Philip J. Gatti

Can neurons in the nucleus ambiguus selectively regulate cardiac rate and atrio-ventricular conduction?

Previous anatomic data have described the distribution of presumptive negative chronotropic and negative dromotropic neurons in the ventro-lateral nucleus ambiguus (NA-VL) following injections of retrograde tracers into physiologically selective parasympathetic intracardiac ganglia. Negative dromotropic neurons were preferentially distributed in the rostral NA-VL (rNA-VL). Negative chronotropic neurons were preferentially distributed in the caudal NA-VL (cNA-VL). Significant numbers of both types of cardio-inhibitory neurons were observed to overlap in an intermediate level of the NA-VL (iNA-VL). In the present report, we have examined the effects of microinjections of the excitatory amino-acid glutamate (GLU) into the cNA-VL and iNA-VL on cardiac rate and AV conduction while recording the electrocardiogram in paced and non-paced cat hearts. The data indicate that: (i) excitation of neurons in the cNA-VL causes a 58 +/- 17% reduction in cardiac rate, without influencing AV conduction; and (ii) excitation of neurons in the iNA-VL causes both a reduction in heart rate (68 +/- 12%) and a decrease in the rate of AV conduction (38 +/- 7%). These physiological results support the anatomical inference that neurons in the cNA-VL that are retrogradely labeled from physiologically selective parasympathetic intracardiac ganglia selectively exhibit negative chronotropic properties. Furthermore, the data indicate that there is a longitudinal cardiotopic organization of both negative chronotropic and negative dromotropic neurons in the NA-VL. This CNS organization mirrors the peripheral organization of functionally selective cardiac components of the vagus nerve. Finally, the data are consistent with the hypothesis that anatomically separated and functionally selective parasympathetic preganglionic vagal motoneurons in the NA independently control cardiac rate and AV conduction.

Cardiotopic organization of the nucleus ambiguus ? An anatomical and physiological analysis of neurons regulating atrioventricular conduction

Previous data indicate that there are anatomically segregated and physiologically independent parasympathetic postganglionic vagal motoneurons on the surface of the heart which are capable of selective control of sinoatrial rate, atrioventricular conduction and atrial contractility. We have injected a retrograde tracer into the cardiac ganglion which selectively regulates atrioventricular conduction (the AV ganglion). Medullary tissues were processed for the histochemical detection of retrogradely labeled neurons by light and electron microscopic methods. Negative dromotropic retrogradely labeled cells were found in a long column in the ventrolateral nucleus ambiguus (NA-VL), which enlarged somewhat at the level of the area postrema, but reached its largest size rostral to the area postrema in an area termed the rostral ventrolateral nucleus ambiguus (rNA-VL). Three times as many cells were observed in the left rNA-VL as compared to the right (P < 0.025). Retrogradely labeled cells were also consistantly observed in the dorsal motor nucleus of the vagus (DMV). The DMV contained one third as many cells as the NA-VL. The right DMV contained twice as many cells as the left (P < 0.05). These data are consistent with physiological evidence that suggests that the left vagus nerve is dominant in the regulation of AV conduction, but that the right vagus nerve is also influential. While recording the electrocardiogram in paced and non-paced hearts, L-glutamate (GLU) was microinjected into the rNA-VL. Microinjections of GLU caused a 76% decrease in the rate of atrioventricular (AV) conduction (P < 0.05) and occasional second degree heart block, without changing heart rate. The effects of GLU were abolished by ipsilateral cervical vagotomy. These physiological data therefore support the anatomical inference that CNS neurons that are retrogradely labeled from the AV ganglion selectively exhibit negative dromotropic properties. Retrogradely labeled negative dromotropic neurons displayed a round nucleus with ample cytoplasm, abundant rough endoplasmic reticulum and the presence of distinctive somatic and dendritic spines. These neurons received synapses from afferent terminals containing small pleomorphic vesicles and large dense core vesicles. These terminals made both asymmetric and symmetric contacts with negative dromotropic dendrites and perikarya, respectively. In conclusion, the data presented indicate that there is a cardiotopic organization of ultrastructurally distinctive negative dromotropic neurons in the NA-VL. This central organization of parasympathetic preganglionic vagal motoneurons mirrors the functional organization of cardioinhibitory postganglionic neurons of the peripheral vagus nerve. These data are further discussed in comparison to a recent report on the light microscopic distribution and ultrastructural characteristics of negative chronotropic neurons in the NA-VL42.(ABSTRACT TRUNCATED AT 400 WORDS)

Vagal control of left ventricular contractility is selectively mediated by a cranioventricular intracardiac ganglion in the cat.

Activation of the vagus nerve leads to decreases in sinoatrial (SA) rate, atrioventricular (AV) conduction, and myocardial contractility. Previous data are consistent with the hypothesis that vagal control of cardiac rate and AV conduction are mediated by two anatomically separated and physiologically independent parasympathetic intracardiac ganglia located in fat pads on the surface of the right and left atria, respectively. These data suggested that vagal control of ventricular contractility might be mediated through another intracardiac ganglion. We examined the ventricles of cat hearts histologically for the presence of ganglia. Multiple small basophilic ganglia composed of a few neurons, and an occasional larger ganglion were found embedded in the epicardial fat surrounding the cranial margin of the anterior surface of the left ventricle, near the juncture with the right ventricle, which we refer to as the CV ganglion. In anesthetized cats, right cervical vagal stimulation decreased SA rate by 44 +/- 5%, decreased the rate of AV conduction by 68 +/- 14%, and reduced ventricular contractility by 19.5 +/- 5.7%. Vagally induced negative inotropism was almost completely prevented by microinjection of a ganglionic blocking drug into the CV ganglion. However, these injections into the CV ganglion did not significantly effect vagally induced decreases in either SA rate or AV conduction. We conclude: (1) that ganglia are found in a fat pad on the surface of the left ventricle of the cat heart and (2) that the CV ganglion selectively mediates the negative inotropic effect of vagal stimulation on the left ventricle. Greater understanding of the physiological functions of intracardiac neuronal circuits may help in developing new strategies to treat disorders of cardiac contractility such as congestive heart failure.

The physiological and anatomical demonstration of functionally selective parasympathetic ganglia located in discrete fat pads on the feline myocardium

Experiments utilizing surgical parasympathectomy of discrete fat pad ganglia on the surface of the heart have suggested that there are two anatomically segregated and physiologically independent parasympathetic intracardiac ganglia which are capable of selective control of sino-atrial (SA) rate and atrio-ventricular (AV) conduction. Some pharmacological data, however, are inconsistent with these conclusions. We have examined the cardiodynamic effects of discrete injections of a ganglionic blocking drug into two fat pads on the surface of the cat heart. These fat pads were shown to contain ganglion cells histologically. It was observed that vagal effects upon cardiac rate are selectively mediated by neurons located in ganglia overlying the right pulmonary veins at the junction of the right atrium and superior vena cava. On the other hand, vagal effects upon AV conduction were selectively mediated by neurons located in a fat pad at the junction of the inferior vena cava and the inferior left atrium. These pharmacological data support the concept that specific intracardiac ganglia are capable of selective control of SA rate and AV conduction.

Parasympathetic neurons in the cranial medial ventricular fat pad on the dog heart selectively decrease ventricular contractility

We hypothesized that selective control of ventricular contractility might be mediated by postganglionic parasympathetic neurons in the cranial medial ventricular (CMV) ganglion plexus located in a fat pad at the base of the aorta. Sinus rate, atrioventricular (AV) conduction (ventricular rate during atrial pacing), and left ventricular contractile force (LV dP/dt during right ventricular pacing) were measured in eight chloralose-anesthetized dogs both before and during bilateral cervical vagus stimulation (20-30 V, 0.5 ms pulses, 15-20 Hz). Seven of these dogs were tested under beta-adrenergic blockade (propranolol, 0.8 mg kg(-1) i.v.). Control responses included sinus node bradycardia or arrest during spontaneous rhythm, high grade AV block or complete heart block, and a 30% decrease in contractility from 2118 +/- 186 to 1526 +/- 187 mm Hg s(-1) (P < 0.05). Next, the ganglionic blocker trimethaphan (0.3-1.0 ml of a 50 microg ml(-1) solution) was injected into the CMV fat pad. Then vagal stimulation was repeated, which now produced a relatively small 5% (N.S., P > 0.05) decrease in contractility but still elicited the same degree of sinus bradycardia and AV block (N = 8, P < 0.05). Five dogs were re-tested 3 h after trimethaphan fat pad injection, at which time blockade of vagally-induced negative inotropy was partially reversed, as vagal stimulation decreased LV dP/dt by 19%. The same dose of trimethaphan given either locally into other fat pads (PVFP or IVC-ILA) or systemically (i.v.) had no effect on vagally-induced negative inotropy. Thus, parasympathetic ganglia located in the CMV fat pad mediated a decrease in ventricular contractility during vagal stimulation. Blockade of the CMV fat pad had no effect on vagally-mediated slowing of sinus rate or AV conduction.

Neural control of left ventricular contractility in the dog heart: synaptic interactions of negative inotropic vagal preganglionic neurons in the nucleus ambiguus with tyrosine hydroxylase immunoreactive terminals

Recent physiological evidence indicates that vagal postganglionic control of left ventricular contractility is mediated by neurons found in a ventricular epicardial fat pad ganglion. In the dog this region has been referred to as the cranial medial ventricular (CMV) ganglion [J.L. Ardell, Structure and function of mammalian intrinsic cardiac neurons, in: J.A. Armour, J.L. Ardell (Eds.). Neurocardiology, Oxford Univ. Press, New York, 1994, pp. 95-114; B.X. Yuan, J.L. Ardell, D.A. Hopkins, A.M. Losier, J.A. Armour, Gross and microscopic anatomy of the canine intrinsic cardiac nervous system, Anat. Rec., 239 (1994) 75-87]. Since activation of the vagal neuronal input to the CMV ganglion reduces left ventricular contractility without influencing cardiac rate or AV conduction, this ganglion contains a functionally selective pool of negative inotropic parasympathetic postganglionic neurons. In the present report we have defined the light microscopic distribution of preganglionic negative inotropic neurons in the CNS which are retrogradely labeled from the CMV ganglion. Some tissues were also processed for the simultaneous immunocytochemical visualization of tyrosine hydroxylase (TH: a marker for catecholaminergic neurons) and examined with both light microscopic and electron microscopic methods. Histochemically visualized neurons were observed in a long slender column in the ventrolateral nucleus ambiguus (NA-VL). The greatest number of retrogradely labeled neurons were observed just rostral to the level of the area postrema. TH perikarya and dendrites were commonly observed interspersed with vagal motoneurons in the NA-VL. TH nerve terminals formed axo-dendritic synapses upon negative inotropic vagal motoneurons, however the origin of these terminals remains to be determined. We conclude that synaptic interactions exist which would permit the parasympathetic preganglionic vagal control of left ventricular contractility to be modulated monosynaptically by catecholaminergic afferents to the NA-VL.

Synaptic interactions of substance P immunoreactive nerve terminals in the baro- and chemoreceptor reflexes of the cat

The neurochemical anatomy and synaptic interactions of morphologically identified chemoreceptor or baroreceptor afferents in the nucleus of the solitary tract (NTS) are poorly understood. A substantial body of physiological and light microscopic evidence suggests that substance P (SP) may be a neurotransmitter contained in first order sensory chemo- or baroreceptor afferents, however ultrastructural support of this hypothesis is lacking. In the present report we have traced the central projections of the carotid sinus nerve (CSN) in the cat by utilizing the transganglionic transport of horseradish peroxidase. Medullary tissues including the commissural NTS (cNTS) were processed for the histochemical visualization of transganglionically labeled CSN afferents and for the immunocytochemical detection of SP by dual labeling light and electron microscopic methods. At the light microscopic level, dense bilateral labeling with TMB was found in the tractus solitarius (TS) and cNTS, caudal to the obex. Rostral to the obex, significant ipsilateral TMB labeling was detected in the dorsal, dorso-lateral, and medial subnuclei of the NTS, as well as in the TS. Significant staining of SP immunoreactive processes was detected in most subnuclei of the NTS. The cNTS was examined by electron microscopy. Either HRP or SP were readily identified in single labeled unmyelinated axons, myelinated axons, and nerve terminals in the cNTS. SP immunoreactivity was also identified in unmyelinated axons, myelinated axons, and nerve terminals in the cNTS which were simultaneously identified as CSN primary afferents. These ultrastructural data support the hypothesis that SP immunoreactive first order neurons are involved in the origination of the chemo- and baroreceptor reflexes. Axo-axonic synapses were observed between CSN primary afferent terminals and: (a) unlabeled nerve terminals; (b) other CSN primary afferent terminals; and (c) terminals containing SP. Axo-axonic synapses were also observed between CSN primary afferents which contained SP, and other SP terminals. These observations may mediate the morphological bases for multiple forms of presynaptic inhibition in the cNTS, including those involved in cardiorespiratory integration. In conclusion, our results indicate that SP immunoreactive nerve terminals may be important in both the origination and the modulation of the chemo- and/or baroreceptor reflexes.

Substance P nerve terminals synapse upon negative chronotropic vagal motoneurons

Previous data indicate that there are anatomically segregated and physiologically independent parasympathetic ganglia on the surface of the heart which are capable of selective control of sino-atrial rate, atrio-ventricular conduction, and atrial contractility. We have injected a retrograde tracer into the cardiac ganglion which selectively regulates heart rate (the SA ganglion). Medullary tissues were processed for the histochemical visualization of retrogradely labeled neurons and for the immunohistochemical detection of the neurotransmitter substance P (SP) by dual labeling light and electron microscopic methods. Negative chronotropic retrogradely labeled cells were found in a long slender column in the ventrolateral nucleus ambiguous (NA-VL) which enlarged somewhat at the level of the area postrema. These cells were found bilaterally, but they were asymmetrically distributed. Half the animals showed a pronounced right side predominance in retrograde labeling, while the other half of the animals showed a lesser left side predominance. These observations may help to explain some of the controversy in the literature concerning the relative influence of the right and left vagus nerves on sinus rate. Ultrastructural examination demonstrated axo-somatic and axo-dendritic contacts between SP nerve terminals and retrogradely labeled negative chronotropic NA-VL neurons. SP immunoreactivity was often associated with large dense-core vesicles in terminals forming either symmetric or asymmetric synapses. These observations provide a potential anatomical substrate for the centrally mediated bradycardia elicited by microinjections of SP into the NA. SP immunoreactive terminals were also observed to make axo-somatic, axo-dendritic, and axo-axonic synapses with unlabeled neurons in NA-VL.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructural circuitry of cardiorespiratory reflexes : there is a monosynaptic path between the nucleus of the solitary tract and vagal preganglionic motoneurons controlling atrioventricular conduction in the cat

We have tested the hypothesis: (1) that presumptive negative dromotropic vagal preganglionic neurons in the ventrolateral nucleus ambiguus (NA-VL) can be selectively labelled from the heart, by injecting one of two fluorescent tracers into the two intracardiac ganglia which independently control sino-atrial (SA) rate or atrioventricular (AV) conduction; i.e., the SA and AV ganglia, respectively. The NA-VL was examined for the presence of single and/or double labelled cells. Over 91% of vagal preganglionic neurons in the NA-VL projecting to either intracardiac ganglion did not project to the second ganglion. Consequently, we also tested the hypothesis: (2) that there is a monosynaptic connection between neurons of the medial, and/or dorsolateral nucleus of the solitary tract (NTS), rostral to obex, and negative dromotropic neurons in the NA-VL. An anterograde tracer was injected into the NTS, and a retrograde tracer into the AV ganglion. The anterograde marker was found in both myelinated and unmyelinated axons in the NA-VL, as well as in nerve terminals. Axo-somatic and axo-dendritic synapses were detected between terminals labelled from the NTS, and retrogradely labelled negative dromotropic neurons in the NA-VL. This is the first ultrastructural demonstration of a monosynaptic pathway between neurons in the NTS and functionally associated (negative dromotropic) cardioinhibitory neurons. The data are consistent with the hypothesis that the neuroanatomical circuitry mediating the vagal baroreflex control of AV conduction may be composed of as few as four neurons in series, although interneurons may also be interposed within the NTS.

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

SummaryTyrosine 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.