Characterization by stimulation of medullary mechanisms underlying gasping neurogenesis

Our purpose was to evaluate the hypothesis that neurons in the lateral tegmental field of the medulla comprise a pattern generator for neurogenesis of gasping. Stimulations in this area produced changes characteristic of pattern generators in other systems. These included shifts in gasping rhythm and refractory periods for eliciting gasps; the latter varied inversely with spontaneous gasping frequency. These responses were recorded from activities of phrenic and hypoglossal nerves of decerebrate, cerebellectomized, vagotomized, paralyzed, and ventilated cats. Gasping followed freezing the brain stem between pons and medulla. In addition to lateral tegmental loci, gasps were elicited by stimulating areas extending lateral to the nucleus ambiguus and medial to the contralateral medulla. These areas are envisaged to contain axons to or from the pattern generator of lateral tegmental field. Finally, stimulations in sites approximating nucleus tractus solitarius and nucleus ambiguus delayed spontaneous gasps and terminated ongoing gasps. Current required to terminate gasps fell during neural inspiration. Our data are consistent with the lateral tegmental field of medulla comprising a central pattern generator for gasping and pacemaker elements being a component of this pattern generator.     

Neurogenesis, control, and functional significance of gasping

Gasps are frequently the first and last breaths of life. Gasping, which is generated by intrinsic medullary mechanisms, differs fundamentally from other automatic ventilatory patterns. A region of the lateral tegmental field of the medulla is critical for the neurogenesis of the gasp but has no role in eupnea. Neuronal mechanisms in separate brain stem regions may be responsible for the neurogenesis of different ventilatory patterns. This hypothesis is supported by the recording of independent respiratory rhythms simultaneously from isolated brain stem segments. Data from fetal and neonatal animals also support gasping and eupnea being generated by separate mechanisms. Gasping may represent the output of a simple but rugged pattern generator that functions as a backup system until the control system for eupnea is developed. Pacemaker elements are hypothesized as underlying the onset of inspiratory activity in gasping. Similar elements, in a different brain stem region, may be responsible for the onset of the eupneic inspiration with neural circuits involving the pons, the medulla, and the spinal cord serving to shape efferent respiratory-modulated neural discharges.     

Persistence of eupnea and gasping following blockade of both serotonin type 1 and 2 receptors in the in situ juvenile rat preparation.

In severe hypoxia or ischemia, normal eupneic breathing is replaced by gasping, which can serve as a powerful mechanism for "autoresuscitation." We have proposed that gasping is generated by medullary neurons having intrinsic pacemaker bursting properties dependent on a persistent sodium current. A number of neuromodulators, including serotonin, influence persistent sodium currents. Thus we hypothesized that endogenous serotonin is essential for gasping to be generated. To assess such a critical role for serotonin, a preparation of the perfused, juvenile in situ rat was used. Activities of the phrenic, hypoglossal, and vagal nerves were recorded. We added blockers of type 1 and/or type 2 classes of serotonergic receptors to the perfusate delivered to the preparation. Eupnea continued following additions of any of the blockers. Changes were limited to an increase in the frequency of phrenic bursts and a decline in peak heights of all neural activities. In ischemia, gasping was induced following any of the blockers. Few statistically significant changes in parameters of gasping were found. We thus did not find a differential suppression of gasping, compared with eupnea, following blockers of serotonin receptors. Such a differential suppression had been proposed based on findings using an in vitro preparation. We hypothesize that multiple neurotransmitters/neuromodulators influence medullary mechanisms underlying the neurogenesis of gasping. In greatly reduced in vitro preparations, the importance of any individual neuromodulator, such as serotonin, may be exaggerated compared with its role in more intact preparations.     

Independent brain stem sites for ventilatory neurogenesis

The purpose was to determine if independent ventilatory rhythms could be generated in each half of completely separated brain stems. In decerebrate, cerebellectomized, vagotomized, paralyzed, and ventilated cats, activities of phrenic, recurrent laryngeal (RLN), and/or hypoglossal nerves were monitored. Midsaggital brain stem divisions were performed by sections and lesions. Eupnea continued following divisions of mesencephalon and pons. Hypoglossal and phrenic activities were eliminated after sections approximating the obex. In most preparations, RLNs discharged with independent rhythms after completion of midsagittal brain stem section and a C1 transection. Independent rhythms were also obtained from each half of medulla following transections at the pontomedullary junction and at C1, and midsagittal medullary divisions. In other animals with transections between pons and medulla and at C1, synchronized RLN and hypoglossal activities persisted after sagittal medullary divisions, 2.0 mm lateral to midline contralaterally. Data demonstrate that there is more than one potential brain stem site for ventilatory neurogenesis. It is hypothesized that there are many such sites, possibly having pacemaker cells, in pons and medulla.     

Medullary regions for neurogenesis of gasping: noeud vital or noeuds vitals?

St. John, Walter M. Medullary regions for neurogenesisof gasping: noeud vital or noeuds vitals? J. Appl.Physiol. 81(5): 1865-1877, 1996.Gasping isa critical mechanism for survival in that it serves as a mechanism forautoresuscitation when eupnea fails. Eupnea and gasping are separablepatterns of automatic ventilatory activity in all mammalian speciesfrom the day of birth. The neurogenesis of the gasp is dependent on thedischarge of neurons in the rostroventral medulla. This gasping centeroverlaps a region termed "the pre-Bötzinger complex."Neuronal activities of this complex, characterized in an in vitro brainstem spinal cord preparation of the neonatal rat, have beenhypothesized to underlie respiratory rhythm generation. Yet, therhythmic activity of this in vitro preparation is markedly differentfrom eupnea but identical with gasping in vivo. In eupnea, medullaryneuronal activities generating the gasp and the identical rhythm of the in vitro preparation are incorporated into a portion of thepontomedullary circuit defining eupneic ventilatory activity. However,these medullary neuronal activities do not appear critical for theneurogenesis of eupnea, per se.

     

Respiratory neural activities after caudal-to-rostral ablation of medullary regions

The purpose is to assess the importance of medullary mechanisms for the neurogenesis of eupnea. Cats that were used were decerebrate, cerebellectomized, vagotomized, paralyzed, and ventilated. Activities of the phrenic, facial, and mylohyoid nerves were monitored. Progressive caudal-to-rostral transections of the spinal cord and medulla were performed. Phrenic activity was eliminated by C1 spinal transections. Only modest changes in facial and mylohyoid activities resulted from transections as far rostral as the level of the dorsal respiratory nucleus. Rhythmic discharges ceased on transections at the pontomedullary junction. However, rhythmic mylohyoid discharges were maintained if protriptyline and strychnine were administered before and during the transection. In other studies rhythmic phrenic, facial, and mylohyoid discharges continued, albeit with an altered rhythm, after destruction of neurons in the dorsal respiratory nucleus by kainic acid. We conclude that caudal medullary mechanisms do not play an essential role in the neurogenesis of breathing movements. Rather, structures in rostral medulla and pons appear necessary for sustaining eupneic neural activities. The concept of multiple brain stem sites for ventilatory neurogenesis is discussed.     

Site of central chemosensitivity in fetal sheep.

The heart rate, blood pressure, and respiratory response to topically applied cyanide on the ventrolateral medullary surface and upper spinal cord was studied on exteriorized sinaortic-denervated fetal lambs under pentobarbital anesthesia. On all sites tested cyanide produced a rapid increase in heart rate and blood pressure (P smaller than 0.05) which was most pronounced from the area adjacent to the nerve roots IX to XI (mean 32%). Respiratory efforts consisting of 1-8 gasps were induced in half the applications to the medulla but never when the pledgets were applied to the spinal cord. The mean delay to response was 43 s (range 13-102 s). After cautery of the chemosensitive areas, topical application of cyanide failed to stimulate gasping, whereas intravenous cyanide or cord clamping still produced a vigorous respiratory response. It is concluded that sympathetic stimulation of the heart and blood vessels can originate centrally in response to local histotoxic hypoxia of the ventral medulla and upper spinal cord. Furthermore, it is proposed that in the apneic fetus histotoxic hypoxia of the medulla initiates respiration possibly by stimulating a special gasping mechanism which is separate from the respiratory center responsible for rhythmic breathing after birth. The responsible neurons must be located at least 2 mm beneath the ventral medullary surface.     

Unique spectral peak in phrenic nerve activity characterizes gasps in decerebrate cats

The respiratory pattern of gasping has been characterized on the phrenic nerve as rapidonset, rapid-rise, large-amplitude bursts of neural activity. Furthermore, medullary sites critical for the neurogenesis of gasping have been identified and are not the sites of identified respiratory neurons, such as the dorsal and ventral respiratory groups. I classified envelopes of phrenic nerve activity as eupneic breaths, or gasps based on the time-domain features of duration, shape, and amplitude. Gasps were elicited by hypoxia and low blood pressure in 9 of 12 decerebrate cats. Inspiratory times were 1.15 +/- 0.43 (SD) for eupneic breaths and 0.55 +/- 0.18s for gasps. The high-frequency peaks in the power spectra of phrenic nerve activity were at 80 +/- 13 Hz for eupneic breaths and at 120 +/- 21 Hz for gasps. Three of the 12 cats developed a breathing pattern that began as a normal breath and terminated in a gasp. Power spectra of the normal portion had eupneic spectral peaks (75 +/- 24 Hz); power spectra of the gasp portion had the high peaks at 110 +/- 23 Hz, a value 1.5 times higher than that for the normal peaks. Although this analysis of peripheral nerve activity cannot distinguish between two central pattern generators at two distinct anatomical sites or one pattern generator operating in two distinct modes, the fact that gasps were much shorter in duration and had markedly higher spectral peaks than control breaths supports the idea that the central pattern generator for gasping is not the central pattern generator for eupnea.     

A trigeminoreticular pathway: implications in pain

Neurons in the caudalmost ventrolateral medulla (cmVLM) respond to noxious stimulation. We previously have shown most efferent projections from this locus project to areas implicated either in the processing or modulation of pain. Here we show the cmVLM of the rat receives projections from superficial laminae of the medullary dorsal horn (MDH) and has neurons activated with capsaicin injections into the temporalis muscle. Injections of either biotinylated dextran amine (BDA) into the MDH or fluorogold (FG)/fluorescent microbeads into the cmVLM showed projections from lamina I and II of the MDH to the cmVLM. Morphometric analysis showed the retrogradely-labeled neurons were small (area 88.7 μm(2)±3.4) and mostly fusiform in shape. Injections (20-50 μl) of 0.5% capsaicin into the temporalis muscle and subsequent immunohistochemistry for c-Fos showed nuclei labeled in the dorsomedial trigeminocervical complex (TCC), the cmVLM, the lateral medulla, and the internal lateral subnucleus of the parabrachial complex (PBil). Additional labeling with c-Fos was seen in the subnucleus interpolaris of the spinal trigeminal nucleus, the rostral ventrolateral medulla, the superior salivatory nucleus, the rostral ventromedial medulla, and the A1, A5, A7 and subcoeruleus catecholamine areas. Injections of FG into the PBil produced robust label in the lateral medulla and cmVLM while injections of BDA into the lateral medulla showed projections to the PBil. Immunohistochemical experiments to antibodies against substance P, the substance P receptor (NK1), calcitonin gene regulating peptide, leucine enkephalin, VRL1 (TPRV2) receptors and neuropeptide Y showed that these peptides/receptors densely stained the cmVLM. We suggest the MDH- cmVLM projection is important for pain from head and neck areas. We offer a potential new pathway for regulating deep pain via the neurons of the TCC, the cmVLM, the lateral medulla, and the PBil and propose these areas compose a trigeminoreticular pathway, possibly the trigeminal homologue of the spinoreticulothalamic pathway.     

Differing activities of medullary respiratory neurons in eupnea and gasping

Our purpose was to compare further eupneic ventilatory activity with that of gasping. Decerebrate, paralyzed, and ventilated cats were used; the vagi were sectioned within the thorax caudal to the laryngeal branches. Activities of the phrenic nerve and medullary respiratory neurons were recorded. Antidromic invasion was used to define bulbospinal, laryngeal, or not antidromically activated units. The ventilatory pattern was reversibly altered to gasping by exposure to 1% carbon monoxide in air. In eupnea, activities of inspiratory neurons commenced at various times during inspiration, and for most the discharge frequency gradually increased. In gasping, the peak discharge frequency of inspiratory neurons was unaltered. However, all commenced activities at the start of the phrenic burst and reached peak discharge almost immediately. The discharge frequencies of all groups of expiratory neurons fell in gasping, with many neurons ceasing activity entirely. These data are consistent with the hypothesis that brain stem mechanisms controlling eupnea and gasping differ fundamentally.     

Regional responsiveness of renal perfusion to activation of the renal nerves

We tested for regional differences in perfusion responses, within the renal medulla and cortex, to renal nerve stimulation in pentobarbital sodium-anesthetized rabbits. Laser-Doppler flux (LDF) was monitored at various depths below the cortical surface (1-15 mm). Basal cortical LDF (1-3 mm, approximately 200-450 U) was greater than medullary LDF (5-15 mm, approximately 70-160 U), but there were no statistically significant differences in basal LDF within these regions. The background LDF signal during aortic occlusion was similar in the cortex (2 mm, 31 U) and outer medulla (7 mm, 31 U), but slightly greater in the inner medulla (12 mm, 44 U). During electrical stimulation of the renal nerves (0.5-8 Hz), cortical LDF and total renal blood flow were similarly progressively reduced with increasing stimulus frequency. Medullary LDF (measured between 5 and 15 mm) was overall less responsive than cortical LDF. For example, 4-Hz stimulation reduced inner medullary LDF (9 mm) by 19 +/- 6% but reduced cortical LDF (1 mm) by 54 +/- 11%. However, medullary LDF responses to nerve stimulation were similar at all depths measured. Our results indicate that while the vascular elements controlling medullary perfusion are less sensitive to the effects of electrical stimulation of the renal nerves than are those controlling cortical perfusion, sensitivity within these vascular territories appears to be relatively homogeneous.     

Retrofacial lesions: effects on CO2-sensitive phrenic and sympathetic nerve activity.

We made unilateral chemical (10- or 50-nl microinjections; 4.7 mM kainic acid) or electrolytic (5-15 mA; 15 s) lesions in a region of the rostral ventrolateral medulla (VLM) caudal to the retrotrapezoid nucleus in 10 decerebrate, paralyzed, vagotomized, and servo-ventilated cats. The lesions were 3.0-4.2 mm lateral to the midline, within 2 mm caudal to the facial nucleus, and within 2.5 mm of the VLM surface. Four control injections (mock cerebrospinal fluid and fluorescent beads alone) produced small and inconsistent effects over 3-5 h. The predominant effect of the lesions was a significant decrease in baseline integrated phrenic nerve amplitude (PNA) (apnea in 2 cases), total respiratory cycle duration, and the response to increased CO2 (slope < 15% of control in 3 cases). The respiratory-related peak amplitude of the integrated sympathetic signal, blood pressure, and the sympathetic nerve activity response to CO2 were also decreased after the majority of lesions. Not all lesions produced all effects, and some lesions resulted in increased PNA and respiratory cycle duration. The lesioned region appears functionally to represent a caudal extension of the retrotrapezoid nucleus containing neurons necessary for normal baseline PNA and CO2 sensitivity. In addition, it contains neurons involved in the determination of resting respiratory frequency and normal sympathetic activity and blood pressure. The pattern of mixed responses among animals suggests that a heterogeneity of function is present within a relatively small VLM region.     

Genesis of rhythmic respiratory activity in pons independent of medulla

We hypothesized that rhythmic respiratory-related activity could be generated in pons independent of medullary mechanisms. In decerebrate, cerebellectomized, vagotomized, paralyzed, and ventilated cats, we recorded efferent activities of the phrenic nerve and mylohyoid branch of the trigeminal nerve. Following transections of the brain stem at the pontomedullary junction, the phrenic and trigeminal nerves discharged with independent rhythms. Spontaneous trigeminal discharges eventually ceased but were reestablished after strychnine, doxapram, and/or protriptyline were administered. In some animals having no spontaneous trigeminal discharges after transection, these discharges appeared, with a rhythm different from the phrenic, following administration of these agents. In other cats having no transections between pons and medulla, these pharmacological agents induced trigeminal and phrenic discharges after kainic acid had been injected into the entire dorsal and ventral medullary respiratory nuclei. Phrenic and trigeminal discharges were linked, indicating survival of bulbospinal neurons or presence of pontospinal units. We conclude that rhythms, similar to respiratory rhythm, can occur by mechanisms in isolated pons. Such mechanisms are hypothesized to be within the pneumotaxic center and may underlie the neurogenesis of eupnea.     

Cough and laryngeal muscle discharges in brainstem lesioned anaesthetized cats

Experiments were carried out to determine whether there are separate drives from the selected neuronal networks of the brainstem affecting the discharge patterns of laryngeal and respiratory pump muscles during cough. Twenty-four non-decerebrate spontaneously breathing cats anesthetized with sodium pentobarbitone were used. Microinjections of kainic acid into the lateral tegmental field of the medulla, medullary midline or pontine respiratory group eliminated the cough evoked by mechanical stimulation of the tracheobronchial and laryngopharyngeal mucosa. These stimuli, in most cases, provoked irregular bursts of discharges in the posterior cricoarytenoid and thyroarytenoid laryngeal muscles (or they had no effect on them). No pattern of laryngeal muscle activities following lesions resembled the laryngeal cough response. Lesions of the target regions did not result in any apparent changes in the eupnoeic pattern of laryngeal activity. Neurons of the medullary lateral tegmental field, raphe nuclei and the pontine respiratory group seem to be indispensable for the configuration of the central cough motor pattern. However, these neurons do not appear to be essential for the discharge patterns of laryngeal motoneurons during eupnoea. The residual laryngeal "cough" responses are probably mediated by an additional motor drive.     

Potential switch from eupnea to fictive gasping after blockade of glycine transmission and potassium channels

This study evaluated possible neuronal mechanisms responsible for the transition from normal breathing (eupnea) to gasping. We hypothesized that a blockade of both inhibitory glycinergic synaptic transmission and potassium channels, combined with an increase in extracellular concentration of potassium, would induce a switch from an eupneic respiratory pattern to gasping. Efferent activities of the phrenic, vagal, and hypoglossal nerves were recorded during eupnea and ischemia-induced gasping in a perfused in situ preparation of the juvenile rat (4-6 wk of age). To block potassium channels, 4-aminopyridine (4-AP, 1-10 microM) was administered. Strychnine (0.2-0.6 microM) was used to block glycinergic neurotransmission. After administrations of 4-AP, excess extracellular potassium (10.25-17.25 mM), and strychnine, the incrementing pattern of eupneic phrenic activity was altered to a decrementing discharge. Hypoglossal and vagal activities became concentrated to the period of the phrenic burst with expiratory activity being reduced or eliminated. These changes in neural activities were similar to those in ischemia-induced gasping. Results are consistent with the concept that the elicitation of gasping represents a switch from a network-based rhythmogenesis for eupnea to a pacemaker-driven mechanism.     

Areal distributions of cortical neurons projecting to different levels of the caudal brain stem and spinal cord in rats

Distributions of corticospinal and corticobulbar neurons were revealed by tetramethylbenzidine (TMB) processing after injections of wheatgerm agglutinin conjugated to horseradish peroxidase (WGA:HRP) into the cervical or lumbar enlargements of the spinal cord, or medullary or pontine levels of the brain stem. Sections reacted for cytochrome oxidase (CO) allowed patterns of labeled neurons to be related to the details of the body surface map in the first somatosensory cortical area (SI). The results indicate that a number of cortical areas project to these subcortical levels: (1) Projection neurons in granular SI formed a clear somatotopic pattern. The hindpaw region projected to the lumbar enlargement, the forepaw region to the cervical enlargement, the whisker pad field to the lower medulla, and the more rostral face region to more rostral brain stem levels. (2) Each zone of labeled neurons in SI extended into adjacent dysgranular somatosensory cortex, forming a second somatotopic pattern of projection neurons. (3) A somatotopic pattern of projection neurons in primary motor cortex (MI) paralleled SI in mediolateral sequence corresponding to the hindlimb, forelimb, and face. (4) A weak somatotopic pattern of projection neurons was suggested in medial agranular cortex (Agm), indicating a premotor field with a rostromedial-to-caudolateral representation of hindlimb, forelimb, and face. (5) A somatotopic pattern of projection neurons representing the foot to face in a mediolateral sequence was observed in medial parietal cortex (PM) located between SI and area 17. (6) In the second somatosensory cortical area (SII), neurons projecting to the brain stem were immediately adjacent caudolaterally to the barrel field of SI, whereas neurons projecting to the upper spinal cord were more lateral. No projection neurons in this region were labeled by the injections in the lower spinal cord. (7) Other foci of projection neurons for the face and forelimb were located rostral to SII, providing evidence for a parietal ventral area (PV) in perirhinal cortex (PR) lateral to SI, and in cortex between SII and PM. None of these regions, which may be higher-order somatosensory areas, contained labeled neurons after injections in the lower spinal cord. Thus, more cortical fields directly influence brain stem and spinal cord levels related to sensory and motor functions of the face and forepaw than the hindlimb. The termination patterns of corticospinal and corticobulbar projections were studied in other rats with injections of WGA:HRP in SI.(ABSTRACT TRUNCATED AT 400 WORDS)     

Areal Distributions of Cortical Neurons Projecting to Different Levels of the Caudal Brain Stem and Spinal Cord in Rats

Distributions of corticospinal and corticobulbar neurons were revealed by tetramethylbenzidine (TMB) processing after injections of wheatgerm agglutinin conjugated to horseradish peroxidase (WGA:HRP) into the cervical or lumbar enlargements of the spinal cord, or medullary or pontine levels of the brain stem. Sections reacted for cytochrome oxidase (CO) allowed patterns of labeled neurons to be related to the details of the body surface map in the first somatosensory cortical area (SI). The results indicate that a number of cortical areas project to these subcortical levels: (1) Projection neurons in granular SI formed a clear somatotopic pattern. The hindpaw region projected to the lumbar enlargement, the forepaw region to the cervical enlargement, the whisker pad field to the lower medulla, and the more rostral face region to more rostral brain stem levels. (2) Each zone of labeled neurons in SI extended into adjacent dysgranular somatosensory cortex, forming a second somatotopic pattern of projection neurons. (3) A somatotopic pattern of projection neurons in primary motor cortex (MI) paralleled SI in mediolateral sequence corresponding to the hindlimb, forelimb, and face. (4) A weak somatotopic pattern of projection neurons was suggested in medial agranular cortex (Ag_m), indicating a premotor field with a rostromedial-to-caudolateral representation of hindlimb, forelimb, and face. (5) A somatotopic pattern of projection neurons representing the foot to face in a mediolateral sequence was observed in medial parietal cortex (PM) located between SI and area 17. (6) In the second somatosensory cortical area (SII), neurons projecting to the brain stem were immediately adjacent caudolaterally to the barrel field of SI, whereas neurons projecting to the upper spinal cord were more lateral. No projection neurons in this region were labeled by the injections in the lower spinal cord. (7) Other foci of projection neurons for the face and forelimb were located rostral to SII, providing evidence for a parietal ventral area (PV) in perirhinal cortex (PR) lateral to SI, and in cortex between SII and PM. None of these regions, which may be higher-order somatosensory areas, contained labeled neurons after injections in the lower spinal cord. Thus, more cortical fields directly influence brain stem and spinal cord levels related to sensory and motor functions of the face and forepaw than the hindlimb.

The termination patterns of corticospinal and corticobulbar projections were studied in other rats with injections of WGA:HRP in SI. Injections in lateral SI representing the face produced dense terminal label in the contralateral trigeminal complex. Injections in cortex devoted to the forelimb and forepaw labeled the contralateral cuneate nucleus and parts of the dorsal horn of the spinal cord. The cortical injections also demonstrated interconnections of parts of SI with some of the other regions of cortex with projections to the spinal cord, and provided further evidence for the existence of PV in rats.     

Transmedulla Neurons in the Sky Compass Network of the Honeybee (Apis mellifera) Are a Possible Site of Circadian Input

Honeybees are known for their ability to use the sun’s azimuth and the sky’s polarization pattern for spatial orientation. Sky compass orientation in bees has been extensively studied at the behavioral level but our knowledge about the underlying neuronal systems and mechanisms is very limited. Electrophysiological studies in other insect species suggest that neurons of the sky compass system integrate information about the polarization pattern of the sky, its chromatic gradient, and the azimuth of the sun. In order to obtain a stable directional signal throughout the day, circadian changes between the sky polarization pattern and the solar azimuth must be compensated. Likewise, the system must be modulated in a context specific way to compensate for changes in intensity, polarization and chromatic properties of light caused by clouds, vegetation and landscape. The goal of this study was to identify neurons of the sky compass pathway in the honeybee brain and to find potential sites of circadian and neuromodulatory input into this pathway. To this end we first traced the sky compass pathway from the polarization-sensitive dorsal rim area of the compound eye via the medulla and the anterior optic tubercle to the lateral complex using dye injections. Neurons forming this pathway strongly resembled neurons of the sky compass pathway in other insect species. Next we combined tracer injections with immunocytochemistry against the circadian neuropeptide pigment dispersing factor and the neuromodulators serotonin, and γ-aminobutyric acid. We identified neurons, connecting the dorsal rim area of the medulla to the anterior optic tubercle, as a possible site of neuromodulation and interaction with the circadian system. These neurons have conspicuous spines in close proximity to pigment dispersing factor-, serotonin-, and GABA-immunoreactive neurons. Our data therefore show for the first time a potential interaction site between the sky compass pathway and the circadian clock.     

A subsidiary fever center in the medullary raphé?

In fever, as in normal thermoregulation, signals from the preoptic area drive both cutaneous vasoconstriction and thermogenesis by brown adipose tissue (BAT). Both of these responses are mediated by sympathetic nerves whose premotor neurons are located in the medullary raphé. EP3 receptors, key prostaglandin E2 (PGE2) receptors responsible for fever induction, are expressed in this same medullary raphé region. To investigate whether PGE2 in the medullary raphé might contribute to the febrile response, we tested whether direct injections of PGE2 into the medullary raphé could drive sympathetic nerve activity (SNA) to BAT and cutaneous (tail) vessels in anesthetized rats. Microinjections of glutamate (50 mM, 60-180 nl) into the medullary raphé activated both tail and BAT SNA, as did cooling the trunk skin. PGE2 injections (150-500 ng in 300-1,000 nl) into the medullary raphé had no effect on tail SNA, BAT SNA, body temperature, or heart rate. By contrast, 150 ng PGE2 injected into the preoptic area caused large increases in both tail and BAT SNA (+60 +/- 17 spikes/15 s and 1,591 +/- 150% of control, respectively), increased body temperature (+1.8 +/- 0.2 degrees C), blood pressure (+17 +/- 2 mmHg), and heart rate (+124 +/- 19 beats/min). These results suggest that despite expression of EP3 receptors, neurons in the medullary raphé are unable to drive febrile responses of tail and BAT SNA independently of the preoptic area. Rather, they appear merely to transmit signals for heat production and heat conservation originating from the preoptic area.