Microinjections of anisomycin into the intermediate cerebellum during learning affect the acquisition of classically conditioned responses in the rabbit

The purpose of this study was to examine the effects of protein synthesis inhibition in the intermediate cerebellum on the acquisition and expression of classically conditioned nictitating membrane responses in the rabbit. Animals were conditioned for three days in a standard delay paradigm. Before each training session, either a solution of anisomycin (a protein synthesis inhibitor) or vehicle was bilaterally injected into the interposed cerebellar nuclear. Following these three training sessions, rabbits were tested to determine whether the previous training under the influence of anisomycin or vehicle resulted in the acquisition of conditioned responses. In this test, animals that were injected previously with the protein synthesis inhibitor exhibited significantly less retention of conditioned responses than rabbits injected with vehicle. Additional experiments demonstrated that anisomycin does not block the expression of conditioned responses during conditioning or in well-trained animals. Microinjections of muscimol at the same sites of the previous drug infusions suppressed the expression of conditioned responses, indicating that the protein synthesis inhibitor was applied to the eyeblink-related parts of cerebellar circuits. The obtained data are the first to demonstrate that a manipulation of cerebellar circuits, which does not affect the performance of learned behavior, can affect the process of learning. These results suggest that the synthesis of new proteins in the intermediate cerebellum participates in the formation of plastic changes responsible for eyeblink conditioning.

Augmentation of postural muscle tone induced by the stimulation of the descending fibers in the midline area of the cerebellar white matter in the acute decerebrate cat

In a reflexively standing acute decerebrate cats, the cerebellar white matter was systematically stimulated and the effects on the level of postural muscle tone were studied. A stimulating microelectrode was placed systematically at 0.1-0.5 mm increments from H + 2 to H - 2 at levels ranging from P7.0 to P8.0 rostrocaudally and mediolaterally from LR0 to L1.5 or R1.5. Stimuli delivered to the restricted region of the cerebellar white matter along its midline resulted in simultaneous and bilateral augmentation of tonic activities in the neck, lumbar back, fore- and hindlimb extensor muscles along with increased levels in the forces exerted by each of the left and the right fore- and hindlimbs. Effective stimulus regions were located in the cerebellar white matter rostral and ventral to the most rostral part of the fastigial nucleus. Microinjection of a retrograde neural tracer, cholera-toxin b subunit conjugated horseradish peroxidase (CTb-HRP), into the lesioned effective stimulus sites resulted in a retrograde labeling of cells in the fastigial nuclei, bilaterally. All these results suggest that the augmentation of postural muscle tone was evoked by a selective activation of fastigiofugal fibers which course through the 'hook bundle'.

Changes in initiation of orienting gaze shifts after muscimol inactivation of the caudal fastigial nucleus in the cat

1. The production of a goal-directed saccadic gaze shift involves the specification of movement amplitude and direction, and the decision to trigger the movement. Behavioural and neurophysiological data suggest that these two functions involve separate processes which may interact. 2. The medio-posterior cerebellar areas are classically assigned a major contribution to the control of saccade metrics, and previous cerebellar lesion studies have revealed marked dysmetria of visually triggered gaze shifts. In contrast, these studies did not provide evidence for a cerebellar role in saccadic initiation. 3. In the present study, we investigated in the head-unrestrained cat the deficits in both the initiation and the metrics control of saccadic gaze shifts following pharmacological inactivation of the caudal part of the fastigial nucleus (cFN). 4. After cFN inactivation, latencies for contraversive gaze shifts increased to about 137 +/- 28% of normal, and latencies for ipsiversive gaze shifts decreased to about 84 +/- 8% of normal. Similar changes in head movement latency were observed, such that the temporal coupling between eye and head components remained largely unaffected. 5. Contraversive gaze shifts were more hypometric as their latency increased. In contrast, the degree of hypermetria in ipsiversive gaze shifts was unrelated to latency. 6. These results suggest a functional role of the medio-posterior cerebellum in gaze shift initiation and in storing information about the target location and/or the desired gaze shift amplitude.

Participation of caudal fastigial nucleus in smooth pursuit eye movements. II. Effects of muscimol inactivation

We studied the effect of temporarily inactivating the caudal fastigial nucleus (CFN) in three rhesus macaques trained to make smooth pursuit eye movements. We injected the gamma-aminobutyric acid A agonist muscimol into one or both CFNs where we had recorded pursuit-related neurons a few minutes earlier. Inactivating the CFN on one side impaired pursuit in one monkey so severely that it could not follow step-ramp targets moving at 20 degrees/s, the target velocity that we used to test the other two monkeys. We tested this monkey with targets moving at 10 degrees/s. In all three monkeys, unilateral CFN inactivation either increased the acceleration of ipsilateral step-ramp pursuit (in 2 monkeys, to 144 and 220% of normal) or decreased the acceleration of contralateral pursuit (in 1 monkey, to 71% of normal). Muscimol injected into both CFNs in two of the monkeys left both ipsilateral and contralateral acceleration nearly normal in both monkeys (101% of normal). Unilateral CFN inactivation also impaired the velocity of maintained pursuit as the monkeys tracked a target moving at a constant velocity or oscillating sinusoidally. Averaged across both types of movements in all three monkeys, gains for ipsilateral, contralateral, upward, and downward pursuit were 94, 67, 84, and 73% of normal, respectively. Unilateral CFN inactivation also impaired the monkeys' ability to suppress their vestibuloocular reflex (VOR). Averaged across the two monkeys VOR gain during suppression increased from 0.06 to 0.25 during yaw rotation and from 0.21 to 0.59 during pitch rotation. Bilateral CFN inactivation reduced pursuit gains in all directions more than unilateral injection did. In the two monkeys tested, ipsilateral, contralateral, upward, and downward gains went from 94, 86, 85, and 74% of normal, respectively, after we inactivated one CFN to 88, 73, 80, and 64% of normal after we also inactivated the second CFN. We can explain many, but not all, of the effects of CFN activation on smooth pursuit with the behavior of CFN neurons, and the assumption that the activity of each CFN neuron helps drive pursuit movements in the direction that best activates that neuron. We conclude that the CFN, like the flocculus-ventral paraflocculus, is a cerebellar region involved in control of smooth pursuit.

Effects of inactivating individual cerebellar nuclei on the performance and retention of an operantly conditioned forelimb movement

These experiments were designed to examine the effects of inactivating separately each of the major cerebellar nuclear regions in cats on the execution and retention of a previously learned, operantly conditioned volitional forelimb movement. The experiments test the postulates that the cerebellar nuclei, and particularly the interposed nuclei, contribute substantially to the spatial and temporal features of the interjoint coordination required to execute the task and that the engram necessary for the retention of this task is not located in any one of the cerebellar nuclei. All cats were trained to perform a task in which they were required to reach for and grasp a vertical bar at the sound of a tone and move the bar to a reward zone through a template consisting of two straight grooves in the shape of an inverted "L." After the task was learned, the effects of inactivating separately each nuclear region (the fastigial, interposed, and dentate nuclei) using muscimol microinjections were determined. Data were analyzed by quantifying several features of the movement's kinematics and by determining changes in the organization of the reaching component of the movement using an application of dimensionality analysis, an analysis that examines the correlation among the changes in joint angles and limb segment positions during the task. The retention of the previously learned task also was assessed after each injection. Injections of each nuclear region affected temporal and spatial features of the learned movement. However, the largest effects resulted from inactivating the interposed nuclei. These effects included an increased length of the reach trajectory, an accentuated deviation of the wrist trajectory from a straight line, cyclic movement of the distal extremity as the target was approached, a difficulty in grasping the bar, altered temporal features of the movement, and a highly characteristic change in the dimensionality measurements. The changes in dimensionality reflected a decreased correlation (linear interdependence) of the joint angular velocities coupled with an increased correlation among the linear velocities of markers located on the joints themselves. Related but less consistent changes in dimensionality resulted from fastigial injections. The motor sequence required to negotiate the template could be executed after the nuclear microinjections, indicating that retention of the motor sequence was not affected by the inactivation of any of the cerebellar nuclei. However, in two of the five animals, some decreases in performance were observed after dentate injection that were not characteristic of changes related to an effect on retention. These data suggest that the cerebellum plays an important role in regulating the consistent, stereotypic organization of complex goal-directed movements, including the temporal correlation among joint angle velocities. The data also indicate that the retention of the task is not dependent on any of the individual cerebellar nuclear regions. Consequently, these structures are unlikely to be critical storage sites for the engram established during the learning of this task.

Single-unit evidence for eye-blink conditioning in cerebellar cortex is altered, but not eliminated, by interpositus nucleus lesions

Many theories of motor learning explain learning-related changes in motor behavior in terms of plasticity in the cerebellar cortex. Empirical evidence, however, does not always appear to be consistent with such formulations. It is the anterior cerebellar interpositus nucleus (aINP) that seems to be essential for acquisition and retention of conditioned eye-blink responses under most circumstances and it has been therefore suggested that the aINP is the critical site of learning-related plasticity during eye-blink conditioning. Supporting this conclusion are studies demonstrating that multiple-unit conditioning-related neural activity patterns observed in many brain regions disappear after aINP lesion. The possibility that the cerebellar cortex may be involved in forming these patterns has not been assessed adequately, however. In the current study, trained rabbits received kainic acid lesions of the INP. After recovery, the animals underwent additional sessions of conditioning during which single-unit activity was recorded from the cerebellar cortex. Our results suggest that the aINP is not the sole site of plasticity during eye-blink conditioning, as a subset of the neurons recorded from lesioned animals demonstrated conditioning-related firing patterns. The lesions did change the character of these firing patterns from those observed in saline controls, however, in ways that can be generally described as a loss of organization. The normal tendency for the population of cortical cells to change firing rate together, for instance, was significantly less noticeable in lesioned animals. These results suggest that the aINP may be involved in the production of important features of conditioned responding, such as system timing function, therefore suggesting the need for more models that incorporate aINP and brain stem feedback as integral to the production of organized neural and behavioral responses.

Respiratory-related neurons of the fastigial nucleus in response to chemical and mechanical challenges

Responses of cerebellar respiratory-related neurons (CRRNs) within the rostral fastigial nucleus and the phrenic neurogram to activation of respiratory mechano- and chemoreceptors were recorded in anesthetized, paralyzed, and ventilated cats. Respiratory challenges included the following: 1 ) cessation of the ventilator for a single breath at the end of inspiration (lung inflation) or at functional residual capacity, 2) cessation of the ventilator for multiple breaths, and 3) exposure to hypercapnia. Nineteen CRRNs having spontaneous activity during control conditions were characterized as either independent (basic, n = 14) or dependent (pump, n = 5) on the ventilator movement. Thirteen recruited CRRNs showed no respiratory-related activity until breathing was stressed. Burst durations of expiratory CRRNs were prolonged by sustained lung inflation but were inhibited when the volume was sustained at functional residual capacity; it was vice versa for inspiratory CRRNs. Multiple-breath cessation of the ventilator and hypercapnia significantly increased the firing rate and/or burst duration concomitant with changes noted in the phrenic neurogram. We conclude that CRRNs respond to respiratory inputs from CO2 chemo- and pulmonary mechanoreceptors in the absence of skeletal muscle contraction.

Reversible inactivation of the cerebellar interpositus nucleus completely prevents acquisition of the classically conditioned eye-blink response

Numerous studies from several laboratories report that temporary inactivation of the cerebellar interpositus nucleus and regions of overlying cortex during eye-blink conditioning completely prevents acquisition of the conditioned eye-blink response (CR) without affecting the ability to learn the CR in subsequent training without inactivation. Recently, these results have been challenged by the suggestion that learning was not completely blocked in these studies. Instead, it has been suggested that low levels of responses on test sessions might represent a retarded form of learning caused by drug effects on cerebellar cortex. The present study was designed to address this issue directly. Very low doses of muscimol were used to selectively inactivate the interpositus nucleus of rabbits during five conditioning sessions. Animals performed no significant levels of CRs during those sessions. Training was continued four more sessions without any inactivations to test whether any learning had occurred during the previous five sessions. Detailed analysis of responses during session six revealed that learning was completely blocked by the low doses of muscimol infused into the interpositus during the first five sessions. Animals subsequently acquired the CR normally. These results confirm and extend the original findings that appropriate lesions (either temporary or permanent) of the interpositus nucleus completely prevent acquisition of the conditioned eye-blink response. Other issues regarding reversible inactivation studies are also discussed.

The physiological effects of serotonin on spontaneous and amino acid-induced activation of cerebellar nuclear cells: an in vivo study in the cat

It is well established that cerebellar efferents originate from neurons located within the cerebellar nuclei. Neurons within these nuclei receive excitatory inputs derived from the axons that arise from cells in several different regions of the brainstem and spinal cord, some of which continue on to terminate as mossy fibers and climbing fibers in the cerebellar cortex. GABA-induced inhibition in the nuclei is derived primarily from Purkinje cells located in the overlying cortex and possibly from axonal collaterals of a population of small, GABAergic nuclear neurons. In addition, a third chemically defined system of afferents that contain the monoamine serotonin forms a dense plexus of fibers throughout the cat's cerebellar nuclei. The intent of this study is to determine the physiological effects of serotonin on the spontaneous activity of cerebellar nuclear cells as well as that induced by application of the excitatory amino acids glutamate and aspartate in an adult in vivo preparation. Iontophoretic application of serotonin in anesthetized preparations suppresses both spontaneous and excitatory amino acid induced activity. In addition, interactions between serotonin and the amino acid analogs quisqualate and NMDA were analyzed; 5HT suppresses the excitatory responses of neurons to both analogs. However, there is a stronger suppressive effect on quisqualate-induced excitation as compared to that elicited by NMDA. In addition to modulating the effects of the excitatory amino acids, serotonin also potentiates the inhibitory effects of GABA. However, the effect was greatest if the neuron was initially preconditioned with GABA. In summary, serotonin acts to suppress amino acid induced activity in cerebellar nuclear neurons and to enhance gABA-mediated inhibition. The net effect is a decrease in nuclear cell activity and consequently in cerebellar output.

Beta-adrenergic modulation of GABAergic inhibition in the deep cerebellar nuclei of F344 rats

The presence of norepinephrine (NE) and NE activated cells, in the deep cerebellar nuclei (DCN) of male F344 rats, was investigated using immunohistochemistry and electrophysiology, during iontophoresis of the beta-adrenergic agonist isoproterenol (ISO). During extracellular electrophysiology, GABA was iontophoretically applied to the cell and ISO was then co-applied in an attempt to modulate the GABAergic inhibition of cell firing in the DCN. Immunohistochemistry was used to detect tyrosine hydroxylase (TH) positive fibers in the DCN. Isoproterenol modulated GABAergic inhibition in 51% of the DCN cells recorded from. In addition, TH-positive fibers that appeared to make contact with DCN cells were found. Therefore, this study demonstrated that functional NE receptors exist in the DCN and NE appears to be present in fibers therein.

Bicuculline-induced circling from the rat superior colliculus is blocked by GABA microinjection into the deep cerebellar nuclei

In a recent electrophysiological experiment, we showed the deep cerebellar nuclei to be a major source of excitatory input to the superior colliculus. Furthermore, target neurons in the colliculus were found, in every case, to receive convergent tonic inhibitory input from the substantia nigra pars reticulata. In the present study, we investigated these effects in the awake rat. We asked whether circling behaviour, induced by unilateral injection of a GABA antagonist into the lateral colliculus, could be suppressed by concurrent cerebellar inactivation. Rats were chronically implanted with bilateral guide cannulae located above the superior colliculus and deep cerebellar nuclei. Bicuculline methiodide (25 pmol) was microinjected unilaterally into intermediate layers of the colliculus at increasing depths until an optimal contralateral circling response was elicited. This behaviour was taken as the "baseline response" and was the first of three treatments. The second was an identical manipulation of the colliculus with a concurrent 200-nl microinjection of 1 M GABA into the contralateral deep cerebellar nuclei. The third was a repeat of BIC alone into the colliculus or, if rotation had been suppressed by more than 50% on test 2, the treatment was collicular BIC plus deep cerebellar saline. This latter treatment was used as a control for possible non-pharmacological injection effects. The effect of cerebellar GABA at 26 sites (17 within cerebellar nuclei and 9 outside) on BIC-induced rotation at 15 collicular sites was studied in ten animals. Only GABA injections at sites that fell within the cerebellar nuclei significantly reduced turning (P < 0.0001). A full behavioural analysis showed that this was a specific suppression of turning, not the result of general motor impairment. These results provide clear behavioral evidence that opposing, convergent influences from the basal ganglia and cerebellum interact in the lateral superior colliculus to control head and body movements. They furthermore suggest that the tonic deep cerebellar excitation of the superior colliculus could be the driving force in the expression of rotation induced by manipulations of the basal ganglia.

GABAergic modulation of complex spike activity by the cerebellar nucleoolivary pathway in rat

1. The role of gamma-aminobutyric acid (GABA) on the pattern generation properties of neuronal ensembles in the olivocerebellar system was studied utilizing multiple electrode recordings of complex spikes (CSs) from rat crus 2a Purkinje cells (PCs). Initially multiple electrode experiments were combined with microinjections of picrotoxin into the inferior olive (IO). To corroborate the picrotoxin findings, the cerebellar nuclei, a major source of the GABAergic terminals in the IO, were chemically lesioned with the use of microinjections of kainic acid and N-methyl-D-aspartate. Both procedures generated comparable results. 2. After intraolivary picrotoxin injection there was an increase in the average firing rate, synchrony, and rhythmicity of spontaneous CS activity. In addition, the neuronal oscillation frequency tended to shift to lower frequencies. 3. The spatial distribution of synchronous CS activity in control conditions displayed a predominantly rostrocaudal orientation. Injection of picrotoxin to the IO disrupted this rostrocaudal organization and led to synchronous CS activity among PCs throughout crus 2a. Similar effects were observed relating to the distribution of CSs evoked via the "climbing fiber reflex," in which antidromic activation of the climbing fibers is followed by a return excitation that is mediated by the gap junctions between olivary neurons. 4. Chemical lesions of the cerebellar nuclei resulted in increased CS average firing rates. The effect of the lesions on CS synchronicity was similar to that following the picrotoxin injections, but greater in magnitude. In contrast to the olivary picrotoxin injections, the cerebellar nuclear lesions did not lead to an enhanced CS rhythmicity. 5. Bilateral recordings from left and right crus 2a demonstrated significant interhemispheric synchronization of CS activity, consistent with a previous report. Both unilateral olivary injections of picrotoxin and unilateral cerebellar nuclear lesions resulted in increased synchronization of CS activity between the left and right crus 2a. 6. We conclude that the cerebellar nucleoolivary projection to the olivary glomeruli modulates the effective electrotonic coupling between olivary neurons, and thereby carves out ensembles of neurons whose activity is synchronized. Thus these two nuclei may form the basis for a flexible and sophisticated motor coordination system able to help generate the many distinct movements that organisms are capable of performing.

Presynaptic GABAB receptors modulate IPSPs evoked in neurons of deep cerebellar nuclei in vitro

1. Recording from deep cerebellar nuclei neurons, we investigated the role of presynaptic gamma-aminobutyric acid-B (GABAB) receptors in the modulation of monosynaptic inhibitory postsynaptic potentials (IPSPs) evoked by stimulation of Purkinje cells in rat slice cultures. 2. Bath application of the GABAB receptor agonist, baclofen (10 and 100 microM) induced two effects in cerebellar nuclei neurons: a postsynaptic hyperpolarization of 4.2 +/- 1.7 (SD) mV and a reduction in the amplitude of evoked IPSPs (30 +/- 10%). 3. When the postsynaptic GABAB response was blocked by filling the electrode with cesium methanesulfonate (2 M), or with a solution containing QX 314 (50 mM), bath application of baclofen (10 microM) reversibly depressed the evoked IPSPs by 36.7 +/- 18.7% and 42 +/- 20.3%, respectively. Under these experimental conditions, baclofen (10 microM) also reduced the amplitude of spontaneous IPSPs (10.2 +/- 9.5%) and decreased their frequency by 45.6 +/- 8.8%, suggesting a presynaptic site of action. 4. The presynaptic action of baclofen was not due to activation of receptors on the somata of Purkinje cells: baclofen (100 microM) failed to alter membrane holding current in Purkinje cells, and it had no effect on the rate of spontaneous action-potential discharge in Purkinje cells in the presence of ionotropic glutamate receptor antagonists (6-cyano-7-nitroquinoxaline-2,3-dione, 20 microM; D-2-amino-5-phosphonovalerate, 40 microM). 5. IPSPs could be evoked by extracellular stimulation of the Purkinje cell layer or by direct stimulation of the fiber bundle connecting Purkinje cells to deep cerebellar neurons. In both situations, baclofen (10 microM) reduced the amplitude of evoked IPSPs by 32.7 +/- 8.8% and 31.2 +/- 10.2%, respectively. 6. These results demonstrate that GABAB receptors are present on the terminals of Purkinje cells. Their activation causes a decrease in the amplitude of evoked IPSPs recorded in deep cerebellar nuclei and also reduces the frequency of spontaneous inhibitory events.

A nitric oxide synthase inhibitor delays the formation of learning-related neural activity in the cerebellar interpositus nucleus during rabbit eyelid conditioning

Inhibiting nitric oxide synthesis with L-nitro arginine methyl ester (L-NAME) has been found to retard the acquisition of the classically conditioned rabbit eyelid response. In the present study, rabbits received an SC injection of either L-NAME (an inhibitor of nitric oxide synthase) or its inactive stereoisomer, D-NAME, an hour before each daily training session. L-NAME retarded both the acquisition of conditioned responses and associated learning-related neural activity in the interpositus nucleus. There was no effect of L-NAME on spontaneous interpositus activity recorded during a period of time before trial onset. These findings indicate that nitric oxide may play a role in neuronal processes associated with the acquisition of the classically conditioned rabbit eyelid responses.

Medullary respiratory neuronal activity modulated by stimulation of the fastigial nucleus of the cerebellum

The ability of the rostral fastigial nucleus (FNr) of the cerebellum to modulate medullary respiratory neuronal activity was examined in 17 anesthetized, paralyzed and ventilated cats. A bipolar stimulating electrode was positioned into the FNr and tungsten microelectrodes used to record units within the nucleus tractus solitarius (NTS), nucleus ambiguus (NA) and nucleus retroambigualis (NRA). Transient stimuli (< 150 microA, 5-200 Hz) were delivered during inspiration or expiration, and the effects noted on medullary neuronal activity and the phrenic neurogram. The results showed that FNr stimulation: (1) modulated inspiratory and expiratory neuronal (ramp-, early- and late-inspiratory and stage I and II expiratory) discharges recorded from the NTS, NA and NRA (n = 67, 14 and 28) when stimuli (> or = 20-50 Hz) were delivered during either the inspiratory or expiratory phases; (2) terminated the burst durations of inspiratory (77%) and expiratory (94%) neurons with stimulus-response latencies of 28.2 +/- 3.1 ms (inspiratory) and 29.4 +/- 3.6 ms (expiratory); (3) elicited changes in phrenic neurogram concomitant with the effects noted on medullary neuronal activities; (4) failed to change heart rate and arterial blood pressure; and (5) did not affect medullary neuronal and phrenic nerve activity following kainic acid injection into the FNr. We conclude that activation of the FNr (likely its cell bodies) can modulate the respiratory output via influences on medullary respiratory-related neurons. The primary cerebellar effect across all sub-types of respiratory neurons was early termination.

Modelling the role of the cerebellar fastigial nuclei in producing accurate saccades: the importance of burst timing

Clinical and experimental data indicate that damage to the cerebellar vermis results in permanent loss of saccadic accuracy. Models of saccade production therefore need to provide a role for the cerebellum. It has been proposed that the vermis adjusts the gain of the saccadic internal feedback loop in response to information about the amplitude of the intended saccade. A model of how the fastigial nuclei (through which vermal output is channelled) influence brainstem saccadic circuitry to achieve this effect was constructed in three stages. (1) The brainstem was represented by a version of Robinson's internal feedback model, which relates excitatory burst neuron discharge to horizontal saccade dynamics. (2) The original model was lesioned to simulate the effects of bilateral inactivation of the fastigial nuclei, namely slow hypermetric saccades. This required reducing the synaptic weight of the internal feedback pathway, and lowering the gain of the excitatory burst neurons. The resultant brainstem-only model served as a preparation for testing the effects of neuronal discharge patterns within the fastigial nuclei. (3) These discharge patterns were simulated using measurements from recent electrophysiological studies. It was found that saccadic accuracy and normal dynamics were restored in the model if the simulated burst from neurons in the contralateral fastigial nucleus were subtracted from the feedback signal (i.e. added to the command signal) early in the saccade, and the burst from neurons in the ipsilateral fastigial nucleus were added to the feedback signal later in the saccade. This pattern corresponds to the observed timing of neuronal bursts in the fastigial nuclei, and accounts qualitatively for the effects of unilateral stimulation and inactivation of both the fastigial nuclei and the cerebellar vermis. This method of producing accurate saccades also contributes to time optimal control, by increasing both saccadic acceleration and deceleration. Appropriate timing of burst onset and duration in the fastigial nuclei is essential for these roles. Evidence concerning the effects of cerebellar damage on fast movements of other parts of the body suggests that the cerebellum may use similar strategies for controlling a wide range of simple movements.

Hypoxic respiratory responses attenuated by ablation of the cerebellum or fastigial nuclei

The general contribution of the cerebellum to hypoxic respiratory responses and the special role of the fastigial nucleus (FN) in the hypoxic respiratory reflex mediated via peripheral chemoreceptors were investigated in anesthetized and spontaneously breathing cats. Seven cats were exposed to isocapnic progressive hypoxia before and after cerebellectomy by decreasing the fractional concentration of end-tidal O2 (FETO2) from 15 +/- 0.3% to 7% while maintaining the pressure of end-tidal CO2 at a constant level of approximately 30 mmHg. Five additional cats inhaled five breaths of pure N2 (transient hypoxia) and received sodium cyanide (50 micrograms iv) before and after thermal lesions of the bilateral FN. The results showed that cerebellectomy or FN lesions failed to alter the respiratory variables (minute ventilation, tidal volume, respiratory frequency, and the peak of integrated diaphragm activity) during eupneic breathing. However, cerebellectomy significantly attenuated minute ventilation (FETO2 < or = 13%) and the peak of integrated diaphragm activity (FETO2 < or = 10%) compared with control. During progressive hypoxia, changes in respiratory frequency were noted earlier (FETO2 < or = 13%) than changes in tidal volume (FETO2 < or = 10%). Similarly, bilateral lesions of the FN resulted in a profound reduction in these respiratory responses to transient hypoxia and sodium cyanide. We conclude that the cerebellum can facilitate the respiratory response to hypoxia and that the FN is an important region in the modulation of the hypoxic respiratory responses, presumably via its effects on inputs from peripheral chemoreceptors.

Pharmacological characterization of pre- and postsynaptic GABAB receptors in the deep nuclei of rat cerebellar slices

Whole-cell current-and voltage-clamp recordings were made from deep nuclear neurons in cerebellar slices from seven- to nine-day-old rats. Baclofen, a GABAB agonist, produced a slow postsynaptic hyperpolarization associated with a decrease in input resistance. The hyperpolarization was G-protein-dependent, blocked by intracellular Cs+ and antagonized by CGP 35348, a GABAB antagonist. In dialysed neurons recorded with Cs+ -containing pipettes, baclofen suppressed deep nuclear neuronal inhibitory postsynaptic potentials and inhibitory postsynaptic currents evoked by electrical stimulations of the Purkinje cell axons. This effect was blocked by CGP 35348, indicating that the suppressions were mediated by presynaptic GABAB receptors. The inability of CGP 35348 or uptake inhibitors (nipecotic acid and NO-711) to alter the decay of inhibitory postsynaptic currents evoked by maximal stimulation suggested that GABAB receptors are not activated by the stimulation of the GABAergic input. Paired-pulse depression of inhibitory postsynaptic currents was not blocked by CGP 35348. Moreover, neither uptake inhibitors nor CGP 35348 produced any significant changes to the whole-cell current produced by a tetanic stimulation of Purkinje cell axons, suggesting that GABAB autoreceptors were also not activated by endogenous GABA release. Our findings indicate that while pre- and postsynaptic GABAB receptors are present in the deep nuclei of the rat cerebellum, they are not activated by electrical stimulation of the Purkinje cell axons.

Adrenergic modulation of hilar neuron activity and granule cell inhibition in the guinea-pig hippocampal slice

To study the effects of norepinephrine on synaptic inhibition in the dentate gyrus, intracellular recordings were made from hilar neurons in the guinea-pig hippocampal slice. The effects of norepinephrine on hilar neurons were compared with changes in the frequency of spontaneous inhibitory postsynaptic potentials recorded from granule cells. Hilar neurons comprised two electrophysiologically distinct groups: type I hilar neurons displayed a pronounced single spike afterhyperpolarization and little spike frequency accommodation, type II hilar neurons had small afterhyperpolarizations and pronounced spike frequency accommodation. The majority of recordings were from type I hilar neurons which are presumably inhibitory to granule cells. In most instances, effects of norepinephrine (2-10 microM) on hilar neurons could be mimicked by the beta-adrenergic agonist isoproterenol (0.1-1 microM). Isoproterenol induced a slight depolarization, blocked a slow afterhyperpolarization and, in type II neurons, reduced spike frequency accommodation. These effects were associated with an increase in the spontaneous discharge rate and an enhancement of spontaneous excitatory and inhibitory postsynaptic potentials. In accordance, isoproterenol and norepinephrine increased the frequency of inhibitory postsynaptic potentials in granule cells. In the presence of the non-N-methyl-D-aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione and the N-methyl-D-aspartate receptor antagonist CGP 37849, isoproterenol and norepinephrine also increased the frequency of Cl- -dependent inhibitory postsynaptic potentials in granule cells. Under this experimental condition, however, norepinephrine reduced the discharge rate of type I hilar neurons through an effect on alpha-receptors. In the presence of GABAA receptor blockers, norepinephrine increased the frequency of spontaneously occurring K(+)-dependent inhibitory postsynaptic potentials in granule cells. Accordingly, the frequency of burst discharges in type I hilar neurons was increased. We suggest that the discrepancy in the effect of norepinephrine on the discharge rate of presumed inhibitory hilar neurons and the frequency of Cl- -dependent inhibitory postsynaptic potentials in granule cells results from a direct effect of norepinephrine on GABAergic terminals because norepinephrine also enhanced the frequency of tetrodotoxin-resistant inhibitory postsynaptic potentials in granule cells. Thus, the net effect of synaptically released norepinephrine on synaptic inhibition in the dentate gyrus will be determined by opposing actions of alpha- versus beta-receptor stimulation at the synapse on hilar neurons.

Pharmacological characterization of somatostatin receptors in rat cerebellar nuclei

Rat cerebellar nuclei contain somatotropin release-inhibiting factor (SRIF) receptors that bind [125I][Leu8,D-Trp22,Tyr25]SRIF-28 but do not bind [125I][Tyr0,D-Trp8]SRIF-14. The aim of the present study was to investigate the pharmacological profile of these receptors by means of binding experiments on tissue sections and quantitative autoradiography. Competition experiments indicated the presence of a single class of [125I][Leu8,D-Trp22,Tyr25]SRIF-28 binding sites in the lateral cerebellar nuclei, showing similar affinities for SRIF-14 and SRIF-28, but low affinity for short-chained analogs. The IC50 values for somatostatin analogs to compete with the binding of [125I][Leu8,D-Trp22,Tyr25]SRIF-28 in the lateral cerebellar nuclei ranked as follows: [Leu8,D-Trp22,Tyr25]SRIF-28 approximately SRIF-14 approximately SRIF-28 < CGP 23996 < D-Phe-Phe-Phe-D-Trp-Lys-Thr-Phe-Thr-NH2 (BIM 23052) < SMS 201-995 approximately N-Ahep-(7-10)SRIF-14-Bzl << MK 678 < D-Phe-Phe-Tyr-D-Trp-Lys-Val-Phe-D-Nal-NH2 (BIM 23056) < D-Phe-c[Cys-Tyr-D-Trp-Lys-Abu-Cys]Nal-NH2 (NC 8-12). Optimum binding of [125I][Leu8,D-Trp22,Tyr25]SRIF-28 did not require divalent cations, and was partly inhibited by guanosine 5' triphosphate. It appears from this study that the rat lateral cerebellar nuclei contain a pure population of receptors exhibiting the same binding characteristics as the recently cloned sstr1 somatostatin receptor. These nuclei could thus provide a useful model in which to investigate the characteristics of native sstr1.

Saccadic dysmetria is similar in patients with a lateral medullary lesion and in monkeys with a lesion of the deep cerebellar nucleus

Some of the clinical hallmarks of lateral medullary infarction--Wallenberg's syndrome--are saccadic dysmetria, smooth pursuit deficit, and lateropulsion of the body. Similar movement disorders are seen in monkeys after local unilateral injection of GABAergic drugs in the caudal fastigial nucleus of monkeys. These include an ipsilateral saccadic hypermetria and a contralateral saccadic hypometria as well as cogwheel smooth pursuit eye movements toward the contralateral side and an ipsiversive lateropulsion of the body. It was previously suggested that the lateral medullary infarction causes a lesion of climbing fibers to the cerebellum. This lack of climbing fiber input increases the activity of ipsilateral Purkinje cells, which consequently provide too much inhibition of the deep cerebellar nuclei.

Effects of muscimol inactivation of the cerebellar interposed-dentate nuclear complex on the performance of the nictitating membrane response in the rabbit

Intracranial microinjections of the GABAA agonist muscimol were used to assess the involvement of the dentato-interposed cerebellar nuclear complex in the performance of the conditioned (CR) and unconditioned (UR) nictitating membrane responses in the rabbit. Specifically, the experiments test the hypothesis that the cerebellar nuclei are involved in the performance of both the CRs and URs. The experiments employed temporary nuclear lesions to disrupt the CRs in order to examine parallel effects on URs. Animals were conditioned in a standard delay conditioning paradigm. Injection sites at which the muscimol application disrupted execution of the CRs were identified in each rabbit. Once these sites were found, the effects of muscimol and saline injections were evaluated while alternating paired trials with unpaired trials in which only the unconditioned stimuli were applied. There are two main findings in the present study. First, the activation of the GABAA receptors in the dentato-interposed cerebellar nuclear region reduced the amplitude and increased the latency of the UR. This change in the UR closely paralleled the disruption of the CR. This observation is consistent with the notion that the cerebellum is involved in the regulation of defensive flexion reflexes. Second, cerebellar nuclear inactivation did not eliminate the tone-induced enhancement of the UR. This finding suggests the presence of cerebellum-independent circuits subserving the intermodal interaction between the conditioned and unconditioned stimuli.

Kainic acid administration in the fastigial nucleus produces differential cardiovascular effects in awake and anesthetized rats

Kainic acid was microinjected or microdialyzed into the rostral medial aspect of the fastigial nucleus to determine its effect on mean arterial pressure and heart rate. This was carried out in both the awake and the anesthetized (alpha-chloralose) rat. In awake animals, kainic acid elicited an initial phasic pressor response which was followed by a long-term elevation of mean arterial pressure that lasted for the duration of the experiment (2 h). Rats anesthetized with alpha-chloralose exhibited only a tonic depressor response. This converted to a pressor response as the rats began to emerge from anesthesia after 2 h. Both the awake and the anesthetized rats exhibited regular phasic changes in mean arterial pressure that was superimposed on the longer term changes in the mean arterial pressure. Similar results were obtained in both the microinjected and the microdialyzed animals. Thus, stimulation of the intrinsic fastigial neurons by kainic acid evokes an elevation of the mean arterial pressure in the awake rat. This is manifested as a decrease in pressure in the anesthetized animal. Thus, stimulation of the cardiovascular region of the fastigial nucleus can increase or decrease mean arterial pressure. It is possible that the direction of the change in mean arterial pressure is dependent on the level of afferent or intrinsic fastigial neural activity.

Activity induced elevations of intracellular calcium concentration in neurons of the deep cerebellar nuclei

1. Depolarization-induced changes in the cytosolic free calcium concentration ([Ca2+]i) were examined in slice-cultured neurons of the deep cerebellar nuclei by combined intracellular and multisite fura-2 recording techniques. 2. Firing of tetrodotoxin (TTX)-sensitive action potentials induced by depolarizing current pulses caused large elevations in somatic as well as proximal dendritic [Ca2+]i. In the dendrites, rise and decay times of [Ca2+]i were faster than in the soma. [Ca2+]i changes associated with depolarizations to < or = -40 mV in the presence of TTX were small compared with changes induced by Na+ spike firing, suggesting that Ca2+ influx through high voltage-activated Ca2+ channels is a major cause for Na+ spike-associated [Ca2+]i increases. 3. During sustained Na+ spike firing at a constant frequency (> 20 Hz), [Ca2+]i approached a constant level, after approximately 1 s in the dendrites and 2 s in the soma, respectively. The amplitude of the attained level was positively correlated with the firing frequency. We suggest that during tonic activity [Ca2+]i reaches a steady state determined by Ca2+ influx and extrusion. 4. TTX-resistant plateau potentials caused substantially greater [Ca2+]i increases in the dendrites than in the soma. In the dendrites, plateau-associated Ca2+ transients were comparable in amplitude to Ca2+ transients triggered by short (50 ms) Na+ spike trains, in the soma, they were considerably smaller. 5. Low-threshold spikes (LTSs) in association with a burst of Na+ spikes induced a sharp increase in [Ca2+]i both in the soma and in dendrites.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of the fastigial nucleus in generalized seizures as demonstrated by GABA agonist microinjections

The cerebellum is electrically and metabolically active during seizures. Numerous studies have also shown that cerebellar electrical stimulation and lesions of the cerebellar cortex or nuclei influence seizure threshold, but there are significant contradictions, with different effects observed even in investigations using the same species and similar seizure types and experimental manipulations. Discrete intracerebral microinjection of neuroactive agents has been used to characterize the way in which other brain regions control seizures, but has not been applied to the cerebellar systems. This approach has advantages because effects are restricted to specific receptors and spare passing axons; experimental variables also can be simply specified and reproduced. We used this method to characterize the role of the cerebellar nuclei in seizures and to determine if observed effects could be reproduced with different agents at different doses. Effects of bilateral control microinjections in the fastigial (medial) cerebellar nucleus were compared with different doses of the GABAA agonist piperidine-4-sulfonic acid and the GABAB agonist (-)baclofen (Bf). Soon after injection, the animals were ataxic. After 4 min, seizures were induced by timed continuous intravenous (i.v.) bicuculline (BIC) infusion. Both GABA agonists produced significant reductions in myoclonic, clonic, and tonic seizure thresholds. Injections just dorsal or anterior to this nucleus and bilateral dentate (lateral) nucleus injections had little effect on seizures. These results demonstrate that the cerebellar system does control seizures, but does not provide support for the early concept that cerebellar stimulation and systemic phenytoin block seizures through inhibition of cerebellar nuclei secondary to Purkinje cell activation.