Collateral damage and compensatory changes after injection of a toxin targeting neurons with the neurokinin-1 receptor in the nucleus tractus solitarii of rat

A role for neurokinin 1 receptor expressing neurons in the paratrigeminal nucleus in bradykinin-evoked cough in guinea-pigs

KEY POINTS

Airway projecting sensory neurons arising from the jugular vagal ganglia terminate centrally in the brainstem paratrigeminal nucleus, synapsing upon neurons expressing the neurokinin 1 receptor. This study aimed to assess the involvement of paratrigeminal neurokinin 1 receptor neurons in the regulation of cough, breathing and airway defensive responses. Lesioning neurokinin 1 receptor expressing paratrigeminal neurons significantly reduced cough evoked by inhaled bradykinin but not inhaled ATP or tracheal mechanical stimulation. The reduction in bradykinin-evoked cough was not accompanied by changes in baseline or evoked respiratory variables (e.g. frequency, volume or timing), animal avoidance behaviours or the laryngeal apnoea reflex. These findings warrant further investigations into targeting the jugular ganglia and paratrigeminal nucleus as a therapy for treating cough in disease.

ABSTRACT

Jugular vagal ganglia sensory neurons innervate the large airways and are thought to mediate cough and associated perceptions of airway irritations to a range of chemical irritants. The central terminals of jugular sensory neurons lie within the brainstem paratrigeminal nucleus, where postsynaptic neurons can be differentiated based on the absence or presence of the neurokinin 1 (NK1) receptor. Therefore, in the present study, we set out to test the hypothesis that NK1 receptor expressing paratrigeminal neurons play a role in cough evoked by inhaled chemical irritants. To test this, we performed selective neurotoxin lesions of NK1 receptor expressing neurons in the paratrigeminal nucleus in guinea-pigs using substance P conjugated to saporin (SSP-SAP). Sham lesion control or SSP-SAP lesion guinea-pigs received nebulised challenges, with the pan-nociceptor stimulant bradykinin or the nodose ganglia specific stimulant adenosine 5'-triphosphate (ATP), in conscious whole-body plethysmography to study cough and associated behaviours. Laryngeal apnoea reflexes and cough evoked by mechanical stimulation of the trachea were additionally investigated in anaesthetised guinea-pigs. SSP-SAP significantly and selectively reduced the number of NK1 receptor expressing neurons in the paratrigeminal nucleus. This was associated with a significant reduction in bradykinin-evoked cough, but not ATP-evoked cough, mechanical cough or laryngeal apnoeic responses. These data provide further evidence for a role of jugular vagal pathways in cough, and additionally suggest an involvement of NK1 receptor expressing neurons in the paratrigeminal nucleus. Therefore, this neural pathway may provide novel therapeutic opportunities to treat conditions of chronic cough.

Reduced responses to glutamate receptor agonists follow loss of astrocytes and astroglial glutamate markers in the nucleus tractus solitarii

Saporin (SAP) or SAP conjugates injected into the nucleus tractus solitarii (NTS) of rats kill astrocytes. When injected in its unconjugated form, SAP produces no demonstrable loss of or damage to local neurons. However bilateral injections of SAP significantly attenuate responses to activation of baroreceptor reflexes that are mediated by transmission of signals through glutamate receptors in the NTS We tested the hypothesis that SAP would reduce cardiovascular responses to activation of NTS glutamate receptors despite its recognized ability to spare local neurons while killing local astrocytes. In animals treated with SAP and SAP conjugates or, as a control, with the toxin 6-hydroxydopamine (6-OHDA), we sought to determine if dose-related changes of arterial pressure (AP) or heart rate (HR) in response to injection into NTS of N-methyl-d-aspartate (NMDA) or α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) were attenuated. Also we quantified changes in immunoreactivity (IR) for EAAT2, EAAC1, and VGluT2 in NTS after SAP and SAP conjugates. Our earlier studies showed that IR for NMDA and AMPA receptors was not changed after injection of SAP We found that EAAT2 and EAAC1, both found in astrocytes, were reduced by SAP or its conjugates but not by 6-OHDA In contrast, VGluT2-IR was increased by SAP or conjugates but not by 6-OHDA AP and HR responses to NMDA and AMPA were attenuated after SAP and SAP conjugate injection but not after 6-OHDA Results of this study are consistent with others that have shown interactions between astroglia and neurons in synaptic transmission mediated by glutamate receptor activation in the NTS.

Chemosensitive Phox2b-expressing neurons are crucial for hypercapnic ventilatory response in the nucleus tractus solitarius

KEY POINTS

Central hypercapnic hypoventilation is highly prevalent in children suffering from congenital central hypoventilation syndrome (CCHS). Mutations of the gene for paired-like homeobox 2b (Phox2b) are aetiologically associated with CCHS and Phox2b is present in central components of respiratory chemoreflex, such as the nucleus tractus solitarius (NTS). Injection of the neurotoxin substance P-saporin into NTS destroys Phox2b-expressing neurons. Impaired hypercapnic ventilatory response caused by this neurotoxin is attributable to a loss of CO₂ -sensitive Phox2b-expressing NTS neurons. A subgroup of Phox2b-expressing neurons exhibits intrinsic chemosensitivity. A background K⁺ channel-like current is partially responsible for such chemosensitivity in Phox2b-expressing neurons. The present study helps us better understand the mechanism of respiratory deficits in CCHS and potentially locates a brainstem site for development of precise clinical intervention.

ABSTRACT

The nucleus tractus solitarius (NTS) neurons have been considered to function as central respiratory chemoreceptors. However, the common molecular marker defined for these neurons remains unknown. The present study investigated whether paired-like homeobox 2b (Phox2b)-expressing NTS neurons are recruited in hypercapnic ventilatory response (HCVR) and whether these neurons exhibit intrinsic chemosensitivity. HCVR was assessed using whole body plethysmography and neuronal chemosensitivity was examined by patch clamp recordings in brainstem slices or dissociated neurons from Phox2b-EGFP transgenic mice. Injection of the neurotoxin substance P-saporin (SSP-SAP) into NTS destroyed Phox2b-expressing neurons. Minute ventilation and tidal volume were both reduced by 13% during exposure to 8% CO₂ in inspired air when ∼13% of the Phox2b-expressing neurons were eliminated. However, a loss of ∼18% of these neurons was associated with considerable decreases in minute ventilation by ≥18% and in tidal volume by≥22% when challenged by ≥4% CO₂ . In both cases, breathing frequency was unaffected. Most CO₂ -activated neurons were immunoreactive to Phox2b. In brainstem slices, ∼43% of Phox2b-expressing neurons from Phox2b-EGFP mice displayed a sustained or transient increase in firing rate during physiological acidification (pH 7.0 or 8% CO₂ ). Such a response was also present in dissociated neurons in favour of an intrinsic property. In voltage clamp recordings, a background K⁺ channel-like current was found in a subgroup of Phox2b-expressing neurons. Thus, the respiratory deficits caused by injection of SSP-SAP into the NTS are attributable to proportional lesions of CO₂ /H⁺ -sensitive Phox2b-expressing neurons.

Neurokinin-1 Receptor-Immunopositive Neurons in the Medullary Dorsal Horn Provide Collateral Axons to both the Thalamus and Parabrachial Nucleus in Rats

It has been suggested that the trigemino-thalamic and trigemino-parabrachial projection neurons in the medullary dorsal horn (MDH) are highly implicated in the sensory-discriminative and emotional/affective aspects of orofacial pain, respectively. In previous studies, some neurons were reported to send projections to both the thalamus and parabrachial nucleus by way of collaterals in the MDH. However, little is known about the chemoarchitecture of this group of neurons. Thus, in the present study, we determined whether the neurokinin-1 (NK-1) receptor, which is crucial for primary orofacial pain signaling, was expressed in MDH neurons co-innervating the thalamus and parabrachial nucleus. Vesicular glutamate transporter 2 (VGLUT2) mRNA, a biomarker for the subgroup of glutamatergic neurons closely related to pain sensation, was assessed in trigemino-parabrachial projection neurons in the MDH. After stereotactic injection of fluorogold (FG) and cholera toxin subunit B (CTB) into the ventral posteromedial thalamic nucleus (VPM) and parabrachial nucleus (PBN), respectively, triple labeling with fluorescence dyes for FG, CTB and NK-1 receptor (NK-1R) revealed that approximately 76 % of the total FG/CTB dually labeled neurons were detected as NK-1R-immunopositive, and more than 94 % of the triple-labeled neurons were distributed in lamina I. In addition, by FG retrograde tract-tracing combined with fluorescence in situ hybridization (FISH) for VGLUT2 mRNA, 54, 48 and 70 % of FG-labeled neurons in laminae I, II and III, respectively, of the MDH co-expressed FG and VGLUT2 mRNA. Thus, most of the MDH neurons co-innervating the thalamus and PBN were glutamatergic. Most MDH neurons providing the collateral axons to both the thalamus and parabrachial nucleus in rats were NK-1R-immunopositive and expressed VGLUT2 mRNA. NK-1R and VGLUT2 in MDH neurons may be involved in both sensory-discriminative and emotional/affective aspects of orofacial pain processing.

Synaptogenesis and Myelination in the Nucleus/Tractus Solitarius: Potential Role in Apnea of Prematurity, Congenital Central Hypoventilation, and Sudden Infant Death Syndrome

Fetuses as early as 15 weeks' gestation exhibit rhythmical respiratory movements shown by real-time ultrasonography. The nucleus/tractus solitarius is the principal brainstem respiratory center; other medullary nuclei also participate. The purpose was to determine temporal maturation of synaptogenesis. Delayed synaptic maturation may explain neurogenic apnea or hypoventilation of prematurity and some cases of sudden infant death syndrome. Sections of medulla oblongata were studied from 30 human fetal and neonatal brains 9 to 41 weeks' gestation. Synaptophysin demonstrated the immunocytochemical sequence of synaptogenesis. Other neuronal markers and myelin stain also were applied. The nucleus/tractus solitarius was similarly studied in fetuses with chromosomopathies, metabolic encephalopathies, and brain malformations. Synapse formation in the nucleus solitarius begins at about 12 weeks' gestation and matures by 15 weeks; myelination initiated at 33 weeks. Synaptogenesis was delayed in 3 fetuses with different conditions, but was not specific for only nucleus solitarius. Delayed synaptogenesis or myelination in the nucleus solitarius may play a role in neonatal hypoventilation, especially in preterm infants and in some sudden infant death syndrome cases.

Astrocytes in the rat nucleus tractus solitarii are critical for cardiovascular reflex control

We have shown that an antibody to dopamine-β-hydroxylase conjugated with saporin (anti-DBH-SAP) damages catecholamine neurons in the nucleus tractus solitarii (NTS) of rat, attenuates arterial baroreflexes, and leads to lability of arterial blood pressure, damage to cardiac myocytes, and, in some animals, sudden death. However, others have shown that injection of 6-hydroxydopamine (6-OHDA), a toxin devoid of saporin, also damaged NTS catecholamine neurons but did not lead to these cardiovascular changes. We found similar cardiovascular changes after injecting a different SAP conjugate to target NTS neurons with neurokinin (NK1) receptors. Because ribosome-inactivating proteins may be toxic to glia, we hypothesized that SAP, a ribosome-inactivating protein, might target glia whose loss could account for physiological changes. We tested this hypothesis by assessing effects on select neurons and on glia in the NTS after exposure to SAP, targeted SAP conjugates, or 6-OHDA. SAP and all SAP conjugates led to loss of immunoreactivity for glial fibrillary acidic protein, a marker for astrocytes, in the NTS while 6-OHDA did not. As reported previously, anti-DBH-SAP selectively killed noradrenergic neurons in the NTS while SAP conjugated to stabilized substance P (SSP-SAP) selectively killed neurons with NK1 receptors. In contrast, SAP produced no demonstrable neuronal damage. All injections led to activation of microglia in the NTS; however, only SAP and its conjugates attenuated cardiovascular reflexes while also producing lability of arterial pressure, damage to cardiac myocytes, and in some animals, sudden death. Thus, NTS astrocytes may play a role in mediating cardiovascular reflex transmission through the NTS.

Sudden death following selective neuronal lesions in the rat nucleus tractus solitarii

In efforts to assess baroreflex and cardiovascular responses in rats in which substance P (SP) or catecholamine transmission had been eliminated we studied animals after bilateral injections into the nucleus tractus solitarii (NTS) of SP or stabilized SP (SSP) conjugated to saporin (SP-SAP or SSP-SAP respectively) or SAP conjugated to an antibody to dopamine-β-hydroxylase (anti-DBH-SAP). We found that SP- and SSP-SAP eliminated NTS neurons that expressed the SP neurokinin-1 receptor (NK1R) while anti-DBH-SAP eliminated NTS neurons expressing tyrosine hydroxylase (TH) and DBH. The toxins were selective. Thus SP- or SSP-SAP did not eliminate TH/DBH neurons and anti-DBH-SAP did not eliminate NK1R neurons in the NTS. Each toxin, however, led to chronic lability of arterial blood pressure, diminished baroreflex function, cardiac ventricular irritability, coagulation necrosis of cardiac myocytes and, in some animals, sudden death associated with asystole. However, when TH/DBH neurons were targeted and eliminated by injection of 6-hydroxydopamine (6-OHDA), none of the cardiovascular or cardiac changes occurred. The studies reviewed here reveal that selective lesions of the NTS lead to altered baroreflex control and to cardiac changes that may lead to sudden death. Though the findings could support a role for SP or catecholamines in baroreflex transmission neither is proven in that NK1R colocalizes with glutamate receptors. Thus neurons with both are lost when treated with SP- or SSP-SAP. In addition, loss of catecholamine neurons after treatment with 6-OHDA does not affect cardiovascular control. Thus, the effect of the toxins may depend on an action of SAP independent of the effects of the SAP conjugates on targeted neuronal types.

Sudden death and myocardial lesions after damage to catecholamine neurons of the nucleus tractus solitarii in rat

Lesions that remove neurons expressing neurokinin-1 (NK1) receptors from the nucleus tractus solitarii (NTS) without removing catecholaminergic neurons lead to loss of baroreflexes, labile arterial pressure, myocardial lesions, and sudden death. Because destruction of NTS catecholaminergic neurons expressing tyrosine hydroxylase (TH) may also cause lability of arterial pressure and loss of baroreflexes, we sought to test the hypothesis that cardiac lesions associated with lability are not dependent on damage to neurons with NK1 receptors but would also occur when TH neurons in NTS are targeted. To rid the NTS of TH neurons we microinjected anti-dopamine β-hydroxylase conjugated to saporin (anti-DBH-SAP, 42 ng/200 nl) into the NTS. After injection of the toxin unilaterally, immunofluorescent staining confirmed that anti-DBH-SAP decreased the number of neurons and fibers that contain TH and DBH in the injected side of the NTS while sparing neuronal elements expressing NK1 receptors. Bilateral injections in eight rats led to significant lability of arterial pressure. For example, on day 8 standard deviation of mean arterial pressure was 16.8 ± 2.5 mmHg when compared with a standard deviation of 7.83 ± 0.33 mmHg in six rats in which phosphate buffered saline (PBS) had been injected bilaterally. Two rats died suddenly at 5 and 8 days after anti-DBH-SAP injection. Seven-treated animals demonstrated microscopic myocardial necrosis as reported in animals with lesions of NTS neurons expressing NK1 receptors. Thus, cardiac and cardiovascular effects of lesions directed toward catecholamine neurons of the NTS are similar to those following damage directed toward NK1 receptor-containing neurons.