Navigation and the underlying brain signals are influenced by various allothetic and idiothetic cues, depending on environmental conditions and task demands. Visual landmarks typically control navigation in familiar environments but, in the absence of landmarks, self-movement cues are able to guide navigation relatively accurately. These self-movement cues include signals from the vestibular system, and may originate in the semicircular canals or otolith organs. Here, we tested the otolithic contribution to navigation on a food-hoarding task in darkness and in light. The dark test prevented the use of visual cues and thus favored the use of self-movement information, whereas the light test allowed the use of both visual and non-visual cues. In darkness, tilted mice made shorter-duration stops during the outward journey, and made more circuitous homeward journeys than control mice; heading error, trip duration, and peak error were greater for tilted mice than for controls. In light, tilted mice also showed more circuitous homeward trips, but appeared to correct for errors during the journey; heading error, trip duration, and peak error were similar between groups. These results suggest that signals from the otolith organs are necessary for accurate homing performance in mice, with the greatest contribution in non-visual environments.

PurposeThe crayfish neuromuscular junction (NMJ) has been used as a study for central nervous system synaptic transmission due to its dual innervation by excitatory and inhibitory neurons to each muscle fiber. Likewise, the NMJs in the walking limb utilize glutamate and GABA as excitatory and inhibitory transmitters, respectively. Many neurotransmitters have been studied at this site, including serotonin (5HT). Application of 5HT locally enhances excitatory junction potential (EJP) amplitudes with excitatory nerve stimulation and can produce spontaneous, repetitive depolarizations of the post-synaptic membrane. This study characterizes automatic EJPs in the presence of 5HT at various concentrations and assesses concentration of threshold for automaticity.MethodsThe excitatory nerve innervating the dactylopodite opener muscle was isolated in first or second walking limbs of locally obtained crayfish (Oncogenes rusticus). Standard intracellular electrophysiological recording techniques were used to study amplitudes of EJPs evoked at 30 Hz. Control EJPs were recorded in Van Herreveld's solution (Van H) and averaged over 10 seconds following stimulus onset. The limb was bathed in 5HT concentrations ranging 0.5 to 500 μM. EJPs were obtained within 30 seconds of application and then again at 5 minutes. Recording were made of automatic EJPs to characterize frequency and amplitude changes.ResultsThreshold of automaticity occurred at 1.0 to 1.3 μM with onset of automaticity occurring from 2-3 min post-application of 5-HT. Automaticity at 5 μM had onset times of 1-2 min, 10 μM at 30 s, 25 μM at 0-30 s, and 50 μM occurring immediately post-application. There were significant trends toward increasing EJP amplitude and frequency which tended to stabilize 30-60 s post-application. In all instances, post-5HT washout resolved automatic EJPs.ConclusionAutomatic EJPs in post-synaptic muscle membrane without application of electrical stimulus in the presence of 5-HT was observed. There was an inversely graded response in time to onset with threshold to automaticity in the range of 1.0 to 1.3 μM 5-HT. Increasing frequency and amplitude tended to be seen within the first 30-60 s of automaticity onset, which then stabilized. Concentrations of 5-HT from 0.5 to 0.9 μM were assessed in three preparations without observing automatic EJPs.

PurposeThe crayfish neuromuscular junction (NMJ) has been used as a model for central nervous system synaptic transmission due to its dual innervation by excitatory (glutamate) and inhibitory (GABA) neurons to each muscle fiber. Local application of 5HT has demonstrated the ability to enhanced excitatory junction potential (EJP) amplitudes with excitatory nerve stimulation and can produce spontaneous, repetitive depolarizations of the post-synaptic membrane. The original purpose of this study was to (1) examine the phenomena of histeresis associated with automatic EJPs. Secondarily, we observed changes in responsiveness to 5HT near the end of local crayfish season, which has been studied and demonstrated by others.MethodsThe excitatory nerve innervating the dactylopodite opener muscle was isolated in first or second walking limbs of locally obtained crayfish (Oncogenes rusticus). Standard intracellular electrophysiological recording techniques were used to study amplitudes of EJPs evoked at 30 Hz. Control EJPs were recorded in Van Herreveld's solution and averaged over 10 seconds following the start of stimulus onset. The limb was bathed in concentrations of 5HT ranging from 0.5 to 500 μM. EJPs were obtained within 30 seconds of application and then again at 5 minutes. Recordings were made of automatic EJPs to characterize frequency and amplitude changes. When assessing for histeresis, the exchange of the bath was done by aspirating out the bath and replacing it with the next lower concentration.Results1. Histeresis was assessable only in one instance, and was not present in a drop from a threshold level of 1.1 μM to subthreshold 1.05 μM. 2. A significant change in responsiveness to 5HT was demonstrated over the last two weeks of locally available crayfish, in comparison to results 2-6 weeks prior. Threshold for automatic EJPs was seen at 1.0 to 1.3 μM from July 9 to Aug 19, all bathing in 5-HT approximately 2-3 minutes before onset. Thereafter, a transition of increasing threshold and onset to automaticity occurred until 8-25-04, at which point no automatic EJP spiking occurred in the presence of 5HT in the range of 1-500 μM.Conclusion(1) Histeresis was not seen in the study when decreasing from a threshold level of 1.1 μM 5HT to 1.05 μM 5HT. (2) The species of crayfish studied demonstrated a change in responsiveness to 5HT in regard to ability to elicit automatic EJPs in the range of 1.0 to 500 μM, which correlates with known seasonal variability in 5HT sensitivity.