Vestibular compensation in guinea pigs given intravenous lidocaine after unilateral labyrinthectomy

The effects of protein kinase C and calmodulin kinase II inhibitors on vestibular compensation in the guinea pig

Previous studies have demonstrated that vestibular compensation, the process of behavioural recovery which occurs following unilateral deafferentation of the vestibular labyrinth (UVD), is correlated with changes in in vitro phosphorylation of various protein substrates in the brainstem vestibular nucleus complex (VNC). The aim of the present study was to investigate the possible causal relationship between protein kinase activity and the induction of the vestibular compensation process, by delivering inhibitors of protein kinase C (PKC) or Ca(2+)/calmodulin-dependent kinase II (CaMKII) into the ipsilateral VNC at the time of the UVD and determining their effects on three static symptoms of UVD, spontaneous nystagmus (SN), yaw head tilt (YHT) and roll head tilt (RHT) in guinea pigs. Infusion of the PKC inhibitor, 3-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-4-(1H-indol-3-yl)-1H-pyrr ole-2,5-dione, HCl (bisindolylmaleimide I, HCl/GF 109203X, HCl) ('Bis I'), at a concentration of 5 or 50 microM, significantly increased SN frequency at the earliest time points (6 and 8 h post-UVD) compared to vehicle controls and the less selective analogue, 2,3-bis(1H-indol-3-yl)-N-methylmaleimide (bisindolylmaleimide V) ('Bis V'). However, the compensation of YHT and RHT was unaffected by the PKC inhibitor. By contrast, the cell-permeable CaMKII inhibitor, myristoylated autocamtide-2 related inhibitory peptide (N-Myr-Lys-Lys-Ala-Leu-Arg-Arg-Gln-Glu-Ala-Val-Asp-Ala-Leu-OH) ('myr-AIP') or the cell-impermeable analogue, autocamtide-2 related inhibitory peptide (N-Lys-Lys-Ala-Leu-Arg-Arg-Cln-Glu-Ala-Val-Asp-Ala-Leu-OH) ('AIP'), failed to alter the compensation of SN, YHT or RHT at any dose compared to vehicle controls. These results implicate PKC-, but not CaMKII-, signal transduction pathways in the initiation of SN compensation in guinea pig.

Molecular mechanisms of recovery from vestibular damage in mammals: recent advances

The aim of this review is to summarise and critically evaluate studies of vestibular compensation published over the last 2 years, with emphasis on those concerned with the molecular mechanisms of this process of lesion-induced plasticity. Recent studies of vestibular compensation have confirmed and extended the previous findings that: (i) compensation of the static ocular motor and postural symptoms occurs relatively rapidly and completely compared to the dynamic symptoms, many of which either do not compensate substantially or else compensate variably due to sensory substitution and the development of sensori-motor strategies which suppress or minimize symptoms; (ii) static compensation is associated with, and may be at least partially caused by a substantial recovery of resting activity in the ipsilateral vestibular nucleus complex (VNC), which starts to develop very quickly following the unilateral vestibular deafferentation (UVD) but does not correlate perfectly with the development of some aspects of static compensation (e.g., postural compensation); and (iii) many complex biochemical changes are occurring in the VNC, cerebellum and even areas of the central nervous system like the hippocampus, following UVD. However, despite many recent studies which suggest the importance of excitatory amino acid receptors such as the N-methyl-D-aspartate receptor, expression of immediate early gene proteins, glucocorticoids, neurotrophins and nitric oxide in the vestibular compensation process, how these various factors are linked and which of them may have a causal relationship with the physiological changes underlying compensation, remains to be determined.

Pharmacology of the vestibular system

In the past year significant advances have been made in our understanding of the neurochemistry and neuropharmacology of the peripheral and central vestibular systems. The recognition of the central importance of excitatory amino acids and their receptors at the level of the hair cells, vestibular nerve and vestibular nucleus has progressed further, and the role of nitric oxide in relation to activation of the N-methyl-D-aspartate receptor subtype is becoming increasingly clear. Increasing evidence suggests that excessive N-methyl-D-aspartate receptor activation and nitric oxide production after exposure to aminoglycoside antibiotics is a critical part of hair cell death, and new pharmacological strategies for preventing aminoglycoside ototoxicity are emerging as a result. Conversely, the use of aminoglycosides to lesion the peripheral vestibular system in the treatment of Meniere's disease has been studied intensively. In the vestibular nucleus, new studies suggest the importance of opioid, nociceptin and glucocorticoid receptors in the control of vestibular reflex function. Finally, the mechanisms of action and optimal use of antihistamines in the treatment of vestibular disorders has also received a great deal of attention.