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S J Fung

Publications and source records attributed to S J Fung.

50 records · Page 3Linked to original sources
PubMed1986

Facilitation of lumbar monosynaptic reflexes by locus coeruleus in the rat.

The present study was initiated to delineate whether species difference exists between cats and rats in the descending influence of locus coeruleus (LC) on spinal motoneuronal activity. In male Sprague-Dawley rats anesthetized with chloral hydrate (400 mg/kg, i.p.), localized activation of LC promoted an exclusive facilitation of lumbar spinal extensor and flexor monosynaptic reflexes (MSRs). Such LC-evoked potentiations may vary in degree (37.5-147.4%), duration (70.6-72.9 ms) and latency (3.0-5.5 ms) among different animals. While minimally affecting the control MSRs, the alpha 1-adrenoceptor blocker prazosin (20 micrograms/kg, i.v.) significantly antagonized the enhancing effect of the LC on MSRs, suggesting the participation of noradrenergic neurotransmission in the process. Since these results are in general agreement with previous observations from our laboratory on the cat, we conclude that the LC exerts similar facilitatory actions on both extensor and flexor motoneuron activity of the hindlimb in at least two animal species, rat and cat.

Animals↗
PubMed1986

Increased efficacy of antidromic and orthodromic activation of cat alpha motoneurons upon arrival of coerulospinal volleys.

The present study demonstrates the enhanced efficacy of impulse initiation among the hindlimb alpha motoneurons of flexor and extensor origins (n = 35) upon electrical stimulation of the locus coeruleus (LC) in decerebrate cats. When combined with the LC-evoked excitatory postsynaptic potential (EPSP), intracellular hyperpolarization-induced partial and total blocks of antidromic invasion were overcome, resulting in full-spike generation in all cells (n = 21). In three other cells, partial blocks, representing the motoneuron refractoriness resulting from double stimulation at close intervals, were relieved by the concomitant LC-EPSP. When an antidromic volley occurred at a time when the somadendritic (SD) membrane was near threshold, LC stimulation was shown to increase the probability of full-spike initiation as well as to shorten the initial segment (IS)-SD delay, suggesting a coerulospinal enhancement of the safety factor for IS-SD impulse conduction. When coincident with the LC-EPSPs, group Ia EPSPs of flexor and extensor origins were demonstrated to reach the threshold of discharging the cells (n = 4). In those cells exhibiting prominent depolarizing synaptic noise (n = 10), LC stimulation was sufficient to cause the cell to fire action potentials presumably by interacting with concomitant excitatory synaptic drive. The present results advocate that the descending LC excitatory drive has engaged in the action potential initiation process of the alpha motoneuron, facilitating its reaching the firing threshold during concurrent depressed membrane excitability as well as subthreshold converging inputs.

Action Potentials↗
PubMed1985

Aging of motoneurons and synaptic processes in the cat.

The aging of spinal cord alpha motoneurons was explored in old cats with intracellular recording techniques to determine the basic membrane properties of these neurons and their monosynaptic response following activation of group Ia afferent fibers. The conduction velocity of the motoneurons' axons decreased in old animals (14 to 15 years of age) compared with adult controls (1 to 3 years of age). The input resistance of the motoneurons increased in the old cats; no change occurred in the resting membrane potential or spike amplitude. There was a reduction in the delay between the initial segment and the somadendritic components of the antidromic spike. The half-width duration of the monosynaptic EPSP in the old cats increased, but its amplitude did not change. These data indicate that a host of different membrane properties of spinal cord motoneurons and their Ia-monosynaptic input are affected by the aging process. Analysis of the results suggests that the degradation of neuronal processes occurs in all motoneurons rather than preferentially affecting a specific population of motoneurons.

Action Potentials↗
PubMed1982

Hyperpolarizing membrane responses induced in lumbar motoneurons by stimulation of the nucleus reticularis pontis oralis during active sleep.

Intracellular recordings were obtained from lumbar motoneurons in unanesthetized, undrugged, normally respiring cats during the states of wakefulness, quiet sleep and active sleep. The objective was to examine the state-dependent control of spinal cord motoneurons exerted by the pontomesencephalic reticular formation. Accordingly, electrical stimulation was applied to the nucleus reticularis pontis oralis while the membrane potential of lumbar motoneurons was recorded. Short latency depolarizing and/or hyperpolarizing potentials were observed throughout sleep and wakefulness; no state-dependent pattern was found in the direction of polarization or amplitude for these early potentials. However, a long latency hyperpolarizing potential emerged exclusively during active sleep; it was characterized by a peak latency of 45 +/- 1 (S.E.M.) ms, a duration of 40 +/- 2 ms, and an amplitude of 3 +/- 0.5 mV. This active sleep-selective potential was capable of inhibiting spontaneous motoneuron activity. These and previously obtained data support the notion that excitation of the nucleus reticularis pontis oralis results in somatomotor inhibition at the level of the spinal cord and brainstem selectively during the state of active sleep.

Animals↗
PubMed1981

Evidence of facilitatory coerulospinal action in lumbar motoneurons of cats.

Functional connectivity of the feline coerulospinal projection was delineated by utilizing the combined approaches of antidromic activation and electrical stimulation. We isolated 25 locus coeruleus (LC) neurons that were electrophysiologically identified and histologically verified and that could be driven by stimulating the spinal cord. Antidromicity of the spike potentials was confirmed by the constant latency, the high frequency (100 Hz) following, fractionation of the initial segment-somatodendritic potential, and collision between the antidromic and the spontaneous orthodromic spikes. The mean conduction speed was 20 +/- 8 m/sec (range = 7 to 32 m/sec). Intracellular studies revealed facilitatory LC actions in 22 lumbar motoneurons (MNs), In 13 MNs, LC activation alone produced slow-rising excitatory postsynaptic potentials (EPSPs) of 3 +/- 12 mV amplitude that lasted 4-30 msec. Six of the 13 MNs discharged action potentials upon LC stimulation. In the remaining 9 MNs, no observable potential change was registered after LC activation. Antecedent LC stimulation consistently potentiated the synaptic efficacy of testing dorsal root shocks. The enhancement of synaptic activation was antagonized by systemic injection of phenoxy-benzamine (3 mg/kg). These results suggest that facilitation of MNs by the LC is at least in part mediated by distal dendritic depolarization. Those MNs that exhibited augmented excitability but no demonstrable EPSPs may have been activated by norepinephrine-mediated synaptic modulation.

Afferent Pathways↗
PubMed1980

Sialic acid: regulation of electrogenesis in cultured heart cells.

Removal of up to 50% of the sialic acid from aggregates of 7-day chick embryo heart cells during incubation in a highly purified preparation of neuraminidase resulted in hyperpolarization of the maximum diastolic potential, reduction in the slope of diastolic depolarization leading to slowing of beating, negative shifts of threshold potential and the voltage at which upstroke velocity was maximal, and an initial increase in the action potential overshoot. These effects were opposite to those induced by phospholipase C, a possible contaminant. The modification of electrical properties by neuraminidase is suggested to result from an enhanced influx of calcium ions that might "screen" or bind specifically to internal negative fixed charges and thereby shift the voltage dependence of conductance and kinetic parameters to more negative potentials. This hypothesis is supported by the results above and by greater uptake of 45Ca following release of sialic acid, augmentation of enzyme-induced changes in elevated Ca2+, and the similarity of effects produced by A23187, and inophore which also increased 45Ca uptake. However, other mechanisms cannot be ruled out at the present time.

Action Potentials↗
PubMedDate not supplied

Coerulospinal cells containing neuropeptide Y in the cat.

Spinally projecting neuropeptide Y (NPY)-immunoreactive cells were sought in the feline locus coeruleus (LC) nuclear complex after horseradish peroxidase (HRP) injection into the lumbar cord; HRP injection was followed by intracerebroventricular colchicine administration. Our results revealed that a significant number (approximately 20% of all descending cells from the LC complex) of spinally projecting NPY-immunoreactive neurons arise from the LC alpha, the subcoeruleus and the Kölliker-Fuse nuclei. Other nonspinally projecting NPY-containing cells were also evident in the laterodorsal tegmental nucleus and the LCd, in addition to those occurring in the aforementioned LC nuclear complex.

Animals↗
PubMedDate not supplied

Immunohistochemical evidence for coexistence of methionine-enkephalin and tyrosine hydroxylase in neurons of the locus coeruleus complex projecting to the spinal cord of the cat.

Previous studies have revealed the presence of pontospinal neurons with either methionine-enkephalin- or tyrosine hydroxylase-like immunoreactivity in the dorsolateral pontine tegmentum of the cat. Using a combined fast blue retrograde transport technique and simultaneous immunofluorescence histochemistry, the present study was designed to reveal the coexistence of enkephalin and tyrosine hydroxylase in cat coerulospinal neurons and to determine if and to what extent the coerulospinal pathway is heterogeneous. Fast blue-labelled neurons with tyrosine hydroxylase- and enkephalin-like immunoreactivities were found in the nucleus locus coeruleus, nucleus subcoeruleus, Kölliker-Fuse nucleus, and the medial and lateral parabrachial nuclei. Approximately 87% of tyrosine hydroxylase-like immunoreactive neurons had enkephalin-like immunoreactivity, whereas about 76% of the enkephalin-like immunoreactive neurons had tyrosine hydroxylase-like immunoreactivity. About 71% of all coerulospinal neurons exhibited both tyrosine hydroxylase- and enkephalin-like immunoreactivities. These findings indicate that coerulospinal activity may lead to spinal cord effects reflecting both norepinephrine and enkephalin activity in most cases but do not rule out each transmitter's isolated functions.

Amidines↗