Central connections of aortic depressor and carotid sinus nerves

Neurochemical transmission of baroreceptor input in the nucleus tractus solitarius

Baroreceptor activation has been found to produce different types of discharge patterns in neurons in the nucleus tractus solitarius (NTS). The contribution of different glutamate receptor subtypes, neuropeptide modulators and input from different baroreceptor subtypes to the generation of firing patterns in NTS barosensitive neurons was examined in a series of studies. Results from these studies indicate that both subtypes of ionotropic glutamate receptors contribute to discharge in barosensitive neurons, and the role of each subtype can vary for different neurons. The neuropeptide neurotensin was found to modulate baroreceptor control of BP and discharge of central barosensitive neurons, both through modulation of baroreceptor afferent input and possibly through release of neurotensin by baroreceptor afferent fibers in the NTS. Finally, selective modulation of input from baroreceptor subtypes indicates that there is some degree of divergent baroreceptor innervation of NTS neurons that could contribute to initiation of their different discharge patterns in response to baroreceptor input.

Discharge patterns of baroreceptor-modulated neurons in the nucleus tractus solitarius

Activity of baroreceptor-modulated neurons in the nucleus tractus solitarius (NTS) was recorded extracellularly during selective pressure stimulation of carotid baroreceptors, using an isolated carotid sinus preparation in anesthetized dogs. One of two different patterns of activity was recorded from individual baro-sensitive neurons in response to slow ramp increases in carotid sinus pressure. The cause of these two distinct firing patterns is not known but preliminary results indicate that it may be due in part to input from different functional types of baroreceptors. These results suggest that some differentiation in blood pressure control may be encoded in the responses of central baro-sensitive neurons in the NTS.

Peripheral chemoreceptor inputs to medullary inspiratory and postinspiratory neurons of cats

The effect of peripheral chemoreceptor activation on inspiratory and postinspiratory medullary neurons was investigated using intracellular recording techniques. Peripheral chemoreceptors were activated by injecting CO2 saturated 1 N bicarbonate solution into the lingual artery or by electrically stimulating the carotid sinus nerve. Injections of 20-300 microliters bicarbonate solution evoked changes in respiratory frequency and in peak phrenic nerve discharge. The membrane potential of inspiratory alpha neurons, whether bulbospinal or not and independent of their anatomic location, was decreased during inspiration. A sequence of compound excitatory and inhibitory effects were observed when the stimulus was given during the postinspiratory and expiratory phases of the respiratory cycle. Inspiratory beta- and late-inspiratory neurons, however, were inhibited by peripheral chemoreceptor activation. Postinspiratory neurons were strongly activated during postinspiration. Neither class of respiratory neurons were shown to receive direct synaptic inputs from the peripheral chemoreceptors as tested by electrical stimulation of the carotid sinus nerve and signal averaging of the respiratory neuron membrane potential. The experiments revealed differential influences of afferent chemoreceptor activity on various components of the respiratory network. We conclude that chemoreceptor afferents activate non-respiratory modulated medullary neurons which, in turn, activate or inhibit various neurons of the medullary respiratory control network. The responses of each type of respiratory neuron to chemoreceptors afferents may then be considered in the context of this direct interaction as well as the network interactions of the various cells.

The carotid sinus connections: a WGA-HRP study in the cat

The neural connections of the carotid sinus were studied by administration of horseradish peroxidase or a lectin conjugate to the adventitia of the carotid sinus of cats. The carotid sinus afferents project exclusively to the nucleus of the tractus solitarius (NTS). Rostral to the obex the projection is mainly ipsilateral with a strong contralateral component caudal to the obex. The carotid sinus projects to several NTS territories that do not receive afferents from the carotid body chemoreceptors; they are: the dorsolateral, the lateral extension of the commissural, the caudal intermediate, the ventrolateral and the gelatinosus subnuclei. In addition the carotid sinus central representation includes territories occupied also by carotid body terminals: dorsal, interstitial, rostral intermediate, medial and the medial part of commissural subnuclei (see previous paper). Labeled cell bodies were found in the petrosal (216 +/- 37, mean +/- S.E.M.) nodose (825 +/- 434) and superior cervical ganglia (3583 +/- 1227) demonstrating the sympathetic efferent innervation of the carotid sinus and a dual sensory innervation via both the glossopharyngeal and vagus nerves.

Baroreceptor inputs to the nucleus tractus solitarius in the cat: postsynaptic actions and the influence of respiration

1. The postsynaptic action of carotid sinus nerve (SN), aortic nerve (AN), superior laryngeal nerve (SLN) and vagal nerve (VN) stimulation has been studied on neurones in the nucleus of the tractus solitarius (NTS) in vivo. 2. Three distinct patterns of postsynaptic responses were evoked by SN stimulation, an EPSP, an EPSP-IPSP sequence and an IPSP, observed separately in individual neurones. This diversity of response was represented in cells proven to receive baroreceptor input by inflation of a balloon-tipped catheter within the ipsilateral carotid sinus. 3. Virtually none of the neurones identified as baroreceptive exhibited pulse-related discharge. 4. A variety of influences to AN, SLN and VN stimulation were observed in neurones receiving baroreceptor afferent information. This wide convergence of input implies that this region of the brain stem is important in the integration of cardiovascular reflexes. 5. The hypothesis was tested that respiratory 'gating' of the baroreceptor reflex is produced by synaptic actions within the NTS. There was an absence of any modification of PSPs by lung inflation and by variations in the timing of the stimulation of the afferent nerves within the respiratory cycle. These observations indicate that respiratory modifications of the baroreceptor reflex must occur at later stages in the reflex pathway.

Post-synaptic activity evoked in the nucleus tractus solitarius by carotid sinus and aortic nerve afferents in the cat

Post-synaptic responses evoked in neurones of the nucleus tractus solitarius by electrical stimulation of the carotid sinus, aortic and vagal nerves, alone or in combination, have been studied in anaesthetized cats using both extracellular and intracellular recording techniques. A total of 292 neurones received an input from at least one of the three nerves tested. The activity of the large majority of these cells (249) could only be shown to be altered by stimulation of one of these nerves and in 222 of these cases this was an excitatory response. These responses showed the expected post-synaptic characteristics including temporal summation and, in intracellular records, a summation of evoked excitatory post-synaptic potentials (e.p.s.p.s). The minimum latency to onset of these responses was variable, both for individual cells and for the population as a whole and varied within the range 2-124 ms. In a small number of cells (twenty-seven), the input was purely inhibitory in nature. In neurones showing a tonic discharge this produced a decrease in the rate of firing. This influence was most marked in intracellular records where membrane hyperpolarizations were noted. Again, the latency to onset was variable, in the range 4-27 ms. Convergent inputs from two or more of the nerves were identified in forty-three neurones. The effects of these were always excitatory. They could be observed both as a facilitation of spike activity recorded extracellularly and as summation of subliminally evoked e.p.s.p.s recorded intracellularly. On the basis of threshold voltages and latency to onset, the afferents to these neurones are indistinguishable from those providing an exclusive input. It can be concluded that at least some of the neurones in the nucleus tractus solitarius and its vicinity receive inputs from more than one source. The implications of these observations on the role of this brain-stem area in cardiorespiratory reflexes is discussed.

Pathways between the nucleus tractus solitarius and neurosecretory neurons of the supraoptic nucleus: electrophysiological studies

In anesthetized cats recordings were made from hypothalamo-neurohypophysial neurons in a supraoptic nucleus (SON) of the hypothalamus. The region of the nucleus tractus solitarius in the medulla, identified electrophysiologically as the site of termination of the first relay neurons of the sinus and aortic nerves, was stimulated with single or short trains of pulses (2-3 at 200 Hz). Out of 133 SON neurons 67 were affected by such stimuli. In 14 cells (21% of 'responsive' neurons) the stimulus produced profound inhibition of SON neuron activity after a latency of 10-30 msec. In another 8 neurons (12%) the inhibitory effect was observed after a longer latency of over 100 msec. An increase in intensity of stimulus merely prolonged or increased the inhibitory effect without changing the response qualitatively. The other 45 (67%) SON neurons were excited by stimulation of the nucleus tractus solitarius. In a small proportion of these neurons (5 cells, 7%) the stimulus evoked discharges, even in spontaneously silent neurosecretory cells, after a latency of 10-20 msec with little fluctuation. In the remaining 40 neurons, i.e. 60% of the 'responsive' neurons, the excitatory effect was observed after a latency of 40-120 msec. Again, changes in intensity of stimulation did not alter the nature of this response. The results indicate that both 'fast' as well as 'slow' pathways between the nucleus tractus solitarius and SON neurons exist and impulses travelling through the latter pathway from the carotid sinus or aortic nerve affect the larger proportion of SON neurons.