The nucleus accumbens (NAc) is part of a forebrain system implicated in reward, motivation, and learning. NAc neurons become activated during various ingestive activities, including salt intake. A subset of neurons within the nucleus tractus solitarius (NTS) shows c-Fos activation during prolonged sodium deprivation in rats. These neurons express mineralocorticoid receptors and the enzyme 11-beta-hydroxysteroid dehydrogenase type 2 (HSD2), which makes them selectively sensitive to aldosterone-an adrenal hormone that modulates sodium appetite. Here we tested whether these neurons project multisynaptically to the core or shell subregions of the NAc. Pseudorabies virus (PRV)-a retrograde transneuronal tracer-was injected into the NAc in rats and after 3-4 days PRV-infected HSD2 neurons were identified. PRV injections into the NAc core yielded greater numbers of PRV-labeled HSD2 neurons than did comparable injections into the NAc shell. Transneuronal labeling was also found in brainstem sites that receive direct projections from HSD2 neurons, namely, lateral parabrachial and prelocus coeruleus nuclei. In other experiments a retrograde neural tracer (cholera toxin beta-subunit) was injected into the NAc. Extensive retrograde labeling was found in the midline thalamus and frontal cortical regions, but no cells were labeled in the NTS or parabrachial region. These findings indicate that the HSD2 neurons project via a multisynaptic pathway to the NAc, which may be relayed sequentially through two sites: the dorsolateral pons and the paraventricular thalamic nucleus. HSD2 neurons may be part of an ascending pathway involved in the salt-seeking behavior of sodium-depleted rats.

The voltage-gated K(+) channel Kv1.3 has been reported to regulate transmitter release in select central and peripheral neurons. In this study, we evaluated its role at the synapse between visceral sensory afferents and secondary neurons in the nucleus of the solitary tract (NTS). We identified mRNA and protein for Kv1.3 in rat nodose ganglia using RT-PCR and Western blot analysis. In immunohistochemical experiments, anti-Kv1.3 immunoreactivity was very strong in internal organelles in the soma of nodose neurons with a weaker distribution near the plasma membrane. Anti-Kv1.3 was also identified in the axonal branches that project centrally, including their presynaptic terminals in the medial and commissural NTS. In current-clamp experiments, margatoxin (MgTx), a high-affinity blocker of Kv1.3, produced an increase in action potential duration in C-type but not A- or Ah-type neurons. To evaluate the role of Kv1.3 at the presynaptic terminal, we examined the effect of MgTx on tract evoked monosynaptic excitatory postsynaptic currents (EPSCs) in brain slices of the NTS. MgTx increased the amplitude of evoked EPSCs in a subset of neurons, with the major increase occurring during the first stimuli in a 20-Hz train. These data, together with the results from somal recordings, support the hypothesis that Kv1.3 regulates the duration of the action potential in the presynaptic terminal of C fibers, limiting transmitter release to the postsynaptic cell.

10.1152/jn.00224.2002. Dopamine (DA) modulates the cardiorespiratory reflex by peripheral and central mechanisms. The aim of this study was to examine the role of DA in synaptic transmission of the nucleus tractus solitarius (NTS), the major integration site for cardiopulmonary reflexes. To examine DA's role, we used whole cell, voltage-clamp recordings in a rat horizontal brain stem slice. Solitary tract stimulation evoked excitatory postsynaptic currents (EPSCs) that were reduced to 70 +/- 5% of control by DA (100 microM). The reduction in EPSCs by DA was accompanied by a decrease in the paired pulse depression ratio with little or no change in input resistance or EPSC decay, suggesting a presynaptic mechanism. The D1-like agonist SKF 38393 Br (30 microM) did not alter EPSC amplitude, whereas the D2-like agonist, quinpirole HCl (30 microM), depressed EPSCs to 73 +/- 4% of control. The D2-like receptor antagonist, sulpiride (20 microM), abolished DA modulation of EPSCs. Most importantly, sulpiride alone increased EPSCs to 131 +/- 10% of control, suggesting a tonic D2-like modulation of synaptic transmission in the NTS. Examination of spontaneous EPSCs revealed DA reversibly decreased the frequency of events from 9.4 +/- 2.2 to 6.2 +/- 1.4 Hz. Sulpiride, however, did not alter spontaneous events. Immunohistochemistry of NTS slices demonstrated that D2 receptors colocalized with synaptophysin and substance P, confirming a presynaptic distribution. D2 receptors also localized to cultured petrosal neurons, the soma of presynaptic afferent fibers. In the petrosal neurons, D2 was found in cells that were TH-immunopositive, suggesting they were chemoreceptor afferent fibers. These results demonstrate that DA tonically modulates synaptic activity between afferent sensory fibers and secondary relay neurons in the NTS via a presynaptic D2-like mechanism.

The nucleus of the solitary tract (NTS) contains a subpopulation of neurons that express the enzyme 11-beta-hydroxysteroid dehydrogenase type 2 (HSD2), which makes them uniquely sensitive to aldosterone. These neurons may drive sodium appetite, which is enhanced by aldosterone. Anterograde and retrograde neural tracing techniques were used to reveal the efferent projections of the HSD2 neurons in the rat. First, the anterograde tracer Phaseolus vulgaris leucoagglutinin was used to label axonal projections from the medial NTS. Then, NTS-innervated brain regions were injected with a retrograde tracer, cholera toxin beta subunit, to determine which sites are innervated by the HSD2 neurons. The HSD2 neurons project mainly to the ventrolateral bed nucleus of the stria terminalis (BSTvl), the pre-locus coeruleus (pre-LC), and the inner division of the external lateral parabrachial nucleus (PBel). They also send minor axonal projections to the midbrain ventral tegmental area, lateral and paraventricular hypothalamic nuclei, central nucleus of the amygdala, and periaqueductal gray matter. The HSD2 neurons do not innervate the ventrolateral medulla, a key brainstem autonomic site. Additionally, our tracing experiments confirmed that the BSTvl receives direct axonal projections from the neighboring A2 noradrenergic neurons in the NTS, and from the same pontine sites that receive major inputs from the HSD2 neurons (PBel and pre-LC). The efferent projections of the HSD2 neurons may provide new insights into the brain circuitry responsible for sodium appetite.

Artemisia extracts have been used as remedies for a variety of maladies related to metabolic and gastrointestinal control. Because the vagal afferent-nucleus of the solitary tract (NST) synapse regulates the same homeostatic functions affected by Artemisia, it is possible that these extracts may have activity at the synaptic level in the NST. Therefore, we evaluated how extracts of three common medicinal Artemisia species, Artemisia santolinifolia (SANT), Artemisia scoparia (SCO), and Artemisia dracunculus L (PMI-5011), modulate the excitability of the glutamatergic vagal afferent-NST synapse. Our in vitro live cell calcium imaging data from prelabeled vagal afferent terminals show that SANT extract is a positive modulator of vagal afferent calcium levels, as the extract significantly increased the calcium signal relative to the time control. Neither SCO nor PMI-5011 extract altered the vagal calcium signals compared to the time control. Furthermore, whole cell voltage-clamp recordings from NST neurons corroborated the vagal terminal calcium data in that SANT extract also significantly increased miniature excitatory postsynaptic current (mEPSC) frequency in NST neurons. These data suggest that SANT extract could be a pharmacologically significant mediator of glutamatergic neurotransmission within the CNS.

Neurons in the rostral nucleus of the solitary tract (rNST) receive taste information from the tongue and relay it mainly to the parabrachial nucleus (PBN) and the medullary reticular formation (RF) through two functionally different neural circuits. To help understand how the information from the rNST neurons is transmitted within these brainstem relay nuclei in the taste pathway, we examined the terminals of the rNST neurons in the PBN and RF by use of anterograde horseradish peroxidase (HRP) labeling, postembedding immunogold staining for glutamate, serial section electron microscopy, and quantitative analysis. Most of the anterogradely labeled, glutamate-immunopositive axon terminals made a synaptic contact with only a single postsynaptic element in PBN and RF, suggesting that the sensory information from rNST neurons, at the individual terminal level, is not passed to multiple target cells. Labeled terminals were usually presynaptic to distal dendritic shafts in both target nuclei. However, the frequency of labeled terminals that contacted dendritic spines was significantly higher in the PBN than in the RF, and the frequency of labeled terminals that contacted somata or proximal dendrites was significantly higher in the RF than in the PBN. Labeled terminals receiving axoaxonic synapses, which are a morphological substrate for presynaptic modulation frequently found in primary sensory afferents, were not observed. These findings suggest that the sensory information from rNST neurons is processed in a relatively simple manner in both PBN and RF, but in a distinctly different manner in the PBN as opposed to the RF.

Direct detection of circulating nutrients by the central nervous system has been implicated in the regulation of energy balance, and the mediobasal hypothalamus is considered as the primary sensing site mediating these effects. Neurons sensitive to energyrelated signals have also been identified outside the hypothalamus, particularly within the caudomedial nucleus of the solitary tract (cmNTS) in brainstem, but the consequences of direct cmNTS nutrient detection on energy balance remain poorly characterized. Here we determined the behavioral and metabolic consequences of direct L-leucine detection by the cmNTS and investigated the intracellular signaling and neurochemical pathways implicated in cmNTS L-leucine sensing in rats. Our results support the distributed nature of central nutrient detection, evidence a role for the cmNTS S6K1 pathway in the regulation of meal size and body weight, and suggest that the cmNTS integrates direct cmNTS nutrient detection with gut-derived, descending forebrain, and adiposity signals of energy availability to regulate food intake.

To meet the challenge to human health posed by obesity, a better understanding of the regulation of feeding is essential. Medications targeting 5-hydroxytryptamine (5-HT; serotonin) 2C receptors (htr2c; 5-HT2CR) improve obesity. Here we probed the functional significance of 5-HT2CRs specifically within the brainstem nucleus of the solitary tract (5-HT2CRNTS) in feeding behavior. Selective activation of 5-HT2CRNTS decreased feeding and was sufficient to mediate acute food intake reductions elicited by the 5-HT2CR agonist obesity medication lorcaserin. Similar to pro-opiomelanocortin neurons expressed within the hypothalamic arcuate nucleus (POMCARC), a subset of POMCNTS neurons co-expressed 5-HT2CRs and were activated by 5-HT2CR agonists. Knockdown of POMCNTS prevented the acute appetite-suppressive effect of lorcaserin, whereas POMCARC knockdown prevented the full anorectic effect. These data identify 5-HT2CRNTS as a sufficient subpopulation of 5-HT2CRs in reducing food intake when activated and reveal that 5-HT2CR agonist obesity medications require POMC within the NTS and ARC to reduce food intake.

Phasic pattern of neuronal activity has been previously described in detail for magnocellular vasopressin neurons in the hypothalamic paraventricular and supraoptic nuclei. This characteristic bistable pattern consists of alternating periods of electrical silence and elevated neuronal firing, implicated in neuropeptide release. Here, with the use of multi-electrode array recordings ex vivo, we aimed to study the firing pattern of neurons in the nucleus of the solitary tract (NTS) - the brainstem hub for homeostatic, cardio-vascular, and metabolic processes. Our recordings from the mouse and rat hindbrain slices reveal the phasic activity pattern to be displayed by a subset of neurons in the dorsomedial NTS subjacent to the area postrema (AP), with the inter-spike interval distribution closely resembling that reported for phasic magnocellular vasopressin cells. Additionally, we provide interspecies comparison, showing higher phasic frequency and firing rate of phasic NTS cells in mice compared to rats. Further, we describe daily changes in their firing rate and pattern, peaking at the middle of the night. Last, we reveal these phasic cells to be sensitive to α 2 adrenergic receptors activation and to respond to electrical stimulation of the AP. This study provides a comprehensive description of the phasic neuronal activity in the rodent NTS and identifies it as a potential downstream target of the AP noradrenergic system.

The parabrachial complex (PB) is critically involved in aversive processes, and chronic pain is associated with amplified activity of PB neurons in rodent models of neuropathic pain. Here, we demonstrate that catecholaminergic input from the caudal nucleus of the solitary tract (cNTScat), a stress responsive region that integrates interoceptive and exteroceptive signals, causes amplification of PB activity and their sensory afferents. We used a virally mediated expression of a norepinephrine (NE) sensor, NE2h, fiber photometry, and extracellular recordings in anesthetized mice to show that noxious mechanical and thermal stimuli activate cNTS neurons. These stimuli also produce prolonged NE transients in PB that far outlast the noxious stimuli. Similar NE transients can be evoked by focal electrical stimulation of cNTS, a region that contains the noradrenergic A2 cell group that projects densely on PB. In vitro, optical stimulation of cNTScat terminals depolarized PB neurons and caused a prolonged increase the frequency of excitatory synaptic activity. A dual opsin approach showed that sensory afferents from the caudal spinal trigeminal nucleus are potentiated by cNTScat terminal activation. This potentiation was coupled with a decrease in the paired pulse ratio (PPR), consistent with an cNTScat-mediated increase in the probability of release at SpVc synapses. Together, these data suggest that A2 neurons of the cNTS generate long lasting NE transients in PB which increase excitability and potentiate responses of PB neurons to sensory inputs. These reveal a mechanism through which stressors from multiple modalities may potentiate the aversiveness of nociceptive stimuli.

The HSD2 (11-beta-hydroxysteroid dehydrogenase type 2-expressing) neurons in the nucleus of the solitary tract (NTS) of the rat are aldosterone-sensitive and have been implicated in sodium appetite. The central nucleus of the amygdala (CeA) has been shown to modulate salt intake in response to aldosterone, so we investigated the connections between these two sites. A prior retrograde tracing study revealed only a minor projection from the HSD2 neurons directly to the CeA, but these experiments suggested that a more substantial projection may be relayed through the parabrachial nucleus. Small injections of cholera toxin beta subunit (CTb) into the external lateral parabrachial subnucleus (PBel) produced both retrograde cell body labeling in the HSD2 neurons and anterograde axonal labeling in the lateral subdivision of the CeA. Also, injections of either CTb or Phaseolus vulgaris leucoagglutinin into the medial subdivision of the CeA labeled a descending projection from the amygdala to the medial NTS. Axons from the medial CeA formed numerous varicosities and terminals enveloping the HSD2 neurons. Complementary CTb injections, centered in the HSD2 subregion of the NTS, retrogradely labeled neurons in the medial CeA. These bidirectional projections could form a functional circuit between the HSD2 neurons and the CeA. The HSD2 neurons may represent one of the functional inputs to the lateral CeA, and their activity may be modulated by a return projection from the medial CeA. This circuit could provide a neuroanatomical basis for the modulation of salt intake by the CeA.

The rostral ventrolateral medulla (RVLM) contains the presympathetic neurons involved in cardiovascular regulation that has been implicated as one of the most important central sites for the antihypertensive action of moxonidine (an α2-adrenergic and imidazoline agonist). Here, we sought to evaluate the cardiovascular effects produced by moxonidine injected into another important brainstem site, the commissural nucleus of the solitary tract (commNTS). Mean arterial pressure (MAP), heart rate (HR), splanchnic sympathetic nerve activity (sSNA) and activity of putative sympathoexcitatory vasomotor neurons of the RVLM were recorded in conscious or urethane-anesthetized, and artificial ventilated male Wistar rats. In conscious or anesthetized rats, moxonidine (2.5 and 5 nmol/50 nl) injected into the commNTS reduced MAP, HR and sSNA. The injection of moxonidine into the commNTS also elicited a reduction of 28% in the activity of sympathoexcitatory vasomotor neurons of the RVLM. To further assess the notion that moxonidine could act in another brainstem area to elicit the antihypertensive effects, a group with electrolytic lesions of the commNTS or sham and with stainless steel guide-cannulas implanted into the 4th V were used. In the sham group, moxonidine (20 nmol/1 μl) injected into 4th V decreased MAP and HR. The hypotension but not the bradycardia produced by moxonidine into the 4th V was reduced in acute (1 day) commNTS-lesioned rats. These data suggest that moxonidine can certainly act in other brainstem regions, such as commNTS to produce its beneficial therapeutic effects, such as hypotension and reduction in sympathetic nerve activity.

Prostaglandin E(2) (PGE(2)) is a prototypical inflammatory mediator that excites and sensitizes cell bodies [Kwong K, Lee LY (2002) PGE(2) sensitizes cultured pulmonary vagal sensory neurons to chemical and electrical stimuli. J Appl Physiol 93:1419-1428; Kwong K, Lee LY (2005) Prostaglandin E(2) potentiates a tetrodotoxin (TTX)-resistant sodium current in rat capsaicin-sensitive vagal pulmonary sensory neurons. J Physiol 56:437-450] and peripheral nerve terminals [Ho CY, Gu Q, Hong JL, Lee LY (2000) Prostaglandin E (2) enhances chemical and mechanical sensitivities of pulmonary C fibers in the rat. Am J Respir Crit Care Med 162:528-533] of primary vagal sensory neurons. Nearly all central nerve terminals of vagal afferents are in the nucleus tractus solitarius (NTS), where they operate with a high probability of release [Doyle MW, Andresen MC (2001) Reliability of monosynaptic sensory transmission in brain stem neurons in vitro. J Neurophysiol 85:2213-2223]. We studied the effect of PGE(2) on synaptic transmission between tractus solitarius afferent nerve terminals and the second-order NTS neurons in brain stem slices of Sprague-Dawley rats. Whole-cell patch recording in voltage clamp mode was used to study evoked excitatory postsynaptic glutamatergic currents (evEPSCs) from NTS neurons elicited by electrical stimulation of the solitary tract (ST). In 34 neurons, bath-applied PGE(2) (200 nM) decreased the evEPSC amplitude by 49+/-5%. In 22 neurons, however, PGE(2) had no effect. We also tested 15 NTS neurons for capsaicin sensitivity. Seven neurons generated evEPSCs that were equally unaffected by PGE(2) and capsaicin. Conversely, evEPSCs of the other eight neurons, which were PGE(2)-responsive, were abolished by 200 nM capsaicin. Furthermore, the PGE(2-)induced depression of evEPSCs was associated with an increase in the paired pulse ratio and a decrease in both the frequency and amplitude of the spontaneous excitatory postsynaptic currents (sEPSCs) and TTX-independent spontaneous miniature excitatory postsynaptic currents (mEPSCs). These results suggest that PGE(2) acts both presynaptically on nerve terminals and postsynaptically on NTS neurons to reduce glutamatergic responses.

Neuronatin (Nnat) is involved in the regulation of cellular molecular signaling and appears to be also linked to metabolic processes. The gastrointestinal peptides cholecystokinin (CCK) and bombesin (BN) have an effect on the short-term inhibition of food intake and induce neuronal activation in different brain nuclei, prominently in the nucleus of the solitary tract (NTS) involved in the modulation of food intake. The aim of the study was to examine if Nnat immunoreactivity is detectable in the NTS, and whether peripheral CCK-8S or BN cause c-Fos activation of Nnat neurons. Non-fasted male Sprague-Dawley rats received an intraperitoneal (i.p.) injection of 5.2 or 8.7 nmol CCK-8S/kg or 26 or 32 nmol BN/kg (n = 4 all groups) or vehicle solution (0.15 M NaCl; n = 7). The number of c-Fos neurons was determined 90 min post injection in the NTS and dorsal motor nucleus of the vagus (DMV). We observed Nnat immunoreactive neurons in the NTS and DMV. CCK-8S (25-fold and 51-fold, p = 0.025 and p = 0.001) and BN (31-fold and 59-fold, p = 0.007 and p = 0.001) at both doses increased the number of c-Fos positive neurons in the NTS. CCK and BN did not show a significant effect in the DMV. Both doses of CCK-8S (24-fold and 48-fold p = 0.011 and p = 0.001) and bombesin (31-fold and 56-fold, p = 0.002 and p = 0.001) increased the number of activated Nnat neurons in the NTS (p = 0.001) compared to the vehicle group, while in the DMV no significant increase of c-Fos activation was detected. In conclusion, i.p. injected CCK-8S or BN induce an increased neuronal activity in NTS Nnat neurons, giving rise that Nnat may play a role in the regulation of food intake mediated by peripheral CCK-8S or BN.

The present study investigated the prenatal development of the cyto- and chemoarchitecture of the human nucleus of the solitary tract from 9 to 35 weeks, by using Nissl staining and immunoreactivity to calbindin, calretinin, tyrosine hydroxylase and GAP-43. The nucleus began to gain heterogeneity and show different subnuclei as early as 13 weeks, and approached cytoarchitectural maturation from 21 to 25 weeks. The subnuclear division pattern observed in the fetal nucleus of the solitary tract at 25 weeks was very similar to that of the adult. Neurons immunoreactive to calbindin first appeared in the medial gastrointestinal area of the nucleus at 13 weeks, particularly within a putative gelatinosus subnucleus, while calretinin immunoreactivity during fetal life suggested the possible presence of a central subnucleus. Tyrosine hydroxylase immunoreactive neurons were seen in the medial subdivisions of the nucleus of the solitary tract as early as 13 weeks, but the population continued to increase until 25 weeks. Strong GAP-43 immunoreactivity was also present in the nucleus of the solitary tract at 13 weeks, especially in the dorsolateral and commissural subnuclei, while at 21 weeks there was a significant decline of GAP-43 expression. Results from the chemoarchitectural study showed that the human nucleus of the solitary tract expressed various neurochemical substances at an early developmental age (13 weeks), even before cellular and neuropil maturation was fully attained. Expression of these factors may play an important role in establishment and integration of viscerosensory function in the nucleus.

The nucleus of the solitary tract (NTS) regulates life-sustaining functions ranging from appetite and digestion to heart rate and breathing. It is also the brain's primary sensory nucleus for visceral sensations relevant to symptoms in medical and psychiatric disorders. To better understand which neurons may exert top-down control over the NTS, here we provide a brain-wide map of all neurons that project axons directly to the caudal, viscerosensory NTS, focusing on a medial subregion with aldosterone-sensitive HSD2 neurons. Injecting an axonal tracer (cholera toxin b) into the NTS produces a similar pattern of retrograde labeling in rats and mice. The paraventricular hypothalamic nucleus (PVH), lateral hypothalamic area, and central nucleus of the amygdala (CeA) contain the densest concentrations of NTS-projecting neurons. PVH afferents are glutamatergic (express Slc17a6/Vglut2) and are distinct from neuroendocrine PVH neurons. CeA afferents are GABAergic (express Slc32a1/Vgat) and are distributed largely in the medial CeA subdivision. Other retrogradely labeled neurons are located in a variety of brain regions, including the cerebral cortex (insular and infralimbic areas), bed nucleus of the stria terminalis, periaqueductal gray, Barrington's nucleus, Kölliker-Fuse nucleus, hindbrain reticular formation, and rostral NTS. Similar patterns of retrograde labeling result from tracer injections into different NTS subdivisions, with dual retrograde tracing revealing that many afferent neurons project axon collaterals to both the lateral and medial NTS subdivisions. This information provides a roadmap for studying descending axonal projections that may influence visceromotor systems and visceral "mind-body" symptoms.

Despite generally being a reinforcing drug of abuse, amphetamine (amph) also produces effects such as hypophagia and conditioned taste avoidance (CTA), which may indicate that amph acts as an aversive homeostatic stressor. Stress-responsive prolactin-releasing peptide (PrRP)-positive noradrenergic and glucagon-like peptide-1 (GLP-1)-positive neurons in the caudal nucleus of the solitary tract (cNTS) are modulated by metabolic state, and are prime candidates for mediating amph-induced hypophagia and CTA. The present study used dual immunolabeling and fluorescent in situ hybridization (RNAscope) to examine acute amph-induced activation of cFos expression in phenotypically-identified cNTS neurons in ad lib-fed vs. overnight-fasted male Sprague Dawley rats. We also examined the impact of food deprivation on amph-induced CTA. Compared to control saline treatment, amph activated significantly more cNTS neurons, including PrRP-negative noradrenergic (NA) neurons, GABAergic neurons, and glutamatergic neurons, but not PrRP or GLP-1 neurons. Amph also increased neural activation within a subset of central cNTS projection targets, including the lateral parabrachial nucleus and central amygdala, but not the paraventricular hypothalamus. Food deprivation did not alter amph-induced neural activation or impact the ability of amph to support CTA. These findings indicate that PrRP-negative NA and other cNTS neurons are recruited by acute amph treatment regardless of metabolic state, and may participate in amph-induced hypophagia and CTA.

The nucleus of the solitary tract (NTS) is a key gateway for meal-related signals entering the brain from the periphery. However, the chemical mediators crucial to this process have not been fully elucidated. We reveal that a subset of NTS neurons containing cholecystokinin (CCK(NTS)) is responsive to nutritional state and that their activation reduces appetite and body weight in mice. Cell-specific anterograde tracing revealed that CCK(NTS) neurons provide a distinctive innervation of the paraventricular nucleus of the hypothalamus (PVH), with fibers and varicosities in close apposition to a subset of melanocortin-4 receptor (MC4R(PVH)) cells, which are also responsive to CCK. Optogenetic activation of CCK(NTS) axon terminals within the PVH reveal the satiating function of CCK(NTS) neurons to be mediated by a CCK(NTS)→PVH pathway that also encodes positive valence. These data identify the functional significance of CCK(NTS) neurons and reveal a sufficient and discrete NTS to hypothalamus circuit controlling appetite.

The depressor response elicited by unilateral low intensity electrical stimulation of the rat ventral medial prefrontal cortex may be mediated by a connection with the solitary tract nucleus. We tested this hypothesis by (i) examining the influence of medial prefrontal cortex stimulation on the induction of Fos-like immunoreactivity in neurons in the medulla oblongata, and (ii) by testing the effect of inhibition of solitary tract nucleus neurons on the medial prefrontal cortex stimulation-evoked depressor response. Depressor responses (>10 mmHg) were elicited by electrical stimulation of the medial prefrontal cortex every minute for 1 h ('Stimulated' group). Control animals were treated identically but did not receive electrical stimulation ('Unstimulated' group). Neurons exhibiting Fos-like immunoreactivity were abundant at the stimulation site which included the infralimbic area, and dorsal peduncular cortex. Medullary Fos-like immunoreactivity observed in the 'Stimulated' and 'Unstimulated' groups exceeded levels observed in untreated rats and was detected in the rostral, caudal and intermediate areas of the ventrolateral medulla, and the commissural, intermediate, medial and lateral regions of the solitary tract nucleus, as well as the medial vestibular nucleus, and the dorsal motor nucleus of the vagus. The number of neurons displaying Fos-like immunoreactivity in the ipsilateral solitary tract nucleus and caudal ventrolateral medulla of the 'Stimulated' group was found to be significantly elevated compared to the contralateral side (P<0.05), and the 'Unstimulated' group bilaterally. Inhibition of solitary tract nucleus neurons using bilateral injections of the GABA(A) receptor agonist muscimol (44 pmol/25 nl) inhibited the sympathetic vasomotor baroreflex and attenuated the depressor and sympathoinhibitory response to medial prefrontal cortex stimulation by 62% and 65%, respectively. These findings suggest that the projection from the medial prefrontal cortex to the solitary tract nucleus is excitatory and support the hypothesis that the depressor response elicited by medial prefrontal cortex stimulation is mediated, in part, by a cortico-solitary projection which activates the intramedullary baroreflex pathway.

Several brain regions are thought to function as important sites of chemoreception including the nucleus of the solitary tract (NTS), medullary raphe and retrotrapezoid nucleus (RTN). In the RTN, mechanisms of chemoreception involve direct H(+)-mediated activation of chemosensitive neurons and indirect modulation of chemosensitive neurons by purinergic signalling. Evidence suggests that RTN astrocytes are the source of CO2-evoked ATP release. However, it is not clear whether purinergic signalling also influences CO2/H(+) responsiveness of other putative chemoreceptors. The goals of this study are to determine if CO2/H(+)-sensitive neurons in the NTS and medullary raphe respond to ATP, and whether purinergic signalling in these regions influences CO2 responsiveness in vitro and in vivo. In brain slices, cell-attached recordings of membrane potential show that CO2/H(+)-sensitive NTS neurons are activated by focal ATP application; however, purinergic P2-receptor blockade did not affect their CO2/H(+) responsiveness. CO2/H(+)-sensitive raphe neurons were unaffected by ATP or P2-receptor blockade. In vivo, ATP injection into the NTS increased cardiorespiratory activity; however, injection of a P2-receptor blocker into this region had no effect on baseline breathing or CO2/H(+) responsiveness. Injections of ATP or a P2-receptor blocker into the medullary raphe had no effect on cardiorespiratory activity or the chemoreflex. As a positive control we confirmed that ATP injection into the RTN increased breathing and blood pressure by a P2-receptor-dependent mechanism. These results suggest that purinergic signalling is a unique feature of RTN chemoreception.