Attenuation of the afferent limb of the baroreceptor reflex in streptozotocin-induced diabetic rats

Diabetic autonomic neuropathy is a common complication following prolonged diabetes. Alterations of cardiovascular reflexes contribute to the increased cardiovascular morbidity and mortality seen in diabetic patients. This study sought to better characterize these complications by investigating the afferent limb of the baroreceptor reflex in an experimental rat model of diabetes. Streptozotocin (STZ)-induced diabetic and euglycemic control rats were studied at 8- and 16-week time points after initiation of the experiment. Activation of the afferent limb of the baroreceptor reflex was assessed by measuring the numbers of c-Fos-immunoreactive (ir) neurons in the CNS site of termination of the baroreceptor afferent neurons, the nucleus of the solitary tract (NTS). Initial experiments established that baseline cardiovascular parameters and NTS expression of c-Fos-ir neurons were not different between diabetic and control rats at either time point. Phenylephrine (PE)-induced activation of baroreceptors resulted in a significant elevation in the numbers of c-Fos-ir neurons in the NTS of control rats. Although diabetic rats showed similar pressor responses to PE, the activation of c-Fos-ir neurons in the NTS of diabetic rats was significantly attenuated. At both 8 and 16 weeks, STZ-induced diabetic rats had significantly fewer c-Fos-ir neurons in the commissural NTS and in the caudal subpostrernal NTS when compared to the non-diabetic control animals receiving PE. These data suggest that STZ-induced diabetes, for a period of 8 and 16 weeks, results in reduced activity in the afferent baroreceptor input to the NTS, and are consistent with diabetes-induced damage to baroreceptor afferent nerves.     

Efferent projections of the nucleus of the solitary tract to peri-locus coeruleus dendrites in rat brain: evidence for a monosynaptic pathway.

Locus coeruleus (LC) neurons respond to autonomic influences, are activated by physiological stressors, and discharge in parallel with peripheral sympathetic nerves. The circuitry underlying modulation of LC activity by physiological manipulations (i.e., hemodynamic stress, hypovolumia) remains unclear. Specifically, monosynaptic projections from primary baroreceptor centers to the LC have been suggested by electrophysiological studies but have not been unequivocally established. Light microscopic anterograde tract-tracing studies have previously shown that neurons originating in the nucleus of the solitary tract (NTS) project to a region of the rostrodorsal pontine tegmentum, which contains noradrenergic dendrites of the LC; however, it is not known whether these NTS efferents specifically target LC dendrites. Therefore, we combined peroxidase labeling of biotinylated dextran amine (BDA) or Phaseolus vulgaris-leucoagglutinin (PHA-L) from the NTS with gold-silver labeling for tyrosine hydroxylase (TH) in the rostrolateral peri-LC region. Injections placed into neighboring nuclei (nucleus gracilis, hypoglossal nucleus) served as controls. Only injections centered in the NTS produced anterograde labeling in peri-LC regions containing TH processes. By electron microscopy, BDA- or PHA-L-labeled axon terminals originating from the NTS contained small, clear, and some large dense-core vesicles and formed heterogeneous synaptic contacts characteristic of both excitatory- and inhibitory-type transmitters. Approximately 19% of the BDA and PHA-L axon terminals examined originating from the commissural portion of the NTS formed synaptic specializations with dendrites exhibiting TH immunoreactivity in the peri-LC. These results demonstrate that neurons projecting from the cardiovascular-related portion of the NTS target noradrenergic dendrites, indicating that barosensitive NTS neurons may directly modulate the activity of LC neurons and may serve to integrate autonomic responses in brain by influencing the widespread noradrenergic projections of the LC. In addition, these findings demonstrate that extranuclear dendrites are an important termination site for afferents to the LC.     

Distribution of neurotensin-immunoreactivity within baroreceptive portions of the nucleus of the tractus solitarius and the dorsal vagal nucleus of the rat

We have examined the distribution of neurotensin immunoreactivity within subnuclear regions of the nucleus of the tractus solitarius (NTS) and the dorsal motor nucleus of the vagus nerve (DVN) in the rat. In order to determine which regions of the NTS were involved in the regulation of baroreceptor reflexes, we mapped the central distribution of the aortic branch of the vagus nerve using transganglionic transport of horseradish peroxidase. Comparison of the pattern of aortic nerve innervation with that of the distribution of neurotensin-immunoreactive cells and fibers shows the dorsomedial nucleus of the NTS both to be the primary site of aortic baroreceptor termination and to contain the highest concentration of neurotensin-immunoreactive elements within the NTS. Neurotensin-immunoreactive fibers are also present in medial regions of the NTS adjacent to the area postrema where they may be involved in the modulation of vagal gastric afferents. Double-label experiments, in which, on the same tissue sections, neurotensin immunohistochemistry was combined with retrograde horseradish peroxidase labeling of DVN neurons, reveal a topographic innervation of vagal preganglionic motoneurons by neurotensin-immunoreactive fibers. The heaviest innervation is of lateral portions of the DVN and adjacent ventral portions of the NTS at the level of the obex, an area which may contain cardiac motoneurons. In this region neurotensin-immunoreactive fibers can be observed in close proximity to retrogradely labeled cells. The concentration of neurotensin elements in a region of the NTS which is involved in the control of baroreceptor reflexes provides a morphological basis for the cardiovascular effects produced by central administration of the peptide. Additional control may be exerted at the level of the motoneuron, as evidenced by apparent neurotensin fiber innervation of presumptive cardiac preganglionic neurons. Similarly, the distribution of neurotensin fibers suggests that the peptide may be acting in gastric regulatory areas of the NTS or on vagal secretomotor neurons to regulate gastric acid secretion.     

The direct effect of insulin on barosensitive neurones in the nucleus tractus solitarii of rats.

The present investigation was designed to determine the direct effect of insulin on the spontaneous discharge of barosensitive neurones in the nucleus tractus solitarii (NTS) of rats anaesthetized with urethane. Microinjection of 20 nl insulin (10 IU/ml) into NTS decreased the spontaneous discharge of 38 of the 52 units studied (73.1%), and this decrease was augmented by increasing the concentration to 40 IU/ml. Microinjections of insulin vehicle, glucose, hydralazine or phenylephrine did not elicit significant changes in the spontaneous discharge of NTS barosensitive neurones. These results demonstrate that insulin inhibits the spontaneous discharge of barosensitive NTS neurones. They suggest that insulin increases sympathetic nervous activity via a central neural mechanism and may play a role in the modulation of cardiovascular information within the NTS.     

Frequency dependence of synaptic vesicle exocytosis in aortic baroreceptor neurons and the role of group III mGluRs

Synaptic transmission between baroreceptor afferents and the nucleus tractus solitarius (NTS) is essential for reflex regulation of blood pressure. High frequency stimulation of the afferents in vivo leads to a decrease in synaptic strength and is generally attributed to reduction in presynaptic neurotransmitter release. It has been hypothesized that during high frequency stimulation glutamate a major neurotransmitter at the baroreceptor afferent terminals inhibits its own release via presynaptic group III metabotropic glutamate receptors (mGluRs). A key player in modulation of presynaptic release is vesicle exocytosis. The present study utilized cultured aortic baroreceptor neurons and the styryl dye FM2-10 to characterize (1) the dependence of exocytosis at these afferent nerve terminals on the frequency of neuronal activation, (2) the effect of duration of stimulation on the rate of exocytosis and (3) the role of mGluRs in the frequency-dependent modulation of exocytosis. Destaining in the FM2-10 loaded boutons during 3 min of stimulation, a measure of exocytosis, progressively decreased with increasing frequency (0.5, 1.0 and 10 Hz). Blockade of group III mGluRs with 300 microM (RS)-cyclopropyl-4-phosphonophenylglycine (CPPG) facilitated exocytosis evoked by 10 Hz stimulation but not at 0.5 Hz. The data suggest that aortic baroreceptor terminals exhibit frequency-dependent depression of exocytosis and support a role for group III mGluRs in the frequency-dependent modulation of exocytosis.     

Monosynaptic projections from the nucleus tractus solitarii to C1 adrenergic neurons in the rostral ventrolateral medulla: Comparison with input from the caudal ventrolateral medulla

The rostral ventrolateral medulla (RVL) contains reticulospinal adrenergic (C1) neurons that are thought to be sympathoexcitatory and that form the medullary efferent limb of the baroreceptor reflex pathway. The RVL receives direct projections from two important autonomic regions, the caudal ventrolateral medulla (CVL) and the nucleus tractus solitarii (NTS). In the present study, we used anterograde tracing from the CVL or the NTS combined with immunocytochemical identification of C1 adrenergic neurons in the RVL to compare the morphology of afferent input from these two autonomic regions into the RVL. NTS (n = 203) and CVL (n = 380) efferent terminals had similar morphology and vesicular content, but CVL efferent terminals were slightly larger than NTS efferent terminals. Overall, efferent terminals from either region were equally likely to contact adrenergic neurons in the RVL (21% for NTS, 25% for CVL). Although efferents from both regions formed both symmetric and asymmetric synapses, NTS efferent terminals were statistically more likely to form asymmetric synapses than CVL efferent terminals. CVL efferent terminals were more likely to contact adrenergic somata than were NTS efferents, which usually contacted dendrites. These findings 1) support the hypothesis that a portion of NTS efferents to the RVL may be involved in sympathoexcitatory, e.g., chemoreceptor, reflexes (via asymmetric synapses), whereas those from the CVL mediate sympathoinhibition (via symmetric synapses); and 2) provide an anatomical substrate for differential postsynaptic modulation of C1 neurons by projections from the NTS and CVL. With their more frequent somatic localization, CVL inhibitory inputs may be more influential than excitatory NTS inputs in determining the discharge of RVL neurons. © 1996 Wiley-Liss, Inc.     

Modulation of the carotid baroreceptor reflex by substance P in the nucleus tractus solitarius

Previous studies have shown that administration of substance P (SP) into the nucleus tractus solitarius (NTS) can evoke a depressor response similar to that produced by activation of the arterial baroreceptors. In addition, some studies have suggested that SP increases the reflex responses to activation of baroreceptor input. The present study was performed to determine the effects of SP on the carotid sinus baroreceptor reflex at the level of the NTS by examining the effects of both exogenous SP microinjected into different rostrocaudal locations in the NTS and blockade of the effects of endogenous SP, through the microinjection of a substance P antagonist (SPa; [D-Pro, D-Trp]-substance P). Changes in pressure in an isolated carotid sinus in anesthetized dogs were used to evoke baroreflex changes in arterial blood pressure (BP) before and after microinjection of SP (0.5 microM) or SPa (10 microM) into barosensitive regions of the NTS. Microinjection of SP or its antagonist did not alter baseline, resting BP but did produce significant changes in baroreflex sensitivity. Microinjection of SP into different rostrocaudal regions of the NTS produced different responses, with rostral and caudal NTS microinjections producing significant increases in sensitivity. No effects on baroreflex sensitivity were obtained in response to SP microinjections into the intermediate NTS. Unlike SP, microinjection of the SPa significantly decreased baroreflex sensitivity at all rostrocaudal levels of the NTS. These data demonstrated that SP has the capability to modulate the carotid baroreflex at the level of the NTS and support a physiological role for endogenously released SP.     

Local and commissural neuropeptide-containing projections of the nucleus of the solitary tract to the dorsal vagal complex in the pigeon

The neuropeptide content of neurons of the nucleus of the solitary tract (NTS), which have local and commissural projections to the dorsal motor nucleus of the vagus (DMNX) and to NTS, were demonstrated in the pigeon (Columba livia) by using a combined fluorescein-bead retrograde-transport-immunofluorescence technique. The specific peptides studied were bombesin, cholecystokinin, enkephalin, galanin, neuropeptide Y, neurotensin, and substance P. Perikarya immunoreactive for bombesin were located in the Medial tier subnuclei of NTS and the caudal NTS. Most galanin- and substance P-immunoreactive cells were found in subnucleus medialis ventralis. Cells immunoreactive for neuropeptide Y were found in the medial tier of NTS and in the lateral tier, especially in subnucleus lateralis dorsalis intermedius. The majority of enkephalin- and neurotensin-immunoreactive cells were found centrally in subnuclei medialis dorsalis and medialis intermedius. Cells immunoreactive for cholecystokinin were located in subnuclei laterolis dorsalis pars anterior, medialis superficialis, and the caudal NTS. Based on the presence of retrogradely labeled cells, numerous neurons of the medial tier of NTS, but extremely few lateral tier NTS neurons, had projections to the ipsilateral and contralateral DMNX and NTS. The number of retrogradely labeled NTS cells was always greater ipsilateral than contralaterally. The percentages of peptide-immunoreactive NTS cells that projected to the ipsilateral and contralateral DMNX were in the ranges of 29–61% and 10–48%, respectively. The percentages of peptide-immunoreactive NTS cells that projected to the contralateral NTS ranged from 13 to 60%. Peptide-immunoreactive NTS cells that have local and commissural projections to DMNX and NTS may act as interneurons in vagovagal reflex pathways and in the integration of visceral sensory and forebrain input to NTS and DMNX. © 1994 Wiley-Liss, Inc.     

Chemical coding and central projections of gastric vagal afferent neurons

Abstract  Vagal afferents that innervate gastric muscle or mucosa transmit distinct sensory information from their endings to the nucleus of the tractus solitarius (NTS). While these afferent subtypes are functionally distinct, no neurochemical correlate has been described and it is unknown whether they terminate in different central locations. This study aimed to identify gastric vagal afferent subtypes in the nodose ganglion (NG) of ferrets, their terminal areas in NTS and neurochemistry for isolectin-B4 (IB4) and calcitonin gene-related peptide (CGRP). Vagal afferents were traced from gastric muscle or mucosa and IB4 and CGRP labelling assessed in NG and NTS. 7 ± 1% and 6 ± 1% of NG neurons were traced from gastric muscle or mucosa respectively; these were more likely to label for CGRP or for both CGRP and IB4 than other NG neurons ( P  <   0.01). Muscular afferents were also less likely than others to label with IB4 ( P  <   0.001). Less than 1% of NG neurons were traced from both muscle and mucosa. Central terminals of both afferent subtypes occurred in the subnucleus gelatinosus of the NTS, but did not overlap completely. This region also labelled for CGRP and IB4. We conclude that while vagal afferents from gastric muscle and mucosa differ little in their chemical coding for CGRP and IB4, they can be traced selectively from their peripheral endings to NG and to overlapping and distinct regions of NTS. Thus, there is an anatomical substrate for convergent NTS integration for both types of afferent input.     

Intracellular recording of lamina X neurons in a horizontal slice preparation of rat lumbar spinal cord

A horizontal slice preparation of postnatal rat lumbar spinal cord has been developed which allows correlative observations of the morphology, electrophysiology, and receptor pharmacology of lamina X neurons. These slices better maintain afferent input and somatodendritic morphology and are amenable to subsequent immunohistochemical processing. Stable intracellular recordings obtained from postnatal day 14–45 animals reveal that a number of different intrinsic membrane conductances contribute to the regulation of excitability in lamina X neurons. In addition, lamina X neurons possess inhibitory GABAergic as well as excitatory glutamate and cholecystokinin receptors. This preparation will be useful in future studies designed to characterize developmental changes in the intrinsic membrane properties, synaptic profiles and neuropeptide responsiveness of lamina X neurons in the rat. Such a characterization is important given that lamina X represents a unique sexually dimorphic region that is a convergence site for somatic and visceral afferent inputs, which includes nociceptive information.     

Plasticity of vagal brainstem circuits in the control of gastric function

Background Sensory information from the viscera, including the gastrointestinal (GI) tract, is transmitted through the afferent vagus via a glutamatergic synapse to neurons of the nucleus tractus solitarius (NTS), which integrate this sensory information to regulate autonomic functions and homeostasis. The integrated response is conveyed to, amongst other nuclei, the preganglionic neurons of the dorsal motor nucleus of the vagus (DMV) using mainly GABA, glutamate and catecholamines as neurotransmitters. Despite being modulated by almost all the neurotransmitters tested so far, the glutamatergic synapse between NTS and DMV does not appear to be tonically active in the control of gastric motility and tone. Conversely, tonic inhibitory GABAergic neurotransmission from the NTS to the DMV appears critical in setting gastric tone and motility, yet, under basal conditions, this synapse appears resistant to modulation. Purpose Here, we review the available evidence suggesting that vagal efferent output to the GI tract is regulated, perhaps even controlled, in an ‘on‐demand’ and efficient manner in response to ever‐changing homeostatic conditions. The focus of this review is on the plasticity induced by variations in the levels of second messengers in the brainstem neurons that form vago‐vagal reflex circuits. Emphasis is placed upon the modulation of GABAergic transmission to DMV neurons and the modulation of afferent input from the GI tract by neurohormones/neurotransmitters and macronutrients. Derangement of this ‘on‐demand’ organization of brainstem vagal circuits may be one of the factors underlying the pathophysiological changes observed in functional dyspepsia or hyperglycemic gastroparesis.     

Direct injection of substance P-antisense oligonucleotide into the feline NTS modifies the cardiovascular responses to ergoreceptor but not baroreceptor afferent input

Substance P (SP) is released from the feline nucleus tractus solitarius (NTS) in response to activation of skeletal muscle afferent input. However, there are differing results about SP release from the rostral NTS in response to baroreceptor afferent input. An anti-sense oligonucleotide to feline SP (SP-asODN) was injected directly into the rostral NTS of chloralose-anesthetized cats to determine whether blood pressure or heart rate responses to ergoreceptor activation (muscle contraction) or baroreceptor unloading (carotid artery occlusion) were sensitive to SP knockdown. Control injections included either buffer alone or a scrambled-sequenced oligonucleotide (SP-sODN). Both muscle contractions and carotid occlusions were performed 3, 6 and 12 h after the completion of the oligonucleotide injections. The cardiovascular responses to contractions were significantly attenuated 3 and 6 h after SP-asODN, but not by the injection of the SP-sODN. The cardiovascular responses to contractions returned to control levels 12 h post anti-sense injection. No detectable release of SP (using antibody-coated microprobes) was measured 3 and 6 h after SP-asODN injections and the expression of SP-immunoreactivity (SP-IR) in the NTS was significantly attenuated, as determined by immunohistochemistry procedures. In contrast, neither the injection of SP-asODN nor the s-ODN attenuated the cardiovascular responses to carotid occlusions, or altered the pattern of release of SP from the brainstem. Injection of the SP-sODN did not affect the expression of SP-IR. These results suggest that the SP involved with mediating the peripheral somatomotor signal input to the rostral NTS comes from SP-containing neurons within the NTS. Our results also suggest that SP in the rostral NTS does not play a direct role in mediating the cardiovascular responses to unloading the carotid baroreceptors. We suggest that the SP released during isometric contractions excites an inhibitory pathway modulating baroreceptor input, thus contributing to the increase in mean blood pressure.     

Spontaneous activity and barosensitivity of the barosensitive neurons in the rostral ventrolateral medulla of hypertensive rats induced by transection of aortic depressor nerves

In order to determine the role of the rostral ventrolateral medulla (RVLM) in the development of neurogenic hypertension, the aortic depressor nerves of rats were transected (tADN) to produce neurogenic hypertension. The rate and pattern of firing of the barosensitive RVLM neurons of the treated rats were studied. In neurogenic hypertensive rats, the RVLM barosensitive neurons exhibited a faster firing rate and a shorter interspike interval (ISI) than the corresponding values of the control and sham groups, indicating an enhanced spontaneous activity of these neurons in the hypertensive rats. The coefficient of variation (cv) and skewness (sk) of the ISI histogram, parameters reflecting the regularity of neuronal firing, were smaller in neurogenic hypertensive than in the control and sham-operated rats. Following tADN, the responsiveness of these neurons to blood pressure changes was attenuated, suggesting a reduced intrinsic barosensitivity of neurons and/or a reduced baroreceptor input. The increase in spontaneous activity and firing regularity of RVLM barosensitive neurons imply an enhancement in the efficacy of outflow from these neurons. The increased efficacy of the outflow from the RVLM barosensitive neurons and the resetting of the baroreflex may contribute to the genesis of neurogenic hypertension.     

Quantitative autoradiography of angiotensin II receptors in the rat solitary-vagal area: effects of nodose ganglionectomy or sinoaortic denervation

The experiments reported here were designed to examine whether angiotensin II (AII) receptors in the rat solitary-vagal area (SVA) are associated with the neuronal components of the baroreceptor reflex. AII receptors were characterized both in membrane preparations from the rat brainstem and by in vitro autoradiography using the radiolabeled AII antagonist [125I]Sar1,Ile8-AII([ 125I]SI-AII). Saturation analysis of [125I]SI-AII binding to membrane preparations from rat brainstem indicated binding to two high affinity sites (Kd1 0.32 nM and Bmax1 5.10 fmol/mg protein, Kd2 0.99 nM and Bmax2 7.94 fmol/mg protein). The rank order competition by unlabeled angiotensin peptides (SI-AII greater than AII greater than AIII greater than AI) in both membrane preparations and by quantitative autoradiography was consistent with the labeling of the brain AII receptor. Autoradiography of the [125I]SI-AII binding in sections through the SVA revealed that the nucleus tractus solitarius (NTS) and the dorsal motor nucleus of the vagus (DMV) were heavily labeled. Bilateral sinoartic denervation, which disrupts primary baroreceptor afferents, resulted in a small decrease in [125I]SI-AII binding in the rostral and intermediate NTS and DMV. Unilateral nodose ganglionectomy, which disrupts completely the vagal afferent input to the NTS and produces retrograde degeneration of the vagal efferent neurons in the DMV, resulted in a marked decrease in [125I]SI-AII binding at all levels of the ipsilateral NTS and 56% decrease within the ipsilateral DMV. These results indicate that AII receptors within the SVA are distributed heterogeneously, with a large portion associated with vagal afferent fibers in the NTS and vagal efferent neurons of the DMV, and a small but significant portion associated with baroreceptor afferents. The majority of AII receptors in the NTS, however, were not affected by these surgical interventions and therefore appear to be located on intrinsic interneurons or non-vagal afferents in the NTS.     

Anatomical substrates for baroreflex sympathoinhibition in the rat

The fundamental neuronal substrates of the arterial baroreceptor reflex have been elucidated by combining anatomical, neurophysiological, and pharmacological approaches. A serial pathway between neurons located in the nuclei of the solitary tract (NTS), the caudal ventrolateral medulla (CVL), and the rostral ventrolateral medulla (RVL) plays a critical role in inhibition of sympathetic outflow following stimulation of baroreceptor afferents. In this paper, we summarize our studies using tract-tracing and electron microscopic immunocytochemistry to define the potential functional sites for synaptic transmission within this circuitry. The results are discussed as they relate to the literature showing: (1) baroreceptor afferents excite second-order neurons in NTS through the release of glutamate; (2) these NTS neurons in turn send excitatory projections to neurons in the CVL; (3) GABAergic CVL neurons directly inhibit RVL sympathoexcitatory neurons; and (4) activation of this NTS-->CVL-->RVL pathway leads to disfacilitation of sympathetic preganglionic neurons by promoting withdrawal of their tonic excitatory drive, which largely arises from neurons in the RVL. Baroreceptor control may also be regulated over direct reticulospinal pathways exemplified by a newly recognized sympathoinhibitory region of the medulla, the gigantocellular depressor area. This important autonomic reflex may also be influenced by parallel, multiple, and redundant networks.     

Neurons in the caudal ventrolateral medulla mediate the arterial baroreceptor reflex by inhibiting barosensitive reticulospinal neurons in the rostral ventrolateral medulla in rabbits.

Participation of the caudal ventrolateral medulla in the arterial baroreceptor reflex was examined in urethane-anesthetized, vagotomized and immobilized rabbits whose aortic nerve was cut bilaterally. The extent of the caudal ventrolateral medulla was mapped by decreases in the renal sympathetic nerve activity and arterial pressure following a local microinjection of a neuroexcitatory amino acid, sodium glutamate (0.075-1.5 nmol). It extended between the levels 1.3 mm rostral and 3.0 mm caudal to the obex. An injection of sodium glutamate into the caudal ventrolateral medulla also diminished spontaneous activity of barosensitive reticulospinal neurons in the rostral ventrolateral medulla. In the 'split medulla preparation' in which the medulla was split along the midsagittal plane to disrupt fiber connections associating both sides, a neurotoxic agent, kainic acid, was injected unilaterally into the rostral ventrolateral medulla. This treatment markedly attenuated responses of renal sympathetic nerve activity and arterial pressure induced by a sodium glutamate injection into the ipsilateral caudal ventrolateral medulla, whereas responses to an injection into the contralateral caudal ventrolateral medulla were totally preserved. In four separate experiments, three to five injections of kainic acid were made unilaterally to cover the whole extent of the caudal ventrolateral medulla. The sympathoinhibitory and depressor responses to stimulation of the ipsilateral aortic nerve were then totally abolished. Simultaneously, the cardiac cycle-related rhythmic fluctuation of renal sympathetic nerve activity, which represented activity of the carotid sinus baroreceptor reflex, was attenuated to the noise level. These results, together with our previous electrophysiological demonstration of barosensitive caudal ventrolateral medulla neurons with axonal projections to the rostral ventrolateral medulla, strongly support the hypothesis that neurons in the caudal ventrolateral medulla mediate the arterial baroreceptor-vasomotor reflex through inhibition of barosensitive reticulospinal neurons in the rostral ventrolateral medulla.     

Fine structure and plasticity of barosensitive neurons in the nucleus of solitary tract

Intravenous phenylephrine (PE) activates neurons in the nucleus of the solitary tract (NTS) whose distribution conforms to those of central projections of the carotid sinus and aortic depressor nerves. This was exploited to permit fine structural characterization of cells presumed to compose the first station in the processing of arterial baroreceptor input, and their responses to stimulation. Rats were perfused at varying intervals after PE injection, and sections through the baroreceptor afferent zone of the NTS prepared for preembedding immunolocalization of Fos-immunoreactivity. Labeled neurons composed a continuous strip extending from the dorsal part of the commissural NTS (NTScom) to the dorsal subnucleus at the level of the area postrema (NTSap). PE-sensitive neurons in these regions were medium-sized, round to ovoid in shape, with scant cytoplasm and an unremarkable complement of organelles. Distinctive features included extensively invaginated nuclei and well-developed Golgi apparati; Fos-ir cells in the NTSap were distinguished from those in NTScom by virtue of better-developed rough endoplasmic reticulum and Golgi, and less convoluted nuclei. Proximal synaptic input to PE-sensitive neurons was sparse and was provided by terminals containing predominantly small, clear synaptic vesicles that formed mainly symmetric junctions with somata and primary dendrites. Prolonged stimulation was accompanied by accentuation of nuclear invaginations, marked accumulation of heterochromatin at their apices, and evidence of enhanced Golgi activity (vesicular budding). These may represent adaptations to facilitate changes in gene expression, to maintain neurotransmitter availability, or both, in the face of a persistent hypertensive challenge.     

Sinoaortic denervation does not prevent differential Pavlovian conditioning of bradycardia in rabbits

Recent findings suggest that descending projections from the amygdaloid central nucleus (ACE) to the nucleus of the solitary tract (NTS) may modulate the baroreceptor reflex and thereby facilitate the expression of the bradycardiac conditioned response (CR) in rabbits. The purpose of the present study was to examine the role of the afferent limb of the baroreceptor reflex in differential Pavlovian conditioning of bradycardia in rabbits. Animals received either aortic denervation, sinoaortic denervation or sham denervation. After recovery from surgery, animals received one differential Pavlovian conditioning session per day over the next 6 days. Sinoaortic denervation abolished the baroreceptor reflex as assessed by intravenous injections of phenylephrine. In addition, sinoaortic denervation increased baseline heart rate (HR), altered the topography of the HR unconditioned response, but did not abolish the HR orienting response or prevent the acquisition of bradycardiac CRs. The findings of the present study suggest that afferent barosensory input is necessary for the expression of the HR CR in rabbits. However, descending ACE projections may still play a role in the HR CR by directly affecting NTS neurons.     

Neurotensin effects on N-type calcium currents among rat pallidal neurons: an electrophysiological and immunohistochemical study

The tridecapeptide neurotensin (NT) is involved in the modulation of dopamine (DA)-mediated functions in the nigrostriatal and mesocorticolimbic pathways. Its relevance in mammalian globus pallidus (GP) is questioned. A recent electrophysiological study on GP slices described NT-mediated robust membrane depolarization, depending upon the suppression of potassium conductance and/or the activation of cation current. Here, we have studied whether NT also affected high-voltage-activated calcium (Ca(2+)) currents, by means of whole-cell recordings on isolated GP neurons. In our hands, the full peptide and the segment NT8-13 reversibly inhibited N-like Ca(2+) current in about 60% of the recorded dissociated neurons, irrespective of their capacitance. The NT-mediated modulation showed no desensitization and was antagonized by the NT1 antagonists SR48692 and SR142948. These results imply an abundant expression of NTS(1) on GP cell somata. Then, we performed a light and immunofluorescence-confocal microscopy study of NTS(1) localization among GP neurons. We found that NTS(1) is localized in about 56% of GP neurons in both subpopulations of neurons, namely parvalbumin positive and negative. We conclude that NT, likely released from the striatal terminals in GP, acts through the postsynaptic NTS(1) preferentially localized in the lateral aspects of the GP. These data suggest a new implication (neither merely presynaptic nor simply "excitatory") for NT in the modulation of GP firing pattern. In addition, NT might have a role in affecting the interplay among the endogenous release of GABA/glutamate and DA. This hypothesis might have implications on both sensori-motor and associative functions of the GP and should be tested in DA-denervated disease models.     

Presynaptic adenosine A2a receptors on soma and central terminals of rat vagal afferent neurons

The dorsal vagal complex of the medulla oblongata is a key centre involved in the regulation of numerous autonomic functions, including cardiovascular control. Adenosine has been implicated as a potential neuromodulator of the baroreceptor reflex, and therefore the current study has investigated the presence and characteristics of adenosine receptors on rat vagal afferent neurons. In the nodose-vagal grease gap preparation, the adenosine A_2a agonist CGS-21680 evoked a depolarisation only in the presence of the selective adenosine A₁ antagonist PACPX. Autoradiography using [³H]NECA (4 nM) with suppression of A₁ binding enabled the first visualisation of high affinity adenosine A₂ receptors in the nucleus tractus solitarius (NTS). Unilateral nodose ganglionectomy resulted in over 90% reduction in binding in the lesioned (ipsilateral) NTS compared to a sham control. Furthermore, local administration of CGS-21680 increased evoked glutamate release in the NTS, as measured by in vivo microdialysis. These data suggest the presence of presynaptic adenosine A_2a receptors on both the soma and central terminals of rat vagal afferent neurons, and thereby support the hypothesis that adenosine may have a modulatory role in the baroreceptor reflex.