F. Viana

Modulation of neonatal rat hypoglossal motoneuron excitability by serotonin

The effects of 5-HT on neonatal rat hypoglossal motoneurons (HMs) were studied in two in vitro slice preparations. Serotonin caused either reversible depolarization or the generation of an inward current (I5-HT) in every cell tested. I5-HT persisted after synaptic blockade. In most of the cells tested, the magnitude of I5-HT was independent of membrane potential (-50 to -120 mV), and 5-HT had little effect on input resistance or slope conductance. In addition, 5-HT significantly reduced the amplitude of the post-spike medium-duration afterhyperpolarization. This reduction probably contributed to the resulting increase in the slope of the relationship describing the steady-state firing frequency response to injected current (f-I) observed in the presence of 5-HT. Thus, 5-HT increases the excitability of neonatal HMs via at least two different postsynaptic mechanisms.

Postnatal changes in rat hypoglossal motoneuron membrane properties.

Intracellular recording techniques were used to characterize changes that take place in rat hypoglossal motoneuronal excitability from early postnatal stages to adulthood. This study focused primarily on the first two weeks of postnatal life, when major changes in the maturation of the neuromuscular system take place. Neonatal hypoglossal motoneurons were identified by their location within the hypoglossal nucleus and by their characteristic electrophysiology. These criteria were supported by antidromic activation and intracellular staining of retrogradely labeled hypoglossal motoneurons. Action potential duration decreased progressively during postnatal development. The reduction was primarily due to a more rapid repolarization, suggesting developmental changes in voltage-dependent potassium conductances. The duration of the calcium-dependent afterhyperpolarization decreased by half during the first two weeks of postnatal life. Changes in subthreshold responses included a decrease in input resistance and an increase in the degree of hyperpolarizing sag and inward rectification with age. Rheobase current was negatively correlated with input resistance, and increased progressively during postnatal development. Membrane time constant decreased almost four-fold over the first two postnatal weeks, suggesting that membrane resistivity is not constant. This decrease in membrane resistivity could account for a large fraction of the change in input resistance and rheobase with age. Thus, the early postnatal development of the rat includes systematic changes in the electrophysiological properties of motoneurons innervating tongue muscles. Some of these modifications are not easily explained by a mere change in neuronal surface area but likely involve changes in the density of expressed ion channels.

Neuromodulation of hypoglossal motoneurons: cellular and developmental mechanisms.

Hypoglossal motoneurons (HMs) in the caudal brainstem have a respiratory-related activity pattern and contribute to control of upper airway resistance. In this review, we focus primarily on signalling mechanisms utilized by neurotransmitters to enhance HM excitability. In particular, we consider: (1) the membrane depolarization induced by a number of different putative transmitters [thyrotropin-releasing hormone (TRH), serotonin (5-HT), norepinephrine (NE)]; and (2) the inhibition of a calcium-dependent spike after hyperpolarization (AHP) by 5-HT and its effect on firing behavior. Potential functional consequences on HM behavior of these different neurotransmitter effects is discussed. In addition, we describe postnatal changes in transmitter effects and suggest potential cellular mechanisms to explain those developmental changes. Most of the data discussed are derived from in vitro electrophysiological recordings performed in preparations from neonatal and adult rats.

Repetitive firing properties of developing rat brainstem motoneurones.

1. The repetitive firing properties of neonatal and adult rat hypoglossal motoneurones (HMs) were investigated in a brainstem slice preparation. Neonatal HMs could be classified into two main groups: (1) neurones with a decrementing or adapting firing pattern (type D); exhibiting an early and a late phase; and (2) neurones with an incrementing or accelerating firing pattern (type I). 2. The pattern of repetitive firing changed markedly during development. While most HMs recorded from young rats (< postnatal day (P) 4) were type D, the majority of HMs recorded during the second postnatal week were type I. In adults (> P21), nearly all HMs had a decrementing firing pattern, characterized by a brief period of adaptation and high steady‐state firing rates. 3. The calcium‐dependent after‐hyperpolarization (AHP) was shortest in type I neonatal HMs, and decreased in amplitude during trains of action potentials (APs). In type D neurones, these same trains caused a slight enhancement of AHP amplitude. In adult HMs, with a decrementing firing pattern, trains of APs also caused summation of the AHP. 4. Type D neonatal HMs showed a progressive prolongation of the AP during repetitive firing. In contrast, type I neonatal HMs had almost no change in AP duration. In adult HMs the AP was short and experienced only a modest increase in duration during fast repetitive firing. 5. The function relating steady‐state firing frequency to injected current (f‐I curve) was linear. The mean steady‐state f‐I slope was significantly higher in neonates than in adults (approximately 30 vs. approximately 20 Hz nA‐1), and was weakly correlated with input resistance. The f‐I slope was negatively correlated with AHP duration in neonatal HMs only. In addition, for a given AHP duration the slope was higher in neonatal HMs. 6. Two threshold behaviours were observed among neonatal HMs: (a) a progressive rhythmic firing threshold, and (b) a sudden transition from subthreshold to regular repetitive firing. Current threshold for repetitive firing was strongly correlated with cell input conductance. Type I neonatal HMs had higher minimal steady firing rates (fmin) than type D HMs. In neonates, fmin was strongly correlated with AHP duration. Adult HMs showed a weaker correlation between these two parameters, and fmin was higher than predicted by AHP duration. 7. In summary, HMs responded to depolarizing current pulses with different firing patterns during postnatal development.(ABSTRACT TRUNCATED AT 400 WORDS)

Postnatal Development of Hypoglossal Motoneuron Intrinsic Properties

This review has provided evidence that marked changes are occurring in ionic currents present in upper airway motoneurons during the early postnatal period. Our results have shown that the density of the LVA Ca2+ current decreases during this period, and this probably reflects a reduced expression of the Ca2+ channel responsible for this current, the so-called T-type channel. These results help to explain the changes in burst firing behavior of HMs during the early postnatal period. We have shown that the fraction of HMs exhibiting burst firing behavior was the greatest among HMs just at or after birth, and disappeared by 10 days of age (Viana et al, 1993). The LVA Ca2+ current contributes to this firing behavior. In contrast to the reduction in the LVA Ca2+ current density with postnatal development, there is an apparent increase in Ih current density during this period. The increase in Ih provides a basis for a number of differences in the electrophysiological properties of adult versus neonate HMs. These include a striking depolarizing sag and overshoot during and immediately after application of hyperpolarizing current pulses in adult HMs. It is of interest that rebound depolarization following hyperpolarization can be observed in neonatal HMs even though there is little Ih present. This response probably reflects the activation of a LVA Ca2+ current. Other differences in neonate versus adult HMs also are in part probably due to differences in Ih current density. Since Ih is active at normal resting membrane potential (approximately -70 mV), Ih may contribute to the lower input resistance of adult compared with neonatal HMs (Haddad et al, 1990; Núñez-Abades et al, 1993; Viana et al, 1994), and the lower apparent membrane resistivity of older HMs (Viana et al 1994). The larger Ih in the adult may be a factor in the shorter spike afterhyperpolarization observed in adult versus neonatal HMs (Viana, et al, 1994). This may be a consequence of the greater amount of Ih activated during the afterhyperpolarization in adult HMs. The larger Ih in adult HMs may also contribute to differences in how synaptic inputs are integrated. For example, inhibitory inputs which hyperpolarize the membrane potential may have their effect lessened due to Ih activation with hyperpolarization. Thus in adult HMs Ih may weaken prolonged or strong hyperpolarizations that occur in response to inhibitory synaptic inputs, while depolarizing responses arising from excitatory synaptic inputs may not be compromised. In contrast, neonatal HMs, which lack a substantial Ih current, do not have the stabilizing influence upon membrane potential that is due to Ih. Therefore, these cells may be more susceptible to such inhibitions. In conclusion, this chapter has described the changes that take place in two ionic currents during postnatal development, and how they contribute to distinct subthreshold and firing properties of neonatal and adult motoneurons.

Projections and terminations of single respiratory axons in the cervical spinal cord of cat.

Position, divergence, branching, and termination patterns of single, respiratory axons were studied in cat cervical spinal cord by injecting horseradish peroxidase (HRP) intra-axonally. We stained 12 axons which were characterized by their firing patterns and by electrical stimulation. Five axons discharged during inspiration (I); the remaining 7 discharged during expiration (E). No injected axon was evoked by stimulating ipsilateral phrenic nerve roots while 7 (4 I, 3 E) of 12 were excited at a short latency from stimulating at a medullary site (on the midline, 1-2 mm rostral to the obex, approximately 3 mm below the dorsal medullary surface) where many bulbospinal respiratory axons decussate. All injected stem axons were located in the ventral and ventrolateral funiculi, traversed in a rostrocaudal direction, and were stained for lengths ranging from 3.6 to 12.4 mm. Mean axonal diameter was 2.9 microns. In 6 axons (4 I, 2 E), 14 collaterals were stained: 1 on each E axon, 2 on one I axon, 3 each on 2 others and 4 on another I axon. Collaterals emerged perpendicularly from the descending stem axon and projected directly to the ventral horn. The average distance between neighboring collaterals was 1.0 mm (n = 7). Collaterals did not arborize until they were near or within the ventral horn. Both en passant and terminaux types of presynaptic boutons were found primarily within the rostrocaudal cylinder that defined the phrenic motor column. In addition, some boutons were located dorsomedial to the phrenic motor column. We conclude that I axons, presumably of medullary origin, have multiple collaterals which terminate primarily in the phrenic motor column. However, the same axon can have terminals in different regions of the ventral horn, which are known to contain dendrites of phrenic motoneurons.