Synaptic properties of serotonergic growth cones in developing rat brain

Abstract

In order to gain insight into the events that take place when serotonergic growth cones are remodeled into synapses, we tested the hypothesis that neurotransmitter-related properties of presynaptic terminals are already present in these growth cones before synaptogenesis begins. The ontogeny of markers for the specific reuptake of 5-HT and for 5-HT-storing synaptic vesicles was studied in isolated growth cone (IGC) fractions from developing rat brain. High- affinity 3H-imipramine binding (a marker for the plasma membrane 5-HT transporter) was significantly enriched in IGC fractions prepared before the beginning of cortical synaptogenesis [embryonic day 15 (E15) and E20]. Radioautography with 3H-imipramine or 3H-paroxetine (another marker for the transporter) confirmed that the 5-HT transporter is present in the cerebral cortex when it contains serotonergic growth cones, but not serotonergic synapses. Specific uptake of 3H-5-HT was found in IGC fractions as early as E15; this uptake was inhibited by fluoxetine. Electron microscopic radioautography demonstrated directly that growth cones were the structures in IGC fractions that took up 3H- 5-HT. The synaptic vesicle protein synaptophysin and a 45 kDa protein found specifically in serotonergic synaptic vesicles, serotonin-binding protein (SBP), were each enriched in IGC fractions from E15 to postnatal day 5; SBP immunoreactivity increased approximately 10-fold between E15 and E20. Endogenous 5-HT was detected in IGC fractions at E15 and increased in amount as development proceeded. The ratio of 5-HT to 5-hydroxyindole acetic acid suggested that 5-HT within growth cones is protected from catabolism by monoamine oxidase. Reserpine-induced depletion of 5-HT, a marker for the vesicular carrier of 5-HT, was apparent in IGC fractions at E20, but not at E15. These data suggest that properties that characterize the presynaptic components of mature serotonergic synapses develop in growth cones before synapses are formed. The early development of these properties may permit neurotransmission to be established rapidly during synaptogenesis or, alternatively, enable 5-HT to play a role in ontogeny.

Early Serotonergic Projections to Cajal-Retzius Cells: Relevance for Cortical Development

“…3D). The ultrastructure of some 5-HTϩ profiles closely resembled that of serotonergic growth cones, as described by Igvy-May et al (1994). They contained small and large densecored vesicles, as well as large vacuoles (Fig.…”

Structural Effects and Neurofunctional Sequelae of Developmental Exposure to Psychotherapeutic Drugs: Experimental and Clinical Aspects

“…Ontogenesis of the rat 5-HT transporter and 5-HT receptors indicates a rapid increase during the perinatal and early postnatal period to reach adult levels by the end of the third postnatal week, with a time course that closely parallels synaptogenesis (Igvy-May et al, 1994). Recently, many investigations have been devoted to the exploration of the neurobehavioral and neurochemical effects of developmental exposure to SSRIs, and most research has focused on fluoxetine because of its high selectivity and negligible affinity for several receptor subtypes.…”

Neonatal Antidepressant Exposure has Lasting Effects on Behavior and Serotonin Circuitry

“…Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in the production of serotonin, and during development, brain serotonin levels and TPH mRNA increases 35-fold in DR between embryonic day 18 and PN22 in rat (Rind et al, 2000). Likewise, the level of SERT mRNA, the primary means of removing serotonin from the synapse and Lasting neonatal SSRI effects on 5HT circuits D Maciag et al terminating serotonin signaling, has been found to be related to synaptogenesis (Ivgy-May et al, 1994;Hansson et al, 1998). In human brain, serotonergic neurons have been detected by 5 weeks of gestation and synaptic levels of serotonin increase during the first 2 years after birth and then decrease to adult levels after the age of 5 years (Sodhi and Sanders-Bush, 2004).…”