Cholinergic and noradrenergic stimulation in the rabbit flocculus have synergistic facilitatory effects on optokinetic responses

Acetylcholine Modulates Cerebellar Granule Cell Spiking by Regulating the Balance of Synaptic Excitation and Inhibition

Sensorimotor integration in the cerebellum is essential for refining motor output, and the first stage of this processing occurs in the granule cell layer. Recent evidence suggests that granule cell layer synaptic integration can be contextually modified, although the circuit mechanisms that could mediate such modulation remain largely unknown. Here we investigate the role of ACh in regulating granule cell layer synaptic integration in male rats and mice of both sexes. We find that Golgi cells, interneurons that provide the sole source of inhibition to the granule cell layer, express both nicotinic and muscarinic cholinergic receptors. While acute ACh application can modestly depolarize some Golgi cells, the net effect of longer, optogenetically induced ACh release is to strongly hyperpolarize Golgi cells. Golgi cell hyperpolarization by ACh leads to a significant reduction in both tonic and evoked granule cell synaptic inhibition. ACh also reduces glutamate release from mossy fibers by acting on presynaptic muscarinic receptors. Surprisingly, despite these consistent effects on Golgi cells and mossy fibers, ACh can either increase or decrease the spike probability of granule cells as measured by noninvasive cell-attached recordings. By constructing an integrate-and-fire model of granule cell layer population activity, we find that the direction of spike rate modulation can be accounted for predominately by the initial balance of excitation and inhibition onto individual granule cells. Together, these experiments demonstrate that ACh can modulate population-level granule cell responses by altering the ratios of excitation and inhibition at the first stage of cerebellar processing.SIGNIFICANCE STATEMENT The cerebellum plays a key role in motor control and motor learning. While it is known that behavioral context can modify motor learning, the circuit basis of such modulation has remained unclear. Here we find that a key neuromodulator, ACh, can alter the balance of excitation and inhibition at the first stage of cerebellar processing. These results suggest that ACh could play a key role in altering cerebellar learning by modifying how sensorimotor input is represented at the input layer of the cerebellum.

Differential regulations of vestibulo-ocular reflex and optokinetic response by β- and α2-adrenergic receptors in the cerebellar flocculus

Norepinephrine modulates synaptic plasticity in various brain regions and is implicated in memory formation, consolidation and retrieval. The cerebellum is involved in motor learning, and adaptations of the vestibulo-ocular reflex (VOR) and optokinetic response (OKR) have been studied as models of cerebellum-dependent motor learning. Previous studies showed the involvement of adrenergic systems in the regulation of VOR, OKR and cerebellar synaptic functions. Here, we show differential contributions of β- and α2-adrenergic receptors in the mouse cerebellar flocculus to VOR and OKR control. Effects of application of β- or α2-adrenergic agonist or antagonist into the flocculus suggest that the β-adrenergic receptor activity maintains the VOR gain at high levels and contributes to adaptation of OKR, and that α2-adrenergic receptor counteracts the β-receptor activity in VOR and OKR control. We also examined effects of norepinephrine application, and the results suggest that norepinephrine regulates VOR and OKR through β-adrenergic receptor at low concentrations and through α2-receptor at high concentrations.

Modulation of Purkinje cell response to glutamate during the sleep-waking cycle

The hypothesis that corticocerebellar responsiveness is modified by the behavioral state was tested in freely moving rats by evaluating the responses of extracellularly recorded Purkinje cells located in the cerebellar posterior vermis to microiontophoretically applied glutamate (8-80 nA for 3-5 s every 30-32 s) during the spontaneous sleep-waking cycle. Rats were chronically implanted for polygraphic recordings so that responses of Purkinje cells to glutamate could be related to the states of quiet waking, slow-wave sleep and paradoxical sleep. Analysis on a population of 33 neurons subjected to alternate periods of sleep and waking showed that the mean response to glutamate was significantly reduced to 75+/-18% during slow-wave sleep with respect to waking. This effect occurred independently on changes of basal firing rate which in sleep was slightly, although significantly, reduced to 94+/-12%. Independence of glutamate response modulation from changes of baseline firing was also observed in a different data set obtained from 19 Purkinje cells which were recorded during a continuous slow-wave sleep period that allowed several consecutive drug applications. In this condition responses to glutamate progressively decreased as sleep proceeded while spontaneous activity remained stable after a slight decrease at the transition from waking to sleep. Spectral analysis performed on the electroencephalogram signal, in particular on epochs centered around each glutamate pulse, revealed that for both data sets the reduction of neuronal responsiveness was related to the intensity of slow-wave sleep and more precisely to the delta and slow oscillation (0.6-4.2 Hz) content of the power spectrum of the electroencephalogram. Spontaneous and glutamate-evoked activity were also evaluated in 23 Purkinje cells during transition from slow-wave sleep to paradoxical sleep. In particular, during paradoxical sleep spontaneous activity became irregular so that for 44 out of 90 glutamate responses quantification was unreliable. The remaining 46 responses were characterized by high variability in amplitude even within the same episode of paradoxical sleep. With respect to the preceding slow-wave sleep values, 17/46 responses increased, 14/46 decreased and 15/46 remained within the 15% limit, giving a mean value of 132%. These data indicate that Purkinje cell response to glutamate is modulated during the spontaneous sleep-waking cycle. We speculate that this modulation depends upon the action of the neuromodulatory systems which diffusely project to the cerebellum, whose function would be to adapt the performance of the cerebellar circuits to changes of the animal state. On the other hand, the phasic changes in amplitude of Purkinje cell response during paradoxical sleep could be due to the interaction between the effects of glutamate application and those exerted by endogenous signals possibly related to the phasic events of this sleep stage.

Light-microscopic distribution and parasagittal organisation of muscarinic receptors in rabbit cerebellar cortex

Recent studies on the effects of intrafloccular injections of muscarinic agonists and antagonists on compensatory eye movements in rabbit, indicate that muscarinic receptors may play a modulatory role in the rabbit cerebellar circuitry. It was previously demonstrated by Neustadt et al. (1988), that muscarinic receptors in rabbit cerebellar cortex are distributed into alternating longitudinal zones of very high and very low receptor density. In the present study, the zonal and cellular distribution of muscarinic receptors in the rabbit cerebellar cortex is investigated in detail using in vitro ligand autoradiography with the non-selective high-affinity antagonist [3H]quinuclidinyl benzilate (QNB), and the M2-specific antagonist [3H]AF-DX384, and immunocytochemistry with a monoclonal antibody specific for the cloned m2 muscarinic receptor protein. [3H]QNB and [3H]AF-DX384 binding sites and m2-immunoreactivity had similar overall distributions: dense labeling occurred in the dendritic arbors of a subset of Purkinje cells that are organized into parasagittal bands. A high level of muscarinic receptor labeling was also observed in a thin substratum of the molecular layer immediately above the Purkinje cell layer of the vestibulo-cerebellar lobules, i.e. the nodulus, the ventral uvula and the flocculus. Labeling in this stratum was associated with densely packed fibres, which were putatively identified as parallel fibres. Also Golgi cells, which were localized in part in the molecular layer, and a subset of mossy fibre rosettes, primarily concentrated in lobule VI, were immunoreactive for the m2 receptor. The parasagittal band of labeled Purkinje cell dendrites were most prominent in the anterior lobe (lobules I-V), in crus 1 and 2, in the flocculus, the ventral paraflocculus and the rostral folium of the nodulus. In other lobules, only infrequent Purkinje cells contained muscarinic receptors. The parasagittal organisation of muscarinic receptors differed from that of zebrin I, a Purkinje cell-specific protein which is often used as a marker of parasagittal parcelation of the cerebellar cortex. In the anterior lobe, however, there was a partial correspondence between muscarinic receptor and zebrin I bands. In the flocculus the distribution of muscarinic-receptor-positive Purkinje cells was related to the distinct white matter compartments as revealed with acetylcholinesterase (AChE) histochemistry. Muscarinic receptor-containing Purkinje cells were located primarily in the floccular zone 1, which is implicated in the control of eye movements about a horizontal axis. In order to relate the distribution of muscarinic receptor labeling to that of cholinergic nerve terminals, [3H]QNB binding sites and sodium-dependent [3H]hemicholinium-3 binding were compared. Sodium-dependent [3H]hemicholinium-3 binding sites mainly occurred in the granule cell layer of the vestibulo-cerebellum, which corresponds well with the distribution of the acetylcholine synthesizing enzyme, choline acetyltransferase (ChAT). However, sodium-dependent [3H]hemicholinium binding complemented, rather than co-localized with, muscarinic receptors which were primarily distributed in the molecular layer of the lobules of the vestibulo-cerebellar lobules. Their functional significance is puzzling, since their distribution does not correspond to that of markers of cholinergic innervation.

Unilateral cholinergic stimulation of the rabbit's cerebellar flocculus: asymmetric effects on optokinetic responses

In previous work, we have demonstrated an acceleration of the buildup of slow-phase velocity of optokinetic nystagmus (OKN) after bilateral floccular injection of the aselective cholinergic agonist carbachol (Tan and Collewijn 1991; Tan et al. 1992a). In the present study we investigated the effects of unilateral floccular injections of carbachol. Such unilateral injections specifically enhanced the buildup of OKN slow-phase velocity in the direction toward the injected flocculus (ipsiversive). During binocular optokinetic stimulation, this enhancement was expressed in the motion of both eyes. Acceleration of the eye contralateral to the injected flocculus increased from 1 to about 2 degrees/s2, while the acceleration of the ipsilateral eye increased from 1 to about 1.5 degrees/s2. In contrast, buildup of contraversive OKN was unchanged. No changes were found in the steady-state OKN and optokinetic afternystagmus (OKAN). Monocular optokinetic stimulation was only effective in the nasal direction, and the effects of unilateral injection of carbachol were disconjugate. Ipsiversive OKN was enhanced only in the contralateral, seeing eye, while the response of the ipsilateral, covered eye was unchanged. We hypothesize that the directionally specific effect of unilateral cholinergic floccular stimulation on OKN is due to enhancement of predominantly the excitatory phase of modulation of the Purkinje cell's simple-spike activity by carbachol, without a marked effect of carbachol on the inhibitory phase of simple-spike modulation.

Muscarinic nature of cholinergic receptors in the cerebellar flocculus involved in the enhancement of the rabbit's optokinetic response

Intrafloccular micro-injection of the aselective cholinergic agonist carbachol enhances the optokinetic reflex (OKR)17. Histochemical and physiological studies have identified cholinergic receptors of the muscarinic as well as nicotinic type in the cerebellar cortex, and both have been implicated in cholinergic transmission. The present study was undertaken to elucidate the receptor type involved in the control of OKR. For that purpose, effects of injections of the nicotinic N1 agonist DMPP on the OKR and vestibulo-ocular reflex (VOR) were compared with injections of the muscarinic agonist betanechol and the aselective cholinergic agonist carbachol. Injection of betanechol mimicked the enhancement of the OKR by carbachol, while DMPP had no effect. We conclude that muscarinic receptors are involved in the positive modulatory action of the cholinergic system in the cerebellar flocculus.

Optokinetic nystagmus in the rabbit and its modulation by bilateral microinjection of carbachol in the cerebellar flocculus

1. In the alert, pigmented rabbit, eye movements were recorded during optokinetic nystagmus (OKN) and during optokinetic after nystagmus (OKAN). These responses were elicited by steps in surround-velocity ranging from 5-110 degrees/s during binocular as well as monocular viewing. 2. In the baseline condition, OKN showed an approximately linear build-up of eye velocity to a steady-state, followed by a linear decay of eye velocity during OKAN after the lights were turned off. Build-up during binocular viewing was characterized by a constant, maximum eye-acceleration (about 1 degree/s2) for stimulus velocities up to 60 degrees/s. OKAN, instead, was characterized by a fixed duration (about 10 s) for stimulus velocities up to 20 degrees/s. Steady-state eye velocity saturated at about 50 degrees/s. 3. Monocular stimulation in the preferred (nasal) direction elicited a build-up that was on average twice as slow as during binocular stimulation. Steady-state velocity during monocular stimulation saturated at about 20 degrees/s. OKAN was of equal duration as during binocular stimulation. In the non-preferred direction, a very irregular nystagmus was elicited without velocity build-up. The stronger response to binocular stimulation, compared to the responses under monocular viewing condition in either nasal and temporal direction suggests potentiation of the signals of either eye during binocular viewing. 4. OKN and OKAN were re-assessed after intra-floccular micro-injection of the nonselective cholinergic agonist carbachol. In the binocular viewing condition, eye-acceleration during build-up was strongly enhanced from 1 degree/s2 before to 2.5 degrees/s2 after injection. The saturation level of steady-state eye velocity was also increased, from 50 degrees/s before to more than 60 degrees/s after carbachol. The duration of OKAN, however, was shortened from 10 s before to 6 s after injection. The response to monocular stimulation in the preferred direction revealed similar changes. 5. The flocculus appears to be involved in the control of the dynamics of OKN in the rabbit. Cholinergic mechanisms affect the floccular control of the rate at which slow-phase velocity can be built up and the rate of decay of eye velocity during OKAN. Cholinergic stimulation of the flocculus enhances the dynamics of OKN, while velocity storage is shortened.