Early postnatal development of thyrotropin-releasing hormone (TRH) expression, TRH receptor binding, and TRH responses in neurons of rat brainstem

Abstract

We investigated the postnatal development of the thyrotropin-releasing hormone (TRH)-containing raphe system in the brainstem of neonatal rats. Postnatal changes in TRH expression in nucleus (n.) raphe obscurus (ROb) and n. raphe pallidus (RPa) were evaluated by in situ hybridization using an 35S-labeled oligonucleotide probe complementary to TRH precursor mRNA. TRH mRNA expression was low at birth [postnatal day 0 (P0)], but was clearly evident by P7 and increased from that time to reach sustained high levels from P14 to P28. Consistent with this postnatal increase in TRH expression, we found increases in the density of TRH-immunoreactive (IR) fibers, which are derived from ROb and RPa, in the hypoglossal nucleus (nXII). TRH-IR fibers in nXII were very sparse at P0, but increased markedly over the first 2 postnatal weeks. The change in TRH innervation of nXII was closely matched by concomitant increases in 3H-methyl-TRH binding in nXII; specific TRH binding increased from very low levels at birth to high levels of P14. Finally, we recorded intracellularly the electrophysiological responses to TRH of hypoglossal motoneurons (HMs; n = 42) of neonatal rats (P0-P21) in a brainstem slice preparation. The response of neonatal HMs to TRH, in contrast to adult HMs, was highly variable. In some neonatal HMs, even at P0, TRH caused a depolarization with a decrease in input conductance (GN) that was characteristic of the response of all adult HMs. However, in other neonatal HMs, TRH was either without effect or caused a slight depolarization with no apparent change in GN, responses that were unlike those of adult HMs. A response was considered typical (i.e., "adult-like") if GN decreased to < 85% of control. The percentage of cells responding in a typical manner increased progressively from 25% at P0-P2 to 100% after P11. In addition, we found that the density of TRH-sensitive current (normalized to cell capacitance) increased with postnatal age in HMs that responded in a typical manner, suggesting that expression of the TRH-sensitive conductance is also developmentally regulated. Together, these data indicate that the TRH raphe neuronal system of the rat brainstem is not fully mature at the time of birth but develops over the first few postnatal weeks. This was true of levels of TRH mRNA in caudal raphe nuclei, density of TRH-IR fibers and 3H-methyl-TRH binding in nXII, and also the manner and magnitude of electrophysiological responses of HMs to exogenously applied TRH.

Excitation of Histaminergic Tuberomamillary Neurons by Thyrotropin-Releasing Hormone

“…However, immature motoneurons displayed activa- tion of a nonselective cationic conductance by TRH and lack of interaction with potassium channels (Bayliss et al, 1994); moreover, their depolarization by TRH was not accompanied by a change in input resistance. This "immature" type of the response, partially explored by Bayliss et al (1992Bayliss et al ( , 1994, bears similarity with the TRH responses in TMN neurons, which were, however, recorded in young adult rodents.…”

Synaptic Control of Motoneuronal Excitability

“…Furthermore, the electrophysiological effects of TRH on motoneurons also change during this developmental period. An increasing fraction of TRHresponsive neurons are found through the early postnatal period, roughly paralleling the increase in receptor binding (78). However, a number of other changes also contribute to the functional maturation of hypoglossal motoneuron responses to TRH.…”

“…It is noteworthy that 5-HT, which modulates Ba 2+ -sensitive leak K + channels in adult motoneurons (526,1228), also fails to target those channels in neonatal hypoglossal motoneurons (91,1228). Finally, there is a progressive increase in TRH current density that matches closely the increase in membrane conductance over the early postnatal period (78,92,1313). This latter effect ensures that the magnitude of the TRH-induced membrane depolarization will be comparable in adult and neonatal hypoglossal motoneurons, at least in the population of neonatal cells that are capable of responding (78) (see Fig.…”

“…For example, a greater number of TRH-immunoreactive fibers form close appositions to respiratory-related motoneurons of the nucleus ambiguus than to either nonrespiratory ambigual motoneurons or to hypoglossal or facial motoneurons (1196). In addition, there are developmental increases in the density of TRH-immunoreactive fibers in brain stem and spinal cord motor pools (78,999); in the hypoglossal nucleus, the increased TRH innervation coincides temporally with the increased expression of TRH receptors by hypoglossal motoneurons (78).…”

“…Expression of the TRH precursor mRNA increases markedly in raphe neurons within the first 2 wk after birth (78), as does the density of TRH-immunoreactive fibers in motor nuclei receiving input from the raphe nuclei (78,999). Concomitant with this elevated innervation, the levels of TRH binding sites also increase to reach sustained high levels in rat hypoglossal motoneurons by postnatal day 14 (78). Thus both the presynaptic and postsynaptic elements of the raphe motoneuronal system mature over the first 2 wk via a process that is remarkably well matched in timing.…”