[3H]5-hydroxytryptamine binding to brain astroglial cells: differences between intact and homogenized preparations and mature and immature cultures

Effects of amyloid-β peptides on the serotoninergic 5-HT1A receptors in the rat hippocampus

A recent [(18)F]MPPF-positron emission tomography study has highlighted an overexpression of 5-HT(1A) receptors in the hippocampus of patients with mild cognitive impairment compared to a decrease in those with Alzheimer's disease (AD) [Truchot, L., Costes, S.N., Zimmer, L., Laurent, B., Le Bars, D., Thomas-Antérion, C., Croisile, B., Mercier, B., Hermier, M., Vighetto, A., Krolak-Salmon, P., 2007. Up-regulation of hippocampal serotonin metabolism in mild cognitive impairment. Neurology 69 (10), 1012-1017]. We used in vivo and in vitro neuroimaging to evaluate the longitudinal effects of injecting amyloid-β (Aβ) peptides (1-40) into the dorsal hippocampus of rats. In vivo microPET imaging showed no significant change in [(18)F]MPPF binding in the dorsal hippocampus over time, perhaps due to spatial resolution. However, in vitro autoradiography with [(18)F]MPPF (which is antagonist) displayed a transient increase in 5-HT(1A) receptor density 7 days after Aβ injection, whereas [(18)F]F15599 (a radiolabelled 5-HT(1A) agonist) binding was unchanged suggesting that the overexpressed 5-HT(1A) receptors were in a non-functional state. Complementary histology revealed a loss of glutamatergic neurons and an intense astroglial reaction at the injection site. Although a neurogenesis process cannot be excluded, we propose that Aβ injection leads to a transient astroglial overexpression of 5-HT(1A) receptors in compensation for the local neuronal loss. Exploration of the functional consequences of these serotoninergic modifications during the neurodegenerative process may have an impact on therapeutics targeting 5-HT(1A) receptors in AD.

Embryonic and postnatal development of the serotonergic raphe system and its target regions in 5-HT1A receptor deletion or overexpressing mouse mutants

The neurotransmitter 5-HT regulates early developmental processes in the CNS. In the present study we followed the embryonic and postnatal development of serotonergic raphe neurons and catecholaminergic target systems in the brain of 5-HT1A receptor knockout (KO) and overexpressing (OE) in comparison with wild-type (WT) mice from embryonic day (E) 12.5 to postnatal day (P) 15.5. Up to P15.5 no differences were apparent in the differentiation and distribution of serotonergic neurons in the raphe area as revealed by the equal number of serotonergic neurons in the dorsal raphe in all three genotypes. However, the establishment of serotonergic projections to the mesencephalic tegmentum and hypothalamus was delayed at E12.5 in KO and OE animals and projections to the cerebral cortex between E16.5 and E18.5 were delayed in OE mice. This delay was only transient and did not occur in other brain areas including septum, hippocampus and striatum. Moreover, OE mice caught up with WT and KO animals postnatally such that at P1.5 serotonergic innervation of the cortex was more extensive in the OE than in KO and WT mice. Tissue levels of 5-HT and of its main metabolite 5-hydroxyindoleacetic acid as well as 5-HT turnover were considerably higher in brains of OE mice and slightly elevated in KO mice in comparison with the WT, starting at E16.5 through P15.5. The initial differentiation of dopaminergic neurons and fibers in the substantia nigra at E12.5 was transiently delayed in KO and OE mice as compared with WT mice, but no abnormalities in noradrenergic development were apparent in later stages. The present data indicate that 5-HT1A receptor deficiency or overexpression is associated with increased 5-HT synthesis and turnover in the early postnatal period. However, they also show that effects of 5-HT1A KO or OE on the structural development of the serotonergic system are at best subtle and transient. They may nonetheless contribute to the establishment of increased or reduced anxiety-like behavior, respectively, in adult mice.

Cajal's hypotheses on neurobiones and neurotropic factor match properties of microtubules and S-100 beta

Cajal described both the morphology and plasticity of neurons. He summarized the structure of neurons as composed of membrane, protoplasm, Golgi apparatus, nucleus, spongioplasm and neurofibrils (cytoskeleton). He initially considered the cytoskeleton as absorbing excitation energy and forming a "conductive pathway in the protoplasm" within the neuron. Later, he viewed the neurofibrillary threads as independent, living entities and called them neurobiones. Cajal recognized neuroplasticity in development, memory, sleep, injury and dementia, as well as after exposure to cold and starvation. He noted cytoskeletal changes during these events. However, he did not causatively connect the plastic changes in neurons with the changes in cytoskeleton. Finally, Cajal proposed a theory of chemoaffinity in 1892, and modified his neurotropic theory over the next 40 years. Today we accept that changes in the cytoskeleton produce changes in neuronal morphology. The properties of the cytoskeleton and neurobione as described by Cajal are similar to those of microtubules. These long intraneuronal neurofibrils are polymers of the protein tubulin and, whilst not being living entities, are highly dynamic, sensitive to environmental stimuli, and stabilized by microtubule associated proteins (MAPs). Furthermore, Cajal was very specific in his characterization of the neurotropic factor derived from Schwann cells. Initially, he thought the chemicals attracted the axonal fibers, but later he wrote that the factor was not attractant but rather was involved in assimilation, growth and ramifications. The neurotropic hypothesis described by Cajal in Degeneration and Regeneration in the Nervous System is more similar to a neurite extension factor (NEF) than to a neurotrophic growth factor or specific chemoaffinity (attractant) molecule. S-100 beta is the major NEF found in PNS Schwann cells and CNS astroglial cells. In summary, the views of Cajal on neuroplasticity, its frequency and function, agree with the modern hypothesis of neuronal instability. This concept states that MAPs regulate microtubule stability by a S-100 beta sensitive phosphorylation processes. Serotonin, by acting on the astroglial 5-HT1A receptor, releases S-100 beta and regulates neuronal morphology and apoptosis. This neuronal-glial connection provides a fresh view for linking neuroplasticity, mental illness, and memory with changes in the cytoskeleton.

Modern views on an ancient chemical: serotonin effects on cell proliferation, maturation, and apoptosis

Evolutionarily, serotonin existed in plants even before the appearance of animals. Indeed, serotonin may be tied to the evolution of life itself, particularly through the role of tryptophan, its precursor molecule. Tryptophan is an indole-based, essential amino acid which is unique in its light-absorbing properties. In plants, tryptophan-based compounds capture light energy for use in metabolism of glucose and the generation of oxygen and reduced cofactors. Tryptophan, oxygen, and reduced cofactors combine to form serotonin. Serotonin-like molecules direct the growth of light-capturing structures towards the source of light. This morphogenic property also occurs in animal cells, in which serotonin alters the cytoskeleton of cells and thus influences the formation of contacts. In addition, serotonin regulates cell proliferation, migration and maturation in a variety of cell types, including lung, kidney, endothelial cells, mast cells, neurons and astrocytes). In brain, serotonin has interactions with seven families of receptors, numbering at least 14 distinct proteins. Of these, two receptors are important for the purposes of this review. These are the 5-HT1A and 5-HT2A receptors, which in fact have opposing functions in a variety of cellular and behavioral processes. The 5-HT1A receptor develops early in the CNS and is associated with secretion of S-100beta from astrocytes and reduction of c-AMP levels in neurons. These actions provide intracellular stability for the cytoskeleton and result in cell differentiation and cessation of proliferation. Clinically, 5-HT1A receptor drugs decrease brain activity and act as anxiolytics. The 5-HT2A receptor develops more slowly and is associated with glycogenolysis in astrocytes and increased Ca(++) availability in neurons. These actions destabilize the internal cytoskeleton and result in cell proliferation, synaptogenesis, and apoptosis. In humans, 5-HT2A receptor drugs produce hallucinations. The dynamic interactions between the 5-HT1A and 5-HT2A receptors and the cytoskeleton may provide important insights into the etiology of brain disorders and provide novel strategies for their treatment.

S-100beta protects cultured neurons against glutamate- and staurosporine-induced damage and is involved in the antiapoptotic action of the 5 HT(1A)-receptor agonist, Bay x 3702

The serotonin (5-HT)(1A) receptor agonists have already been shown to protect cultured neurons from excitotoxic as well as from apoptotic damage [B. Ahlemeyer, J. Krieglstein, Stimulation of 5-HT(1A) receptors inhibits apoptosis induced by serum deprivation in cultured neurons from chick embryo, Brain Res. 777 (1997) 179-186. ; B. Ahlemeyer, A. Glaser, C. Schaper, I. Semkova, J. Krieglstein, The 5-HT(1A) receptor agonist, Bay x 3702, inhibited apoptosis induced by serum deprivation in cultured neurons, Eur. J. Pharmacol. 370 (1999) 211-216.; J.H.M. Prehn, M. Welsch, C. Backhauss, J. Nuglisch, F. Ausmeier, C. Karkoutly, J. Krieglstein, Effects of serotonergic drugs in experimental brain ischemia: evidence for a protective role of serotonin in cerebral ischemia, Brain Res. 630 (1993) 110-120.; I. Semkova, P. Wolz, J. Krieglstein, Neuroprotective effect of 5-HT(1A) receptor agonist, Bay x 3702, demonstrated in vitro and in vivo, Eur. J. Pharmacol. 359 (1998) 251-260.; B. Suchanek, H. Struppeck, T. Fahrig, The 5-HT(1A) receptor agonist, Bay x 3702, prevents staurosporine-induced apoptosis, Eur. J. Pharmacol. 355 (1998) 95-101.] and to increase the release of the neurotrophic protein, S-100beta [P.M. Whitaker-Azmitia, R. Murphy, E.C. Azmitia, Stimulation of astroglial 5-HT(1A) receptors releases the serotonergic growth factor, protein S-100, and alters astroglial morphology, Brain Res. 497 (1989) 80-86. ; P.M. Whitaker-Azmitia, R. Murphy, E.C. Azmitia, S-100 protein is released from astroglial cells by stimulation of 5-HT(1A) receptors, Brain Res. 528 (1990) 155-158.]. In this study, we tried to find out whether S-100beta can protect cultured neurons from glutamate- and staurosporine-induced damage and whether the neuroprotective activity of the highly selective 5-HT(1A) receptor agonist, Bay x 3702, is mediated by an induction of S-100beta. Extracellularly added S-100beta (1-10 ng/ml) reduced staurosporine-induced damage in pure neuronal cultures from chick embryo telencephalon as well as in mixed neuronal/glial cultures from neonatal rat hippocampus. In addition, S-100beta (1 ng/ml) reduced neuronal death induced by exposure to glutamate (0.25 mM, 30 min) in mixed neuronal/glial cultures from neonatal rat hippocampus. In cultured rat cortical astrocytes, a 24 h-treatment with Bay x 3702 (1 nM) increased the S-100beta content in the culture medium from 2.2+/-0.3 (controls) to 6.2+/-0.7 ng/ml. In the adult rat, a 4 h-infusion of 4 microg/kg Bay x 3702 (i.v.) was found to increase the S-100beta content in the striatum 6 h after the beginning of the infusion to 153+/-37 microg/g compared with 60+/-20 microg/g in vehicle-treated rats. Bay x 3702 had no effect on the S-100beta content in the rat hippocampus. Finally, we tried to block the protective effect of Bay x 3702 against staurosporine-induced damage in mixed neuronal/glial cultures from rat neonatal hippocampus by anti-S-100beta antibodies. We found only a partial blockade, although the antibodies fully blocked the antiapoptotic effect of S-100beta itself demonstrating that the antibody was effective in blocking neuroprotection by S-100beta. Thus, we conclude that S-100beta was able to protect cultured neurons against glutamate- and staurosporine-induced damage. Furthermore, S-100beta mediated partially the protective effect of the 5-HT(1A) receptor agonist, Bay x 3702, against staurosporine-induced apoptosis in mixed neuronal/glial cultures from neonatal rat hippocampus.

Prenatal cocaine exposure potentiates 5-HT(2a) receptor function in male and female rat offspring

We have reported previously prenatal cocaine-induced functional deficits in serotonergic terminals, and gender-specific supersensitivity of postsynaptic 5-HT(1A) receptor-mediated hormone responses in offspring. This study investigates the effects of prenatal exposure to cocaine on postsynaptic 5-HT(2A) receptor-mediated responses in prepubescent male and female offspring. Pregnant rats were administered saline or (-)cocaine (15 mg/kg, s.c., b.i.d) from gestational day 13 through 20. Changes in 5-HT(2A) receptor function in offspring were assessed by differences in the ability of DOI [4-iodo, 2,5-dimethoxyphenyl-isopropylamine; 2. 0 mg/kg, s.c.] to elevate plasma levels of the hormones ACTH, corticosterone and renin. Basal hormone levels in male and female progeny were unaffected by prenatal cocaine exposure. However, prenatal exposure to cocaine significantly potentiated the magnitude of the ACTH response to DOI in both male (+19%) and female (+43%) progeny. Similarly, the DOI-induced elevation of plasma renin was markedly potentiated in male (+51%) and female (+83%) cocaine-exposed offspring. Although DOI significantly elevated corticosterone levels in both male and female offspring, the magnitude of corticosterone responses was not altered by prenatal exposure to cocaine. Densities of agonist ((125)I-DOI)-labeled receptors in hypothalamus and cortex were unaltered by prenatal exposure to cocaine. These data indicate prenatal cocaine-induced supersensitivity of postsynaptic 5-HT(2A) receptor function in male and female offspring without changes in receptor density. Synapse: 35:163-172, 2000.

Cultured astrocytes express messenger RNA for multiple serotonin receptor subtypes, without functional coupling of 5-HT1 receptor subtypes to adenylyl cyclase

The literature describing the expression of 5-HT receptor subtypes by astrocytes is controversial and incomplete. It is clear that primary cultures of astrocytes express receptors of the 5-HT2 family coupled to phospholipase C and of the 5-HT7 receptor family positively coupled to adenylyl cyclase. Cultured astrocytes have also been reported to express receptors of the 5-HT1 family, although the exact subtypes present are unknown. In the present study we have investigated which of the known rat G-protein coupled 5-HT receptor mRNAs are expressed by cultured astrocytes. Reverse transcriptase-polymerase chain reaction (RT-PCR) revealed expression of 5-HT1A, 5-HT1B, 5-HT1D, 5-HT1F, 5-HT2A, 5-HT2B, 5-HT2C, 5-HT5B, 5-HT6 and 5-HT7 receptor mRNAs in astrocytes derived from 2-day old rats and cultured for 10-12 days. Messenger RNAs for 5-HT4 and 5-HT5A receptors were not detected. The functional expression of 5-HT1 receptor subtypes was investigated by measuring the ability of 5-HT1 receptor agonists: 8-OH-DPAT (5-HT1A receptors), RU24969 (5-HT1A, 5-HT1B, 5-HT1D, and 5-HT1F receptors) or sumatriptan (5-HT1B, 5-HT1D, and 5-HT1F receptors) to modulate forskolin or isoproterenol stimulated cAMP production. These compounds, at concentrations up to 10 microM, did not significantly attenuate cAMP production. These results indicate that although astrocytes express mRNA for each of the five 5-HT1 receptor subtypes which have been isolated from the rat, these receptors are not coupled to the inhibition of adenylyl cyclase.

5-HT1A receptor agonist reverses adrenalectomy-induced loss of granule neuronal morphology in the rat dentate gyrus

Adrenal steroids are important for maintaining neuronal maturation in the adult rats. Two weeks after bilateral adrenalectomy (ADX), hippocampal MAP-2 (microtubule associated protein-2) and calbindin immunoreactivity (IR) decreased in the molecular layer of the superior blade of the dentate gyrus. The molecular and granular cell layer at the lateral tip of the superior blade decreased in width by 32% and 50%, respectively. The granule neurons showed reduced staining with Nissl and an anti-calbindin antibody. These changes suggested a loss of the mature neuronal morphology. In this same localized regions, two glial proteins, glial fibrillary acidic protein (GFAP) and S-100 beta showed dramatically reduced immunoreactivity. These effects induced by ADX were reduced within 72 hrs by ipsapirone (1 mg/kg), a 5HT1A receptor agonist. Loss of adult neuronal morphology by ADX, and reversal by the 5HT1A agonist, may be evidence of the trophic importance of the 5HT1A receptor in granule neurons of hippocampus.

Neural and astroglial effects of a chronic parachlorophenylalanine-induced serotonin synthesis inhibition

Serotonin (5HT) is one of the classical neurotransmitters expressed earlier in the embryonic rat brain, and it was proposed as a developmental signal in the central nervous system. In the adult brain, 5HT seems to be involved in neuronal plasticity. It was postulated that S-100 protein, a glial neurotrophic factor, could be modulated by 5HT probably through the glial 5HT1A receptors. In a model of chronic inhibition of endogenous 5HT synthesis produced by the daily administration of parachlorophenylalanine (PCPA) for 2 wk, we have studied by immunohistochemical methods and digital morphometric analysis the expression of two proteins present in rat brain astrocytes: glial fibrillary acidic protein (GFAP) and S-100 protein. The effectiveness of the PCPA treatment was tested by the use of specific anti-5HT antibodies that showed absence of 5HT fibers in 5HT innervation areas like frontal cortex and hippocampus. Different effects of PCPA treatment on serotoninergic raphe nuclei were observed: dorsal raphe nucleus (DRN) seemed to be more sensitive to the PCPA's action than ventral raphe nucleus (VRN). In DRN and in the two 5HT innervation areas studied, glial cells responded to the 5HT depletion induced by PCPA showing astrocytes with large and tortuous processes. Astrocytes from 5HT-depleted regions showed higher immunostaining for S-100 protein than controls. There was not any modification in optical density of S-100 protein immunostaining in VRN, the area less sensitive to PCPA treatment. These observations indicated that astrocytes are sensitive to the 5HT level, and in presence of low 5HT concentration in the intercellular space, astrocytes could react by synthesizing glial proteins like GFAP and S-100 protein.

5-HT1a receptors mediate the neurotrophic effect of serotonin on developing dentate granule cells

We have previously reported that neonatal (P3) serotonin (5-HT) depletion results in a significant decrease in the number of dendritic spines per 50 microns of dendritic length on dentate granule cells. This effect is specific and permanent. Neither total dendritic length nor the number of dendritic segments is affected by 5-HT depletion. The area dentata contains a dense 5-HT1a receptor population that is present in the at birth. Therefore, 5-HT1a receptors represented a likely candidate for the mediation of the effects of 5-HT on developing granule cells. The present study used the drugs buspirone and NAN-190, which have been shown to be an agonist and antagonist respectively at postsynaptic 5-HT1a receptors in vivo, to test the idea that neurotrophic actions of 5-HT result from 5-HT1a receptor stimulation. Following 5-HT depletion with PCA, pups received daily injections of buspirone (1.0 mg/kg) from P5 to P14. Granule cell morphology was then studied using intracellular filling with Neurobiotin on P14, P21 and P60. Buspirone treatment prevented the loss of dendritic spines previously shown to follow 5-HT depletion with PCA. No other morphological parameters were significantly changed by buspirone treatment. Naive pups received daily injections of NAN-190 from P3 to P14. One group received 1.0 mg/kg while a second group received 3.5 mg/kg. Both doses of NAN-190 resulted in dendritic spine loss comparable to that obtained with neonatal PCA treatment. This loss was permanent suggesting that the first two postnatal weeks may represent a critical period for the action of 5-HT on developing granule cells. Significant, dose-dependent changes in total dendritic length and number of dendritic segments reminiscent of the effects of norepinephrine depletion were also observed in NAN-190-treated rats. We suspect that this change is the result of the action NAN-190 at alpha receptors and is therefore distinct from the specific effect of 5-HT on the number of dendritic spines. The NAN-190 experiment also shows that the loss of dendritic spines is a function of decreased stimulation of 5-HT1a receptors and not the loss of 5-HT terminal membrane.

Identification of 5-hydroxytryptamine receptors positively coupled to adenylyl cyclase in rat cultured astrocytes

1. 5-Hydroxytryptamine (5-HT) elicited a dose-dependent stimulation of intracellular adenosine 3': 5'-cyclic monophosphate (cyclic AMP) accumulation in cultured astrocytes derived from neonatal rat (Sprague Dawley) thalamic/hypothalamic area with a potency (pEC50) of 6.68 +/- 0.08 (mean +/- s.e. mean). 2. In order to characterize the 5-HT receptor responsible for the cyclic AMP accumulation the effects of a variety of compounds were investigated on basal cyclic AMP levels (agonists) and 5-carboxamidotryptamine (5-CT) stimulated cyclic AMP levels (antagonists). The rank order of potency for the agonists investigated was 5-CT (pEC50 = 7.81 +/- 0.09) > 5-methoxytryptamine (5-MeOT) (pEC50 = 6.86 +/- 0.36) > 5-HT (pEC50 = 6.68 +/- 0.08). The following compounds, at concentrations up to 10 microM, did not affect basal cyclic AMP levels 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), cisapride, sumatriptan, DOI and RU 24969. The rank order of potency of antagonists was methiothepin (pKi = 7.98 +/- 0.25) > mesulergine (pKi = 7.58 +/- 0.18) > ritanserin (pKi = 7.20 +/- 0.24) > clozapine (pKi = 7.03 +/- 0.19) > mianserin (pKi = 6.41 +/- 0.19). The following compounds, at concentrations up to 10 microM, were inactive: ketanserin, WAY100635, GR127935. This pharmacological profile is consistent with that of 5-HT7 receptor subtype-mediated effects. 3. The cultured astrocytes exhibited regional heterogeneity in the magnitude of cyclic AMP accumulation (Emax). Cells cultured from the thalamic/hypothalamic area had significantly higher Emax values (588 +/- 75% and 572 +/- 63% of basal levels for 5-CT and 5-HT, respectively) compared to brainstem (274 +/- 51% and 318 +/- 46%, respectively) and colliculus astrocytes (244 +/- 15% and 301 +/- 24%, respectively). No significant differences in pEC50 (for either 5-HT or 5-CT) values were observed. 4. Reverse transcriptase-polymerase chain reaction (RT-PCR) with primers specific for the 5-HT7 receptor confirmed expression of messenger RNA for this receptor subtype by the cultured astrocytes derived from all regions investigated. Primers specific for the 5-HT6 receptor also amplified a cDNA fragment from the same samples. 5. From these findings, we conclude that astrocytes cultured from a number of brain regions express functional 5-HT receptors positively coupled to adenylyl cyclase and that the level of receptor expression or the efficiency of receptor coupling is regionally-dependent. The pharmacological profile of the receptor on thalamic/hypothalamic astrocytes suggests that the 5-HT7 receptor is the dominant receptor that is functionally expressed even though astrocyte cultures have the capacity to express both 5-HT6 and 5-HT7 receptor messenger RNA.

Neuro-glial neurotrophic interaction in the S-100 beta retarded mutant mouse (Polydactyly Nagoya). III. Transplantation study

The hippocampus and caudo-dorsal cortex of the homozygote of polydactyly mutant mouse (Polydactyly Nagoya, Pdn/Pdn) were markedly reduced in S-100 beta positive astrocytes and serotonergic fibers as compared to the heterozygote (Pdn/+) and wild type (+/+) [39]. The Pdn/Pdn mice die within 2 days after birth, so it is impossible to examine postnatal changes. To demonstrate the developmental change of Pdn/Pdn hippocampal tissue, we transplanted hippocampal pieces of neonatal Pdn/Pdn and +/+ mice into the right and left hippocampus of the same adult +/+ mice, respectively, and immunocytochemically examined them. Two weeks after transplantation, +/+ hippocampal tissue contained a large number of glial fibrillary acidic protein (GFAP) and S-100 beta positive astrocytes and a number of serotonergic fibers. While Pdn/Pdn hippocampal tissue contained numerous GFAP positive astrocytes, S-100 beta positive astrocytes and serotonergic fibers were not observed. Two months after transplantation, GFAP and S-100 beta were expressed in the Pdn/Pdn hippocampal tissue similar to the +/+ tissue. Serotonergic fibers were distributed in the +/+ tissue, while no serotonergic fibers were observed in the Pdn/Pdn transplant tissue. In contrast, no difference was observed in the tyrosine hydroxylase positive fibers between Pdn/Pdn and +/+ grafts. The expression of 5-HT1A receptor-like immunoreactivity was higher in the +/+ tissue than that of Pdn/Pdn tissue. The present results suggest that the expression of S-100 beta in the astrocytes of early stage of transplantation is a critical for fiber ingrowth of serotonergic neurons and expressions of 5-HT1A receptor.

Expression of serotonin receptors in human fetal astrocytes and glioma cell lines: a possible role in glioma cell proliferation and migration

Expression of seven serotonin or 5-hydroxytryptamine (5-HT) receptors (5-HT1D alpha, 5-HT1E, 5-HT2, 5-HT1A, 5-HT1C, 5-HT1D beta, and 5-HT6) was investigated in human normal fetal astrocytes and eight glioma cell lines by reverse transcription and polymerase chain reaction (RT-PCR). No expression of 5-HT1D beta and 5-HT6 was observed in any of the cell lines studied. The 5-HT1D alpha receptor was found to be expressed in two human glioma cell lines but not in normal astrocytes. In addition, only three glioma cell lines expressed the 5-HT1E receptor. The 5-HT1C receptor was expressed in six glioma cell lines but not in normal astrocytes while the 5-HT1A was found to be expressed in normal astrocytes from the left hemisphere and in six glioma cell lines but not in normal astrocytes from the cerebellum. Interestingly, the 5-HT2 receptor was expressed in all cells studied but very weakly in normal astrocytes. The effect of 5-HT on glioma cell proliferation, migration, and invasion was also investigated. Serotonin was found to positively modulate these three processes in vitro. These results suggest that 5-HT may play an important role in the control of the biological properties of human glioma cells.

Prenatal cocaine delays astroglial maturation: immunodensitometry shows increased markers of immaturity (vimentin and GAP-43) and decreased proliferation and production of the growth factor S-100

Exposure to cocaine during fetal development has been demonstrated to produce a variety of brain and behavioral changes. Cocaine is a potent releaser of a variety of neurotransmitters, such as serotonin, which act as developmental signals. Since serotonin plays an important role in astroglial maturation, migration, and growth factor production (e.g. S-100 beta), we proposed that these properties of astroglial cells will be altered in a brain prenatally exposed to cocaine. To observe cocaine's effects on astroglial development, we performed immunocytochemical analyses of a variety of developmental protein makers including BrdU, Gap-43, vimentin, and S100 beta. Our results demonstrate that prenatal cocaine administration produces decreased cell proliferation as measured by BrdU staining, retarded neurite outgrowth as ascertained by increased Gap-43 immunoreactivity, increased density of vimentin-positive radial glial cells, and diminished tissue S100 beta immunoreactivity. Overall, these results suggest that cocaine delays astroglial development. This delay would have profound effects on neuronal development and outgrowth and, thus, development of the entire brain.

Characterization of the 5-hydroxytryptamine2A receptor-activated cascade in rat C6 glioma cells

We have investigated the identity and intracellular cascade of responses resulting from activation of the endogenous 5-hydroxytryptamine receptor in the C6 rat glioma cell line. Sequence analysis of reverse transcription-polymerase chain reaction products derived from C6 glioma cell messenger RNA revealed complete homology with a portion of the rat 5-hydroxytryptamine2A receptor. The binding of [3H]ketanserin to cell membranes demonstrated a significant correlation with the 5-hydroxytryptamine2A receptor in rat frontal cortex. On intact cells, 5-hydroxytryptamine stimulated a concentration-dependent increase in phosphatidyl inositide turnover and intracellular [Ca2+] mediated by 5-hydroxytryptamine2A receptors. In whole-cell patch-clamp recordings, 5-hydroxytryptamine induced an outward current mediated predominantly by K+ ions (reversal potential = -80 mV). Using caged molecules containing Ca2+ or inositol 1,4,5-trisphosphate in the patch electrode solution, we found that rapid photolytic release of Ca2+ and particularly inositol 1,4,5-trisphosphate within the cytosol induced an outward current with characteristics similar to those seen after application of 5-hydroxytryptamine. Comparison between differentiated and undifferentiated cells revealed significantly higher receptor density and maximal phosphoinositide response to 5-hydroxytryptamine in undifferentiated cells but the associated rise in [Ca2+]i and activation of an outward current was observed more frequently in differentiated cells. Prolonged exposure of the cells to 5-hydroxytryptamine led to a decrease in all responses and to the down-regulation of receptor number. We conclude that the rat C6 glioma cell expresses a 5-hydroxytryptamine2A receptor identical to that found in rat brain and that stimulation of the receptor in C6 cells leads to the activation of Ca2+ activated K+ channels via phosphoinositide hydrolysis and subsequent rise in cytosolic Ca2+ ion concentration. However, the contrasting effects of differentiation on receptor number and phosphoinositide response to 5-hydroxytryptamine compared to Ca2+ release and conductance change indicate that a complex relationship exists between the component parts of the receptor-activated cascade.

Vasoactive intestinal peptide increases intracellular calcium in astroglia: synergism with alpha-adrenergic receptors

In type I astrocytes from rat cerebral cortex, vasoactive intestinal peptide (VIP) at concentrations below 1 nM evoked an increase in intracellular calcium ion concentration. This response, however, was observed in only 18% of the astrocytes examined. alpha-Adrenergic stimulation with phenylephrine or norepinephrine also resulted in an intracellular calcium response in these cells and the threshold sensitivity of astrocytes to phenylephrine was vastly different from cell to cell. Treatment of these astrocytes with VIP (0.1 nM) together with phenylephrine at subthreshold concentrations produced large increases in intracellular Ca2+ concentration ([Ca2+]i) and oscillations. The continued occupation of the alpha-adrenergic receptor was required for sustained synergism. Both alpha-receptor stimulation and stimulation with the mixture of agonists induced the cellular calcium response by triggering release of calcium from cellular stores, since the response persisted in the absence of extracellular calcium. Furthermore, thapsigargin pretreatment, which depletes intracellular stores, abolished the agonist-induced [Ca2+]i response. VIP (0.1 nM) and phenylephrine were found to increase cellular levels of inositol phosphates; however, there was no apparent additivity in this response when the agonists were added together. These observations suggest a calcium-mediated second messenger system for the high-affinity VIP receptor in astrocytes and that alpha-adrenergic receptors act synergistically with the VIP receptor to augment an intracellular calcium signal. The synergism between diverse receptor types may constitute an important mode of cellular signaling in astroglia.

Localization of 5-HT1A receptors to astroglial cells in adult rats: implications for neuronal-glial interactions and psychoactive drug mechanism of action

Although tissue culture studies have shown a variety of neurotransmitter receptors on astroglial cells, verifying these observations in adult animals has been difficult and rarely accomplished. In the current study we have used double immunocytochemistry to localize 5-HT1a receptors to astroglial cells in fixed sections of adult rat brain. The astroglial cells were identified using an antibody raised against the astroglial-specific protein glial fibrillary acidic protein (GFAP). To label the 5-HT1a receptor, we used an antibody we recently raised against a unique peptide sequence occurring in the second extracellular loop of the receptor. Our results show that the 5-HT1a receptor occurs in relatively high abundance on astroglial cells. There is regional specificity, the receptor being much more commonly found in septum and hippocampus than striatum. There are also intraregional differences in that even within a single brain region one astrocyte may have very high levels of the receptor while an adjacent cell has none. We propose that the cellular localization of this receptor could have significance in understanding the mechanism of action of 5-HT1a receptor active drugs in alleviating anxiety and depression. The mechanism may be through the release of a neurotrophic agent, S-100 beta, from astrocytes. This factor may then cause regeneration or sprouting of neuronal terminals which have been lost due to a disease process.

Calcium dependence of serotonin-evoked conductance in C6 glioma cells

Whole-cell membrane currents and imaging of intracellular calcium concentrations ([Ca2+]i) were used to investigate the role of calcium in a response to serotonin of C6 glioma cells. Activation of a high-affinity serotonin receptor induced a transient rise in calcium concentration in these cells and activated a predominantly potassium conductance, with a small chloride component. Perfusion of the cytoplasm with an internal solution containing high calcium concentration induced similar but prolonged increase of membrane conductance. The responsiveness of C6 cells to serotonin was negatively correlated with the concentration of the unbound calcium chelator BAPTA when BAPTA-buffered calcium-containing intracellular solutions were used. Responses to serotonin persisted in the absence of external calcium, decreased gradually, and then recovered partially after replenishment of extracellular calcium. These findings substantiate the direct role of intracellular calcium in mediating the serotonin response, and indicate that serotonin-induced release of calcium from intracellular stores is sufficient for the activation of conductance in the C6 glioma cell line.

A role for glial cells in activity-dependent central nervous plasticity? Review and hypothesis

Activity-dependent plasticity relies on changes in neuronal transmission that are controlled by coincidence or noncoincidence of presynaptic and postsynaptic activity. These changes may rely on modulation of neural transmission or on structural changes in neuronal circuitry. The present overview summarizes experimental data that support the involvement of glial cells in central nervous activity-dependent plasticity. A role for glial cells in plastic changes of synaptic transmission may be based on modulation of transmitter uptake or on regulation of the extracellular ion composition. Both mechanisms can be initiated via neuronal-glial information transfer by potassium ions, transmitters, or other diffusible factor originating from active neurons. In addition, the importance of changes in neuronal circuitry in many model systems of activity-dependent plasticity is summarized. Structural changes in neuronal connectivity can be influenced or mediated by glial cells via release of growth or growth permissive factors on neuronal activation, and by active displacement and subsequent elimination of axonal boutons. A unifying hypothesis that integrates these possibilities into a model of activity-dependent plasticity is proposed. In this model glial cells interact with neurons to establish plastic changes; while glial cells have a global effect on plasticity, neuronal mechanisms underlie the induction and local specificity of the plastic change. The proposed hypothesis not only explains conventional findings on activity-dependent plastic changes, but offers an intriguing possibility to explain several paradoxical findings from studies on CNS plasticity that are not yet fully understood. Although the accumulated data seem to support the proposed role for glial cells in plasticity, it has to be emphasized that several steps in the proposed cascades of events require further detailed investigation, and several "missing links" have to be addressed by experimental work. Because of the increasing evidence for glial heterogeneity (for review see Wilkin et al., 1990) it seems to be of great importance to relate findings on glial populations to the developmental stage and topographical origin of the studied cells. The present overview is intended to serve as a guideline for future studies and to expand the view of "neuro" physiologists interested in activity-dependent plasticity. Key questions that have to be addressed relate to the mechanisms of release of growth and growth-permissive factors from glial cells and neuronal-glial information transfer. It is said that every complex problem has a simple, logical, wrong solution. Future studies will reveal the contribution of the proposed simple and logical solution to the understanding of central nervous plasticity.

Serotonin promotes region-specific glial influences on cultured serotonin and dopamine neurons

To test the hypothesis that glia mediate interactions between embryonic serotonergic (5-HT) neurons and dopamine neurons, we studied the effects of 5-HT in co-cultures of E14 raphe neurons of mesencephalic dopamine neurons and radial glia/astrocytes derived from the same (homotypic) or opposite (heterotypic) brain region using a dose (10(-5) M) that would produce 5-HT uptake into glial cells as well as activate 5-HT receptors. Morphometric analysis of 5-HT and tyrosine hydroxylase (TH) immunoreactive neurons revealed regional differences in the effects of 5-HT (and nialamide) on survival, cell soma size, and dendrite-like neurite outgrowth in neuronal-glial co-cultures. In general, 5-HT had more significant effects on both types of monoamine neuron when they were cultured with mesencephalic glia (GSN). Stimulatory effects of 5-HT on growth of TH neurons in GSN cultures suggest that developing raphe axons, which reach the mesencephalon during the early differentiation of these neurons, may enhance the influence of local glial-derived trophic factors. Likewise, the promotion of 5-HT neuronal survival in these cultures suggests that glial factors in the mesencephalon may contribute to the support of 5-HT neurons in addition to the influences of raphe glia. The inhibitory effects of 5-HT on neurite outgrowth by raphe neurons in GSN co-cultures indicates enhanced sensitivity of these neurons to the inhibitory effects of 5-HT in the presence of mesencephalic glia. The region-specific effects of 5-HT and nialamide in glial co-cultures suggest that raphe and mesencephalic glia may express different capacities for 5-HT uptake, receptors, and/or monoamine oxidase (MAO) activities. These characteristics could be important for the specificity of growth-regulatory influences of glial cells on the development of brain monoamine neurons.

Neurotransmitters as Neurotrophic Factors: a New Set of Functions

At the start of this review, factors were deemed trophic if they stimulated mitosis, permitted neural cell survival, promoted neurite sprouting and growth cone motility, or turned on a specific neuronal phenotype. The in vitro evidence from cell cultures is overwhelming that both neurotransmitters and neuropeptides can have such actions. Furthermore, the same chemical can exert several of these effects, either on the same or on different cell populations. Perhaps the most striking example is that of VIP, which can stimulate not only mitosis, but also survival and neurite sprouting of sympathetic ganglion neuroblasts (Pincus et al., 1990a,b). The in vivo data to support the in vitro experiments are starting to appear. A role for VIP in neurodevelopment is supported by in vivo studies that show behavioral deficits produced in neonatal rats by treatment with a VIP antagonist (Hill et al., 1991). The work of Shatz' laboratory (Chun et al., 1987; Ghosh et al., 1990) suggests that neuropeptide-containing neurons, transiently present, serve as guideposts for thalamocortical axons coming in to innervate specific cortical areas. Along similar lines, Wolff et al. (1979) demonstrated gamma-aminobutyric acid-accumulating glia in embryonic cortex that appeared to form axoglial synapses and suggested the possibility that gamma-aminobutyric acid released from the glia might play a role in synaptogenesis by increasing the number of postsynaptic thickenings. Meshul et al. (1987) have provided evidence that astrocytes can regulate synaptic density in the developing cerebellum. The work of Zagon and McLaughlin (1986a,b, 1987) has shown that naltrexone, an antagonist of the endogenous opioid peptides, affects both cell number and neuronal sprouting. Lauder's laboratory (Lauder et al., 1982) has shown a role for 5-HT in regulation of the proliferation of numerous cell types. These studies illustrate another important point, that neurotransmitters and neuropeptides function in communication not only between neurons, but also between neurons and glial cells, and between glial cells. Given that astrocytes can express virtually all of the neural receptors and can produce at least some of the neurotransmitters and neuropeptides, they must now be considered equal partners in the processes of intercellular communication in the nervous system, including the trophic responses. The actions of neurotransmitters and neuropeptides have to be considered in terms of a broad spectrum of actions that range from the trophic actions described in this review, to the classic transmitter actions, to potential roles in neurotoxicity.(ABSTRACT TRUNCATED AT 400 WORDS)

5,7-Dihydroxytryptamine injections increase glial fibrillary acidic protein in the hypothalamus of adult rats

The distribution and levels of glial fibrillary acidic protein (GFAP) were determined in the adult rat hypothalamus following axotomy of serotonin (5-HT) neurons. Seven days after unilateral intrahypothalamic injection of the 5-HT neurotoxin, 5,7-dihydroxytryptamine, there was a marked increase in the number of GFAP-labelled astrocytes in the ipsilateral hypothalamus of 5,7-DHT-treated as compared to sham-treated rats. In addition, levels of GFAP were significantly increased 7 days after 5,7-DHT injection.

Postsynaptic 5-HT1 receptors and offensive aggression in rats: a combined behavioural and autoradiographic study with eltoprazine

The present study was designed to assess whether the antiaggressive effects of eltoprazine are mediated via presynaptic and/or postsynaptic 5-HT1 receptors. We describe the effects of central 5-HT depletion 1) on the behaviour of resident TMD-S3 rats in a territorial situation, 2) on the efficacy of eltoprazine to inhibit offensive aggression, and 3) on the 5-HT1A, 5-HT1B and 5-HT1C receptor binding in brains of rats previously used in behavioural studies. Male resident rats were given combined 5,7-dihydroxytryptamine (5,7-DHT) injections into the dorsal and median raphe nuclei. Two to four weeks after the lesions, rats were confronted with an intruder Wiser rat in their home cage for a 10-min period. The 5,7-DHT treatment resulted in a modest reduction of offensive behaviour, while having no effects on other social and nonsocial behaviours. Oral administration of eltoprazine (1 mg/kg) specifically reduced offensive aggression in both sham- and 5,7-DHT-lesioned animals, leaving social interest and exploration intact or even increasing it. A low dose (0.3 mg/kg) of eltoprazine did not affect the behavioural repertoire of sham-operated rats, whereas this dose significantly reduced offense behaviours in the 5,7-DHT-lesioned residents. Quantitative autoradiographic studies 5 weeks after 5,7-DHT treatment revealed a significant increase in radioligand binding to 5-HT1A, 5-HT1B and 5-HT1C sites in many brain regions studied, except for the raphe nuclei where [3H]8-OH-DPAT binding to 5-HT1A sites was markedly reduced. The concentrations of 5-HT and 5-HIAA in frontal cortex were reduced to approximately 10% of controls. The results indicate that serotonin has a stimulatory rather than an inhibitory influence on offensive aggressive behaviour. Central 5-HT depletion does not prevent the antiaggressive effects of eltoprazine, indicating a role for postsynaptic 5-HT1 receptors in the modulation of offensive aggression. The 5,7-DHT-induced overall upregulation of 5-HT1A, 5-HT1B and 5-HT1C binding sites suggests that these three receptor subtypes receive a tonic serotonergic influence. It is conceivable that this postsynaptic 5-HT1 receptor supersensitivity is reflected by the increased efficacy of eltoprazine to inhibit offensive aggression.

Activity of hippocampal extract on development of [3H]5-HT high-affinity uptake in dissociated microcultures

Specific and localized lesions of the 5-HT fibers in the hippocampus induce homotypic collateral sprouting and enhance serotonergic fiber outgrowth from adult neurons and transplanted fetal tissue. In this study, hippocampal extracts were prepared and applied to primary cultures of fetal serotonergic neurons. The effects of plating density and serum additives were examined. The growth of the serotonergic neurons in the rostral brainstem dissociated cultures were estimated by measuring the specific uptake of [3H]5-HT. The results indicate the presence of a trypsin-sensitive factor which is active when prepared fresh at dilutions up to 1/10,000. The factor is higher in hippocampus than cerebellum. Young male tissue contained more activity than either female or aged hippocampus. Although both positive and negative effects are described, higher dilutions of factor (1/1,000) were generally stimulatory in high density cultures while lower dilutions (1/10) were inhibitory in low density cultures. Specific removal of 5-HT hippocampal afferents with fornix-fimbria microinjections of 5,7-dihydroxytryptamine resulted in an initial loss of activity (2 days and 2 weeks) followed by an enhanced activity (2 months) compared to normal hippocampal extract. Several possibilities are discussed as to the identity of the serotonergic growth factor from hippocampal supernatant.

Stimulation of astroglial 5-HT1A receptors releases the serotonergic growth factor, protein S-100, and alters astroglial morphology

Stimulation of astroglial 5-HT1A receptors causes astroglial cells to acquire a more mature morphology and to release a factor (or factors) which promotes growth of serotonergic neurons. By using an antibody-blocking approach, we have shown that at least one of the growth-promoting factors thus released is the astroglial-specific protein S-100. This may be a particularly important observation, in view of studies implicating S-100 in both Down's syndrome and Alzheimer's disease.

S-100B but not NGF, EGF, insulin or calmodulin is a CNS serotonergic growth factor

The effects of S-100B, nerve growth factor (NGF), epidermal growth factor (EGF) and insulin were tested in cultured mesencephalic neurons. Only chronic S-100B showed enhancement (maximal at 3.2 ng/ml is 171%) after 3 days of incubation of the [3H]5-HT uptake capacity by serotonergic neurons. A single application at initial plating of S-100B (maximal at 5 ng/ml is 185%), but not calmodulin, increased the development of the [3H]5-HT uptake capacity by the cultured serotonergic neurons. Morphometric analysis of cultured 5-HT immunoreactive (IR) neurons showed an increase (135 and 147%) in neurite length 30 h after S-100B application of 16 and 3.2 ng/ml (respectively). These results suggest that S-100B is a serotonergic growth factor in the mammalian brain.

Stimulation of astroglial serotonin receptors produces culture media which regulates growth of serotonergic neurons

Our work has been concerned with the role of high affinity serotonin receptors in regulating the development of the serotonergic system. In previous studies, we have found evidence that these receptors occur on astroglial cells and that their number is developmentally linked. The current work is aimed at investigating the mechanism by which these receptors may regulate serotonin neuronal growth. Primary cultures of astroglial cells were exposed to serotonin (5-HT) or the selective receptor agonists 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-D-PAT, for 5-HT1a receptors) or trifluoro-methyl-phenyl-piperazine (TFMPP) and m-chlorophenylpiperazine (mCPP) (for 5-HT1b receptors). Media was collected after 4 or 24 h, and added to primary cultures of serotonergic neurons. Growth was determined by specific uptake of radiolabeled serotonin into the cultures. Our results show the presence of a factor(s) in the glial-conditioned media which can be stimulatory or toxic to serotonin neurons, depending on the neuronal plating density. This factor is significantly present after 24 h, is found in both brainstem and cortical astroglial-conditioned media and appears to be linked to the 5-HT1a receptor. Thus, it appears possible that the serotonergic neuronal system can regulate its own development through an action on astroglial cells.

On the sensitivity of intact cells to perturbation by ethanol

A comparison was made of ethanol's effects on the order of plasma membranes in intact cells and some isolated membrane preparations. Order was assessed by steady-state fluorescence polarization techniques using the non-permeant probe, TMA-DPH. The data show that two cultured cells, rat neonatal astroglial and N2A neuroblastoma, were sensitive to significant ethanol-induced disordering within the anesthetically relevant range (100 - 200 mM). Human erythrocytes, cultured fibroblasts and homogenized astroglial cells required higher ethanol concentrations (greater than 250 mM) to produce a similar effect. Intact erythrocytes were approximately twice as sensitive as erythrocyte ghost membranes to ethanol-induced perturbation. The neonatal glial and N2A cells were approximately five times more sensitive than synaptic membranes to ethanol effects. DMPC and DMPC + cholesterol liposomes and myelin membranes were insensitive to ethanol's effects. The incorporation of 10 mole % ganglioside GM1 sensitized the liposomes to ethanol-induced perturbation.

Neurite extension on dibutyryl cyclic AMP-stimulated astrocytes

Although both central and peripheral neurons successfully regenerate cut axons along peripheral nerve and other suitable substrates, axonal elongation through the mature central nervous system (CNS) is limited. It has been proposed that the presence of reactive astrocytes formed in response to CNS injury act as a barrier to axonal regeneration. In contrast, in vitro, astrocytes in a flat or unstimulated state have been shown to be a preferred substrate for neurite extension. We have investigated whether induced modifications of astrocytes alter the capacity of these cells to act as a substrate for axonal elongation. Treatment with dibutyryl cyclic AMP (dBcAMP) results in a marked morphological and biochemical change in astrocytes, considered by some to be a model of reactive astrocytosis. Retinal and dorsal root ganglia explants from embryonic mice were cultured on top of untreated glial monolayers and those treated with dBcAMP. The subsequent neuritic growth was measured at 48 h. No difference was found between the groups, indicating that astrocytes are an excellent substrate for axonal growth, even after they develop a stellate shape and high levels of glial fibrillary acidic protein.

Use of tissue culture models to study neuronal regulatory trophic and toxic factors in the aged brain

Dementia is believed to result from the loss of selective neurons within the brain, but approaches for systematic study of that degenerative process are hampered by the complexity of the neuronal milieu. Tissue culture models provide a means to reduce dramatically the variables inherent in the study of neuronal plasticity. Three levels of complexity can be described: cellular and molecular diversity; primary and secondary interconnections; and finally, the dynamics influenced by age. The following review discusses the advantages and disadvantages of tissue culture models for the detailed study of neuronal trophic and toxic factors. Our selection of factors is broadened to include ions, intermediate metabolites, antioxidants, steroids, neuropeptides, gangliosides, metals, neurotransmitters, brain extracts, and protein molecules. Most of these factors have been shown to be altered in the aged brain, to have a significant effect on cultured neurons, or both. This multilevel analysis provides the reader with an overview of the events regulating neuronal survival, differentiation and death. An understanding of these basic questions is necessary to sequence the molecular events resulting in neuronal death.

Glial and neuronal opioid receptors: apparent positive cooperativity observed in intact cultured cells

Opioid receptors were characterized in glial and neuronal homogeneous cultures of embryonic chick forebrain, using [3H]naloxone as a labelled ligand. Binding experiments were performed on intact cells. The specific binding of [3H]naloxone reached equilibrium after 1 min. The apparent dissociation constants were estimated as 0.51 nM for glial and 0.63 nM for neuronal cells. Equilibrium measurements indicated the apparent positive cooperativity of the binding, resulting in Hill coefficients of 2.61 for glial and 2.04 for neuronal cells. Competition of unlabelled naloxone for specific binding sites resulted in maximum-shape curves in glial cells if measured at low receptor occupancy. This supports the positive cooperativity of ligand binding. Opioid agonists, ethylketocyclazocine (EKC), morphine and [D-Ala2,L-Leu5]enkephalin (DALA), provoked biphasic competition curves in both cell types with a characteristic maximum at low competitor concentrations. The possible physiological role of glial opioid receptors in neuron-glia communication and the significance of cooperativity is discussed.

Temporal effects of intrahypothalamic 5,7-dihydroxytryptamine: relationship between serotonin levels and [3H]serotonin binding

The relationship between serotonin (5-HT) levels and [3H]5-HT binding in discrete hypothalamic areas was examined in separate groups of animals at various times, following unilateral intrahypothalamic injection of 5,7-dihydroxytryptamine (5,7-DHT). Seven days post-5,7-DHT lesion, 5-HT levels were significantly decreased in both the ipsilateral and contralateral ventromedial and dorsomedial hypothalamic nuclei (VMN, DMN). In the lateral hypothalamic area (LHA), 5-HT levels were significantly decreased only ipsilaterally. Fifty days postlesion, 5-HT levels in the ipsilateral VMN remained significantly below sham, while the DMN and LHA returned to sham values. Seven days after 5,7-DHT there was a significant increase in [3H]5-HT labeling densities in the ipsilateral and contralateral ventromedial hypothalamic area as well as in the ipsilateral LHA. In contrast, in the dorsomedial hypothalamic area there was no increase in [3H]5-HT binding. Fifty days postlesion, no significant differences in [3H]5-HT binding between 5,7-DHT and sham were observed in any areas examined. This data provides further evidence for the regeneration of 5-HT fibers in the hypothalamus and demonstrates that the relationship between [3H]5-HT binding and 5-HT levels varies from one hypothalamic area to another.

Postnatal changes in serotonin receptors following prenatal alterations in serotonin levels: further evidence for functional fetal serotonin receptors

Recent work by ourselves and others has indicated that serotonin (5-HT) acts as a regulator of neuronal growth in fetal tissue, probably through an action on the high affinity 5-HT receptors known to be present. In order for our hypothesis to be correct, these receptors must be shown to be functional in fetal tissue. Furthermore, since 5-HT has a dual role in neuronal development, these receptors must be functional both in the region of the serotonergic cell body (brainstem) and in projection areas (forebrain). We have tested the functional status of fetal 5-HT receptors by testing their ability to adapt to changes in the level of 5-HT. Pregnant rats were treated with saline, p-chlorophenylalanine (a 5-HT depletor) or 5-methoxytryptamine (a 5-HT agonist) and the characteristics of the high affinity 5-HT receptors in the rat pups determined using a binding assay with [3H]5-HT. Our results show that both forebrain and brainstem receptors respond to alterations in transmitter level in a manner similar to adult brain. Thus, fetal 5-HT receptors are functional and capable of playing a role in neuronal development.