Activation of mossy and climbing fiber pathways to the cerebellar cortex by stimulation of the fornix in the rat

Is the cerebellum ready for navigation?

Spatial navigation required the acquisition of at least two complementary processes: the organization of the spatial representation of the environment (declarative learning) and the acquisition of a motor behaviour adapted to the specific context (procedural learning). The potential role of the cerebellum in spatial navigation is part of the debate concerning its role in cognitive function. Experiments ranging from cerebellar patients to animal models have indicated that cerebellar damage affects the processing of spatial information. The main unresolved issue concern the interpretation of these deficits. Is the cerebellum involved in both declarative and procedural components of navigation? Could all deficits in navigation paradigms be interpreted by a deficit in a motor-dependant process? The purpose of this review is to examine different results coming from anatomical data, experimental paradigms and models in order to give a critical answer to this question.

The effects of cerebellar damage on maze learning in animals

The role of the cerebellum in spatial learning has recently been investigated in genetically and non-genetically lesioned animal models, particularly in water mazes, in view of the minimal impact such lesions exert on swimming movements. A dissociation between place and cued learning in the Morris water maze has been observed in several models, including cerebellar mutant mice (Rora(sg), Nna1(pcd-1J), nervous), rats with lesions of either the lateral cerebellar cortex or the dentate nucleus, and rats with selective Purkinje cell loss caused by intracerebroventricular injections of OX-7-saporin, confirming the hypothesis that cerebellar damage may cause a cognitive deficit independently of fine motor control. In addition, the results of hemicerebellectomized rats indicate the probable involvement of the cerebellum in working memory and the procedural aspect of maze learning. The findings of impaired maze learning in cerebellar-lesioned mice and rats are concordant with those of deficient visuospatial functions in patients with cerebellar atrophy. The spatial deficits may be ascribed to altered metabolic activity in cerebellar-related pathways.

Rapid olfactory processing implicates subcortical control of an olfactomotor system

Sniffs are modulated in response to odor content. Higher concentrations of odor induce lesser-volume sniffs. This phenomenon implicates a neural feedback mechanism that measures sensory input (odor concentration) and modulates motor output (sniffing) accordingly. Here we used air-dilution olfactometry to probe the time course of this olfactomotor mechanism. A stainless-steel computer-controlled olfactometer, equipped with mass flow controllers, temperature and humidity control, and on-line photo-ionization detection, was coupled to a highly sensitive pneumatotachograph that measured nasal flow. The olfactometer was used to generate four ascending concentrations of the odorants propionic acid and phenethyl alcohol. Sniff volume was inversely related to odor concentration (P > 0.0001). Sniffs were uniform and concentration independent for the initial 150 ms but acquired a concentration-dependent flowrate as early as 160 ms following sniff onset for propionic acid (P > 0.05) and 260 ms for phenethyl alcohol (P > 0.05). Considering that odorant transduction takes around 150 ms and odorant-induced cortical evoked potentials have latencies of around 300 ms, the rapid motor adjustments measured here suggest that olfactomotor sniff feedback control is subcortical and may rely on neural mechanisms similar to those that modulate eye movements to accommodate vision and ear movements to accommodate audition.

The hippocampus as an olfacto-motor mechanism: were the classical anatomists right after all?

The relations between behavior, olfactory input (monitored by recording the activity of the olfactory mucosa), and the spontaneous field potentials of the dentate gyrus were studied in freely moving rats. Bursts of 30-80 Hz (gamma) waves were elicited in the dentate gyrus when a rat sniffed at a variety of objects but were not elicited by auditory, somesthetic, or visual inputs and were not related to the occurrence of locomotion. The presence of gamma wave activity was associated with an enhancement of the population spike elicited in the dentate gyrus by stimulation of the perforant path. Odorized air blown into a nostril via a cannula, inserted under light urethane anesthesia, elicited a gamma wave response bilaterally in the dentate gyrus. These and other data were reviewed to support the general hypothesis that the hippocampus is primarily an olfacto-motor mechanism and does not play any unique role in learning and memory, cognitive mapping, or emotion. The role of the hippocampus in the control of some forms of motor activity is supported by numerous anatomical and electrophysiological studies, studies of the effect of hippocampal lesions on behavior, and studies of the effects of electrical or chemical stimulation of the hippocampus on behavior.

Visuospatial abilities

The importance of the hippocampus and its anatomical connections, including the medial septum, thalamic nuclei, and neocortical regions in many spatial tasks including the Morris water maze, has been emphasized. Studies in mutant mice with cerebellar atrophy and in rats with electrolytic lesions of the cerebellum have indicated that the cerebellum has a role in visuospatial and visuomotor processes in the Morris maze. Directional deficits in the water have also been noted in rats whose cerebellum was exposed to X-rays during different developmental stages. Cerebellar interactions with the superior colliculus, the hippocampus, and the neocortex via thalamic nuclei are suggested to be the basis of the cerebellar modulation of directional sense in maze tests.

Effects of midline and lateral cerebellar lesions on motor coordination and spatial orientation

Rats were lesioned in the midline cerebellum, comprising the vermis and fastigial nucleus, or the lateral cerebellum, comprising the cerebellar hemispheres and dentate nucleus, and evaluated in a series of motor and non-motor learning tests. Rats with midline lesions had difficulty in maintaining their equilibrium on a bridge and were slower before turning upward and traversed less squares on an inclined grid. They were not impaired for muscle strength when suspended from a horizontal wire. Rats with lateral lesions had milder deficits on the bridge and were not affected in the other two tests. In the Morris water maze test, rats with lateral lesions were deficient in spatial orientation, whereas rats with midline lesions were deficient in visuomotor coordination. Lateral lesions had no effects on visual discrimination learning. These results illustrate the differential influence of midline as opposed to lateral cerebellar regions on both motor and non-motor behaviors. Fastigial nucleus lesions decreased the time spent in equilibrium and latencies before falling on the bridge and the distance travelled along the inclined grid but had no effect on muscle strength when suspended from the horizontal string. Quadrant entries and escape latencies were higher in rats with fastigial lesions during the hidden platform condition of the Morris water maze but not during the visible platform condition. It is concluded that fastigial-lesioned rats are impaired in equilibrium and spatial orientation but with repeated trials learn to improve their performances.

Cerebellar contributions to instrumental learning

There is emerging evidence that the cerebellum is involved in spatial and nonspatial instrumental learning tasks. Cerebellar-lesioned animals have deficits in water maze learning tasks that may be explained by two-way interactions with higher order brain regions. There is suggestive evidence that cerebellar modulation extends to shock avoidance and discrimination learning. Although this evidence needs to be confirmed by a wider range of lesion methods and choice of learning tasks, it is in line with the hypothesis that the cerebellum affects cognitive processes and is not strictly concerned with motor control and the acquisition and retention of conditioned reflexes.

Organization of cingulo-ponto-cerebellar connections in the cat

This study deals with three different aspects of the organization of connections from the cingulate gyrus to the cerebellum. (1) With the use of wheat germ agglutinin-horseradish peroxidase as a retrograde tracer, the distribution of cingulate neurons projecting to the pontine nuclei was studied. Retrogradely labeled cells were found in layer 5 in all parts of the cingulate gyrus. Average densities of cingulo-pontine cells were similar in the different cytoarchitectonic subdivisions, although some density gradients were observed. The projection was found to be remarkably strong. Average densities of corticopontine cells in the cingulate gyrus ranged from 500-700 cells per mm2 cortical surface, and the total number of neurons was in the range of 75,000-105,000 (n = 4). (2) A topographical organization of terminal fields of fibers originating in different parts of the cingulate gyrus was demonstrated with the combined use of anterograde degeneration and anterograde transport of wheat germ agglutinin-horseradish peroxidase. Terminal fibers originating in different zones of the cingulate gyrus were distributed in a patchy mosaic within a narrow band along the ventromedial aspect of the pontine nuclei. (3) We confirm, with the combined use of lesions in the cingulate gyrus and injections of wheat germ agglutinin-horseradish peroxidase in the ventral paraflocculus, that there is considerable overlap between terminal fibers originating in the cingulate gyrus, and cells retrogradely labeled from the ventral paraflocculus. The role of the ventral paraflocculus as a receiver of "limbic" input is discussed.

Effects of electrical stimulation and reversible lesions of the olivocerebellar pathway on Purkinje cell activity in the flocculus of the cat

The activity of Purkinje cells (P-cells) in the flocculus of 8 lightly anesthetized cats and one alert cat was recorded for periods of up to several hours each. The resting simple spike (SS) rate in the anesthetized cats was 37 +/- 21 Hz (mean +/- standard deviation), similar to that observed in the alert cat. Complex spikes (CSs) were evoked by an electrode placed in the climbing fiber (CF) decussation or inferior olive (IO). For each P-cell, SS activity was suppressed completely at or above a cut-off frequency of evoked CSs; median cut-off rate was 5 Hz (range 1-10 Hz, 15 cells). Reversible lesions of the CF pathway were made by microinjection of 1-10 microliter of saturated lidocaine into the IO or CF decussation. This abolished spontaneous CS activity and produced two reversible effects on SS discharge: (1) an increase in mean SS rate of 98%, from 23 +/- 13 to 40 +/- 18 Hz (11 cells); and (2) a decrease of 50% in the variability of SS firing rate. Similar effects were observed in two P-cells whose CF axons were lesioned mechanically. These results show that electrical stimulation and reversible lesions in areas previously shown to alter the gain of the vestibulo-ocular reflex (VOR) also alter CF input to the flocculus, suggesting that the gain changes were caused by changes in CS rate. This study confirms and extends the observation that a reciprocal relationship exists between CS rate and SS background rate, and therefore further suggests that the changes in the gain of the VOR might be due to changes in SS background rate.

Electrophysiologic identification of projections from the midbrain to the paraflocculus and midvermis in the rat

Regions of the midbrain in the rat were stimulated electrically with bipolar electrodes to identify responsive, single neurons in the parafloccular lobule of the cerebellum. Eighty four percent (44/52) of the cells recorded in the paraflocculus showed evidence of a modulation in simple spike discharge activity (mossy fiber activations) following stimulation with a bipolar electrode whose tip was placed in the ventral layers of the contralateral superior colliculus. Mossy fiber (MF) evoked responses were indicated by the presence of an excitation followed by an inhibition of simple spike frequency at latencies of 5-16 msec and by the demonstration of responsiveness to stimulus frequencies up to 50 Hz. Ten percent (4/41) of identified Purkinje cells in the paraflocculus demonstrated activation of complex spike potentials following stimulation of regions in the ventral superior colliculus. Experiments involving stimulation of the midbrain and visual cortices indicated that 70% of the parafloccular neurons are responsive to inputs from both the cortex and deep regions of the colliculus. Electrophysiologic evidence also is presented that demonstrates the existence of a midbrain projection to midvermal lobules VI and VII of the cerebellum.

A topographic analysis of limbic and somatic inputs to the cerebellar cortex in the rat

The effects of electrical stimulation of the fornix, snout, and limbs on the activity of electrophysiologically identified Purkinje cells were investigated. Extracellularly recorded responses were analyzed according to: latency; mode of termination as mossy or climbing fibers; and, whether excitatory or inhibitory responses were evoked. The positions of responsive cells were transposed onto a planar representation of the Purkinje cell layer of the rat cerebellum derived from a series of sagittal sections. Fornix responsive cells were found in the vermis of lobules IV, V, VI, and VII as well as in the paravermal regions of lobules II--VIIb. In addition, some evidence was obtained for a projection to lobulus simplex. Snout stimulation activated cells mainly in the vermis of lobule VI and also in adjacent lobules III, IV, V, and VIIa. A few responsive cells were also found in the paravermal regions of lobules V, VI, and VIIa and in the lateral portion of lobulus simplex. Forelimb responsive cells were located mainly in the vermis of lobules IV--VIa and in the paravermal protion of lobules III--VII. The few hindlimb cells encountered were located in the vermis and paravermis of lobules II--IV. The topographical overlap of the fornix projection with areas in receipt of forelimb, snout, neck, and auditory-visual teleceptor input suggests that the fornix-mediated information may modulate cerebellar circuits involved in postural adjustment, general orienting, and exploratory motor behavior.