GLP-1 neurons form a local synaptic circuit within the rodent nucleus of the solitary tract

Glutamatergic neurons that express pre-proglucagon (PPG) and are immunopositive (+) for glucagon-like peptide-1 (i.e., GLP-1+ neurons) are located within the caudal nucleus of the solitary tract (cNTS) and medullary reticular formation in rats and mice. GLP-1 neurons give rise to an extensive central network in which GLP-1 receptor (GLP-1R) signaling suppresses food intake, attenuates rewarding, increases avoidance, and stimulates stress responses, partly via GLP-1R signaling within the cNTS. In mice, noradrenergic (A2) cNTS neurons express GLP-1R, whereas PPG neurons do not. In this study, confocal microscopy in rats confirmed that prolactin-releasing peptide (PrRP)+ A2 neurons are closely apposed by GLP-1+ axonal varicosities. Surprisingly, GLP-1+ appositions were also observed on dendrites of PPG/GLP-1+ neurons in both species, and electron microscopy in rats revealed that GLP-1+ boutons form asymmetric synaptic contacts with GLP-1+ dendrites. However, RNAscope confirmed that rat GLP-1 neurons do not express GLP-1R mRNA. Similarly, Ca2+ imaging of somatic and dendritic responses in mouse ex vivo slices confirmed that PPG neurons do not respond directly to GLP-1, and a mouse crossbreeding strategy revealed that <1% of PPG neurons co-express GLP-1R. Collectively, these data suggest that GLP-1R signaling pathways modulate the activity of PrRP+ A2 neurons, and also reveal a local "feed-forward" synaptic network among GLP-1 neurons that apparently does not use GLP-1R signaling. This local GLP-1 network may instead use glutamatergic signaling to facilitate dynamic and potentially selective recruitment of GLP-1 neural populations that shape behavioral and physiological responses to internal and external challenges.

Alterations in the Solitary Tract Nucleus of the Rat Following Perinatal Food Restriction and Subsequent Nutritional Rehabilitation

Newborn of altricial species maintain functional gustatory communication with the mother because the neural substrate and the capacity to discriminate and promote gustofacial responses are already operating. Because little is known about the effects of perinatal food restriction upon gustatory neuronal brain stem structures, we characterized neuronal Golgi-Cox alterations of the solitary tract rostral portion (NSTr) where gustatory information is known to convey in neonatal Wistar rats. Pre-and neonatally undernourished rats exhibited a general reduction in the number and extension of distal dendrites particularly in small neurons but little effect upon perikarya measurements of the NSTr neuronal population. By contrast, in nutritional and sensory rehabilitated rats the number of distal dendrites increased, although the dendritic extensions were less affected compared to perinatally underfed and control subjects. The data indicate that perinatal food restriction interferes with the NSTr dendritic arbor organization, while nutritional and sensorial rehabilitation given by normally lactating dams induced plastic changes presumably modifying the integrative processes underlying early taste discriminative capabilities. Moreover, since perinatal food restriction is a powerful stressor influence and the NST forms a part of a complex system underlying adaptive stress responses, the neuronal alterations observed here may be partly due to this noxious perinatal influence.

Interactions between orexin-immunoreactive fibers and adrenaline or noradrenaline-expressing neurons of the lower brainstem in rats and mice

Orexins are expressed in neurons of the dorsolateral hypothalamus and their axons widely distribute throughout the central nervous system. The noradrenergic cell groups of the lower brainstem belong to the targets of these orexin projections. Double immunostainings for orexin and phenylethanolamine N-methyltransferase (PNMT), as well as orexin and tyrosine hydroxylase (TH) were applied to demonstrate the orexinergic innervation of catecholamine cell groups in the lower brainstem of the mouse and the rat. In various densities, networks of orexin-positive fibers and terminals were present on neurons of each adrenaline (C1, C2, C3) and noradrenaline (locus coeruleus, A1, A2, A4, A5 and A7) cell groups. The most dense networks of orexin fibers and terminals were detected in the locus coeruleus, the subcoeruleus area, and in the nucleus of the solitary tract. By using confocal microscope to analyze triple immunostainings we could detect that about two-third of the orexin-PNMT or orexin-TH immunopositive close contacts contained synaptophysin (a presynapse-specific protein) in the C1, C2 and C3 adrenaline, or in the A1, A2 noradrenaline cell groups, respectively. Orexin-immunopositive axons in the C1, C2, as well as A1, A2 and A6 cell groups have been examined by an electron microscope. Relatively few asymmetrical (excitatory) synaptic contacts could be demonstrated between PNMT- or TH-positive dendrites and orexin terminals, although the vast majority of orexin-positive axons was located in juxtaposition to PNMT- or TH-positive neurons.

Variations in blood pressure and heart rate in conscious rats with cervical lymphatic blockade

The possible effects of cervical lymphatic blockade (CLB) on a series of parameters in conscious freely moving rats were analysed. Blood pressure (BP) and heart rate (HR) for conscious male Sprague-Dawley rats at 1, 3, 7, 11, 15 and 21 days after a CLB or a sham operation were monitored continuously for 24 hours with a computerized recording system. Since BP and HR were subjected to spontaneous variations, blood pressure variability (BPV) and heart rate variability (HRV) were expressed as the standard deviation of beat-to-beat BP and HR values. The baroreflex sensitivities (BRS) were determined by measuring the heart period (HP = 60,000/HR) prolongation in response to the elevation in BP induced by an intravenous administration of phenylephrine at 1, 7, 15 and 21 days after the CLB or sham operation. Compared with those in sham-operated rats, the values of systolic blood pressure (SBP), diastolic blood pressure (DBP), mean arterial blood pressure (MAP), HR and BRS in CLB rats were significantly lower, whereas the values of BPV and HRV were markedly raised in CLB rats at different time points. Furthermore, the impaired ultrastructure in the dorsomedial nucleus of the solitary tract (dmNTS) including degeneration, apoptosis and necrosis in neurons and gliacytes, were apparent from the 1st to 15th day but the changes were most significant at 7th day after CLB operation. Structural changes appeared to be closely related to functional changes of the dmNTS at each time point. Thus, in CLB conscious rats, a significant decline in blood pressure accompanied by dysfunction in its regulation might be due to the impaired structure in the dmNTS.

Colocalization of neurokinin-1, N-methyl-D-aspartate, and AMPA receptors on neurons of the rat nucleus tractus solitarii

Substance P (SP) and glutamate are implicated in cardiovascular regulation by the nucleus tractus solitarii (NTS). Our earlier studies suggest that SP, which acts at neurokinin 1 (NK1) receptors, is not a baroreflex transmitter while glutamate is. On the other hand, our recent studies showed that loss of NTS neurons expressing NK1 receptors leads to loss of baroreflex responses and increased blood pressure lability. Furthermore, studies have suggested that SP may interact with glutamate in the NTS. In this study, we sought to test the hypothesis that NK1 receptors colocalize with glutamate receptors, either N-methyl-d-aspartate (NMDA) receptors or AMPA receptors or both in the NTS. We performed double-label immunofluorescent staining for NK1 receptors and either N-methyl-d-aspartate receptor subunit 1 (NMDAR1) or AMPA specific glutamate receptor subunit 2 (GluR2) in the rat NTS. Because vesicular glutamate transporter 2 (VGLUT2) containing fibers are prominent in portions of the NTS where cardiovascular afferent fibers terminate, we also performed double-label immunofluorescent staining for NK1 receptors and VGLUT2. Confocal microscopic images showed that NK1 receptors-immunoreactivity (IR) and NMDAR1-IR colocalized in the same neurons in many NTS subnuclei. Almost all NTS neurons positive for NK1 receptor-IR also contained NMDAR1-IR, but only 53.4% to 74.8% of NMDAR1-IR positive neurons contained NK1 receptors-IR. NK1 receptor-IR and GluR2-IR also colocalized in many neurons in NTS subnuclei. A majority of NK1 receptor-IR positive NTS neurons also contained GluR2-IR, but only 45.8% to 73.9% of GluR2-IR positive NTS neurons contained NK1 receptors-IR. Our results also showed that fibers labeled for VGLUT2-IR were in close apposition to fibers and neurons labeled for NK1 receptor-IR. The data support our hypothesis, provide an anatomical framework for glutamate and SP interactions, and may explain the loss of baroreflexes when NTS neurons, which could respond to glutamate as well as SP, are killed.

Glial coverage of the small cell somata in the rat nucleus of tractus solitarius during postnatal development

Astrocytes are thought to be active participants in synaptic plasticity in the developing nervous system. Previous studies suggested that axosomatic synapses decreased in number on the small cells of the rat caudal nucleus of tractus solitarius (cNTS) toward the end of the first postnatal week. Astrocytes might be involved in this phenomenon. We examined the morphological development of astrocytic processes around the small cell soma in the rat cNTS using light and electron microscopy. Glial fibrillary acidic protein (GFAP), glutamate-aspartate transporter (GLAST), and glutamate transporter-1 (GLT-1)-positive structures within the cNTS became more intensely stained as development proceeded. GLAST-positive structures encompassed calbindin-positive small cell somata after postnatal day 10. Electron microscopic observations indicated that astrocytic processes encompass the small cell soma, while the number of axosomatic synapses decreases as development proceeds. The timing for glial coverage of the small cell soma appears to be consistent with the decrease in axosomatic synapses on the small cells. These observations imply that astrocytes may participate actively in regulating the decrease of axosomatic synapses on small cells in the cNTS during postnatal development.

Ultrastructure of primary afferent terminals and synapses in the rat nucleus of the solitary tract: comparison among the greater superficial petrosal, chorda tympani, and glossopharyngeal nerves

The greater superficial petrosal (GSP), chorda tympani (CT), and glossopharyngeal (IX) nerves terminate in overlapping patterns in the brainstem in the rat nucleus of the solitary tract (NTS). There is one region, in particular, that receives overlapping inputs from all three nerves and is especially plastic during normal and experimentally altered development. To provide the requisite data necessary ultimately to delineate the circuitry in this region, we characterized the morphology of the synaptic inputs provided by the GSP, CT, and IX nerves through transmission electron microscopy. Although all three nerves had features characteristic of excitatory nerve terminals, ultrastructural analysis revealed dimorphic morphologies differentiating IX terminals from GSP and CT terminals. IX terminals had a larger area than GSP and CT terminals, and more synapses were associated with IX terminals compared with GSP and CT terminals. Additionally, IX terminals formed synapses most often with spines, as opposed to GSP and CT terminals, which formed synapses more often with dendrites. IX terminals also exhibited morphological features often associated with synaptic plasticity more often than was seen for GSP and CT terminals. These normative data form the basis for future studies of developmentally and environmentally induced plasticity in the rodent brainstem.

Fos expression in the brainstem nuclei evoked by nasal air-jet stimulation in rats

BACKGROUND

Noxious stimulation of the nasal mucosa may induce protective reflexes in the upper airway in rats. Previously, we have reported that nasal air-jet stimulation increases the activities of the laryngeal muscles in decerebrate cats; however, the neuronal mechanism of this phenomenon still is not clarified.

METHODS

After the application of nasal air-jet stimulation for 2 hours, we investigated the distribution of Fos-positive cells (FPCs) throughout the medulla compared with sham-operated rats using Fos immunoreactivity.

RESULTS

FPCs in the spinal trigeminal nucleus, the parvocellular reticular nucleus, and the nucleus of the solitary tract were more frequent than the sham-operated rats.

CONCLUSION

These results suggest that the afferents induced by air-jet stimulation were conveyed to these FPCs and that some of these cells might participate in the augmentation of laryngeal muscle activities during nasal air-jet stimulation.

Subcellular distributions of adenosine A1 and A2A receptors in the rat dorsomedial nucleus of the solitary tract at the level of the area postrema

Adenosine A1 and A2A receptors mediate distinct cardiovascular components of defense reactions that are ascribed, in part, to opposing actions within the nucleus tractus solitarius. To assess the cellular sites of relevance to these actions, we examined the light and electron microscopic immunolabeling of adenosine A1 and A2A receptors in the rat dorsomedial nucleus of the solitary tract at the level of the area postrema (dmNTS-AP), a region crucial for cardiovascular regulation involving vagal baroreceptor afferents. Immunoreactivity for each receptor was independently localized to distinct segments of plasma membranes and endomembranes in somatodendritic, axonal, and glial profiles. The dendritic labeling for each receptor also was detected within and near asymmetric, excitatory-type synapses. Of all peroxidase labeled profiles exclusive of somata, approximately 58% were A1- and 39% were A2A-labeled dendrites. Dendrites and astrocytic glia were the profiles that most often expressed both subtypes of adenosine receptors. The axonal labeling for A2A receptors was seen mainly in unmyelinated axons, whereas the A1 receptors were prominently localized within axon terminals. These terminals often formed single or multisynaptic excitatory-type junctions or single symmetric synapses on dendrites, a few of which expressed A1 and A2A receptors. These results provide the first ultrastructural evidence that A1 and A2A receptors have distributions conductive to their dual involvement in modulating the output of single neurons and glial function in the dmNTS-AP, where the predominate presynaptic effects of adenosine are mediated through A1 receptors.

Cranial visceral afferent pathways through the nucleus of the solitary tract to caudal ventrolateral medulla or paraventricular hypothalamus: target-specific synaptic reliability and convergence patterns

Cranial visceral afferents activate central pathways that mediate systemic homeostatic processes. Afferent information arrives in the brainstem nucleus of the solitary tract (NTS) and is relayed to other CNS sites for integration into autonomic responses and complex behaviors. Little is known about the organization or nature of processing within NTS. We injected fluorescent retrograde tracers into two nuclei to identify neurons that project to sites involved in autonomic regulation: the caudal ventrolateral medulla (CVLM) or paraventricular nucleus of the hypothalamus (PVN). We found distinct differences in synaptic connections and performance in the afferent path through NTS to these neurons. Anatomical studies using confocal and electron microscopy found prominent, primary afferent synapses directly on somata and dendrites of CVLM-projecting NTS neurons identifying them as second-order neurons. In brainstem slices, afferent activation evoked large, constant latency EPSCs in CVLM-projecting NTS neurons that were consistent with the precise timing and rare failures of monosynaptic contacts on second-order neurons. In contrast, most PVN-projecting NTS neurons lacked direct afferent input and responded to afferent stimuli with highly variable, intermittently failing synaptic responses, indicating polysynaptic pathways to higher-order neurons. The afferent-evoked EPSCs in most PVN-projecting NTS neurons were smaller and unreliable but also often included multiple, convergent polysynaptic responses not observed in CVLM-projecting neurons. A few PVN-projecting NTS neurons had monosynaptic EPSC characteristics. Together, we found that cranial visceral afferent pathways are structured distinctly within NTS depending on the projection target. Such, intra-NTS pathway architecture will substantially impact performance of autonomic or neuroendocrine reflex arcs.

Pattern differentiation of excitatory and inhibitory synaptic inputs on distinct neuronal types in the rat caudal nucleus of the tractus solitarius

Region- and size-specific neuronal organizations of the caudal nucleus of the tractus solitarius (cNTS) were investigated, followed by analyses of excitatory and inhibitory synaptic input patterns onto specific cell types by patch clamp recordings and immunoelectron microscopy. Cell size distribution and numerical density of cNTS neurons were examined in subregions at levels of the area postrema. In the subpostremal and dorsomedial subnuclei, characterized by the presence of dense glutamatergic and sparse GABAergic somata, small calbindin neurons constituted 42% of the total cells. The medial subnucleus contained large numbers of glutamatergic, GABAergic, and catecholaminergic somata and large tyrosine hydroxylase-containing cells constituted 13% in this region. In total, small neurons (<150 microm2) represented about 80% of the cell population in the cNTS. Predominant excitatory postsynaptic currents were observed in the adult small neurons, while inhibitory postsynaptic currents were more evident in larger neurons, irrespective of subnuclear location. This distinct differentiation of postsynaptic current patterns was not evident in neonates. GABAergic synapses were more frequently associated with dendrites of large catecholaminergic cells (73%) than with those of small calbindin-containing cells (10%) in adults. These results indicate that differential synaptic input patterns were developmentally established in distinct small and large neurons.

Detection of angiotensin II mediated nitric oxide release within the nucleus of the solitary tract using electron-paramagnetic resonance (EPR) spectroscopy

We previously identified an action of nitric oxide (NO) within the nucleus tractus solitarii (NTS) that attenuates the cardiac component of the baroreceptor reflex. In the present study we have tested the hypothesis that angiotensin II (AngII), acting on angiotensin type 1 receptors (AT1R), can release NO within the NTS and that its actions are mediated by soluble guanylate cyclase (sGC). Utilising cryogenic electron paramagnetic resonance (EPR), we have detected NO release in brainstem samples following AngII, but not saline, microinjections into the NTS. In these experiments, we confirmed that both AngII and a NO donor (diethylamine NONOate) in the NTS both depressed the baroreflex bradycardia. In additional studies, we showed that the latter effects were both sensitive to blockade of sGC using 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ). To initiate studies to resolve the cellular source of NO released by angiotensin II in the NTS, we performed immunohistochemical/electron microscopy studies on the distribution of AT1R. We found AT1R located on NTS neurones and blood vessels. Since a rise in intracellular calcium [Ca]i levels is prerequisite for nNOS activation, we imaged responses in [Ca]i in NTS neurones during exposure to AngII in vitro using confocal microscopy. Our data indicate a paucity of neurones showing changes in [Ca]i when exposed to AngII (200 nM). We suggest that AngII-induced release of NO is from non-neuronal sites. With the presence of AT1R on blood vessel endothelial cells we propose that AngII released NO in the NTS is due to activation of endothelial nitric oxide synthase located within the endothelium. The present study supports the novel concept that AngII can trigger NO release in the NTS by a mechanism of vascular-neuronal signalling that affects central neuronal networks regulating cardiovascular function.

Vesicular glutamate transporters type 1 and 2 expression in axon terminals of the rat nucleus of the solitary tract

The nucleus of the solitary tract is the site of termination of primary afferent fibers running in the facial, glossopharyngeal and vagus nerves. The present study was performed to map the distribution of glutamatergic axons terminals in the rat nucleus of the solitary tract using immunodetection of vesicular glutamate transporter 1 and vesicular glutamate transporter 2. The two vesicular glutamate transporters were differentially distributed among nucleus of the solitary tract subdivisions. Vesicular glutamate transporter 1 immunoreactivity was mostly found in the lateral part of the nucleus (ventrolateral, interstitial and intermediate subdivisions) whereas vesicular glutamate transporter 2 labeling was distributed throughout the nucleus of the solitary tract. Electron microscope examination indicated that vesicular glutamate transporter immunoreactivity was localized in axon terminals filled with round synaptic vesicles. After injection of cholera toxin B subunit in sensory ganglia, anterograde labeling was found in vesicular glutamate transporter 1, as well as vesicular glutamate transporter 2-immunoreactive boutons. Double immunolabeling experiments allowed distinctions between terminals expressing either vesicular glutamate transporter 1 or vesicular glutamate transporter 2 or both vesicular glutamate transporter 1 and vesicular glutamate transporter 2 immunoreactivities. The latter population, expressing both transporters immunolabeling, completely disappeared after deafferentation induced by removal of sensory ganglia. This study indicates that vesicular glutamate transporter content identifies three different subpopulations of glutamatergic boutons in the nucleus of the solitary tract and provides definitive evidence that primary afferent neurons contribute glutamatergic terminals to the nucleus of the solitary tract.

Fine structural survey of the intermediate subnucleus of the nucleus tractus solitarii and its glossopharyngeal afferent terminals

The intermediate subnucleus of the nucleus tractus solitarii (imNTS) receives somatosensory inputs from the soft palate and pharynx, and projects onto the nucleus ambiguus, thus serving as a relay nucleus for swallowing. The ultrastructure and synaptology of the rat imNTS, and its glossopharyngeal afferent terminals, have been examined with cholera toxin-conjugated horseradish peroxidase (CT-HRP) as an anterograde tracer. The imNTS contained oval or ellipsoid-shaped, small to medium-sized neurons (18.2 x 11.4 microm) with little cytoplasm, few cell organelles and an irregularly shaped nucleus. The cytoplasm often contained one or two nucleolus-like stigmoid bodies. The average number of axosomatic terminals was 1.8 per profile. About 83% of them contained round vesicles and formed asymmetric synaptic contacts (Gray's type I), while about 17% contained pleomorphic vesicles and formed symmetric synaptic contacts (Gray's type II). The neuropil contained small or large axodendritic terminals, and about 92% of them were Gray's type I. When CT-HRP was injected into the nodose ganglion, many labeled terminals were found in the imNTS. All anterogradely labeled terminals contacted dendrites but not somata. The labeled terminals were usually large (2.69+/-0.09 mum) and exclusively of Gray's type I. They often contacted more than two dendrites, were covered with glial processes, and formed synaptic glomeruli. A small unlabeled terminal occasionally made an asymmetric synaptic contact with a large labeled terminal. The large glossopharyngeal afferent terminals and the neurons containing stigmoid bodies characterized the imNTS neurons that received pharyngeal afferents.

Angiotensin II AT-1A receptor immunolabeling in rat medial nucleus tractus solitarius neurons: subcellular targeting and relationships with catecholamines

The angiotensin II AT-1A receptor (AT-1A) is the major mediator of the hypertensive actions of angiotensin II (ANG II) in the medial nucleus of the solitary tract (mNTS). The localization of the AT-1A receptor at surface or intracellular sites is an important determinant of its signaling properties, including intercellular or intracrine communication. However, the spatial localization of this protein, particularly within small distal or intermediate size dendrites of mNTS neurons, is unknown. Within the mNTS, ANG II and catecholamines interact in the regulation of autonomic function; however, it is unknown if AT-1A receptors are present at functional sites in catecholamine containing dendrites, or are contacted by catecholamine containing axon terminals. We compared surface and intracellular distributions of the AT-1A receptor in dendritic processes from the mNTS using immunogold electron microscopy in conjunction with immunoperoxidase labeling for tyrosine hydroxylase (TH) and morphometric analysis. Collapsed across all AT-1A-labeled dendritic profiles, immunogold labeling was more frequent in intracellular sites as compared with the plasma membrane. Small (<0.6 microm) dendritic profiles contained a higher ratio of particles associated with the surface membrane when compared with larger profiles. Approximately 27% of all AT-1A receptor-labeled dendritic profiles also contained labeling for TH. Approximately 12% of dendritic profiles single labeled for the AT-1A receptor were contacted by TH containing axons or axon terminals. The present results provide the first quantitative demonstration of select plasmalemmal and intracellular localizations of AT-1A receptors in dendritic processes of mNTS neurons, including those containing TH, or contacted by catecholaminergic axon terminals. These results suggest that AT-1A receptors are positioned for modulation of catecholamine signaling in the mNTS.

Vagal stimulation modifies parameters of heterochromatin in the nuclei of vagosolitary complex neurons of medulla oblongata in rats

New data were obtained on modification of heterochromatin parameters in the nuclei of medulla oblongata neurons in Wistar rats after stimulation of the vagus nerve: decrease in the area of heterochromatin regions and redistribution of chromocenters within the neuronal nuclear system. It was concluded that realization of the viscero-visceral reflex is associated with rearrangement of chromatin in neurons involved in transmission of the corresponding information.

Mu-opioid receptor is present in dendritic targets of Endomorphin-2 axon terminals in the nuclei of the solitary tract

Endomorphins represent a group of endogenous opioid peptides with high affinity for the mu-opioid receptor. In the brainstem, Endomorphin-2 is found in trigeminal dorsal horn and the nuclei of the solitary tract, suggesting its presence in both nociceptive and visceral primary afferents. If Endomorphin-2 were an endogenous ligand for the mu-opioid receptor, we would expect to find the receptor at cellular sites in close association with the peptide. We used dual-labeling immunocytochemistry combined with electron microscopy to examine interactions between Endomorphin-2-immunoreactive and mu-opioid receptor-immunoreactive profiles within the nuclei of the solitary tract in the rat. Endomorphin-2-immunoreactivity was found primarily in unmyelinated axons and axon terminals in nuclei of the solitary tract and the majority of these terminals contained dense core vesicles. Endomorphin-2-immunoreactive axon terminals often formed asymmetric synapses with dendritic spines lacking mu-opioid receptor-immunoreactivity, but mu-opioid receptor-immunoreactivity was found in many of the larger dendritic targets of Endomorphin-2-immunoreactive terminals. Thus, mu-opioid receptor-immunoreactivity was found in the postsynaptic targets of Endomorphin-2-immunoreactive axon terminals, consistent with the hypothesis that Endomorphin-2 is an endogenous ligand for this receptor within the nuclei of the solitary tract. A small number of Endomorphin-2-immunoreactive somata, dendrites, and axon terminals also contained mu-opioid receptor-immunoreactivity. Cells that contain both the opioid peptide and its receptor may be a substrate for potential autoregulation of nuclei of the solitary tract neurons by opioid ligands. Finally, using tract tracing and confocal microscopy, we found Endomorphin-2-immunoreactivity in a subset of vagal afferents. Together these findings support the hypothesis that Endomorphin-2 is a ligand for the mu-opioid receptor within nuclei of the solitary tract and that the peptide is at least partially derived from primary visceral afferents.

Decreased plasma membrane targeting of NMDA-NR1 receptor subunit in dendrites of medial nucleus tractus solitarius neurons in rats self-administering morphine

Opioid abuse is associated with repeated administration and escalation of dose that can result in profound adaptations in homeostatic processes. Potential cellular mechanisms and neural sites mediating opiate-dependent adaptations may involve NMDA-dependent synaptic plasticity within brain areas participating in behaviors related to consumption of natural reinforcers, as well as affective-autonomic integration, notably the medial nucleus tractus solitarius (mNTS). NMDA-dependent synaptic plasticity may be mediated by changes in the intracellular and surface targeting of NMDA receptors, particularly in postsynaptic sites including spines or small distal dendrites. High-resolution immunogold electron microscopic immunocytochemistry combined with morphometry were used to measure changes in targeting of the NMDA-NR1 (NR1) receptor subunit between intracellular and plasmalemmal sites in dendrites of neurons of the intermediate mNTS of rats self-administering escalating doses of morphine (EMSA). In control and EMSA rats, the density of plasmalemmal and cytosolic gold particles was inversely related to profile size. Collapsed across all NR1-labeled dendrites, rats self-administering morphine had a lower number of plasmalemmal gold particles per unit surface area (7.1 +/- 0.8 vs. 14.4 +/- 1 per 100 microm), but had a higher number of intracellular gold particles per unit cross-sectional area (169 +/- 6.1 vs. 148 +/- 5.1 per 100 microm2) compared to saline self-administering rats. Morphometric analysis showed that the decrease in plasma membrane labeling of NR1 was most robust in small dendritic profiles (<1 microm), where there was a reciprocal increase in the density of intracellular particles. These results indicate that the plasmalemmal distribution of the essential NR1 subunits in distal sites may prominently contribute to NMDA receptor-dependent modulation of neural circuitry regulating homeostatic processes, and targeting of these proteins can be prominently affected by morphine self-administration.

A novel central pathway links arterial baroreceptors and pontine parasympathetic neurons in cerebrovascular control

1. We tested the hypothesis that arterial baroreceptor reflexes modulate cerebrovascular tone through a pathway that connects the cardiovascular nucleus tractus solitarii with parasympathetic preganglionic neurons in the pons. 2. Anesthetized rats were used in all studies. Laser flowmetry was used to measure cerebral blood flow. We assessed cerebrovascular responses to increases in arterial blood pressure in animals with lesions of baroreceptor nerves, the nucleus tractus solitarii itself, the pontine preganglionic parasympathetic neurons, or the parasympathetic ganglionic nerves to the cerebral vessels. Similar assessments were made in animals after blockade of synthesis of nitric oxide, which is released by the parasympathetic nerves from the pterygopalatine ganglia. Finally the effects on cerebral blood flow of glutamate stimulation of pontine preganglionic parasympathetic neurons were evaluated. 3. We found that lesions at any one of the sites in the putative pathway or interruption of nitric oxide synthesis led to prolongation of autoregulation as mean arterial pressure was increased to levels as high as 200 mmHg. Conversely, stimulation of pontine parasympathetic preganglionic neurons led to cerebral vasodilatation. The second series of studies utilized classic anatomical tracing methods to determine at the light and electron microscopic level whether neurons in the cardiovascular nucleus tractus solitarii, the site of termination of baroreceptor afferents, projected to the pontine preganglionic neurons. Fibers were traced with anterograde tracer from the nucleus tractus solitarii to the pons and with retrograde tracer from the pons to the nucleus tractus solitarii. Using double labeling techniques we further studied synapses made between labeled projections from the nucleus tractus solitarii and preganglionic neurons that were themselves labeled with retrograde tracer placed into the pterygopalatine ganglion. 4. These anatomical studies showed that the nucleus tractus solitarii directly projects to pontine preganglionic neurons and makes asymmetric, seemingly excitatory, synapses with those neurons. These studies provide strong evidence that arterial baroreceptors may modulate cerebral blood flow through direct connections with pontine parasympathetic neurons. Further study is needed to clarify the role this pathway plays in integrative physiology.

Receptor targeting in medullary nuclei mediating baroreceptor reflexes

The sympathoinhibitory component of the baroreceptor reflex prominently involves glutamatergic visceral afferents terminating in the nuclei of the solitary tract (NTS) and C1 adrenergic neurons of the rostral ventrolateral medulla (RVLM). As reviewed, we have used electron microscopic immunocytochemical dual labeling in these regions to precisely analyze (1) the cellular sites for synergistic or opposing responses attributed to activation of different receptor subtypes on single neurons and (2) interactions involving monoaminergic neurons identified by their content of neurotransmitter synthesizing enzymes, vesicular monoamine transporter, and frequent coexpression of endogenous opioid peptides. The summarized results provide important cellular substrates for N-methyl-D-aspartate (NMDA)-mediated glutamatergic transmission and activation of either serotoninergic (5-HT2A), adrenergic (alpha 2A), or mu- or delta opioid receptors within the baroreceptor reflex circuit.

Serotonin transporters (SERTs) within the rat nucleus of the solitary tract: subcellular distribution and relation to 5HT2A receptors

Serotonergic transmission is terminated by serotonin transporter (SERT)-mediated uptake following activation of serotonin receptors, several subtypes of which are present in the medial nucleus of the solitary tract (mNTS) at the area postrema level. In this region, serotonin (5HT) is a major modulator of the baroreceptor reflex and also affects gastric motility. This serotonin is derived from multiple sources including local neurons and inputs from raphe and visceral vagal afferents. To determine the relevant functional sites for serotonin uptake in the mNTS, we examined the electron microscopic localization of SERTs using both immunoperoxidase and immunogold labeling in rat brain. In addition, we combined these methods for dual labeling of SERTs and 5HT2A receptors to detect whether the SERT in this region was located near or at a distance from the sites of activation of these G-protein coupled receptors. Intensive SERT immunolabeling was seen on plasma membranes of axons and morphologically heterogeneous axon terminals that formed symmetric or asymmetric synapses on dendrites without detectable 5HT2A immunoreactivity (IR). 5HT2A-IR was, however, located in other nearby neuronal and glial profiles, some of which apposed intensively SERT-labeled terminals or terminals containing lower intensity of SERT immunolabeling. In somatodendritic profiles, co-expression of SERT and 5HT2A receptor immunolabeling was seen near synapses and Golgi lamellae. Our results suggest that in the mNTS 5HT activates 5HT2A receptors at a distance from SERT-mediated uptake sites in diverse cell types including some that express both 5HT2A receptors and SERTs.

Neurokinin-1 receptors in the rat nucleus tractus solitarius: pre- and postsynaptic modulation of glutamate and GABA release

Neurokinins such as substance P and neurokinin A have long been thought to act as neurotransmitters or modulators in the nucleus tractus solitarius. However, the role and location of the receptors for these peptides have remained unclear. We examined the consequences of activation of the neurokinin-1 (NK1) receptor subtype in the rat nucleus tractus solitarius using whole-cell patch clamp recordings in brain slices. Application of delta-Ala-Phe-Phe-Pro-MeLeu-D-Pro[spiro-gamma-lactam]-Leu-Trp-NH2 (a specific NK1 agonist) or neurokinin A resulted in depolarization, evident as a slow inward current, mediated by direct postsynaptic NK1 receptor activation. The effect was conserved in the presence of tetrodotoxin, and protein kinase C-dependent since it was blocked by 2-[1-(3-dimethylaminopropyl)indol-3-yl]-3-(indol-3-yl)maleimide, a specific protein kinase C inhibitor. In addition, an increase in the frequency and amplitude of spontaneous excitatory postsynaptic currents was observed, reflecting increased glutamate release induced by NK1 receptor activation. This effect was abolished by tetrodotoxin, suggesting that it resulted from increased firing in afferent neurons, subsequent to somatodendritic excitation via NK1 receptors. Furthermore, spontaneous inhibitory postsynaptic currents were increased in frequency and amplitude showing that GABA release was promoted by NK1 receptor activation. However, amplitude of miniature inhibitory postsynaptic currents was unaltered by NK1 receptor activation, but the increase in frequency persisted. These findings suggest that NK1 receptors are located on presynaptic terminals as well as at somatodendritic sites of GABAergic neurons. The increase in GABA release was also shown to be protein kinase C-dependent. The data presented here show NK1 receptors in the rat nucleus tractus solitarius are present both excitatory and inhibitory neurons. Activation of these receptors can result in increases in release of both GABA and glutamate, suggesting a crucial modulatory role for NK1 receptors in the rat nucleus tractus solitarius.

[Changes in the ultrastructure of the rat bulbar nuclei caused by kainic acid administration]

The effect of kainic acid (KA) injection into the commissural area of nucleus tractus solitarii (NTS) of rats was studied to evaluate its influence on the ultrastructure of neurons and gliocytes at the site of injection and in the ventrolateral medulla (VM). 30 min after KA injection, in the perikarya of neurons in rostral and caudal NTS and VM the increased number of ribosomes, lysosomes, lipofuscin granules and edema of mitochondria were noted. Giant mitochondria were found in terminals and axons of myelin fibers. 14 days after KA injection, the increased volume of Golgi complex cisterns was found in single neurons in NTS rostral parts, while the neurons of caudal NTS demonstrated pronounced degenerative and destructive changes. During this period, the volume of mitochondria and of vacuoles was increased in dendrites within caudal NTS and VM, where degenerating processes of the nervous cells were found. In gliocytes of caudal NTS, the hypertrophy of cytoplasm was noted with the accumulation of dilated cisterns of endoplasmic reticulum and redistribution of nuclear chromatin. In the same areas of NTS the reaction of endotheliocytes and pericytes, perivascular edema and accumulation of microgliocytes were found. The results obtained indicate the reactive changes in VM in response to intracentral injection of neurotoxin.

Subcellular localization of neuronal nitric oxide synthase in the rat nucleus of the solitary tract in relation to vagal afferent inputs

In the nucleus of the solitary tract (NTS), nitric oxide (NO) modulates neuronal circuits controlling autonomic functions. A proposed source of this NO is via nitric oxide synthase (NOS) present in vagal afferent fibre terminals, which convey visceral afferent information to the NTS. Here, we first determined with electron microscopy that neuronal NOS (nNOS) is present in both presynaptic and postsynaptic structures in the NTS. To examine the relationship of nNOS to vagal afferent fibres the anterograde tracer biotinylated dextran amine was injected into the nodose ganglion and detected in brainstem sections using peroxidase-based methods. nNOS was subsequently visualised using a pre-embedding immunogold procedure. Ultrastructural examination revealed nNOS immunoreactivity in dendrites receiving vagal afferent input. However, although nNOS-immunoreactive terminals were frequently evident in the NTS, none were vagal afferent in origin. Dual immunofluorescence also confirmed lack of co-localisation. Nevertheless, nNOS immunoreactivity was observed in vagal afferent neurone cell bodies of the nodose ganglion. To determine if these labelled cells in the nodose ganglion were indeed vagal afferent neurones nodose ganglion sections were immunostained following application of cholera toxin B subunit to the heart. Whilst some cardiac-innervating neurones were also nNOS immunoreactive, nNOS was never detected in the central terminals of these neurones. These data show that nNOS is present in the NTS in both pre- and postsynaptic structures. However, these presynaptic structures are unlikely to be of vagal afferent origin. The lack of nNOS in vagal afferent terminals in the NTS, yet the presence in some vagal afferent cell bodies, suggests it is selectively targeted to specific regions of the same neurones.

Ultrastructure of the central subnucleus of the nucleus tractus solitarii and the esophageal afferent terminals in the rat

The central subnucleus of the nucleus tractus solitarii (ceNTS) receives afferent projections from the esophageal wall and projects to the nucleus ambiguus, thus serving as a relay nucleus for peristalsis of the esophagus. Here we examine the synaptic organization of the ceNTS, and its esophageal afferents by using transganglionic anterograde transport of cholera toxin-conjugated horseradish peroxidase (CT-HRP). When CT-HRP was injected into the subdiaphragmatic esophagus, many anterogradely labeled terminals were found only in the ceNTS. The ceNTS was composed of round or oval-shaped, small neurons (14.7x8.7 micro m) containing sparse organelles and an irregularly shaped nucleus. The average number of axosomatic terminals was only 1.3 per section cut through the nucleolus. Most of them (92%) contained round vesicles and formed asymmetric synaptic contacts (Gray's type I), and a few (8%) contained pleomorphic vesicles and formed symmetric synaptic contacts (Gray's type II). All anterogradely labeled terminals contacted dendrites but not the neuronal somata. The labeled terminals were large (2.55+/-0.07 micro m) and exclusively Gray's type I. More than half of them (60%) contacted small dendrites (less than 1 micro m in diameter), and contained dense-cored vesicles. More than 40% of the labeled terminals contacted two to four dendrites, thus forming a synaptic glomerulus. Sometimes a labeled terminal that contacted an unlabeled terminal by an adherent junction was found within the glomerulus. The large terminals and these complex synaptic relations appeared to characterize the esophageal afferent projections in the ceNTS.

Ultrastructural localization of serotonin 2A and N-methyl-D-aspartate receptors in somata and dendrites of single neurons within rat dorsal motor nucleus of the vagus

Both glutamate and serotonin are potent modulators of autonomic functions involving the nucleus of the solitary tract (NTS) and the dorsal motor nucleus of the vagus (DMNV) at the level of the area postrema. Moreover, many of the dendrites in this NTS region express both N-methyl-D-aspartate (NMDA) and serotonin (5HT) 2A receptors, and some of these dendrites may arise from the adjacent DMNV. Thus, single neurons in DMNV may also express both receptors. To test this hypothesis, we used electron microscopic immunocytochemistry for dual localization of the essential R1 subunit of the NMDA receptor (NR1) and the 5HT2A receptor in rat intermediate DMNV, a region serving mainly gastrointestinal functions. Gold particles representing NR1 and peroxidase reaction product for 5HT2A receptors were seen in the cytoplasm, as well as on distinct segments of the plasma membrane of many dendrites. Of the NR1-labeled dendrites, 31% (254/814) also contained 5HT2A immunoreactivity; among the 5HT2A-labeled dendrites, 52% (254/485) expressed NR1. The 5HT2A labeling was also present in numerous small unmyelinated axons, axon terminals, and glial processes. These profiles were largely without NR1 immunoreactivity, although NR1 was detected in some of the dendrites postsynaptic to 5HT2A-labeled terminals. Our results suggest that calcium entry through NMDA channels and 5HT2A receptor activation may dramatically affect postsynaptic excitability of single neurons in the DMNV. In addition, the findings also indicate that the 5HT2A receptor is strategically positioned for involvement in modulation of the presynaptic release of neurotransmitters affecting the postsynaptic activity of DMNV neurons responsive to NMDA activation.

Co-localization of AMPA receptor subunits in the nucleus of the solitary tract in the rat

AMPA-type glutamate receptors in the caudal portions of nucleus of the solitary tract (NTS) are critical for responses to excitatory afferents from the viscera, including baroreceptors. Using immunocytochemistry combined with electron microscopy, the cellular distributions of different AMPA receptor subunits in the caudal NTS were found to be distinct. GluR2/3 was found at pre- and postsynaptic sites, and in astrocytic glia; while GluR1 was found primarily in small dendrites and spines. In dual-labeling studies, GluR1 and GluR2 were co-localized in large dendrites, but GluR1 was more often found alone in dendritic spines. Therefore, single neurons in the NTS contain both subunits, but there is differential trafficking of GluR1 to potential sites for synaptic plasticity.

Alpha-2A-adrenergic receptors are present on neurons in the central nucleus of the amygdala that project to the dorsal vagal complex in the rat

The descending pathway between the central nucleus of the amygdala (CeA) and the dorsal vagal complex (DVC) is an important substrate for autonomic functions associated with emotion. Activity in this circuit is crucially modulated by catecholamines and agonists of the alpha-2A-adrenergic receptor (alpha(2A)-AR), which relieve cardiovascular and gastrointestinal symptoms associated with experience of aversive stimuli. The subcellular distribution of alpha(2A)-AR within the CeA, however, has not been characterized. It is also not known if any alpha(2A)-AR-expressing neurons in the CeA project to the dorsal vagal complex. In order to address these questions, we examined the immunocytochemical labeling of alpha(2A)-AR in the CeA of rats receiving microinjection of the retrograde tracer fluorogold (FG) into the dorsal vagal complex at the level of the area postrema, an area involved in cardiorespiratory and gastrointestinal functions. Of all alpha(2A)-AR-labeled profiles in the CeA, the majority were either dendrites (42%) or somata (24%). alpha(2A)-AR labeling was often present on the plasmalemma in dendrites and was mainly found in endosome-like organelles in somata. Of all alpha(2A)-AR immunoreactive somata, 62% also contained immunolabeling for FG and 23% of all dendrites also showed labeling for the retrograde tracer. The intracellular distribution of alpha(2A)-AR did not differ in somata or dendrites with or without detectable FG. The remaining singly labeled alpha(2A)-AR profiles consisted of axons (11%), axon terminals (12%), and glial processes (13%). In numerous instances, alpha(2A)-AR-labeled glia or axon terminals were apposed to DVC projecting neurons. Together, this evidence suggests that the principal site for alpha(2A)-AR activation is at extrasynaptic sites on dendrites of CeA neurons, many of which project to the DVC and also show endosomal receptor labeling. In addition, these results indicate that activation of alpha(2A)-AR in the CeA may influence the activity of DVC projecting neurons through indirect mechanisms, including changes in presynaptic transmitter release or glial function. These results suggest that alpha(2A)-AR agonists in the CeA may modulate numerous processes including stress-evoked autonomic reactions and feeding behavior.

Direct projections from the cardiovascular nucleus tractus solitarii to pontine preganglionic parasympathetic neurons: a link to cerebrovascular regulation

Peripheral or central interruption of the baroreflex or the parasympathetic innervation of cerebral vessels leads to similar changes in regulation of cerebral blood flow. Therefore, we sought to test the hypothesis that the cardiovascular nucleus tractus solitarii, the site of termination of arterial baroreceptor nerves, projects to pontine preganglionic neurons whose stimulation elicits cerebral vasodilatation. The current study utilized both light and electron microscopic techniques to analyze anterograde tracing from the cardiovascular nucleus tractus solitarii to preganglionic parasympathetic neurons in the pons. We further used retrograde tracing from that same pontine region to the cardiovascular nucleus tractus solitarii and evaluated the confluence of tracing from the cardiovascular nucleus tractus solitarii to pontine preganglionic neurons labeled retrogradely from the pterygopalatine ganglia. The cardiovascular nucleus tractus solitarii projected to pontine preganglionic parasympathetic neurons, but more rostral and caudal regions of nucleus tractus solitarii did not. In contrast, all three regions of nucleus tractus solitarii projected to the nucleus ambiguus and dorsal motor nucleus of the vagus. Although not projecting to pontine preganglionic parasympathetic neurons, regions lateral, rostral, and caudal to cardiovascular nucleus tractus solitarii sent projections through the pons medial to the preganglionics. The study establishes the presence of a direct monosynaptic pathway from neurons in the cardiovascular nucleus tractus solitarii to pontine preganglionic parasympathetic neurons that project to the pterygopalatine ganglia, the source of nitroxidergic vasodilatory innervation of cerebral blood vessels. It provides evidence that activation of those preganglionic neurons can cause cerebral vasodilatation and increased cerebral blood flow. Finally, it demonstrates differential innervation of medullary and pontine preganglionic parasympathetic neurons by different regions of the nucleus tractus solitarii.

Successive episodes of synapses production in the developing rat nucleus tractus solitarii

In the rat nucleus tractus solitarii (NTS), synaptogenesis is thought to occur both pre- and postnatally. The present study was performed to precisely define the timetable of synapse formation in the NTS after birth. Changes in synapse morphology and densities were analyzed between postnatal day 3 (P3) and P28 using electron microscopy and ethanol phosphotungstic acid (E-PTA) staining. The proportion of morphologically immature synapses was high at P3 (38%) and P14 (30%) and low (8-14%) at the other ages investigated (P7, P21, and P28). Synaptic density significantly increased between P7 and P14 (60%) and between P21 and P28 (54%), but did not significantly change between P3 and P7 and between P14 and P21. Mean synaptic diameter also increased over the first postnatal month. Significant increases in synaptic size occurred between P3 and P7 (28%) and between P14 and P21 (15%). The present data indicate that, in the NTS, synaptogenesis occurs over a protracted period of time and involves distinct successive episodes of synapse production.

Evidence for peptide co-transmission in retrograde- and anterograde-labelled central nucleus of amygdala neurones projecting to NTS

Synaptic terminals in the nucleus of the solitary tract (NTS) from axons originating in the central nucleus of the amygdala (CeA) are known to contain gamma-aminobutyric acid (GABA) immunoreactivity. Here, we have investigated whether such projections contain neuropeptides as putative co-transmitters. Somata in the medial and lateral CeA that were retrogradely labelled with cholera toxin B (CTb) injected into the commissural NTS were found to be immunoreactive for GABA, somatostatin (SOM), neurotensin (NT), vasoactive intestinal polypeptide (VIP) and nitric oxide synthase (NOS). Subpopulations of fibres in the NTS that were anterogradely labelled with biotin dextran amine (BDA) injected into the CeA and examined using both fluorescence and electron microscopy appeared to colocalise somatostatin, but not other neuropeptides. Their varicosities were observed in proximity to NTS neurones that were immunoreactive for the somatostatin receptor sst2A subtype, substance P (SP) NK1 receptor, and the GABAA receptor alpha3, beta1 and gamma2 subunits. This morphological evidence is consistent with the possibility of GABA-somatostatin co-transmission at synapses of some of the CeA projection neurones to NTS that might inhibit cardiovascular reflex responses in response to fear or emotion-related stimuli.

[Ultrastructure of neurons and glial cells in the solitary tract nucleus and in the ventrolateral sections of the rat medulla oblongata after subdiaphragmatic vagotomy]

Using electron microscopy, complex remodelling of neuronal and glial cell ultrastructure was demonstrated in the solitary tract nucleus and ventrolateral medulla at different time intervals following bilateral subdiaphragmatic truncal vagotomy. Four weeks after vagotomy phagocytizing microgliocytes were demonstrated in the vicinity of nuclei of degenerating neurons within the rostral, but not the caudal, areas of solitary tract nucleus and ventrolateral medulla. It is suggested that the ultrastructural changes observed in the medulla after vagotomy depend on the distribution pattern of vagal projections to the caudal brainstem.

Somatostatin immunoreactivity in axon terminals in rat nucleus tractus solitarii arising from central nucleus of amygdala: coexistence with GABA and postsynaptic expression of sst2A receptor

Axon terminals synapsing on neurones in the nucleus tractus solitarii (NTS) that originate from the central nucleus of the amygdala (CeA) have been shown to contain gamma-aminobutyric acid (GABA) immunoreactivity. Here we investigated whether such terminals also contain somatostatin (SOM), a neuropeptide found in axons distributed throughout the NTS and in somata in the CeA, and known to modulate cardiovascular reflexes when microinjected into the NTS. With fluorescence microscopy, SOM immunoreactivity was seen in the varicosities of some axons throughout the NTS that were anterogradely labelled with biotin dextran amine injected into the CeA. Such varicosities were frequently observed in close proximity to dendrites of NTS neurones that were immunoreactive for the SOM receptor sst(2A) subtype, and in many cases also for catecholamine synthesising enzymes. In the caudal, cardioregulatory zone of NTS, SOM immunoreactivity was localised by electron microscopic pre-embedding gold labelling to boutons containing dense-cored and clear pleomorphic vesicles and forming symmetrical synapses, mostly onto dendrites. Additional post-embedding gold labelling for GABA suggested that a subpopulation (29%) of GABAergic terminals sampled in this area of NTS contained SOM. Almost all boutons anterogradely labelled from the amygdala were GABA-immunoreactive (-IR) and 21% of these were SOM-IR. A similar proportion of these boutons (22%) formed synapses onto dendrites containing immunoreactivity for the SOM receptor sst(2A) subtype. These observations provide evidence that some of the GABAergic projection neurones in the CeA that inhibit baroreceptor reflex responses in the NTS in response to fear or emotional stimuli could release SOM, which might modulate the activity of NTS neurones via an action on sst(2A) receptors.

Differential distribution of 5HT2A and NMDA receptors in single cells within the rat medial nucleus of the solitary tract

Activation of serotonin (5-hydroxytryptamine; 5HT) receptors of the 2A subtype (5HT2A) in the intermediate portion of the medial nucleus tractus solitarius (mNTS) produces marked hypotension and bradycardia. This portion of the mNTS receives major input from glutamatergic baroreceptor afferents. Thus, the cardiorespiratory effects of 5HT2A agonists may be attributed, in part, to interactions involving the glutamatergic target neurons, some of which express N-methyl-D-aspartate (NMDA) glutamate receptors. To determine the functional sites for activation of 5HT2A receptors and their relationship to NMDA receptors in this region, we used electron microscopic immunocytochemistry for the localization of antipeptide antisera selectively recognizing each receptor protein in the intermediate mNTS in rat brain. Of 1,052 5HT2A-labeled profiles, 38% were dendrites and dendritic spines, 27% were unmyelinated axons, 14% were axon terminals, and 11% were glial processes. These 5HT2A-labeled profiles frequently contained NR1 gold particles with dendrites comprising 68% of the total dual-labeled profiles. In dendrites, the 5HT2A immunoreactivity was localized to cytoplasmic organelles or discretely distributed on synaptic or extrasynaptic segments of the plasma membrane. In contrast, NR1 immunoreactivity was prominently localized to postsynaptic junctions and these were distinct from the 5HT2A receptor labeling when coexpressed in the same dendrites. Dendrites containing both receptors composed 56% (224/399) of the total 5HT2A-labeled dendrites and 34% (224/659) of the total NR1-labeled dendrites. Our results provide the first ultrastructural evidence that in the intermediate mNTS, 5HT2A receptor agonists may affect the postsynaptic excitability of many of the same neurons that show NMDA-evoked responses to glutamate.

Alpha(2A)-adrenergic receptors are present in mu-opioid receptor containing neurons in rat medial nucleus tractus solitarius

Agonists of the alpha-2A-adrenergic- (alpha(2A)-AR) and the mu-opioid-receptor (muOR) jointly affect autonomic functions that are also disregulated in animals undergoing withdrawal from chronic administration of the muOR agonist morphine. Cardiovascular and gastrointestinal reflexes are mediated, in part, by the medial nucleus of the solitary tract (mNTS) at caudal (cNTS) and intermediate (iNTS) subregions. Together, this evidence suggests that alpha(2A)-AR and muOR may be colocalized within many of the same neuronal profiles in both the intermediate and caudal mNTS. In order to examine whether alpha(2A)-AR and muOR are present within common somata, dendrites, or axon terminals in the mNTS, we used electron microscopic immunocytochemistry for the detection of antisera against each receptor at intermediate and caudal levels of this brain region. Most of the dually labeled profiles were somata and dendrites. Of all dual-labeled profiles in the iNTS 49% were somata and were 47% dendrites, whereas in the cNTS 61% were somata and 32% were dendrites. Within dual-labeled profiles, the intracellular distribution of alpha(2A)-AR and muOR differed. MuOR was more frequently associated with the plasmalemma, whereas alpha(2A)-AR was often affiliated with vesicular organelles. Few axon terminals, and even fewer glia, contained both markers. We also frequently observed single-labeled alpha(2A)-AR glia that apposed exclusively muOR-containing dendrites or axon terminals. These findings indicate that somata and dendrites contain functional sites for convergent muOR and alpha(2A)-AR activation. In addition, each receptor is positioned for involvement in intercellular signaling between apposed neurons and glia. Activation of alpha(2A)-AR on muOR-containing somata or dendrites, or on glia apposed to muOR-containing neurons, may help to account for the efficacy of alpha(2A)-AR agonists in relieving some of the autonomic symptoms of opiate withdrawal.

Association of extracellular acetylcholinesterase with gustatory nerve terminal fibers in the nucleus of the solitary tract

Acetylcholinesterase (AChE) staining is associated with terminal fields of the glossopharyngeal and chorda tympani nerves in the nucleus of the solitary tract (NST). To address AChE function at these sites, the location of the staining was examined at the fine structural level in combination with the labeling of chorda tympani nerve fibers with biotinylated dextran in golden Syrian hamsters. AChE staining was located in the endoplasmic reticulum of geniculate ganglion neuronal somata, and extracellularly, surrounding labeled chorda tympani terminal fibers and boutons in the NST. Neuronal profiles adjacent to these labeled fibers were stained less intensely, whereas most non-adjacent profiles were unstained. The location of staining is consistent with the secretion of AChE into the extracellular space by primary afferent chorda tympani fibers. AChE staining was reduced in the dextran-labeled chorda tympani fibers and terminals as well as adjacent non-labeled profiles 2 weeks following nerve transection and dextran application. The distribution of staining outside synapses and the loss of staining following denervation is suggestive of a non-cholinergic role for AChE in the intact gustatory system.

Comparative fine structural distribution of endopeptidase 24.15 (EC3.4.24.15) and 24.16 (EC3.4.24.16) in rat brain

Endopeptidase 24.15 (EP24.15) and 24.16 (EP24.16) are closely related metalloendopeptidases implicated in the metabolism of several neuropeptides and widely expressed in mammalian brain. To gain insight into the functional role of these two enzymes in the central nervous system, we examined their cellular and subcellular distribution in rat brain by using electron microscopic immunogold labeling. In all areas examined, EP24.15 and EP24.16 immunoreactivity were observed in selective subpopulations of neuronal and glial cells. Subcellular localization of EP24.15 in neurons revealed that this enzyme was predominantly concentrated in the nucleus, whereas EP24.16 was almost exclusively cytoplasmic. The amount of EP24.15 found in the nucleus was inversely correlated with that found in the cytoplasm, suggesting that the enzyme could be mobilized from one compartment to the other. Within the cytoplasm, EP24.15 and EP24.16 immunoreactivity showed comparable distributional patterns. Both enzymes were detected throughout perikarya and dendrites, as well as within axons and axon terminals. In all neuronal compartments, EP24.15 and EP24.16 showed a major association with membranes of neurosecretory elements, including Golgi cisternae, tubulovesicular organelles, synaptic vesicles, and endosomes. However, whereas EP24.15 always faced the cytoplasmic face of the membranes, EP24.16 was observed on both cytoplasmic and luminal sides, suggesting that the latter was more likely to contribute to the processing of peptides or to the degradation of internalized ligands. Taken together, the present results suggest that EP24.15 could play a major role in the hydrolysis of intranuclear substrates, whereas EP24.16 would be predominantly involved in the processing and inactivation of signaling peptides.

Changes in GABA-immunoreactivity during development of the rostral subdivision of the nucleus of the solitary tract

GABA plays an important role in the processing of gustatory information in the rostral nucleus of the solitary tract. The following study used post-embedment immunohistochemistry in the rat brainstem to localize GABA at both the light and electron microscopic levels to characterize the developmental distribution of GABA and synaptogenesis of GABA-immunoreactive terminals in the rostral nucleus of the solitary tract. During the first postnatal week, GABA is present in the rostral nucleus of the solitary tract, but less of it is synaptic than any time later in development. Of the few synaptic terminals present at postnatal day 1, less than 20% are GABA-immunoreactive. This proportion more than doubles to reach adult levels by postnatal day 10. By weaning (postnatal day 20), GABA-immunoreactive cells are found in nearly the same density as in the adult. Development continues after weaning and is characterized by a disproportionate loss of non-GABA-containing cells. Finally, one previously identified subtype of GABA-immunoreactive terminal matures very late during the postweaning phase of development. The study provides the first analysis of the development of GABA-related circuitry in the rostral nucleus of the solitary tract using anatomical methods. These data provide the background with which to view the emerging physiology of developing taste neurons.

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.

mu-opioid receptors are present in vagal afferents and their dendritic targets in the medial nucleus tractus solitarius

Ligands of the mu-opiate receptor (MOR) are known to influence many functions that involve vagal afferent input to the nucleus tractus solitarius (NTS), including cardiopulmonary responses, gastrointestinal activity, and cortical arousal. The current study sought to determine whether a cellular substrate exists for direct modulation of vagal afferents and/or their neuronal targets in the NTS by ligands of the MOR. Anterograde tracing of vagal afferents arising from the nodose ganglion was achieved with biotinylated dextran amine (BDA), and the MOR was detected by using antipeptide MOR antiserum. The medial subdivision of the intermediate NTS was examined by electron microscopy for the presence of peroxidase-labeled, BDA-containing vagal afferents and immunogold MOR labeling. MOR was present in both presynaptic axon terminals and at postsynaptic sites, primarily dendrites. In dendrites, MOR immunogold particles usually were located along extrasynaptic portions of the plasma membrane. Of 173 observed BDA-labeled vagal afferent axon terminals, 33% contained immunogold labeling for MOR within the axon terminal. Many of these BDA-labeled terminals formed asymmetric, excitatory-type synapses with dendrites, some of which contained MOR immunogold labeling. MORs were present in 19% of the dendrites contacted by BDA-labeled terminals but were present rarely in both the vagal afferent and its dendritic target. Together, these results suggest that MOR ligands modulate either the presynaptic release from or the postsynaptic responses to largely separate populations of vagal afferents in the intermediate NTS. These results provide a cellular substrate for direct actions of MOR ligands on primary visceral afferents and their second-order neuronal targets in NTS.

Rostrocaudal variation in targeting of N-methyl-D-aspartate and mu-opioid receptors in the rat medial nucleus of the solitary tract

N-methyl-D-aspartate (NMDA) receptors and mu-opioid receptors (MOR) have been implicated in gustatory and cardiorespiratory visceral reflexes, respectively involving second order sensory neurons in rostral and intermediate portions of the medial nucleus of the solitary tract (mNTS). To determine whether there are cellular sites suggesting functional interaction involving NMDA receptors and MOR in these regions, we examined their ultrastructural immunocytochemical localization by using antisera recognizing the functional subunit of NMDA receptors (NR1) or MOR in rat brain. In both mNTS subdivisions, NR1 labeling was prominently seen along membranes of cytoplasmic organelles in somata and large dendrites, as well as on asymmetric postsynaptic densities in small dendrites and dendritic spines. Many of these profiles also contained MOR immunoreactivity that was mainly distributed along extrasynaptic plasma membranes. Quantitative regional comparison showed that dendrites composed 64% (167 of 261) and 35% (137 of 390) of the dually labeled structures in the rostral and intermediate mNTS, respectively. In contrast, only 11% (28 of 261) of the total dually labeled profiles in the rostral, but 46% (180 of 390) of those in the intermediate mNTS were axon terminals. Many of the terminals containing NR1 and/or MOR were large and formed asymmetric synapses with multiple targets, resembling those features of known visceral afferents. Our results suggest that opioids, active at MOR in mNTS, modulate excitatory visceral reflexes involving mainly postsynaptic NMDA receptors in the rostral region. In addition, they suggest that similar mechanisms exist in the intermediate mNTS, where both NMDA receptors and MOR may differentially regulate the presynaptic release of glutamate from the visceral afferents.

Cardiac afferents to the nucleus of the tractus solitarius: A WGA-HRP study in the rat

Central distribution of the sensory fibers of the heart was investigated in the rat by the use of transganglionic transport of horseradish peroxidase (HRP). After the left intercostal thoracotomy was done under deep anesthesia and artificial respiration, wheat germ agglutinin-conjugated HRP (WGA-HRP) was injected into the left and right ventricular walls and the apex of the heart. HRP-labeled fibers were observed to be distributed to the dorsomedial portion of the medulla oblongata through the vagal nerve. The labeled fibers were present in various subnuclei of the nucleus of the tractus solitarius (NTS) bilaterally at the level of +0.36 to -1.74 mm to the obex. However, the most conspicuous feature in the present study was that the labeled fibers were exclusively confined to the medial, ventrolateral and commissural NTS with some distribution to the dorsolateral NTS. Although the labeling in the medial and ventrolateral NTS was observed to extend rostrocaudally, it was of interest that the labeling in the medial NTS was divided into the ventral and dorsal parts at the level around the obex. Accumulation of the labeled fibers in the commissural NTS was found at the level caudal to the obex and these fibers were traced to the caudal portion of its subnucleus with a gradual decrease in number. This pattern of distribution of cardiac afferents in the NTS was considered to be peculiar to the rat, because it was quite different from that reported previously in the cat.

Distribution of AMPA receptor subunits GluR1-4 in the dorsal vagal complex of the rat: a light and electron microscope immunocytochemical study

The dorsal vagal complex, localized in the dorsomedial medulla, includes the nucleus tractus solitarii (NTS), the dorsal motor nucleus of the vagus nerve (DMN) and the area postrema (AP). The distribution of AMPA-preferring glutamate receptors (AMPA receptors) within this region was investigated using immunohistochemistry and antibodies recognizing either one (GluR1 or GluR4) or two (GluR2 and GluR3) AMPA receptors subunits. The distribution of GluR1 immunoreactivity showed high contrast of staining between strongly and lightly labeled areas. Labeling was intense in the AP and weak in the NTS, except for its medial and dorsalmost parts which exhibited moderate staining. Almost no GluR1 immunoreactivity was found in the DMN. GluR2/3 immunolabeling was present in the entire dorsal vagal complex. This labeling was strong in the AP, the DMN and the medial half of the NTS and moderate in the lateral half of the NTS, except for the interstitial subdivision which exhibited intense staining. Labeling induced by the GluR4 antibody was very weak throughout the dorsal vagal complex. Ultrastructural examination showed that GluR1 and GluR2/3 immunoreactivity was localized in neuronal cell bodies and dendrites. No labeled axon terminal or glial cell body was found. Immunoperoxidase staining in labeled cell bodies and dendrites was associated with intracellular organelles (microtubules, mitochondria, cisternae of the endoplasmic reticulum,.) and/or parts of the plasma membrane. Plasma membrane labeling was often associated with asymmetrical synaptic differentiations. No labeled symmetrical synapse was found using either GluR1 or GluR2/3 antibody. The present results show that AMPA receptors have a widespread distribution in neuronal perikarya and dendrites of the rat dorsal vagal complex. They suggest differences in subunit composition between AMPA receptors localized in the NTS, the DMN and the AP. Ultrastructural data are consistent with the fact that AMPA receptors associated with the plasma membrane are mostly synaptic receptors. However, they also suggest the existence of a large intracellular pool of receptor subunits in neuronal soma and dendrites.

Glycine-immunoreactive synaptic terminals in the nucleus tractus solitarii of the cat: ultrastructure and relationship to GABA-immunoreactive terminals

Postembedding immunogold labeling methods applied to ultrathin and semithin sections of cat dorsomedial medulla showed that neuronal perikarya, dendrites, myelinated and nonmyelinated axons, and axon terminals in the nucleus tractus solitarii contain glycine immunoreactivity. Light microscopic observations on semithin sections revealed that these immunoreactive structures were unevenly distributed throughout the entire nucleus. At the electron microscopic level, synaptic terminals with high levels of glycine-immunoreactivity, assumed to represent those releasing glycine as a neurotransmitter, were discriminated from terminals containing low, probably metabolic levels of glycine-immunoreactivity, by a quantitative analysis method. This compared the immunolabeling of randomly sampled terminals with a reference level of labeling derived from sampling the perikarya of dorsal vagal neurones. The vast majority of these "glycinergic" terminals contained pleomorphic vesicles, formed symmetrical synaptic active zones, and targeted dendrites. They appeared to be more numerous in areas of the nucleus tractus solitarii adjoining the tractus solitarius, but rather scarce caudally, medially, ventrally, and in the dorsal motor vagal nucleus. In a random analysis of the entire nucleus tractus solitarii, 26.2% of sampled terminals were found to qualify as glycine-immunoreactive. In contrast, boutons immunoreactive for gamma-aminobutyric acid (GABA) were more evenly distributed throughout the dorsal vagal complex and accounted for 33.7% of the synaptic terminals sampled. A comparison of serial ultrathin sections suggested three subpopulations of synaptic terminals: one containing high levels of both GABA- and glycine-immunoreactivities (21% of all terminals sampled), one containing only GABA-immunoreactivity (12.7%), and relatively few terminals (5.2%) that were immunoreactive for glycine alone. These results were confirmed by dual labeling of sections using gold particles of different sizes. This study reports the first analysis of the ultrastructure of glycinergic nerve terminals in the cat dorsal vagal complex, and the pattern of coexistence of glycine and GABA observed provides an anatomical explanation for our previously reported inhibitory effects of glycine and GABA on neurones with cardiovascular and respiratory functions in the nucleus tractus solitarii.

Presence of mu-opioid receptors in targets of efferent projections from the central nucleus of the amygdala to the nucleus of the solitary tract

Opioids acting at mu-opioid receptors (MORs) within the nucleus of the solitary tract (NTS) potently modulate autonomic functions that are also known to be influenced by inputs from the central nucleus of the amygdala (CEA). In addition, many of the physiological effects of MOR agonists have been attributed to interactions with neurons that contain gamma-aminobutyric acid (GABA), one of the neurotransmitters present in CEA-derived terminals and their targets in the medial NTS. Together, these observations suggest that MORs are present at pre- or postsynaptic sites within the CEA to NTS circuitry. To test this hypothesis, we combined anterograde transport of biotinylated dextran amine (BDA) with immunogold-silver localization of an antipeptide antiserum against the MOR in the NTS of adult rats. In animals receiving bilateral CEA injections of BDA, anterogradely labeled axons were seen throughout the rostrocaudal NTS. Electron microscopy of the medial NTS at rostral and intermediate levels showed anterograde BDA-labeling in many small unmyelinated axons and axon terminals, none of which contained detectable MOR. The BDA-labeled axon terminals formed mainly symmetric, inhibitory-type synapses with somata and dendrites. Over half of the somatic and approximately 10% of the dendritic targets showed nonsynaptic plasmalemmal immunogold labeling for MOR. The BDA-labeled axon terminals were also frequently apposed by other small axons that contained MORs. These results suggest that within the medial NTS, MOR agonists modulate the postsynaptic inhibition produced by CEA afferents and also play a role in the presynaptic release of other neurotransmitters.

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.

Ultrastructure of glutamate and GABA immunoreactive axon terminals of the rat nucleus tractus solitarius, with a note on infralimbic cortex afferents

The principal fast neurotransmitters in the CNS are glutamate and GABA. Our aim was to provide a baseline account on the ultrastructure of the axon terminals immunoreactive to glutamate or GABA present in the nucleus tractus solitarius (NTS) of the rat. In addition, we wanted to complete our study of cortico-solitary afferents at the electron microscopic level, by analyzing the inputs from the infralimbic cortex. Using post-embedding immunogold, we found that nearly 61% of the axon terminals were glutamatergic, and 36% were GABAergic in the rat visceral NTS. In general, axons making asymmetric synaptic contacts were enriched in glutamate, compared to axons involved in symmetric synapses. In contrast, the vast majority of the GABAergic axon terminals made symmetric synaptic contacts. We could discern five types of glutamatergic and two types of GABAergic axon terminals that differed in their fine structure. Afferents from the infralimbic cortex were small, with clear synaptic vesicles and no dense core vesicles; they made asymmetric contacts with fine dendrites, and were glutamatergic. We conclude that most axon terminals in the NTS use glutamate or GABA as fast transmitters, in addition to being a heterogeneous population of morphological types.

Activation of neuropeptide FF neurons in the brainstem nucleus tractus solitarius following cardiovascular challenge and opiate withdrawal

Neuropeptide FF (NPFF), a morphine modulatory peptide, is localized within discrete autonomic regions including the brainstem nucleus tractus solitarius (NTS) and the parabrachial nucleus (PBN). We investigated the activation of NPFF neurons in the NTS of rats induced by cardiovascular challenge and centrally generated opiate withdrawal. For hypotensive stimulation, we used systemic infusions of sodium nitroprusside (NP) or hemorrhage (HEM), and hypertension was achieved by intravenous phenylephrine (PHENYL) or angiotensin II (AII). In rats that received continuous intracerebroventricular injections of morphine, intraperitoneal injections of naloxone precipitated behavioural signs of opioid withdrawal. Activated NTS neurons were identified by using a combined immunohistochemistry for Fos and NPFF, and neurons projecting to the PBN were determined with a retrograde tracer. HEM, administration of vasoactive drugs, and opiate withdrawal produced a very robust activation of NTS neurons. In NP and HEM groups, 25.6+/-3.2% and 7.6+/-1.3% of NPFF neurons were activated, respectively. Lesser numbers of NPFF neurons were activated in the PHENYL (4.6+/-1.6%) and AII (2.4+/-0.8%) groups. However, following opiate withdrawal, virtually no Fos expression was observed in NPFF neurons. NPFF neurons activated during NP infusion constituted the largest number of cells projecting to the PBN. This study shows that NPFF neurons in NTS that project to the PBN respond selectively to NP as opposed to other cardiovascular challenges or opiate withdrawal. These data support an emerging and important role for NPFF in the context of central cardiovascular regulation.

Cellular and subcellular distribution of substance P receptor immunoreactivity in the dorsal vagal complex of the rat and cat: a light and electron microscope study

Immunoreactivity for the substance P receptor (NK1 receptor) has been investigated by light and electron microscopy in the dorsal vagal complexes of adult rats and cats. The general pattern of NK1 immunoreactivity was similar for both rat and cat. Numerous NK1-immunoreactive neurons were present in the area postrema, the nucleus of the solitary tract, and the dorsal motor nucleus of the vagus nerve. The density of labelled neurons differed between the subnuclei of the nucleus of the solitary tract. Overall, the efferent neurons of the dorsal motor nucleus of the vagus nerve highly expressed NK1 when compared to neurons in the nucleus of the solitary tract. The results are discussed with reference to the viscerotopic organisation of the dorsal vagal complex. Ultrastructural analysis demonstrated that NK1 immunoreactivity was present only at the membrane surface of somatic and dendritic profiles of neurons. No labelling was found in axon terminals, axons, or glial processes. NK1 immunoreactivity, as revealed by a preembedding immunogold technique in serial ultrathin sections, was preferentially located at nonsynaptic sites. A semiquantitative study suggested that the density of NK1 receptors is statistically higher at membrane sites free of any contact (synaptic or not) with axon terminals. The subcellular localisation of NK1 immunoreactivity was similar for neurons of both rat and cat. These results suggest that in the dorsal vagal complex, substance P might act on NK1 receptors through a process of volume transmission.