Comparison of the effects of ACTH-(4-10) on medial vestibular nucleus neurons in brainstem slices from labyrinthine-intact and compensated guinea pigs

Heterotrimeric guanosine triphosphate-binding protein-coupled modulatory actions of motilin on K+ channels and postsynaptic γ-aminobutyric acid receptors in mouse medial vestibular nuclear neurons

Some central nervous system neurons express receptors of gastrointestinal hormones, but their pharmacological actions are not well known. Previous anatomical and unit recording studies suggest that a group of cerebellar Purkinje cells express motilin receptors, and motilin depresses the spike discharges of vestibular nuclear neurons that receive direct cerebellar inhibition in rats or rabbits. Here, by the slice-patch recording method, we examined the pharmacological actions of motilin on the mouse medial vestibular nuclear neurons (MVNs), which play an important role in the control of ocular reflexes. A small number of MVNs, as well as cerebellar floccular Purkinje cells, were labeled with an anti-motilin receptor antibody. Bath application of motilin (0.1 μm) decreased the discharge frequency of spontaneous action potentials in a group of MVNs in a dose-dependent manner (K(d) , 0.03 μm). The motilin action on spontaneous action potentials was blocked by apamin (100 nm), a blocker of small-conductance Ca(2+) -activated K(+) channels. Furthermore, motilin enhanced the amplitudes of inhibitory postsynaptic currents (IPSCs) and miniature IPSCs, but did not affect the frequencies of miniature IPSCs. Intracellular application of pertussis toxin (PTx) (0.5 μg/μL) or guanosine triphosphate-γ-S (1 mm) depressed the motilin actions on both action potentials and IPSCs. Only 30% of MVNs examined on slices obtained from wild-type mice, but none of the GABAergic MVNs that were studied on slices obtained from vesicular γ-aminobutyric acid transporter-Venus transgenic mice, showed such a motilin response on action potentials and IPSCs. These findings suggest that motilin could modulate small-conductance Ca(2+) -activated K(+) channels and postsynaptic γ-aminobutyric acid receptors through heterotrimeric guanosine triphosphate-binding protein-coupled receptor in a group of glutamatergic MVNs.

The contribution of the intrinsic excitability of vestibular nucleus neurons to recovery from vestibular damage

Damage to the peripheral vestibular system results in a syndrome of ocular motor and postural abnormalities that partially and gradually abate over time in a process known as 'vestibular compensation'. The first, rapid, phase of compensation has been associated with a recovery of spontaneous resting activity in the ipsilateral vestibular nucleus complex (VNC), as a consequence of neuronal and synaptic plasticity. Increasing evidence suggests that normal VNC neurons in labyrinthine-intact animals, as well as ipsilateral VNC neurons following unilateral vestibular deafferentation (UVD), rely to some extent on intrinsic pacemaker activity provided by voltage-dependent conductances for their resting activity. Modification of this intrinsic pacemaker activity may underlie the recovery of resting activity that occurs in ipsilateral VNC neurons following UVD. This review summarizes and critically evaluates the 'intrinsic mechanism hypothesis', identifying discrepancies amongst the current evidence and suggesting experiments that may test it further.

An in vitro investigation of the effects of the ACTH/MSH(4-9) analogue, Org 2766, on guinea pig medial vestibular nucleus neurons

Vestibular compensation is a process of CNS plasticity that is correlated to a return of resting activity in medial vestibular nucleus (MVN) neurons ipsilateral to a peripheral vestibular deafferentation. Systemic administration of melanocortin peptides accelerates the compensation process; the ACTH/MSH(4-9) analogue, Org 2766, accelerates this process at smaller doses than ACTH/MSH(4-10). The present study investigated the effect of Org 2766 on MVN neurons in vitro using extracellular single-cell recording. Org 2766 was less potent at the neuronal level than ACTH/MSH(4-10). When Org 2766 and ACTH/MSH (4-10) were tested consecutively on the same neuron, the response was often different. Org 2766 and ACTH/MSH (4-10) may have a different mode and/or site of action.

alpha-Difluoromethylornithine delays behavioral recovery and induces decompensation after unilateral labyrinthectomy

Biochemical and pharmacologic studies suggest a role for the ornithine decarboxylase-polyamine system as a modulator of behavioral changes during vestibular compensation. alpha-Difluoromethylornithine specifically blocks the rate-limiting step of polyamine biosynthesis. To assess the effects of alpha-difluoromethylornithine on the acute phase of postural compensation, guinea pigs were divided into groups subjected to either unilateral labyrinthectomy only (n = 7), alpha-difluoromethylornithine (500 mg/kg/day) for 4 days before labyrinthectomy (n = 10), equivalent volumes of saline for 4 days before labyrinthectomy (n = 8), and sham operations (n = 5). Yaw head tilt and roll head tilt, trunk curvature, and air-righting reflex were measured at baseline and at regular intervals up to 4 weeks. alpha-Difluoromethylornithine significantly delayed recovery of normal air-righting but had no effect on yaw head tilt, roll head tilt, and trunk curvature. We also evaluated effects of alpha-difluoromethylornithine in compensated guinea pigs. Fully compensated animals from phase 1 were randomly assigned to receive alpha-difluoromethylornithine (500 mg/kg/day) or saline once daily for 4 days. Only 33% of alpha-difluoromethylornithine animals maintained air-righting, compared with 100% of saline-treated animals (p = 0.003). Maximum trunk curvature was greater in the alpha-difluoromethylornithine group (p = 0.02). Thus alpha-difluoromethylornithine not only delayed the time course for postural recovery after unilateral labyrinthectomy, it also transiently disrupted the maintenance of the compensated state.

A dose-response analysis of the beneficial effects of the ACTH-(4-9) analogue, Org 2766, on behavioural recovery following unilateral labyrinthectomy in guinea-pig

1. After removal of the peripheral vestibular receptors in one inner ear (unilateral labyrinthectomy, UL), oculo-motor and postural symptoms occur but disappear over time in a process of recovery known as vestibular compensation. 2. ACTH-(4-10), a fragment of the adrenocorticotrophic hormone (ACTH) molecule, which is devoid of corticotrophic activity, has been shown to enhance vestibular compensation. The present study investigated the effect of the ACTH-(4-9) analogue, Org 2766, on vestibular compensation in guinea-pig. Org 2766 is reported to be more potent behaviourally than ACTH-(4-10). 3. After UL, Org 2766 was delivered via an osmotic minipump implanted s.c. to 30 animals randomly assigned to one of five conditions: 1, 5, 10, 20 or 40 nmol kg-1 Org 2766, every 4 h for 52 h post-UL. Although infusion was continuous, in the present study the doses are expressed as nmol per 4 h in order to compare the results to a previous study in which animals received a discrete dose of ACTH-(4-10) at the end of each 4 h period. All animals were compared to saline controls (n = 6). 4. Three symptoms of UL, spontaneous ocular nystagmus, roll head tilt and yaw head tilt, were measured every 4 h for 52 h, beginning at 10 h post-UL. 5. Rates of infusion of 1, 5 and 10 nmol kg-1 accelerated spontaneous nystagmus compensation; 20 nmol kg-1 produced a significant decrease in the frequency of spontaneous nystagmus, as well as accelerating its compensation; 40 nmol kg-1 had no significant effect on spontaneous nystagmus compensation. 6. In comparison to the effects of Org 2766 on spontaneous nystagmus compensation, Org 2766 had little effect on the compensation of the postural symptoms, yaw head tilt and roll head tilt. Only 5 and 40 nmol kg-1 produced a significant change in postural compensation, and this was a reduction in the rate of roll head tilt compensation.7. At the optimal infusion rate of 20 nmol kg-1 every 4 h, Org 2766 produced a similar effect on spontaneous nystagmus compensation to that of ACTH-(4-10). However, Org 2766 was effective in accelerating spontaneous nystagmus compensation at much smaller doses per 4 h period than ACTH-(4-10). Org 2766 did not have the same effect on. postural compensation as it had on the compensation of spontaneous nystagmus.

Comparison of the effects of NMDA antagonists on medial vestibular nucleus neurons in brainstem slices from labyrinthine-intact and chronically labyrinthectomized guinea pigs

The responses of ipsilateral medial vestibular nucleus (MVN) neurons in brainstem slices from guinea pigs compensated for a unilateral labyrinthectomy (UL), to the N-methyl-D-asparate (NMDA) receptor/channel antagonists CPP and MK801, were compared with those of MVN neurons in brainstem slices from labyrinthine-intact guinea pigs observed in a previous study. The average resting activity of ipsilateral MVN neurons from compensated animals was significantly higher than that for MVN neurons from labyrinthine-intact animals; however, there were no significant differences in the average magnitude of the decrease in firing rate from baseline in response to CPP or MK801 and the only significant difference in the number of responses was to MK801, where fewer ipsilateral MVN neurons from compensated animals responded with a decrease in firing rate. These results suggest that vestibular compensation is not associated with an up-regulation or increased affinity of NMDA receptors in the MVN ipsilateral to the UL.