.

Ontogeny of methamphetamine-induced neurotoxicity and associated hyperthermic response

Methamphetamine (MA) administration to adult rats results in neurotoxicity characterized by depletion of caudate-putamen (CP) dopamine (DA) and serotonin (5-HT) and an accompanying increase in glial fibrillary acidic protein (GFAP) content. The severity of MA-induced neurotoxicity correlates with the accompanying thermoregulatory response, i.e., a hyperthermic response facilitates neurotoxicity while a hypothermic response is neuroprotective. In the following study, the thermoregulatory and neurotoxic effects of MA administration (4 x 10 mg/kg) were investigated in developing rats at postnatal days (PND) 20, 40 and 60. Rats at PND 20 and PND 40 were administered MA at ambient temperatures of 22 degrees C and 30 degrees C; and PND 60 rats were administered MA at 22 degrees C only. Temperatures were measured and thermal responses were compared by calculating the total thermal response (TTR) induced by MA treatment. MA administration to PND 60 rats at 22 degrees C induced a hyperthermic response, resulted in a 47% reduction of neostriatal DA and a 49% increase of GFAP content. Administration of MA to PND 40 rats at 22 degrees C failed to induce a hyperthermic response and did not result in reduced DA or increased GFAP. However, administration of MA to PND 40 rats at 30 degrees C induced hyperthermia, reduced neostriatal DA by 54% and increased GFAP by 70%. MA administration to PND 20 rats at either 22 degrees C or 30 degrees C did not result in DA depletion or increased GFAP, even though MA administration to PND 20 rats at 30 degrees C induced hyperthermia. These results demonstrate that the induction of hyperthermia is necessary to exhibit MA-induced neurotoxicity at PND 40; however, PND 20 rats are resistant to the DA depleting effects of MA despite the induction of hyperthermia.

The effects of bupropion in vivo in the neostriatum of 5-day-old and adult rats

Infusion of six concentrations of the dopamine uptake inhibitor bupropion into the neostriatum increased extracellular dopamine in a dose-dependent manner in 5-day-old and adult rats. There was no age-related difference when calculated as a percentage of predrug dopamine baseline levels, but the absolute increase of dopamine was greater in the adult rats. Bupropion had only a minor effect on extracellular levels of DOPAC.

Serotonin transporter expression in rat brain regions and blood platelets: aging and glucocorticoid effects

Hyperactivity of the hypothalamus-pituitary-adrenal axis is more common in elderly depression than in younger cohorts and glucocorticoids are known to influence serotonergic systems. The current study explores the interaction of glucocorticoids with aging on serotonin transporter expression and function. Continuous infusions of dexamethasone (26 days) reduced transporter expression in the aged brain but the ability of imipramine to inhibit synaptosomal [3H]serotonin uptake was unimpaired. These effects were unique to aged animals, as prior work with young adults found no effects of dexamethasone on transporter expression. In contrast to the effects in the brain, there were no differences in platelet transporter expression between young and old rats nor did dexamethasone treatment affect the values in the aged group: thus, the platelet may not reliably model these aspects of CNS function. The results suggest that there are basic biologic differences in the effects of glucocorticoids in aged vs. young brain that could contribute to lowered effectiveness to antidepressants in elderly depression; if transport capacity is already reduced by the effects of increased glucocorticoids, further inhibition of transport by antidepressants would have proportionally less impact on synaptic serotonin concentrations.

Age-related differences in the chronic and acute response to cocaine in the rat

Behavioral sensitization is known to occur in adult animals after the chronic intermittent administration of cocaine. Dopaminergic pathways in the brain, such as the nigrostriatal and mesoaccumbens projections play a vital role in this phenomenon. These pathways are rudimentary in the 1st week of life, indicating that the developing animal may be unable to respond to cocaine in the same manner as an adult. In the present study, we report that the acute response to cocaine is remarkably similar between week-old and adult rats. Pups do not, however, show locomotor sensitization to acute cocaine after chronic cocaine-administration as adults.

Do glucocorticoids contribute to the abnormalities in serotonin transporter expression and function seen in depression? An animal model

Adrenocorticosteroids and serotonergic neurons exert reciprocal regulatory actions, and both are abnormal in depression. We evaluated whether glucocorticoids influence the serotonin transporter in rat platelets and brain by infusing dexamethasone for 26 days, sufficient for replacement of the entire platelet population. Effectiveness was verified by measurement of plasma dexamethasone levels, adrenal atrophy, and growth inhibition. At the end of the infusion, we examined [3H]paroxetine binding to platelet, hippocampal, and cerebrocortical membranes, and [3H]serotonin uptake into platelets and synaptosomes. Dexamethasone slightly reduced platelet [3H]paroxetine binding (12%) and had no effect on binding in brain. Platelet [3H]serotonin uptake was unaffected, but synaptosomal uptake was significantly reduced. In neither platelets nor synaptosomes did dexamethasone alter imipramine's ability to inhibit uptake. Thus, elevated glucocorticoids are not responsible for reduced platelet serotonin transporter expression in depression, nor for resistance to imipramine's effect in platelets in elderly depression; however, reduced synaptosomal [3H]serotonin uptake indicates that glucocorticoids can affect transport efficiency, even when the number of transporter molecules is unaltered.

Ontogeny of behavioral sensitization to cocaine

The ontogeny of the behavioral effects of acute cocaine administration and behavioral sensitization to cocaine in rat pups was investigated. Acute behavior stimulating effects of cocaine were observed in pups as young as 7 postnatal days (PND) old, although they needed a higher dose of cocaine than adult rats to evoke the same motor effects. An adult dose-response curve pattern of stereotypy and locomotion to acute cocaine treatment was observed at PND 21, and of rearing at PND 28. Rats aged PND 7, 14, 21, 28, and 56 received repeated injections of saline or cocaine (15 mg/kg) twice a day for 5 consecutive days. After a 3-week period of abstinence, sensitization to a challenge dose of cocaine was assessed. Cocaine-induced stereotyped behavior was enhanced significantly only in rats in which cocaine pretreatment was initiated on PND 21, 28, and 56, but not earlier on PND 7 and 14. Adult female rats given repeated cocaine injections on PND 56-60 showed significantly greater sensitization than males, but no such sex difference was observed in pups given cocaine repeatedly on PND 21-25 or 28-32. These results show clearly that cocaine-induced behavioral sensitization in rats occurred only when subchronic cocaine administration was commenced on PND 21 or later.

In search of a mechanism for receptor-mediated neurobehavioral teratogenesis by nicotine: catecholamine release by nicotine in immature rat brain regions

Nicotine disrupts central nervous system development through interactions with nicotinic cholinergic receptors found in immature brain, leading to discoordination of target cell replication and differentiation. However, it is unclear whether the net result is achieved by nicotine's actions on its specific target cells, or indirectly through receptor-mediated release of other neurotransmitters, such as catecholamines, that possess neurotrophic properties. In the current study, developing rats (1, 7, 14 and 21 days old) were challenged acutely with nicotine (0.3 mg/kg) and the release of catecholamines was evaluated in vivo (AMPT method) in three brain regions that differ in nicotinic receptor concentrations. Nicotine did not stimulate catecholamine release at birth, but developed the capacity to do so in parallel with the ontogeny of nicotinic cholinergic receptors in the midbrain+brainstem and in the forebrain. In the cerebellum, which remains poor in nicotinic receptors, no response was obtained at any age. Superimposed on this general pattern, changes in sensitivity to nicotine were also seen that corresponded to ontogenetic changes in endogenous cholinergic tone, suggesting that receptor desensitization occurs normally during developmental stages in which neuronal activity is high. The absence of a catecholamine response to nicotine at birth in the rat indicates that neurobehavioral teratology associated with fetal nicotine exposure does not reflect secondary actions mediated through catecholamines. However, because brain development in the neonatal rat corresponds to fetal stages in man, the onset of these mechanisms may be relevant to human fetal exposure.

Similar properties of fetal and adult amine transporters in the rat brain

A variety of drug classes, including psychomotor stimulants and antidepressants, interact with monoamine transporters in order to exert their effects. Although these transporters have been extensively characterized in the adult brain, little is known about uptake mechanisms in the fetal system. High affinity dopamine (DA) and serotonin (5-HT) uptake in the striatum and frontal cortex, respectively, were examined in rat fetuses (embryonic day 20; E-20). These results were then compared to uptake in adult rat synaptosomal preparations of the same regions. The data indicate that the fetal (E-20) uptake mechanism is sodium-dependent. Furthermore, the potency of various agents to inhibit transporter function was assessed. These drugs produced a concentration-dependent inhibition of uptake, and the resulting IC50 values were not significantly different from those obtained in the adult preparations. Our results provide evidence that the affinity of monoamine uptake inhibitors for fetal (E-20) DA and 5-HT transporters is similar to that observed with adult transporters. This observation has broad implications when considering neuronal development and in utero exposure to drugs that exert their effects through these transporters.

Cocaine binding sites in fetal rat brain: implications for prenatal cocaine action

Binding of [3H]cocaine to membrane preparations from whole fetal rat brain was studied. High-affinity binding (10 nM cocaine) was detected as early as gestational day (GD) 15 and steadily increased across subsequent development. Saturation studies comparing [3H]cocaine binding at GD20 and adulthood yielded similar KD values, and LIGAND analyses favored a two-site model if an extended range of [3H]cocaine concentrations was used. Various monoamine uptake inhibitors displaced labeled cocaine with potencies consistent with the idea that [3H]cocaine labels the dopamine (DA), serotonin (5-HT), and possibly also the norepinephrine (NE) transporters in whole fetal brain preparations. Synaptosomal DA uptake was well developed by GD20, as was the potency of cocaine to inhibit such uptake. The results indicate that functional, monoamine transporter related cocaine binding sites are present in the fetal rat brain. Such sites are likely to play an important role in mediating the direct interactions of prenatally-administered cocaine with developing monoaminergic systems in both animals and humans.

A molecular recognizing system of serotonin in rat fetal axonal growth cones: uptake and high affinity binding

Axonal growth cone particles (AGCP) isolated from prenatal and postnatal rat brain had different high-affinity 5-HT uptake characteristics. In postnatal AGCP the uptake behaves as in the adult rat brain, while in the prenatal AGCP the uptake characteristics seem to be in a transitional stage. Also in prenatal AGCP we observed specific, high-affinity 5-HT binding sites. These results support the idea of an important role for 5-HT during axogenesis.

Administration of tryptophan-enriched diets to pregnant rats retards the development of the serotonergic system in their offspring

It is well established that an increased availability of tryptophan stimulates serotonin synthesis not only in the adult but also in the developing brain. In order to study the influence of a permanently increased supply of tryptophan on the developing central 5-HT-system, female rats were fed a tryptophan-enriched diet from mating throughout pregnancy and lactation. The effect of this dietary regime was assessed by measurements of neurochemical markers of 5-HT innervation in the developing brain of their offspring. A diminished content of 5-HT, a decreased activity of tryptophan hydroxylase and a reduction of crude synaptosomal high-affinity 5-HT uptake was found in the cortex and in the brain stem of 5 day old rat pups of mothers fed the tryptophan-enriched diet. The postnatal increase of all three markers of serotonergic innervation in the offspring of these mothers was retarded. Both the initial depletion and the delayed maturation of 5-HT content, of tryptophan hydroxylase activity and of synaptosomal serotonin uptake were more pronounced in the cortex than in the brain stem. Apparently, the increased dietary intake of tryptophan throughout pregnancy and lactation caused a delayed outgrowth of 5-HT axons and/or reduced collateral sprouting and synapse formation in the brain of the developing offspring.

Development of monoaminergic neurotransmitters in fetal and postnatal rat brain: analysis by HPLC with electrochemical detection

The monoamines dopamine, norepinephrine, epinephrine, and serotonin and their major metabolites 3,4-dihydroxyphenylacetic acid, homovanillic acid, 3-methoxy-4-hydroxyphenylethylene glycol, and 5-hydroxyindoleacetic acid were measured in the CNS of the rat during development from fetal day 18 to young adult. The catecholamines, serotonin, and their major metabolites remained low during fetal life. Concentrations measured in total brain started to increase around birth till the end of the fourth week of life after which steady-state levels were measured. Our results suggest that although monoamine systems are already morphologically well developed during late gestational life, they probably become a significant functional system only around birth and early postnatal life.

Postnatal ontogenesis of dopaminergic and serotoninergic systems in rat caudate nucleus

The development and sex differences of the catecholaminergic and serotoninergic systems were studied in rat caudate nucleus by measuring dopamine, serotonin and their metabolites in male and female pups from birth up to day 30 of life. Dopamine increased regularly during the entire period studied, while its metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), increased sharply only after day 9 of life. At birth, serotonin levels were 50% of the juvenile value; its metabolite, 5-hydroxyindoleacetic acid (5-HIAA), increased greatly after day 6 of life. No difference was observed in male and female neurotransmitter ontogenesis. The results indicated that dopaminergic and serotoninergic systems were not mature in the caudate nucleus at birth. A functional immaturity of enzyme systems involved in neurotransmitter synthesis and/or catabolism is suggested.

Ontogeny of the enhanced behavioral response to amphetamine in amphetamine-pretreated rats

Repeated administration of amphetamine to adult rats results in enhanced behavioral responses to subsequent amphetamine exposure. These experiments were designed to determine the earliest age at which behavioral sensitization to amphetamine could be detected. Rats from both sexes (n = 6-8/group) at ages of 1, 7, 21 or 49 postnatal days (PNDs) were injected with either d-amphetamine sulfate (5 mg/kg) or saline, SC, twice daily for 5 consecutive days. Stereotyped behavior and locomotor activity responses to a challenge dose of d-amphetamine (2.5 mg/kg), or saline, IP, were assessed for a total of 90 min, 15 days after the last dose of pretreatment. Amphetamine-induced stereotyped behavior was significantly enhanced only when amphetamine pretreatment was initiated at PND 49, but not at the earlier ages of PND 1, 7 or 21. There was no apparent sex difference in this effect. Correspondingly, amphetamine-induced locomotor activity was reduced in both sexes of the same age group (PND 49), but not in groups pretreated earlier, when compared to the saline-pretreated rats. These results suggest that amphetamine sensitization may be a late-developing effect, one which occurs sometime after the 3rd week of postnatal life.

Development of haloperidol-induced dopamine release in the rat striatum using intracerebral dialysis

Haloperidol-induced dopamine (DA) release and metabolism were studied in the rat striatum at 10-11, 21-22, and 35-36 days of age using intracerebral dialysis and HPLC with electrochemical detection. There was an age-related increase in basal DA release and extracellular levels of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), with the greatest increases occurring between 10-11 and 21-22 days of age. Haloperidol (0.1 mg/kg, i.p.) significantly increased DA release at each age compared to control. Also, haloperidol produced a significantly greater increase in DA release at 10-11 days than at 21-22 or 35-36 days of age when expressed as percentage of predrug release. Haloperidol increased DA release over 60 min to 235%, 138%, and 158% above baseline at 10-11, 21-22, and 35-36 days of age, respectively, after which time the levels remained relatively constant. Haloperidol significantly increased extracellular DOPAC and HVA levels at each age compared to controls, but there were no significant differences in DOPAC or HVA levels between ages in response to haloperidol. The results indicate that, at 10 days of age, DA release in the striatum is physiologically functional and that the regulatory feedback control of DA release and metabolism in the striatum develops prior to 10 days of age.

Development and ageing of the rat nigrostriatal dopamine system studied with fast cyclic voltammetry

Fast cyclic voltammetry at carbon fibre microelectrodes was used to measure electrically stimulated dopamine release in the striatum of anaesthetised young, adult, and senescent Wistar rats. By alteration of stimulus parameters and by use of nomifensine, investigation of dopamine release, uptake, and compartmentalisation within the striatum was possible. The rate of dopamine release was highest in adult rats. No difference was observed between young and old animals. The size of the releasable (newly synthesised) dopamine pool was also largest in the adult group, again with no significant difference occurring between young and aged rats. The rate of dopamine uptake was highest in adult rats, although, when expressed as a function of dopamine release, young and aged rats showed proportionally greater uptake. Nomifensine (10 mg/kg i.p.) increased dopamine release by mobilising the inert storage pool to a greater extent in young than in adult rats, whereas the effect of the drug on uptake was similar in all age groups. The functional significance and possible explanation of these results are discussed.

Rapid maturation of synaptic functions of prenatal serotoninergic neurons in short-term cultures: absence of sex differences and hormone effects

Serotonin is believed to modulate neuronal differentiation during early stages of brain development. In order to assess basic functional requirements for such a role, it was investigated how early serotoninergic neurons mature with respect to transmitter storage and stimulus-secretion coupling. Dissociated cell cultures were raised from embryonic rat rhombencephalon obtained at gestational day 14 and cultured for 3-8 days, which may roughly correspond to the prenatal period in vivo. Because of a possible involvement of serotonin in processes leading to sexual differentiation of the brain, gender-specific cultures were raised in addition and treated with sex steroids. Sensitivity of [3H]serotonin uptake to fluoxetine could already be observed at 3 days in vitro. Vesicular storage as probed with reserpine and nigericin, and the capability of releasing preaccumulated serotonin in a Ca2+-dependent manner were also present as early as 3 days in vitro. Seven per cent of the pre-accumulated transmitter could be released per minute upon stimulation with 54 mM K+. Immunocytochemical and autoradiographic preparations demonstrated that, after the same short culture period, the neurons had formed large fiber networks. No differences could be detected regarding any of the above parameters between female and male serotonin neurons and between cultures treated with and without estradiol, testosterone and dihydrotestosterone. It is concluded that, in contrast to other neuronal phenotypes, serotoninergic neurons are functionally mature when or shortly after they are taken into culture, i.e. around gestational day 14. The functional competence of prenatal serotonin systems should be a prerequisite for their suspected role in modulating neural development at pre and postsynaptic sites. The present results provide no evidence for the occurrence of a sexual dimorphism of serotonin neurons at this early developmental stage.

Correlation between (3H)dopamine specific uptake and (3H)GBR 12783 specific binding during the maturation of rat striatum

The development of the specific uptake of dopamine in the rat striatum during the early postnatal period is compared with the ontogenetic changes of the specific binding of (3H)GBR 12783 to the site of uptake inhibition. During maturation, the increase in the specific binding of (3H)GBR 12783 parallels the increase in the specific uptake of dopamine. (3H)GBR 12783 specific binding sites increase in number from day 1 postpartum until 40 days, when they reach the adult level. In 40 day-old rats, the weight of the striatum represents 80% of adult values. The affinity of (3H)GBR 12783 for the inhibition site is similar in membrane preparations obtained from 6 day-old pups and adults; this results in a same ability of the inhibitor to block the specific uptake of dopamine into synaptosomes obtained from pups or adult rats. These data support the hypothesis of the existence of a single molecular entity including both the inhibition site and the carrier itself.

Perinatal dietary supplementation with a soy lecithin preparation: effects on development of central catecholaminergic neurotransmitter systems

Previous work has shown that exposure of developing rats to soy lecithin preparations (SLP) influences macromolecular constituents of immature brain cells and causes abnormal behavioral patterns. To determine if synaptic mechanisms are adversely affected by SLP, we examined the developmental characteristics of noradrenergic and dopaminergic pathways in discrete brain regions. Although transmitter levels were unaffected, the utilization rates of both catecholamines were profoundly disturbed in an age-dependent, regionally-selective manner. Utilization tended to be subnormal in the preweanling stage, but demonstrated a postweaning elevation in cerebellum and midbrain + brainstem. Enhanced utilization persisted in the latter region only, and cerebral cortex actually showed a lowered utilization rate in adulthood (60 days of age). Transmitter uptake capabilities were also affected by developmental exposure to SLP, as was tyrosine hydroxylase activity. The patterns of effects on these two variables indicated that the altered transmitter utilization rate probably reflected a change in impulse activity in the affected neuron populations, with promotion of activity in the midbrain + brainstem and reduced activity in the cerebral cortex. These data indicate that dietary supplementation with SLP throughout perinatal development alters synaptic characteristics in a manner consistent with disturbances in neural function.

Effects of neonatal hypoxia on brain development in the rat: immediate and long-term biochemical alterations in discrete regions

To evaluate the sensitivity of immature brain tissue to hypoxic insult, neonatal rats were exposed to 7% O2 for 2 h at critical stages of development (1, 8, 15, 23 days of postnatal age); the immediate and long-term impact of hypoxia was then assessed in cerebellum, cerebral cortex and midbrain through measurement of ornithine decarboxylase (ODC) activity, a biochemical determinant of cellular injury and subsequent maturation, and through measurements of protein synthesis, growth and synaptosomal uptake of norepinephrine (an index of noradrenergic synaptogenesis). In one-day-old rats, hypoxia caused stimulation of protein synthesis and short-term suppression of ODC activity which persisted for several hours after termination of low O2 exposure; over the ensuing days, there was a prolonged elevation of enzyme activity and a subsequent, regionally selective increase in synaptosomal uptake of norepinephrine without changes in brain growth. In contrast, hypoxia in 8-day-old rats produced signs of metabolic injury, with a short-term elevation of ODC throughout the brain and reduced protein synthetic rates, eventual shortfalls in brain regional growth and no net increase in synaptosomal uptake. The effects of hypoxia on brain regional growth in 8-day-old animals appeared to represent an age-specific effect, as low as O2 conditions in older animals did not affect growth (animals made hypoxic at 15 or 23 days), but did produce an eventual reduction in synaptosomal uptake (hypoxia at 15 days). Differences between one-day-old and 8-day-old rats were also apparent in cerebral responses simply to a 2-h separation from the dam under normoxic conditions. These results support the view that cellular development and synaptogenesis are compromised when neonatal brain tissue is exposed to hypoxic conditions, and that there are critical periods of sensitivity in which processes undergoing rapid maturational change are particularly vulnerable.

Ontogenesis of [3H]serotonin binding sites in the hypothalamus of the female rat: relation to serotonin-induced LH release in moxestrol-pretreated rats

The release of luteinizing hormone (LH) evoked by exogenous serotonin (5 mg/kg, i.p.) was measured at different developmental ages in the female rat. Since LH response to serotonin is modulated by estrogen, moxestrol, a synthetic compound which does not bind to alpha-fetoprotein, was administered 48 h before serotonin or saline injections. Serotonin was ineffective in releasing LH in 5-day females pretreated with moxestrol; the response increased abruptly at 12 and 18 days and from then onwards decreased gradually. This is in clear contrast to the development of the serotonin-induced prolactin release, which is absent in the first postnatal week, becomes evident on day 12 and increases gradually as the rat approaches puberty. In a second group of experiments, the ontogenesis of hypothalamic-preoptic suprachiasmatic [3H]serotonin binding was studied in 5-, 16-, 27- and 37-day-old female rats. A specific serotonergic binding site could be quantified at 5 days (Bmax 4.31 : 1.89 fmol/micrograms DNA; Kd: 2.74: 1.17 nM). A gradual increase of both Bmax and Kd was observed as the animal matured: the number of binding sites almost doubled from day 16 to day 27, and increased even further by 37 days (Bmax 24.21 : 5.16 fmol/micrograms DNA). These data indicate: a specific [3H]serotonin binding site in hypothalamic-preoptic suprachiasmatic area is present in the first postnatal week of the female rat; Bmax and Kd values increase from 5 to 37 days of age.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of sodium-dependent, high-affinity serotonin uptake in rat spinal cord synaptosomes

Synaptosomal accumulation of [3H]serotonin was used to determine if the rat spinal cord possesses a high-affinity neuronal uptake system for serotonin. Two temperature-dependent accumulation processes were found, one sodium-dependent, the second sodium-independent. Sodium-dependent [3H]serotonin accumulation was linear with sodium concentrations up to 143 mM, was associated with the purified synaptosomal fraction (P2B), and decreased 76% by osmotic lysis, 88% by sonication, and 96% by 0.1% Triton X-100. Drug inhibition studies demonstrated fluoxetine to be the most potent inhibitor of this system (IC50 0.075 microM) while desipramine (IC50 0.43 microM) and nomifensine (IC50 0.95 microM) were less potent. Kinetic analysis revealed that sodium-dependent accumulation in purified synaptosomes was saturable at low [3H]serotonin concentrations (Ku = 50 nM, Vmax = 4 pmol/mg protein/min). Sodium-independent [3H]5-HT accumulation was substantially less sensitive to fluoxetine, desipramine and nomifensine. While sodium-independent accumulation was not significantly affected by osmotic lysis, it was markedly increased by prior sonication of tissue. Also, in contrast to sodium-dependent accumulation, sodium-independent accumulation was evenly distributed in all tissue fractions, and was not saturable at low [3H]serotonin concentrations. It is concluded that sodium-dependent [3H]serotonin accumulation reflects uptake into spinal serotonergic nerve terminals while sodium-independent accumulation probably reflects a temperature-sensitive binding to membrane fragments. Comparison to brain uptake of serotonin and the necessity for using 37 degrees C sodium-free blanks rather than 0 degree C blanks in spinal cord homogenates is discussed.

Exposure to methylmercury in utero: effects on biochemical development of catecholamine neurotransmitter systems

Administration of methylmercury to pregnant rats resulted in major alterations in synaptic dynamics of brain dopamine systems in the offspring which were prominent even at doses of the organomercurial which did not produce acute toxicity, fetal or neonatal death, low birth weight or reduced litter sizes. The abnormalities were typified by shortfalls in both the levels and turnover rate of the transmitter in vivo, accompanied by elevations in synaptic uptake as assessed in synaptosomal preparations in vitro. These effects were not apparent in the immediate postnatal period but instead showed a delayed onset beginning at about the time of weaning. Methylmercury exposure displayed selectivity in that central noradrenergic systems showed only the synaptic uptake alterations without changes in transmitter levels or turnover; targeted interactions were also apparent in peripheral sympathetic pathways to the heart and kidney. The threshold dose required to elicit damage to biochemical development of neurotransmitter systems was the same as that to alter more generalized cellular development, as assessed through measurements of brain ornithine decarboxylase activity. These studies indicate that neurochemical damage produced by prenatal exposure of the developing organism to methylmercury involves transmitter-selective alterations in synaptic dynamics and function which may contribute to adverse behavioral outcomes; the underlying mechanisms, however, do not necessarily reflect actions of the organomercurial which are primary or specific to these particular neuronal tissues.

Effects of neonatal mercuric chloride administration on growth and biochemical development of neuronal and non-neuronal tissues in the rat: comparison with methylmercury

Mercuric chloride (HgCl2) was administered at 3 dosage levels (0.5, 1, and 2.5 mg/kg s.c.) daily to newborn rats beginning at 1 day of age and continuing through weaning. HgCl2 produced a dose-dependent inhibition of body growth which was apparent only after weaning and which worsened as the animals approached adulthood; brain growth was also adversely affected, but less so than was body weight. Growth of other tissues (heart, kidney, liver) was influenced in a different manner, with initial increases over control organ weights and a subsequent decline toward normal (kidney) or to subnormal levels (heart, liver). Examination of ornithine decarboxylase (ODC) activity, an index of cellular maturation, confirmed that HgCl2 produced separable types of effects in different organ systems. Although the patterns of growth alterations caused by HgCl2 resembled those seen with methylmercury, the effects on ODC were dissimilar, suggesting that there were some differences in the underlying biochemical mechanisms. In addition to causing generalized alterations of cellular development, HgCl2 produced specific effects on catecholamine neurotransmitter systems, with increases in brain norepinephrine levels and turnover as well as elevations in synaptosomal uptake capabilities for norepinephrine. Dopamine levels and turnover were slightly reduced or unchanged, but synaptosomal uptake was still elevated. Target-specific effects were also apparent in the peripheral sympathetic nervous system, where renal but not cardiac norepinephrine was elevated in the HgCl2 group. Again, some but not all of these changes can be produced by comparable exposure to methylmercury; many of the neuronal effects in animals exposed to HgCl2 were in the opposite direction from those seen with the organomercurial.

Increase in dopamine uptake in rat striatal synaptosomes after an acute in vivo administration of organic and inorganic lead

Adult male Wistar rats were administered acute toxic doses of lead (Pb), triethyl lead (TriEL) or tetraethyl lead (TEL) by gavage. The ability of striatal, hypothalamic and cortical synaptosomes to take up tritiated monoamines was assayed 24 hours later. Pb, TriEL as well as TEL increased dopamine (DA) uptake into striatal synaptosomes at least at some dose level. Pb increased, but TEL decreased 5-hydroxytryptamine(5-HT) uptake into hypothalamic synaptosomes, while TEL increased noradrenaline (NA) uptake into cortical synaptosomes. After a 3 week administration to initially 4 week old rats of even toxic doses of Pb in drinking water, monoamine uptake was not significantly affected. On the contrary, the neurotoxicity of TEL was cumulative in that a much lower dose decreased 5-HT uptake when divided over a 3 week period than acutely. In vitro TriEL inhibited DA uptake (IC50; 0.8 microM) into striatal and 5-HT uptake (5.0 microM) into hypothalamic synaptosomes but TEL and delta-aminolevulinic acid did not. The results suggest that dopaminergic and serotonergic neurones differ in their response to alkyl lead in vivo. The differences could be due to basic differences in the neurochemical behaviour of these two types of nerve endings.

Pre- and postnatal ontogeny and characterization of dopaminergic D2, serotonergic S2, and spirodecanone binding sites in rat forebrain

The ontogeny of binding sites for [3H] spiperone was studied in time-pregnant rats. Binding of [3H]spiperone to fresh homogenates of pre- and postnatal rat forebrain was characterized by Scatchard analysis and competition experiments with a number of dopaminergic and serotonergic agonists and antagonists and additional substances. A convenient discrimination of three high-affinity sites, i.e., the dopaminergic D2, serotonergic S2, and spirodecanone (Sd) sites, was obtained with l-(-)sulpiride and cis-flupenthixol. The analgesic R5573 was found not to be specific for the Sd site but to interact with all three sites. The three binding sites became detectable in sequential order. S2 and D2 binding sites were first found at embryonic days 15.75 and 17.75, respectively. The Sd site did not appear before postnatal day 8. All three binding sites reached adult values at approximately postnatal day 30. During the prenatal period, the increase in the number of D2 binding sites paralleled the rise in forebrain dopamine concentrations. The kinetics of D2 and S2 sites were the same at embryonic day 19.75 and postnatal day 30. These observations provide evidence for the presence of the receptor substrate for actions of neuroleptics on dopaminergic and serotonergic systems during fetal life.

Stimulation of ornithine decarboxylase by histamine or norepinephrine in brain regions of the developing rat: evidence for biogenic amines as trophic agents in neonatal brain development

Ornithine decarboxylase (ODC) initiates the synthesis of polyamines which play key roles in regulation of cellular development. Intracisternal administration of histamine or norepinephrine to developing rats produced age-dependent stimulation of ODC in brain. In cerebral cortex and ponsmedulla, stimulation by norepinephrine was demonstrable at postnatal day 7 and maximum stimulation occurred at about day 9. In contrast, cerebellum showed no initial reactivity to norepinephrine but still developed a large peak of response capability by day 9. In all 3 regions, the response declined rapidly thereafter during the period of major synaptogenesis of noradrenergic pathways. With histamine, none of the regions displayed ODC reactivity at 7 days postnatally; stimulation appeared by day 9, peaked at about day 11 and then declined rapidly. Thus, the trophic effect of histamine or norepinephrine toward ODC activity is present or develops postnatally and appears to terminate with synaptogenesis and onset of neurotransmitter properties of the amines.

Ontogenetic development of the striatal [3H]spiperone binding: regulation by sodium and guanine nucleotide in rats

Ontogenetic development of specific [3H]spiperone binding to crude synaptic membranes and its regulation by Na+ and GTP was investigated in the rat striatum. (d)-Butaclamol more effectively inhibited [3H]spiperone binding than (l)-butaclamol. The ratio of inhibitory activity of (d)- and (l)-butaclamol for [3H]spiperone binding was not different between 1-, 7-, and 70-day-old animals but eight- to ninefold lower at 18 days of gestation than during the postnatal period. A Scatchard plot of specific binding indicated the presence of two types of binding: low-affinity (KD = 1.51 nM) and high-affinity (KD = 0.09 nM) binding on day 70. Only one component (KD = 0.075 nM) was observed on days 1 and 7 and both types of binding were found on day 15. Bmax gradually increased with age and reached a peak on day 30, followed by a decline on days 70 and 360. Na+, 100 mM, significantly increased specific binding on days 1, 7, 15, and 70. GTP, 50 microM, completely reversed the Na+-induced decrease in IC50 of apomorphine on both days 15 and 70, but not on day 7. It is suggested that receptors could recognize ligand stereospecificity on day 1. The density in dopamine receptors in the striatum reaches a peak on day 30, followed by a decrease on days 70 and 360. In addition, regulation by Na+ and GTP in agonist binding to dopamine receptors seems to become functional between 1 and 2 weeks after birth.

Development of alpha-noradrenergic and Dopaminergic receptor systems depends on maturation of their presynaptic nerve terminals in the rat brain

To study the relationship between ontogeny of rat brain catecholamine nerve terminals and the receptor systems for the catecholamine transmitters, the developmental patterns of synaptosomal uptake mechanisms were compared with those of alpha-noradrenergic and dopaminergic receptor-mediated effects. Uptakes of [(3)H]dopamine or [(3)H]norepinephrine into dopaminergic and noradrenergic nerve terminals were low during the 1st week postpartum and increased rapidly during the 2nd week. A similar pattern was obtained for ontogeny of dopaminergic receptor binding sites, as evaluated by [(3)H]domperidone binding. Stimulation of incorporation of (33)P(i) into brain phospholipids (elicited by intracisternal injection of dopamine), which is mediated by dopaminergic receptors, was shown to be highly correlated with the maturation of both receptor binding sites and presynaptic nerve terminal uptake. A similar result was seen with norepinephrine, in that the synaptosomal uptake mechanism and norepinephrine-induced stimulation (33)P(i) incorporation into phospholipids, an alpha-noradrenergic effect, developed in a parallel fashion. To test the hypothesis that development of the receptor systems is linked to nerve terminal ontogeny, presynaptic nerve terminals were destroyed in neonates by intracisternal administration of 6-hydroxydopamine. The lesions prevented the maturational increase in the number of dopamine receptor binding sites and produced a defect in development of the dopamine- and norepinephrine-induced stimulation of (33)P(i) incorporation. The results suggest that ontogeny of both dopaminergic and alpha-noradrenergic receptor systems depend upon development of the presynaptic nerve terminals containing the transmitters.

Ontogenetic development of high potassium- and acetylcholine-induced release of dopamine from striatal slices of the rat

The high K+- and acetylcholine (ACh)-induced release of [3H]dopamine (DA) were investigated in whole brain slices of fetuses and striatal slices of postnatal rats. A high K+-induced, Ca2+-dependent release of [3H]DA was demonstrated in brain on day 18 of gestation and the amount of release from the striatum increased during postnatal development. ACh induced [3H]DA release in striatal slices from 3-, 7- and 70-day-old rats but not from 1-day-old animals. ACh-induced [3H]DA release on both day 7 and day 70 was dependent on Ca2+ and significantly reduced by the addition of 5 x 10(-6) M atropine or 10(-5) M hexamethonium. It is suggested that DA is taken up into functionally releasable pools at a late fetal stage and that from around 3 days after birth, ACh may begin to regulate DA release by stimulating muscarinic and/or nicotinic receptors located on DA nerve terminals in the striatum.

Ontogenesis of uptake and deamination of 5-hydroxytryptamine, dopamine and beta-phenylethylamine in isolated perfused lung and lung homogenates from rats

1. Uptake of 5-hydroxytryptamine (5-HT) and beta-phenylethylamine (PEA) was studied in perfused lung from male rats between 10 and 70 days old. 2. Monoamine oxidase (MAO) activity towards 5-HT, PEA and dopamine was studied in homogenate preparations of lung from rats aged between 5 and 80 days. 3. Uptake of 5-HT (10 microM) decreased throughout the age range studied but uptake of PEA (50 microM) increased for the first 30 days and beyond this age it decreased. Metabolites formed for both amines reflected the changes in uptake. 4. MAO activity deaminating 5-HT is well developed by day 10 and reaches its maximum by day 40. For dopamine and PEA, MAO activity remained low until day 20, and the developed rapidly, reaching a maximum by day 40 for dopamine; activity towards PEA did not reach a maximum by day 80. 5. These results show that uptake and MAO activity changes with age and thus the lung responds like other tissues. 6. These results also demonstrate the independent development of uptake and MAO activity towards 5-HT, PEA and dopamine.

TRH-induced behavioral arousal in developing rats pretreated with 6-hydroxydopa

The behavioral effect of thyrotropin releasing hormone (TRH) was investigated in the developing rat pretreated with 6-OHDOPA at birth. An IP injection of TRH (20 mg/kg) increased walking with sniffing, rearing, body shaking, grooming, chewing and licking in the 7-, 14-, 20- and 30-day-old as well as in the adult rat. TRH-induced locomotor stimulation began a few minutes after the injection and lasted for approximately 60 min. But on Day 7, TRH produced locomotor stimulation betwen 1.5 hr and 3.5 hr after the injection. Neonatal treatment with 6-OHDOPA markedly potentiated TRH-induced locomotor stimulation and behavioral arousal in the 7-day-old rat but not in the 14-day-old and adult rat. The marked potentiation of TRH-induced locomotor stimulation by 6-OHDOPA in the 7-day-old rat was reduced by alpha-flupenthixol (pA2=5.9) and phenoxybenzamine (pA2=4.4). These results suggest that central dopamine neurons are involved in TRH-induced behavioral arousal in the infant rat.