Afferent pathway and neuromodulation of superficial and deeper thoracic spinal neurons receiving noxious pulmonary inputs in rats

Regulation of breathing by cardiopulmonary afferents

This chapter broadly reviews cardiopulmonary sympathetic and vagal sensors and their reflex functions during physiologic and pathophysiologic processes. Mechanosensory operating mechanisms, including their central projections, are described under multiple sensor theory. In addition, ways to interpret evidence surrounding several controversial issues are provided, with detailed reasoning on how conclusions are derived. Cardiopulmonary sensory roles in breathing control and the development of symptoms and signs and pathophysiologic processes in cardiopulmonary diseases (such as cough and neuroimmune interaction) also are discussed.

Sensory nerves in lung and airways

Sensory nerves innervating the lung and airways play an important role in regulating various cardiopulmonary functions and maintaining homeostasis under both healthy and disease conditions. Their activities conducted by both vagal and sympathetic afferents are also responsible for eliciting important defense reflexes that protect the lung and body from potential health-hazardous effects of airborne particulates and chemical irritants. This article reviews the morphology, transduction properties, reflex functions, and respiratory sensations of these receptors, focusing primarily on recent findings derived from using new technologies such as neural immunochemistry, isolated airway-nerve preparation, cultured airway neurons, patch-clamp electrophysiology, transgenic mice, and other cellular and molecular approaches. Studies of the signal transduction of mechanosensitive afferents have revealed a new concept of sensory unit and cellular mechanism of activation, and identified additional types of sensory receptors in the lung. Chemosensitive properties of these lung afferents are further characterized by the expression of specific ligand-gated ion channels on nerve terminals, ganglion origin, and responses to the action of various inflammatory cells, mediators, and cytokines during acute and chronic airway inflammation and injuries. Increasing interest and extensive investigations have been focused on uncovering the mechanisms underlying hypersensitivity of these airway afferents, and their role in the manifestation of various symptoms under pathophysiological conditions. Several important and challenging questions regarding these sensory nerves are discussed. Searching for these answers will be a critical step in developing the translational research and effective treatments of airway diseases.

Inhalation of a pulmonary irritant modulates activity of lumbosacral spinal neurons receiving colonic input in rats

The purpose of the present study was to determine whether an intraspinal nociceptive pathway from the lungs modulated activity of spinal neurons that also received afferent input from the colon. Extracellular potentials of single lumbosacral (L6-S2) spinal neurons were recorded in pentobarbital-anesthetized, paralyzed, and ventilated male rats. The lower airways and lungs were irritated by injecting ammonia vapor over a 30% NH(4)OH solution into the inspiratory line of the ventilator (0.5 ml, 20 s). Graded colorectal distension (CRD; 20-60 mmHg, 20 s) was produced by air inflation of a balloon. Inhaled ammonia (IA) altered activity of 31/51 (61%) lumbosacral spinal neurons responding to noxious CRD (60 mmHg, 20 s). In contrast, IA changed activity of 3/30 (10%) spinal neurons with somatic fields that did not respond to colorectal inputs. IA decreased activity of 16/31 (52%) spinal neurons and increased activity of the other 15 neurons with colorectal input. Multiple patterns of viscerovisceral convergent spinal neurons with excitatory and inhibitory responses to CRD and IA were observed; 87% (27/31) of the viscerovisceral convergent neurons also responded to innocuous and/or noxious stimuli of somatic fields. Bilateral cervical vagotomy abolished responses to IA in 2/8 tested neurons, indicating that the remaining 6 neurons had input originating from sympathetic afferent fibers. Rostral C1 spinal transection did not abolish inhibitory responses to IA in 4/4 neurons, but L2 transection eliminated inhibitory responses to IA in 3/3 neurons. These results indicated that irritation of the lower airways modulated activity of lumbosacral spinal neurons with colorectal input. It might contribute to intraspinal cross talk between the colon and lungs.

Characterization of thoracic spinal neurons with noxious convergent inputs from heart and lower airways in rats

Respiratory symptoms experienced in some patients with cardiac diseases may be due to convergence of noxious cardiac and pulmonary inputs onto neurons of the central nervous system. For example, convergence of cardiac and respiratory inputs onto single solitary tract neurons may be in part responsible for integration of regulatory and defensive reflex control. However, it is unknown whether inputs from the lungs and heart converge onto single neurons of the spinal cord. The present aim was to characterize upper thoracic spinal neurons responding to both noxious stimuli of the heart and lungs in rats. Extracellular potentials of single thoracic (T3) spinal neurons were recorded in pentobarbital anesthetized, paralyzed, and ventilated male rats. A catheter was placed in the pericardial sac to administer bradykinin (BK, 10 microg/ml, 0.2 ml, 1 min) as a noxious cardiac stimulus. The lung irritant, ammonia, obtained as vapor over a 30% solution of NH(4)OH was injected into the inspiratory line of the ventilator (0.5-1.0 ml over 20 s). Intrapericardial bradykinin (IB) altered activity of 58/65 (89%) spinal neurons that responded to inhaled ammonia (IA). Among those cardiopulmonary convergent neurons, 81% (47/58) were excited by both IA and IB, and the remainder had complex response patterns. Bilateral cervical vagotomy revealed that vagal afferents modulated but did not eliminate responses of individual spinal neurons to IB and IA. The convergence of pulmonary and cardiac nociceptive signaling in the spinal cord may be relevant to situations where a disease process in one organ influences the behavior of the other.

Identification of an immune-responsive mesolimbocortical serotonergic system: potential role in regulation of emotional behavior

Peripheral immune activation can have profound physiological and behavioral effects including induction of fever and sickness behavior. One mechanism through which immune activation or immunomodulation may affect physiology and behavior is via actions on brainstem neuromodulatory systems, such as serotonergic systems. We have found that peripheral immune activation with antigens derived from the nonpathogenic, saprophytic bacterium, Mycobacterium vaccae, activated a specific subset of serotonergic neurons in the interfascicular part of the dorsal raphe nucleus (DRI) of mice, as measured by quantification of c-Fos expression following intratracheal (12 h) or s.c. (6 h) administration of heat-killed, ultrasonically disrupted M. vaccae, or heat-killed, intact M. vaccae, respectively. These effects were apparent after immune activation by M. vaccae or its components but not by ovalbumin, which induces a qualitatively different immune response. The effects of immune activation were associated with increases in serotonin metabolism within the ventromedial prefrontal cortex, consistent with an effect of immune activation on mesolimbocortical serotonergic systems. The effects of M. vaccae administration on serotonergic systems were temporally associated with reductions in immobility in the forced swim test, consistent with the hypothesis that the stimulation of mesolimbocortical serotonergic systems by peripheral immune activation alters stress-related emotional behavior. These findings suggest that the immune-responsive subpopulation of serotonergic neurons in the DRI is likely to play an important role in the neural mechanisms underlying regulation of the physiological and pathophysiological responses to both acute and chronic immune activation, including regulation of mood during health and disease states. Together with previous studies, these findings also raise the possibility that immune stimulation activates a functionally and anatomically distinct subset of serotonergic neurons, different from the subset of serotonergic neurons activated by anxiogenic stimuli or uncontrollable stressors. Consequently, selective activation of specific subsets of serotonergic neurons may have distinct behavioral outcomes.

Spinal cord stimulation modulates intraspinal colorectal visceroreceptive transmission in rats

Previous studies have shown that spinal cord stimulation (SCS) of upper lumbar segments decreases visceromotor responses to mechanical stimuli in a sensitized rat colon and reduces symptoms of irritable bowel syndrome in patients. SCS applied to the upper cervical spinal dorsal column reduces pain of chronic refractory angina. Further, chemical stimulation of C1-C2 propriospinal neurons in rats modulates the responses of lumbosacral spinal neurons to colorectal distension. The present study was designed to compare the effects of upper cervical and lumbar SCS on activity of lumbosacral neurons receiving noxious colorectal input. Extracellular potentials of L6-S2 spinal neurons were recorded in pentobarbital anesthetized, paralyzed and ventilated male rats. SCS (50 Hz, 0.2 ms) at low intensity (90% of motor threshold) was applied to the dorsal column of upper cervical (C1-C2) or upper lumbar (L2-L3) ipsilateral spinal segments. Colorectal distension (CRD, 20 mmHg, 40 mmHg, 60 mmHg, 20s) was produced by air inflation of a latex balloon. Results showed that SCS applied to L2-L3 and C1-C2 segments significantly reduced the excitatory responses to noxious CRD from 417.6+/-68.0 to 296.3+/-53.6 imp (P<0.05, n=24) and from 336.2+/-64.5 to 225.0+/-73.3 imp (P<0.05, n=18), respectively. Effects of L2-L3 and C1-C2 SCS lasted 10.2+/-1.9 and 8.0+/-0.9 min after offset of CRD. Effects of SCS were observed on spinal neurons with either high or low-threshold excitatory responses to CRD. However, L2-L3 or C1-C2 SCS did not significantly affect inhibitory neuronal responses to CRD. C1-C2 SCS-induced effects were abolished by cutting the C7-C8 dorsal column but not by spinal transection at cervicomedullary junction. These data demonstrated that upper cervical or lumbar SCS modulated responses of lumbosacral spinal neurons to noxious mechanical stimulation of the colon, thereby, proved two loci for a potential therapeutic effect of SCS in patients with irritable bowel syndrome and other colonic disorders.