Superior colliculus and active navigation: role of visual and non-visual cues in controlling cellular representations of space.

To begin investigation of the contribution of the superior colliculus to unrestrained navigation, the nature of behavioral representation by individual neurons was identified as rats performed a spatial memory task. Similar to what has been observed for hippocampus, many superior collicular cells showed elevated firing as animals traversed particular locations on the maze, and also during directional movement. However, when compared to hippocampal place fields, superior collicular location fields were found to be more broad and did not exhibit mnemonic properties. Organism-centered spatial coding was illustrated by other neurons that discharged preferentially during right or left turns made by the animal on the maze, or after lateralized sensory presentation of somatosensory, visual, or auditory stimuli. Nonspatial movement-related neurons increased or decreased firing when animals engaged in specific behaviors on the maze regardless of location or direction of movement. Manipulations of the visual environment showed that many, but not all, spatial cells were dependent on visual information. The majority of movement-related cells, however, did not require visual information to establish or maintain the correlates. Several superior collicular cells fired in response to multiple maze behaviors; in some of these cases a dissociation of visual sensitivity to one component of the behavioral correlate, but not the other, could be achieved for a single cell. This suggests that multiple modalities influence the activity of single neurons in superior colliculus of behaving rats. Similarly, several sensory-related cells showed dramatic increases in firing rate during the presentation of multisensory stimuli compared to the unimodal stimuli. These data reveal for the first time how previous findings of sensory/motor representation by the superior colliculus of restrained/anesthetized animals might be manifested in freely behaving rats performing a navigational task. Furthermore, the findings of both visually dependent and visually independent spatial coding suggest that superior colliculus may be involved in sending visual information for establishing spatial representations in efferent structures and for directing spatially-guided movements.

Reversible inactivation of the lateral dorsal thalamus disrupts hippocampal place representation and impairs spatial learning.

Place-specific discharge of hippocampal cells was monitored while rats performed daily 15 trials of a spatial memory task. During the intertrial interval between trials 5 and 6, the lateral dorsal nucleus of the thalamus (LDN) was reversibly inactivated. Choice accuracy on the maze became impaired, and many hippocampal place fields became disrupted. These data support the proposition that the LDN passes onto hippocampus important (spatial) information that is used for accurate maze navigation.

Analgesic effects of intraventricular and intrathecal injection of morphine and ketocyclazocine in the infant rat.

Little is known of the neural bases of analgesia in immature animals. This experiment examined the effects of intracerebroventricular (i.c.v.) and intrathecal (i.t.) administration of morphine or ketocyclazocine in tests of antinociception in rats aged 3 to 14 days of age. Analgesia tests were conducted using both thermal and mechanical (pressure) noxious stimuli applied to the forepaw, hindpaw or tail. In the 3-day-old morphine-injected i.c.v. produced analgesia in the forepaws when either the mechanical or thermal noxious stimulus was used. There was no effect when the hindpaw or tail was tested. At 10 days of age, when the mechanical stimulus was used, morphine was analgesic in tests on all three appendages but was only effective in the forepaw when the thermal stimulus was used. Morphine was fully effective in all tests with both stimuli at 14 days of age. Ketocyclazocine had no consistent effect when given i.c.v. When injected i.t., morphine produced analgesia in the forepaws in the thermal test at 4 days of age and in all appendages by 10 days. When the mechanical test was used, morphine was effective in all appendages at all ages tested. Ketocyclazocine was analgesic at all appendages for the mechanical stimulus at all ages but was only transiently effective in the thermal test. The results demonstrate differential development of analgesia mediated at different levels of the neural axis and are consistent with the development of descending inhibitory that may mediate analgesia induced by i.c.v. injections of morphine. Neural mechanisms that are involved in the analgesic effects of these drugs against the two types of stimuli are also developmentally distinct.