Muscarinic receptors in the cerebellar vermis modulate the gain of the vestibulospinal reflexes in decerebrate cats
Andre, P.; Pompeiano, O.; Manzoni, D.; D'Ascanio, P.
Archives Italiennes de Biologie 130(3): 213-245
1992
ISSN/ISBN: 0003-9829 PMID: 1354955 Document Number: 389220
The Purkinje (P)-cells of the cerebellar vermis, which exert a prominent influence on posture as well as on the gain of vestibulospinal (VS) reflexes, are under the control not only of the classic mossy fibers and climbing fibers which liberate excitatory amino acids as neurotransmitter, but also of cholinergic afferents. The role of these afferents was investigated in precollicular decerebrate cats by using the method of local microinjection of cholinergic agents into appropriate areas of the cerebellar cortex. Unilateral injection into the vermal cortex of the culmen of the non-selective cholinergic agonist carbachol (0.25 .mu.l at 0.5 .mu.g/.mu.l saline) produced a postural asymmetry, characterized by a slight decrease of the extensor tonus in the ipsilateral forelimb and an increased tonus in the contralateral forelimb. Moreover, the gain of the EMG responses of the ipsilateral and the contralateral triceps brachii to animal tilt increased significantly, while no significant changes in the phase angle of the responses were observed. These effects started 5-10 min after the injection and persisted for at least 2 hours before disappearing. Similar but smaller effects were obtained after injection of eserine, an inhibitor of acetylcholinesterase. Thus, the effects could be produced by increasing the naturally present amount of acetylcholine (ACh). The changes in posture and gain of the VS reflexes described above utilized in part at least muscarinic receptors, since effects similar to those induced by carbachol injection were also obtained after unilateral microinjection into the vermal cortex of the culmen of the muscarinic agonist bethanechol (0.25 .mu.l at 0.1 .mu.g/.mu.l saline). On the other hand opposite effects, characterized by an increased postural activity in the ipsilateral forelimb associated with a decreased activity in the contralateral forelimb, as well as by a reduced gain of the EMG responses of the triceps brachii of both sides to animal tilt were observed in other experiments after local microinjection of the muscarinic antagonist scopolamine (0.25 .mu.l at 4-8 .mu.g/.mu.l saline). Evidence for muscarinic supersensitivity was obtained following repetitive injections of scopolamine into the cerebellar vermis. The area which upon injection of the cholinergic agents modified the postural activity as well as the gain of the vs reflexes was located within the third and/or the fourth folium rostral to the fissura prima (culmen), at the laterality of 1.4-1.8 mm with respect to the midline. This area correponded to the zone B of the cerebellar cortex, which receives a labyrinth input and projects to the ipsilateral lateral vestibular nucleus, where it exerts a prominent inhibitory influence. The results indicate the important role of the cholinergic system in the cerebellar control of the VS reflexes. Since most of the P-cells of the cerebellar vermis fire out-of-phase with respect to the related lateral VS neurons during animal tilt, it appears that ACh may act through muscarinic receptors by enhancing the amplitude of their modulation to labyrinth stimulation, thus increasing the gain of the VS reflexes.