NMDA receptor subunit NR1-immunoreactivity in the rat pons and brainstem and colocalization with Fos induced by nasal stimulation

The 5-HT1F receptor as the target of ditans in migraine - from bench to bedside

Migraine is a leading cause of disability in more than one billion people worldwide, yet it remains universally underappreciated, even by individuals with the condition. Among other shortcomings, current treatments (often repurposed agents) have limited efficacy and potential adverse effects, leading to low treatment adherence. After the introduction of agents that target the calcitonin gene-related peptide pathway, another new drug class, the ditans - a group of selective serotonin 5-HT_1F receptor agonists - has just reached the international market. Here, we review preclinical studies from the late 1990s and more recent clinical research that contributed to the development of the ditans and led to their approval for acute migraine treatment by the US Food and Drug Administration and the European Medicines Agency.

Neurochemistry of the Kölliker-Fuse nucleus from a respiratory perspective

The Kölliker-Fuse nucleus (KF) is a functionally distinct component of the parabrachial complex, located in the dorsolateral pons of mammals. The KF has a major role in respiration and upper airway control. A comprehensive understanding of the KF and its contributions to respiratory function and dysfunction requires an appreciation for its neurochemical characteristics. The goal of this review is to summarize the diverse neurochemical composition of the KF, focusing on the neurotransmitters, neuromodulators, and neuropeptides present. We also include a description of the receptors expressed on KF neurons and transporters involved in each system, as well as their putative roles in respiratory physiology. Finally, we provide a short section reviewing the literature regarding neurochemical changes in the KF in the context of respiratory dysfunction observed in SIDS and Rett syndrome. By over-viewing the current literature on the neurochemical composition of the KF, this review will serve to aid a wide range of topics in the future research into the neural control of respiration in health and disease.

The Mammalian Diving Response: Inroads to Its Neural Control

The mammalian diving response (DR) is a remarkable behavior that was first formally studied by Laurence Irving and Per Scholander in the late 1930s. The DR is called such because it is most prominent in marine mammals such as seals, whales, and dolphins, but nevertheless is found in all mammals studied. It consists generally of breathing cessation (apnea), a dramatic slowing of heart rate (bradycardia), and an increase in peripheral vasoconstriction. The DR is thought to conserve vital oxygen stores and thus maintain life by directing perfusion to the two organs most essential for life-the heart and the brain. The DR is important, not only for its dramatic power over autonomic function, but also because it alters normal homeostatic reflexes such as the baroreceptor reflex and respiratory chemoreceptor reflex. The neurons driving the reflex circuits for the DR are contained within the medulla and spinal cord since the response remains after the brainstem transection at the pontomedullary junction. Neuroanatomical and physiological data suggesting brainstem areas important for the apnea, bradycardia, and peripheral vasoconstriction induced by underwater submersion are reviewed. Defining the brainstem circuit for the DR may open broad avenues for understanding the mechanisms of suprabulbar control of autonomic function in general, as well as implicate its role in some clinical states. Knowledge of the proposed diving circuit should facilitate studies on elite human divers performing breath-holding dives as well as investigations on sudden infant death syndrome (SIDS), stroke, migraine headache, and arrhythmias. We have speculated that the DR is the most powerful autonomic reflex known.

Perineural Injection of the Ethmoidal Nerve of Horses

OBJECTIVE

To describe the anatomical location of the ethmoidal nerve in the equine periorbital region and to determine the accuracy of a technique used to deposit local anesthetic solution adjacent to the nerve.

STUDY DESIGN

Prospective experimental study.

ANIMALS

Adult equine cadaver heads (n=6 for preliminary dissection; n=15 [30 nerve sites] for evaluation of injection technique).

METHODS

Preliminary dissections were performed to identify landmarks for injection of local anesthetic adjacent to the ethmoidal nerve. For evaluation of the injection technique a 20 gauge 6 cm spinal needle was inserted into the rostromedial aspect of the supraorbital fossa, where the caudal aspect of the zygomatic process emerges from the frontal bone caudal and medial to the globe. The needle was inserted to its hub at an angle of 110° to the long axis of the head using a protractor, in both a sagittal plane and a transverse plane, and 0.5 mL of new methylene blue dye was injected as a marker. The ethmoidal nerve was identified by dissection immediately after each injection and inspected for proximity of the dye.

RESULTS

Dye was observed surrounding the nerve in 27 of 30 sites (90%) or lying within 5 mm of the nerve at the other 3 sites.

CONCLUSION

The described technique of perineural injection of the ethmoidal nerve proved reliable and simple. Anesthetizing the ethmoidal nerve may be helpful in desensitizing portions of the ipsilateral paranasal sinuses and nasal cavity innervated by the nerve when sinonasal surgery is performed in the standing horse.

The Kölliker-Fuse nucleus: a review of animal studies and the implications for cranial nerve function in humans

To review the scientific literature on the relationship between Kölliker-Fuse nucleus (KF) and cranial nerve function in animal models, with view to evaluating the potential role of KF maturation in explaining age-related normal physiologic parameters and developmental and acquired impairment of cranial nerve function in humans. Medical databases (Medline and PubMed). Studies investigating evidence of KF activity responsible for a specific cranial nerve function that were based on manipulation of KF activity or the use of neural markers were included. Twenty studies were identified that involved the trigeminal (6 studies), vagus (9), and hypoglossal nerves (5). These pertained specifically to a role of the KF in mediating the dive reflex, laryngeal adductor control, swallowing function and upper airway tone. The KF acts as a mediator of a number of important functions that relate primarily to laryngeal closure, upper airway tone and swallowing. These areas are characterized by a variety of disorders that may present to the otolaryngologist, and hence the importance of understanding the role played by the KF in maintaining normal function.

Trigeminal Medullary Dorsal Horn Neurons Activated by Nasal Stimulation Coexpress AMPA, NMDA, and NK1 Receptors

Afferent information initiating the cardiorespiratory responses during nasal stimulation projects from the nasal passages to neurons within the trigeminal medullary dorsal horn (MDH) via the anterior ethmoidal nerve (AEN). Central AEN terminals are thought to release glutamate to activate the MDH neurons. This study was designed to determine which neurotransmitter receptors (AMPA, kainate, or NMDA glutamate receptor subtypes or the Substance P receptor NK1) are expressed by these activated MDH neurons. Fos was used as a neuronal marker of activated neurons, and immunohistochemistry combined with epifluorescent microscopy was used to determine which neurotransmitter receptor subunits were coexpressed by activated MDH neurons. Results indicate that, during nasal stimulation with ammonia vapors in urethane-anesthetized Sprague-Dawley rats, activated neurons within the superficial MDH coexpress the AMPA glutamate receptor subunits GluA1 (95.8%) and GluA2/3 (88.2%), the NMDA glutamate receptor subunits GluN1 (89.1%) and GluN2A (41.4%), and NK1 receptors (64.0%). It is therefore likely that during nasal stimulation the central terminals of the AEN release glutamate and substance P that then produces activation of these MDH neurons. The involvement of AMPA and NMDA receptors may mediate fast and slow neurotransmission, respectively, while NK1 receptor involvement may indicate activation of a nociceptive pathway.

Bilateral sectioning of the anterior ethmoidal nerves does not eliminate the diving response in voluntarily diving rats

The diving response is characterized by bradycardia, apnea, and increased peripheral resistance. This reflex response is initiated by immersing the nose in water. Because the anterior ethmoidal nerve (AEN) innervates the nose, our hypothesis was that intact AENs are essential for initiating the diving response in voluntarily diving rats. Heart rate (HR) and arterial blood pressure (BPa) were monitored using implanted biotransmitters. Sprague-Dawley rats were trained to voluntarily swim 5 m underwater. During diving, HR decreased from 480 ± 15 to 99 ± 5 bpm and BPa increased from 136 ± 2 to 187 ± 3 mmHg. Experimental rats (N = 9) then received bilateral AEN sectioning, while Sham rats (N = 8) did not. During diving in Experimental rats 7 days after AEN surgery, HR decreased from 478 ± 13 to 76 ± 4 bpm and BPa increased from 134 ± 3 to 186 ± 4 mmHg. Responses were similar in Sham rats. Then, during nasal stimulation with ammonia vapors in urethane-anesthetized Experimental rats, HR decreased from 368 ± 7 to 83 ± 4 bpm, and BPa increased from 126 ± 7 to 175 ± 4 mmHg. Responses were similar in Sham rats. Thus, 1 week after being sectioned the AENs are not essential for initiating a full cardiorespiratory response during both voluntary diving and nasal stimulation. We conclude that other nerve(s) innervating the nose are able to provide an afferent signal sufficient to initiate the diving response, although neuronal plasticity within the medullary dorsal horn may be necessary for this to occur.

Suppression of the cough reflex by α 2-adrenergic receptor agonists in the rabbit

The α₂-adrenergic receptor agonist clonidine has been shown to inhibit citric acid-induced cough responses in guinea pigs when administered by aerosol, but not orally. In contrast, oral or inhaled clonidine had no effect on capsaicin-induced cough and reflex bronchoconstriction in humans. In addition, intravenous administration of clonidine has been shown to depress fentanyl-induced cough in humans. We investigated the effects of the α₂-adrenergic receptor agonists, clonidine and tizanidine, on cough responses induced by mechanical and chemical (citric acid) stimulation of the tracheobronchial tree. Drugs were microinjected (30–50 nL) into the caudal nucleus tractus solitarii (cNTS) and the caudal ventral respiratory group (cVRG) as well as administered intravenously in pentobarbital sodium-anesthetized, spontaneously breathing rabbits. Bilateral microinjections of clonidine into the cNTS or the cVRG reduced cough responses at 0.5 mmol/L and abolished the cough reflex at 5 mmol/L. Bilateral microinjections of 0.5 mmol/L tizanidine into the cNTS completely suppressed cough responses, whereas bilateral microinjections of 5 mmol/L into the cVRG only caused mild reductions in them. Depressant effects on the cough reflex of clonidine and tizanidine were completely reverted by microinjections of 10 mmol/L yohimbine. Intravenous administration of clonidine (80–120 μg/kg) or tizanidine (150–300 μg/kg) strongly reduced or completely suppressed cough responses. These effects were reverted by intravenous administration of yohimbine (300 μg/kg). The results demonstrate that activation of α₂-adrenergic receptors in the rabbit exerts potent inhibitory effects on the central mechanism generating the cough motor pattern with a clear action at the level of the cNTS and the cVRG.

Modulation of the cough reflex by GABA(A) receptors in the caudal ventral respiratory group of the rabbit

We have previously shown that the caudal ventral respiratory group (cVRG) is a possible site of action of some antitussive drugs and plays a crucial role in determining both the expiratory and inspiratory components of the cough motor pattern. In addition, it has been reported that medullary expiratory neurons of the cVRG are subject to potent GABAergic gain modulation. This study was devoted to investigate the role of cVRG GABA_A receptors in the control of baseline respiratory activity and cough responses to mechanical and chemical (citric acid) stimulation of the tracheobronchial tree. To this purpose, bilateral microinjections (30–50 nl) of bicuculline or muscimol were performed into the cVRG of pentobarbital sodium-anesthetized, spontaneously breathing rabbits. Bicuculline (1 mM) increased peak abdominal activity and respiratory frequency due to decreases in T_E. Cough responses were potentiated mainly owing to increases in the cough number. The recovery was observed within ~2 h. On the contrary, muscimol (0.3 mM) abolished abdominal activity and decreased respiratory frequency due to increases in T_E. In addition, cough responses were progressively reduced and completely suppressed within ~20 min. Partial recovery of cough responses was achieved after ~3 h or within ~5 min following bicuculline microinjections at the same locations. The sneeze reflex induced by mechanical stimulation of the nasal mucosa persisted following bicuculline and muscimol microinjections. However, the number and intensity of expiratory thrusts were enhanced by bicuculline and suppressed by muscimol. The results provide evidence that a potent GABA_A-mediated inhibitory modulation is exerted at the level of the cVRG not only on respiratory activity, but also on cough and sneeze reflex responses.

Animal models for investigating the central control of the Mammalian diving response

Pioneering studies by Per Scholander indicated that the diving response consists of reflexly induced apnea, bradycardia and an alteration of blood flow that maintains perfusion of the heart and brain. More recently field physiological studies have shown that many marine animals can adjust cardiorespiratory aspects of their diving response depending upon the behavioral situation. This could suggest that the very labile heart rate during diving is under direct cortical control. However, the final control of autonomic nervous system functioning resides within the brainstem and not the cortex. Many physiologists regard the brain as a "black box" where important neuronal functioning occurs, but the complexity of such functioning leaves systematic investigation a daunting task. As a consequence the central control of the diving response has been under-investigated. Thus, to further advance the field of diving physiology by understanding its central neuronal control, it would be first necessary to understand the reflex circuitry that exists within the brainstem of diving animals. To do this will require an appropriate animal model. In this review, two animals, the muskrat and rat, will be offered as animal models to investigate the central aspects of the diving response. Firstly, although these rodents are not marine animals, natural histories indicate that both animals can and do exploit aquatic environments. Secondly, physiological recordings during natural and simulated diving indicate that both animals possess the same basic physiological responses to underwater submersion that occur in marine animals. Thirdly, the size and ease of housing of both animals makes them attractive laboratory research animals. Finally, the enormous amount of scientific literature regarding rodent brainstem autonomic control mechanisms, and the availability of brain atlases, makes these animals ideal choices to study the central control of the mammalian diving response.

Activation of brainstem neurons by underwater diving in the rat

The mammalian diving response is a powerful autonomic adjustment to underwater submersion greatly affecting heart rate, arterial blood pressure, and ventilation. The bradycardia is mediated by the parasympathetic nervous system, arterial blood pressure is mediated via the sympathetic system and still other circuits mediate the respiratory changes. In the present study we investigate the cardiorespiratory responses and the brainstem neurons activated by voluntary diving of trained rats, and, compare them to control and swimming animals which did not dive. We show that the bradycardia and increase in arterial blood pressure induced by diving were significantly different than that induced by swimming. Neuronal activation was calculated after immunohistochemical processing of brainstem sections for Fos protein. Labeled neurons were counted in the caudal pressor area, the medullary dorsal horn, subnuclei of the nucleus tractus solitarii (NTS), the nucleus raphe pallidus (RPa), the rostroventrolateral medulla, the A5 area, the nucleus locus coeruleus, the Kölliker–Fuse area, and the external lateral and superior lateral subnuclei of the parabrachial nucleus. All these areas showed significant increases in Fos labeling when data from voluntary diving rats were compared to control rats and all but the commissural subnucleus of the NTS, A5 area, and RPa were significantly different from swimming rats. These data provide a substrate for more precise experiments to determine the role of these nuclei in the reflex circuits driving the diving response.

Suppression of the cough reflex by inhibition of ERK1/2 activation in the caudal nucleus tractus solitarii of the rabbit

The caudal nucleus tractus solitarii (cNTS), the predominant site of termination of cough-related afferents, has been shown to be a site of action of some centrally acting antitussive agents. A role of ERK1/2 has been suggested in acute central processing of nociceptive inputs. Because pain and cough share similar features, we investigated whether ERK1/2 activation could also be involved in the central transduction of tussive inputs. For this purpose, we undertook the present research on pentobarbital sodium-anesthetized, spontaneously breathing rabbits by using microinjections (30–50 nl) of an inhibitor of ERK1/2 activation (U0126) into the cNTS. Bilateral microinjections of 25 mM U0126 caused rapid and reversible reductions in the cough responses induced by both mechanical and chemical (citric acid) stimulation of the tracheobronchial tree. In particular, the cough number and peak abdominal activity decreased. Bilateral microinjections of 50 mM U0126 completely suppressed the cough reflex without affecting the Breuer-Hering inflation reflex, the pulmonary chemoreflex, and the sneeze reflex. These U0126-induced effects were, to a large extent, reversible. Bilateral microinjections of 50 mM U0124, the inactive analog of U0126, at the same cNTS sites had no effect. This is the first study that provides evidence that ERK1/2 activation within the cNTS is required for the mediation of cough reflex responses in the anesthetized rabbit. These results suggest a role for ERK1/2 in the observed effects via nontranscriptional mechanisms, given the short time involved. They also may provide hints for the development of novel antitussive strategies.

Ionotropic glutamate receptors in the external lateral parabrachial nucleus participate in processing cardiac sympathoexcitatory reflexes

Stimulation of cardiac sympathetic afferents during myocardial ischemia with metabolites such as bradykinin (BK) evokes sympathoexcitatory reflex responses and activates neurons in the external lateral parabrachial nucleus (elPBN). The present study tested the hypothesis that this region in the pons processes sympathoexcitatory cardiac reflexes through an ionotropic glutamate receptor mechanism. The ischemic metabolite BK (0.1-1 μg) was injected into the pericardial space of anesthetized and bilaterally vagotomized or intact cats. Hemodynamic and renal sympathetic nerve activity (RSNA) responses to repeated administration of BK before and after unilateral 50-nl microinjections of kynurenic acid (Kyn; 25 mM), 2-amino-5-phosphonopentanoic acid (AP5; 25 mM), and 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzol(F)quinoxaline (NBQX; 10 mM) into the elPBN were recorded. Intrapericardial BK evoked significant increases in mean arterial pressure (MAP) and RSNA in seven vagotomized cats. After blockade of glutamate receptors with the nonselective glutamate receptor antagonist Kyn, the BK-evoked reflex increases in MAP (50 ± 6 vs. 29 ± 2 mmHg) and RSNA (59 ± 8.6 vs. 29 ± 4.7%, before vs. after) were significantly attenuated. The BK-evoked responses returned to pre-Kyn levels 85 min after the application of Kyn. Similarly, BK-evoked reflex responses were reversibly attenuated by blockade of glutamate N-methyl-d-aspartate (NMDA) receptors with AP5 (n = 5) and α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors with NBQX (n = 5). In contrast, we observed that the repetitive administration of BK evoked consistent reflex responses including MAP and RSNA before and after microinjection of 50 nl of the artificial cerebrospinal fluid vehicle into the elPBN in five animals. Microinjection of glutamate receptor antagonists into regions outside the elPBN did not alter BK-induced reflex responses. Microinjection of Kyn into the elPBN reversibly attenuated BK-induced reflex responses in four vagus intact animals. These data are the first to show that NMDA and AMPA ionotropic glutamate receptors in the elPBN play an important role in processing cardiac excitatory reflex responses.

Decrease in the expression of N-methyl-D-aspartate receptors in the nucleus tractus solitarii induces antinociception and increases blood pressure

N-methyl-D-aspartate receptors (NMDAR) have a role in cardiovascular control at the nucleus tractus solitarii (NTS), eliciting increases or decreases in blood pressure (BP), depending on the area injected with the agonists. In spite of the association between cardiovascular control and pain modulation, the effects of manipulating NMDAR in pain responses have never been evaluated. In this study, we decreased the expression of NMDAR in the NTS using gene transfer to target receptor subunits and evaluate long-term effects. Seven days after the injection of lentiviral vectors containing the NR1a subunit cDNA of NMDAR, in antisense orientation, into the intermediate NTS of Wistar rats, BP was measured, and the formalin test of nociception was performed. The antisense vector induced a decrease of NR1 expression in the NTS and elicited BP rises and hypoalgesia. Antisense vectors inhibited formalin-evoked c-Fos expression in the spinal cord, indicating decreased nociceptive activity of spinal neurons. Using a time-course approach, we verified that the onset of both the increases in BP and the hypoalgesia was at 4 days after vector injection into the NTS. The injection of NMDA into the NTS reversed the effects of antisense vectors in pain behavioral responses and spinal neuronal activation and decreased BP and heart rate. The present study shows that the NR1 subunit of the NMDAR at the NTS is critical in the regulation of tonic cardiovascular and nociceptive control and shows an involvement of the nucleus in the modulation of sustained pain.

Role of excitatory amino acids in the mediation of tracheobronchial cough induced by citric acid inhalation in the rabbit

We investigated the role of ionotropic glutamate receptors located within the caudal portions of the nucleus tractus solitarii (cNTS) and the caudal ventral respiratory group (cVRG) in the mediation of coughing evoked by citric acid inhalation in spontaneously breathing rabbits under pentobarbitone anaesthesia. Bilateral microinjections (30-50nl) of 10mM CNQX and 10mM D-AP5 were performed to block non-NMDA and NMDA receptors, respectively. An attempt was also made to investigate the effects of ionotropic glutamate receptor blockade within the cVRG on sneezing induced by mechanical stimulation of the nasal mucosa. Blockade of non-NMDA receptors within the cNTS abolished coughing and associated tachypneic responses, while blockade of NMDA receptors only reduced cough responses. Blockade of non-NMDA receptors within the cVRG always abolished spontaneous rhythmic abdominal activity as well as coughing and associated tachypneic responses; blockade of NMDA receptors only reduced spontaneous rhythmic abdominal activity and coughing. As to sneezing, blockade of non-NMDA receptors within the cVRG suppressed the expiratory thrusts without affecting the inspiratory preparatory bursts, while blockade of NMDA receptors only strongly attenuated the expiratory thrusts. This study is the first to provide evidence that ionotropic glutamate receptors, and especially non-NMDA receptors, are involved in the mediation of coughing induced by citric acid inhalation and to suggest that citric acid-activated cough-related afferents terminate within the cNTS. Present data also corroborate the notion that the cVRG is involved in the generation of the whole cough motor pattern, but seems to represent merely an expiratory output system for sneezing.

Postnatal emergence of synaptic plasticity associated with dynamic adaptation of the respiratory motor pattern

The shape of the three-phase respiratory motor pattern (inspiration, postinspiration, late expiration) is controlled by a central pattern generator (CPG) located in the ponto-medullary brainstem. Synaptic interactions between and within specific sub-compartments of the CPG are subject of intensive research. This review addresses the neural control of postinspiratory activity as the essential determinant of inspiratory/expiratory phase duration. The generation of the postinspiratory phase depends on synaptic interaction between neurones of the nucleus tractus solitarii (NTS), which relay afferent inputs from pulmonary stretch receptors, and the pontine Kölliker-Fuse nucleus (KF) as integral parts of the CPG. Both regions undergo significant changes during the first three postnatal weeks in rodents. Developmental changes in glutamatergic synaptic functions and its modulation by brain-derived neurotrophic factor may have implications in synaptic plasticity within the NTS/KF axis. We propose that dependent on these developmental changes, the CPG becomes permissive for short- and long-term plasticity associated with environmental, metabolic and behavioural adaptation of the breathing pattern.

Blockade of hindbrain NMDA receptors containing NR2 subunits increases sucrose intake

We have previously shown that blockade of N-methyl-d-aspartate (NMDA) receptors in the caudal brain stem delays satiation and increases food intake. NMDA receptors are heterodimers made up of distinct, but different, ion channel subunits. The NR2 subunits of the NMDA receptor contain the binding site for glutamate. About half of vagal afferents express immunoreactivity for NMDA NR2B subunit and about half of the NR2B expressing afferents also express NMDA NR2C or NR2D subunits. This suggests that increased food intake may be evoked by interference with glutamate binding to NMDA channels containing the NR2B subunit. To test this, we measured deprivation-induced intake of 15% sucrose solution following fourth ventricle and intra-nucleus of the solitary tract (intra-NTS) injections of Conantokin G (Con G; NR2B blocker), d-3-(2-carboxypiperazin-4-yl)-1-propenyl-1-phosphoric acid (d-CPPene; NR2B/2A blocker), and (+/-)-cis-1-(phenanthren-2yl-carbonyl)piperazine-2,3-dicarboxylic acid (PPDA; NR2D/C blocker). Fourth ventricular administration of Con G (5, 20, 40, 80 ng), d-CPPene (3.0, 6.25, 12.5, 25, 50, 100 ng), and PPDA (300, 400 ng) increased sucrose intake significantly compared with control. Likewise, injections of Con G (10 ng), d-CPPene (5 ng, 10 ng), and PPDA (0.5, 1.0, 2.5, 5.0 ng) directly into the NTS significantly increased sucrose intake. These results show that hindbrain injection of competitive NMDA antagonists with selectivity or preference for the NMDA receptor NR2B or NR2C subunits increases food intake.

Emergence of brain-derived neurotrophic factor-induced postsynaptic potentiation of NMDA currents during the postnatal maturation of the Kolliker-Fuse nucleus of rat

The Kölliker-Fuse nucleus (KF) contributes essentially to respiratory pattern formation and adaptation of breathing to afferent information. Systems physiology suggests that these KF functions depend on NMDA receptors (NMDA-R). Recent investigations revealed postnatal changes in the modulation of glutamatergic neurotransmission by brain-derived neurotrophic factor (BDNF) in the KF. Therefore, we investigated postnatal changes in NMDA-R subunit composition and postsynaptic modulation of NMDA-R-mediated currents by BDNF in KF slice preparations derived from three age groups (neonatal: postnatal day (P) 1-5; intermediate: P6-13; juvenile: P14-21). Immunohistochemistry showed a developmental up-regulation of the NR2D subunit. This correlated with a developmental increase in decay time of NMDA currents and a decline of desensitization in response to repetitive exogenous NMDA applications. Thus, developmental up-regulation of the NR2D subunit, which reduces the Mg(2+) block of NMDA-R, causes these specific changes in NMDA current characteristics. This may determine the NMDA-R-dependent function of the mature KF in the control of respiratory phase transition. Subsequent experiments revealed that bath-application of BDNF progressively potentiated these repetitively evoked NMDA currents only in intermediate and juvenile age groups. Pharmacological inhibition of protein kinase C (PKC), as a downstream component of the BDNF-tyrosine kinase B receptor (trkB) signalling, prevented BDNF-induced potentiation of NMDA currents. BDNF-induced potentiation of NMDA currents in later developmental stages might be essential for synaptic plasticity during the adaptation of the breathing pattern in response to peripheral/central commands. The lack of plasticity in neonatal neurones strengthens the hypothesis that the respiratory network becomes permissive for activity-dependent plasticity with ongoing postnatal development.

Developmental changes in the BDNF-induced modulation of inhibitory synaptic transmission in the Kölliker-Fuse nucleus of rat

The Kölliker-Fuse nucleus (KF), part of the pontine respiratory group, is involved in the control of respiratory phase duration, and receives both excitatory and inhibitory afferent input from various other brain regions. There is evidence for developmental changes in the modulation of excitatory inputs to the KF by the neurotrophin brain-derived neurotrophic factor (BDNF). In the present study we investigated if BDNF exerts developmental effects on inhibitory synaptic transmission in the KF. Recordings of inhibitory postsynaptic currents (IPSCs) in KF neurons in a pontine slice preparation revealed general developmental changes. Recording of spontaneous and evoked IPSCs (sIPSCs, eIPSCS) revealed that neonatally the gamma-aminobutyric acid (GABA)ergic fraction of IPSCs was predominant, while in later developmental stages glycinergic neurotransmission significantly increased. Bath-application of BDNF significantly reduced sIPSC frequency in all developmental stages, while BDNF-mediated modulation on eIPSCs showed developmental differences. The eIPSCs mean amplitude was uniformly and significantly reduced following BDNF application only in neurons from rats younger than postnatal day 10. At later postnatal stages the response pattern became heterogeneous, and both augmentations and reductions of eIPSC amplitudes occurred. All BDNF effects on eIPSCs and sIPSCs were reversed with the tyrosine kinase receptor-B inhibitor K252a. We conclude that developmental changes in inhibitory neurotransmission, including the BDNF-mediated modulation of eIPSCs, relate to the postnatal maturation of the KF. The changes in BDNF-mediated modulation of IPSCs in the KF may have strong implications for developmental changes in synaptic plasticity and the adaptation of the breathing pattern to afferent inputs.

Effects of anesthetics on hypoglossal nerve discharge and c-Fos expression in brainstem hypoglossal premotor neurons

This study examined the effects of anesthesia on the hypoglossal nerve and diaphragm activities and on c-Fos expression in brainstem hypoglossal premotor neurons (pmXII). Experiments were performed in 71 rats by using halothane inhalation, pentobarbital sodium, or mixtures of alpha-chloralose and urethane or ketamine and xylazine. First, various cardiorespiratory parameters were measured in the rats (n = 31) during both awake and anesthetized conditions. The volatile anesthetic halothane, but not the other anesthetics, was always associated with a strong phasic inspiratory activity in the hypoglossal nerve. Second, a double-immunohistochemical study was performed in awake and anesthetized rats (n = 40) to gauge the level of activity of pmXII neurons. Brainstem pmXII neurons were identified after microiontophoresis of the retrograde tracer Fluoro-Gold in the right hypoglossal motor nucleus. Patterns of c-Fos expression at different brainstem levels were compared in five groups of rats (i.e., awake or anesthetized with halothane, pentobarbital, chloralose-urethane, and ketamine-xylazine). Sections were processed for double detection of c-Fos protein and Fluoro-Gold by using the standard ABC method and a two-color peroxidase technique. Anesthesia with halothane induced the strongest c-Fos expression in a restricted pool of pmXII located in the pons at the level of the Kölliker-Fuse nucleus and the intertrigeminal region. The results demonstrated a major effect of halothane in inducing changes in hypoglossal activity and revealed a differential expression of c-Fos protein in pmXII neurons among groups of anesthetized rats. We suggest that halothane mediates changes in respiratory hypoglossal nerve discharge by altering activity of premotor neurons in the Kölliker-Fuse and intertrigeminal region.

Involvement of NMDA receptors in Zif/268 expression in the trigeminal nucleus caudalis following formalin injection into the rat whisker pad

We investigated the involvement of N-methyl-D-aspartate (NMDA) glutamate receptor in the expression of the proteins Zif/268 and c-Fos elicited by painful stimuli. To this purpose, the effect of the administration of MK-801, an NMDA receptor antagonist, on Zif/268 and c-Fos expression following a noxious stimulus, represented by formalin injection into the whisker pad of rats, was examined in neurons of the trigeminal nucleus caudalis. Furthermore, the co-localization of formalin injection-evoked Zif/268 and c-Fos expression and subunit 1 of the NMDA receptor (NR1) was studied in this nucleus. Zif/268 or c-Fos immunoreactivity elicited by formalin injection was significantly reduced by pretreatment with MK-801 in the superficial layer of the trigeminal nucleus caudalis; more than 40% of the neurons expressing Zif/268 and c-Fos in this layer were also immunolabeled by NR1. On the other hand, there was little effect of MK-801 administration on Zif/268 and c-Fos immunoreactivity in the nucleus proprius and deep lamina V of the trigeminal nucleus caudalis, while most neurons expressing Zif/268 or c-Fos in these two regions were labeled by NR1. These results point out differences between the superficial and deeper layers of the trigeminal nucleus caudalis in the involvement of NMDA receptor in the mechanisms underlying the expression of protein products of immediate early genes induced by painful stimuli.

NMDA Receptor 1 Expression in the Brainstem of Human Infants and Its Relevance to the Sudden Infant Death Syndrome (SIDS)

The N-methyl-D-aspartate (NMDA) glutamatergic receptor is widely expressed in the brain during the early postnatal period and, among other functions is involved in cardiorespiratory control and in cell death by excitotoxic mechanisms. This study examined NMDA receptor-1 (NR1) expression in the human infant brainstem and assessed whether expression differed between non-SIDS and SIDS infants. NRI mRNA was identified using non-radioactive in situ hybridization and quantified by optical density. NRI protein was identified by immunohistochemistry and quantified by cellular counting. Eight nuclei of the mid-medulla and 2 nuclei of the rostral pons were studied. NRI mRNA and protein were expressed in all nuclei studied, confirming that the NMDA receptor is widely distributed in the human infant brainstem. Compared to non-SIDS infants (n = 10). SIDS infants (n = 15) had increased mRNA in 6 nuclei of the mid-medulla (p < 0.05 for all) while protein was increased in the dorsal motor nucleus of the vagus (p = 0.04) and decreased in the nucleus of the spinal trigeminal tract (p = 0.03). No differences were observed in the rostral pons. This preliminary study suggests that abnormalities of the glutamatergic system are present in SIDS victims. Further studies will be required to delineate these abnormalities and to investigate potential underlying mechanisms and sequelae.

Role of excitatory amino acid inputs to the rostral ventrolateral medulla in cardiovascular regulation

1. Excitatory amino acid (EAA)-mediated neural transmission in the rostral ventrolateral medulla (RVLM) is important for many cardiovascular reflexes, although the receptor subtypes involved vary depending on the specific response. 2. Although injection of the EAA ionotropic receptor antagonist kynurenic acid into the RVLM has no effect on baseline arterial pressure, this lack of effect appears to result from EAA inputs to RVLM exciting both excitatory and inhibitory mechanisms within the RVLM. 3. The balance between EAA-mediated excitation and inhibition of RVLM neurons may be shifted to excitation in experimental models of hypertension. 4. The excitatory influence that EAA inputs to the RVLM have on vasomotor neurons in the RVLM may involve a sarthran-sensitive intermediary in the RVLM.

Food-related gastrointestinal signals activate caudal brainstem neurons expressing both NMDA and AMPA receptors

Vagal mechano- and chemosensors in the gastrointestinal tract and the portal-hepatic axis signaling the arrival of nutrients are major determinants of the satiation process. Although glutamate and its various receptor subtypes have been shown to transmit gustatory and cardiovascular sensory information at the level of the solitary nucleus (nucleus tractus solitarius; NTS), their involvement in the transmission of gastrointestinal satiety signals is not clear. Gastrointestinal sensors were stimulated by gastric balloon distension or by intraduodenal infusion of either linoleic acid or glucose in chronically catheterized, non-anesthetized rats, leading to activation of second order neurons in the NTS as detected by c-Fos immunohistochemistry. Subsequent (double)-immunohistochemistry for either NMDA or AMPA glutamate receptors was used to determine the percentage of activated neurons expressing a particular receptor subtype. Gastric distension and duodenal nutrient stimuli produced slightly, but significantly different patterns of c-Fos induction in the dorsal vagal complex. Expression of NMDA receptors, as detected by a NR2ab subunit-specific antibody, was abundant throughout the dorsal medulla. The percentage of neurons in the NTS activated by gastric distension (63.9+/-2.9%), linoleic acid (62.8+/-1.4%), and glucose (64.1+/-1.4%), expressing NMDA receptor was similar. Expression of AMPA receptors, as detected by a GLUR2/3 subunit-specific antibody, was equally abundant throughout the dorsal medulla. Again, the percentage of activated neurons expressing GLUR2/3 was similar for the gastric distension (59.8-65.6%) and duodenal linoleic acid (60.6-67.0%) stimuli, and for the various subnuclei of the NTS. Finally, GLUR1-specific immunoreactivity was much less abundant, with only a small percentage of distension-activated (4.4+/-0.4%) and linoleic acid-activated (5.1+/-0.4%) neurons expressing this receptor subunit. The results suggest a widespread, general involvement of both NMDA and AMPA receptors in primary afferent signal transmission at the level of the NTS, with no differential recruitment of the examined receptor subtypes by the different gastrointestinal sensory stimuli. This may indicate a high degree of convergence among sensory signals, or alternatively, the presence of other transmission systems such as peptides referring sensory specificity to second order neurons.

Qualitative light and electron microscope study of glutamate receptors in the caudal spinal trigeminal nucleus of the rat

Though ionotropic and metabotropic glutamate receptors have recently been recognized to play important roles in the transmission of orofacial nociceptive impulses, their detailed distribution in the spinal trigeminal nucleus has not been systematically investigated. There is also controversy regarding the electron microscope localization of metabotropic receptors. We therefore undertook this investigation to address the above-mentioned issues in the caudal part of the spinal trigeminal nucleus, using light and electron microscope immunocytochemistry, to provide baseline information for the development of agonists and antagonists of these receptors in the clinical treatment of orofacial pain. The results showed some moderately to strongly stained glutamate receptor 1 neurons, and many strongly stained glutamate receptor 2/3 neurons in lamina II of the nucleus, suggesting that the latter may play an important role in orofacial pain processing, with the former playing a minor role. The metabotropic glutamate receptor 1 immunoreactive product was localized mostly in dendrites, while most of the metabotropic glutamate receptors 2/3 immunoreactive product was deposited in axon terminals containing synaptic vesicles of different shapes, suggesting that glutamate receptors 2/3 may control the release of both excitatory and inhibitory neurotransmitters.

Functional and chemical anatomy of the afferent vagal system

The results of neural tracing studies suggest that vagal afferent fibers in cervical and thoracic branches innervate the esophagus, lower airways, heart, aorta, and possibly the thymus, and via abdominal branches the entire gastrointestinal tract, liver, portal vein, billiary system, pancreas, but not the spleen. In addition, vagal afferents innervate numerous thoracic and abdominal paraganglia associated with the vagus nerves. Specific terminal structures such as flower basket terminals, intraganglionic laminar endings and intramuscular arrays have been identified in the various organs and organ compartments, suggesting functional specializations. Electrophysiological recording studies have identified mechano- and chemo-receptors, as well as temperature- and osmo-sensors. In the rat and several other species, mostly polymodal units, while in the cat more specialized units have been reported. Few details of the peripheral transduction cascades and the transmitters for signal propagation in the CNS are known. Glutamate and its various receptors are likely to play an important role at the level of primary afferent signaling to the solitary nucleus. The vagal afferent system is thus in an excellent position to detect immune-related events in the periphery and generate appropriate autonomic, endocrine, and behavioral responses via central reflex pathways. There is also good evidence for a role of vagal afferents in nociception, as manifested by affective-emotional responses such as increased blood pressure and tachycardia, typically associated with the perception of pain, and mediated via central reflex pathways involving the amygdala and other parts of the limbic system. The massive central projections are likely to be responsible for the antiepileptic properties of afferent vagal stimulation in humans. Furthermore, these functions are in line with a general defensive character ascribed to the vagal afferent, paraventricular system in lower vertebrates.