Antagonists-resistant calcium currents in rat embryo motoneurons

Ca2+ channels diversity of cultured rat embryo motoneurons was investigated with whole-cell current recordings. In 5-20 mM Ba2+, the whole-cell currents were separated in low- (LVA) and high-voltage-activated (HVA) current. The LVA current was evident since the first day in culture, while the HVA component was small and increased with time. Recordings after 4 days revealed approximately 20% L-, approximately 45% N- and approximately 35% P- and R-type currents. P-type currents were revealed only in 40% of motoneurons, in which 20-200 nM omega-Aga-IVA caused 20% irreversible block of total current. The remaining 60% of cells were insensitive even to higher doses of the toxin (500 nM in 5 mM Ba2+), suggesting weak expression and heterogeneous distribution of P-type channels compensated by high densities of HVA Ca2+ channels resistant to all the antagonists (R-type). A significant residual current could also be resolved after prolonged applications of 5 microM omega-CTx-MVIIC, which allowed separation of N- and P-type currents by the distinct onset of toxin block. The antagonists-resistant current reveals biophysical characteristics typical of HVA channels, but distinct from the alphaE channel. The current activates around -20 mV in 20 mM Ba2+; inactivates slowly and independently of Ca2+; is blocked by low [Cd2+] and high [Ni2+]; and is larger with Ba2+ than Ca2+. The uncovered R-type calcium current can account for part of the presynaptic Ca2+ current controlling neurotransmitter release at the mammalian neuromuscular junction whose activity is resistant to DHP-and omega-CTx-GVIA, and displays anomalous sensitivity to omega-Aga-IVA and omega-CTx-MVIIC.     

Organization of motoneurons in the dorsal hypoglossal nucleus that innervate the retrusor muscles of the tongue in the rat

This anatomical investigation was prompted by the incomplete knowledge of the myotopic organization of the dorsal subdivison of the hypoglossal nucleus. Intrinsic muscle motoneurons were not segregated and labeled previously with regard to the lateral division of the hypoglossal nerve. Also, motoneuron number and cell size, in relation to the individual retrusor tongue musculature, were rarely addressed previously. Retrograde labeling ofretrusor muscle motoneurons in the dorsal subdivision of the rat hypoglossal nucleus was done. Cholera toxin conjugate horseradish peroxidase (CTHRP) was injected into the retrusor tongue muscles with only the lateral division of the hypoglossal nerve intact. The dorsal subdivision of the hypoglossal nucleus contained approximately 800 motoneurons ranging in cell body size from 19 to 41 microm. When either the styloglossus, hyoglossus, superior longitudinal, or inferior longitudinal muscle was isolated and injected with CTHRP, a separate motoneuron pool for each muscle was seen. The extrinsic muscle motoneurons, styloglossus and hyoglossus, were found rostrolateral and caudolateral respectively. In contrast, the intrinsic superior and inferior longitudinal muscle motoneurons were found more central and medial in the nucleus. Extrinsic muscle motoneurons were larger (approximately 30 microm) than intrinsic muscle motoneurons (approximately 26 microm; P < .0001). Intrinsic muscle motoneurons account for a great majority of the motoneurons in the dorsal aspect of the hypoglossal nucleus and their axons have been shown to be contained in the lateral (retrusor) division of the hypoglossal nerve. This study revealed the myotopic organization of the retrusor subdivision of the rat hypoglossal nucleus.     

Role of GABAB receptors in the control of hypoglossal motoneurons in vivo

There is little information on GABAB receptor-mediated effects on orofacial motoneurons. We recorded the inspiratory activity from both hypoglossal (XII) nerves in urethane-anesthetized, paralyzed, vagotomized and artificially ventilated rats. A GABAB receptor agonist, baclofen, or antagonist, CGP-35348, was microinjected into one XII nucleus. Baclofen rapidly reduced the XII nerve activity in a dose-dependent manner by over 50%. The antagonist caused a delayed suppression of activity by 40%. We conclude that: (1) GABAB receptors within the XII nucleus may suppress the activity of inspiratory XII motoneurons, but they are not tonically active under the conditions of our experiment; (2) there is a net endogenous excitatory effect in XII motoneurons that is mediated by GABAB receptors located in the reticular formation surrounding the XII nucleus.     

Intensity and frequency dependence of laryngeal afferent inputs to respiratory hypoglossal motoneurons

Inspiratory hypoglossal motoneurons (IHMs) mediate contraction of the genioglossus muscle and contribute to the regulation of upper airway patency. Intracellular recordings were obtained from antidromically identified IHMs in anesthetized, vagotomized cats, and IHM responses to electrical activation of superior laryngeal nerve (SLN) afferent fibers at various frequencies and intensities were examined. SLN stimulus frequencies <2 Hz evoked an excitatory-inhibitory postsynaptic potential (EPSP-IPSP) sequence or only an IPSP in most IHMs that did not change in amplitude as the stimulus was maintained. During sustained stimulus frequencies of 5-10 Hz, there was a reduction in the amplitude of SLN-evoked IPSPs with time with variable changes in the EPSP. At stimulus frequencies >25 Hz, the amplitude of EPSPs and IPSPs was reduced over time. At a given stimulus frequency, increasing stimulus intensity enhanced the decay of the SLN-evoked postsynaptic potentials (PSPs). Frequency-dependent attenuation of SLN inputs to IHMs also occurred in newborn kittens. These results suggest that activation of SLN afferents evokes different PSP responses in IHMs depending on the stimulus frequency. At intermediate frequencies, inhibitory inputs are selectively filtered so that excitatory inputs predominate. At higher frequencies there was no discernible SLN-evoked PSP temporally locked to the SLN stimuli. Alterations in SLN-evoked PSPs could play a role in the coordination of genioglossal contraction during respiration, swallowing, and other complex motor acts where laryngeal afferents are activated.     

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.     

Differential expression of calbindin and calmodulin in motoneurons after hypoglossal axotomy

Interferon-gamma (IFN-gamma) is a structurally labile cytokine that rapidly denatures upon exposure to acid or heat. Here we show that both acid-denatured (pH 2) and thermally inactivated (50 degrees C) porcine IFN-gamma can be rescued with the Escherichia coli GroEL/ES chaperonin system and ATP, and reassembled into bioactive dimers. At 35 degrees C, spontaneous refolding of acid-denatured IFN-gamma was found to be dependent on the presence of guanidinium hydrochloride (0.15-0.25 M) or NaCl (0.1-0.2 M). Under non-permissive reaction conditions for regain of native structure (low-ionic-strength buffer at 35 degrees C), the yield of IFN-gamma refolded with GroEL/ES/ATP increased about 30-fold above the level of spontaneous refolding. In the absence of GroES, GroEL captured IFN-gamma in a folding-competent complex. Under these conditions, both ATP and alpha-casein induced release of IFN-gamma from GroEL but with the released protein tending to partition into sedimentable aggregates. Only in the presence of GroES, did ATP induce complete discharge of IFN-gamma from GroEL, with the released protein refolded into a conformation that is (a) immunoreactive/bio-active, (b) resistant to precipitation and (c) in a dimeric configuration. Chicken egg albumin and 90-kDa heat-shock protein were inactive in the exertion of any protective effect against physicochemical stress. The precise amount of protein refolded to the native state at different times of the folding reaction was determined by alpha-casein quenching and ELISA. The former is based on the conversion by excess alpha-casein of any population of unfolded IFN-gamma into one that escapes antibody recognition by subsequent ELISA. Since the native dimers, however, are not affected by alpha-casein quenching, immunoreactivity is directly proportional to the yield of correctly refolded protein. The validity of this approach was confirmed by measurement of biological activity. GroEL/ES-meditated reactivation amounted to > 80% both by ELISA and antiviral assay.     

Characterization of outward currents in neurons of the avian nucleus magnocellularis

Characterization of outward currents in neurons of the avian nucleus magnocellularis. J. Neurophysiol. 80: 2824-2835, 1998. Neurons of the nucleus magnocellularis (NM) preserve the timing of auditory signals through the convergence of a variety of voltage- and ligand-gated ion channels. To understand better how these channels interact, we have characterized the kinetics, voltage sensitivity, and pharmacology of outward currents of NM neurons in brain slices. The reversal potential (Erev) of outward currents varied with potassium concentration as expected for currents carried by potassium. However, Erev was consistently more positive than the Nernst potential for potassium (EK). Deviation of Erev from the calculated EK most likely arose from potassium accumulation in extracellular spaces by potassium conductances active at rest and during depolarizing steps. Three outward potassium currents were studied that varied in voltage and pharmacological sensitivity. A tetraethylammonium (TEA)-sensitive, high-threshold current was activated within 1-5 ms of the onset of depolarization, with a half-maximal activation voltage (V1/2) of -19 mV. It was blocked partially by 4-aminopyridine (4-AP) and was the dominant ionic conductance of NM neurons. A dendrotoxin-I (DTX) and 4-AP-sensitive, low-threshold current had a V1/2 of -58 mV, rapid activation kinetics, and only partial inactivation, with decay time constants between 20 and 100 ms. A rapidly inactivating current was observed that was resistant to TEA and DTX and was blocked by intracellular Cs+. The transient current was inactivated almost completely at the resting potential. The onset of inactivation was fastest at potentials negative to those that caused activation. When intracellular K+ was replaced by Cs+, large inward and outward currents were obtained that corresponded respectively to the above-mentioned DTX- and TEA-sensitive currents. Outward, TEA-sensitive current was carried by Cs+, with a PCs/PK of approximately 0.1. In current-clamped neurons, DTX induced repetitive firing and increased membrane time constant near rest but had little effect on action potential duration. These studies indicate that a low-threshold, DTX-sensitive current plays a key role in making NM neurons highly responsive to the onset and offset of synaptic stimuli.     

Increased spike-frequency adaptation and tea sensitivity in dorsal root fibers after sciatic nerve injury

Excitability of rat dorsal root axons were studied 3 weeks after injury to the sciatic nerve. Whole nerve recordings were obtained from injured and control nerves in a sucrose gap chamber. Constant current depolarization pulses (30-200 ms) applied approximately 50% above the stimulus strength required for maximal amplitude compound action potentials (CAPs) evoked a burst of action potentials in the dorsal root which displayed spike adaptation. The depolarization-induced burst response of the dorsal roots was greatly reduced after crush or transection of the sciatic nerve. However, application of the potassium channel blocker, tetraethylammonium (TEA), restored the burst discharge in injured dorsal root axons. Brief tetanic stimulation of the dorsal root also induced an afterhyperpolarization (AHP) that was twice as large in the transection group as compared to the control group, and which was blocked by TEA. There were no changes seen in the amplitude of the compound action potential, frequency-following characteristics, refractory properties, or 4-AP sensitivity in the dorsal roots after peripheral nerve injury. These results suggest that there is enhanced spike adaptation that occurs at the same time as an increase in the sensitivity to the potassium channel blocker, TEA, in axon regions proximal to the site of nerve injury and have implications for the pathophysiology of nerve injury.     

Characteristics of a fast transient outward current in guinea pig trigeminal motoneurons

A fast transient voltage dependent outward current (TOC) in trigeminal motoneurons (TMNs) was studied in guinea pig brainstem slices by use of sharp electrodes in combination with single electrode voltage clamp techniques. In solutions containing TTX, low Ca2+/Mn2+ and 20 mM TEA this current activated around -55 to -60 mV from holding potentials negative to resting potential, obtained its peak amplitude within 5 ms and decayed as a single exponential with a time constant of 6-8 ms. Half maximal values for inactivation and activation were -72 and -37 mV, respectively. Bath application of 5 mM 4-AP suppressed this current by approximately 90% and eliminated the early depolarizing transient membrane rectification observed in response to a constant depolarizing current pulse, prolonged the action potential duration, and reduced the threshold voltage and delay to onset of the action potential. It is suggested that this current resembles the typical A-current observed in many CNS neurons and, as a result of its voltage and time dependent properties, could contribute to control of motoneuronal discharge and timing of burst onset during rhythmical jaw movements. Therefore, any cellular models of masticatory activity should include the properties of this current.     

Non-NMDA receptor-mediated modulation of voltage-activated outward currents in chick neurones

The effect of kainate on the voltage-activated current of cultured embryonic chick telencephalic neurones was studied by whole-cell voltage clamp recording. In addition to opening non-NMDA receptor coupled ion channels, kainate produced additional effects, with a slower time course: it modulated voltage-activated currents. These effects were blocked by the non-NMDA receptor antagonists CNQX (6-cyano-7-nitroquinoxaline-2,3-dione) and GYKI 53784 ((-)-1-(4-aminophenyl)-3- methylcarbamoyl-4-methyl-7,8-methylenedioxy-3,4- dihydro-5H-2,3-benzodiazepine), indicating that non NMDA receptors are involved. It has been shown that the reversible inhibition of outward potassium currents was partly due to a decrease of current amplitude and partly to a marked shift of the inactivation curve towards more negative potentials.     

Control of the delayed outward potassium currents in bursting pace-maker neurones of the snail, Helix pomatia

The net outward current in bursting pace-maker neurones of the snail (Helix pomatia) during sustained and repeated voltage clamp pulses was studied. The properties of currents remaining in cobalt-Ringer or after TEA injection were compared with those in untreated cells. 2. With sustained voltage clamp depolarizations the net outward current first increases to a maximum at 150 msec and then declines to 60% or less of its peak intensity. This depression, which is greater during repetition of short pulses (e.g. 100 msec pulses at 0-5 sec intervals), represents a true decrease in the outward flow of K (designated IK) and is not due to a decreased driving force resulting from extracellular K accumulation. The steady-state current-voltage (I-V) relationship for IK is N-shaped (Heyer & Lux, 1976). 3. A component of IK persists when Ca and Mg in the medium are replaced by Co (ICo-res). With voltage clamp depolarizations ICo-res increases rapidly to a maximum and then partially inactivates with voltage dependent time constants of hundredths or tenths of seconds. Repolarization removes the inactivation. Thus, repeated stimulation with short pulses does not increase the depression of ICo-res-ICo-res (e.g. measured during voltage steps from holding potentials of -50 to near 0 mV) is smaller in test pulses preceded by depolarization and larger in pulses preceded by hyperpolarization. The steady state I-V relationship is not N-shaped. ICo-res is blocked by intracellular injection of tetraethylammonium (TEA). 4. Repeated voltage clamp depolarization to near 0 mV with 100 msec pulses for neurones with large Ca currents in normal Ringer produces a long-term depression which is maximal with 300-400 msec repolarizations (to -50 mV) between pulses. This corresponds with stimulus parameters for the maximum Ca current (Heyer & Lux, 1976). Intracellular injection of Ca2+ (also Ba2+ and Co2+) likewise reduces the total net outward current and especially the delayed outward current under voltage clamp. 5. The component of IK which is removed by Co is identified as Ca dependent and designated IK(Ca). With single voltage clamp pulses IK(Ca) follows the approximate time course and voltage dependence of the slow inward Ca current (Iin slow; Heyer & Lux, 1976). Several lines of evidence suggest that Ca ions moving through the membrane activate IK(Ca). 6. Part of IK cannot be blocked by intracellular TEA injection. In different neurones the magnitude of the IK component resistant to TEA (ITEA-res) is approximately proportional to the relative magnitudes of Iin slow.ITEA-res does not inactivate with sustained depolarization and shows pronounced long-term depression with repetitive stimulation at intermediate intervals and an increased outward current at the onset of the second and subsequent pulses following short repolarizations. The steady-state I-V relationship is N-shaped. ITEA-res is abolished by extracellular Co. 7. A net inward current with low depolarizations can be measured after TEA injection...     

Development of glycinergic synaptic transmission to rat brain stem motoneurons

Using an in vitro rat brain stem slice preparation, we examined the postnatal changes in glycinergic inhibitory postsynaptic currents (IPSCs) and passive membrane properties that underlie a developmental change in inhibitory postsynaptic potentials (IPSPs) recorded in hypoglossal motoneurons (HMs). Motoneurons were placed in three age groups: neonate (P0-3), intermediate (P5-8), and juvenile (P10-18). During the first two postnatal weeks, the decay time course of both unitary evoked IPSCs [mean decay time constant, taudecay = 17.0 +/- 1.6 (SE) ms in neonates and 5.5 +/- 0.4 ms in juveniles] and spontaneous miniature IPSCs (taudecay = 14.2 +/- 2.4 ms in neonates and 6.3 +/- 0.7 ms in juveniles) became faster. As glycine uptake does not influence IPSC time course at any postnatal age, this change most likely results from a developmental alteration in glycine receptor (GlyR) subunit composition. We found that expression of fetal (alpha2) GlyR subunit mRNA decreased, whereas expression of adult (alpha1) GlyR subunit mRNA increased postnatally. Single GlyR-channels recorded in outside-out patches excised from neonate motoneurons had longer mean burst durations than those from juveniles (18.3 vs. 11.1 ms). Concurrently, HM input resistance (RN) and membrane time constant (taum) decreased (RN from 153 +/- 12 MOmega to 63 +/- 7 MOmega and taum from 21.5 +/- 2.7 ms to 9.1 +/- 1.0 ms, neonates and juveniles, respectively), and the time course of unitary evoked IPSPs also became faster (taudecay = 22.4 +/- 1.8 and 7.7 +/- 0.9 ms, neonates vs. juveniles, respectively). Simulated synaptic currents were used to probe more closely the interaction between IPSC time course and taum, and these simulations demonstrated that IPSP duration was reduced as a consequence of postnatal changes in both the kinetics of the underlying GlyR channel and the membrane properties that transform the IPSC into a postsynaptic potential. Additionally, gramicidin perforated-patch recordings of glycine-evoked currents reveal a postnatal change in reversal potential, which is shifted from -37 to -73 mV during this same period. Glycinergic PSPs are therefore depolarizing and prolonged in neonate HMs and become faster and hyperpolarizing during the first two postnatal weeks.     

Estimation of outward currents in isolated human atrial myocytes using inactivation time course analysis

The aim was to investigate outward currents in single, isolated, human, atrial myocytes and to determine the relative contribution of individual current components to the total outward current. Currents were recorded using the whole-cell patch-clamp technique at 36-37 degreesC. Individual outward current components were estimated from recordings of total outward current using a mathematical procedure based on the inactivation time course of the respective currents. This method allows estimation of outward currents without the use of drugs or conditioning voltage-clamp protocols to suppress individual current components. A rapidly activating and partially inactivating total outward current was recorded when myocytes were voltage clamped at potentials positive to -20 mV (peak current density 24. 0+/-0.97 pA/pF at +40 mV; n=107 cells, 33 patients). This total outward current comprised three overlapping currents: a rapidly inactivating, transient, outward current (Ito1) a slowly and partially inactivating current (ultrarapid delayed rectifier, IKur) and a third current component which most probably reflects a non selective cation current (not characterized). The average current densities at +40 mV were 8.92+/-0.44 pA/pF for Ito1 and 15.1+/-0.72 pA/pF for IKur (n=107 cells). Recovery from inactivation was bi-exponential for both currents and was faster for Ito1. A slowly activating delayed rectifier current (IK) was not found. The current densities of peak Ito1 and IKur varied strongly between individual myocytes, even in those from the same patient. The ratio IKur/Ito1 was 0.5-6.9 with a mean of 1.98+/-0.11 (n=107 cells), suggesting that IKur is the main repolarizing current. The amplitudes of the total outward current, Ito1 and IKur, and the ratio of the latter two were independent of patient age (16-87 years).     

Descending projections from the nucleus raphe obscurus to pudendal motoneurons in the male rat

Previous physiological and behavioral studies have shown that the nucleus raphe obscurus (nRO) modulates pelvic floor reflex function (Yamanouchi and Kakeyama [1992] Physiol. Behav. 51:575-579; Beattie et al. [1996] Soc. Neurosci. Abstr. 22:722.4; Holmes et al. [1997] Brain Res. 759:197-204). In the present study, small injections of fluorescent tracers were used to investigate direct descending projections from the rostral and caudal portions of the brainstem nRO to retrogradely labeled pudendal motoneurons (MN) in the male rat. The caudal nRO projects into the ventral and lateral funiculi of the spinal cord, with arborizations in the thoracic intermediolateral cell column; in laminae VII, IX, and X of the lumbosacral cord; and in the sacral parasympathetic nucleus (SPN). Many identified external anal sphincter and ischiocavernosus MNs appeared to be in direct apposition with fibers originating from the caudal nRO; and more than half of the bulbospongiosus MNs that were identified appeared to receive such descending input. In addition to the nRO spinal autonomic and pudendal motoneuronal targets, projections were observed to regions of the intermediate gray that contain interneurons organizing the pelvic floor reflexes and to MN pools that are involved in functionally related somatic activities. Finally, several neurons in the lumbar enlargement were labeled retrogradely with FluoroRuby after injections into the nRO and the immediately adjacent reticular formation. Thus, the nRO may be in a position to modulate the coordinated actions of autonomic preganglionic and functionally related skeletal MN activity involved in sexual and eliminative reflex functions.     

Colocalization of substance P or enkephalin in serotonergic neuronal afferents to the hypoglossal nucleus in the rat

The serotonergic innervation of the hypoglossal nucleus originates from the caudal raphe nuclei. Non-serotonergic neurons in the caudal raphe nuclei also project to the hypoglossal nucleus. We employed a triple-fluorescence technique to determine whether the substance P- or the enkephalin-containing neurons in the caudal raphe nuclei that projected to the hypoglossal nucleus also contained serotonin. Rhodamine latex microspheres were injected into the hypoglossal nucleus, and then serotonin and peptide dual-immunofluorescence was performed to colocalize perikarya containing serotonin, substance P, and rhodamine microspheres; or perikarya containing serotonin, enkephalin, and rhodamine microspheres. Our results demonstrate that most substance P-containing neuronal afferents to the hypoglossal nucleus colocalize serotonin. In contrast, few enkephalin-containing neuronal afferents to the hypoglossal nucleus also contain serotonin. These data suggest that substance P projections to the hypoglossal nucleus are a subset of serotonergic projections and that limited overlap exists between the populations of enkephalinergic and serotonergic neuronal afferents to the hypoglossal nucleus. Either substance P- or enkephalin-containing somata account for a very small proportion of non-serotonergic caudal raphe projections to the hypoglossal nucleus. Finally, these data demonstrate the medial tegmental field origins of the substance P projections and the enkephalin projections to the hypoglossal nucleus.     

Contribution of ingestive behaviors to taste-aversion learning in the rat.

Conducted 3 experiments with male Sprague-Dawley rats (N = 132). Ss tasting but not ingesting a flavored solution prior to toxicosis acquired weaker aversions to the flavor than Ss that actively consumed the CS during conditioning. Taste was isolated from ingestion either by curarizing the Ss or by infusing a flavored solution very rapidly into the oral cavity of non-water-deprived Ss. Control groups showed that the facilitatory effect of ingestion on taste-aversion learning did not depend on the consumption of very much of the CS solution. (20 ref.) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Tetrahydroberberine blocks membrane K+ channels underlying its inhibition of intracellular message-mediated outward currents in acutely dissociated CA1 neurons from rat hippocampus

In the patch-clamp perforated whole-cell recording mode, tetrahydroberberine (THB), a novel dopamine (DA) receptor antagonist, inhibits not only DA-induced outward K+ currents, but also acetylcholine-, caffeine- or strychnine-induced outward current. However, THB does not affect either GABA- or glycine-induced Cl- currents, or non-NMDA receptor agonist-induced cation currents. As expected for a K+ channel blocker, THB evokes a downward current deflection accompanied by a decrease of conductance. It is concluded that the direct blockade of membrane K+ channels by THB underlies its inhibition of intracellular message-mediated outward currents.     

Calcitonin gene-related peptide and alpha-CGRP mRNA expression in cranial motoneurons after hypoglossal nerve injury during postnatal development

Changes in calcitonin gene-related peptide (CGRP) immunoreactivity and alpha-CGRP mRNA expression were determined in the hypoglossal nucleus after the nerve was crushed or transected in rats at 10, 14 and 21 days postnatal. alpha-CGRP mRNA expression was determined in normal, noninjured, hypoglossal nuclei at the three ages and after both injuries in 10 and 21 days postnatal rats. Reinnervation and neuronal survival were assayed. Although the three age groups expressed comparable levels of alpha-CGRP mRNA and its peptide in intact, hypoglossal nuclei, axonal injury produced age-dependent alterations in alpha-CGRP mRNA and CGRP. In the 21 days postnatal rats, changes in alpha-CGRP mRNA and peptide mimicked those reported in adult motoneurons after the same injuries. CGRP was elevated until reinnervation after nerve crush, whereas biphasic elevations occurred after nerve transection. In 21 days postnatal rats, increases in alpha-CGRP mRNA preceded elevations of the peptide but a greater increase resulted initially after nerve transection. An upregulation of alpha-CGRP mRNA also developed initially after both injuries in 10 days postnatal rats but subsequent elevations of alpha-CGRP mRNA did not materialize. In contrast, CGRP immunoreactivity did not increase after either injury in 10 days postnatal rats and, in fact decreased. Levels of CGRP immunoreactivity did not differ from normal amounts after either nerve injury in 14 days postnatal rats. Substantial neuronal cell loss occurred after each injury in 10 and 14 days postnatal rats but was not found in 21 days postnatal rats. Tongue reinnervation by surviving motoneurons was established after all injury paradigms except 10 days postnatal transection. The current findings demonstrate an age-dependent correlation between injury-induced expression of CGRP and hypoglossal motoneuron survival.     

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.     

Subcompartmental organization of the ventral (protrusor) compartment in the hypoglossal nucleus of the rat

The extent and myotopic organization of the ventral (protrusor) compartment of the hypoglossal nucleus (nXII) in the rat is controversial. Of particular concern is the location of motoneurons that innervate the intrinsic (verticalis, transversus) as compared to extrinsic (genioglossus) tongue protrusor muscles. These issues were investigated with retrograde transport, lesion/degeneration/immunocytochemical, and classic Golgi staining techniques. Results from these experiments demonstrate the following: (1) the ventral compartment extends the entire rostrocaudal length of nXII and is organized into three longitudinally oriented subcompartments, one medial and one lateral within the boundaries of nXII, and one outside the confines of nXII, defined as the lateral accessory subcompartment; 2) the medial and lateral subcompartments contain motoneurons that innervate the intrinsic (verticalis, transversus) and extrinsic (genioglossus) tongue protrusor muscles, respectively, while the lateral accessory subcompartment innervates the geniohyoid muscle; (3) ventral subcompartments are unequal in size and vary along the rostrocaudal dimension of nXII. The medial subcompartment is largest caudally and smallest rostrally, while the converse is true for the lateral subcompartment. By contrast, the lateral accessory subcompartment is present only along the caudal one-half of nXII; (4) medial and lateral subcompartments are further organized into smaller subgroups. Medial and centromedial subgroups are discernible within the medial subcompartment, lateral and centrolateral subgroups within the lateral subcompartment. Both medial and lateral subgroups extend throughout the rostrocaudal length of nXII, whereas the centromedial and centrolateral subgroups are present only along the middle two-thirds of nXII where they form a central motoneuron band; (5) there is an inverse myotopic organization within the medial and lateral subcompartments such that proximal and distal portions of intrinsic and extrinsic protrusor muscles receive innervation from rostral and caudal motoneurons, respectively; and (6) there is a correlation between motoneuron morphology (size, shape and dendritic field domains), subcompartment localization, and myotopic specificity. Motoneurons in the medial subcompartment are small (mean = 23.08 microns), round to globular, with dendrites oriented medially, dorsomedially, dorsolaterally, and caudally, whereas lateral subcompartment motoneurons are large (mean = 29.49 microns), round to triangular, with dendrites directed mainly mediolaterally and dorsally. These data are relevant to understanding the functional organization of nXII and the motor control of the tongue. Results are further discussed relative to the convergence of multifunctional afferent systems in the ventromedial subcompartment of nXII.