Neural activity with cardiac periodicity in medulla oblongata of cat

'Aortic baroreceptor' neurons in the nucleus tractus solitarius in rats: convergence of cardiovascular inputs as revealed by heartbeat-locked activity

Rat aortic depressor nerve (ADN) contains only baroreceptor afferents. We identified 'aortic baroreceptor' neurons in the nucleus tractus solitarius (NTS) as those responding to electrical stimulation of the ADN and attempted to demonstrate convergence of cardiovascular mechanoreceptor inputs in these 'baroreceptor' neurons. In chloralose-urethane-anesthetized rats, ADN stimulation evoked either short or long latency responses (SLR, LLR) in 193 neurons of the NTS. 28 (SLR, 15; LLR, 13) demonstrated ongoing activities with cardiac rhythm despite the fact that the ADN had been cut peripherally. In 12 (SLR, 5; LLR, 7) of the 28 neurons, heartbeat-locked activity was abolished by carotid occlusion (CO), and augmented by methoxamine-induced blood pressure elevation, indicating that the heartbeat-locked activity originated from carotid sinus baroreceptors (CSB). In 11 neurons (SLR, 6; LLR, 5), the heartbeat-locked activity was not affected by CO but was abolished by topical application of lidocain on the ipsilateral cervical vagus, suggesting that the heartbeat-locked activity originated mostly from cardiac mechanoreceptors. The origin of the heartbeat-locked activity of the remaining 5 neurons could not be determined. The onsets, as well as peaks of the heartbeat-locked activity of vagal origin appeared significantly earlier than those of CSB origin. In conclusion, NTS neurons receive converging projection not only from the two major arterial baroreceptors but also from the arterial baroreceptors and cardiac mechanoreceptors, thereby integrating sensory information of vascular and cardiac origins.

Spinal and trigeminal projections to the nucleus of the solitary tract: a possible substrate for somatovisceral and viscerovisceral reflex activation

This study used the retrograde transport of a protein-gold complex to examine the distribution of spinal cord and trigeminal nucleus caudalis neurons that project to the nucleus of the solitary tract (NST) in the rat. In the spinal grey matter, retrogradely labeled cells were common in the marginal zone (lamina I), in the lateral spinal nucleus of the dorsolateral funiculus, in the reticular part of the neck of the dorsal horn (lamina V), around the central canal (lamina X), and in the region of the thoracic and sacral autonomic cell columns. The pattern of labeling closely resembled that seen for the cells at the origin of the spinomesencephalic tract and shared some features with that of the spinoreticular and spinothalamic tracts. Labeled cells in lamina IV of the dorsal horn were only observed when injections spread dorsally, into the dorsal column nuclei, and are thus not considered to be at the origin of the spinosolitary tract. They are probably neurons of the postsynaptic fibers of the dorsal column. Retrogradely labeled cells were also numerous in the superficial laminae of the trigeminal nucleus caudalis, through its rostrocaudal extent. The pattern of marginal cell labeling appeared to be continuous with that of labeled neurons in the paratrigeminal nucleus, located in the descending tract of trigeminal nerve. Since the NST is an important relay for visceral afferents from both the glossopharyngeal and vagus nerves, we suggest that the spinal and trigeminal neurons that project to the NST may be part of a larger system that integrates somatic and visceral afferent inputs from wide areas of the body. The projections may underlie somatovisceral and/or viscerovisceral reflexes, perhaps with a significant afferent nociceptive component.

Arterial pressure- and cardiac rhythm-related single-neuron activities in the nucleus reticularis gigantocellularis of the rat

This communication presents an analysis of 531 extracellularly recorded spontaneously active nucleus reticularis gigantocellularis (NRGC) single-neurons in pentobarbital anesthetized rats, with particular reference to their cardiovascular-related activities. Fifty-nine percent of the gigantocellular neurons evaluated had temporal relationship with induced or spontaneous arterial pressure fluctuations. Twenty-eight percent of the NRGC neurons manifested definite discharge patterns and periodicity with reference to the cardiac cycle. It is discussed that these gigantocellular neurons may serve as a primary component of the cardiovascular regulatory mechanism. Alternatively, they may coordinate the necessary circulatory support for activities varying from respiration to somatomotor functions or sleep-wakefulness.

Evidence that L-glutamic acid mediates baroreceptor function in the cat

The possible role of L-glutamic acid (L-glu) as a neurotransmitter of baroreceptor afferent neurons was investigated in the cat by monitoring the changes in three indices of baroreceptor function seen with the L-glu antagonists L-glutamic acid diethyl ester (GDEE) and 1-hydroxy-3-amino-pyrrolidone-2-(HA-966). Baroreceptor function was determined from a) the computer summed inhibition of sympathetic nerve discharge (SND) evoked by electrical stimulation of vagal baroreceptor afferent pathways, b) the locking of SND to the cardiac cycle, and c) the sympathoinhibitory response to i.v. pressor doses of phenylephrine. Direct bilateral microinjections of GDEE (20 micrograms) and HA-966 (4 micrograms) into the region of the nucleus tractus solitarii (NTS) resulted in immediate, marked reductions in the SND inhibitory response to vagal stimulation, a loss in SND locking to the cardiac cycle, a shift in the arterial pulse/SND phase relation, and a diminished sympathoinhibitory response to phenylephrine. Control microinjections of isotonic saline (1 mu 1/NTS) were devoid of these effects. The vagal induced sympathoinhibitory response was restored after NTS microinjections of GDEE by increasing the intensity of the vagal stimulus, or by directly stimulating the NTS injection site, suggesting that the impairment in baroreceptor function seen with this L-glu antagonist was independent of mechanical or local anesthetic effects. These data strongly suggest that L-glu may act as a neurotransmitter of baroreceptor afferent neurons in the NTS of the cat.

Problems associated with the identification of brain stem neurons responsible for sympathetic nerve discharge

A study of the role of lower brain stem neurons generating sympathetic nerve discharge and reactions. A discussion of the problem and requirements for its resolution is followed by a description of approaches made and discussion of reported findings as well as new observations. There is a discussion of the possible origin of respiratory linked rhythms in sympathetic nerve discharge. Spike-triggered averaging was used to relate medullary neuron and inferior cardiac sympathetic nerve discharge under varying external and internal circumstance. Data obtained indicate that it is possible to identify and distinguish reticular neurons which govern sympathetic nerve discharge from other functional types.

Action of the morphinometic agent, fentanyl, on the nucleas tractus solitarii and the nucleus ambiguus cardiovascular neurons

Using extracellular recordings, the discharges of blood pressure dependent neurons were detected in the nucleus tractus solitarii (NTS) and in the nucleus ambiguus (NA) of anesthetized dogs. Type A blood pressure dependent neurons (sympathoinhibitory or vagoexcitatory neurons) were detected when their firing rates were increased during i.v. noradrenaline and when their firing rates were reduced or abolished during bilateral carotid occlusion. Type B blood pressure dependent neurons (sympathoexcitatory neurons or vagoinhibitory neurons) were identified when their firing rates were reduced or when firing was abolished during the rise in blood pressure induced by noradrenaline and when their firing rates were increased during bilateral carotid occlusion. Baroreceptor neurons were identified when neuronal activity with cardiac rhythm, was detected in the NTS. Fentanyl (20 micrograms/kg i.v.) decreased the spontaneous firing rate of type A neurons found in the NTS. In contrast, fentanyl altered in two opposite ways the firing rate of neurons classified as type A and found in the NA. In the first group of type A neurons, fentanyl induced a marked increase in firing rates. A second group of type A neurons in the NA responded to fentanyl by a reduction in firing rates. Fentanyl reduced the firing rate of type B neurons detected in the NTS and the NA. Fentanyl (20 micrograms/kg i.v.) induced biphasic changes in the firing rate of baroreceptor neurons detected in the NTS. The effects of fentanyl on blood pressure, heart rate and firing rate of all types of cardiovascular neurons were antagonized by naloxone.

Discharge of neurons in the parabrachial pons related to the cardiac cycle: changes during different sleep-waking states

Neuronal activity in the nucleus parabrachialis medialis (NPBM) was examined in unanesthetized, unrestrained cats during each sleep-waking state. Discharge related to the cardiac cycle was assessed by calculating cross-correlation functions between neuronal and cardiac events (using the peak of the EKG 'R' wave as a reference). Of the 60 NPBM neurons examined, the activity of 20 cells showed discharge relationships to the cardiac cycle. Firing of NPBM neurons in phase with the cardiac cycle was observed during each sleep-waking state, but was most apparent during awake (AW) and rapid eye movement (REM) sleep states. State-related changes in the correlation of NPBM neuronal discharge to the cardiac cycle included variations in the extent of rhythmic modulation of activity, and shifts in the phase relationship to the cardiac reference. The predominant phase relationship of NPBM neuronal activity to the cardiac cycle was an increased probability of discharge during a period ranging from 50 msec preceding to 50 msec following the peak of the EKG 'R' wave.

Neuronal activity with cardiac rhythm in the nucleus of the solitary tract in cats and dogs. II. Activity modulation in relation to the respiratory cycle

In anaesthetized, spontaneously breathing cats and dogs extracellular recordings were made of the spontaneous activities of 28 single neurones with cardiac rhythm. Histological exmaination confirmed that the neurones were situated in the mediodorsal portion of the nucleus of the solitary tract. The neurones, which had already been analysed in respect to their pulse-rhythmical pattern27, were investigated by means of inspiration triggered histograms. The neurones could be grouped according to the respiratory modulations of their activities. Increased activities occurred during (a) the rise of the respiratory blood pressure wave, (b) the respiratory rest period, and (c) the inspiration. Three neurones did not exhibit respiratory modulation. The respiratory modulations of groups a and b were most probably caused by changing inputs from cardiovascular receptors in the course of respiration. It could be shown by adequate stimulation and elimination of the vagus nerves that the neurones of group c were influenced by lung inflation receptors as well as by cardiovascular receptors.

Neuronal activity with cardiac rhythm in the nucleus of the solitary tract in cats and dogs. I. Different discharge patterns related to the cardiac cycle

In anaesthetized, spontaneously breathing cats and dogs, extracellular recordings were made of the spontaneous activities of 28 single neurones with cardiac rhythm. It was confirmed by histological examination that the neurones were situated in the mediodorsal portion of the nucleus of the solitary tract. Different discharge patterns in relation to the heart cycle were demonstrated using R-deflection triggered histograms. Most of the neurones discharged with systolic bursts. In the cat, about half the neurones showed two to three activity peaks in the histograms which could occur at any time of the heart cycle. The height of the various peaks sometimes changed in different ways in the course of the respiratory cycle. It can be concluded that different cardiovascular afferents converge to single neurones. In some neurones it could be shown by elimination of the vagus nerves that the pulse-rhythmical discharge pattern was induced mainly by vagal inputs, mainly by glossopharyngeal inputs or by inputs from both nerves. Two-thirds of the neurones discharging with systolic bursts could be enhanced by increasing the arterial blood pressure. In some of these neurones a proportionality between the number of impulses per heart cycle and the mean arterial blood pressure was found.

Neuronal activity with relation to cardiac rhythm in the lower brain stem of the dog

Extracellular recordings of reticular neurons in the lower brain stem were performed in anesthetized dogs. Thirty out of 131 neurons showed rhythms of about 2-4 c/sec which were similar to heart or to EEG rhythms. By means of post-event-time histograms, correlations to the EEG delta-theta rhythm could be found in some neurons. These results were published elsewhere. Fixed relations of the neuronal activity to the cardiac rhythm could be verified in 7 neurons. Two neurons showed maxima 70 msec and 120 msec, respectively, after the beginning of the pulse wave. Two neurons showed minima after 110 msec and 130 msec, respectively. Three pulse-rhythmical neurons could not be classified in this way. This rhythmicity was changing or vanishing during the course of registration, but it could be strengthened experimentally by blood-pressure increase. It was shown that these neurons receive inputs from cardiovascular afferents and therefore are blood-pressure dependent neurons. From the finding that pulse-rhythmical modulation is different at different times under the same blood-pressure levels, it is assumed that there are varying strong influences from other systems to these neurons. The fact that these neurons are influenced by cardiovascular afferents does not mean, however, that they are cardiovascular neurons exclusively. It is suggested that blood-pressure reticular neurons may belong to the cardiovascular system as well as to other brain stem systems.

Spontaneous and synaptic excitation of paramedian reticular neurones in the decerebrate cat

1. In decerebrate cats neurones in the region of the paramedian reticular nucleus were identified by responses to stimulation of implanted cerebellar electrodes. Approximately one half were antidromically activated and one half orthodromically. 2. Somatic stimulation and electrical stimulation of both hind limb and cranial nerves activated many of these cells. There was no correlation between the effects of these stimuli on cell firing and on blood pressure. 3. A number of rhythms in spontaneous firing were observed. One fifth of cells fired with the rhythm of efferent activity in the recurrent laryngeal nerve. In animals showing cycles of stability and instability in blood pressure corresponding phases of activity and inactivity in the firing of paramedian reticular neurones were observed. 4. Most paramedian reticular cells showed bursts of firing preceding abrupt rises in blood pressure but this was also observed with cells lateral to this area. 5. One third of cells studied showed changes in firing rate correlated with the changes in blood pressure which followed I.V. acetylcholine and noradrenaline. 6. Bilateral carotid artery occlusion, which always produced a rise in blood pressure, had little effect on cell firing.