Response properties and functional organization of neurons in midline region of medullary reticular formation of cats

Blood pressure changes alter tracheobronchial cough: computational model of the respiratory-cough network and in vivo experiments in anesthetized cats

We tested the hypothesis, motivated in part by a coordinated computational cough network model, that alterations of mean systemic arterial blood pressure (BP) influence the excitability and motor pattern of cough. Model simulations predicted suppression of coughing by stimulation of arterial baroreceptors. In vivo experiments were conducted on anesthetized spontaneously breathing cats. Cough was elicited by mechanical stimulation of the intrathoracic airways. Electromyograms (EMG) of inspiratory parasternal, expiratory abdominal, laryngeal posterior cricoarytenoid (PCA), and thyroarytenoid muscles along with esophageal pressure (EP) and BP were recorded. Transiently elevated BP significantly reduced cough number, cough-related inspiratory, and expiratory amplitudes of EP, peak parasternal and abdominal EMG, and maximum of PCA EMG during the expulsive phase of cough, and prolonged the cough inspiratory and expiratory phases as well as cough cycle duration compared with control coughs. Latencies from the beginning of stimulation to the onset of cough-related diaphragm and abdominal activities were increased. Increases in BP also elicited bradycardia and isocapnic bradypnea. Reductions in BP increased cough number; elevated inspiratory EP amplitude and parasternal, abdominal, and inspiratory PCA EMG amplitudes; decreased total cough cycle duration; shortened the durations of the cough expiratory phase and cough-related abdominal discharge; and shortened cough latency compared with control coughs. Reduced BP also produced tachycardia, tachypnea, and hypocapnic hyperventilation. These effects of BP on coughing likely originate from interactions between barosensitive and respiratory brainstem neuronal networks, particularly by modulation of respiratory neurons within multiple respiration/cough-related brainstem areas by baroreceptor input.

Re-evaluation of the afferent connections of the pituitary in the weakly electric fish Apteronotus leptorhynchus: an in vitro tract-tracing study

The pituitary plays a key role in the interaction between the brain and the endocrine system. We re-examined the afferent connections of the pituitary in the weakly electric fish Apteronotus leptorhynchus using the in vitro application of dextran-tetramethylrhodamine to the pituitary. The resultant retrograde labeling was analyzed. Application of the tracer to the rostral part, but not the caudal part, of the pituitary labels hypothalamic cells in the anterior division of the periventricular nucleus, the suprachiasmatic nucleus, and the nucleus tuberis lateralis pars anterior. Application of the tracer to either the rostral or caudal parts of the pituitary labels hypothalamic cells in the posterior division of the periventricular nucleus (RPPp), the nucleus hypothalamus caudalis (Hc), the nucleus hypothalamus anterioris, the ventral hypothalamic nucleus, and the central nucleus of the inferior lobe. Furthermore, cells in the rostral two-thirds of the brainstem reticular formation (RF) project to the entire rostrocaudal extent of the pituitary. The largest projections to the pituitary are from Hc, PPp, and RF. Of the cells in Hc that project to the pituitary, almost all (96%) are small and the remainder are medium-sized. Of the cells in PPp that project to the pituitary, about half are small or medium-sized (44% and 56%, respectively). In Hc and PPp, about one-third to one-half of the cells that project to the pituitary are markedly elongated. The cells in RF that project to the pituitary are small (4%), medium-sized (89%), or large (7%) and about four-fifths of these cells are markedly elongated. With regard to the RF projections, the pituitary may receive copies of motor instructions and sensory information supplied by collaterals of the descending and ascending projection systems of RF cells. Thus, the ongoing motor activity of the animal and the ensuing sensory feedback from this activity could directly influence the pituitary.

Effects of temperature and volume on intraperitoneal saline-induced changes in blood pressure, nociception, and neural activity in the rostroventral medulla

ON and OFF cells of the rostral ventromedial medulla are hypothesized to modulate nociception with ON cells facilitating pain and OFF cells inhibiting pain. The current study analyzed the effects of intraperitoneal saline at different volumes and temperatures on nociception (tail flick reflex), blood pressure, and the activity of ON and OFF cells in lightly anesthetized rats. At large volumes (20 cc/kg), room temperature saline excited 7/11 ON cells and inhibited 10/12 OFF cells for 2-5 min. In contrast, large volumes (20 cc/kg) of body temperature saline (37 degrees C) excited only 1/10 ON cells and inhibited only 3/13 OFF cells, and small volumes (1-2 cc/kg) of room temperature saline excited only 3/10 ON cells and inhibited only 4/11 OFF cells. Tail flick latency increased following saline administration at large volumes with a significant effect of time, but not temperature. The excitation of ON cells and inhibition of OFF cells indicate that cold intraperitoneal saline could be painful and the increase in tail flick latency may indicate a diffuse noxious inhibitory control. It is also possible that the changes in ON and OFF cell activity caused a hyperalgesia that was masked by a simultaneous hypoalgesia that was mediated independent of the ON and OFF cells. Because intraperitoneal saline may produce pain or hyperalgesia, care should be used when saline is used experimentally or clinically.

Potential role of medullary raphe-spinal neurons in cutaneous vasoconstriction: an in vivo electrophysiological study

In rabbits, raphe magnus/pallidus neurons form a link in the CNS pathway regulating changes in cutaneous blood flow elicited by nociceptive stimulation and activation of the central nucleus of the amygdala. To characterize relevant raphe-spinal neurons, we performed extracellular recordings from the rostral medullary raphe nuclei in anesthetized, paralyzed, mechanically ventilated rabbits. All studied neurons were antidromically activated from the dorsolateral funiculus of the spinal cord (C(8)-T(2)). Of 129 studied neurons, 40% were silent. The remaining neurons discharged spontaneously at 0.3-29 Hz. Nociceptive stimulation (lip squeeze with pliers) excited 63 (49%), inhibited 9 (7%), and did not affect 57 (44%) neurons. The same stimulation also elicited falls in ear pinna blood flow. In neurons activated by the stimulation, the increase in discharge preceded the fall in flow. Electrical stimulation of the spinal trigeminal tract excited 61/63 nociception-activated neurons [onset latencies range: 6-75 ms, mean: 28 +/- 3 (SE) ms], inhibited 9/9 nociception-inhibited neurons (onset latencies range: 9-85 ms, mean: 32 +/- 10 ms), and failed to affect 55/57 neurons insensitive to nociceptive stimulation. Neurons insensitive to nociceptive/trigeminal stimulation were also insensitive to nonnociceptive tactile stimulation and to electrical stimulation of the amygdala. They were either silent (32/45) or discharged regularly at low frequencies. They possessed long-duration action potentials (1.26 +/- 0.08 ms) and slow-conducting axons (6.0 +/- 0.5 m/s). These neurons may be serotonergic raphe-spinal cells. They do not appear to be involved in nociceptive-related cutaneous vascular control. Of the 63 neurons sensitive to nociceptive and trigeminal tract stimulation, 35 also responded to tactile stimulation (wide receptive field). These neurons possessed short action potentials (0.80 +/- 0.03 ms) and fast-conducting axons (30.3 +/- 3.1 m/s). In this subpopulation, electrical stimulation of the amygdala activated nearly all neurons tested (10/12), with a mean onset latency of 34 +/- 3 ms. The remaining 28 neurons sensitive to nociceptive and trigeminal stimulation did not respond to tactile stimuli and were mainly unaffected by amygdala stimulation. It may be that fast-conducting raphe-spinal neurons, with wide multimodal receptive fields and with input from the central nucleus of the amygdala, constitute the bulbo-spinal link in the CNS pathway regulating cutaneous blood flow in response to nociceptive and alerting stimuli.

Behavioral correlates of the activity of serotonergic and non-serotonergic neurons in caudal raphe nuclei

We investigated the behavioral correlates of the activity of serotonergic and non-serotonergic neurons in the nucleus raphe pallidus (NRP) and nucleus raphe obscurus (NRO) of unanesthetized and unrestrained cats. The animals were implanted with electrodes for recording single unit activity, parietal oscillographic activity, and splenius, digastric and masseter electromyographic activities. They were tested along the waking-sleep cycle, during sensory stimulation and during drinking behavior. The discharge of the serotonergic neurons decreased progressively from quiet waking to slow wave sleep and to fast wave sleep. Ten different patterns of relative discharge across the three states were observed for the non-serotonergic neurons. Several non-serotonergic neurons showed cyclic discharge fluctuations related to respiration during one, two or all three states. While serotonergic neurons were usually unresponsive to the sensory stimuli used, many non-serotonergic neurons responded to these stimuli. Several non-serotonergic neurons showed a phasic relationship with splenius muscle activity during auditory stimulation. One serotonergic neuron showed a tonic relationship with digastric muscle activity during drinking behavior. A few non-serotonergic neurons exhibited a tonic relationship with digastric and/or masseter muscle activity during this behavior. Many non-serotonergic neurons exhibited a phasic relationship with these muscle activities, also during this behavior. These results suggest that the serotonergic neurons in the NRP and NRO constitute a relatively homogeneous population from a functional point of view, while the non-serotonergic neurons form groups with considerable functional specificity. The data support the idea that the NRP and NRO are implicated in the control of somatic motor output.

Respiratory neuronal assemblies

This review describes results from in vivo experiments on brain stem network mechanisms that control breathing. Multi-array recording technology and computational methods were used to test predictions derived from simulations of respiratory network models. This highly efficient approach has the advantage that many simultaneously recorded neurons are subject to shared stimulus, history, and state-dependent conditions. Our results have provided evidence for concurrent or parallel network interactions in the generation and modulation of the respiratory motor pattern. Recent data suggest that baroreceptors, chemoreceptors, nociceptors, and airway cough receptors shape the respiratory motor pattern, at least in part, through a system of shared coordinated 'multifunctional' neurons distributed in the brain stem. The 'gravity method' for the analysis and representation of multi-neuron data has demonstrated respiratory phase-dependent impulse synchrony among neurons with no respiratory modulation of their individual firing rates. The detection of this emergent property motivated the development of pattern detection methods that subsequently identified repeated transient configurations of these 'correlational assemblies'. These results support the view that information can be 'coded' in the nervous system by spike timing relationships, in addition to firing rate changes that traditionally have been measured by neurophysiologists.

Responses of simultaneously recorded respiratory-related medullary neurons to stimulation of multiple sensory modalities

This study addresses the hypothesis that multiple afferent systems share elements of a distributed brain stem network that modulates the respiratory motor pattern. Data were collected from 18 decerebrate, bilaterally vagotomized, paralyzed, artificially ventilated cats. Up to 28 neurons distributed in the rostral and caudal ventral respiratory group, nucleus tractus solitarius, and raphe obscurus were recorded simultaneously with microelectrode arrays. Phases of the respiratory cycle and inspiratory drive were assessed from integrated efferent phrenic nerve activity. Carotid chemoreceptors were stimulated by injection of CO2-saturated saline solution via the external carotid artery. Baroreceptors were stimulated by increased blood pressure secondary to inflation of an embolectomy catheter in the descending aorta. Cutaneous nociceptors were stimulated by pinching a footpad. Four hundred seventy-four neurons were tested for respiratory modulated firing rates and responses; 403 neurons were tested with stimulation of all 3 modalities. Chemoreceptor stimulation and pinch, perturbations that tend to increase respiratory drive, caused similar responses in 52 neurons; 28 responded oppositely. Chemoreceptor and baroreceptor stimulation resulted in similar primary responses in 45 neurons; 48 responded oppositely. Similar responses to baroreceptor stimulation and pinch were recorded for 38 neurons; opposite effects were measured in 26 neurons. Among simultaneously recorded neurons, distinct combinations of firing rate changes were evoked in response to stimulation of the different modalities. The results show a functional convergence of information from carotid chemoreceptors, baroreceptors, and cutaneous nociceptors on respiratory-modulated neurons distributed in the medulla. The data are consistent with the hypothesis that brain stem neurons have overlapping memberships in multifunctional groups that influence the respiratory motor pattern.

Multimodal medullary neurons and correlational linkages of the respiratory network

This study addresses the hypothesis that multiple sensory systems, each capable of reflexly altering breathing, jointly influence neurons of the brain stem respiratory network. Carotid chemoreceptors, baroreceptors, and foot pad nociceptors were stimulated sequentially in 33 Dial-urethan-anesthetized or decerebrate vagotomized adult cats. Neuronal impulses were monitored with microelectrode arrays in the rostral and caudal ventral respiratory group (VRG), nucleus tractus solitarius (NTS), and n. raphe obscurus. Efferent phrenic nerve activity was recorded. Spike trains of 889 neurons were analyzed with cycle-triggered histograms and tested for respiratory-modulated firing rates. Responses to stimulus protocols were assessed with peristimulus time and cumulative sum histograms. Cross-correlation analysis was used to test for nonrandom temporal relationships between spike trains. Spike-triggered averages of efferent phrenic activity and antidromic stimulation methods provided evidence for functional associations of bulbar neurons with phrenic motoneurons. Spike train cross-correlograms were calculated for 6,471 pairs of neurons. Significant correlogram features were detected for 425 pairs, including 189 primary central peaks or troughs, 156 offset peaks or troughs, and 80 pairs with multiple peaks and troughs. The results provide evidence that correlational medullary assemblies include neurons with overlapping memberships in groups responsive to different sets of sensory modalities. The data suggest and support several hypotheses concerning cooperative relationships that modulate the respiratory motor pattern. 1) Neurons responsive to a single tested modality promote or limit changes in firing rate of multimodal target neurons. 2) Multimodal neurons contribute to changes in firing rate of neurons responsive to a single tested modality. 3) Multimodal neurons may promote responses during stimulation of one modality and "limit" changes in firing rates during stimulation of another sensory modality. 4) Caudal VRG inspiratory neurons have inhibitory connections that provide negative feedback regulation of inspiratory drive and phase duration.

Medullary raphe neurones and baroreceptor modulation of the respiratory motor pattern in the cat

1. Perturbations of arterial blood pressure change medullary raphe neurone activity and the respiratory motor pattern. This study sought evidence for actions of baroresponsive raphe neurones on the medullary respiratory network. 2. Blood pressure was perturbed by intravenous injection of an alpha1-adrenergic receptor agonist, unilateral pressure changes in the carotid sinus, or occlusion of the descending aorta in thirty-six Dial-urethane-anaesthetized, vagotomized, paralysed, artificially ventilated cats. Neurones were monitored with microelectrode arrays in two or three of the following domains: nucleus raphe obscurus-nucleus raphe pallidus, nucleus raphe magnus, and rostral and caudal ventrolateral medulla. Data were analysed with cycle-triggered histograms, peristimulus time and cumulative sum histograms, cross-correlograms and spike-triggered averages of efferent phrenic nerve activity. 3. Prolongation of the expiratory phase and decreased peak integrated phrenic amplitude were most frequently observed. Of 707 neurones studied, 310 had altered firing rates during stimulation; changes in opposite directions were monitored simultaneously in fifty-six of eighty-seven data sets with at least two baroresponsive neurones. 4. Short time scale correlations were detected between neurones in 347 of 3388 pairs. Seventeen pairs of baroresponsive raphe neurones exhibited significant offset correlogram features indicative of paucisynaptic interactions. In correlated raphe-ventrolateral medullary neurone pairs with at least one baroresponsive neurone, six of seven ventrolateral medullary decrementing expiratory (E-Decr) neurones increased their firing rate during baroreceptor stimulation. Thirteen of fifteen ventrolateral medullary inspiratory neurones correlated with raphe cells decreased their firing rate during baroreceptor stimulation. 5. The results support the hypothesis that raphe neuronal assemblies transform and transmit information from baroreceptors to neurones in the ventral respiratory group. The inferred actions both limit and promote responses to sensory perturbations and match predictions from simulations of the respiratory network.

Primary motor cortex influences on the descending and ascending systems

The motor cortex plays a crucial role in the co-ordination of movement and posture. This is possible because the pyramidal tract fibres have access both directly and through collateral branches to structures governing eye, head, neck trunk and limb musculature. Pyramidal tract axons also directly reach the dorsal laminae of the spinal cord and the dorsal column nuclei, thus aiding in the selection of the sensory ascendant transmission. No other neurones in the brain besides pyramidal tract cells have such a wide access to different structures within the central nervous system. The majority of the pyramidal tract fibres that originate in the motor cortex and that send collateral branches to multiple supraspinal structures do not reach the spinal cord. Also, the great majority of the corticospinal neurones that emit multiple intracraneal collateral branches terminate at the cervical spinal cord level. The pyramidal tract fibres directed to the dorsal column nuclei that send collateral branches to supraspinal structures also show a clear tendency to terminate at supraspinal and cervical cord levels. These facts suggest that a substantial co-ordination between descending and ascending pathways might be produced by the same motor cortex axons at both supraspinal and cervical spinal cord sites. This may imply that the motor cortex co-ordination will be mostly directed to motor responses involving eye-neck-forelimb muscle synergies. The review makes special emphasis in the available evidence pointing to the role of the motor cortex in co-ordinating the activities of both descending and ascending pathways related to somatomotor integration and control. The motor cortex may function to co-operatively select a unique motor command by selectively filter sensory information and by co-ordinating the activities of the descending systems related to the control of distal and proximal muscles.

Integration in the ventral medulla and coordination of sympathetic, pain and arousal functions

The nucleus paragigantocellularis lateralis (PGi) in the rostral ventral medulla is implicated in several functions including cardiovascular control, respiration, pain and analgesia. More recent studies implicate this region in alertness and attention as well, by virtue of its prominent projections to the nucleus locus coeruleus (LC). To investigate information that is integrated in the PGi, we used tract tracing to examine brain and spinal projections to this region. Afferents to PGi were found to be functionally diverse and topographically organized. Projections to the retrofacial PGi are primarily autonomic in nature. A wider range of inputs were found to target the rostral (juxtafacial) aspect of the PGi, including brain nuclei involved in the processing of somatosensory and auditory stimuli, as well as autonomic areas. Efferent projections to the LC were also examined in detail. Neuropharmacology experiments revealed that the PGi provides a potent excitatory amino acid input to the LC and an inhibitory input acting at alpha 2 receptors on LC neurons. PGi neurons projecting to the LC stained for markers of adrenaline, enkephalin, GABA and corticotropin releasing factor. Finally, some PGi neurons collateralize to innervate both the LC and the spinal cord. These results suggest that the LC may function in parallel to peripheral autonomic systems providing a cognitive complement to sympathetic function, and that the PGi may integrate a wide range of inputs to facilitate adaptive responses to urgent environmental events.

Responses of feline raphespinal neurons to urinary bladder distension

Effects of distending the urinary bladder were studied on extracellular activity of 77 raphespinal neurons in 19 alpha-chloralose anesthetized cats. Neurons were activated antidromically from thoracic spinal cord; recording sites were located in nucleus raphe magnus (NRM). Mean conduction velocity was 48 +/- 2 m/s. Urinary bladder distension (UBD) increased activity in 12 cells and decreased activity in 17 cells. Spontaneous bladder contractions also affected activity in raphespinal neurons responsive to UBD. Noxious pinch stimulus applied to proximal hindlimbs or forelimbs either increased or decreased activity in 28 raphespinal neurons. No cells were excited both by UBD and pinching of skin and deep tissues of the limbs. Thus, excitatory viscerosomatic convergence was not observed with the stimuli tested in raphespinal neurons examined in this study. Urinary bladder input to descending projection neurons in NRM might participate in descending modulation of dorsal horn neurons. In addition, micturition reflexes might be affected by urinary bladder input to these neurons.

Responses of neurons in the caudal medullary raphe nuclei of the cat to stimulation of the vestibular nerve

In the decerebrate cat, recordings were made from neurons in the caudal medullary raphe nuclei to determine if they responded to electrical stimulation of the vestibular nerve and thus might participate in vestibulosympathetic reflexes. Many of these cells projected to the upper thoracic spinal cord. The majority (20/28) of raphespinal neurons with conduction velocities between 1 and 4 m/s received vestibular inputs; 13 of the 20 were inhibited, and 7 were excited. Since many raphespinal neurons with similar slow conduction velocities are involved in the control of sympathetic outflow, as well as in other functions, these cells could potentially relay vestibular signals to sympathetic preganglionic neurons. The onset latency of the vestibular effects was long (median of 15 ms), indicating the inputs were polysynaptic. In addition, 34 of 42 raphespinal neurons with more rapid conduction velocities (6-78 m/s) also received long-latency (median of 10 ms) labyrinthine inputs; 26 were excited and 8 were inhibited. Although little is known about these rapidly-conducting cells, they do not appear to be involved in autonomic control, suggesting that the function of vestibular inputs to raphe neurons is not limited to production of vestibulosympathetic reflexes. One hypothesis is that raphe neurons are also involved in modulating the gain of vestibulocollic and vestibulospinal reflexes; this possibility remains to be tested.

Antinociception produced by electrical stimulation of vagal afferents: independence of cervical and subdiaphragmatic branches

Expt. 1 showed that electrical stimulation of either the main dorsal or ventral branch of the subdiaphragmatic vagus could produce inhibition of the nociceptive tail-flick reflex in lightly anesthetized rats. The antinociception produced by electrical stimulation of the dorsal subdiaphragmatic vagus was eliminated by resection of the right cervical vagus, but relatively unaffected by resection of the left cervical vagus. The opposite effects for cervical vagal resection were obtained with electrical stimulation of the ventral branch of the subdiaphragmatic vagus. These results indicate that the antinociceptive effects of subdiaphragmatic vagal stimulation are mediated via uncrossed afferents traveling in the cervical vagus to activate an inhibitory spinopetal system. These findings are consistent with the established anatomy of vagal afferents. Expt. 2 showed that degeneration of the dorsal subdiaphragmatic vagus did not alter the threshold intensity of right cervical vagal stimulation necessary to produce inhibition of the tail-flick reflex. These results demonstrate that the antinociceptive effects of cervical vagal stimulation are primarily due to activation of the cardiopulmonary component of the nerve, rather than the subdiaphragmatic component. The second experiment also demonstrated that the subdiaphragmatic branch of the vagus can be selectively degenerated with ricin while leaving the cervical branch intact, even though the cell bodies of both sets of afferents are located within the nodose ganglion. These data are discussed in terms of vagal afferents and their role in the modulation of nociceptive transmission.

Involvement of the raphe in the respiratory effects of gigantocellular area activation

Previous reports indicate that the nucleus reticularis gigantocellularis (NGC) of the brainstem reticular formation is involved in inhibitory respiratory and cardiovascular reflexes. Stimulation of portions of the nearby bulbar raphe complex, specifically the raphe magnus (RM), have also been shown to suppress phrenic activity and to decrease blood pressure and heart rate. Since synaptic connectivity between the NGC and the RM has been demonstrated, we hypothesized that the RM may be involved in the cardiopulmonary effects of NGC stimulation. This study found that electrolytic lesions in the raphe magnus attenuated the inhibitory respiratory effects but not the cardiovascular suppression due to NGC stimulation. Lesions in the raphe magnus also lowered resting blood pressure and resting breath frequency. We conclude that the RM may mediate part of the NGC-mediated respiratory effects.

Antinociception and cardiovascular responses produced by electrical stimulation in the nucleus tractus solitarius, nucleus reticularis ventralis, and the caudal medulla

In experiment 1, quantitative regional comparisons of the antinociceptive and cardiovascular responses produced by electrical stimulation in the caudal medulla, including regions such as the nucleus tractus solitarius (NTS), nucleus reticularis ventralis (NRV), nucleus reticularis gigantocellularis (NRGC), nucleus reticularis paragigantocellularis (NRPGC), nucleus raphe obscurus (NRO), and medial portions of the lateral reticular nucleus (LRN), were made in the rat. Electrical stimulation in all of these regions resulted in inhibition of the nociceptive tail-flick reflex, although the threshold intensity for inhibition was greater for sites in NTS compared to many sites ventral to the NTS. Antinociception was generally accompanied by an increase in mean arterial blood pressure, with the exception of sites in the NRO, where depressor responses were evoked by stimulation. Detailed comparisons between the NTS and NRV revealed that greater intensities of electrical stimulation were required to produce antinociception for sites in the NTS as compared to the NRV. There were no significant differences in threshold intensities for antinociception as a function of rostrocaudal subdivisions of the NTS, but the lateral subdivision of the NTS was significantly more efficacious than the medial subdivision. This mediolateral difference within NTS was primarily due to stimulation in medial sites producing overt movements in some animals, probably due to stimulation of adjacent midline nuclei or pathways. Within the NRV, thresholds for inhibition of the tail-flick reflex were greater for sites in the dorsal subdivision as compared to the ventral subdivision, which contains spinopetal projections from the NRM. The slopes of the lines of recruitment for inhibition of the tail-flick reflex at stimulation sites in either the NTS or NRV were both very steep, similar to other forms of antinociception. In experiment 2, the pulse duration of electrical stimulation was varied for sites of stimulation in the lateral NTS and NRV to generate strength-duration curves. This experiment confirmed that stimulation sites in the lateral NTS required greater current intensities to inhibit the tail-flick reflex than sites in the NRV. However, the chronaxies derived from the strength-duration functions for the NTS or NRV were both approximately 170 microseconds, indicating that the antinociceptive effects in these regions may not be exclusively due to the stimulation of fibers of passage. These results are discussed in terms of the role of the NTS, NRV, and caudal medulla in the modulation of nociceptive responses and cardiovascular function.

Diverse afferents converge on the nucleus paragigantocellularis in the rat ventrolateral medulla: retrograde and anterograde tracing studies

The nucleus paragigantocellularis in the ventrolateral medulla has been implicated in cardiovascular, pain, and analgesic functions; and it has also been found to be a major afferent to the pontine nucleus locus coeruleus. In the present study, afferents to the nucleus paragigantocellularis were identified in the rat by means of the retrograde tracers wheat germ agglutinin-conjugated horseradish peroxidase or Fluoro-Gold. Projections to the nucleus paragigantocellularis arise from a wide variety of nuclei with autonomic, visceral, and sensory-related functions. Major afferents with consistent and robust retrograde labeling include most laminae of the spinal cord, the caudal lateral medulla, the contralateral paragigantocellularis, the nucleus of the solitary tract, the A1 area, the lateral parabrachialis, the Kölliker-Fuse nucleus, the periaqueductal gray, and a preoculomotor nucleus in the ventral central gray, the supraoculomotor nucleus. Other notable afferents, seen only after large caudal injections into the nucleus paragigantocellularis, include the lateral hypothalamus, the paraventricular nucleus of the hypothalamus, and the medial prefrontal cortex. Minor afferents include the gigantocellular nucleus, the area postrema, the caudal raphe groups, the inferior colliculus, the A5 area, and the locus coeruleus. The projection from the supraoculomotor nucleus, not previously reported as an afferent to the ventrolateral medulla, was confirmed with anterograde tracing by means of Phaseolus vulgaris-leucoagglutinin. Iontophoretic deposits of Phaseolus vulgaris-leucoagglutinin into the nucleus of the solitary tract (commissuralis level) or into the periaqueductal gray also yielded terminal fiber labeling in the nucleus paragigantocellularis. Fibers from the supraoculomotor nucleus and the nucleus of the solitary tract were densest in the lateral aspect of the nucleus paragigantocellularis (corresponding to the rostroventrolateral reticular nucleus), while fibers from the periaqueductal gray were more medially located. Previous studies have defined inputs to the rostral ventrolateral medulla from the cochlear nucleus as well as from the colliculi. In the present study, deposits of wheat germ agglutinin-conjugated horseradish peroxidase or Phaseolus vulgaris-leucoagglutinin into the cochlear nucleus or the superior colliculus yielded only sparse anterograde labeling in the nucleus paragigantocellularis, but heavily labeled adjacent areas. The inferior collicular injections yielded strong but restricted anterograde labeling in the rostromedial paragigantocellularis, medial to the facial nucleus. These results indicate that the paragigantocellularis area receives inputs from diverse brain structures. Neurons in the nucleus paragigantocellularis afferent to the locus coeruleus, being distributed throughout this region, may provide a channel where several types of information are integrated and transmitted to the extensive locus coeruleus noradrenergic efferent network...

Rapid desensitization and down-regulation of 5-HT2 receptors by DOM treatment

The regulation of the 5-HT2 receptor-mediated head twitch response and of 5-HT2 receptor binding in the frontal cortex was studied in rats treated repeatedly with the 5-HT2 agonist 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM) (2.5 mg/kg, s.c.). Four injections in 24 h produced a near maximal reduction in the behaviour (-70%) and in the Bmax for [3H]ketanserin binding (-41%). The KD values tended to increase slightly. 5-HT2 receptors reappeared, with half-lives of 5.5 to 3 days. In view of the reported anomalous 5-HT2 receptor regulation by antagonists and the regular regulation by agonists, we propose a refinement in the receptor regulation theory.

Putative nociceptive modulating neurons in the rostral ventromedial medulla of the rat: firing of on- and off-cells is related to nociceptive responsiveness

In the unstimulated, lightly anesthetized rat, both on- and off-cells exhibit alternating periods of silence and activity lasting from several seconds to a few minutes. In the preceding paper, we showed that the active periods of all cells of the same class are always in phase, whereas the firing of cells of different classes is invariably out of phase. Thus, the pattern of firing of any single on- or off-cell provides a useful indication of the excitability of all on- and off-cells in the rostral ventromedial medulla (RVM). In this study, we measured the latency of the tail flick response (TF) at set intervals while recording from TF-related neurons in RVM, and were able to demonstrate a significant relationship between the spontaneous firing of both on- and off-cells and the latency of the TF response. If noxious heat is applied at a time when an off-cell is spontaneously active (or an on-cell is silent), the TF latency is longer than if the TF trial falls during a period in which the off-cell is silent (or the on-cell is active). This correlation between on- and off-cell firing and changes in TF latency is consistent with a nociceptive modulatory role for either or both cell classes. These findings support the hypothesis that off-cells inhibit and on-cells facilitate spinal nociceptive transmission and reflexes.

How visual inputs to the ponto-bulbar reticular formation are used in the synthesis of premotor signals during orienting

The primate superior colliculus (SC) is known as a structure subserving the transformation of visual information into "commands" for orienting eye movements. Collicular burst neurons discharging with short lead times in relation to visually triggered or spontaneous saccades are supposed to be the output elements linking the SC to immediately premotor pattern generators. In this paper we summarize some data available for the cat's SC neurones, identified as tecto-reticulo-spinal projection cells (TRSN), and reticulospinal neurones (RSN), identified as receiving excitatory collicular input. Some TRSNs respond to visual stimuli in the absence of orienting movements and, hence, their signals cannot be regarded as motor "commands", in spite of their proven connections with premotor pools in the brain stem and with the spinal cord. Moreover, a small fraction of RSNs belonging to polysynaptic descending collicular pathways also displays visual responses dissociated from movement, in addition to discharges related to the performance of orienting eye-head synergies. The processes of visual to motor transformation, assumed by current models as being definitively accomplished in the SC, appear thus to be partially performed in the reticular network incorporating the overlapping collaterals of tectal projection cells and their target neurons in the reticular core. It is concluded that, at least as for visuomotor transformations underlying orienting movements in the cat, the deep division of the SC and the brain stem reticular formation represent an ensemble, rather than a sequence of hierarchically arranged levels of processing.

Medullary substrates of descending spinal inhibition activated by intravenous administration of [D-Ala2]methionine enkephalinamide in the rat

Two experiments were performed in pentobarbital-anesthetized rats to provide information about the locus of the descending inhibitory system(s) in the brainstem engaged by [D-Ala2]methionine enkephalinamide (DALA) activation of vagal afferents. In Expt. 1, local anesthetic block of either the medial medullary nucleus raphe magnus (NRM) or the combined NRM-lateral medullary reticular formation (MRF) region failed to affect DALA-induced inhibition of the tail-flick reflex or reflex bradycardia. The hypotensive action of DALA was marginally enhanced in rats with a combined NRM-MRF local anesthetic block. However, inhibition of the tail-flick reflex produced by electrical stimulation in the NRM-MRF was eliminated by the local anesthetic. In Expt. 2, local anesthetic blocks of regions at the level of and caudal to obex were effective in blocking DALA-induced inhibition of the tail-flick reflex. These regions included lateral sites approximately 0.4 mm rostral to obex to -2.4 mm caudal to obex, and midline sites approximately -2.4 to -3.2 mm caudal to obex. These findings suggest that brainstem areas mediating DALA-induced antinociception are lateral to the midline at the level of the obex. The spinopetal efferents mediating antinociception then course medially and ventrally, and finally course bilaterally in the dorsolateral funiculi before terminating in the spinal dorsal horn.

Serotoninergic and non-serotoninergic responses of phrenic motoneurones to raphe stimulation in the cat

Experiments were conducted on anaesthetized, paralysed and artificially ventilated cats in order to determine if the altered discharge pattern of phrenic motoneurones recorded during stimulation of medullary raphe nuclei (raphe magnus (r. magnus), raphe obscurus (r. obscurus), raphe pallidus (r. pallidus] are related to release of serotonin (5-hydroxytryptamine, 5-HT) at synapses on respiratory neurones. Effects of 5-HT released by the neurotoxin p-chloroamphetamine (PCA) on spontaneous activity of phrenic motoneurones were also examined. In addition, responses of phrenic motoneurones to 5-HT applied by micro-electrophoresis were recorded. Stimulation (100 Hz) of r. magnus or r. obscurus depressed the spontaneous inspiratory discharges of phrenic motoneurones. Administration of the 5-HT receptor antagonists cinanserin, methysergide or methergoline reduced, but did not abolish, the inhibition. Inhibition of the neuronal reuptake of 5-HT with fluoxetine enhanced the inhibition and reduced the peak inspiratory action potential frequency of spontaneous discharges. Stimulation (100 Hz) of r. pallidus produced increased firing of phrenic motoneurones. Firing of phrenic motoneurones was evoked even during the normally quiescent expiratory phase of spontaneous respiratory activity. Antagonists at 5-HT receptors reduced the degree of tonic firing, resulting in partial restoration of expiratory pauses during r. pallidus stimulation. Inhibition of 5-HT reuptake, on the other hand, resulted in increased tonic firing. Release of 5-HT by PCA produced a rapid and severe reduction of phrenic nerve activity. Activity was restored by 5-HT receptor antagonists. In reserpine-treated cats, effects of stimulating medullary raphe nuclei were still pronounced, however 5-HT receptor antagonists had no effect on the responses. These results, along with the observation that 5-HT receptor antagonists are only partially effective in non-reserpinized cats, indicate that non-serotoninergic influences contribute as well to the responses evoked by raphe stimulation. Micro-electrophoretic application of 5-HT by large ejecting currents (100-200 nA) had weak stimulatory effects on twenty of forty-five phrenic motoneurones, which exhibited small increases in peak inspiratory discharge frequency during 5-HT application. Ejecting currents less than 100 nA were without effect. It is concluded that 5-HT analogues and agents which release endogenous 5-HT after parenteral administration do not act directly at synapses on the soma membranes of phrenic motoneurones.(ABSTRACT TRUNCATED AT 400 WORDS)

Responses of phrenic motoneurones of the cat to stimulation of medullary raphe nuclei

Responses of phrenic motoneurones to stimulation of the three medullary raphe nuclei (raphe magnus (r. magnus), raphe obscurus (r. obscurus) and raphe pallidus (r. pallidus] were recorded in anaesthetized and decerebrated cats. Stimulation of r. magnus or r. obscurus depressed phrenic motoneurones. Stimulation at 100 Hz reduced action potential frequency within each inspiratory burst, without appreciable changes in inspiratory duration, or number of inspiratory bursts per unit time. The depression was proportional to the stimulus intensity (40-160 microA) and frequency (12-100 Hz) and lasted throughout the period of stimulation. Intracellular recording revealed concomitant depression of central respiratory drive potentials (c.r.d.p.s) and increased membrane input resistance during r. obscurus or r. magnus stimulation. In motoneurones which discharged action potentials during expiratory as well as inspiratory phases following intracellular chloride injection, stimulation of r. magnus or r. obscurus depressed cell firing during both phases. Both c.r.d.p.s and reversed inhibitory post-synaptic potentials (i.p.s.p.s) were depressed. These findings indicate that the depression is not related to post-synaptic inhibition of phrenic motoneurones. Stimulation (100 Hz) of r. pallidus produced discharges of action potentials in phrenic motoneurones. Stimulation lengthened the duration of each inspiratory discharge in proportion to stimulus intensity. Continuous firing occurred throughout the period of stimulation with maximal intensities. Intracellular recordings revealed sustained depolarization and reduction in membrane input resistance during the discharge. Responses were recorded extracellularly from medullary inspiratory neurones of the dorsal respiratory group (d.r.g.) and ventral respiratory group (v.r.g.) and from vagal axons which fired in phase with phrenic nerve activity. Responses to raphe stimulation were similar to those recorded from phrenic motoneurones. Evidence is presented that the responses are not related to stimulation of decussating bulbo-spinal axons from d.r.g. or v.r.g. neurones. It is suggested that medullary respiratory neurones receive inhibitory and excitatory synaptic inputs from medullary raphe neurones. Hypercapnia (5% CO2 in O2) or hypoxia (15% O2 in N2) reduced markedly the inhibition produced during stimulation of r. obscurus or r. magnus, and restored expiratory-linked silent periods during stimulation of r. pallidus. Activation of Hering-Breuer or baroreceptor reflexes did not alter responses to r. pallidus stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

[D-Ala2]-methionine enkephalinamide (DALA): characterization of antinociceptive, cardiovascular, and autonomic nervous system actions in conscious and pentobarbital-anesthetized rats

The antinociceptive, cardiovascular, and autonomic nervous system actions of [D-Ala2]-methionine enkephalinamide (DALA) were assessed in conscious and pentobarbital-anesthetized rats. Intravenous administration of DALA inhibited the tail-flick reflex evoked by noxious radiant heat in both conscious and pentobarbital-anesthetized rats. In general, the magnitude of the inhibition was not significantly affected by the presence of pentobarbital-anesthesia. DALA also induced hypotension and bradycardia in these rats, but these responses were significantly attenuated by pentobarbital-anesthesia. Arterial blood flows were initially decreased and vascular resistances increased in mesenteric, renal, and hindquarter beds following DALA administration, but these parameters rapidly returned to baseline levels except in the hindquarter bed where flows increased significantly above baseline levels. All of the changes in blood flows were significantly greater in conscious compared to pentobarbital-anesthetized rats. In intact pentobarbital-anesthetized rats, DALA induced inhibition of renal sympathetic nerve activity. However, DALA induced a pressor response with a brief increase in renal sympathetic nerve activity in rats with bilateral vagotomy. Similarly, in the conscious rat with ganglionic blockade, DALA induces a brief pressor response. These outcomes indicate that the brief hypotension observed in the intact rat following administration of DALA is probably the net outcome of a large vagally-induced decrease in sympathetic tone and a small increase in sympathetic tone of either neurogenic or peripheral origin. These outcomes are discussed in terms of cardiovascular-somatosensory interactions.

Interaction between naloxone and serotonin in the control of the cardiovascular system in hemorrhaged cats

We investigated the mechanism for the pressor effect of intravenous administration of naloxone in pentobarbital-anesthetized cats. Comparisons were made between groups of hemorrhaged animals that received either naloxone or an equivalent volume of saline after 1 h of hemorrhage. Two other groups of hemorrhaged animals were depleted of serotonin by pretreatment with para-chlorophenylalanine 40-48 h before the experiment. One group of serotonin-depleted animals received naloxone after 60 min of hemorrhage and the other group received saline. Animals with normal brain serotonin content showed a significant pressor effect following naloxone when compared with animals given saline. Animals with reduced brain serotonin content also had a pressor response following naloxone administration. Serotonin-depleted animals showed an increase in maximum left ventricular dP/dt following naloxone administration when compared to serotonin-depleted animals given saline. Our data are consistent with the hypothesis that naloxone can exert a pressor effect in hemorrhaged cats by actions at central and at peripheral sites. In cats with normal serotonin values, the peripheral action of naloxone is predominant. In serotonin-depleted animals given naloxone, central and peripheral sites contribute to the pressor effect.