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

Synapse formation and spontaneous activity in rat brainstem neurons in primary culture

The correlation between synaptogenesis and onset of spontaneous action potentials was assessed in rat brainstem cells up to 29 days in primary culture. Cells exhibited different stages of maturation followed by electron microscopy and patch clamp recordings. Terminal boutons with no preferential orientation of presynaptic vesicles appeared after 2 days in culture. After 5 days, preferential orientation of presynaptic vesicles and thickening of postsynaptic membranes were observed. The spontaneous discharge of action potentials, single or bursting, was observed after 7 days in vitro. This was followed by the expression of a 128-pS K(+) channel starting at 13 days in vitro. A 69-pS K(+) channel was also present throughout the duration of the cultures. These results suggest that spontaneous discharge of action potentials does not occur before synapses are formed and K(+) channel types develop differentially in brainstem neurons in vitro.

Serotonin promotes the differentiation of glutamate neurons in organotypic slice cultures of the developing cerebral cortex

The monoamines serotonin (5-HT), noradrenaline (NA), and dopamine (DA), which are present in the developing brain apparently before they assume their neurotransmitter functions, are regarded as strong candidates for a role in the maturation of the cerebral cortex. Here we sought to investigate their effects on the generation and differentiation of cortical cell types. Slice cultures, prepared from the cortices of embryonic day (E) 14, E16, and E19 rat fetuses, were kept in defined medium or in defined medium plus 5-HT for 7 d. E16 cortices were also exposed to NA or DA for the same period. At the end of this period, the proportions of the neuronal [glutamate (Glu)-, GABA-, calbindin-, calretinin-labeled], glial (GFAP), and neuroepithelial (nestin) cell types were estimated for all conditions. We found that in E16 cultures, application of 5-HT, but not of NA or DA, significantly increased the proportion of Glu-containing neurons without affecting the overall neuronal population or the proportions of any other cell types. A similar effect was observed in co-cultures of E16 cortex with slices through the midbrain raphe nuclei of E19 rats. The total amount of cortical Glu, as measured with HPLC, was also increased in these co-cultures. To investigate whether the effect of 5-HT was the result of changes in cell proliferation, we exposed slices to bromodeoxyuridine (BrdU) and found that the proportion of BrdU-labeled cells was similar in the 5-HT-treated and control slices. These results indicate that 5-HT promotes the differentiation of cortical Glu-containing neurons without affecting neuroepithelial cell proliferation.

Endothelin-1 expression in myenteric neurons cultured from rat small intestine

Endothelin is a potent vasoactive peptide. More recently, endothelin-1 (ET-1) has been found in neural tissues such as spinal cord, brain and peripheral ganglion cells. Inagaki (Gastroenterology 101 (1991) 47) reported evidence of ET-1-like immunoreactivity in enteric neurons, but there are no reports of ET-1 peptide or mRNA expression specifically in myenteric neurons. Using a primary culture of myenteric neurons, we set out to evaluate ET-1 peptide and mRNA expression. Myenteric neurons were cultured using a dissection and enzyme dispersion technique. ET-1 reactivity was localized to neurons and ET-1 levels from cells and media were assayed by radioimmunoassay under a variety of media conditions or with depolarizing buffer or veratridine (75 microM). Prepro ET-1 mRNA expression was determined by Northern analysis of total RNA utilizing a rat ET-1 cDNA. ET-1 immunoreactivity was observed almost exclusively in myenteric neurons. Cells contained 0.78 pg/micrograms protein and did not vary with variations in media conditions. Basal release/secretion into media occurred but was not enhanced by depolarizing media or veratridine. High levels of ET-1 mRNA expression were identified. These results of high level constitutive expression of ET-1 linked with previous reports of ET-1 modulation of cholinergic intestinal smooth muscle contraction suggest a neuromodulatory role.

FAST and SLOW ipsilateral and contralateral spinal reflexes in the neonate rat are modulated by 5-HT

1. Three ipsilateral (MSR, PSR, IPSI SLOW) and two contralateral segmental reflexes (CON FAST, CON SLOW) were recorded from L4 or L5 ventral roots of the neonate rat spinal cord in vitro. MSR, PSR and CON FAST were evoked from lower threshold afferents; more intense stimulation evoked IPSI SLOW and CON SLOW. 2. Kainate/AMPA receptors were involved in mediation of MSR, PSR, CON FAST, IPSI SLOW and CON SLOW and NMDA receptors in mediation of CON FAST, IPSI SLOW and CON SLOW. 3. All five reflexes were depressed by 5-HT (IC50 1.2-7.9 microM; order of sensitivity, CON SLOW > CON FAST = IPSI SLOW > MSR = PSR); and by 5-CT (IC50 1.9-8.8 nM; order of sensitivity, MSR > IPSI SLOW = CON FAST = CON SLOW > PSR). alpha-Me-5-HT also depressed all five reflexes. 4. Dipropyl-5-CT selectively depressed MSR and CON SLOW (IC50 90-170 nM) but was less potent than 5-CT. 8-OH-DPAT selectively depressed MSR (IC50 1.1 microM), IPSI SLOW and CON SLOW (IC50 5.7-7.6 microM), while methylsergide depressed only MSR (IC50 26 nM). 5. Phenyl biguanide and m-chlorophenyl biguanide (5-HT3 receptor agonists) had no significant effects on any reflex. 6. It is concluded that a 5-HT1-like receptor mediates depression of the MSR. A different receptor or a mixed population of receptors, but not 5-HT3 receptors, mediate inhibition of PSR, CON FAST, IPSI SLOW and CON SLOW.

Serotonin and nialamide differentially regulate survival and growth of cultured serotonin and catecholamine neurons

In this morphometric analysis of immunoreactive serotonin (5-HT) and tyrosine hydroxylase (TH) neurons in culture, 5-HT and the MAO inhibitor nialamide influenced the survival, cell body size and neurite outgrowth of embryonic day 14 (E14) 5-HT neurons after treatment from 1-3 days in vitro (DIV), but did not significantly affect E14 or E15 TH neurons of either the noradrenergic or dopaminergic phenotype. These treatments had minimal effects on 5-HT neurons derived from E15 embryos. The stimulatory effects of 5-HT on survival and somal growth of E14 5-HT neurons was in contrast to its inhibitory effects on neurite outgrowth, suggesting trophic and inhibitory autoregulation of different cellular compartments of developing 5-HT neurons. The decreased sensitivity of E15 5-HT neurons to these treatments, despite similar viability and growth of these neurons in control cultures, suggests the existence of a critical period for this regulation during the initial period of serotonergic neurogenesis when these neurons are forming the bilateral B4-9 raphe complex. The lack of significant effects of 5-HT on TH neurons suggests differential sensitivities of 5-HT and TH neurons to developmental regulation by this neurotransmitter.

Sexual differentiation of monoaminergic neurons--genetic or epigenetic?

It is currently believed that sexual differentiation of the brain is mediated entirely by the epigenetic action of gonadal steroids during a critical period of development. Ingrid Reisert and Christoph Pilgrim review sexual dimorphisms of monoaminergic systems, which also appear to be generated by sex steroids. However, there are a number of observations that are not explainable by the 'androgen theory of sexual differentiation'. Results obtained from cultures of embryonic rat brain tissue appear to indicate that dopaminergic neurons may develop morphological and functional sex differences in the absence of sex steroids. Hormone-independent and -dependent developmental processes may affect diencephalic and mesencephalic dopaminergic neurons in a regionally diverse fashion. Factors other than sex steroids need to be examined. It is possible that some sexual dimorphisms in the nervous system may develop under primary genetic control.

Sex-specific schedule in steroid response of rhombencephalic catecholaminergic neurons in vitro

Morphological differentiation of tyrosine hydroxylase-immunoreactive neurons was investigated in dissociated cell cultures of rhombencephalon of male and female day 14 rat embryos grown in the presence or absence of sex steroids. Numbers of cells were counted and morphometrical measurements carried out of soma size and length of tyrosine hydroxylase-immunoreactive neurites (processes). Subtle sex differences in length of stained neurites, which were not yet present after 3 days in vitro, were observed after 6 days in cultures grown in the absence of sex steroids. Female tyrosine hydroxylase-immunoreactive neurites could be traced over longer distances than male ones. Daily treatment of cultures with testosterone or 17 beta-estradiol resulted in an increase of lengths of stained neurites of female neurons after 3 days and of male neurons after 6 days in vitro. Regarding cell numbers or soma size, there were no differences between genders or between controls and hormone-treated cultures. It is concluded that sex steroids promote the outgrowth of neurites from noradrenergic neurons within a gender-specific time frame. It appears that the critical period for developmental effects of sex steroids differs between males and females.