Prolonged augmentation of respiratory discharge in hypoglossal motoneurons following superior laryngeal nerve stimulation

Experiments were conducted to investigate long-lasting effects of brief superior laryngeal nerve (SLN) stimulation on respiratory discharge in the hypoglossal nerve. In paralyzed, decerebrate and artificially ventilated cats, SLN stimulation (Hz, 3-5, s, 3-5 times threshold for inhibition of phrenic nerve discharge) immediately increase hypoglossal activity. Following stimulation, the amplitude of respiratory activity in the hypoglossal nerve was augmented (478 +/- 205%), and slowly decayed to prestimulus levels with a time constant of 106 +/- 16 s. In contrast, phrenic nerve activity was completely inhibited during the SLN stimulation and for several seconds thereafter. After activity resumed, phrenic burst frequency remained depressed (33 +/- 6%). Stimulation of the carotid sinus nerve elicited similar effects on hypoglossal nerve activity. Intracellular recordings from hypoglossal motoneurons indicated that SLN stimulation increased central respiratory drive potentials (CRDPs) following a stimulus train, but had inconsistent effects on resting membrane potential. Intracellular depolarizing current pulses (5-15 nA; 2 s) had no prolonged effects on membrane potential or CRDPs. The possible role of serotonin in prolonged augmentation of hypoglossal activity following SLN stimulation was investigated. Intracellular injection of horseradish peroxidase (HRP) into hypoglossal motoneurons and immunohistochemistry for serotonin revealed some close appositions between serotonin immunoreactive boutons and HRP-labeled neurons, but such appositions were sparse. Pretreatment with methysergide had little effect on prolonged augmentation of hypoglossal discharge following SLN stimulation. These results indicate that: (1) SLN stimulation causes prolonged augmentation of hypoglossal activity probably via increased synaptic inputs to hypoglossal motoneurons; and (2) serotonin is not necessary in the underlying mechanism.

Intracellular recording from respiratory neurones in the perfused 'in situ' rat brain

The study describes an arterially perfused in situ rat brain preparation, which uses an 'open circuit' flow of blood substitute with or without an oxygen carrier (2% perfluorotributylamine). The respiratory motor output was recorded from the phrenic and hypoglossal nerves, and could be maintained for up to 11 h from the start of perfusion (temperature of perfusate: 27-30 degrees C). The preparation allowed stable intracellular recordings from respiratory neurons in the brain stem and cervical spinal cord, and should be suitable for other studies which cannot be performed in standard whole animal models. The advantages of this approach compared with other in vitro or perfused in situ preparations are discussed.

An intracellular study of respiratory neurons in the rostral ventrolateral medulla of the rat and their relationship to catecholamine-containing neurons

Intracellular recording and labelling with Lucifer yellow of respiratory neurons in the rostral ventrolateral medulla were carried out in urethane-anaesthetised rats. A combined immunofluorescence and immunoperoxidase technique enabled an assessment of the tyrosine hydroxylase immunoreactivity, as well as an examination of the morphology of inspiratory and expiratory neurons in this part of the medulla oblongata. The results demonstrate: a) that respiratory neurons in the rostral ventrolateral medulla of the rat are intermingled with catecholamine-containing neurons of the C1 cell group, but are not themselves catecholamine-containing; b) that many non-spinally projecting respiratory neurons have axonal arborisations within the ventrolateral medulla in the same region as the C1 cell group, other respiratory neurons, and neurons reported to have a cardiovascular function; and c) that the dendrites of respiratory neurons in this region radiate throughout the ventrolateral medulla and frequently approach the ventral surface.

GABA-immunoreactive boutons make synapses with inspiratory neurons of the dorsal respiratory group

Intracellular labelling with horseradish peroxidase (HRP) combined with gamma-aminobutyric (GABA) immunocytochemistry was used to assess the GABAergic input to inspiratory bulbospinal neurons of the dorsal respiratory group in the cat. The relationship between GABA-immunoreactive (GABA-IR) boutons and intracellularly labelled neurons was examined at the light microscopic and ultrastructural levels. At the light microscopic level, GABA-IR boutons were frequently found in close apposition to dendrites and cell bodies of labelled neurons. The presence of synapses was confirmed with electron microscopy. In addition, synaptic specializations were observed between immunoreactive boutons and unlabelled terminals which in turn formed synaptic contacts with HRP-labelled dendrites, a finding consistent with presynaptic inhibition. These results demonstrate a direct GABAergic input to a functionally defined population of medullary respiratory neurons, and suggest involvement of this neurotransmitter in the control of these neurons.

Extensive monosynaptic inhibition of ventral respiratory group neurons by augmenting neurons in the Bötzinger complex in the cat

Axonal projections and synaptic connectivity of expiratory Bötzinger neurons with an augmenting firing pattern (Bot-Aug neurons) to neurons in the ipsilateral ventral respiratory group (VRG) were studied in anaesthetized cats. Antidromic mapping revealed extensive axonal arborizations of Bot-Aug neurons (24 of 45) to the rostral or caudal VRG, with some having arbors in both regions. Of 234 pairs of neurons studied with intracellular recording and spike-triggered averaging, monosynaptic inhibitory postsynaptic potentials (IPSPs) were evoked in 49/221 VRG neurons by 38/98 Bot-Aug neurons. The highest incidence of monosynaptic inhibition was found in inspiratory bulbospinal neurons (10 of 23 tested). Evidence was also found for monosynaptic inhibition, by a separate group of Bot-Aug neurons, of expiratory bulbospinal neurons (12/58), while excitatory postsynaptic potentials (EPSPs) were identified in another two of these neurons. In addition, monosynaptic IPSPs were recorded from 13 of 53 identified laryngeal motoneurons, and from 14 of 100 respiratory propriobulbar neurons. Presumptive disynaptic IPSPs were recorded from 11 of the 221 VRG neurons. We conclude that Bot-Aug neurons exert widespread inhibition on all major neuron categories in the ipsilateral VRG, and should be regarded as an important element in shaping the spatiotemporal output pattern of both respiratory motoneurons and premotor neurons.

Serotonin immunoreactive boutons form close appositions with respiratory neurons of the dorsal respiratory group in the cat

The aim of this study was to examine the location of serotonin immunoreactive boutons on both the soma and dendrites of neurons in the dorsal respiratory group by using a combination of intracellular recording and labelling and immunohistochemistry. Inspiratory neurons in the ventrolateral nucleus of the solitary tract (vl-NTS) were intracellularly labelled with horseradish peroxidase (HRP) in anaesthetised adult cats. The morphology of 23 neurons, all antidromically activated from the contralateral C3 spinal segment, was examined. Six neurons displayed pronounced dendritic arborizations outside the vl-NTS, with prominent dorsal and/or medial projections. The dendrites of the remaining neurons were almost entirely confined to the vl-NTS. Intramedullary axon collaterals were detected in four of nineteen examined axons. Serotoninergic fibres were immunohistochemically demonstrated in the NTS, and the apposition of immunoreactive boutons to the HRP-filled neurons examined at the light microscopic level. Boutons were identified in close apposition with the somata, proximal and distal dendrites of these neurons. However, the density of contacts was found to be substantially less than in a previous study of phrenic motoneurons (Lipski et al: Soc. Neurosci. Abst. 14:379, '88; Pilowsky et al: J. Neurosci. in press, '90). The relative paucity of contacts of serotonin immunoreactive boutons with premotor inspiratory neurons of the dorsal respiratory group indicates that the serotoninergic system affects respiratory pathways mainly at the level of respiratory motoneurons or at brainstem sites outside the vl-NTS.

Substrate properties of analogs of myo-inositol

The hydrolysis of the minor cell membrane lipid phosphatidylinositol-4,5-bisphosphate mediates the action of many growth factors and hormones. As an approach to the development of specific inhibitors of this process, we have synthesized a series of analogs of myo-inositol and have evaluated their ability to serve as substrates for phosphatidylinositol synthetase. Modification at the 2-, 3-, or 4-positions produced compounds unable to serve as substrates, but several 5-modified analogs retained activity as substrates of phosphatidylinositol synthetase. The product formed from 5-deoxy-5-fluoro-myo-[3H]inositol by phatidylinositol synthetase was hydrolyzed by phospholipase D and gave 5-deoxy-5-fluoro-myo-inositol as the radiolabeled product. Two analogs, 5-deoxy-myo-inositol and 5-deoxy-5-fluoro-myo-inositol, were shown to permeate L1210 leukemia cells and be incorporated into cellular phospholipid. Analysis of the radiolabeled lipids formed on incubation of L1210 cells with 5-deoxy-5-fluoro-myo-[3H]inositol indicated that the fradulent lipid formed was further phosphorylated to the monophosphate but not to the diphosphate form.

Antidromic mapping of descending axons of respiratory bulbospinal neurons in the nucleus tractus solitarius of the rabbit

Antidromic mapping of the descending axons of the respiratory bulbospinal neurons in the region of the nucleus tractus solitarius (NTS) was performed on rabbits anesthetized with urethane. Among 177 units tested, 29 out of 87 inspiratory (I), 27 out of 84 expiratory (E) and 2 out of 6 phase-spanning units were identified as bulbospinal. A prominent feature of the bulbospinal pathway from the NTS in the rabbit is the abundance of ipsilateral descending axons. The axons rising from one side are situated in the ventrolateral and ventral funiculi of both sides. The axonal conduction velocities are about 25-35 m/s. Both I and E bulbospinal neurons can be divided into R alpha and R beta types according to 'no I inflation' and 'maintained E inflation' tests.

Respiration related neurons in the region of the nucleus tractus solitarius of the rabbit

Systematic mapping of respiration-related units (RRUs) in the region of the nucleus tractus solitarius (NTS) was performed on rabbits anesthetized with urethane. Of 523 RRUs 295 (56%) were inspiratory (I), 130 (25%) expiratory (E) and 98 (19%) phase-spanning (PS). Such a large number of E and PS RRUs in NTS have not been reported in the cat. Six out of 16 I RRUs 6 of 13 E RRUs, and 1 of 3 PS RRUs had projections to contralateral cervical spinal cord. The 'no I inflation' test was conducted on 76 I and 38 E RRUs, of which 29 I RRUs were identified as I alpha and 31 as I beta. Thirteen E RRUs were inhibited and 6 facilitated by preceding lung inflation. Almost all types of respiratory neurons so far reported from different nuclei of cat's brainstem have been located in the NTS region of the rabbit.

The role of the human caudate nucleus in acupuncture analgesia

The role of the caudate nucleus in acupuncture analgesia was studied in 17 patients receiving caudate stimulation through chronic implanted electrodes to relieve intractable pain caused by late malignancy. Electrical stimulation of the head of the caudate nucleus provided relief from intractable pain in all 17 patients. The pain and pain tolerance threshold were elevated, the alteration in skin galvanic activity, respiratory movement and fingertip plethysmography elicited by a given painful stimulus were depressed. The analgesia required a period of induction and persisted for some time after cessation of stimulation; it showed no obvious segmental topography. These characteristics are similar to those observed in acupuncture analgesia. The effect of caudate stimulation was similar to that of electric needling in depressing the late component of the somatosensory evoked potentials (SEPs) recorded over the scalp and from the centrum medianum of the thalamus. Evoked potentials could be recorded from the caudate nucleus when an acupuncture point was electrically stimulated. The evoked potential record was a complex wave of positive and negative components, the peak latencies of main components being 100-180 msec for positive wave and 148-332 msec for negative wave components. The present study supports the supposition that the caudate nucleus may play a role in acupuncture analgesia. The possibility that the effect of caudate stimulation is brought on by inhibiting the activity of the medial thalamus is discussed.