Carotid sinus nerve terminals which are tyrosine hydroxylase immunoreactive are found in the commissural nucleus of the tractus solitarius

Summary Tyrosine hydroxylase immunoreactive sensory neurons in the petrosal ganglion selectively innervate the carotid body via the carotid sinus nerve. Central projections of the carotid sinus nerve were traced with horseradish peroxidase. The commissural nucleus of the tractus solitarius was examined by dual labelling light and electron microscopy. Dense bilateral labelling with horseradish peroxidase was found in the tractus solitarius and commissural nucleus of the tractus solitarius. Horseradish peroxidase was found in unmyelinated axons, myelinated axons, and nerve terminals. About 88% of horseradish peroxidaselabelled carotid sinus nerve axons were unmyelinated. Tyrosine hydroxylase immunoreactivity was identified in unmyelinated axons, myelinated axons, dendrites, perikarya, and nerve terminals. Most tyrosine hydroxylase immunoreactive axons (93%) in the commissural nucleus of the tractus solitarius were unmyelinated. Tyrosine hydroxylase immunoreactivity was simultaneously identified in carotid sinus nerve unmyelinated axons, myelinated axons, and nerve terminals. These double-labelled terminals comprised 28% of the number of tyrosine hydroxylase immunoreactive terminals in the commissural nucleus of the tractus solitarius, and 55% of transganglionically-labelled terminals. Therefore, there are both central and peripheral sources of tyrosine hydroxylase immunoreactive nerve terminals in the commissural nucleus of the tractus solitarius. These data support the hypothesis that peripheral tyrosine hydroxylase immunoreactive neurons are involved in the origination of the chemoreceptor reflex. Axo-axonic synapses between peripheral carotid sinus nerve afferent terminals and central terminals containing tyrosine hydroxylase immunoreactivity were observed in 22% of the axo-axonic synapses observed. Thus, central tyrosine hydroxylase immunoreactivity neurons are involved in the modulation of the chemo- and/or baroreceptor reflexes. Synaptic contacts were not observed between carotid sinus nerve afferents and tyrosine hydroxylase immunoreactive perikarya of dendrites. Catecholaminergic neurons are thus unlikely to be the second order neurons of either the chemo- or baroreceptor reflex in the commissural nucleus of the tractus solitarius.     

N-methyl-D-aspartate receptors are present in vagal afferents and their dendritic targets in the nucleus tractus solitarius.

N-Methyl-D-aspartate receptors are present in the nodose ganglion, which contains the cell bodies of vagal afferents, and in the nucleus tractus solitarius, where these afferent fibers terminate. This suggests that N-methyl-D-aspartate receptors are located presynaptically on visceral vagal afferents and/or their target neurons in the nucleus tractus solitarius. To test this hypothesis, we combined anterograde transport of biotinylated dextran amine, following injections into the left nodose ganglion, with electron microscopic immunogold labeling of antipeptide antiserum against the R1 subunit of the N-methyl-D-aspartate receptor in the nucleus tractus solitarius of rat brain. Within the medial nucleus tractus solitarius, the N-methyl-D-aspartate receptor R1 immunoreactivity was seen in dendrites (39% of 639 profiles), axons and axon terminals (41%), and a few neuronal perikarya and glia. Many vagal afferent axons and terminals (40% of 468 profiles) contained N-methyl-D-aspartate receptor R1 immunogold labeling. In addition, 42% of the dendrites contacted by vagal afferent terminals (n = 206) contained N-methyl-D-aspartate receptor R1 immunoreactivity. In axons and dendrites, the gold particles were occasionally seen within asymmetric postsynaptic junctions or at non-synaptic sites on the plasma membrane. More commonly, however, N-methyl-D-aspartate receptor R1 labeling was seen on membranes of vesicular cytoplasmic organelles, suggesting that there is abundant N-methyl-D-aspartate receptor protein available for activity-dependent mobilization to the plasmalemma. Since many vagal afferents are glutamatergic, our results implicate N-methyl-D-aspartate receptors in autoregulation of the presynaptic release and postsynaptic responses to glutamate at the level of the first central synapse in the nucleus tractus solitarius.     

Synaptic connections of central carotid sinus afferents in the nucleus of the tractus solitarius of the rat. II. Connections with substance P-immunoreactive neurons

Summary A combined transganglionic transport and immunocytochemical technique was used to study the synaptic morphology of central carotid sinus afferents and substance P-immunoreactive neurons in the commissural subnucleus of the nucleus of the tractus solitarius in rats. A large population of substance P-immunoreactive neurons (88.32%) were seen in close association with central carotid sinus afferents by light microscopy. However, many labelled central carotid sinus afferents appeared not associated with substance P-immunoreactive neurons in the nucleus of the tractus solitarius. Substance P-immunoreactive neurons were spindle, pear, or oval-shaped with a short axis ranging from 5 to 11 m. Their long axis was oriented predominantly in a lateral-medial direction along the path of the central carotid sinus afferents from the solitary tract to the midline. Synaptic contacts between central carotid sinus afferents and substance P-structures, including dendritic profiles of different calibers and somas, were readily found by electron microscopy. Many central carotid sinus afferents were also found in synaptic contact with non-immunoreactive dendrites and somas. Appositions between central carotid sinus afferents and unlabelled axon terminals were common, but only in a few cases were morphological manifestations of synapses revealed. In the latter, the substance P-immunoreactive terminals appeared mostly presynaptic but postsynaptic ones were also encountered. Our data provide the evidence that some of the substance P-immunoreactive cells in the nucleus of the tractus solitarius are 2nd order neurons of the carotid sinus afferent pathway. The possibility that some of the substance P-immunoreactive neurons in the nucleus of the tractus solitarius may be interneurons and mediate carotid sinus afferent inputs to catecholaminergic neurons in the nucleus of the tractus solitarius is considered. Our findings also provide an anatomical substrate for a possible presynaptic modulatory role of central carotid sinus afferents on the inputs from other brain centers to the substance P-neurons in the nucleus of the tractus solitarius.     

Role of the ventrolateral region of the nucleus of the tractus solitarius in processing respiratory afferent input from vagus and superior laryngeal nerves

Summary The role of respiratory neurons located within and adjacent to the region of the ventrolateral nucleus of the tractus solitarius (vlNTS) in processing respiratory related afferent input from the vagus and superior laryngeal nerves was examined. Responses in phrenic neural discharge to electrical stimulation of the cervical vagus or superior laryngeal nerve afferents were determined before and after lesioning the vlNTS region. Studies were conducted on anesthetized, vagotomized, paralyzed and artificially ventilated cats. Arrays of 2 to 4 tungsten microelectrodes were used to record neuronal activity and for lesioning. Constant current lesions were made in the vlNTS region where respiratory neuronal discharges were recorded. The region of the vlNTS was probed with the microelectrodes and lesions made until no further respiratory related neuronal discharge could be recorded. The size and placement of lesions was determined in subsequent microscopic examination of 50 m thick sections. Prior to making lesions, electrical stimulation of the superior laryngeal nerve (4–100 A, 10 Hz, 0.1 ms pulse duration) elicited a short latency increase in discharge of phrenic motoneurons, primarily contralateral to the stimulated nerve. This was followed by a bilateral decrease in phrenic nerve discharge and, at higher currents, a longer latency increase in discharge. Stimulation of the vagus nerve at intensities chosen to selectively activate pulmonary stretch receptor afferent fibers produced a stimulus (current) dependent shortening of inspiratory duration. Responses were compared between measurements made immediately before and immediately after each lesion so that changes in response efficacy due to lesions per se could be distinguished from other factors, such as slight changes in the level of anesthesia over the several hours necessary in some cases to complete the lesions. Neither uni- nor bi-lateral lesions altered the efficacy with which stimulation of the vagus nerve shortened inspiratory duration. The short latency excitation of the phrenic motoneurons due to stimulation of the superior laryngeal nerve was severely attenuated by unilateral lesions of the vlNTS region ipsilateral to the stimulated nerve. Neither the bilateral inhibition nor the longer latency excitation due to superior laryngeal nerve stimulation was reduced by uni- or bi-lateral lesions of the vlNTS region. These results demonstrate that extensive destruction of the region of the vlNTS: a) does not markedly affect the inspiratory terminating reflex associated with electrical stimulation of the vagus nerve in a current range selective for activation of pulmonary stretch receptor afferents, and b) abolishes the short-latency increase, but not the bilateral decrease or longer latency increase in phrenic motoneuronal discharge which follows stimulation of the superior laryngeal nerve. We conclude that respiratory neurons in the region of the vlNTS do not play an obligatory role in the respiratory phase transitions in this experimental preparation. Neurons in the vlNTS region may participate in other reflexes, such as the generation of augmented phrenic motoneuronal discharge in response to activation of certain superior laryngeal or vagus nerve afferents.     

Barosensory cells in the nucleus tractus solitarius receive convergent input from group III muscle afferents and central command

Some neural mechanism must prevent the full expression of the baroreceptor reflex during static exercise because arterial blood pressure increases even though the baroreceptors are functioning. Two likely candidates are central command and input from the thin fiber muscle afferents evoking the exercise pressor reflex. Recently, activation of the mesencephalic locomotor region, an anatomical locus for central command, was found to inhibit the discharge of nucleus tractus solitarius cells that were stimulated by arterial baroreceptors in decerebrated cats. In contrast, the effect of thin fiber muscle afferent input on the discharge of nucleus tractus solitarius cells stimulated by baroreceptors is not known. Consequently in decerebrated unanesthetized cats, we examined the responses of barosensory nucleus tractus solitarius cells to stimulation of thin fiber muscle afferents and to stimulation of the mesencephalic locomotor region, a maneuver which evoked fictive locomotion. We found that electrical stimulation of either the mesencephalic locomotor region or the gastrocnemius nerve at current intensities that recruited group III afferents inhibited the discharge of nucleus tractus solitarius cells receiving baroreceptor input. We also found that the inhibitory effects of both gastrocnemius nerve stimulation and mesencephalic locomotor region stimulation converged onto the same barosensory nucleus tractus solitarius cells. We conclude that the nucleus tractus solitarius is probably the site whereby input from both central command and thin fiber muscle afferents function to reset the baroreceptor reflex during exercise.     

Converging and diverging cholinergic inputs from submucosal neurons amplify activity of secretomotor neurons in guinea-pig ileal submucosa.

The organization of synaptic connections between guinea-pig ileal submucosal neurons was examined using intracellular recordings from single or pairs of submucosal neurons. Synaptic inputs were elicited by stimulating cholinergic neurons using pressure-pulse application of 5-hydroxytryptamine (5-HT) in ganglia adjacent to those where intracellular recordings were obtained. In addition, when pairs of intracellular recordings were obtained, one neuron was activated by intracellular stimulation and synaptic responses were recorded in the other neuron. Neurobiotin-filled microelectrodes were employed to characterize cells electrophysiologically and immunohistochemically. Recordings were obtained from 176 (173 S-type and three AH-type) neurons; 81% of cells were classified as vasoactive intestinal peptide (VIP) neurons. No fast excitatory postsynaptic potentials and only rare slow excitatory postsynaptic potentials were recorded following intracellular stimulation of paired S-type neurons. However, when paired intracellular recordings were obtained from neurons within the same ganglion and 5-HT was applied to an adjacent ganglion, this stimulation evoked synchronized fast excitatory postsynaptic potentials in 94% of pairs. In contrast, when cell bodies of VIP-VIP pairs were located in different ganglia, fast synaptic activation evoked by 5-HT stimulation was not synchronized in 87% of pairs. When intracellular recordings were obtained from a single neuron and two separate ganglia were stimulated by 5-HT pressure-pulse activation, fast excitatory postsynaptic potentials originating from both sources were recorded in the same VIP neuron. Morphological study of 34 S-type and three AH-type horseradish peroxidase-labeled neurons was conducted. AH-type neurons had multiple axonal branches with dense arborization of collaterals containing numerous varicosities in three to nine ganglia, whereas axons of S-type neurons exhibited relatively rare collaterals and varicosities within adjacent ganglia.These results demonstrate that cholinergic neurons provide both diverging and converging inputs to VIP neurons, providing a mechanism to enhance activation of VIP secretomotor neurons. The axonal projections of AH-type neurons suggest they are likely candidates to provide diverging inputs to multiple VIP neurons.     

Afferent and efferent connections of brainstem locomotor regions: study by means of horseradish peroxidase transport technique

Afferent and efferent connections of the hypothalamic and mesencephalic locomotor regions and also the bulbar locomotor strip were studied in cat using retrograde (horseradish peroxidase) transport technique. To study the sources of afferent projections, the enzyme microinjections were performed exactly into the same brain sites eliciting treadmill locomotion by means of electrical stimulation. When studying efferent projections horseradish peroxidase labeled neurons were revealed within locomotor regions after enzyme microinjections into different brain structures. Experimental data have shown that the hypothalamic and mesencephalic locomotor regions have mutual afferent and efferent projections with numerous brain areas including interconnections. Apart from the entopeduncular nucleus, the great number of different sensory nuclei are noted: among the sources of afferent projections are the nucleus tractus spinalis nervi trigemini, nucleus cuneatus, nucleus tractus solitarius and vestibular nuclei. In addition, after horseradish peroxidase injection into the mesencephalic locomotor region labeled neurons were found in the cochlear nuclei. Direct descending neuronal projections of the hypothalamic and mesencephalic locomotor regions are distributed mainly in the ipsilateral brainstem. Only a few of them reach the lumbar spinal cord. The most marked efferent projections of given regions are those to the brainstem reticular formation. After horseradish peroxidase injection into a functionally identified bulbar locomotor strip, labeled neurons were revealed in different stem regions mainly caudal to the enzyme injection site. The existence of a locomotor strip as an independent structural formation is called into question. When studying the locomotor region connections, the structural heterogeneity of these regions is revealed. Transitory fibers of ascending tracts are presumably within their limits side by side with neurons. The role of these fibers in stepping initiation by electrical stimulation of locomotor regions remains uncertain.     

Light and electron microscopy of contacts between primary afferent fibres and neurones with axons ascending the dorsal columns of the feline spinal cord

In addition to primary afferent fibres, the dorsal columns of the cat spinal cord contain ascending second-order axons which project to the dorsal column nuclei. The aim of the present study was to obtain morphological evidence that certain primary afferent axons form monosynaptic contacts with cells of origin of this postsynaptic dorsal column pathway. In ten adult cats, neurones with axons ascending the dorsal columns were retrogradely labelled with horseradish peroxidase using a pellet implantation method in the thoracic dorsal columns. In the lumbosacral regions of the same animals, primary afferent fibres were labelled intra-axonally with ionophoretic application of horseradish peroxidase. Tissue containing labelled axons was prepared for light and combined light and electron microscopy. Ultrastructural examination demonstrated that slowly adapting (Type I), hair follicle, Pacinian corpuscle and group Ia muscle spindle afferents formed monosynaptic contacts with labelled cells and light microscopical analysis suggested that they also received monosynaptic input from rapidly adapting (Krause) afferents. This evidence suggests that sensory information from large-diameter cutaneous and muscle spindle afferent fibres is conveyed disynaptically via the postsynaptic dorsal column pathway to the dorsal column nuclei. Some of the input to this pathway is probably modified in the spinal cord as the majority of primary afferent boutons forming monosynaptic contacts were postsynaptic to other axon terminals. The postsynaptic dorsal column system appears to constitute a major somatosensory pathway in the cat.     

Intracellular analysis of respiratory-modulated hypoglossal motoneurons in the cat

Intracellular recordings from hypoglossal motoneurons in the brainstem of cats are described, along with postsynaptic potentials evoked by superior laryngeal, vagal and carotid sinus nerve stimulation. The study concentrates on hypoglossal motoneurons with respiratory-related discharge, which can be categorized into inspiratory, inspiratory/early-expiratory and expiratory patterns. Seven cells were labelled with horseradish peroxidase, their location and morphology are described. Stimulation of laryngeal receptors by balloon inflation or by water injection into the larynx, or mimicked by electrical stimulation of the superior laryngeal nerve results in enhanced postinspiratory activity in those cells (inspiratory/early-expiratory, expiratory) already receiving postinspiratory excitation; or actually produces a wave of postinspiratory depolarization in cells (inspiratory) previously quiescent during that period. It is concluded that the firing pattern of the respiratory-modulated hypoglossal motoneurons is unlikely to be static but depends on other factors, one of these being the level of ongoing, or previous laryngeal receptor stimulation.     

Inputs from regularly and irregularly discharging vestibular nerve afferents to secondary neurons in the vestibular nuclei of the squirrel monkey. II. Correlation with output pathways of secondary neurons

1. Intracellular recordings were made from secondary neurons in the vestibular nuclei of barbiturate-anesthetized squirrel monkeys. Monosynaptic excitatory postsynaptic potentials (EPSPs) evoked by stimulation of the ipsilateral vestibular nerve (Vi) were measured. An electrophysiological paradigm, described in the preceding paper (26), was used to determine the proportion of irregularly (I) and regularly (R) discharging Vi afferents making direct connections with individual secondary neurons. The results were expressed as a % I index, an estimate for each neuron of the percentage of the total Vi monosynaptic input that was derived from I afferents. The secondary neurons were also classified as I, R, or M cells, depending on whether they received their direct Vi inputs predominantly from I or R afferents or else from a mixture (M) of both kinds of Vi fibers. The neurons were located in the superior vestibular nucleus (SVN) or in the rostral parts of the medical or lateral (LVN) vestibular nuclei. 2. Antidromic activation or reconstruction of axonal trajectories after intrasomatic injection of horseradish peroxidase (HRP) was used to identify three classes of secondary neurons in terms of their output pathways: 1) cerebellar-projecting (Fl) cells innervating the flocculus (n = 26); 2) rostrally projecting (Oc) cells whose axons ascended toward the oculomotor (IIIrd) nucleus (n = 27); and 3) caudally projecting (Sp) cells with axons descending toward the spinal cord (n = 13). Two additional neurons, out of 21 tested, could be antidromically activated both from the level of the IIIrd nucleus and from the spinal cord. 3. The Vi inputs to the various classes of relay neurons differed. As a class, Oc neurons received the most regular inputs. Sp neurons had more irregular inputs. Fl neurons were heterogeneous with similar numbers of R, M, and I neurons. The mean values (+/- SD) of the % I index for the Oc, Fl, and Sp neurons were 34.7 +/- 24.7, 51.9 +/- 30.4, and 61.8 +/- 18.0%, respectively. Only the Oc neurons had a % I index that was similar to the proportion of I afferents (34%) in the vestibular nerve (cf. Ref. 26). 4. The commissural inputs from the contralateral vestibular nerve (Vc) also differed for the three projection classes. Commissural inhibition was most common in Fl cells: 22/25 (88%) of the neurons had Vc inhibitory postsynaptic potentials (IPSPs) and 1/25 (4%) had a Vc EPSP. In contrast, Vc inputs were only observed in approximately half the Oc and Sp neurons.(ABSTRACT TRUNCATED AT 400 WORDS)     

Monosynaptic circuitry of trigeminal proprioceptive afferents coordinating jaw movement with visceral and laryngeal activities in rats

Jaw movement is intimately related to various oromotor and visceral functions such as feeding, swallowing, vocalization, respiration and cardiac function. Neuronal circuitry that links jaw movement and these visceral and oromotor functions is largely unknown. The purpose of this study is to determine whether the trigeminal proprioceptive and jaw-muscle spindle afferents send axons to and synapse with the motoneurons innervating visceral organs and laryngeal muscles utilizing multiple double-labeling and physiological approaches. Double labeling of anterograde tracing combined to retrograde transport was performed by injection of biotinylated dextran amine into the mesencephalic trigeminal nucleus and cholera toxin B subunit or horseradish peroxidase into the vagus nerve and the recurrent laryngeal nerve. Mesencephalic trigeminal nucleus neuronal terminals contacted with visceral and laryngeal muscle motoneurons in the ambiguus nucleus and the nearby intermediate reticular zone. By electron microscopic observation, we confirmed that mesencephalic trigeminal nucleus terminals made asymmetric axodendritic synapses with these motoneurons. Double labeling of physiologically characterized jaw muscle spindle afferent neurons combined with anti-choline acetyltransferase immunohistochemistry showed that jaw-muscle spindle afferent boutons closely contacted choline acetyltransferase-immunoreactive motoneurons in the ambiguus nucleus and intermediate reticular zone. This report is the first to demonstrate that the trigeminal proprioceptive afferents synapsed upon visceral and laryngeal muscle motoneurons and provide neuronal networks for the jaw-visceral and jaw-laryngeal coordination.     

Electrophysiological and morphological characterization of cytochemically-defined neurons in the caudal nucleus of tractus solitarius of the rat.

Morphological and electrophysiological properties of calbindin D-28k-, GABA- and dopamine-beta-hydroxylase-immunopositive neurons were investigated in the caudal nucleus of tractus solitarius of rats, using a patch-clamp whole-cell recording combined with intracellular staining and immunocytochemistry. Calbindin D-28K- and GABA-positive neurons had a small cell body (10.9+/-0.3 microm in diameter) and were distributed throughout the caudal nucleus of tractus solitarius. Double fluorescence immunocytochemistry revealed that calbindin- and GABA-positive neurons formed distinct subpopulations. Calbindin- and GABA-positive neurons double stained for biocytin showed extensive axon collaterals within the nucleus of tractus solitarius and some calbindin-positive, but not GABA-positive neurons, had also projection axons leaving the nucleus of tractus solitarius. Dopamine-beta-hydroxylase-immunopositive neurons had a small (10.8+/-0.3 microm) or large (17.2+/-0.4 microm) cell body. Neurons with a small cell body were observed in the dorsomedial nucleus at the level of the area postrema, and in the area postrema, while neurons with a large cell body were observed in the medial nucleus throughout the caudal nucleus of tractus solitarius. Double fluorescence immunocytochemistry revealed that almost all small dopamine-beta-hydroxylase-positive neurons were also immunoreactive for calbindin, while large dopamine-beta-hydroxylase-positive neurons were not. Double staining for dopamine-beta-hydroxylase and biocytin showed that neurons with a small cell body had moderate axon collaterals. On the contrary, neurons with a large cell body had few, if any, axon collaterals and a projection axon which could leave the nucleus of tractus solitarius. Following stimulation of the tractus solitarius, all neurons with a small cell body exhibited a polysynaptic excitatory response (type I neurons), while dopamine-beta-hydroxylase-immunopositive neurons with a large cell body exhibited a monosynaptic excitatory response (type II neurons) or an excitatory followed by an inhibitory response (type III neurons). Spontaneous and evoked excitatory postsynaptic currents of (type I neurons) calbindin- or GABA-positive neurons were reversibly blocked by 6-cyano-7-nitroquinoxaline-2,3-dione. Spontaneous and evoked inhibitory postsynaptic currents of type III neurons were reversibly blocked by bicuculline. Type II neurons showed no spontaneous excitatory nor inhibitory postsynaptic currents. It was concluded that the three kinds of chemically-defined neurons formed distinct neuronal subpopulations in the caudal nucleus of tractus solitarius in terms of synaptic responses and morphological characteristics such as cell size and axonal trajectory.     

Fine structure of synapses associated with characterized postsynaptic dorsal column neurons in the cat

Fourteen dorsal horn neurons with axons projecting through the dorsal columns were identified either by electrophysiological methods (and subsequently injected with horseradish peroxidase) or by retrograde labelling with horseradish peroxidase in cats. All neurons were contacted by small (less than 2 micron) boutons containing spherical or elongated agranular vesicles. One neuron with its soma located in lamina III received additional contacts from central elements of glomerular complexes. Neurons with somata located more ventrally (deep lamina IV and V) were also postsynaptic to large (greater than 2 microns) electron lucent profiles which formed multiple synapses with the labelled cells. Some boutons presynaptic to postsynaptic dorsal column neurons were themselves postsynaptic to profiles containing pleiomorphic agranular vesicles at axoaxonic synapses. They also occasionally participated in triadic complexes. It is concluded that the synaptic arrangements formed by boutons in association with postsynaptic dorsal column neurons differ significantly from those associated with spinocervical neurons. Such differences might provide the anatomical substrate for the observed receptive field characteristics of these neurons.     

Effects of hypothalamic stimulation on activity of dorsomedial medulla neurons that respond to subdiaphragmatic vagal stimulation

1. Effects of hypothalamic stimulation on activity of dorsomedial medulla neurons that responded to subdiaphragmatic vagal stimulation were investigated in urethan-anesthetized rats. 2. Extracellular recordings were made from 231 neurons in the nucleus of the tractus solitarius (NTS) that fired repetitively in response to single-pulse subdiaphragmatic vagal stimulation and from 320 neurons in the dorsal motor nucleus of the vagal nerve (DMV) that responded antidromically to subdiaphragmatic vagal stimulation. The mean latencies of responses to subdiaphragmatic vagal stimulation were 90.3 +/- 17.1 ms (mean +/- SD) for NTS neurons, and 90.8 +/- 11.2 ms for DMV neurons. This indicated that both afferent and efferent subdiaphragmatic vagal fibers were thin and unmyelinated and had a conduction velocity of approximately 1 m/s. 3. In extracellular recordings from 320 DMV neurons, marked inhibition preceded the antidromic response and subdiaphragmatic vagal stimulation evoked orthodromic spikes in only a few neurons. 4. Intracellular recordings from 66 DMV neurons revealed inhibitory postsynaptic potentials (IPSPs) before the antidromic responses. These IPSPs suppressed spontaneous firing and prevented excitatory postsynaptic potentials (EPSPs) from generating action potentials. 5. Stimulation in all hypothalamic loci studied, the ventromedial hypothalamic nucleus (VMH), the lateral hypothalamic area (LHA), and the paraventricular nucleus (PVN), induced responses with similar characteristics of excitation alone or excitation followed by inhibition in most NTS and DMV neurons. 6. No reciprocal effect of VMH and LHA stimulation was observed on NTS and DMV neurons. 7. Intracellular recordings from DMV neurons revealed monosynaptic EPSPs in response to stimulation of the VMH, the LHA, and the PVN. 8. PVN stimulation evoked significantly more responses in NTS and DMV neurons than VMH stimulation and more responses in DMV neurons than LHA stimulation. This suggests a difference in the number of connections between each hypothalamic site and the dorsomedial medulla. 9. The same dorsomedial medulla neurons were tested with VMH and LHA stimulation. The respective mean latencies of the antidromic and the orthodromic NTS neuron responses were 37.3 +/- 3.2 and 39.6 +/- 12.9 ms for VMH stimulation and 29.8 +/- 5.3 and 31.8 +/- 8.7 ms for LHA stimulation. The mean latencies of the orthodromic DMV neuron responses were 39.4 +/- 8.3 ms for VMH stimulation and 31.1 +/- 5.2 ms for LHA stimulation. The estimated conduction velocity from the VMH to the dorsomedial medulla was approximately 0.25 m/s and from the LHA it was approximately 0.33 m/s, which was significantly faster.(ABSTRACT TRUNCATED AT 400 WORDS)     

Action of the efferent vestibular system on primary afferents in the toadfish, Opsanus tau

Spinalized toadfish were held in a lucite chamber and perfused through the mouth with running seawater. Primary vestibular afferents and vestibular efferent axons and somas were studied with glass microelectrodes. Vestibular semicircular canal afferent and efferent axons were visually identified and penetrated with glass microelectrodes. Afferents responded to pulses of injected current with trains of action potentials, whereas efferents responded with only a single spike. This differential response to injected current served to further distinguish these two classes of nerve fibers that share the same canal nerve for part of their course. When current pulses were injected into efferent somadendritic recording sites, cells responded with trains of action potentials similar to those seen in other central nervous system neurons. Semicircular canal afferents were spontaneously active and occupied the same spectrum of regularity as vestibular afferents recorded in other species. Behavioral arousal evoked by lightly touching the fish on the snout or over the eye resembled spontaneous arousal observed in the field and consisted of eye withdrawal, fin erection, and attempted swimming. Efferent vestibular neurons were spontaneously active and increased their frequency of discharge when the fish was behaviorally aroused. Most efferents were briskly activated by behavioral arousal, but the time constant of the decay of their responses was variable ranging from 100 to 600 ms. Not only touch, but multimodal stimuli were capable of increasing the level of spontaneous activity of efferent vestibular neurons. The shortest latency to behavioral activation was 160 ms. Vestibular primary afferents also manifested increase in neuronal activity with behavioral activation. Irregularly discharging afferents were much more responsive than regularly discharging afferents. One rare case of transient inhibition in a regularly discharging afferent is illustrated. Severing the efferent vestibular nerve blocked behavioral activation in vestibular primary afferents. Electrical stimulation of the efferent vestibular nerve produced excitatory postsynaptic potentials (EPSPs) at latencies within the monosynaptic range in vestibular primary afferents. These monosynaptic EPSPs could produce action potentials in primary afferents or could sum with subthreshold depolarizations produced by current passed through the microelectrode to initiate impulses.(ABSTRACT TRUNCATED AT 400 WORDS)     

Projection of single pulmonary stretch receptors to solitary tract region

1. Central projections of single slowly adapting pulmonary stretch receptors were mapped in the medulla by the technique of spike-triggered averaging of extracellular field potentials. Discharge of pulmonary stretch receptors was recorded in continuity from the nodose ganglion; this activity provided the trigger for an averaging computer. 2. These pulmonary stretch receptors were characterized by a linear increase in firing rate in response to increases in transpulmonary pressure, an adaptation index, and peripheral axonal and intramedullary conduction velocities. 3. In accordance with the terminology used by Munson and Sypert (21), three types of electrical potentials were observed for the projection of a pulmonary stretch receptor in the medulla. Axonal potentials were recorded when the brain stem electrode was in the vicinity of the afferent axon. Terminal potentials were recorded when the electrode was adjacent to terminations of the afferent axon. Focal synaptic potentials were recorded when the electrode was near postsynaptic units receiving input from the pulmonary stretch receptor. Maxima of terminal potentials were recorded in a region 1 mm rostral to the obex in the medial nucleus of the tractus solitarius (six cases), in the ventrolateral nucleus of the tractus solitarius (three cases), and in an area just dorsolateral to the tractus solitarius (two cases). Focal synaptic potentials for five pulmonary stretch receptors were observed in a region 1 mm rostral to obex. Maxima of these potentials were recorded in the medial nucleus of tractus solitarius (two cases), in the ventrolateral nucleus of tractus solitarius (two cases), and in an area just dorsolateral to the tractus solitarius (one case). 4. Occasionally both terminal and focal synaptic potentials were observed for the same pulmonary afferent. The difference in the latencies of these potentials fell within the range previously reported for monosynaptic connections of muscle spindle Ia and group II afferents for alpha-motoneurons. This suggests that the afferents of pulmonary stretch receptors have monosynaptic connections with neurons in the medial nucleus of the tractus solitarius, in the ventrolateral nucleus of the tractus solitarius, and in an area dorsolateral to the tractus solitarius.     

Codeine presynaptically inhibits the glutamatergic synaptic transmission in the nucleus tractus solitarius of the guinea pig

Although codeine is the most prominent and centrally acting antitussive agent, the precise sites and mode of its action have not been fully understood yet. In the present study, we examined the effects of codeine on synaptic transmission in second-order neurons of the nucleus tractus solitarius (NTS), which is the first central relay site receiving tussigenic afferent fibers, by using whole-cell patch-clamp recordings in guinea-pig brainstem slices. Codeine (0.3-3 mM) significantly decreased the amplitude of excitatory postsynaptic currents (EPSCs) evoked by electrical stimulation of the tractus solitarius in a naloxone-reversible and concentration-dependent manner, but it had no effect on the decay time of evoked EPSCs (eEPSCs). The inhibition of eEPSCs was accompanied by an increased paired-pulse ratio of two consecutive eEPSCs. The inward current induced by application of AMPA remained unchanged after codeine application. A voltage-sensitive K+ channel blocker, 4-aminopyridine (4-AP) attenuated the inhibitory effect of codeine on eEPSCs. These results suggest that codeine inhibits excitatory transmission from the primary afferent fibers to the second-order NTS neurons through the opioid receptors that activate the 4-AP sensitive K+ channels located at presynaptic terminals.     

The distribution and excitability of myelinated aortic nerve afferent terminals

The location and excitability of aortic nerve terminals have been investigated in cats and rabbits using the technique of antidromic activation from within the medulla. Histological reconstructions of the ‘responsive sites’ showed that the primary afferent terminals of the aortic nerve were restricted to caudal regions of the dorsomedial medulla, in the vicinity of, but not always within the nucleus of the tractus solitarius.Antidromic stimulation was used to test whether the excitability of aortic nerve afferent terminals was altered by prior stimulation of sinus nerve afferents. No appreciable presynaptic influences were seen.     

大鼠孤束核内5－羟色胺能轴突终末的突触联系

用顺行演变与免疫组织化学结合的双标技术，电镜下观察大鼠孤束核内５－羟色胺样免疫反应（５－ＨＴ－ＬＩ）轴突终末的突触联系，特别是与迷走神经初级传入（溃变）终末的关系。发现：１．５－ＨＴ－ＬＩ轴突终末和迷走神经初级传入终末终止于同一个树突形成轴－树突触；２．５－ＨＴ－ＬＩ轴突终末与树突形成轴－树突触；３．５－ＨＴ－ＬＩ轴突终末与迷走神经初级传入终末或非标记的轴突终未形成轴一轴相贴（ａｘｏ－ａｘｏｎｉｃ－ｃｏｎｔａｃｔ）。以上结果提示，孤束核内的５－ＨＴ能神经成分可能通过突触后、突触前机制调控经迷走神经传入的信息。