Differential effects ofp-chlorophenylalanine on indoleamines in brainstem nuclei and spinal cord of rats. II. Identification of immunohistochemically stained structures using computer-assisted image enhancement techniques

Following intraperitoneal injection ofp-chlorophenylalanine (PCPA, 400 and 600 mg/kg) on 3 consecutive days, the brainstem and lumbar cord of rats were removed, frozen-sectioned and immunohistochemically stained (PAP method) for serotonin (5-HT). Using computer-assisted image analysis, the density of 5-HT staining in control, 400 and 600 mg/kg PCPA groups was determined. The mean number of pixels (representing 5-HT staining) was determined in 6 areas in the brainstem containing 5-HT cell bodies (nuclei raphe pallidus, raphe obscurus, rostral and caudal raphe magnus, raphe dorsalis and paragigantocellularis lateralis) and in the dorsal and ventral spinal cord. The resultssuggest a differential depletion of 5-HT within brainstem nuclei following PCPA treatment in that the most marked dose-related reductions were observed in nucleus raphe obscurus and caudal magnus. Furthermore, a computer program designed to isolate terminal structures in the spinal cord identified a differential depletion of 5-HT terminals in the dorsal horn versus the ventral horn. The present study describes 3 analytical approaches combining immunohistochemistry with the computer-assisted image analysis technique and allows comparison between groups of animals which received the same or different drug treatments.     

Effects of enkephalin and 5-hydroxytryptamine on solitary tract neurones involved in respiration and respiratory reflexes

Both enkephalin and 5-hydroxytryptamine (5-HT) have been implicated in neural mechanisms underlying the central control of respiration. In view of the role of the nuclei tractus solitarii (NTS) in respiratory regulation, we carried out a study in artificially ventilated, chloralose-anaesthetized cats of the effects of the microiontophoretic application of [D-Ala2, Met5]-enkephalinamide (DAME) and 5-HT on functionally identified NTS neurones implicated in the control of respiration and respiratory tract reflexes. The neurones examined belonged primarily to two groups: respiratory neurones having a rhythmic activity in phase with the simultaneously recorded phrenic nerve rhythm (viz inspiratory neurones), and presumed reflex interneurones which had no rhythmic activity but which could be orthodromically excited by vagus nerve (X) or superior laryngeal nerve (SLN) stimulation. Application of DAME produced a slow, prolonged depression in 9 of 20 respiratory neurones and 4 of 5 reflex interneurones tested: the remaining neurones showed no change in activity with DAME. The DAME-induced depression was reversed by the intravenous administration of naloxone in the 4 neurones tested. The predominant effect of 5-HT on the 26 respiratory neurones examined was a facilitatory effect of slow onset and long duration; this was noted in 15 neurones. A small number (n = 2) showed a prolonged depression, and the remaining 9 were unaffected. With the reflex interneurones, in contrast, facilitation was not seen with the application of 5-HT; instead, 4 were depressed and 4 unaffected. These findings on functionally identified NTS neurones provide support for the view that both endogenous 5-HT and opiate-related mechanisms are involved in the control of respiration and respiratory tract reflexes. Our findings suggest that they may operate, at least in part, by differing modulatory actions on neurones in the NTS involved in these respiratory-related functions, with enkephalin exerting depressive actions and 5-HT having both facilitatory and depressive effects.     

Inhibition of sympathetic activity by stimulating in the raphe nuclei and the role of 5-hydroxytryptamine in this effect

The possibility that the putative transmitter 5-hydroxytryptamine (5-HT) is involved in the mediation of long latency to onset raphe-spinal inhibition of sympathetic preganglionic neurones was investigated in anaesthetized cats by stimulating sites located in nucleus raphe pallidus and obscurus and recording sympathetic discharge in T₃ or T₁₀ white rami evoked either reflexively or by intraspinal stimulation at cervical level. Several putative 5-HT anttagonists were administered intravenously (i.V.) or topically to the spinal cord. In 7 cats lysergic acid diethylamide (LSD) in a dose range 25–50 μg/kg i.v. or 0.6 μg topically, reversibly reduced the raphe spinal inhibition by 40–100%. Topical application was more effective than i.v. administration. In 5 cats stimulating within the ventromedial reticular formation at sites unlikely to involved 5-HT neurons produced a short latency to onset inhibition which was unaffected by LSD. Methysergide, cinanserin and cyproheptadine depressed sympathetic discharge in the absence of brain stimulation in cats with CNS intact and in unanaesthetized decerebrate spinal cats. The results are discussed in the light of the known actions of the putative 5-HT antagonists.     

Evidence for a serotonergically mediated sympathoexcitatory response to stimulation of medullary raphe nuclei

The cardiovascular role of serotonin (5-HT) containing neurons in the midline medullary raphe nuclei was studied in anesthetized cats. High frequency electrical stimulation of nucleus (n.) raphe (r.) pallidus, n.r. obscurus and n.r. magnus produced both pressor and depressor responses. Single shock stimulation of pressor sites produced an excitatory evoked potential of sympathetic nervous discharge (SND) recorded from the inferior cardiac nerve. Conversely, single shock stimulation of vasodepressor sites resulted in a computer-summed inhibition of SND. The mean conduction velocity in the sympathoexcitatory medullo-spinal pathway to sympathetic preganglionic neurons was calculated to be 1.24 m/s. The 5-HT antagonists methysergide and metergoline blocked the excitation of sympathetic activity evoked from medullary raphe nuclei. In contrast, these agents failed to alter the sympathoexcitatory response to electrical stimulation of lateral medulla pressor sites or the sympathoinhibitory response elicited by raphe stimulation. The 5-HT uptake inhibitor chlorimipramine increased the duration of the sympathoexcitatory response evoked from the raphe but not from the lateral medulla. Finally, mid-collicular transection did not effect the excitation of sympathetic activity elicited by stimulation of medullary raphe nuclei. These data suggest that serotonergic neurons in the midline medullary raphe nuclei provide an excitatory input to sympathetic neurons in the spinal cord.     

Anatomical and functional connections of neurons of the rostral medullary raphe of the rabbit

Single cell recordings were made from neurons in the rostral medullary raphe (RMR) of the rabbit. The recording sites were ones that had been shown to yield pressor responses from electrical stimulation and by pressure injections of glutamate. Electrical stimulation of the intermediolateral (IML) region of the spinal cord led to antidromic activation of 12 of the 100 cells studied. Eleven of these cells were located in raphe pallidus or raphe magnus, and one cell was located in raphe obscurus. These findings were consistent with the results of horseradish peroxidase (HRP) histochemistry experiments. Injections of HRP into the IML led to heavy cell body labeling in raphe pallidus and raphe magnus, but sparse labeling in raphe obscurus. Cells in the RMR could be orthodromically activated by electrical stimulation of the putative defense area of the periaqueductal (PAG) but not by stimulation of putative defense areas in the hypothalamus. Most of these cells were located in raphe pallidus or raphe magnus. Similarly, HRP injections into raphe pallidus and raphe magnus led to heavy cell body labeling in the PAG but not the hypothalamus; no cell body labeling was found in the PAG when injections were made into raphe obscurus.     

Medullary substrates mediating antinociception produced by electrical stimulation of the vagus

Electrical stimulation of afferents of the right cervical vagus inhibited the tail-flick reflex elicited by noxious heat in barbiturate-anesthetized rats. This inhibitory effect was eliminated in rats receiving local anesthetic blockade of either the nucleus tractus solitarii (NTS), the lateral reticular nuclei, the nucleus raphe magnus-medullary reticular formation, or nucleus raphe obscurus regions of the medulla. Similarly, the vasodepressor and bradycardic effects of vagal stimulation were either attenuated or eliminated by local anesthetic blockade of these regions. Microinjection of the non-specific glutamate antagonist gamma-D-glutamylglycine (DGG) into the NTS region also eliminated vagally evoked inhibition of the tail-flick reflex, hypotension, and bradycardia. Conversely, microinjection of glutamate into the NTS region resulted in inhibition of the tail-flick reflex, hypotension, and bradycardia. These findings with DGG and glutamate are consistent with the view that glutamate serves as a neurotransmitter of the primary vagal afferents mediating these antinociceptive and cardiovascular responses. These results are discussed in terms of vagal afferent influences on somatosensory, somatomotor, and cardiovascular function.     

Differential effects ofp-chlorophenylalanine on indoleamines in brainstem nuclei and spinal cord of rats. I. Biochemical and behavioral analysis

The involvement of endogenous serotonergic pathways in the mediation of antinociception has been indicated by electrophysiological, pharmacological and behavioral experiments. However, manipulation of the indole pathway, either by lesioning of raphe nuclei or drug intervention, often produces disparate results. In particular, serotonin (5-HT) synthesis inhibition withp-chlorophenylalanine (PCPA) has been reported to produce either hyperalgesia or analgesia, depending upon the type of pain measurement examined. In the present study, we sought to evaluate the effects of PCPA on (1) behavioral responses to noxious stimulation, and (2) levels of serotonin, tryptophan and 5-hydroxyindoleacetic acid (5-HIAA) in raphe nuclei (pallidus, obscurus, magnus and dorsalis) and spinal cord regions by HPLC with electrochemical detection. Treatment of rats with 400 or 600 mg/kg of PCPA for 3 consecutive days resulted in significant elevations in pain thresholds assessed by tail withdrawal from radiant heat as well as vocalization to electric shock of the tail. The effect of PCPA on vocalization threshold was particularly striking, for the majority of animals showed a nociceptive-specific attenuation of this response. Although the PCPA induced changes in indole content of the various raphe nuclei were not unequivocally dose-dependent, differential reductions of serotonin and 5-HIAA were clearly detected in the various raphe regions. Nuclei raphe pallidus and obscurus were depleted of 5-HT and 5-HIAA to the greatest extent, whereas levels detected in nuclei raphe magnus and dorsalis were reduced by 30–40% from control values. Metabolism of 5-HT and 5-HIAA appeared unaffected by PCPA in all regions examined except the dorsal portion of the spinal cord. These findings collectively suggest that the effects of PCPA are not uniform throughout the central nervous system and raise the possibility that discrepancies in the behavior literature may be attributed to drug-induced changes in some, but not all serotonergic pathways.     

Inhibitory modulation of chemoreflex bradycardia by stimulation of the nucleus raphe obscurus is mediated by 5-HT3 receptors in the NTS of awake rats

Several studies demonstrated the involvement of 5-hydroxytryptamine (5-HT) and its different receptor subtypes in the modulation of neurotransmission of cardiovascular reflexes in the nucleus tractus solitarii (NTS). Moreover, anatomic evidence suggests that nucleus raphe obscurus (ROb) is a source of 5-HT-containing terminals within the NTS. In the present study we investigated the possible changes in the cardiovascular responses to peripheral chemoreceptor activation by potassium cyanide (KCN, i.v.) following ROb stimulation with L-glutamate (10 nmol/50 nL) and also whether 5-HT3 receptors in the caudal commissural NTS are involved in this neuromodulation. The results showed that stimulation of the ROb with L-glutamate in awake rats (n=15) produced a significant reduction in the bradycardic response 30 s after the microinjection (-182+/-19 vs -236+/-10 bpm; Wilcoxon test) but no changes in the pressor response to peripheral chemoreceptor activation (43+/-4 vs 51+/-3 mmHg; two-way ANOVA) in relation to the control. Microinjection of 5--HT3 receptors antagonist granisetron (500 pmol/50 nL), but not the vehicle, into the caudal commissural NTS bilaterally prevented the reduction of chemoreflex bradycardia in response to microinjection of L-glutamate into ROb. These data indicate that 5-HT-containing projections from ROb to the NTS play an inhibitory neuromodulatory role in the chemoreflex evoked bradycardia by releasing 5-HT and activating 5-HT3 receptors in the caudal NTS.     

Cardiovascular responses elicited by electrical and chemical stimulation of the rostral medullary raphe of the rabbit

Electrical stimulation of the rostral medullary raphe (RMR) of the rabbit elicited pressor responses that were accompanied by tachycardia or bradycardia. Stimulation of dorsal sites (the dorsal raphe obscurus) evoked a pressor/tachycardia response and stimulation of ventral sites (the ventral raphe obscurus, raphe magnus and raphe pallidus) produced a pressor/bradycardia response. Electrical stimulation of the RMR after sinoaortic denervation led to an increase in the magnitude of the pressor response elicited from all stimulation sites, a decrease in the magnitude of the bradycardia produced by stimulation at the ventral sites, but had no effect upon the magnitude of the tachycardia observed from stimulation of the dorsal sites. These findings suggest that electrical stimulation of the dorsal sites leads to inhibition of the cardiomotor component of the baroreceptor reflex. The results of vagal blockade experiments demonstrated that baroreceptor attenuation of the pressor responses at ventral sites was mediated primarily by parasympathetic input to the heart. Chemical stimulation of the RMR with L-glutamate also led to a pressor/tachycardia response at the dorsal sites and a pressor/brachycardia response at the ventral sites. This finding provides evidence that neuronal cell bodies, not axon of passage, mediated the responses elicited by electrical stimulation.     

Distribution of enkephalin-immunoreactive cell bodies in relation to serotonin-containing neurons in the raphe nuclei of the cat: Immunohistochemical evidence for the coexistence of enkephalins and serotonin in certain cells

In this study we have examined the distribution of enkephalin-like immunoreactive (ELI) cell bodies in the rat raphe nuclei pallidus (NRP), obscurus (NRO), magnus (NRM), pontis and dorsalis (NRD) after intratissular or intraventricular administration of colchicine. All the raphe nuclei examined were observed to contain ELI cell bodies along their whole caudorostral extent. By comparing consecutive sections treated separately with anti-5-HT and enkephalin-antiserum it was observed that certain 5-HT cells in each raphe nucleus contain ELI material. A quantitative estimation was attempted. In NRP and NRo approximately half of the total immunoreactive neuronal population appeared to be immunoreactive for both 5-HT and the enkephalins. In NRM the proportion would be one-third, whereas it seemed almost negligible in NRD. Among the 5-HT cells, approximately two-thirds might be ELI in NRP and NRO, and one-half in NRM.     

Evidence for cholecystokinin-like immunoreactive neurons in the rat medulla oblongata which project to the spinal cord

The distribution of cholecystokinin-like immunoreactive (CCK-LI) neurons has been mapped in the rat medulla after local and intracerebroventricular colchicine injections. CCK-positive neurons were found in the nucleus raphe magnus, nucleus raphe pallidus, nucleus raphe obscurus, nucleus paragigantocellularis pars α, and a population of ventral medullary neurons. Combined retrograde tracing with the fluorescent dye True Blue and indirect immunofluorescence for visualizing CCK neurons suggested that there was a CCK-LI system originating in the medulla and projecting to the spinal cord. Additional double labelling experiments established that some of these CCK-LI containing neurons also contain 5-HT.     

Serotonergic projections to the rostroventrolateral medulla from midbrain and raphe nuclei

Double-label fluoresence immunohistochemistry was performed to define serotonergic projections from the raphe and midbrain to the sympathoexcitatory region of the rostroventrolateral medulla (RVLM). Immunolabelling of cholera toxin B subunit retrogradely transported from the pressor region of the RVLM was combined with serotonin (5-HT) immunohistochemistry. Major sources of serotonergic input to the RVLM were shown to include the raphe obscurus, raphe pallidus and raphe magnus with a minor contribution from the ventrolateral, lateral and ventral regions of the periaqueductal gray matter, and the dorsal raphe nucleus. Serotonergic modulation of sympathoexcitatory neurons may establish patterns of sympathetic nerve activity evident in many aspects of cardiovascular regulation.     

Burst-firing activity of presumed 5-HT neurones of the rat dorsal raphe nucleus: electrophysiological analysis by antidromic stimulation

We recently reported raphe neurones which frequently fired spikes in short bursts. However, the action potentials were broad and the neurones fired in a slow and regular pattern, suggesting they were an unusual type of 5-hydroxytryptamine (5-HT) neurone. In the present study, we investigated whether these putative burst-firing 5-HT neurones project to the forebrain and whether all spikes fired in bursts propagate along the axon. In anaesthetised rats, electrical stimulation of the medial forebrain bundle evoked antidromic spikes in both burst-firing neurones and in single-spiking, classical 5-HT neurones recorded in the dorsal raphe nucleus. Although the antidromic spike latency of the single-spiking and burst-firing neurones showed a clear overlap, burst-firing neurones had a significantly shorter latency than single-spiking neurones. For both burst-firing neurones and classical 5-HT neurones, antidromic spikes made collisions with spontaneously occurring spikes. Furthermore, in all burst-firing neurones tested, first, second and third order spikes in a burst could be made to collide with an antidromic spike. Interestingly, in a small number of burst-firing neurones, antidromic stimulation evoked spike doublets, similar to those recorded spontaneously. From these data we conclude that burst-tiring neurones in the dorsal raphe nucleus project to the forebrain, and each spike generated by the burst propagates along the axon and could thereby release transmitter (5-HT).     

Studies on the respiratory control mechanism of medullary raphe nuclei and their serotonergic system

Respiratory control mechanism of the medullary raphe nuclei were studied with some references to their serotonergic mechanisms. Anesthetized, paralyzed and artificially ventilated cats were used and their phrenic nerve efferent activity was always observed as an indicator of central respiratory activity. Following results were obtained. 1) Electrical stimulation of medullary raphe nuclei, namely, nucleus raphe magnus, obscurus and pallidus, produced dominantly inhibitory responses in the phrenic nerve activity, while raphe stimulation in the pons and more rostral portion did not produce any respiratory responses. The blood pressure was depressed by raphe stimulation, too, almost in parallel to the respiratory inhibition. These inhibitory responses in respiration and blood pressure were partially antagonized by cyproheptadine (0.3-0.5 mg/kg i.v.) and methysergide (0.3-0.5 mg/kg i.v.). 2) Raphe stimulation inhibited remarkably activities of the medullary inspiratory and expiratory neurons, similarly. 3) In the experiment, where single shot stimulus was added to the raphe nuclei at the various time point in the respiratory phase, raphe stimulation showed the retardative effect of inspiratory switching, in addition to the inhibitory effect of phrenic burst activity. 4) The mechanism of respiratory inhibition produced by raphe stimulation was analyzed by evoked potentials in the averaged phrenic nerve activity. The post-stimulus averaged potentials of the phrenic nerve consist of the depolarizing potentials of about 10 msec duration and the subsequent hyperpolarizing potentials of several 10 msec duration, the duration time depending on the stimulus intensity. When stimulation was given in high frequency, the post-stimulus averaged potential became flattened, and the phrenic burst activity was inhibited almost completely. But in the case of stimulation in ventral parts of the raphe nuclei, the initial depolarizing potential was comparatively more prominent, and when high frequency stimulation was given, continuous firing was observed in the phrenic nerve activity. At the time of the continuous firing, respiratory rhythmicity was disappeared completely. 5) Propranolol (0.3-1.0 mg/kg i.v.), which have been recognized to have 5-HT1 antagonistic activity, reduced the hyperpolarizing potentials of the post-stimulus averaged potentials, and methysergide (0.3-1.0 mg/kg i.v.), 5-HT1 and 5-HT2 antagonist, reduced both depolarizing and hyperpolarizing potentials. These phenomena would suggest strongly that hyperpolarizing and depolarizing potentials are related to the 5-HT1 and 5-HT2 receptors, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

A comparative study of selective stimulation of raphe nuclei in the cat in inhibiting dorsal horn neuron responses to noxious stimulation

Consistent inhibition of cord nociceptive neurons was obtained at low levels of stimulation (5 V or 50–100 μA) within raphe magnus. Less consistently and with higher stimulus intensities, inhibition was observed on stimulating raphe pallidus. Still less frequently, and generally only with stimulation in the 20 V or 500 μA range, inhibition was observed in raphe dorsalis, raphe obscurus and centralis superior. No inhibition could be obtained by stimulation in linearis intermedias or linearis rostralis. Nearly all midline sites where inhibition of cord nociceptive neurons was observed were those within or in immediate proximity to raphe nuclei.     

Serotoninergic and enkephalinergic cell groups in the reticular formation of the bat ray and two skates

The distribution of cells which were immunohistochemically positive for leu-enkephalin (LENK+) or serotonin (5-HT+), two substances widely distributed in the reticular formation, was determined in two species of skates (Raja binoculata and Raja rhina) and a bat ray (Myliobatis californica). The Rajoids are closely related to the Rhinobatoids which contains Platyrhinoidis, an elasmobranch that does not have a nucleus raphe dorsalis. Myliobatis was chosen for an outgroup comparison. Most of the nuclei that were 5-HT+ were also LENK+. The greatest number of labeled cells was in the hypothalamus bordering the third ventricle and in the neurointermediate lobe of the pituitary. The mesencephalon was rich in cells in the ventral tegmental area bordering the red nucleus. In both genera, there were numerous 5-HT+ and LENK+ fusiform cells paralleling the ventral surface of the metencephalon and myelencephalon. These cells were located in several reticular nuclei but were especially prominent in nucleus reticularis (n.r.) pontis oralis, n.r. magnocellularis, and n.r. paragigantocellularis lateralis. The latter nucleus contained fewer LENK+ than 5-HT+ cells. In both genera, 5-HT+ and LENK+ cells were located in raphe pallidus, raphe obscurus, raphe magnus, raphe centralis superior, and raphe linearis. Minor differences in distribution of the remaining 5-HT+ and LENK+ cell groups were found, but these representative elasmobranchs lack a dorsal raphe nucleus which, in mammals, is the largest serotoninergic group.     

Origins of serotonergic projections to the spinal cord in rat: An immunocytochemical-retrograde transport study

The origins of the serotonergic projections to the spinal cord in the rat were determined by employing the retrograde cell marker HRP coupled with the unlabeled antibody, peroxidase-antiperoxidase immunocytochemical method of Sternberger. Large numbers of stained neurons (> 70%) in the medullary raphe nuclear complex were found to contain both HRP retrogradely transported from the spinal cord and positive 5-HT staining. These serotonergic cell groups, including the nucleus raphe obscurus, raphe pallidus, raphe magnus, and the ventral parts of the reticular formation, project to all spinal cord levels. In addition, some neurons contained HRP granules, but were unstained for 5-HT, suggesting that they may contain other non-serotonergic neurotransmitters. More rostrally in the midbrain reticular formation, many 5-HT neurons were found to have projections exclusively to the cervical spinal cord. These findings indicate that the descending serotonin inputs to the spinal cord originate not only from the serotonergic neurons located in the medullary raphe complex, but also from other new sources located in the central gray and reticular formation of the midbrain.     

Serotonergic axonal contacts on identified cat trigeminal motoneurons and their correlation with medullary raphe nucleus stimulation

The innervation of the trigeminal motor nucleus by serotonergic fibers with cell bodies in the raphe nuclei pallidus and obscurus suggests that activation of this pathway may alter the excitability of trigeminal motoneurons. Thus, we recorded intracellular responses from cat jaw-closing (JC) and jaw-opening (JO) α-motoneurons evoked by raphe stimulation and used a combination of intracellular staining of horseradish peroxidase (HRP) and immunohistochemistry at the light and electron microscopic levels to examine the distribution of contacts made by serotonin (5-HT)-immunoreactive boutons on the two motoneurons types. Electrical stimulation applied to the nucleus raphe pallidus-obscurus complex induced a monosynaptic excitatory postsynaptic potential (EPSP) in JC (masseter) α-motoneurons and an EPSP with an action potential in JO (mylohyoid) α-motoneurons. The EPSP rise-times (time to peak) and half widths were significantly longer in the JC than in the JO motoneurons. The EPSPs were suppressed by systemic administration of methysergide (2 mg/kg). Six JC and seven JO α-motoneurons were well stained with HRP. Contacts were seen between 5-HT-immunoreactive boutons and the motoneurons. The JC motoneurons received a significantly larger number of the contacts than did the JO motoneurons. The contacts were distributed widely in the proximal three-fourths of the dendritic tree of JC motoneurons but were distributed on more proximal dendrites in the JO motoneurons. At the electron microscopic level, synaptic contacts made by 5-HT-immunoreactive boutons on motoneurons were identified. The present study demonstrated that JC motoneurons receive stronger 5-HT innervation, and this correlates with the fact that raphe stimulation caused larger EPSPs among these neurons than among JO motoneurons. J. Comp. Neurol. 384:443–455, 1997. © 1997 Wiley-Liss, Inc.     

Activity of medullary serotonergic neurons in freely moving animals

In the mammalian brain, serotonergic neurons in the medulla (n. raphe magnus, obscurus, and pallidus) send dense projections into the spinal cord, especially to the dorsal horn, intermediolateral column, and ventral horn. We have conducted a series of studies examining the single unit activity of these neurons in behaving cats. The experiments were directed at determining whether changes in unit activity were related to pain (n. raphe magnus), autonomic activity (n. raphe obscurus and pallidus), or motor activity (n. raphe obscurus and pallidus). The strongest relationship was between neuronal activity and motor output, especially tonic and repetitive motor activity. We hypothesize that the primary functions of this motor-related activity are to facilitate motor output, suppress processing of some forms of afferent activity, and to coordinate autonomic functioning with the current motor demand.     

Afferent fiber connections from the raphe nuclei to the cerebellar paramedian lobule. A histochemical study in the rabbit utilizing horseradish peroxidase as a retrograde marker

The distribution of labelled cells within the raphe nuclei has been studied after iontophoretic injections of horseradish peroxidase into various folia of the cerebellar paramedian lobule. Retrogradely marked neurons of all sizes and shapes were found in more caudally located raphe nuclei: obscurus, pallidus, magnus and pontis, as well as single neurons in nucleus centralis superior and dorsal raphe nucleus. The nuclei raphe pontis and obscurus send the greatest number of fibers to the paramedian lobule. Most of the projection takes origin from raphe neurons located in midline and ipsilaterally. Folia c and d are the recipients of most afferents from the raphe nuclei (pallidus, obscurus, pontis, magnus). On the other hand, folia f and e appear to receive contribution from nuclei raphe pontis and pallidus, while folia a and b from nuclei obscurus and magnus. Additionally single neurons in nucleus raphe dorsalis contribute to the projection onto the folia f and e, and from nucleus centralis superior to the folia c, d and occasionally to the sublobule a. The studies are discussed in relation to other investigations of afferent-efferent connections and already known functional role of the raphe nuclei.