Processing of vestibular inputs by the medullary lateral tegmental field of conscious cats: implications for generation of motion sickness

The dorsolateral reticular formation of the caudal medulla, the lateral tegmental field (LTF), participates in generating vomiting. LTF neurons exhibited complex responses to vestibular stimulation in decerebrate cats, indicating that they received converging inputs from a variety of labyrinthine receptors. Such a convergence pattern of vestibular inputs is appropriate for a brain region that participates in generating motion sickness. Since responses of brainstem neurons to vestibular stimulation can differ between decerebrate and conscious animals, the current study examined the effects of whole-body rotations in vertical planes on the activity of LTF neurons in conscious felines. Wobble stimuli, fixed-amplitude tilts, the direction of which moves around the animal at a constant speed, were used to determine the response vector orientation, and also to ascertain whether neurons had spatial–temporal convergence (STC) behavior (which is due to the convergence of vestibular inputs with different spatial and temporal properties). The proportion of LTF neurons with STC behavior in conscious animals (25 %) was similar to that in decerebrate cats. Far fewer neurons in other regions of the feline brainstem had STC behavior, confirming findings that many LTF neurons receive converging inputs from a variety of labyrinthine receptors. However, responses to vertical plane vestibular stimulation were considerably different in decerebrate and conscious felines for LTF neurons lacking STC behavior. In decerebrate cats, most LTF neurons had graviceptive responses to rotations, similar to those of otolith organ afferents. However, in conscious animals, the response properties were similar to those of semicircular canal afferents. These differences show that higher centers of the brain that are removed during decerebration regulate the labyrinthine inputs relayed to the LTF, either by gating connections in the brainstem or by conveying vestibular inputs directly to the region.     

Effects of visceral inputs on the processing of labyrinthine signals by the inferior and caudal medial vestibular nuclei: ramifications for the production of motion sickness

Neurons located in the caudal aspect of the vestibular nucleus complex have been shown to receive visceral inputs and project to brainstem regions that participate in generating emesis, such as nucleus tractus solitarius and the “vomiting region” in the lateral tegmental field (LTF). Consequently, it has been hypothesized that neurons in the caudal vestibular nuclei participate in triggering motion sickness and that visceral inputs to the vestibular nucleus complex can affect motion sickness susceptibility. To obtain supporting evidence for this hypothesis, we determined the effects of intragastric infusion of copper sulfate (CuSO₄) on responses of neurons in the inferior and caudal medial vestibular nuclei to rotations in vertical planes. CuSO₄ readily elicits nausea and emesis by activating gastrointestinal (GI) afferents. Infusion of CuSO₄ produced a >30 % change in spontaneous firing rate of approximately one-third of neurons in the caudal aspect of the vestibular nucleus complex. These changes in firing rate developed over several minutes, presumably in tandem with the emetic response. The gains of responses to vertical vestibular stimulation of a larger fraction (approximately two-thirds) of caudal vestibular nucleus neurons were altered over 30 % by administration of CuSO₄. The response gains of some units went up, and others went down, and there was no significant relationship with concurrent spontaneous firing rate change. These findings support the notion that the effects of visceral inputs on motion sickness susceptibility are mediated in part through the caudal vestibular nuclei. However, our previous studies showed that infusion of CuSO₄ produced larger changes in response to vestibular stimulation of LTF neurons, as well as parabrachial nucleus neurons that are believed to participate in generating nausea. Thus, integrative effects of GI inputs on the processing of labyrinthine inputs must occur at brain sites that participate in eliciting motion sickness in addition to the caudal vestibular nuclei. It seems likely that the occurrence of motion sickness requires converging inputs to brain areas that generate nausea and vomiting from a variety of regions that process vestibular signals.     

Transcerebellar inhibitory interaction between the bilateral vestibular nuclei and its modulation by cerebellocortical activity

Summary In decerebrate, unanesthetized cats, the brain stem was longitudinally cut at the midline from its dorsal to ventral surface with the cerebellum kept intact, eliminating neural interactions between the bilateral vestibular nuclei through the brain stem.Extracellular spike potentials of vestibular type I neurons identified by horizontal rotation were distinctly inhibited by contralateral vestibular nerve stimulation. This crossed inhibition was abolished by removal of the medial part of the cerebellum, indicating that the inhibition was mediated through the cerebellum. Neither aspiration of the flocculus on the recording side nor intravenous administration of picrotoxin eliminated transcerebellar crossed inhibition, suggesting that it is mediated through the cerebellar nuclei. When the fastigial, interposite and dentate nuclei were stimulated, inhibition of vestibular type I neurons was produced only from the contralateral fastigial nucleus. Cerebellocortical stimulation which inhibited fastigial type I neurons suppressed transcerebellar crossed inhibition. Effective sites for suppression of transcerebellar crossed inhibition were localized to lobules VI and VIIa in the vermal cortex on the side of labyrinthine stimulation.Intracellular recordings were made from type I neurons in the medial vestibular nucleus. Stimulation of the contralateral vestibular nerve and the contralateral fastigial nucleus produced IPSPs in these neurons with the shortest latency of 3.8 msec and 1.8 msec, respectively. The difference between these two latency values approximates the shortest latency of spike initiation of fastigial type I neurons in response to vestibular nerve stimulation. It is postulated that transcerebellar crossed inhibition is mediated through the fastigial nucleus on the side of labyrinthine stimulation.     

Regional blood flow in genetically obese (ob/ob) mice

1. Experiments have been undertaken on 11 decerebrate cats to investigate the effects of natural vestibular stimulation on the activity of cerebellar fastigial neurons. 2. From recordings in the rostral portion of the nucleus during sinusoidal lateral (roll) and horizontal (yaw) rotation, distinctive patterns of response were observed. 3. The majority of neurons sensitive to vestibular stimulation showed responses to a single modality of vestibular activation. During lateral tilt some neurones showed positional sensititivy, others gave responses related tothe velocity of movement. Other neurones responded in phase with the velocity of movement in the horizontal plane. 4. Aside from these neuronal responses, others provided indications of a convergence of inputs from different sets of vestibular receptors. In particular, several neurons showed a pattern of response that indicated tht they received inputs from otolith receptors and ampullar receptors of the vertical canal. At low velocities of movement their response was positional but with inreasing velocity the magnitude of the response increased and there was a marked phase shift of the discharge towards head velocity. 5. Neurons responding to horizontal rotation often showed positional responses during lateral tilt. There were also indications of a convergence of ampullar inputs from both vertical and horizontal canals. 6. The neural pathways mediating these resonses are discussed in consideration of previous neuroanatomical and neurophysiological data. We consider it likely that several pathways may act to evoke the patterns of response observed, and a role of the cerebellar cortex is indicated.     

Projections of the group y of the vestibular nuclei and the dentate and fastigial nuclei of the cerebellum to the interstitial nucleus of Cajal

Experiments were performed to study the projection of the group y of the vestibular nuclei and the dentate and fastigial nuclei of the cerebellum to the interstitial nucleus of Cajal (INC) in cats by using retrograde axonal transport of horseradish peroxidase (HRP) and electrophysiological methods; and to study the vestibular responses of such projection neurons. Following injections of HRP into the unilateral INC, with partial involvement of the surrounding reticular formation, including the nucleus of Darkschewitsch (ND), many retrogradely labeled neurons were found in the dorsal part of the group y nucleus contralateral to the injection site. Labeled cells were also seen in the contralateral dentate nucleus, frequently in its caudal-ventral part, and in the contralateral fastigial nucleus at all rostrocaudal levels, but most frequently in its caudal part. In electrophysiological experiments performed on cats anesthetized with alpha-chloralose or N2O and paralyzed with gallamine, group y, dentate and fastigial nuclei neurons were antidromically activated by weak stimuli that were confined to the contralateral INC. Depth-threshold curves for antidromic activation of such neurons revealed that the lowest threshold points were within the INC, but not in the ND. The INC-projecting neurons in the group y and dentate nuclei did not respond to electrical stimulation of the ipsilateral or contralateral vestibular nerve, indicating that they do not receive direct labyrinthine inputs. On the contrary, many fastigial neurons projecting to the INC responded to labyrinthine stimulation, suggesting that they may be involved in the vestibular reflexes. These results suggest a difference in properties of INC-projecting neurons in these nuclei.     

The effects of stimulating the cerebellar nodulus in the cat on the responses of vestibular neurons

In a first series of experiments, recordings were obtained from cat abducens and trochlear motorneurons and from axons of secondary vestibular neurons terminating in these motor nuclei, and the effects of cerebellar nodulus stimulation on utricular- and canal-evoked responses in these neurons were studied. Ultricular activation of vestibular axons recorded in the ipsilateral VIth and contralateral IVth nuclei was probably monosynaptically inhibited by nodular stimulation provided conditioning-test intervals were in the range between 0-10 ms and the test stimuli were close to threshold intensities. Of the vestibular axons activated by stimulation of the semicircular canal nerves only those evoked by the horizontal canal stimulation and recorded in the ipsilateral VIth nucleus were weakly inhibited. When the vestibular stimuli were strong enough to produce clear field potentials in the motor nuclei and/or postsynaptic potentials in motorneurons, nodular stimulation had practically no effect on their amplitudes. It is concluded that inhibition of vestibuloocular transmission is weak as compared to floccular inhibition studied previously. In a second series of experiments, recordings were obtained from vestibular neurons which were activated antidromically and/or transsynaptically by stimulation of the contralateral fastigial nucleus, and the effects of ipsilateral nodular stimulation on these responses were studied. It was found that nodular stimulation inhibited both antidromic as well as transsynaptic fastigial activations of vestibular neurons. Most of these vestibular neurons were located in the descending vestibular nucleus and received polysynaptic vestibular and spinal inputs. It is concluded that in addition to its weak inhibitory effect on vestibuloocular transmission the nodulus exerts a powerful inhibition on vestibular neurons transmitting vestibular and spinal inputs to cerebellar nuclei and/or cortex. It is suggested that the nodulus controls cerebellar projecting vestibular neurons which carry vestibular and spinal information to the cerebellum. The vestibular, proprioceptive and visual information which is present in the nodulus may aid the role of the nodulus in controlling body posture.     

Functional organization of vestibulofastigial projection in the horizontal semicircular canal system in the cat

Spike potentials of fastigial nucleus neurons were recorded extracellularly in decerebrate, unanesthetized cats. The neurons responding to head rotation in the horizontal plane with a type I fashion were located mainly in the middle and caudal regions of the fastigial nucleus. Three fourth of these fastigial type I neurons were antidromically activated by stimulation of the contralateral vestibular nuclei. These neurons were excited transsynaptically from the ipsilateral vestibular nerve or nuclei. Intra cellular recordings were made from those neurons which were located in the caudal half of the fastigial nucleus and were activated antidromically from the contralateral vestibular nuclei. Stimulation of the ipsilateral vestibular nerve produced EPSPs in these neurons with latencies of 1.0-6.6 msec. The shortest conduction time along primary vestibular aggerents from the labyrinth to the ipsilateral fastigial nucleus was 0,7 msec. The EPSPs with the shortest latency of 1.0 msec were therefore postulated to be due to monosynaptic connections of primary vestibular afferents with fastigial neurons. Stimulation of ipsilateral vestibular nuclei also produced monosynaptic EPSPs in fastigial neurons. These EPSPs were facilitated by conditioning stimulation of the ipsilateral vestibular nerve, indicating the existence of polysynaptic activation of fastigial neurons from the ipsilateral vestibular nerve through the vestibular nuclei.     

Responses of vestibular nucleus neurons to tilt following chronic bilateral removal of vestibular inputs

Recordings were made from the vestibular nuclei of decerebrate cats that had undergone a combined bilateral labyrinthectomy and vestibular neurectomy 49-103 days previously and allowed to recover. Responses of neurons were recorded to tilts in multiple vertical planes at frequencies ranging from 0.05 to 1 Hz and amplitudes up to 15 degrees. Many spontaneously active neurons were present in the vestibular nuclei; the mean firing rate of these cells was 43 +/- 5 (SEM) spikes/s. The spontaneous firing of the neurons was irregular: the coefficient of variation was 0.86 +/- 0.14. The firing of 27% of the neurons was modulated by tilt. The plane of tilt that elicited the maximal response was typically within 25 degrees of pitch. The response gain was approximately 1 spikes/s/degree across stimulus frequencies. The response phase was near stimulus position at low frequencies, and lagged position slightly at higher frequencies (average of 35 +/- 9 degrees at 0.5 Hz). The source of the inputs eliciting modulation of vestibular nucleus activity during tilt in animals lacking vestibular inputs is unknown, but could include receptors in the trunk or limbs. These findings show that activation of vestibular nucleus neurons during vertical rotations is not exclusively the result of labyrinthine inputs, and suggest that limb and trunk inputs may play an important role in graviception and modulating vestibular-elicited reflexes.     

Convergence of limb,visceral, and vertical semicircular canal or otolith inputs onto vestibular nucleus neurons

The major goal of this study was to determine the patterns of convergence of non-labyrinthine inputs from the limbs and viscera onto vestibular nucleus neurons receiving signals from vertical semicircular canals or otolith organs. A secondary aim was to ascertain whether the effects of non-labyrinthine inputs on the activity of vestibular nucleus neurons is affected by bilateral peripheral vestibular lesions. The majority (72%) of vestibular nucleus neurons in labyrinth-intact animals whose firing was modulated by vertical rotations responded to electrical stimulation of limb and/or visceral nerves. The activity of even more vestibular nucleus neurons (93%) was affected by limb or visceral nerve stimulation in chronically labyrinthectomized preparations. Some neurons received non-labyrinthine inputs from a variety of peripheral sources, including antagonist muscles acting at the same joint, whereas others received inputs from more limited sources. There was no apparent relationship between the spatial and dynamic properties of a neuron's responses to tilts in vertical planes and the non-labyrinthine inputs that it received. These data suggest that non-labyrinthine inputs elicited during movement will modulate the processing of information by the central vestibular system, and may contribute to the recovery of spontaneous activity of vestibular nucleus neurons following peripheral vestibular lesions. Furthermore, some vestibular nucleus neurons with non-labyrinthine inputs may be activated only during particular behaviors that elicit a specific combination of limb and visceral inputs.     

Convergence of gastric vagal and cerebellar fastigial nuclear inputs on glycemia-sensitive neurons of lateral hypothalamic area in the rat

Gastric vagal and cerebellar fastigial nuclear afferents have been implicated in the regulation of food intake by their communication with lateral hypothalamic area (LHA), which is generally referred to be the feeding center. This study was designed to examine the possible convergence of the inputs from the gastric vagal trunks and cerebellar fastigial nucleus (FN) on the LHA neurons. Among recorded 191 LHA neurons, 99 (51.8%) responded to the stimulation of the gastric vagal trunks, of which 55 (55.6%) also responded to the cerebellar FN stimulation. Of 62 LHA neurons that responded to the gastric vagal stimulation, 43 (69.4%) showed an inhibitory response to the intravenous glucose application indicating they were glycemia-sensitive neurons. When the gastric vagal trunks and cerebellar FN were stimulated simultaneously, a summation of the responses usually could be seen in the recorded LHA neurons (16/20, 80%). Moreover, of 45 LHA neurons that responded to both of the gastric vagal trunks and FN stimuli, 30 (66.7%) were identified to be glycemia-sensitive neurons. These results demonstrated that gastric vagal afferents could reach glycemia-sensitive neurons of the LHA, and that the inputs from cerebellar FN and gastric vagal trunks could converge onto glycemia-sensitive neurons in the LHA. According to the facts that gastric vagal inputs and blood glucose level may transmit meal-related visceral signals and FN may forward the somatic information to the LHA, we suggest that an integration of the somatic-visceral response related to the food intake may take place in the LHA following the gastric vagal and cerebellar FN afferent inputs and the integration may play an important role in the short-term regulation of feeding behavior.     

Properties of cerebellar fastigial neurons during translation, rotation, and eye movements

The most medial of the deep cerebellar nuclei, the fastigial nucleus (FN), receives sensory vestibular information and direct inhibition from the cerebellar vermis. We investigated the signal processing in the primate FN by recording single-unit activities during translational motion, rotational motion, and eye movements. Firing rate modulation during horizontal plane translation in the absence of eye movements was observed in all non-eye-movement-sensitive cells and 26% of the pursuit eye-movement-sensitive neurons in the caudal FN. Many non-eye-movement-sensitive cells recorded in the rostral FN of three fascicularis monkeys exhibited convergence of signals from both the otolith organs and the semicircular canals. At low frequencies of translation, the majority of these rostral FN cells changed their firing rates in phase with head velocity rather than linear acceleration. As frequency increased, FN vestibular neurons exhibited a wide range of response dynamics with most cells being characterized by increasing phase leads as a function of frequency. Unlike cells in the vestibular nuclei, none of the rostral FN cells responded to rotational motion alone, without simultaneously exhibiting sensitivity to translational motion. Modulation during earth-horizontal axis rotation was observed in more than half (77%) of the neurons, although with smaller gains than during translation. In contrast, only 47% of the cells changed their firing rates during earth-vertical axis rotations in the absence of a dynamic linear acceleration stimulus. These response properties suggest that the rostral FN represents a main processing center of otolith-driven information for inertial motion detection and spatial orientation.     

Effects of stimulation of frontal cortex,superior colliculus,and neck muscle afferents on interstitiospinal neurons in the cat

Summary Interstitiospinal neurons were activated by antidromic stimulation of the spinal cord ventromedial funiculus at C₁ and C₄ in cerebellectomized cats under chlor alose anesthesia. Neurons responding only to C₁ were classified as N cells and those responding both to C₁ and C₄ were classified as D cells, as in previous experiments (Fukushima et al. 1980a). Vestibular branching interstitiospinal and reticulospinal neurons were also identified as in the previous experiments.Stimulation of the ipsilateral pericruciate cortex evoked firing in 31% of N cells, 41% of D cells and 35% of vestibular branching neurons, while stimulation of the contralateral cortex excited 6% of N cells, 29% of D cells and 14% of vestibular branching neurons. Response latencies ranged from 2 to 15 ms after the effective pulse. By measuring the thresholds of activation of these neurons while changing the depth of the stimulating electrodes, and by mapping the cortical areas, it was shown that the lowest threshold areas were in the frontal eye fields and the anterior sigmoid gyrus near the presylvian sulcus (Area 6). Stimulation of the latter area often evoked neck or shoulder muscle contraction.Stimulation in the deep layers of the ipsilateral superior colliculus evoked firing in about 20% of interstitiospinal neurons and about 42% of vestibular branching neurons, with typical latencies 2–3 ms after the effective pulse, while stimulation of the contralateral superior colliculus was rarely effective. N cells and D cells responded similarly. Thresholds for activation were high in the intermediate tectal layers and declined as the electrodes entered the underlying tegmentum. This suggests that the superior colliculus is not the main source of synaptic inputs to these neurons. Low threshold points were found above the deep fiber layer when stimulating electrodes were inserted into the pretectum.Stimulation of the C₂ biventer cervicis nerve excited about 8% of N cells, 18% of D cells, and 15% of vestibular branching neurons bilaterally with typical latencies around 10 ms. Similar results were obtained when C₂ splenius nerves were stimulated. The fibers responsible for such excitation are probably group II, since stimuli stronger than 1.8 times threshold of the lowest threshold fibers were needed to evoke excitation. Response decrement was often observed when stimuli were repeated at 1/s, while no such decrement was observed at the rate of 1/3 s.When the convergence of cortical and labyrinthine excitatory inputs was studied, 36% of interstitiospinal neurons received single inputs either from the pericruciate cortex or from the labyrinth, 22% of neurons received convergent excitation from both and the remaining 42% did not respond to either stimulus. Although vestibular branching neurons rarely received labyrinthine inputs, they frequently showed convergence of excitation to stimulation of the frontal cortex, superior colliculus and vestibular nuclei.Supported in part by a Grant-in-Aid for Scientific Research (No. 477063) from The Ministry of Education, Science, and Culture of Japan     

Visual sensory substitution in vestibular compensation: neuronal substrates in the alert cat

The purpose of this study was to investigate adaptive changes in the activity of vestibular nuclei neurons unilaterally deprived of their primary afferent inputs when influenced by visual motion cues. These neuronal changes might account for the established role that vision plays in the compensation for posturo-kinetic deficits after the loss of vestibular inputs. Neuronal recordings were made in alert, non-paralysed cats that had undergone unilateral vestibular nerve sections. The unit responses collected in both Deiters' nuclei were compared to those previously recorded in intact cats. We analysed the extracellular activity of Deiters' nucleus neurons, as well as the optokinetic reflex (OKR) evoked during sinusoidal translation of a whole-field optokinetic stimulus in the vertical plane. In intact cats, we found the unit firing rate closely correlated with the visual surround translation velocity, and the relationship between the discharge rate and the motion frequency was tuned around an optimal frequency. The maximum firing rate modulation was generally below the 0.25 Hz stimulus frequency; unit responses were weak or even absent above 0.25 Hz. From the 4th day to the end of the 3rd week after ipsilateral deafferentation, a majority of cells was found to display maximum discharge modulation during vertical visual stimulation at 0.50 Hz, and even at 0.75 Hz, indicating that the frequency bandwidth of the visually induced responses of deafferented vestibular nuclei neurons had been extended. Consequently, the frequency-dependent attenuation in the sensitivity of vestibular neurons to visual inputs was much less pronounced. After the first 3 weeks postlesion, the unit response characteristics were very similar to those observed prior to the deafferentation. On the nucleus contralateral to the neurectomy, the maximum modulation of most cells was tuned to the low frequencies of optokinetic stimulation, as also seen prior to the lesion. We found, however, a subgroup of cells displaying well-developed responses above 0.50 Hz. Under all experimental conditions, the neuronal response phase still remained closely correlated with the motion velocity of the vertical sinusoidal visual pattern. We hypothesize that Deiters' neurons deprived of their primary afferents may transiently acquire the ability to code fast head movements on the basis of visual messages, thus compensating, at least partially, for the loss of dynamic vestibular inputs during the early stages of the recovery process. Since the overall vertical OKR gain was not significantly altered within the 0.0125 Hz–1 Hz range of stimulation after the unilateral neurectomy, it can be postulated that the increased sensitivity of deafferented vestibular neurons to visual motion cues was accounted for by plasticity mechanisms operating within the deafferented Deiters' nucleus. The neuroplasticity mechanisms underlying this rapid and temporary increase in neuronal sensitivity are discussed.     

Responses of thoracic spinal interneurons to vestibular stimulation

Vestibular influences on outflow from the spinal cord are largely mediated via spinal interneurons, although few studies have recorded interneuronal activity during labyrinthine stimulation. The present study determined the responses of upper thoracic interneurons of decerebrate cats to electrical stimulation of the vestibular nerve or natural stimulation of otolith organs and the anterior and posterior semicircular canals using rotations in vertical planes. A majority of thoracic interneurons (74/102) responded to vestibular nerve stimulation at median latencies of 6.5 ms (minimum of ~3 ms), suggesting that labyrinthine inputs were relayed to these neurons through trisynaptic and longer pathways. Thoracic interneuronal responses to vertical rotations were similar to those of graviceptors such as otolith organs, and a wide array of tilt directions preferentially activated different cells. Such responses were distinct from those of cells in the cervical and lumbar enlargements, which are mainly elicited by ear-down tilts and are synchronous with stimulus position when low rotational frequencies are delivered, but tend to be in phase with stimulus velocity when high frequencies are employed. The dynamic properties of thoracic interneuronal responses to tilts were instead similar to those of thoracic motoneurons and sympathetic preganglionic neurons. However, the preferred tilt directions of the interneurons were more heterogeneous than thoracic spinal outputs, showing that the outputs do not simply reflect an addition of local interneuronal activity.     

Properties of utricular-activated vestibular neurons that project to the contralateral vestibular nuclei in the cat

The properties of utricular (UT)-activated vestibular neurons that send axons to the contralateral vestibular nuclei (commissural neurons) were investigated intracellularly or extracellularly in decerebrate cats. A total of 27 vestibular neurons were orthodromically activated by stimulation of UT nerves and antidromically activated by stimulation of the contralateral vestibular nuclei. All neurons tested were classified as vestibulospinal (VS), vestibulooculospinal (VOS), vestibuloocular (VO), and unidentified vestibular neurons (V) after antidromic stimulation of the spinal cord and oculomotor/trochlear nuclei. Most UT-activated commissural neurons (20/27) received monosynaptic inputs. Twelve of 27 commissural neurons were located in the medial vestibular nucleus, 5 were in the lateral vestibular nucleus, 10 were in the descending vestibular nucleus, and no commissural neurons were recorded in the superior vestibular nucleus. Seven of 27 neurons were commissural VS neurons, 9 of 27 were commissural VOS neurons, and 11 of 27 were commissural V neurons. No commissural VO neurons were found. All VOS neurons and 3 VS neurons issued descending axons via the medial vestibulospinal tract. We also studied convergent inputs from the posterior semicircular canal (PC) nerve onto UT-activated commissural neurons. Five of 27 UT-activated commissural neurons received converging inputs from the PC nerves. Electronic Publication     

Convergence patterns of the posterior semicircular canal and utricular inputs in single vestibular neurons in cats

The convergence of the posterior semicircular canal (PC) and utricular (UT) inputs in single vestibular nuclei neurons was studied intracellularly in decerebrate cats. A total of 160 vestibular neurons were orthodromically activated by selective stimulation of the PC and the UT nerve and classified according to whether or not they were antidromically activated from the spinal cord and oculomotor nuclei into vestibulospinal (VS), vestibulooculospinal (VOS), vestibuloocular (VO), and unidentified vestibular neurons. Fifty-three (33%) of 160 vestibular neurons received convergent inputs from both the PC and UT nerves. Seventy-nine (49%) vestibular neurons responded to PC inputs alone, and 28 (18%) neurons received inputs only from the UT nerve. Of 53 convergent neurons, 8 (15%) were monosynaptically excited from both nerves. Thirty-five (66%) received monosynaptic excitatory inputs from the PC nerve and polysynaptic excitatory or inhibitory inputs from the UT nerve, or vice versa. Approximately one-third of VS and VOS neurons received convergent inputs. A majority of the VS neurons descended to the spinal cord through the lateral vestibulospinal tract, while almost all the VOS neurons descended to the spinal cord through the medial vestibulospinal tract. The convergent neurons were found in all vestibular nuclei but more in the lateral nucleus and descending nucleus. The VS neurons were more numerous than VO neurons or VOS neurons.     

Responses of rat lateral hypothalamic neuronal activity to fastigial nucleus stimulation

1. The aim of this study was investigation of neuronal mechanisms underlying inputs from the fastigial nucleus (FN) to the lateral hypothalamic area (LHA). 2. In male anesthetized rats, 295 extracellular and 82 intracellular recordings of LHA responses to electrical stimulation of the FN, which elicited stimulus-locked pressor responses, were examined. 3. Contralateral FN stimulation evoked three types of responses in 48% of spontaneously firing LHA neurons: inhibition with 11 +/- 6 (SD) ms latency followed by excitation (30%), excitation with 15 +/- 12.5 ms latency (14%), and excitation followed by inhibition with 6 +/- 4 ms latency (4%). 4. Contralateral FN stimulation after transection of the inferior cerebellar peduncle (ICP), which resulted in a substantial fall of the fastigial pressor response, also evoked the three types of responses. These responses were unaffected by transection of the ICP. 5. Neuronal activity was recorded intracellularly from 82 LHA neurons, of which 36 (44%) responded to FN stimulation. Of the 36 neurons, 24 showed inhibitory postsynaptic potentials (IPSPs) with a mean latency of 7.5 +/- 2 ms. Of the 24 neurons, 16 were checked for change in IPSP latency with stimulus intensity, and 11 were considered to be monosynaptically connected since their latencies were constant when FN stimulation intensity was changed. The remaining 12 exhibited excitatory postsynaptic potentials (EPSPs) with a longer latency of 10.5 +/- 3 ms, which indicated polysynaptic conduction. The reversal potentials of the IPSP and EPSP were estimated to be about -77 mV and -13 mV, respectively. 6. Most glucose-sensitive neurons (78%), which were identified by their inhibition in response to electrophoretically applied glucose, were inhibited by FN stimulation, whereas only 7% of the glucose-insensitive neurons responded to such stimulation. 7. From the results, it was concluded that LHA neurons receive inhibitory monosynaptic and excitatory polysynaptic inputs from the FN via the superior cerebellar peduncle. These connections may contribute to hypothalamic modulation of feeding behavior.     

Fastigial nucleus modulation of medullary parasolitary neurons

Input from the cerebellar fastigial nuclei to neurons at the lateral margin of the nuclei of the solitary tract, particularly to the area identified as the nucleus parasolitarius was investigated in acutely prepared, anesthetized dogs. Fastigial nucleus stimulation led to short latency excitation of nucleus parasolitarius units often followed by prolonged inhibition of spontaneous activity. Excitation from deep skeletal muscle afferents, converged on 25% of the spontaneously active units excited from the fastigial nuclei; these afferents originated primarily from the ipsilateral forelimb muscles. This study provides electrophysiological evidence for fastigial modulation of neurons previously demonstrated autoradiographically to receive presumed monosynaptic fastigial nucleus efferents. The convergence of forelimb muscle afferent information tentatively identified as being from Group Ia or Group II pressure stretch receptors suggests that the nucleus parasolitarius may be an integrative area for cerebellar, sensorimotor and/or autonomic information.     

Fastigial nucleus projections to the brain stem in beagles: Pathways for autonomic regulation

Efferent connections from a portion of the cerebellar fastigial nucleus were investigated using autoradiography. Bipolar stimulating electrodes were placed in the fastigial nucleus of anesthetized beagles and the area that produced increases in blood pressure and heart rate was localized. A mixture of [³H]leucine and [³H]proline (4:1) was injected into the area and autoradiograms of transported material were prepared. Injections filled the rostral and various parts of the caudal fastigial nucleus. Labeled axons reached the brain stem via two routes, the ipsilateral juxtarestiform body and the contralateral uncinate fasciculus. Ventral portions of the lateral vestibular nucleus were labeled bilaterally, projections to the inferior vestibular and medial vestibular nuclei are contralateral. Nucleus tractus solitarius was heavily labeled on the side opposite the injection. The contralateral medial reticular formation contained many labeled terminals and axons. Label was found in the nucleus reticularis ventralis, lateral reticular nucleus, nucleus gigantocellularis, nucleus pontis caudalis and the paramedian reticular nucleus. No terminal labeling was found in nucleus parvocellularis or nucleus ambiguus.Stimulation of the rostral fastigial nucleus produces increases in blood pressure and heart rate by generalized sympathoexcitation. Many cell groups which facilitate the activity of preganglionic sympathetic neurons do not receive direct fastigial input. It is suggested that sympathoexcitation resulting from stimulation of the fastigial nucleus occurs through multisynaptic connections in the brain stem.     

The inhibitory effect of the cerebellar fastigial stimulation on ADH secretion

1. Both cardiovascular and antidiuretic hormone (ADH) responses to some neural inputs were examined in paralysed anaesthetized cats.

2. Carotid occlusion elicited cardiovascular responses and increased ADH secretion. When the electrical stimulation of discrete loci of the cerebellar fastigial nucleus (fastigial pressor area) was superimposed on carotid occlusion, cardiovascular responses were further facilitated, while ADH secretion was inhibited.

3. The fastigial stimulation alone elicited facilitory cardiovascular responses composed of hypertension and tachycardia, and the fastigial pressor response (FPR), but did not evoke any consistent ADH response.

4. These facts indicate that cerebellar modulation of ADH secretion occurs not directly via the hypothalamo-hypophysial system but through the lower brain stem to which both carotid sinus nerves and outflows from the fastigial pressor area project.

5. We conclude that the fastigial pressor area is specific for not only cardiovascular and other autonomic responses but pituitary hormonal response.