More light on brains

Fine tuning of olfactory orientation behaviour by the interaction of oscillatory and single neuronal activity

We used a simple sensory and motor system to investigate the neuronal mechanisms of olfactory orientation behaviour. The main olfactory organs of terrestrial molluscs, the experimental animals used in this work, are located on the tips of their tentacles, which display complex movements when they explore a new environment. By reconstructing the trajectory of the tentacle tip ('nose') movements in three dimensions in freely moving snails, we showed that the protracted tentacles performed continuous scanning, both spontaneously and in response to odours. Odour applications elicited a brief startle-like quiver of the tentacle in a concentration-independent manner as well as a concentration-dependent contraction. Previous work showed that activation of an identified cerebral motoneuron, MtC3, produces tentacle contraction. Here we showed that in semi-intact preparations, MtC3 responded to odours in a concentration-dependent manner, similar to the tentacle contraction response to the same odours in intact animals. This observation suggests that MtC3 is involved in the central control of the scanning area by regulating the tentacle length. Using voltage-sensitive dyes and imaging, we demonstrated that during the hyperpolarizing phases of oscillations in the procerebral lobe, the main olfactory centre of the CNS of terrestrial molluscs, MtC3 spike frequency significantly decreased. We also showed that direct activation of the procerebral lobe resulted in the phasic inhibition of MtC3. This is therefore an example of an olfactory system in which the interaction of oscillatory and single neuronal activity plays an important role in the fine tuning of orientation behaviour to suit the particular odour environment.

Optical recording of responses to odor in olfactory structures of the nervous system in the terrestrial mollusk Helix

Electrophysiological methods and optical methods based on the use of potential-sensitive dyes were used to record stable rhythmic oscillations of local potentials in the olfactory structure (procerebrum) of the pulmonate mollusk Helix: these oscillations were generally similar to those previously observed in slugs. Odor had the effect of transiently altering rhythmic oscillations to generate an individual pattern. This is the first study describing the recording of procerebrum potentials evoked by presentation of odor, with mapping of the areas of propagation of these potentials relative to the areas of propagation of rhythmic oscillations. The boundary of the propagation of the evoked potential was essentially similar to the projection of the neuropil, and rhythmic oscillations were recorded in the projection layer of procerebrum cell bodies. Evoked potential waves appeared in areas corresponding to the site at which the olfactory nerve enters the cerebral ganglion (of which the procerebrum forms a part) and were propagated in the procerebrum neuropil towards the cell body layer. Evoked potentials did not provoke out-of-phase waves of rhythmic oscillations.

Optical recording of odor-evoked responses in the olfactory brain of the naïve and aversively trained terrestrial snails

Regular spontaneous oscillations were recorded both electro- and optophysiologically using a voltage-sensitive absorption dye in the olfactory part of the brain (procerebral lobe of the cerebral ganglia) of the gastropod mollusk Helix lucorum. Odor application caused transient changes in procerebral oscillations, and an odor-evoked potential was recorded in the procerebrum (PC). The wave of evoked potential originated near the place of olfactory nerve entrance into the PC and propagated via the procerebral neuropile toward the cell body layer. The spread of the odor-evoked potential corresponded roughly to the neuropile area, whereas the spontaneous oscillations were recorded in the cell body layer of the PC and were not observed in the neuropile. Evoked potential did not produce additional events intercalated into the ongoing spontaneous oscillations. Changes in parameters of spontaneous oscillations to the repeated presentations of the same odor were variable. To estimate the role of spontaneous oscillations in odor encoding, we trained the snail to avoid cineole, using paired presentations of cineole and electric shock. Elaboration of conditioned aversion to cineole applications resulted in distinct pairing-specific changes in behavior of the snails and procerebral activity. Responses to odor (cineole) applications were not different in amplitude or frequency of spontaneous oscillations in control and trained snails, whereas ratio of amplitudes of the same oscillation wave in proximal and distal regions of the procerebrum was significantly different in control and aversively trained snails, reflecting changes in neural firing in certain areas of the olfactory lobe.

Voltage-sensitive magnetic gels as magnetic resonance monitoring agents

Many polymer gels undergo a volume phase transition in electric fields. We report here the synthesis of a polyelectrolyte gel that incorporates magnetic particles of iron oxide into the polymer network; these particles couple the gel volume transition to the NMR relaxation times of the surrounding water. We find that the water proton relaxation rates (T2(-1)) in aqueous suspensions of particles of the magnetic gel increase significantly in an electric field. Such changes can also be induced by the hyperpolarization of red blood cells in the suspension, presumably as a result of the electric fields at the cell membrane surfaces. These gels may play a part in magnetic resonance imaging analogous to that of voltage-sensitive dyes in optical imaging.