Neurophysiological and behavioral maturation of cerebellar function studied with tremorogenic drugs

Quantitative behavioral models for high-resolution measurement and characterization of tremor in rodents

Tremor is an involuntary, rhythmic movement disorder. Despite its prevalence, the underlying pathophysiology remains poorly understood and effective treatment options are limited. Animal models are essential in enhancing our understanding of the mechanisms of tremorogenesis and developing new therapeutic interventions. Although tremor is amenable to measurement by automated systems, visual observation is still the most prevalent method for recording tremor in animal studies. This review gives a brief summary of two behavioral methods that enable quantitative measurement of forelimb tremor (the press-while-licking task) and whole-body tremor (the force-plate actometer) in rodents. These methods utilize force transducer and computing technologies to generate high-resolution force-time waveforms for automated detection and characterization of tremor. The focus will be on the sensitive, precise, and quantitative measurement of tremors induced in rodents by low-dose pharmacological agents, brain lesion, physical training, and genetic mutations. The methods reviewed here provide new tools that can facilitate preclinical assessment of treatment strategies for tremor.

A change in the pattern of activity affects the developmental regression of the Purkinje cell polyinnervation by climbing fibers in the rat cerebellum

Pattern of activity during development is important for the refinement of the final architecture of the brain. In the cerebellar cortex, the regression from multiple to single climbing fiber innervation of the Purkinje cell occurs during development between postnatal days (P) 5 and 15. However, the regression is hampered by altering in various ways the morpho-functional integrity of the parallel fiber input. In rats we disrupted the normal activity pattern of the climbing fiber, the terminal arbor of the inferior olive neurons, by administering harmaline for 4 days from P9 to P12. At all studied ages (P15-87) after harmaline treatment multiple (double only) climbing fiber EPSC-steps persist in 28% of cells as compared with none in the control. The ratio between the amplitudes of the larger and the smaller climbing fiber-evoked EPSC increases in parallel with the decline of the polyinnervation factor, indicating a gradual enlargement of the synaptic contribution of the winning climbing fiber synapse at the expense of the losing one. Harmaline treatment had no later effects on the climbing fiber EPSC kinetics and I/V relation in Purkinje cells (P15-36). However, there was a rise in the paired-pulse depression indicating a potentiation of the presynaptic mechanisms. In the same period, after harmaline treatment, parallel fiber-Purkinje cell electrophysiology was unaffected. The distribution of parallel fiber synaptic boutons was also not changed. Thus, a change in the pattern of activity during a narrow developmental period may affect climbing fiber-Purkinje cell synapse competition resulting in occurrence of multiple innervation at least up to 3 months of age. Our results extend the current view on the role of the pattern of activity in the refinement of neuronal connections during development. They suggest that many similar results obtained by different gene or receptor manipulations might be simply the consequence of disrupting the pattern of activity.

Animal models of tremor

Animal models of tremor have been widely used in experimental neurology, because they are an indispensable requirement for understanding the pathophysiology of human tremor disorders and the development of new therapeutic agents. This review focuses on three approaches to produce tremor in animals (application of tremorgenic drugs, experimental central nervous system lesions, study of genetic mutants) and their use in simulating tremor syndromes of humans. Whereas harmaline induces a postural/kinetic tremor in animals that shares some features with human essential tremor/enhanced physiological tremor, MPTP tremor is the best model available for rest tremor in people. The tremor following experimental lesion of the ventromedial tegmentum in primates closely resembles Holmes tremor in humans, whereas cerebellar intention tremor is mimicked by cooling of the lateral cerebellar nuclei. The "campus syndrome," discovered in a breed of Pietrain pigs, might be a useful model of human orthostatic tremor. However, no animal model has yet been generated that exactly recreates all features of any of the known tremor disorders in humans. Problems encountered when comparing tremor in animals and humans include differing tremor frequencies and the uncertainty, if specific transmitter abnormalities/central nervous system lesions seen in animal tremor models are characteristic for their human counterparts. The search for adequate tremor models continues.

Development and properties of spontaneous oscillations of the membrane potential in inferior olivary neurons in the rat

Previous experiments have shown that the conductances that are thought to underlie the spontaneous oscillations of the membrane potential in inferior olivary (IO) neurons of the mature rat are present at 2 days postnatal (PN). In this study intracellular recordings of transmembrane potentials were made in 209 IO neurons in brainstem slices from rats aged 4-25 days PN. Membrane potential oscillations were not found before 9 days PN but were observed in 41% of neurons examined at 10-15 days PN and in 95% of neurons after 16 days PN. These oscillations exhibited a wide range of frequencies (0.5-9.5 Hz) and amplitudes (2-23 mV). The waveforms of the oscillations in different neurons varied from having 2-3 non-harmonic frequencies of unequal amplitude to being almost sinusoidal. There was a positive relationship between the age of the animal and the frequency (P less than 0.0001) and amplitude (P less than 0.002) of the oscillations. The timecourse of development of the membrane potential oscillations is consistent with ultrastructural data which indicate that neuro-neuronal gap junctions in the rat IO nucleus mature between 10 and 15 days PN. Exposure of the neurons to pharmacological agents that induce oscillations in adult IO neurons failed to induce oscillations in neurons less than 9 days PN. Our findings support the hypothesis that oscillations of the membrane potential in IO neurons depend not only on specific membrane conductances but also on electrotonic coupling between the neurons.

Enhanced sensitivity to quipazine in the genetically dystonic rat (dt)

Both normal and genetically dystonic (dt) rats show a high-frequency forepaw tremor in response to systemic administration of the serotonin (5-HT) agonist quipazine at 8 days of age. The response declines with age in normal, but not dystonic, rats. By 16 days of age and after the development of a generalized movement disorder, the dystonic rat exhibits enhanced sensitivity to the tremorogenic effects of the drug in comparison with normal rats. Tremor was blocked by pretreatment with ketanserin, suggesting that it is mediated by 5-HT2 receptors. The dystonic rat has previously been shown to be insensitive to the tremorogenic effects of harmaline, a drug presumed to act indirectly through serotonergic neurons. This finding, coupled with the increased sensitivity to quipazine, suggests the presence of an abnormality in serotonergic systems in the mutants. Since there is evidence of abnormality in the olivo-cerebellar system in the dystonic rat, the alternative hypothesis that a nonserotonergic defect in the olivo-cerebellar system accounts for both the failure of behavioral response to harmaline and the persistent expression of a response to quipazine is also discussed.

Device for quantitating tremor activity in mice: antitremor activity of atropine versus soman- and oxotremorine-induced tremors

A tremor monitor was constructed and tested that quantitates soman- and oxotremorine-induced tremors. The device consisted of a force transducer, from which was suspended a plastic beaker containing a mouse. The signal from the force transducer was fed into a tremor monitor, which was essentially a low-pass filter, and quantitated using the Applecounter from Columbus Instruments (Columbus, Ohio) with the data stored on floppy diskette in a personal computer or on a magnetic tape cassette. The tremor monitor demonstrated a dose response relationship to both soman- and oxotremorine-induced tremors and the antagonism of these tremors by various doses of atropine. In addition, the duration of action of a dose of atropine (17.4 mg/kg, ip) normally used in the therapy of organophosphate poisoning was assessed using the atropine antagonism of oxotremorine-induced tremors as an indicator. This device permits the unbiased determination of the tremor activity of a drug and allows the use of statistical analysis on the data produced.

A new method for quantification of tremors in mice

A procedure for quantifying whole-body tremors in mice was developed. Displacement of a free-floating platform by animal movement created a change in resistance across a strain gauge. The resulting resistive changes were assessed as voltage fluctuations by a wheatstone bridge and converted into an intensity-frequency profile by a series of calculations involving Fourier transformations. Movement profiles of normal mice were characterized by high-intensity, low-frequency peaks. Administration of oxotremorine, 2.5 mg/kg, ip, produced numerous high-frequency, high-intensity peaks within 5 min of administration. The peak frequency at 5 min after oxotremorine was approximately 15 Hz, and within 30 min the intensity-frequency profile had returned to control levels. In dose-response studies, a slight decrease in exploratory activity and initiation of low-level tremors was observed after 0.5 mg/kg. The most severe tremors were detected after a dose of 1.0 mg/kg. The peak frequency remained at approximately 15 Hz at 1.0 to 5.0 mg/kg, but varied greatly in intensity. This study describes an inexpensive, quantitative, sensitive, and rapid technique for quantifying tremors in mice.

Drug-induced activation of the inferior olivary nucleus in young rabbits. Differential effects of harmaline and quipazine

Ontogenic evolution of behavioural and electrophysiological responses to the serotonergic agents, quipazine and harmaline, was studied in the maturing rabbit in normal and pretreated conditions. As regards behavioural effects, tremor induced by quipazine was present from the first postnatal day and was antagonized by methysergide, but not by p-chlorophenylalanine (PCPA) or pretreatment with 5,7-dihydroxytryptamine (5,7-DHT). In contrast, tremor induced by harmaline could not be elicited before the second postnatal week and was partially antagonized by methysergide and 5,7-DHT, but not by PCPA. Electrophysiological studies of cell activity in the inferior olivary nucleus revealed a similar dependency on age since rhythmic activation of the inferior olivary nucleus could be registered from the first postnatal day with quipazine and only from the 8th postnatal day with harmaline; drug interactions with methysergide, PCPA and 5,7-DHT were the same as for the behavioural observations. It is suggested that quipazine directly activates serotonin receptors which are already present at birth, whereas harmaline requires the presence of serotonergic fibres for such activation.

High affinity uptake and Ca2+-dependent release of glutamic acid in the developing cerebellum

This study was undertaken in order to evaluate the postulated role of glutamic acid as the neurotransmitter for the parallel fibers of the cerebellar cortex. We studied the Ca2+-dependent release and the high affinity uptake of glutamic acid in the developing cerebellum. The Ca2+-dependent release of glutamic acid from cerebellar molecular layer during development closely follows the time course of parallel fibers synaptogenesis. Little glutamic acid release was observed at 15 days, then it increased to the adult values at the 21st postnatal day. In the rat the bulk of synapses of the parallel fibers appear between the 15th and the 21st postnatal days, the time at which the nerve terminals of the climbing fibers, the other excitatory input to the Purkinje cells, are already developed. An enhanced Na+-dependent, high affinity uptake of glutamic acid was observed in the developing cerebellum relative to the adult rat. That this higher accumulation of glutamic acid is not related to a releasable pool is suggested by the fact that an enhanced glutamic acid, Ca2+-dependent release relative to the adult was not observed. These results support the view that glutamic acid is the transmitter for the cerebellar parallel fibers.

Studies on the possible sites of chlordecone-induced tremor in rats

A series of psychopharmacological agents were administered to adult male Fischer-344 rats pretreated with a tremorigenic dose of chlordecone in an attempt to elucidate the involvement of spinal and supraspinal processes in the mediation and/or expression of chlordecone-induced tremor. Agents effective in attenuating the frequency of tremor were chlordiazepoxide, muscimol, and mecamylamine; quipazine exacerbated the tremor. Catecholaminergic agents including yohimbine, clonidine, propranolol, and haloperidol did not affect the frequency of chlordecone-induced tremor. Disinhibition of postsynaptic inhibitory sites in the spinal cord with strychnine and antagonism of spinal and supraspinal polysynaptic pathways with mephenesin exacerbated and attenuated the effects of chlordecone, respectively. Destruction of the climbing fibers with 3-acetylpyridine effectively blocked harmine, but not chlordecone-induced tremor, suggesting that chlordecone does not act through this pathway.