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.

1. We sought to outline the brainstem circuit responsible for the increase in sympathetic tone caused by chemical stimulation of the nasal passages with ammonia vapour. Experiments were performed in alpha-chloralose-anaesthetized, paralysed and artificially ventilated rats. 2. Stimulation of the nasal mucosa increased splanchnic sympathetic nerve discharge (SND), elevated arterial blood pressure (ABP), raised heart rate slightly and inhibited phrenic nerve discharge. 3. Bilateral injections of the broad-spectrum excitatory amino acid receptor antagonist kynurenate (Kyn) into the rostral part of the ventrolateral medulla (RVLM; rostral C1 area) greatly reduced the effects of nasal mucosa stimulation on SND (-80 %). These injections had no effect on resting ABP, resting SND or the sympathetic baroreflex. 4. Bilateral injections of Kyn into the ventrolateral medulla at the level of the obex (caudal C1 area) or into the nucleus tractus solitarii (NTS) greatly attenuated the baroreflex and significantly increased the baseline levels of both SND and ABP. However they did not reduce the effect of nasal mucosa stimulation on SND. 5. Single-unit recordings were made from 39 putative sympathoexcitatory neurons within the rostral C1 area. Most neurons (24 of 39) were activated by nasal mucosa stimulation (+65.8 % rise in discharge rate). Responding neurons had a wide range of conduction velocities and included slow-conducting neurons identified previously as C1 cells. The remaining putative sympathoexcitatory neurons were either unaffected (n = 8 neurons) or inhibited (n = 7) during nasal stimulation. We also recorded from ten respiratory-related neurons, all of which were silenced by nasal stimulation. 6. In conclusion, the sympathoexcitatory response to nasal stimulation is largely due to activation of bulbospinal presympathetic neurons within the RVLM. We suggest that these neurons receive convergent and directionally opposite polysynaptic inputs from arterial baroreceptors and trigeminal afferents. These inputs are integrated within the rostral C1 area as opposed to the NTS or the caudal C1 area.

Stimulation of the nasal passages with ammonia vapors can initiate a nasopharyngeal response that resembles the diving response. This response consists of a sympathetically mediated increase in peripheral vascular resistance, parasympathetically mediated bradycardia and an apnea. The current study investigated the role of the anterior ethmoidal nerve (AEN) in the nasopharyngeal response in the rat, as it is thought that the AEN provides the main sensory innervation of the nasal passages. When both AENs were intact, nasal stimulation caused significant bradycardia, hypertension, and apnea and produced Fos label ventrally within the ipsilateral medullary dorsal horn (MDH) and paratrigeminal nucleus just caudal to the obex. This labeling presumably represents activation of second-order trigeminal neurons. When only one AEN was intact, the nasopharyngeal response was slightly attenuated, and a similar pattern of Fos labeling was only seen in the trigeminal nucleus ipsilateral to the intact AEN. The trigeminal labeling contralateral to the intact AEN was significantly reduced. When both AENs were cut, the nasopharyngeal response to nasal stimulation consisted of only a slight apnea and an increase in arterial pressure; the resultant Fos labeling within the trigeminal nucleus was significantly reduced. Cutting both AENs but not stimulating the nasal passages also produced some Fos labeling within the trigeminal nucleus. These findings suggest that a single AEN can provide sufficient afferent input to initiate the cardiorespiratory changes consistent with the nasopharyngeal response. We conclude that the AEN provides a unique afferent contribution that is capable of producing the diving response.

The distribution of labeled neurons in the brain and spinal cord was studied after injecting the Bartha strain of pseudorabies virus (PRV) into the sciatic nerve to provide a baseline for studying neural circuitry after spinal cord injury (SCI) and regeneration. Following a single injection of viral particles into the left sciatic nerve, PRV labeling was found in the spinal cord at 2 days post-injection (p.i.). Increasing complexity in viral labeling from the spinal cord to supraspinal regions became apparent with increasing survival time. In brain regions, several neuronal groups that regulate sympathetic outflow, such as the rostroventrolateral medulla, the lateral paragigantocellular nuclei, and the A5 cells, were densely labeled. However, relatively sparse labeling was noticed in the lateral vestibular nuclei, the red nucleus and the motor cortex whose spinal projections regulate somatic motor function, although those areas were abundantly labeled with Fast blue (FB) in a double-labeling experiment in which FB was co-injected into the lumbar cord. The pattern of viral labeling became more complex beyond 5 days p.i. when increased numbers of cell groups were labeled with PRV but not FB. In addition, some infected neurons started to lyse, as evidenced by a decrease in viral labeling at 7 days p.i. Thus, the 5th day post-viral injection would appear to be an appropriate survival time to obtain maximal labeling with acceptable specificity. We suggest that transneuronal labeling using PRV should be appropriate for studying multi-neural circuitry after SCI and regeneration.