Postnatal developmental changes of tryptophan hydroxylase activity in serotonergic cell bodies and terminals of rat brain

Postnatal changes in the number of serotonin-immunoreactive cells in midbrain raphe nuclei of male rats

To clarify the developmental changes in serotonergic neurons in the subdivisions of the dorsal (DR) and median raphe (MR) nuclei before puberty, the extent of the nuclei and the number of serotonin (5-HT) immunoreactive (ir) cells were measured in 5-, 15-, and 30-day-old rats and 8-week-old (adult) castrated male rats. The brains were fixed and 50 μm frozen sections prepared. After immunostaining for 5-HT, the number of 5-HT-ir cells in a 0.2 × 0.2 mm frame in the dorsal, ventral and lateral subdivisions of the DR (dDR, vDR and lDR, respectively) and MR were counted. Total numbers of 5-HT-ir cells counted in the frame of three sections in each rat were expressed as the number of cells per cubic millimeter (density). The results indicated that the densities of 5-HT-ir cells in the MR were almost the same in all age groups. On the other hand, among the subdivisions of the DR, the mean density of 5-HT-ir cells in 15-day-old rats was higher than that in the 5-day-old group in the lDR only. The area of the three sections of the DR and of the MR was also measured. The area of the DR in 15-day-old rats was found to be twice that in the 5-day-old rats, and differed from the area in 30-day-old rats and adults. There were no differences among the age groups in the areas of the MR. The results indicate that the expression of 5-HT in the lDR and extent of the DR increased to adult levels from days 5 to 15 after birth. In the dDR, vDR and MR, expression of 5-HT at postnatal day 5 was at adult levels already.

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 opposite effect of a 5-HT1B receptor agonist on 5-HT synthesis, as well as its resistant counterpart, in an animal model of depression

Flinders Sensitive Line (FSL) rat is as an animal model of depression with altered parameters of the serotonergic (5-HT) system function (5-HT synthesis rates, tissue concentrations, release, receptor density and affinity), as well as an altered sensitivity of these parameters to different 5-HT based antidepressants. The effects of acute and chronic treatments with the 5-HT(1B) agonist, CP-94253 on 5-HT synthesis, in the FSL rats and the Flinders Resistant Line (FRL) controls were measured using α-[(14)C]methyl-L-tryptophan (α-MTrp) autoradiography. CP-94253 (5mg/kg), or an adequate volume of saline, was injected i.p. as a single dose in the acute experiment or delivered via the subcutaneously implanted osmotic minipump (5 mg/kg/day for 14 days) in the chronic experiment. The acute treatment with CP-94253 significantly decreased the 5-HT synthesis in both the FRL and FSL rats, with a more widespread effect in the FRL rats. Chronic treatment with CP-94253 significantly decreased 5-HT synthesis in the FRL rats, while 5-HT synthesis in the FSL rats was significantly increased throughout the brain. In both the acute and chronic experiment, the FRL rats had higher brain 5-HT synthesis rates, relative to the FSL rats. The shift in the direction of the treatment effect from acute to chronic, using the 5-HT(1B) agonist, CP-94253, on 5-HT synthesis in the FSL model of depression, with an opposite effect on the control FRL rats, suggests the differential adaptation of the 5-HT system in the FSL and FRL rats to chronic stimulation of 5-HT(1B) receptors.

Postnatal changes in tryptophan hydroxylase and serotonin transporter immunoreactivity in multiple brainstem nuclei of the rat: implications for a sensitive period

Previously, we found that the brainstem neuronal network in normal rats undergoes abrupt neurochemical, metabolic, and physiological changes around postnatal days (P) 12-13, a critical period when the animal's response to hypoxia is also the weakest. This has special implications for sudden infant death syndrome (SIDS), insofar as seemingly normal infants succumb to SIDS when exposed to respiratory stressors (e.g., hypoxia) during a narrow postnatal window. Because an abnormal serotonergic system has recently been implicated in SIDS, we conducted a large-scale investigation of the 5-HT-synthesizing enzyme tryptophan hydroxylase (TPH) and serotonin transporter (SERT) with semiquantitative immunohistochemistry in multiple brainstem nuclei of normal rats aged P2-21. We found that 1) TPH and SERT immunoreactivity in neurons of raphé magnus, obscurus, and pallidus and SERT in the neuropil of the pre-Bötzinger complex, nucleus ambiguus, and retrotrapezoid nucleus were high at P2-11 but decreased markedly at P12 and plateaued thereafter until P21; 2) SERT labeling in neurons of the lateral paragigantocellular nucleus (LPGi) and parapyramidal region (pPy) was high at P2-9 but fell significantly at P10, followed by a gradual decline until P21; 3) TPH labeling in neurons of the ventrolateral medullary surface was stable except for a significant fall at P12; and 4) TPH and SERT immunoreactivity in a number of other nuclei was relatively stable from P2 to P21. Thus, multiple brainstem nuclei exhibited a significant decline in TPH and SERT immunoreactivity during the critical period, suggesting that such normal development can contribute to a narrow window of vulnerability in postnatal animals.

Developmental regulation of tryptophan hydroxylase messenger RNA expression and enzyme activity in the raphe and its target fields

Tryptophan hydroxylase is the rate-limiting enzyme in the synthesis of serotonin and during development, brain serotonin levels and tryptophan hydroxylase activities increase. Increased tryptophan hydroxylase activity could result from alterations in tryptophan hydroxylase messenger RNA levels, translation, and/or post-translational regulation. Tryptophan hydroxylase messenger RNA levels in the dorsal raphe nucleus increased 35-fold between embryonic day 18 and postnatal day 22, measured by quantitative in situ hybridization, then decreased by 40% between postnatal days 22 and 61. These changes correlated with tryptophan hydroxylase enzyme activities in the raphe nuclei as expected, but not in cortical or hippocampal targets. Tryptophan hydroxylase messenger RNA expression in the nucleus raphe obscuris increased 2.5-fold between postnatal days 8 and 22 but did not correlate with enzyme activity in the spinal cord. Using an in vitro model of serotonergic raphe neuron differentiation, serotonergic differentiation was associated with an increase in both tryptophan hydroxylase promoter activity and protein expression. In vivo, tryptophan hydroxylase messenger RNA levels per single cell and per brain section were correlated during development up to postnatal day 22, but not beyond for both the dorsal raphe nucleus and nucleus raphe obscuris. Between postnatal days 22 and 61 single cell levels of tryptophan hydroxylase messenger RNA in the dorsal raphe nucleus did not change yet the levels per brain section significantly decreased by 40%. During the same period in the nucleus raphe obscuris, tryptophan hydroxylase messenger RNA levels per single cell signifcantly increased by 30% yet levels per brain section did not change. Comparison of tryptophan hydroxylase messenger RNA levels per cell and per brain section indicated a serotonergic loss between postnatal days 22 and 61 in both the dorsal raphe nucleus and nucleus raphe obscuris and may reflect either a loss of neurotransmitter phenotype or cell death. This study is the first to characterize the expression of brain tryptophan hydroxylase messenger RNA during rat development. In addition, this study is the first to report the activity of tryptophan hydroxylase in the spinal cord and hippocampus in the embryonic and neonatal rat. Together, the data provide a better understanding of the intricate relationship between patterns of tryptophan hydroxylase messenger RNA expression and enzyme activity.

Immortalization of neuroendocrine pinealocytes from transgenic mice by targeted tumorigenesis using the tryptophan hydroxylase promoter

Tryptophan hydroxylase (TPH) is the first enzyme in both serotonin and melatonin biosynthesis in neuroendocrine cells of the pineal gland. The lack of immortalized neuroendocrine pineal cell lines has been a major obstacle to the study of the tissue-specific and circadian regulation of TPH gene expression in the pineal gland. Previously, we demonstrated that a 6.1 kb 5' upstream region of the mouse TPH gene directs the restricted expression of a lacZ reporter gene to the pineal gland and the raphe nuclei of transgenic mice. Therefore, to develop TPH-expressing pineal cell lines we first established transgenic mice carrying a construct consisting of 6.1 kb of 5' flanking region fused to the SV40 T-antigen. These animals developed highly invasive pineal tumors and died at 12-15 weeks of age. The pineal tumors obtained from the transgenic mice were utilized to establish the immortalized pinealocyte-derived cell lines. These cells express two marker enzymes, TPH and serotonin N-acetyltransferase (NAT). In pineal gland TPH and NAT expressions have been known to be regulated during circadian cycle. The two established cell lines therefore promise to be a valuable in vitro model system for the study of the rhythmic nature of the pineal function at molecular level in mammal.

Alcohol exposure during development alters hypothalamic neurotransmitter concentrations

The effect of exposure to alcohol during a period roughly equivalent to the human third trimester on neurotransmitter content in the rat hypothalamus was examined. The alcohol exposure was accomplished via an artificial rearing procedure. The alcohol group was exposed to 5 g/kg/day of ethanol from postnatal day (PD) 4 to 10. There was an artificially reared control group not exposed to alcohol and a normally reared control group. Noradrenaline, dopamine, homovanillic acid (HVA), 3,4-dihydroxyphenylacetic acid (DOPAC), serotonin, and 5-hydroxyindoleacetic acid (5-HIAA) concentrations were measured using high performance liquid chromatography with electrochemical detection in juvenile and adult rats. There were no effects in juvenile rats. In adult rats, alcohol exposure from PD 4 to 10 increased hypothalamic content of noradrenaline, dopamine, serotonin and 5-HIAA. While adult females had greater amounts of hypothalamic serotonin and 5-HIAA than adult males, there were no interactions of sex with alcohol exposure. These results suggest that hypothalamic function is seriously disrupted by alcohol exposure during development.

Drastic and selective hyperinnervation of central serotonergic neurons in a lethal neurodevelopmental mouse mutant, Anorexia (anx)

The autosomal recessive lethal anorexia mutation in mice (anx/anx) causes starvation in preweanlings. In addition, this murine neurodevelopmental mutant shows other distinct phenotypic characteristics and dysfunctional behaviors. Previous studies strongly suggested that the mutation results in elevated serotonergic stimulation, because these traits are characteristic of such overstimulation and because brain serotonin is believed to have an inhibitory effect on feeding behavior. In this report, we show extensive serotonergic hyperinnervation in normal target fields (hippocampus, cortex, olfactory bulb and cerebellum) of mutant mice. Despite the extensive hyperinnervation, the normal laminar organization of the brain was retained. The specificity of the mutation to the serotonergic system was confirmed by demonstration of normal catecholaminergic innervation in the central nervous system (CNS), and this specificity was especially striking in a common target field, the cerebellum. Serotonergic hyperinnervation in these mutant preweanling mice may represent the underlying etiology of increased serotonergic stimulation which leads to anorexic starvation, abnormal behavior, and premature death.

Developmental changes in the brain-stem serotonergic nuclei of teleost fish and neural plasticity

1. During early ontogeny, the serotonergic neurons in the brain stem of the three-spined stickleback shows a temporal and spatial developmental pattern that closely resembles that of amniotes. 2. However, in the adult fish, only the midline nuclei of the rostral group (dorsal and median raphe nuclei) and the dorsal lateral tegmental nucleus are consistently serotonin-immunoreactive (5-HTir), whereas the groups of the upper and lower rhombencephalon (raphe pontis, raphe magnus, and raphe pallidus/obscurus nuclei) are variable and, when present, contain relatively small numbers of 5-HTir neurons. 3. Using specific antisera against tryptophan 5-hydroxylase and aromatic L-amino acid decarboxylase, we have shown that the lateral B9 group and the groups of the upper and lower rhombencephalon are consistently present in adult sticklebacks. The results are discussed in relation to other known instances of neurotransmitter plasticity or transient neurotransmitter expression in teleost fish. 4. While there are several instances of transient expression of neurotransmitter markers by discrete neuronal populations, there is so far no evidence of changes from one neurotransmitter phenotype to another in the brain of teleost fish. However, there are indications of plasticity of expression of catecholamines and indoleamines, and their respective synthesizing enzymes, as reflected in age-dependent changes and variation between individuals of different physiological status. 5. As the brain grows continuously in teleost fish, and new neurons are added from proliferative regions, synaptic connections may be expected to undergo remodeling in all brain regions throughout life. Thus, the teleostean brain may be considered a suitable model for experimental studies of different aspects of neural plasticity.

A 6.1 kb 5' upstream region of the mouse tryptophan hydroxylase gene directs expression of E. coli lacZ to major serotonergic brain regions and pineal gland in transgenic mice

Tryptophan hydroxylase (TPH) catalyzes the first step of serotonin biosynthesis in serotonergic neurons and neuroendocrine cells. Serotonin influences diverse vital physiological functions and is thought to play an important role in several human psychiatric disorders. To localize DNA element(s) important for serotonergic tissue-specific expression of TPH, 6.1 kb of the 5' flanking region of the mouse TPH gene was fused to the coding region of the E. coli lacZ gene, and expression of the resulting fusion gene was analyzed in transgenic mice. The 6.1 kb of 5' flanking sequence was able to direct the expression of a lacZ reporter gene to serotonergic tissues in six lines of transgenic mice. A high level of lacZ expression in transgenic mice carrying the fusion gene was detected in the pineal gland as well as a moderate level of lacZ expression in serotonergic brain regions such as the median and dorsal raphe nuclei, the nuclei raphe magnus and raphe pallidus. In contrast, a smaller 5' flanking sequence of 1.1 kb directed no detectable serotonergic tissue-specific lacZ expression in five lines of transgenic mice. These results presented in this paper suggest first that DNA elements critical to serotonergic tissue-specific expression reside between -6.1 kb and -1.1 kb of 5' flanking region of the mouse TPH gene, but second that this region confers a restricted tissue-specific expression.

Changes in activity and mRNA for rat tryptophan hydroxylase and aromatic L-amino acid decarboxylase of brain serotonergic cell bodies and terminals following neonatal 5,7-dihydroxytryptamine

In the present study, we examined time-dependent changes in activity, mRNA and immunoreactivity of the serotonin biosynthetic enzymes, tryptophan hydroxylase (TPH) and aromatic L-amino acid decarboxylase (AADC) in dorsal raphe nucleus (DRN), caudal brainstem and hypothalamus, following intracisternal injection of 5,7-dihydroxytryptamine (5,7-DHT) in neonatal rats. TPH activity in central serotonergic cell bodies and terminals was reduced to 20-30% of control levels at 1-8 weeks after neonatal, low-dose 5,7-DHT injection (24 micrograms free base). In contrast, AADC activity was either not changed or decreased to 40% of control levels, depending on the region. In situ hybridization and immunocytochemical staining indicated that 5,7-DHT caused a marked reduction in TPH and AADC message levels as well as the number of 5-HT and AADC-immunoreactive cells within the DRN as early as 1 week after 5,7-DHT. Even 15 weeks after drug administration recovery did not occur. This apparent neuronal loss was region-specific suggesting that some serotonergic neurons are more resistant to neonatal 5,7-DHT treatment than others. Taken together, these studies indicate that neonatal treatment with 5,7-DHT produces a marked and permanent (up to 15 weeks) reduction in the number of central serotonergic neurons.

A morphometric study of age-related changes in serotonin-immunoreactive cell groups in the brain of the coho salmon, Oncorhynchus kisutch Walbaum

In the coho salmon there is a transient increase in total brain concentrations of serotonin during smolt transformation which occurs midlife, just before down-stream migration to the ocean. There is also a gradual age-related increase in total brain serotonin concentrations. These increases may be due to reorganization of the central serotonergic system, changes in serotonin turnover, or both. They may be related to the specific physiological conditions during different life stages of salmon, or to ongoing growth and plastic changes of the brain. In the present study we have compared serotonin-immunoreactive (5-HTir) cell groups in 1-year-old freshwater presmolt and 2-year-old seawater postsmolt salmon. Our data indicate a continuous growth of the 5-HTir cell groups in terms of an increase in numbers of 5-HTir neurons in the cell groups of the pretectum and the brain stem, and an increase in the volumes of such neurons and cell groups. However, when related to the increase in total brain volume, i.e., the volume that may be innervated by the 5-HTir neurons, the ratio of 5-HTir neurons per mm3 decreased. The largest decreases were observed in the median raphe nucleus (P less than 0.005) and the B9 group (P less than 0.05). The ratio of volumes of the brain nuclei containing 5-HTir neurons relative to total brain volume was remarkably constant when comparing pre- and postsmolt brains: only the pretectal nucleus showed a significant decrease (P less than 0.01) in relative volume. The total volume of 5-HTir neurons increased in postsmolts (P less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for decreased transport of tryptophan hydroxylase in Alzheimer's disease

Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in the synthesis of serotonin and a specific marker for serotonergic neurons. These neurons are affected in Alzheimer's disease (AD) in several ways: serotonin is decreased in axon terminals, serotonin neurons accumulate neurofibrillary protein, and these neurons are lost in AD brains. One subcellular mechanism which may underlie degeneration of neurons in AD is decreased axonal transport with accumulation of enzymes and their potentially toxic metabolites in the cell body. To determine whether there is a defect in axonal transport in serotonin neurons in AD we measured TPH activity, serotonin and its oxidative metabolite 5-hydroxyindoleacetic acid (5-HIAA) in dorsal raphe cell bodies from Alzheimer and control cases. TPH activity is increased 4.7-fold in raphe neuron cell bodies in Alzheimer brains. Serotonin and 5-HIAA are increased by 4.0- and 2.0-fold, respectively in Alzheimer compared to control raphe cell bodies. In contrast, in synaptic terminals of the amygdala 5-HT and 5-HIAA were decreased by 41% and 50%, respectively in the same AD cases. We propose that the accumulation of TPH and its products in the raphe perikarya in AD results from a diminished transport of TPH to axon terminals. The accumulation of oxidative metabolites of serotonin may contribute to the degeneration of serotonergic neurons in AD.

Strain differences between albino and pigmented rats in monoamine-synthesizing enzyme activities of brain, retina and adrenal gland

The present study compared the activities of some of the monoamine synthesizing enzymes in several brain regions, the retina as well as adrenal gland of albino Sprague-Dawley (SD) and Long-Evans hooded (LE) rats. Brainstem, hypothalamic and retinal tyrosine hydroxylase (TH) activity were significantly higher in LE than in SD. In addition to higher enzyme activity, a larger number of TH-immunoreactive perikarya as well as a higher concentration of TH-immunoreactive processes were observed in the retina of LE rats. There was no strain difference in TH activity of caudate nucleus (CN) and substantia nigra (SN). In contrast to brain regions and retina, adrenal TH activity was markedly higher in SD than in LE animals. Aromatic L-amino acid decarboxylase (AADC) activity of both the brainstem and adrenal gland in the LE strain was lower than in SD animals. No differences in the AADC activity of hypothalamus, SN and CN were found between LE and SD strains. Phenylethanolamine N-methyltransferase (PNMT) activity of the hypothalamus, retina and adrenal gland of LE strains was significantly lower than in SD rats. In spite of the difference in the enzyme activity, there were no marked morphological changes observed in PNMT-immunostaining patterns between the retina of LE and SD rats. Tryptophan hydroxylase activity of both the brainstem and hypothalamus did not exhibit strain differences.(ABSTRACT TRUNCATED AT 250 WORDS)

Behavioural deficits and serotonin depletion in adult rats after transient infant nasal viral infection

Dysfunction of subcortical serotoninergic neurons has been implicated in some behaviour disturbances. The serotoninergic neurons in the dorsal and median raphe project widely in the brain. They innervate the olfactory bulbs and can be targets for exogenous agents attacking the olfactory epithelium and bulbs. We report here an injury to the serotoninergic neurons after intranasal infection in 12-day-old rats with a temperature-sensitive mutant of vesicular stomatitis virus. The brain infection was focal and transient. Viral antigens could no longer be detected 13-15 days after infection. In spite of this the animals, as adults, had a severe serotonin depletion in the cerebral cortex and hippocampus, and showed abnormal locomotor and explorative behaviour as well as learning deficits. The neocortex was histologically intact and parameters related to other neurotransmitters such as dopamine, noradrenaline, GABA and acetylcholine showed no marked changes. A relatively selective damage to serotoninergic nuclei as a result of virus neuroinvasion through a natural portal of entry, may constitute a new pathogenetic mechanism for cortical dysfunction and behavioural deficits.

Tryptophan hydroxylase activity in hypothalamus and brainstem of neonatal and adult rats treated with hydrocortisone or parachlorophenylalanine

The present study has been undertaken to determine whether glucocorticoid, and parachlorophenylalanine (PCPA, a tryptophan hydroxylase inhibitor) affects tryptophan hydroxylase (TPH) levels in brainstem and hypothalamus of neonatal and adult rats. Our results show that: (1) administration of hydrocortisone causes small but significant increases in TPH activity of neonatal brainstem: (2) treatment with PCPA plus glucocorticoid results in a marked decrease of TPH activity in brainstem and hypothalamus of both neonatal and adult rats.

Tryptophan hydroxylase activity and 5-hydroxytryptamine-immunoreactive cells in the superior cervical ganglion of hydrocortisone-treated neonatal rats

Early postnatal glucocorticoid injections led to a large increase in the number of 5-hydroxytryptamine(5-HT)-immunoreactive small cells in the rat superior cervical ganglion. Tryptophan hydroxylase (TPH) activity in ganglia from animals treated with glucocorticoids was not significantly different from saline-injected controls. Both 5-HT immunoreactivity and TPH activity were decreased in ganglia from animals treated with glucocorticoid and the TPH inhibitor parachlorophenylalanine, but not in animals treated with 5-HT uptake inhibitor fluoxetine. These results suggest that 5-HT is synthesized in the small intensely fluorescent (SIF) cells.

Immunohistochemical distribution of serotonin in spinal autonomic nuclei: II. Early and late postnatal ontogeny in the rat

These studies reveal that the postnatal ontogeny of serotonin (5HT) in the sympathetic nuclei of the rat spinal cord is protracted; the adult complement of 5HT-immunoreactive fibers is not achieved until at least 60 days of age. As descending serotonin fibers innervate and demarcate the distribution of preganglionic sympathetic nuclei, rostral-caudal and temporal gradients exist. Additionally, a heterogeneous segmental 5HT ontogenetic pattern is observed in sympathetic nuclei. Most serotonin fibers in laminae VII and X are unorganized at birth except for some sympathetic nuclei in high thoracic regions where the 5HT sympathetic pattern is being initiated. By postnatal day 6 the framework of the 5HT pattern is established in all sympathetic nuclei, and by postnatal day 16 a pattern is formed, which develops into the compact adult state by postnatal day 60. The protracted period of sympathetic 5HT development corresponds with the length of time it takes for the autonomic nervous system to mature. In addition, 5HT intraspinal cell bodies are observed at all time points examined, except for the day of birth, and are found in the same regions as adult 5HT neurons, i.e., dorsal or lateral to the central canal in laminae VII and X and in all spinal segments except cervical levels. Many of the 5HT neurons are pericanalicular and bipolar in appearance. Multipolar 5HT neurons are first observed on postnatal day 45.

Ontogenetic and pharmacological dissociation of various components of locomotor activity and habituation in the rat

Sprague-Dawley-derived male rats were used to investigate locomotor activity and habituation in an open field as a joint function of developmental age (2-6 weeks), pattern of test exposure (single 30-min test vs three 5-min tests at 24-hr intervals), and treatment conditions (i.p. saline, d-amphetamine sulfate 1 mg/kg, or scopolamine hydrocloride 0.5 mg/kg). No-drug animals showed low activity levels in both tests at the end of the second week, intermediate response rates at the end of the third week, and a typical adult-like pattern at later ages (high initial activity followed by marked within-session or between-session habituation). Amphetamine effects varied considerably depending jointly on age and type of test. At the end of the second week, the drug hyperactivity was much more marked in successive brief tests than in the single extended test. One week later, the response increase was rather uniform in both tests. At the end of the fourth week, the sensitivity profile was reversed, consisting of a large drug effect in the extended test but not in successive brief tests. Scopolamine was still without effects at this age, while a typical hyperactivity was produced by the drug in 6-week-old animals. These data show that, at least in the rat strain used, the functional maturation of muscarinic regulatory systems is not a necessary condition either for the appearance of an adult-like response pattern, or for the occurrence of the age- and test-related changes of the amphetamine profile.(ABSTRACT TRUNCATED AT 250 WORDS)