Postnatal Development of Serotonergic Innervation, 5-HT_1AReceptor Expression, and 5-HT Responses in Rat Motoneurons

Abstract

We compared the electrophysiological responses to serotonin (5-HT) of neonatal and juvenile rat hypoglossal motoneurons (HMs) by using intracellular recording techniques in a brainstem slice preparation. In neonatal HMs (ՅP8), 5-HT caused a substantial decrease in the amplitude of spike afterhyperpolarization (AHP) that was associated with an increase in the minimal repetitive firing frequency (F min ). Previous work has shown that this effect of 5-HT was mediated by the 5-HT 1A receptor and may be secondary to inhibition of N-and P/Q-type calcium channels. In contrast to results from neonates, we found that 5-HT did not inhibit the AHP in juvenile HMs (Ն P20). Application of a cocktail of calcium channel toxins (-Conotoxin-GVIA and -Agatoxin-IVA) to juvenile HMs substantially inhibited the AHP, indicating that calcium entry through N-and P/Q-type channels supports the AHP in juvenile HMs, as it does in neonates. In addition, intracellular injection of the long-lasting GTP analog GTP␥S induced an agonist-independent increase in F min similar to that seen in neonates in the presence of 5-HT. Together, these results suggested that intracellular mechanisms downstream of the 5-HT 1A receptor capable of inhibiting the AHP were intact in juvenile HMs. Therefore, we investigated the possibility that age-related changes in effects of 5-HT on the AHP resulted from altered expression of the 5-HT 1A receptor. To this end, we performed ligand-binding autoradiography using [ 3 H]8-OH-DPAT, a 5-HT 1A agonist, and in situ hybridization using radiolabeled oligonucleotide probes specific for the 5-HT 1A receptor. The two approaches gave remarkably similar results. The highest levels of 5-HT 1A receptor expression were found in neonatal HMs, with maximal binding and hybridization at approximately postnatal day 7 (P7) and only low levels of receptor expression by P28. Finally, immunohistochemistry for 5-HT revealed that these developmental changes in 5-HT 1A receptor expression occurred coincident with a postnatal increase in serotonergic innervation of the hypoglossal nucleus (nXII). Together, these findings indicate that developmental changes occur in the serotonergic innervation of nXII and in the expression of 5-HT 1A receptors in HMs during the early postnatal period, resulting in markedly different effects of 5-HT on firing behavior in neonatal and juvenile HMs.

Synaptic Control of Motoneuronal Excitability

“…Without exception, NE decreases the current necessary to induce repetitive firing, even after compensating for the NE-induced depolarization; this is due to the decreased input conductance that follows from inhibition of a leak K + current. This effect is similar to that induced by TRH or 5-HT (75,1228). In a subset of motoneurons, a decrease in the spike AHP is observed, and in these cells, there is also an increased slope of the firing frequency response to current inputs (958).…”

“…However, hypoglossal motoneurons from juvenile animals respond to 5-HT with a depolarization associated with an increase in input resistance and show no effect of 5-HT on the spike AHP (1228). The discrepancy with regard to the spike AHP is explained by a reduction in the expression of 5-HT 1A receptors in juvenile animals (79,1228). In neonatal animals, activation of 5-HT 1A receptors leads to a reduction in N-and P-type Ca 2+ currents, which in turn leads to reduced spike AHP (73).…”

“…Compared with adults, there is marked reduction in the spike AHP in these motoneurons (73,91,1228). However, hypoglossal motoneurons from juvenile animals respond to 5-HT with a depolarization associated with an increase in input resistance and show no effect of 5-HT on the spike AHP (1228). The discrepancy with regard to the spike AHP is explained by a reduction in the expression of 5-HT 1A receptors in juvenile animals (79,1228).…”

“…Exogenous application of 5-HT to hypoglossal motoneurons from neonatal rodents gives rise to a spike-initiating depolarization, with no clear change in membrane input resistance (91,1228). Compared with adults, there is marked reduction in the spike AHP in these motoneurons (73,91,1228).…”

Changes in the control of gastric motor activity during metamorphosis in the amphibian Xenopus laevis, with special emphasis on purinergic mechanisms

“…Developmental changes have previously been found in 5-HT innervation and receptor (5HT 1A ) expression during the early postnatal period in rat motor neurons (Talley et al, 1997) and in respiratory motor control in the amphibian Rana catesbeiana during metamorphosis (Kinkead et al, 2002). As in Rana respiratory control and in adult Xenopus gut (Johansson, 2003), the biphasic effect of 5-HT in the juvenile stomach is manifested as contraction at low 5-HT concentrations followed by relaxation at higher concentrations.…”