Effect of sinus denervation and vagotomy on c-fos expression in the nucleus tractus solitarius after exposure to CO2

Nucleus Tractus Solitarius Neurons Activated by Hypercapnia and Hypoxia Lack Mu Opioid Receptor Expression

Impaired chemoreflex responses are a central feature of opioid-induced respiratory depression, however, the mechanism through which mu opioid receptor agonists lead to diminished chemoreflexes is not fully understood. One brainstem structure involved in opioid-induced impairment of chemoreflexes is the nucleus of the solitary tract (NTS), which contains a population of neurons that express mu opioid receptors. Here, we tested whether caudal NTS neurons activated during the chemoreflex challenge express mu opioid receptors and overlap with neurons activated by opioids. Using genetic labeling of mu opioid receptor-expressing neurons and cFos immunohistochemistry as a proxy for neuronal activation, we examined the distribution of activated NTS neurons following hypercapnia, hypoxia, and morphine administration. The main finding was that hypoxia and hypercapnia primarily activated NTS neurons that did not express mu opioid receptors. Furthermore, concurrent administration of morphine with hypercapnia induced cFos expression in non-overlapping populations of neurons. Together these results suggest an indirect effect of opioids within the NTS, which could be mediated through mu opioid receptors on afferents and/or inhibitory interneurons.

Modulation of hypercapnic respiratory response by cholinergic transmission in the commissural nucleus of the solitary tract

The nucleus of the solitary tract (NTS) is an important area of the brainstem that receives and integrates afferent cardiorespiratory sensorial information, including those from arterial chemoreceptors and baroreceptors. It was described that acetylcholine (ACh) in the commissural subnucleus of the NTS (cNTS) promotes an increase in the phrenic nerve activity (PNA) and antagonism of nicotinic receptors in the same region reduces the magnitude of tachypneic response to peripheral chemoreceptor stimulation, suggesting a functional role of cholinergic transmission within the cNTS in the chemosensory control of respiratory activity. In the present study, we investigated whether cholinergic receptor antagonism in the cNTS modifies the sympathetic and respiratory reflex responses to hypercapnia. Using an arterially perfused in situ preparation of juvenile male Holtzman rats, we found that the nicotinic antagonist (mecamylamine, 5 mM), but not the muscarinic antagonist (atropine, 5 mM), into the cNTS attenuated the hypercapnia-induced increase of hypoglossal activity. Furthermore, mecamylamine in the cNTS potentiated the generation of late-expiratory (late-E) activity in abdominal nerve induced by hypercapnia. None of the cholinergic antagonists microinjected in the cNTS changed either the sympathetic or the phrenic nerve responses to hypercapnia. Our data provide evidence for the role of cholinergic transmission in the cNTS, acting on nicotinic receptors, modulating the hypoglossal and abdominal responses to hypercapnia.

Muscimol dialysis into the caudal aspect of the Nucleus tractus solitarii of conscious rats inhibits chemoreception

We studied the effects on chemoreception of bilateral focal inhibition of the caudal Nucleus tractus solitarii (cNTS) by microdialysis of muscimol (0.5 mM) in rats during wakefulness and NREM sleep at two temperatures, 24 degrees C and 30 degrees C, just below and within the thermoneutral zone, respectively. Body temperature and VO2 did not differ at these two temperatures. The CO2 response (% increase in V(E)/VO2) did not differ at 24 degrees C vs. 30 degrees C and muscimol inhibited the CO2 response equally at both temperatures. In contrast, the hypoxic response (% increase in V(E)/VO2) was greater at 30 degrees C than at 24 degrees C and muscimol inhibited it only at 30 degrees C. These effects were similar in wakefulness and NREM sleep. We conclude that: (1) ambient temperature can affect the V(E)/VO2 response to hypoxia but not hypercapnia and (2) at 24 degrees C muscimol in the cNTS affects the CO2 response but not the hypoxic response providing indirect support for the presence of chemoreception within the NTS.

Cortical representation of Hering's nerve: a possible anatomical pathway for seizure cessation following electrical stimulation

OBJECTIVE

We recently found that Hering's nerve stimulation (HNS) effectively blunts seizure activity. This study characterizes the cortical areas that are activated by HNS.

METHODS

Hering's nerve (HN) was stimulated in 3 dogs and 6 pigs, and then the brains of the animals were removed. The insular and mesial temporal cortices were removed and evaluated for increased neuronal activity by examining Fos-like activity.

RESULTS

In both the dogs and pigs, Fos activity was elevated in the anterior insular and mesial temporal cortices. Increased cortical activity was not noted in adjacent areas such as the frontal cortex.

CONCLUSION

This study demonstrates that the mesial temporal and anterior insular cortices are selectively activated by stimulation of HN, thus suggesting cortical loci at which HNS might blunt seizure activity.