Brainstem afferents to the rostral (juxtafacial) nucleus paragigantocellularis: integration of exteroceptive and interoceptive sensory inputs in the ventral tegmentum

Differential cholinergic innervation of lemniscal versus non-lemniscal regions of the inferior colliculus

The inferior colliculus (IC), a midbrain hub for integration of auditory information, receives dense cholinergic input that could modulate nearly all aspects of hearing. A key step in understanding cholinergic modulation is to identify the source(s) and termination patterns of cholinergic input. These issues have not been addressed for the IC in mice, an increasingly important model for study of hearing. We examined cholinergic inputs to the IC in adult male and female mice. We used retrograde tracing and immunochemistry to identify three sources of cholinergic innervation of the mouse IC: the pedunculopontine tegmental nucleus (PPT), the laterodorsal tegmental nucleus (LDT) and the lateral paragigantocellular nucleus (LPGi). We then used Cre-dependent labeling of cholinergic neurons in normal-hearing ChAT-Cre mice to selectively label the cholinergic projections to the IC from each of the cholinergic sources. Labeling of cholinergic projections from the PPT and LDT revealed cholinergic axons and boutons terminating throughout the IC, with the ipsilateral projection being denser. Electron microscopic examination showed that these cholinergic axons can form traditional synaptic junctions with IC neurons. In separate experiments, selective labeling of cholinergic projections from the LPGi revealed bilateral projections to the IC. The LPGi axons exhibited relatively equal densities on ipsilateral and contralateral sides, but on both sides the terminations were largely restricted to the non-lemniscal regions of the IC (i.e., the dorsal cortex, lateral cortex and intercollicular tegmentum). We conclude first that cholinergic axons can form traditional synapses in the IC. In addition, lemniscal and non-lemniscal regions of the IC receive different patterns of cholinergic innervation. The lemniscal IC (IC central nucleus) is innervated by cholinergic neurons in the PPT and the LDT whereas the non-lemniscal "shell" areas of the IC are innervated by the PPT and LDT and by cholinergic neurons in the LPGi. DATA AVAILABILITY: Data will be made available on request.

Characterization of three cholinergic inputs to the cochlear nucleus

Acetylcholine modulates responses throughout the auditory system, including at the earliest brain level, the cochlear nucleus (CN). Previous studies have shown multiple sources of cholinergic input to the CN but information about their relative contributions and the distribution of inputs from each source is lacking. Here, we used staining for cholinergic axons and boutons, retrograde tract tracing, and acetylcholine-selective anterograde tracing to characterize three sources of acetylcholine input to the CN in mice. Staining for cholinergic axons showed heavy cholinergic inputs to granule cell areas and the dorsal CN with lighter input to the ventral CN. Retrograde tract tracing revealed that cholinergic cells from the superior olivary complex, pontomesencephalic tegmentum, and lateral paragigantocellular nucleus send projections to the CN. When we selectively labeled cholinergic axons from each source to the CN, we found surprising similarities in their terminal distributions, with patterns that were overlapping rather than complementary. Each source heavily targeted granule cell areas and the dorsal CN (especially the deep dorsal CN) and sent light input into the ventral CN. Our results demonstrate convergence of cholinergic inputs from multiple sources in most regions of the CN and raise the possibility of convergence onto single CN cells. Linking sources of acetylcholine and their patterns of activity to modulation of specific cell types in the CN will be an important next step in understanding cholinergic modulation of early auditory processing.

Multiple Sources of Cholinergic Input to the Superior Olivary Complex

The superior olivary complex (SOC) is a major computation center in the brainstem auditory system. Despite previous reports of high expression levels of cholinergic receptors in the SOC, few studies have addressed the functional role of acetylcholine in the region. The source of the cholinergic innervation is unknown for all but one of the nuclei of the SOC, limiting our understanding of cholinergic modulation. The medial nucleus of the trapezoid body, a key inhibitory link in monaural and binaural circuits, receives cholinergic input from other SOC nuclei and also from the pontomesencephalic tegmentum. Here, we investigate whether these same regions are sources of cholinergic input to other SOC nuclei. We also investigate whether individual cholinergic cells can send collateral projections bilaterally (i.e., into both SOCs), as has been shown at other levels of the subcortical auditory system. We injected retrograde tract tracers into the SOC in gerbils, then identified retrogradely-labeled cells that were also immunolabeled for choline acetyltransferase, a marker for cholinergic cells. We found that both the SOC and the pontomesencephalic tegmentum (PMT) send cholinergic projections into the SOC, and these projections appear to innervate all major SOC nuclei. We also observed a small cholinergic projection into the SOC from the lateral paragigantocellular nucleus of the reticular formation. These various sources likely serve different functions; e.g., the PMT has been associated with things such as arousal and sensory gating whereas the SOC may provide feedback more closely tuned to specific auditory stimuli. Further, individual cholinergic neurons in each of these regions can send branching projections into both SOCs. Such projections present an opportunity for cholinergic modulation to be coordinated across the auditory brainstem.

Locus coeruleus spiking differently correlates with S1 cortex activity and pupil diameter in a tactile detection task

We examined the relationships between activity in the locus coeruleus (LC), activity in the primary somatosensory cortex (S1), and pupil diameter in mice performing a tactile detection task. While LC spiking consistently preceded S1 membrane potential depolarization and pupil dilation, the correlation between S1 and pupil was more heterogeneous. Furthermore, the relationships between LC, S1, and pupil varied on timescales of sub-seconds to seconds within trials. Our data suggest that pupil diameter can be dissociated from LC spiking and cannot be used as a stationary index of LC activity.

Involvement of glutamate receptors of the paragigantocellularis lateralis nucleus in the pain modulatory effect of 17β-estradiol in male rats

The pain modulatory role of the paragigantocellularis lateralis nucleus (LPGi) and the 17β-estradiol has thoroughly been probed. This study investigates the contribution of ionotropic glutamate receptors in pain modulatory effect of intra-LPGi injection of 17β-estradiol. For this purpose, the LPGi nucleus cannulation was performed and drugs were injected into this nucleus, 15 min prior to the formalin test. The duration of formalin-induced flexing and licking behaviors was recorded for 60 min immediately after formalin injection. The results showed that the flexing behavior is significantly decreased by intra-LPGi injection of 0.8 µmol 17β-estradiol duringboth phases of formalin test (P < 0.001). However, 17β-estradiol attenuated the licking duration only in the second phase (P < 0.001). Interestingly, NMDA and AMPA/kainate receptor antagonists (AP5 and CNQX, respectively) significantly counteracted the analgesic effect of intra-LPGi injection of 17β-estradiol in both phases of the formalin test (P < 0.001). Consequently, the revealing results showed that the analgesic effect of intra-LPGi injection of 17β-estradiol on acute inflammatory pain might be mediated via the activation of ionotropic glutamate receptors.

Analgesic Effect of 17β-Estradiol on Nucleus Paragigantocellularis Lateralis of Male Rats Mediated Via GABAA Receptors

Introduction:

Beside its autonomic functions, the nucleus paragigantocellularis lateralis (LPGi) is involved in the descending pain modulation. 17β-Estradiol is a neuroactive steroid found in several brain areas such as LPGi. Intra-LPGi microinjection of 17β-estradiol can elicit the analgesic responses. 17β-Estradiol modulates nociception by binding to estrogenic receptors as well as allosteric interaction with other membrane-bound receptors like GABA_A receptors. This study aimed to examine the role of GABA_A receptors in the pain modulating effect of intra-LPGi injection of 17β-estradiol.

Methods:

To study the antinociceptive effects of 17β-estradiol, cannulation into the LPGi nucleus of male Wistar rats was performed. About 500 nL of drug was administered 15 minutes prior to formalin injection (50 μL of 4%). Then, formalin-induced flexing and licking behaviors were recorded for 60 minutes. For evaluating the role of GABA_A receptors in the estradiol-induced pain modulation, 17β-estradiol was administered into the LPGi nucleus 15 minutes after the injection of 25 ng/μL bicuculline (the GABA_A receptor antagonist). Then, the formalin-induced responses were recorded.

Results:

The results of the current study showed that intra-LPGi injection of 17β-estradiol decreased the flexing duration in both phases of formalin test (P<0.001); but it only attenuated the second phase of licking behavior (P<0.001). 17β-estradiol attenuated the second phase of formalin test of both behaviors (P<0.001). Bicuculline prevented the antinociceptive effect of intra-LPGi 17β-estradiol in both first and second phases of formalin-induced responses (P<0.001).

Conclusion:

According to the results of this study, the analgesic effect of intra-LPGi 17β-estradiol on the formalin-induced inflammatory pain might be mediated via GABA_A receptors.

Regulation of neurological and neuropsychiatric phenotypes by locus coeruleus-derived galanin

Decades of research confirm that noradrenergic locus coeruleus (LC) neurons are essential for arousal, attention, motivation, and stress responses. While most studies on LC transmission focused unsurprisingly on norepinephrine (NE), adrenergic signaling cannot account for all the consequences of LC activation. Galanin coexists with NE in the vast majority of LC neurons, yet the precise function of this neuropeptide has proved to be surprisingly elusive given our solid understanding of the LC system. To elucidate the contribution of galanin to LC physiology, here we briefly summarize the nature of stimuli that drive LC activity from a neuroanatomical perspective. We go on to describe the LC pathways in which galanin most likely exerts its effects on behavior, with a focus on addiction, depression, epilepsy, stress, and Alzheimer׳s disease. We propose a model in which LC-derived galanin has two distinct functions: as a neuromodulator, primarily acting via the galanin 1 receptor (GAL1), and as a trophic factor, primarily acting via galanin receptor 2 (GAL2). Finally, we discuss how the recent advances in neuropeptide detection, optogenetics and chemical genetics, and galanin receptor pharmacology can be harnessed to identify the roles of LC-derived galanin definitively. This article is part of a Special Issue entitled SI: Noradrenergic System.

Orienting and reorienting: the locus coeruleus mediates cognition through arousal

Mood, motivation, attention, and arousal are behavioral states having a profound impact on cognition. Behavioral states are mediated though the peripheral nervous system and neuromodulatory systems in the brainstem. The noradrenergic nucleus locus coeruleus is activated in parallel with the autonomic system in response to biological imperatives. These responses can be spontaneous, to unexpected salient or threatening stimuli, or they can be conditioned responses to awaited behaviorally relevant stimuli. Noradrenaline, released in forebrain structures, will facilitate sensory processing, enhance cognitive flexibility and executive function in the frontal cortex, and promote offline memory consolidation in limbic structures. Central activation of neuromodulatory neurons and peripheral arousal, together, prepare the organism for a reorientation or reset of cortical networks and an adaptive behavioral response.

Arousal from sleep in response to intermittent hypoxia in rat pups is modulated by medullary raphe GABAergic mechanisms

Arousal is an important defense against hypoxia during sleep. Rat pups exhibit progressive arousal impairment (habituation) with multiple hypoxia exposures. The mechanisms are unknown. The medullary raphe (MR) is involved in autonomic functions, including sleep, and receives abundant GABAergic inputs. We hypothesized that inhibiting MR neurons with muscimol, a GABA(A) receptor agonist, or preventing GABA reuptake with nipecotic acid, would impair arousal and enhance arousal habituation and that blocking GABA(A) receptors with bicuculline would enhance arousal and attenuate habituation. Postnatal day 15 (P15) to P25 rat pups were briefly anesthetized, and microinjections with aCSF, muscimol, bicuculline, or nipecotic acid were made into the MR. After a ∼30-min recovery, pups were exposed to four 3-min episodes of hypoxia separated by 6 min of normoxia. The time to arousal from the onset of hypoxia (latency) was determined for each trial. Latency progressively increased across trials (habituation) in all groups. The overall latency was greater after muscimol and nipecotic acid compared with aCSF, bicuculline, or noninjected controls. Arousal habituation was reduced after bicuculline compared with aCSF, muscimol, nipecotic acid, or noninjected pups. Increases in latency were mirrored by decreases in chamber [O2] and oxyhemoglobin saturation. Heart rate increased during hypoxia and was greatest in muscimol-injected pups. Our results indicate that the MR plays an important, not previously described, role in arousal and arousal habituation during hypoxia and that these phenomena are modulated by GABAergic mechanisms. Arousal habituation may contribute to sudden infant death syndrome, which is associated with MR serotonergic and GABAergic receptor dysfunction.

Activation of 5-HT1A receptors in the paragigantocellularis lateralis decreases shivering during cooling in the conscious piglet

Activation of 5-HT1A receptors in the medullary raphé decreases sympathetic outflow to thermoregulatory mechanisms, including brown adipose tissue (BAT), thermogenesis, and peripheral vasoconstriction when these mechanisms are previously activated with leptin, prostaglandins, or cooling. These same mechanisms are also inhibited during rapid eye movement (REM) sleep. It is not known whether shivering is also modulated by medullary raphé neurons. We previously showed in the conscious piglet that activation of 5-HT1A receptors with 8-OH-DPAT (DPAT) in the paragigantocellularis lateralis (PGCL), a medullary region lateral to the midline raphé that contains 5-HT neurons, decreases heart rate, body temperature and muscle activity during non-rapid eye movement (NREM) sleep. We therefore hypothesized that activation of 5-HT1A receptors in the PGCL would also attenuate shivering and peripheral vasoconstriction during cooling. During REM sleep in a cool environment, shivering, carbon dioxide production, and body temperature decreased, and ear capillary blood flow and ear skin temperature increased. Shivering associated with rapid cooling was attenuated after dialysis of DPAT into the PGCL. In animals maintained in a continuously cool environment, dialysis of DPAT into the PGCL attenuated shivering and decreased body temperature, but there were no significant increases in ear capillary blood flow or ear skin temperature. We conclude that both naturally occurring REM sleep and exogenous activation of 5-HT1A receptors in the PGCL are associated with a suspension of shivering during cooling. Our data are consistent with the hypothesis that 5-HT neurons in the PGCL facilitate oscillating spinal motor circuits involved in shivering but are less involved in modulating sympathetically mediated thermoregulatory mechanisms.

Convergence of multiple pelvic organ inputs in the rat rostral medulla

Electrophysiological recordings were used to investigate the degree of pelvic/visceral convergent inputs onto single medullary reticular formation (MRF) neurons. A total of 94 MRF neurons responsive to bilateral electrical stimulation of the pelvic nerve (PN) in 12 urethane-anaesthetized male rats were tested for responses to mechanical stimulation of the urinary bladder, urethra, colon and penis, and electrical stimulation of the dorsal nerve of the penis (DNP) and abdominal branches of the vagus. Responses to distension of the bladder were found for 51% (n = 48) of the MRF neurons tested. Of these 48, 71% responded to urethral infusion, 81% responded to colon distension, 100% responded to penile stimulation (and DNP), and 85% responded to vagal stimulation, with 62% responding to stimulation of all four of these territories. This high degree of visceral convergence (i.e. 62%) in a subset of PN-responsive MRF neurons is significantly greater than for the subset of PN-responsive MRF neurons that did not respond to urinary bladder distension (i.e. out of the 46 remaining neurons, none responded to all four of the other pelvic/visceral stimuli combined). These results suggest that the neurons processing information from the urinary bladder at this level of the neural axis are likely to be important for mediating interactions between different visceral organs for the coordination of multiple pelvic/visceral functions.

Inhibition of serotonergic neurons in the nucleus paragigantocellularis lateralis fragments sleep and decreases rapid eye movement sleep in the piglet: implications for sudden infant death syndrome

Serotonergic receptor binding is altered in the medullary serotonergic nuclei, including the paragigantocellularis lateralis (PGCL), in many infants who die of sudden infant death syndrome (SIDS). The PGCL receives inputs from many sites in the caudal brainstem and projects to the spinal cord and to more rostral areas important for arousal and vigilance. We have shown previously that local unilateral nonspecific neuronal inhibition in this region with GABA(A) agonists disrupts sleep architecture. We hypothesized that specifically inhibiting serotonergic activity in the PGCL would result in less sleep and heightened vigilance. We analyzed sleep before and after unilaterally dialyzing the 5-HT1A agonist (+/-)-8-hydroxy-2-(dipropylamino)-tetralin (8-OH-DPAT) into the juxtafacial PGCL in conscious newborn piglets. 8-OH-DPAT dialysis resulted in fragmented sleep with an increase in the number and a decrease in the duration of bouts of nonrapid eye movement (NREM) sleep and a marked decrease in amount of rapid eye movement (REM) sleep. After 8-OH-DPAT dialysis, there were decreases in body movements, including shivering, during NREM sleep; body temperature and heart rate also decreased. The effects of 8-OH-DPAT were blocked by local pretreatment with N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexane-carboxamide, a selective 5-HT1A antagonist. Destruction of serotonergic neurons with 5,7-DHT resulted in fragmented sleep and eliminated the effects of subsequent 8-OH-DPAT dialysis on REM but not the effects on body temperature or heart rate. We conclude that neurons expressing 5-HT1A autoreceptors in the juxtafacial PGCL are involved in regulating or modulating sleep. Abnormalities in the function of these neurons may alter sleep homeostasis and contribute to the etiology of SIDS.

Systemic administration of WIN 55,212-2 increases norepinephrine release in the rat frontal cortex

Cannabinoid agonists modulate a variety of behavioral functions by activating cannabinoid receptors that are widely distributed throughout the central nervous system. In the present study, norepinephrine efflux was assessed in the frontal cortex of rats that received a systemic administration of the cannabinoid agonist, WIN 55,212-2. The synthetic cannabinoid agonist dose-dependently increased the release of norepinephrine in this brain region. Pretreatment with the cannabinoid receptor antagonist, SR 141716A, blocked the increase in norepinephrine release. To identify sites of cellular activation, immunocytochemical detection of c-Fos was combined with detection of the catecholamine synthesizing enzyme, tyrosine hydroxylase (TH), in the brainstem nucleus locus coeruleus (LC), a region that is the sole source of norepinephrine to the frontal cortex. Systemic administration of WIN 55,212-2 significantly increased the number of c-Fos immunoreactive cells within TH-containing neurons in the LC compared to vehicle-treated rats. Pretreatment with SR 141716A inhibited the WIN 55,212-2 induced c-Fos expression, while the antagonist alone did not affect c-Fos expression. Taken together, these data indicate that systemically administered cannabinoid agonists stimulate norepinephrine release in the frontal cortex by activating noradrenergic neurons in the coeruleo-frontal cortex pathway. These effects may partially underlie changes in attention, arousal and anxiety observed following exposure to cannabis-based drugs.

Gene expression in autonomic areas of the medulla and the central nucleus of the amygdala in rats during and after space flight

During space flight astronauts show vestibular-related changes in balance, eye movements, and spontaneous and reflex control of cardiovascular, respiratory and gastrointestinal function, sometimes associated with space motion sickness. These symptoms undergo compensation over time. Here we used changes in the expression of two immediate-early gene (IEG) products to identify cellular and molecular changes occurring in autonomic brainstem regions of adult male albino rats killed at different times during the Neurolab Space Mission (STS-90). Both direct effects of gravitational changes, as well as indirect effects of gravitational changes on responses to light exposure were examined. Regions under the direct control of vestibular afferents such as the area postrema and the caudal part of the nucleus of the tractus solitarius (NTSC) were both directly and indirectly affected by gravity changes. These areas showed no changes in the expression of IEG products during exposure to microgravity with respect to ground controls, but did show a significant increase 24 h after return to 1 G (gravity). Exposure to microgravity significantly inhibited gene responses to light exposure seen after return to 1 G. A similar direct and indirect response pattern was also shown by the central nucleus of the amygdala, a basal forebrain structure anatomically and functionally related to the NTS. The rostral part of the NTS (NTSR) receives different afferent projections than the NTSC. This region did not show any direct gravity-related changes in IEG expression, but showed an indirect effect of gravity on IEG responses to light. A similar pattern was also obtained in the intermediate reticular nucleus and the parvocellular reticular nucleus. Two other medullary reticular structures, the dorsal and the ventral medullary reticular nuclei showed a less well defined pattern of responses that differed from those seen in the NTSC and NTSR. The short- and long-lasting molecular changes in medullary and basal forebrain gene expression described here are thought to play an important role in the integration of autonomic and vestibular signals that ultimately regulate neural adaptations to space flight.

Region-specific changes in NMDA receptor mRNA induced by chronic morphine treatment are prevented by the co-administration of the competitive NMDA receptor antagonist LY274614

The steady-state mRNA levels of the NMDA receptor NR1 subunit were determined by a quantitative solution hybridization assay in selected CNS regions associated with antinociception in the rat. Tissues were obtained by microdissection from rats treated chronically with morphine alone or in combination with LY274614, a competitive NMDA receptor antagonist. Morphine treatment for 7 days resulted in the development of tolerance to morphine's analgesic effect and produced a significant decrease in the steady-state NR1 mRNA levels in the spinal cord dorsal horn (by 16%), and an elevation in nucleus raphe magnus and medial thalamus (by 26 and 38%, respectively). The NR1 mRNA levels were unchanged in the lateral paragigantocellular nucleus, locus coeruleus, periaqueductal grey, and sensorimotor cortex. NMDA receptor binding in the spinal cord measured with [3H]MK-801 was reduced approximately 50% by chronic morphine treatment. Co-administration of LY274614 (s.c. at 24 mg/kg/24 h via an osmotic pump) not only attenuated the development of morphine tolerance but also prevented the changes in the NR1 mRNA levels induced by chronic morphine administration. Neither a 7-day infusion of LY274614 nor an acute injection of morphine (10 mg/kg, s.c.) changed the NR1 mRNA levels. These results suggest that changes in the expression of the NR1 mRNA induced by chronic morphine in three CNS regions involved in antinociception are associated with the development of morphine tolerance and in the spinal cord, morphine tolerance is associated with the downregulation of NMDA receptors.

Short-term (Fos) and long-term (FRA) protein expression in rat locus coeruleus neurons during the neurolab mission: contribution of altered gravitational fields, stress, and other factors

Changes in immediate-early gene (IEG) expression during and after space flight were studied in the rat locus coeruleus (LC) during the NASA Neurolab mission. The LC sends widespread projections throughout the brain and releases the neuromodulator norepinephrine. LC neurons respond to natural vestibular stimulation; their firing rate also increases during waking and decreases or ceases during sleep. LC neurons express IEGs such as c-fos after activation. Adult male albino Fisher 344 rats were killed at four mission time points, and the number of Fos- and Fos-related antigen (FRA)-positive LC cells were counted in flight and ground-based control rats. Half of the subjects at each time point were exposed to light for 60 min prior to killing to standardize their sleep-waking state. FRA-expressing cells were more numerous than Fos-expressing cells in both flight- and ground-based subjects. The difference between FRA- and Fos-expressing cells within individuals was significantly larger 24 h after landing in subjects exposed to both space flight and light pulse than in all other subjects at any mission time point. Fos and FRA responses scaled in proportion to the maximum response observed in any single individual showed similar patterns of variation. Analysis of the scaled and combined responses showed that LC IEG levels responded to both gravity changes and light pulses. Subjects exposed to either single stimulus had equivalent responses, significantly greater than those of control subjects maintained in dim light. The combination of gravity change and light pulse gave significantly higher LC responses than either stimulus alone 24 h after takeoff, and to a lesser extent after 12 days in space; the highest responses were obtained 24 h after landing. By 14 days after landing, animals exposed to space flight and light pulse responded no differently than ground-based subjects. No difference in LC IEG expression was clearly attributable to changes in the sleep-waking state of subjects. Activity of noradrenergic LC neurons has been previously shown to modulate IEG expression in target structures. The increased IEG LC activity (seen most especially 24 h after landing) may reflect large-scale activation of noradrenergic neurons that may serve as a trigger for molecular changes in target structures, and be critical for adaptation to gravity changes.

Descending spinal projections from the rostral gigantocellular reticular nuclei complex

Electrophysiological and physiological studies have suggested that the ventral medullary gigantocellular reticular nuclei (composed of the gigantocellular ventralis and pars alpha nuclei as well as the adjacent lateral paragigantocellular nucleus; abbreviated Gi-LPGi complex) provide descending control of pelvic floor organs (Mackel [1979] J. Physiol. (Lond.) 294:105-122; Hubscher and Johnson [1996] J. Neurophysiol. 76:2474-2482; Hubscher and Johnson [1999] J. Neurophysiol. 82:1381-1389; Johnson and Hubscher [1998] Neuroreport 9:341-345). Specifically, this complex of paramedian reticular nuclei has been implicated in the inhibition of sexual reflexes. In the present study, an anterograde fluorescent tracer was used to investigate direct descending projections from the Gi-LPGi complex to retrogradely labeled pudendal motoneurons (MN) in the male rat. Our results demonstrated that, although a high density of arborizations from Gi-LPGi fibers appears to be in close apposition to pudendal MNs, this relationship also applies to other MNs throughout the entire spinal cord. The Gi-LPGi also projects to spinal autonomic regions, i.e., both the intermediolateral cell column and the sacral parasympathetic nucleus, as well as to regions of the intermediate gray, which contain interneurons involved in the organization of pelvic floor reflexes. Lastly, throughout the length of the spinal cord, numerous neurons located primarily in laminae VII-X, were retrogradely labeled with Fluoro-Ruby after injections into the Gi-LPGi. The diffuse descending projections and arborizations of this pathway throughout the spinal cord suggest that this brainstem area is involved in the direct, descending control of a variety of spinal activities. These results are in contrast with our observations of the discrete projections of the caudal nucleus raphe obscurus, which target the autonomic and somatic MNs involved specifically in sexual and eliminative functions (Hermann et al. [1998] J. Comp. Neurol. 397:458-474).

Fos and FRA protein expression in rat nucleus paragigantocellularis lateralis during different space flight conditions

The nucleus paragigantocellularis lateralis (LPGi) exerts a prominent excitatory influence over locus coeruleus (LC) neurons, which respond to gravity signals. We investigated whether adult albino rats exposed to different gravitational fields during the NASA Neurolab Mission (STS-90) showed changes in Fos and Fos-related antigen (FRA) protein expression in the LPGi and related cardiovascular, vasomotor, and respiratory areas. Fos and FRA proteins are induced rapidly by external stimuli and return to basal levels within hours (Fos) or days (FRA) after stimulation. Exposure to a light pulse (LP) 1 h prior to sacrifice led to increased Fos expression in subjects maintained for 2 weeks in constant gravity (either at approximately 0 or 1 G). Within 24 h of a gravitational change (launch or landing), the Fos response to LP was abolished. A significant Fos response was also induced by gravitational stimuli during landing, but not during launch. FRA responses to LP showed a mirror image pattern, with significant responses 24 h after launch and landing, but no responses after 2 weeks at approximately 0 or 1 G. There were no direct FRA responses to gravity changes. The juxtafacial and retrofacial parts of the LPGi, which integrate somatosensory/acoustic and autonomic signals, respectively, also showed gravity-related increases in LP-induced FRA expression 24 h after launch and landing. The neighboring nucleus ambiguus (Amb) showed completely different patterns of Fos and FRA expression, demonstrating the anatomical specificity of these results. Immediate early gene expression in the LPGi and related cardiovascular vasomotor and ventral respiratory areas may be directly regulated by excitatory afferents from vestibular gravity receptors. These structures could play an important role in shaping cardiovascular and respiratory function during adaptation to altered gravitational environments encountered during space flight and after return to earth.

The retrotrapezoid nucleus (RTN): local cytoarchitecture and afferent connections

The retrotrapezoid nucleus (RTN) provides a source of tonic drive to respiratory neurons and is one of many sites for central chemoreception. Here we evaluate in the rat the local neuronal cytoarchitecture in the RTN histologically 2-4 h after neurobiotin injection and the afferent connections to the RTN 24 h after injection. Our neurobiotin injections often overlapped the RTN and the adjacent neurons of the parapyramidal region, so we group these two regions together in this study. The RTN is made up of small and medium sized neurons and has a low neuronal density compared to other nuclei. The organization of the RTN is reticular in nature and there are prominent small neurons at the ventral medullary border. Adjacent to the pyramids there are medium sized neurons with connections to the raphé pallidus. Major afferent connections include the regions of the dorsal and ventral respiratory groups, the medullary raphé, the contralateral parapyramidal and RTN regions, portions of the nucleus paragigantocellularis lateralis, and portions of the reticular fields. Other sources of input include the Kölliker-fuse nucleus, subceruleus, A5 region, and the paralemniscal zone.

Anatomic relationships of the human nucleus paragigantocellularis lateralis: a DiI labeling study

The nucleus paragigantocellularis lateralis (PGL) is located in the rostral ventrolateral medulla (RVLM), a brainstem region that regulates homeostatic functions, such as blood pressure and cardiovascular reflexes, respiration. central chemosensitivity and pain. In the present study, we examined anatomic relationships of the human nucleus paragigantocellularis lateralis using a bidirectional lipophilic fluorescent tracer, 1,1'-dioctadecyl-3,3.3',3'-tetramethylindocarbocyanine perchlorate (DiI), in nine postmortem human fetal midgestational brainstems. The areas which were labeled by diffusion of DiI from the nucleus paragigantocellularis lateralis included the arcuate nucleus (ARC) of the medulla, caudal raphe (nucleus raphe obscurus and pallidus), hilum and amiculum of the inferior olive, bilateral "reticular formation" (including the nucleus paragigantocellularis lateralis, nucleus gigantocellular-is and the intermediate reticular zone (IRZ)). vestibular and cochlear nuclei, cells and fibers at the floor of the fourth ventricle with morphologic features of tanycytes, parabrachial nuclei (PBN), medial lemniscus, lateral lemniscus, inferior cerebellar peduncle and cerebellar white matter, central tegmental tract, and the capsule of the red nucleus. This pattern of DiI labeling bears many similarities with the pattern of connections of the nucleus paragigantocellularis lateralis previously demonstrated by tract-tracing methods in experimental animals, and is consistent with the role of the nucleus paragigantocellularis lateralis in central regulation of homeostatic functions. In contrast to the animal studies, however, we did not demonstrate connections of the nucleus paragigantocellularis lateralis with the nucleus of the tractus solitarius (nTS) (only connections with the rostral subdivision were examined), locus coeruleus, or the periaqueductal gray (PAG) in the human midgestational brainstem. In our previous studies, six medullary areas showed reduced serotonin receptor binding in a subset of victims of sudden infant death syndrome (SIDS). The present study demonstrated DiI labeling in all of these six areas, suggesting that they are interconnected.

Gating of vagal inputs by sciatic afferents in nonspinally projecting neurons in the rat rostral ventrolateral medulla oblongata

Integration and coordination of somato-visceral sensory information is crucial to achieve adaptive behavioural responses. We have recently shown that sensory vagal and somato-sensory (sciatic nerve) inputs converge in neurons of the rostral ventrolateral medulla oblongata, which was implicated in adjusting visceral activities to changing somatic performances. In the present study, the neuronal mechanism of interaction between sciatic and vagal sensory inputs was examined in the rostral ventrolateral medulla oblongata using in vivo intracellular recording and labelling. Conditioning stimulation of the contralateral sciatic nerve (2 V) led to a time-dependent inhibition of responses to vagal stimulation (100 microA) in each RVLM neuron that received convergent sciatic and vagal sensory inputs (n = 50). None of these neurons had direct spinal projections, and only 8% of them exhibited a visible response to stimulation of the aortic depressor nerve. A significant attenuation of the amplitude of vagal test responses was present for up to 800 ms of conditioning delay, although the duration of this sciatico-vagal inhibition was greatly dependent on the intensity of both stimuli. The electrophysiological data indicated that sciatico-vagal inhibition is mediated presynaptically, via activation of GABAB receptors. Morphological evidence of axo-axonic interactions that may underlie sciatico-vagal inhibition was subsequently found in the electron microscope. It is suggested that during movements of the hindleg, activation of sciatic sensory fibres leads to re-patterning of neuronal activity in RVLM neurons via inhibition of visceral sensory inputs. Sciatico-vagal inhibition is likely to affect the activity of those RVLM neurons that modulate higher neuronal activities via ascending projections.

Glutamatergic influences on the nucleus paragigantocellularis: contribution to performance in avoidance and spatial memory tasks

Stimulation of the locus coeruleus (LC) and the subsequent release of norepinephrine contribute to memory consolidation processes. Excitatory input to the LC is derived primarily from neurons in the nucleus paragigantocellularis (PGi). The authors examined the effects of activating the pathway between PGi and the LC on memory. Rats received vehicle or the excitatory amino acid glutamate (25, 50, or 100 nmol/0.5 microl) into PGi after training in an inhibitory avoidance (IA) or delayed matching-to-sample (DMS) task. Rats given the 100-nmol dose had significantly longer retention latencies on a 48-hr IA retention test. Rats treated with the 50- or 100-nmol dose made significantly more correct responses than controls on an 18-hr DMS retention test. Results suggest that encoding and storage of memory for emotional and spatial events may be enhanced by activation of neuronal circuits afferent to the LC.

Posttraining inactivation of excitatory afferent input to the locus coeruleus impairs retention in an inhibitory avoidance learning task

These experiments examined whether the nucleus paragigantocellularis (PGi) contributes to memory storage processing via its ascending excitatory influence on locus coeruleus (LC) neuronal activity. Activation of the LC leads to memory enhancement and also results in a widespread release of norepinephrine in target structures, such as the amygdala and hippocampus. Infusion of norepinephrine into either structure also improves memory for several types of learned responses. Thus, the capacity for norepinephrine to modulate memory within limbic structures may be contingent upon the functional connections between PGi and the LC. To examine this hypothesis, male Sprague-Dawley rats were implanted with cannula aimed above PGi (Experiments 1 and 2) or 1.5 mm dorsal or medial to PGi (Experiment 3). Immediately following inhibitory avoidance training (0.45 mA, 0. 5 s), phosphate-buffered saline, lidocaine (Experiment 1), or 12.5 or 25 nmol/0.5 microl of the GABA agonist muscimol (Experiment 2) was infused into PGi. On a retention test given 48 h later, the latency to reenter the footshock compartment was significantly shorter for subjects given either lidocaine or 12.5 or 25.0 nmol of muscimol compared to controls. In Experiment 3, infusion of lidocaine or muscimol into areas 1.5 mm dorsal or medial to PGi did not significantly alter retention, indicating that the memory impairment observed in Experiments 1 and 2 was site specific and not due to the spread of drug to cell groups surrounding PGi. These findings suggest that PGi may serve a vital function in relaying biologically relevant information to forebrain structures involved in memory via its excitatory influence on the LC.

Brain stem lesion size determined by DEAD red or conjugation of neurotoxin to fluorescent beads

Neurotoxin microinjected into the retrotrapezoid nucleus of anesthetized rats decreases phrenic activity and eliminates the response to CO2. In unanesthetized rats, such treatment has no effect on awake, resting breathing and decreases CO2 sensitivity by 40% (M. Akilesh, M. Kamper, A. Li, and E. E. Nattie. J. Appl. Physiol. 82: 469-479, 1997). One important factor in explaining these disparate results is the actual size of the anatomic lesion. In the present study, we injected ibotenic acid into the retrotrapezoid nucleus of anesthetized rats and evaluated lesion size by using two new approaches: 1) DEAD red, a fluorescent probe that enters impaired cells through leaky membranes and binds to nucleic acids, and 2) conjugation of toxin to fluorescent beads. With the use of DEAD red, the region containing labeled dying cells was 313 +/- 104 nl (n = 4), six times larger than the initial injected volume, and the physiological effects on phrenic amplitude, the CO2 response, and blood pressure began within minutes and were substantial. With conjugated toxin, in theory, neuronal damage would be limited to the region of detectable fluorescence (49 +/- 10 nl; n = 4). Effects on phrenic amplitude, CO2 sensitivity, and blood pressure were absent until approximately 2 h postinjection. Control experiments, with 2 h of in vitro incubation of the neurotoxin-microbead conjugate and injection of the supernatant after centrifugation, showed similar results that suggest release of conjugated neurotoxin. We conclude that DEAD red provides a useful means to monitor neuronal impairment in acute studies in vivo. Conjugation of neurotoxin to microbeads may be less reliable in this regard.

Nervousness and pain sensitivity: I. A positive correlation

Neuroanatomical studies suggest a close interrelationship between brainstem centers regulating arousal and pain sensitivity. Nervousness, as assessed with a Visual Analog Scale, and pain sensitivity, as assessed with a cold pressor test, were used to clarify whether a physiological association of nervousness and pain sensitivity can be found in healthy subjects. Forty healthy volunteers were included in the study. We demonstrate a significant positive correlation between self-rated nervousness and pain threshold. These data suggest that there is a coupling between nervousness and endogenous pain control. Based on the results, a nervousness-pain-threshold quotient was calculated as a possible measure of the interrelationship of the endogenous pain control system to autonomic activity. A different nervousness-pain-threshold quotient, indicating a different coupling, may provide information on changes in accessory neurophysiologic functions.

Periaqueductal gray matter input to cardiac-related sympathetic premotor neurons

The periaqueductal gray matter (PAG) serves as the midbrain link between forebrain emotional processing systems and motor pathways used in the defense reaction. Part of this response depends upon PAG efferent pathways that modulate cardiovascular-related sympathetic outflow systems, including those that regulate the heart. While it is known that the PAG projects to vagal preganglionic neurons, including possibly cardiovagal motoneurons, no information exists on the PAG circuits that may affect sympathetically mediated cardiac functions and, thus, the purpose of this study was to use neuroanatomical methods to identify these pathways. First, viral transneuronal retrograde tracing experiments were performed in which pseudorabies virus (PRV) was injected into the stellate ganglion of rats. After 4 days survival, five PAG regions contained transynaptically infected neurons; these included the dorsomedial, lateral and ventrolateral PAG columns as well as the Edinger-Westphal and precommissural nuclei. Second, the descending efferent PAG projections were studied with the anterograde axonal marker Phaseolus vulgaris leuco-agglutinin (PHA-L) with a particular focus on determining whether the PAG projects to the intermediolateral cell column (IML). Almost no axonal labeling was found throughout the thoracic IML suggesting that the PAG modulates sympathetic functions by indirect pathways involving synaptic relays through sympathetic premotor cell groups, especially those found in the medulla oblongata. This possibility was examined by a double tracing study. PHA-L was first injected into either the lateral or ventrolateral PAG and after 6 days, PRV was injected into the ipsilateral stellate ganglion. After an additional 4 days survival, a double immunohistochemical procedure for co-visualization of PRV and PHA-L was used to identify the sympathetic premotor regions that receive an input from the PAG. The PAG innervated specific groups of sympathetic premotor neurons in the hypothalamus, pons, and medulla as well as providing reciprocal intercolumnar connections within the PAG itself (Jansen et al., Brain Res. 784 (1998) 329-336). The major route terminates in the ventral medulla, especially within the medial region which contains sympathetic premotor neurons lying within the raphe magnus and gigantocellular reticular nucleus, pars alpha. Both serotonergic and non-serotonergic sympathetic premotor neurons in these two regions receive inputs from the PAG. Weak PAG projections to sympathetic premotor neurons were found in the rostral ventrolateral medulla (including to C1 adrenergic neurons), locus coeruleus, A5 cell group, paraventricular and lateral hypothalamic nuclei. In summary, both the lateral and ventrolateral PAG columns appear to be capable of modulating cardiac sympathetic functions via a series of indirect pathways involving sympathetic premotor neurons found in selected sites in the hypothalamus, midbrain, pons, and medulla oblongata, with the major outflow terminating in bulbospinal regions of the rostral ventromedial medulla.

Expression of c-fos in the rat brainstem after exposure to hypoxia and to normoxic and hyperoxic hypercapnia

In this study, Fos immunohistochemistry was used to map brainstem neuronal pathways activated during hypercapnia and hypoxia. Conscious rats were exposed to six different gas mixtures: (a) air; (b) 8% CO2 in air; (c) 10% CO2 in air; (d) 15% CO2 in air; (e) 15% CO2 + 60% O2, balance N2; (f) 9% O2, balance N2. Double-staining was performed to show the presence of tyrosine hydroxylase. Hypercapnia, in a dose-dependent way caused Fos expression in the following areas: caudal nucleus tractus solitarius (NTS), with few labeled A2 noradrenergic neurons; noradrenergic A1 cells and noncatecholaminergic neurons in the caudal ventrolateral medulla; raphe magnus and gigantocellular nucleus pars alpha (GiA); many noncatecholaminergic (and relatively few C1) neurons in the lateral paragigantocellular nucleus (PGCl), and in the retrotrapezoid nucleus (RTN); locus coeruleus (LC), external lateral parabrachial and Kölliker-Fuse nuclei, and A5 noradrenergic neurons at pontine level; and in caudal mesencephalon, the ventrolateral column of the periaqueductal gray (vlPAG). In most of these nuclei, hypoxia also induced Fos expression, albeit generally less than after hypercapnia. However, hypoxia did not cause labeling in RTN, juxtafacial PGCl, GiA, LC, or vlPAG. After normoxic hypercapnia, more labeled cells were present in NTS and PGCl than after hyperoxic hypercapnia. Part of the observed labeling could be caused by stress- or cardiovascular-related sequelae of hypoxia and hypercapnia. Possible implications for the neural control of breathing are also discussed, particularly with regard to the finding that several nuclei, not belonging to the classical brainstem respiratory centres, contained labeled cells.

Neurons in rostral ventrolateral medulla mediate vestibular inhibition of locus coeruleus in rats

The effects of caloric vestibular stimulation on the central noradrenergic neurons system were examined in the rat. In urethane-anesthetized rats, caloric stimulation inhibited the spontaneous activity of noradrenergic locus coeruleus neurons and increased systemic blood pressure. Electrical and chemical lesions in the ventrolateral medulla attenuated both the locus coeruleus inhibition and the blood pressure increase in response to caloric stimulation. Neither the neuronal inhibition nor the pressor effect was attenuated by any deafferentation of the forebrain or baroreceptors, or lesioning of the nucleus tractus solitarius. These findings indicate that the caloric stimulation-induced locus coeruleus inhibition is mediated by neurons in the ventrolateral medulla, and that these neurons also mediate the vestibulo-pressor responses. The locus coeruleus inhibition via the ventrolateral medulla is, however, considered to be independent of ventrolateral medulla-mediated systemic pressor effect. Collectively these findings suggest that the ventrolateral medulla is the major origin of inhibitory vestibular input to the noradrenergic neurons of the locus coeruleus, and that the ventrolateral medulla plays an important role in the vestibulo-autonomic response.

Effects of unilateral lesions of retrotrapezoid nucleus on breathing in awake rats

In anesthetized rats, unilateral retrotrapezoid nucleus (RTN) lesions markedly decreased baseline phrenic activity and the response to CO2 (E. E. Nattie and A. Li. Respir. Physiol. 97:63-77, 1994). Here we evaluate the effects of such lesions on resting breathing and on the response to hypercapnia and hypoxia in unanesthetized awake rats. We made unilateral injections [24 +/- 7 (SE) nl] of ibotenic acid (IA; 50 mM), an excitatory amino acid neurotoxin, in the RTN region (n = 7) located by stereotaxic coordinates and by field potentials induced by facial nerve stimulation. Controls (n = 6) received RTN injections (80 +/- 30 nl) of mock cerebrospinal fluid. A second control consisted of four animals with IA injections (24 +/- 12 nl) outside the RTN region. Injected fluorescent beads allowed anatomic identification of lesion location. Using whole body plethysmography, we measured ventilation in the awake state during room air, 7% CO2 in air, and 10% O2 breathing before and for 3 wk after the RTN injections. There was no statistically significant effect of the IA injections on resting room air breathing in the lesion group compared with the control groups. We observed no apnea. The response to 7% CO2 in the lesion group compared with the control groups was significantly decreased, by 39% on average, for the final portion of the 3-wk study period. There was no lesion effect on the ventilatory response to 10% O2. In this unanesthetized model, other areas suppressed by anesthesia, e.g., the reticular activating system, hypothalamus, and perhaps the contralateral RTN, may provide tonic input to the respiratory centers that counters the loss of RTN activity.

Increased activity of bulbospinal cardiovascular neurons in the rat rostral ventrolateral medulla upon emergence from anaesthesia

The rostral ventrolateral medulla (RVLM) is part of the vasomotor centre which controls the cardiovascular system and may therefore be critical to the genesis of postoperative hypertension. This area is probably a common site of termination of different inputs involved in the baroreflex. It contains at least two classes of neurons exhibiting spontaneous activities and projecting to sympathetic preganglionic neurons located in the intermediolateral cell-column (IML) of the spinal cord. The first class of neurons corresponds to cells with slow axonal conduction velocities (< 0.8 m s-1) and which contain immunoreactive phenylethanolamine-N-methyltransferase (CI cells); the second class, characterized by faster conduction velocities (2.5-8 m s-1), is considered as glutamatergic, although the C1 cells may also release glutamate alongside catecholamine. The purpose of the present study was to investigate the involvement of the "fast-conducting' RVLM barosensitive bulbospinal (RVLM-BB) neurons in the hypertension occurring upon emergence from halothane anaesthesia. Rats were anaesthetized with halothane, paralysed, and their lungs mechanically ventilated. Avoidable pain, distress or discomfort was consistently avoided as required by the fundamental principles of ethical animal research. Hence, all pressure points and surgical wounds, as well as tracheal tube were carefully covered or infiltrated with adequate local anaesthetic. Control experiments have been performed, allowing us to assert that hypertension accompanying halothane withdrawal was not due to suffering (see Discussion). Under halothane anaethesia, fast conducting (2.7 +/- 1.0 m s-1) RVLM-BB neurons (n = 10) exhibited a continuous discharge (8.4 +/- 7.5 Hz). Five minutes after discontinuing halothane, in increase in arterial blood pressure was recorded (AP 19 +/- 6 mmHg), which was accompanied by an increase in the unitary activities (n = 8.43 +/- 23%). Afterwards, both AP and unitary activity frequencies further increased to reach a maximum value at the end of the sequence (34 +/- 9 mmHg and 161 +/- 120% respectively, n = 10). After resumption of halothane administration, both AP and unitary activities fall down to the baseline level within 5 min (n = 10). This study shows that emergence from halothane anaesthesia reversibly induces RVLM-BB units activation, suggesting that a putative glutamatergic bulbospinal pathway may be involved in the genesis of hypertension occurring upon emergence from anaesthesia. These data may therefore contribute to better understanding of postoperative hypertension and to improve its pharmacological treatment in man.

Afferent sources of substance P in the C1 area of the rat rostral ventrolateral medulla

To examine the potential extrinsic sources of substance P (SP)-containing terminals in the C1 area of the RVL, immunoperoxidase localization of SP was combined with retrograde transport of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). In all cases examined, several neurons containing SP and WGA-HRP were found in the nucleus raphe pallidus and a few dual labeled neurons were found in the nucleus of the solitary tract and the paraventricular nucleus of the hypothalamus. In some cases, one or two double labeled neurons were detected in the lateral hypothalamic area and nucleus raphe obscurus. The results demonstrate that although several brain structures contribute SP afferents to the RVL, the nucleus raphe pallidus is the major extrinsic source.

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.

Preoptic projections to Barrington's nucleus and the pericoerulear region: architecture and terminal organization

The medial preoptic area (MPO), a sexually dimorphic region, plays a pivotal role in neuroendocrine function and reproductive behavior. We recently reported that MPO projects heavily to the midbrain periaqueductal gray (PAG). We also noted that MPO projects to the dorsolateral pontine tegmentum. Here we identified the cells of origin of the MPO-->tegmental projection and delineated the terminal organization of MPO projections to Barrington's nucleus, locus coeruleus (LC), and the rostromedial pericoerulear region (pLCrm). Correlative cyto- and chemoarchitectonic studies were done to define better the nuclear groups of the dorsolateral pontine tegmentum. Retrograde tracing revealed that MPO neurons projecting to the dorsolateral pontine tegmentum are preferentially distributed in distinct subregions of MPO, including the sexually dimorphic medial preoptic nucleus (MPN). Anterograde tracing with wheat germ agglutinin-horseradish peroxidase or Phaseolus vulgaris leucoagglutinin demonstrated considerable target specificity in projections from MPO to the dorsolateral pontine tegmentum. Barrington's nucleus receives a dense focal input along its entire rostrocaudal axis. In addition, pLCrm is heavily targeted by MPO inputs; pLCrm contains a concentrated plexus of extranuclear dendrites of LC neurons. The lateral dorsal tegmental (LDT) nucleus and LC proper receive only sparse input from MPO. MPO projections to Barrington's nucleus could regulate micturition reflexes during reproductive behavior. The MPO-->pLCrm projection could influence noradrenergic LC neurons in relation to reproductive and/or gonadal steroid function. Given the strong established connections from olfactory structures to MPO, it is possible that the MPO-->LC pathway provides an anatomical substrate for olfactory modulation of arousal.

Retrotrapezoid nucleus lesions decrease phrenic activity and CO2 sensitivity in rats

In chloralose-urethane anesthetized, paralyzed, and ventilated rats, we measured the effects of unilateral lesions in the region ventral and ventromedial to the facial nucleus, the retrotrapezoid nucleus (RTN), on eucapnic phrenic activity and the response to increased end-tidal CO2. Chemical (kainic acid injections; 4.7 mM; 10-100 nl) and electrolytic (5-15 mA; 5-15 sec) lesions, anatomically demonstrated to be in the RTN, resulted in a progressive decrease in the amplitude of the integrated phrenic nerve activity from baseline levels of 49-59% of maximum to values of 21-32% of maximum over 30 to 120 min. There were no consistent effects on frequency or on blood pressure. The initial slope of the response to hypercapnia was decreased by 86-92%. Bilateral carotid body ablation did not alter the general pattern of the responses. As in the cat, unilateral RTN lesions decrease baseline phrenic amplitude and virtually abolish the response to hypercapnia. We hypothesize that the RTN region provides; (1) a source of tonic activity which maintains eucapnic ventilatory output, and (2) allows expression of the response to hypercapnia.

Direct projections from the ventrolateral medulla oblongata to the limbic forebrain: anterograde and retrograde tract-tracing studies in the rat

Neurons in the ventrolateral medulla oblongata, a brain region implicated in central vasomotor regulation, have previously been reported to project to some forebrain limbic structures. The aim of the present study was (1) to describe the termination pattern of ventral medullary afferents in forebrain limbic areas using anterograde tract tracing, and (2) to determine the location and some morphological characteristics of the projection neurons using retrograde tract tracing from selected forebrain sites. Following ionophoretic microinjections of the anterograde tract tracer Phaseolus vulgaris leucoagglutinin into the rostral ventrolateral medulla, labelled afferents were observed in the hippocampus, entorhinal and retrosplenial cortices, dorsal septum, nucleus accumbens, and the medial prefrontal cortex. Anterogradely labelled axons, ascending from the caudal ventrolateral medulla, could be traced only to the rostral aspects of the investigated forebrain limbic structures. Here, the main target of the ascending projection was in the ventral septum. However, labelled terminals were also present in the nucleus accumbens, the dorsolateral septum, and in the infralimbic cortex. The density of the ventrolateral medullary projections into all examined forebrain areas was low. The location of the cells in the ventral medulla oblongata which give rise to direct forebrain projections was examined using retrograde tract tracing with wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP). Following WGA-HRP injections into the septo-accumbens region, retrogradely labelled cells were present in both the rostral and caudal ventrolateral medulla. When the tract tracer injection was restricted to the ventral region of the septal complex, the labelled cells were concentrated in the caudal aspects of the ventrolateral medulla (and the nucleus of the solitary tract). Following tracer injections into the anterior cingulate cortex or the hippocampus or the entorhinal cortex, retrogradely labelled cells in the medulla oblongata were predominantly in the rostral ventrolateral medulla. As a first attempt to reveal the chemical nature of the projection cells, the contribution of tyrosine hydroxylase-immunoreactive cells to the innervation of the septo-accumbens area was also investigated: tyrosine hydroxylase-immunoreactive cells of both the caudal ventrolateral medulla and the nucleus of the solitary tract were found to contribute to the innervation of the septo-accumbens area. The distribution of retrogradely labelled cells as well as the termination pattern of the anterogradely labelled terminals indicated that the innervation of the various forebrain limbic areas arises from cells, diffusely distributed in the rostral and/or the caudal ventrolateral medulla oblongata.(ABSTRACT TRUNCATED AT 400 WORDS)

Response of locus coeruleus neurons to footshock stimulation is mediated by neurons in the rostral ventral medulla

While it is well documented that locus coeruleus neurons are potently activated by foot-pinch or sciatic nerve stimulation, little is known about the circuit producing this sensory response. Previous work in our laboratory has identified the medullary nucleus paragigantocellularis as a major excitatory afferent to the locus coeruleus. Here, we use local microinjections into the paragigantocellularis to test whether this nucleus is a link in the pathway mediating the activation of locus coeruleus neurons by subcutaneous footpad stimulation, or footshock, in anesthetized rats. Lidocaine HCl microinjected into the paragigantocellularis reversibly attenuated footshock-evoked activation of 50 out of 56 locus coeruleus cells, with responses in 20 cells completely blocked. Microinjections of GABA into the paragigantocellularis reduced the footshock-evoked responses of 17 out of 27 locus coeruleus cells (seven complete blocks); microinjections of the GABAB agonist baclofen had no effect (0 out of 11 cells blocked). Microinjections of a synaptic decoupling cocktail of manganese and cadmium also attenuated locus coeruleus activation in eight out of nine cells with two complete blocks. With each agent, the most effective injection placement for complete blockade of responses was the ventromedial paragigantocellularis; injections bordering this region attenuated responses, while those outside of the paragigantocellularis (dorsal medullary reticular formation, nucleus tractus solitarius, or facial nucleus), or vehicle injections, were ineffective. These results are consistent with previous findings that pharmacologic blockade of paragigantocellularis-evoked locus coeruleus activity also blocks footshock-evoked responses of locus coeruleus neurons [Ennis and Aston-Jones (1988) J. Neurosci. 8, 3644-3657], and support the view that this somatosensory response, and perhaps other sensory-evoked responses of locus coeruleus neurons, involve the nucleus paragigantocellularis.