Peter Thier — Page 2 | PubMed Health

PubMed2002

The role of the oculomotor vermis in the control of saccadic eye movements.

The oculomotor vermis is a part of the posterior cerebellum, characterized by a low threshold (<10 micro A) for evoked saccades. It comprises vermal lobuli VIc and VIIA. Many Purkinje cells in this area show eye position or saccade-related responses or combinations of the two and usually lack responses to the presentation of visual targets, guiding the oculomotor behavior. The saccade-related responses are directionally selective and show preferences for saccade amplitude or duration, which differ widely between cells. However, at the population level, these saccade-related Purkinje cells give a very precise account of the timing of the saccadic eye movement and, specifically, of the time it ends. This population signal might therefore contribute to determining the end of the saccadic eye movement. Furthermore, by changing the duration of the population response, the amplitude of the saccade could be changed. In other words, saccadic adaptation could be a consequence of changing a representation of time in the cerebellum.

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PubMed2002

Serotonergic control of cerebellar mossy fiber activity by modulation of signal transfer by rat pontine nuclei neurons.

Serotonergic modulation of precerebellar nuclei may be crucial for the function of the entire cerebellar system. To study the effects of serotonin (5-HT) on neurons located within the pontine nuclei (PN), the main source of cerebellar mossy fibers, we performed standard intracellular recordings from PN neurons in a slice preparation of the rat pontine brain stem. Application of 5 microM 5-HT significantly altered several intrinsic membrane properties of PN neurons. First, it depolarized the somatic membrane potential by 6.5 +/- 3.5 mV and increased the apparent input resistance from 49.5 +/- 14.6 to 62.7 +/- 21.1 MOmega. Second, 5-HT altered the I-V relationship of PN neurons: it decreased the inward rectification in hyperpolarizing direction, but increased it when depolarizing currents were applied. Third, it decreased the rheobase from 0.32 +/- 0.14 to 0.24 +/- 0.14 nA without affecting the firing threshold. Finally, the amplitude of medium-duration after hyperpolarizations was reduced from -14.9 +/- 2.0 to -12.3 +/- 2.4 mV. Together, these 5-HT effects on the intrinsic membrane properties result in an increase in excitability and instantaneous firing rate. In addition, application of 5 microM 5-HT also modulated postsynaptic potentials (PSPs) evoked by electric stimulations within the cerebral peduncle. The amplitude, maximal slope, and integral of these PSPs were reduced to 46.2 +/- 23.4%, 45.7 +/- 23.7%, and 61.4 +/- 28.4% of the control value, respectively. In contrast, we found no change in the decay and voltage dependence of PSPs. To test modulatory effects on short-term synaptic facilitation, we applied pairs of electrical stimuli at intervals between 10 and 1,000 ms. 5-HT selectively enhanced the paired-pulse facilitation for interstimulus-intervals >20 ms. The alteration of paired-pulse facilitation points to a presynaptic site of action for 5-HT effects on synaptic transmission. Pharmacological experiments suggested that pre- and postsynaptic effects of 5-HT were mediated by two different kinds of 5-HT receptors: changes in intrinsic membrane properties were blocked by the 5-HT(2) receptor antagonist cinanserin while the reduction of PSPs was prevented by the 5-HT(1) receptor antagonist cyanopindolol. In conclusion, 5-HT increases the excitability of PN neurons but decreases the synaptic transmission on them. The selective enhancement of synaptic facilitation may, however, allow high-frequency inputs to effectively drive PN neurons, thus the PN may act as a high-pass filter during periods of 5-HT release.

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