Dissociations between behavioural recovery andrestoration of vestibular activity intheunilabyrinthecto mized guinea-pig

1.Intheguinea-pig, aunilateral labyrinthectomy induces postural disturbances andanocular nystagmus which abate ordisappear overtime. These behavioural changes areaccompanied by aninitial collapse andasubsequent restoration ofthespontaneous activity intheneurones ofthe ipsilateral vestibular nuclei. Recently, ithasbeenshownthatthevestibular neuronal activity remained collapsed overatleast 10hwhereas itsrestoration wascomplete 1weekafter the lesion. Theaimsofthis study weretodetermine whenrestoration ofspontaneous activity inthe partially deafferented vestibular neurones started andtocompare thetimecourses ofthe behavioural andneuronal recoveries inguinea-pigs thathadundergone a unilateral labyrinthectomy. 2.Neuronal discharge measurements weremadeusing chronic extracellular recording ofsingle unit activity. After a left labyrinthectomy, electrodes wereplaced onthesite ofthedestroyed labyrinth toenable stimulation oftheleft vestibular nerve. Behavioural measurements included chronic recording ofeyemovements bythescleral search coiltechnique. Aftera left labyrinthectomy, lateral deviation ofthehead, twisting ofthehead, andeyevelocity oftheslow phases ofthenystagmus weremeasured. 3.Theneuronal activity oftherostral part ofthevestibular nuclear complex onthelesioned side was recorded inalert guinea-pigs over4hrecording sessions between 12and72hafter thelesion. 4. Thecriterion usedtoselect vestibular neurones foranalysis wastheir recruitment byanelectric shock onthevestibular nerve. Inaddition, inorder toexplore auniform population, wefocused onneurones recruited atmonosynaptic latencies (0-85-1-15 ms). 5. Foreachrecording period, themeanresting rate wascalculated animal byanimal andthegrand meanofthese individual resting ratemeanswascalculated. Previously, adeline inthegrand meanresting ratefrom35-8+ 6-0spikes s-1(control state) to7-1+ 4-2spikes s-' during the first 4hafter labyrinthectomy hadbeenshown. Inthepresent study, thefirst sign ofrecovery wasobserved during the12-16hrecording period whentheresting rategrand meanincreased to16-3 + 3-9spikes s-l. Thisgrandmeanactivity didnotchange significantly during the following 12h.Thereafter, restoration ofneuronal activity improved andwascomplete 1week after thelesion. 6.Although theabatement ofthevestibular symptoms roughly paralleled therestoration of neuronal activity inthevestibular nuclei, somediscrepancies between thetimecourses ofboth phenomena emerged. Animportant step inpostural recovery (the animals managed tostand up) andamajor partoftheabatement ofthenystagmus occurred before therecovery ofvestibular neuronal activity. Inaddition, lateral deviation oftheheaddisappeared while restoration ofthe neuronal activity wasincomplete, butsignificant headtwisting wasstill evident whenvestibular resting rates hadrecovered completely.