Naoko Nisimaru

Destruction of inferior olive induces rapid depression in synaptic action of cerebellar Purkinje cells

THE climbing fibre afferents (CFAs) are a structure unique to the cerebellar cortex; they originate, presumably solely, from the inferior olive (IO) and make an extensive, excitatory synaptic contact with dendrites of cerebellar Purkinje cells (P cells)1. The importance of the CFAs in cerebellar functions has been emphasised in connection with the learning process which may occur in the cerebellar cortex2,3, and also in connection with the development and maintenance of normal dendritic structures of P cells4–7. An interesting recent finding is that after destruction of the IO electrical stimulation of the cerebellar cortex fails to elicit the normal inhibitory synaptic action of P cells on their target neurones in vestibular nuclei8,9. This not only urges reinterpretation of the symptoms arising from destruction of the IO10,11, but also raises a basic general problem of remote interaction in neuronal networks. Here, we report that the depression of the P-cell action develops unexpectedly fast following destruction of the IO, and also present evidence indicating that this remote effect is mediated largely by a non-impulse transmission process, presumably an axonal flow, in CFAs and P-cell axons.

Parasagittal zonal pattern of olivo-nodular projections in rabbit cerebellum

Injection of horseradish peroxidase (HRP) into the nodulus of the rabbit retrogradely labeled cells in 5 subdivisions of the contralateral inferior olive. Localized HRP injections at each of the medial, intermediate and lateral parts of the nodulus revealed 6 longitudinal zones, two zones in each part, projected differentially from the 5 olivary subdivisions: (i) the medial-most zone projected from the beta nucleus, (ii) the lateral zone of the medial part and (iii) the lateral zone of the intermediate part both from the dorsal cap, (iv) the medial zone of the intermediate part from the ventrolateral outgrowth, (v) the medial zone of the lateral part from the dorsomedial cell column, and (vi) the lateral-most zone from the rostrolateral part of the medial accessory olive. A complication is that the zones (v) and (vi) seemed to cover the dorsal lamina of the nodulus, but not the ventral lamina. The lateral part of the ventral nodulus seemed to be projected from the dorsal cap and may therefore be a lateral extension of zone (iv). There was an indication that the rostral-most area of the medial accessory olive also projects to the nodulus, but a specific receptive zone for this projection was unclear. The present results suggest that the small lateral area of the nodulus previously found to be involved in cardiovascular control is projected from the rostrolateral part of the medial accessory olive and/or the dorsomedial cell column.

Characteristics of monocarboxylates as energy substrates other than glucose in rat brain slices and the effect of selective glial poisoning - a 31P NMR study.

In rat brain slices we examined the differences in the levels of high-energy phosphates in the presence of various energy substrates by using 31P NMR with a time resolution of 4 min at 25 degrees C. In parallel experiments we recorded population excitatory postsynaptic potentials (EPSPs) from granule cells in rat hippocampal slices. During high K(+) stimulation (8 min) phosphocreatine (PCr) decreased to a low level and recovered to the control level in standard artificial cerebrospinal fluid (ACSF) in about 10 min. Population EPSPs disappeared following high-K(+) stimulation and recovered in standard ACSF. In iodoacetic acid (IAA)-pretreated slices, whereas glucose was unable to support energy metabolism, the PCr level, which decreased following high-K(+) stimulation, recovered in ACSF containing lactate or pyruvate. The half-time of recovery of PCr levels in ACSF containing lactate was longer than that containing glucose. Population EPSPs in standard ACSF were maintained for more than 1 h, but those in ACSF containing lactate decreased gradually by about half in 40 min. In IAA-pretreated slices, when further treated with fluorocitrate (100 microM) for 2 h, the recovery of the PCr level in ACSF containing lactate after high-K(+) stimulation was completely abolished, whereas the recovery of the PCr level in ACSF containing pyruvate was unaffected. These results indicate that neurons can utilize pyruvate as well as glucose, but not lactate, as exogenous energy substrates, and that lactate may be metabolized to pyruvate in glial cells and transported to neurons to be utilized as an energy substrate.

A depressant area in the lateral nodulus-uvula of the cerebellum for renal sympathetic nerve activity and systemic blood pressure in the rabbit

Electric stimulation at the nodulus and uvula of the cerebellum with a train of 5-10 pulses caused a marked depression of the renal sympathetic nerve activity. The effective sites were located in a small lateral region extending through the dorsal nodulus and encroaching the ventral uvula, at 2.9-3.7 mm lateral to the midline. The depression occurred predominantly contralaterally. Stimulation at this region with pulse trains lasting for several seconds caused a transient reduction of systemic blood pressure.

Orexin-neuromodulated cerebellar circuit controls redistribution of arterial blood flows for defense behavior in rabbits

We investigated a unique microzone of the cerebellum located in folium-p (fp) of rabbit flocculus. In fp, Purkinje cells were potently excited by stimulation of the hypothalamus or mesencephalic periaqueductal gray, which induced defense reactions. Using multiple neuroscience techniques, we determined that this excitation was mediated via beaded axons of orexinergic hypothalamic neurons passing collaterals through the mesencephalic periaqueductal gray. Axonal tracing studies using DiI and biotinylated dextran amine evidenced the projection of fp Purkinje cells to the ventrolateral corner of the ipsilateral parabrachial nucleus (PBN). Because, in defense reactions, arterial blood flow has been known to redistribute from visceral organs to active muscles, we hypothesized that, via PBN, fp adaptively controls arterial blood flow redistribution under orexin-mediated neuromodulation that could occur in defense behavior. This hypothesis was supported by our finding that climbing fiber signals to fp Purkinje cells were elicited by stimulation of the aortic nerve, a high arterial blood pressure, or a high potassium concentration in muscles, all implying errors in the control of arterial blood flow. We further examined the arterial blood flow redistribution elicited by electric foot shock stimuli in awake, behaving rabbits. We found that systemic administration of an orexin antagonist attenuated the redistribution and that lesioning of fp caused an imbalance in the redistribution between active muscles and visceral organs. Lesioning of fp also diminished foot shock-induced increases in the mean arterial blood pressure. These results collectively support the hypothesis that the fp microcomplex adaptively controls defense reactions under orexin-mediated neuromodulation.

Climbing fiber responses evoked in lobule VII of the posterior cerebellum from a vagal nerve in rabbits

The vagal afferent projection to the cerebellar posterior lobe was examined in decerebrate rabbits with electrophysiological techniques. Electrical stimulation of a vagal nerve at the cervical region evoked field potential responses on both sides of the posterior vermal cortex from lobule VI to VIII. The most prominent potentials appeared in the ipsilateral medial vermis of lobule VIIa. These potentials had a latency of 15.6 ms, and were negative in the molecular layer and positive in the granular layer. At a stimulation rate of 20 Hz, their amplitude was reduced by 70% of the control value at 1 Hz. Superimposed on these field potentials, unitary complex spikes were recorded at the depths of Purkinje cell layers. These results indicate that the medial vermal cortex of lobule VII receives vagal afferent signals via climbing fibers, which presumably convey cardiovascular information.

Cerebellar control of the cardiovascular responses during postural changes in conscious rabbits

In conscious control rabbits, tilting the head 30 degrees up from a position 30 degrees down induced initially an inhibition in the renal sympathetic nerve activity (RSNA), however this inhibition immediately released and became a transient increase. Following these responses in RSNA, blood pressure (BP) initially decreased but recovered to the control level within 3-5 s. After bilateral destruction of the lateral nodulus-uvula in the cerebellum, in contrast, the same tilting of the head caused an immediate large increase in RSNA without early inhibition, which was sustained at a high level. BP increased transiently, but then decreased and remained at a level lower than the control. These results indicate that the timing and duration of this transient increase in RSNA during tilting the head up are controlled by the lateral nodulus-uvula and may be important in the rapid adaptation of blood pressure. In addition, this suggests that the lateral nodulus-uvula may play an important role in the cardiovascular control under conditions of consciousness during changes in head position and body posture.

Projection of cardiovascular afferents to the lateral nodulus-uvula of the cerebellum in rabbits

Vagal and aortic afferent projections to the nodulus and uvula of the cerebellar vermis were examined in anesthetized and paralysed rabbits. Electrical stimulations of vagal and aortic nerves at a frequency of 1/s produced field potentials in the lateral cortical region of the contralateral nodulus-uvula with a latency of 6.5-19 ms. These potentials were negative in the molecular layer and positive in the granular layer. At a higher stimulation rate (30/s), these potentials were reduced by 60-65% of the control value. These results indicate that the lateral cortical region of the nodulus-uvula receives contralateral vagal and aortic afferent signals via climbing fibers. Electrical stimulation of vagal nerves also produced field potentials in the lateral region of the ipsilateral nodulus-uvula with a latency of 6-10.5 ms. Their laminal profile was characteristic of mossy fiber responses. At a higher stimulation rate (30/s), the reduction of these potentials was only 35%. Thus the lateral nodulus-uvula also receives the ipsilateral vagal afferent signals via mossy fibers. Following injection of horseradish peroxidase into the small areas of the lateral nodulus, labeled cells were found in bilateral intercalatus nucleus, prepositus hypogrossal nucleus, Roller nucleus, medial fascicullus longitudinalis, and medial and lateral vestibular nuclei. These nuclei may contain precerebellar neurons in the pathway from the vagal nerve to the lateral nodulus-uvula via mossy fibers.

Olivocerebellar projection to the cardiovascular zone of rabbit cerebellum

Climbing fiber responses were evoked in the medial vermal cortex of lobule VIIa by stimulation of the contralateral medial accessory olive (MAO) in anesthetized, paralyzed rabbits. Effective stimulating sites were localized in a small medial part of the caudal MAO, at 0.4-1.6 mm rostral from the caudal pole of the MAO (total length of the MAO, 4.2 mm). Stimulation of this MAO area induced depression in renal sympathetic nerve activity and this depressant response disappeared after ablation of lobule VIIa. Following injections of horseradish peroxidase into the small areas of lobule VIb, VIc, VIIa or VIIb, retrogradely labeled cells were found in corresponding small particular regions of the MAO: lobule VIb to the most caudal part, lobule VIc to the next caudal, lobule VIIa to the most rostral within the caudal MAO, and lobule VIIb further rostrally to the intermediate MAO. There was a clear disparity between the medial halves of lobules VI and VII projected from the medial MAO and the lateral halves from the lateral MAO. These results show that climbing fiber projections to lobules VI and VII are topographically organized, and that the medial region of lobule VIIa, related to cardiovascular function, receives climbing fibers from a localized small medial region of the caudal MAO.

Simultaneous measurements of lactate and blood flow during hypoxia and recovery from hypoxia in a localized region in the brain of the anesthetized rabbit

We observed simultaneous changes in lactate level and regional blood flow (rBF) in the brain of the anesthetized rabbit by using localized proton magnetic resonance spectroscopy (1H MRS) and laser Doppler flowmetry. The volume of interest of 0.5 ml for 1H MRS contained mostly thalamic nuclei. During hypoxia peak area for lactate increased up to 57% of that from N-Acetylaspartate. While the rBF increased during hypoxia up to 260% of the control, oxygen delivery (rBF x arterial oxygen content) decreased. In the normoxic recovery period following hypoxia, the rBF recovered slowly and a consequent overshoot of oxygen delivery was observed. The multiple and stepwise linear regression analyses revealed that the averaged decrease in oxygen delivery during hypoxia was the most significant independent variable for the increase in lactate during hypoxia (correlation coefficient; r2 = 0.68) and also that the increase in lactate during hypoxia was the most significant independent variable for the time for half-recovery of rBF (r2 = 0.75). These results suggest that the increase in lactate during hypoxia is due to the deficiency of oxygen delivery and that the increase in lactate during hypoxia prolongs the period of enhancement of rBF during recovery from hypoxia.