POSTNATAL-DEVELOPMENT OF SEROTONERGIC INNERVATION, 5-HT1A RECEPTOR EXPRESSION, AND 5-HT RESPONSES IN RAT MOTONEURONS

We compared the electrophysiological responses to serotonin (5-HT) of neonatal and juvenile rat hypoglossal motoneurons (HMs) by using intra cellular recording techniques in a brainstem slice preparation. In neo natal HMs (less than or equal to P8), 5-HT caused a substantial decrea se in the amplitude of spike afterhyperpolarization (AHP) that was ass ociated with an increase in the minimal repetitive firing frequency (F -min). Previous work has shown that this effect of 5-HT was mediated b y the 5-HT1A 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 (greater than or eq ual to P20). Application of a cocktail of calcium channel toxins (omeg a-Conotoxin-GVIA and omega-Agatoxin-IVA) to juvenile HMs substantially inhibited the AHP, indicating that calcium entry through N- and P/Q-t ype channels supports the AHP in juvenile HMs, as it does in neonates. In addition, intracellular injection of the long-lasting GTP analog G TP gamma S induced an agonist-independent increase in F-min similar to that seen in neonates in the presence of 5-HT. Together, these result s suggested that intracellular mechanisms downstream of the 5-HT1A rec eptor capable of inhibiting the AHP were intact in juvenile HMs. There fore, we investigated the possibility that age-related changes in effe cts of 5-HT on the AHP resulted from altered expression of the 5-HT1A receptor. To this end, we performed ligand-binding autoradiography usi ng [H-3]8-OH-DPAT, a 5-HT1A agonist, and in situ hybridization using r adiolabeled oligonucleotide probes specific for the 5-HT1A receptor. T he two approaches gave remarkably similar results. The highest levels of 5-HT1A receptor expression were found in neonatal HMs, with maximal binding and hybridization at approximately postnatal day 7 (P7) and o nly low levels of receptor expression by P28. Finally, immunohistochem istry for 5-HT revealed that these developmental changes in 5-HT1A rec eptor expression occurred coincident with a postnatal increase in sero tonergic innervation of the hypoglossal nucleus (nXII). Together, thes e findings indicate that developmental changes occur in the serotonerg ic innervation of nXII and in the expression of 5-HT1A receptors in HM s during the early postnatal period, resulting in markedly different e ffects of 5-HT on firing behavior in neonatal and juvenile HMs.