Biomedical subjects
Kenneth J Lohmann
Publications and source records attributed to Kenneth J Lohmann.
Immunochemical and electrophysiological analyses of magnetically responsive neurons in the mollusc Tritonia diomedea.
Tritonia diomedea uses the Earth's magnetic field as an orientation cue, but little is known about the neural mechanisms that underlie magnetic orientation behavior in this or other animals. Six large, individually identifiable neurons in the brain of Tritonia (left and right Pd5, Pd6, Pd7) are known to respond with altered electrical activity to changes in earth-strength magnetic fields. In this study we used immunochemical, electrophysiological, and neuroanatomical techniques to investigate the function of the Pd5 neurons, the largest magnetically responsive cells. Immunocytochemical studies localized TPeps, neuropeptides isolated from Pd5, to dense-cored vesicles within the Pd5 somata and within neurites adjacent to ciliated foot epithelial cells. Anatomical analyses revealed that neurites from Pd5 are located within nerves innervating the ipsilateral foot and body wall. These results imply that Pd5 project to the foot and regulate ciliary beating through paracrine release. Electrophysiological recordings indicated that, although both LPd5 and RPd5 responded to the same magnetic stimuli, the pattern of spiking in the two cells differed. Given that TPeps increase ciliary beating and Tritonia locomotes using pedal cilia, our results are consistent with the hypothesis that Pd5 neurons control or modulate the ciliary activity involved in crawling during orientation behavior.
Disruption of magnetic orientation in hatchling loggerhead sea turtles by pulsed magnetic fields.
Loggerhead sea turtles (Caretta caretta) derive both directional and positional information from the Earth's magnetic field, but the mechanism underlying magnetic field detection in turtles has not been determined. One hypothesis is that crystals of biogenic, single-domain magnetite provide the physical basis of the magnetic sense. As a first step toward determining if magnetite is involved in sea turtle magnetoreception, hatchling loggerheads were exposed to pulsed magnetic fields (40 mT, 4 ms rise time) capable of altering the magnetic dipole moment of biogenic magnetite crystals. A control group of turtles was treated identically but not exposed to the pulsed fields. Both groups of turtles subsequently oriented toward a light source, implying that the pulsed fields did not disrupt the motivation to swim or the ability to maintain a consistent heading. However, when swimming in darkness under conditions in which turtles normally orient magnetically, control turtles oriented significantly toward the offshore migratory direction while those that were exposed to the magnetic pulses did not. These results are consistent with the hypothesis that at least part of the sea turtle magnetoreception system is based on magnetite. In principle, a magnetite-based magnetoreception system might be involved in detecting directional information, positional information, or both.
The physics and neurobiology of magnetoreception.
Diverse animals can detect magnetic fields but little is known about how they do so. Three main hypotheses of magnetic field perception have been proposed. Electrosensitive marine fish might detect the Earth's field through electromagnetic induction, but direct evidence that induction underlies magnetoreception in such fish has not been obtained. Studies in other animals have provided evidence that is consistent with two other mechanisms: biogenic magnetite and chemical reactions that are modulated by weak magnetic fields. Despite recent advances, however, magnetoreceptors have not been identified with certainty in any animal, and the mode of transduction for the magnetic sense remains unknown.
Animal behaviour: geomagnetic map used in sea-turtle navigation.
Migratory animals capable of navigating to a specific destination, and of compensating for an artificial displacement into unfamiliar territory, are thought to have a compass for maintaining their direction of travel and a map sense that enables them to know their location relative to their destination. Compasses are based on environmental cues such as the stars, the Sun, skylight polarization and magnetism, but little is known about the sensory mechanism responsible for the map sense. Here we show that the green sea-turtle (Chelonia mydas) has a map that is at least partly based on geomagnetic cues.
Identifiable neurons inhibited by Earth-strength magnetic stimuli in the mollusc Tritonia diomedea.
Diverse animals use the Earth's magnetic field as an orientation cue, but little is known about the sensory, processing and motor elements of the neural circuitry underlying magnetic orientation behavior. The marine mollusc Tritonia diomedea has both a magnetic compass sense and a simple nervous system accessible to electrophysiological analysis. Previous studies have revealed that four identifiable neurons, known as LPd5, RPd5, LPd6 and RPd6, respond with enhanced electrical activity to changes in Earth-strength magnetic fields. Here we report that two additional neurons, LPd7 and RPd7, are inhibited by magnetic stimuli. Cobalt fills of the Pd7 neurons indicated that two prominent neurites emerge from the soma and project to the periphery through the ipsilateral cerebral nerves CeN6 and CeN3; in some cases, a third neurite was visible in CeN2. The nerves extend to the anterior region of the animal where they innervate the lateral body walls, oral veil and mouth region. Action potentials in the Pd7 neurons propagate from the central ganglia toward the periphery. Thus, the Pd7 cells have characteristics of efferent neurons. The precise function of these cells during magnetic orientation behavior, however, remains to be determined.
Navigation and seasonal migratory orientation in juvenile sea turtles.
Juvenile loggerhead and green turtles that inhabit inshore waters of North Carolina, USA undertake long seasonal migrations, after which they often return to specific feeding areas. In addition, juvenile turtles are capable of homing to specific sites after being displaced. As a first step towards investigating the navigational mechanisms that underlie these movements, juvenile turtles were captured in coastal waters of North Carolina and displaced 30-167 km along circuitous routes while deprived of visual cues. At the testing location, turtles were tethered in a circular arena and permitted to swim while their orientation was monitored. Between May and September, when juvenile loggerhead and green turtles inhabit feeding areas along the North Carolina coast, turtles oriented in directions that corresponded closely with the most direct route back to their capture locations. During October and November, however, both loggerhead and green turtles oriented southward, a direction consistent with the migratory paths of turtles beginning their autumn migration. The results demonstrate for the first time that both homing and migratory orientation can be elicited in juvenile turtles under laboratory conditions in which orientation cues can be readily manipulated. In addition, the results provide evidence that juvenile loggerheads can assess their position relative to a goal using local cues available at the test site and are therefore capable of map-based navigation.
True navigation and magnetic maps in spiny lobsters.
Animals are capable of true navigation if, after displacement to a location where they have never been, they can determine their position relative to a goal without relying on familiar surroundings, cues that emanate from the destination, or information collected during the outward journey. So far, only a few animals, all vertebrates, have been shown to possess true navigation. Those few invertebrates that have been carefully studied return to target areas using path integration, landmark recognition, compass orientation and other mechanisms that cannot compensate for displacements into unfamiliar territory. Here we report, however, that the spiny lobster Panulirus argus oriented reliably towards a capture site when displaced 12-37 km to unfamiliar locations, even when deprived of all known orientation cues en route. Little is known about how lobsters and other animals determine position during true navigation. To test the hypothesis that lobsters derive positional information from the Earth's magnetic field, lobsters were exposed to fields replicating those that exist at specific locations in their environment. Lobsters tested in a field north of the capture site oriented themselves southwards, whereas those tested in a field south of the capture site oriented themselves northwards. These results imply that true navigation in spiny lobsters, and perhaps in other animals, is based on a magnetic map sense.
Identification of magnetically responsive neurons in the marine mollusc Tritonia diomedea.
Behavioral experiments have demonstrated that the marine mollusc Tritonia diomedea can use the Earth's magnetic field as an orientation cue. Little is known, however, about the neural mechanisms that underlie magnetic orientation behavior in this or any other animal. In previous studies, two neurons in the brain of Tritonia, known as LPd5 and RPd5, were shown to respond with enhanced electrical activity to changes in earth-strength magnetic fields. We report evidence that two additional neurons, known as LPd6 and RPd6, also respond with increases in electrical activity when the magnetic field around the animal is altered. Anatomical analyses revealed that prominent neurites from the Pd6 cells are located within two ipsilateral nerves, pedal nerves 1 and 2. These nerves extend to the periphery of the animal and innervate tissues of the anterior ipsilateral foot and body wall. Electrophysiological recordings demonstrated that action potentials generated by the Pd6 cells propagate from the central ganglia toward the periphery. These results imply that the Pd6 cells play an efferent role in the magnetic orientation circuitry. Given that these cells contain cilio-excitatory peptides and that Tritonia crawls using ciliary locomotion, the Pd6 neurons may control or modulate cilia used in crawling, turning, or both.
Magnet-induced disorientation in hatchling loggerhead sea turtles.
Laboratory experiments have indicated that hatchling loggerhead sea turtles (Caretta caretta) are able to establish and maintain courses using information from the Earth's magnetic field. In previous experiments, turtles were tested in relatively uniform magnetic fields generated by large coil systems surrounding an orientation arena. In this study, we investigated the orientation behavior of hatchlings with either magnets or magnetically inert brass bars attached to their carapaces. Control turtles (with brass bars) oriented significantly towards the east whereas turtles bearing magnets were not significantly oriented as a group. The two distributions were statistically different. These results indicate that magnetic orientation behavior of hatchling sea turtles can be disrupted by attaching a small magnet to the carapace. This finding may prove useful both in field experiments and in efforts to localize magnetoreceptors.
Use of multiple orientation cues by juvenile loggerhead sea turtles Caretta caretta.
Although the orientation cues used by hatchling sea turtles have been studied extensively, little is known about the mechanisms of orientation and navigation that guide older turtles. To investigate the orientation cues used by juvenile loggerheads Caretta caretta L., captured turtles were tethered in a water-filled arena located outdoors. Turtles tested under these conditions established and maintained headings in specific directions in the absence of wave cues, familiar landmarks and chemical gradients. Distorting the magnetic field around the anterior part of a turtle's body did not disrupt orientation if vision remained unimpaired. Similarly, eliminating visual cues by attaching frosted goggles did not disrupt orientation if the magnetic environment was undisturbed. However, when turtles experienced a simultaneous disruption of magnetic and visual cues, their orientation was altered. These results imply that sea turtles, like migratory birds and homing pigeons, are able to maintain headings using multiple sources of directional information.