The hippocampal-mamillary system: anterograde and retrograde amnesia

Long-term memory loss and the effects of tianeptine in young and aged mice

Previous experiments have shown that tianeptine, a new psychotropic agent with antidepressant properties, improves performance in several learning and memory tasks in mice. In more recent investigations, tianeptine was found to completely alleviate working memory deficits produced by long-term alcohol intoxication and to prevent abnormal memory loss in aged animals (Jaffard et al, 1991b, 1991c). The aim of this paper was to examine how this last finding may be integrated in our understanding of brain mechanisms involved in memory loss. For this purpose, we present a brief review of experimental data and theories which, at different levels of analysis, seems to be relevant to this issue. Together with the subsequent examination of the conditions in which tianeptine was found to improve long-term retention, we suggest that: i) long-term memory loss would be largely determined by the initial encoding of information, so that ii) tianeptine would help aged animals to use spatial mapping or configural associations more efficiently at the time of initial acquisition. This, in turn, suggests that one of the main brain target sites for tianeptine in enhancing memory is the hippocampal formation.

Re-evaluating the role of the mammillary bodies in memory

Although the mammillary bodies were among the first brain regions to be implicated in amnesia, the functional importance of this structure for memory has been questioned over the intervening years. Recent patient studies have, however, re-established the mammillary bodies, and their projections to the anterior thalamus via the mammillothalamic tract, as being crucial for recollective memory. Complementary animal research has also made substantial advances in recent years by determining the electrophysiological, neurochemical, anatomical and functional properties of the mammillary bodies. Mammillary body and mammillothalamic tract lesions in rats impair performance on a number of spatial memory tasks and these deficits are consistent with impoverished spatial encoding. The mammillary bodies have traditionally been considered a hippocampal relay which is consistent with the equivalent deficits seen following lesions of the mammillary bodies or their major efferents, the mammillothalamic tract. However, recent findings suggest that the mammillary bodies may have a role in memory that is independent of their hippocampal formation afferents; instead, the ventral tegmental nucleus of Gudden could be providing critical mammillary body inputs needed to support mnemonic processes. Finally, it is now apparent that the medial and lateral mammillary nuclei should be considered separately and initial research indicates that the medial mammillary nucleus is predominantly responsible for the spatial memory deficits following mammillary body lesions in rats.

Lesions of the mammillothalamic tract impair the acquisition of spatial but not nonspatial contextual conditional discriminations

This study examined the influence of selective mammillothalamic tract lesions in rats on the acquisition of two kinds of contextual conditional discrimination: one involving two contexts (A and B) that differed in their visuo-spatial properties and another involving two contexts (C and D) that differed in temperature. In contexts A (and C) presentations of a tone were paired with food whereas presentations of a clicker were not; and in contexts B (and D) presentations of the clicker were paired with food whereas those of the tone were not. Mammillothalamic tract lesions disrupted initial acquisition of the conditional discrimination involving visual contexts (A and B), but not the formally equivalent discrimination involving thermal contexts (C and D). These results provide support for the suggestion that mammillothalamic tract lesions disrupt visuo-spatial encoding.

Evidence of a spatial encoding deficit in rats with lesions of the mammillary bodies or mammillothalamic tract

The present study sought to identify the role of the mammillary bodies and their projections to the anterior thalamic nuclei for spatial memory. Rats with either selective, neurotoxic mammillary body lesions or discrete mammillothalamic tract lesions were tested on various spatial working memory tasks. Tests using the T-maze, radial-arm maze, and water maze were manipulated to compare three possible theories of mammillary body function by increasing proactive interference, increasing retention interval, and taxing the rapid processing of novel spatial stimuli. On T-maze alternation and radial-arm maze tasks, both lesion groups were initially impaired but seemed to recover. Transfer tests revealed, however, a more permanent change in performance, suggesting a failure to use distal (allocentric) cues. Consistent with this, both groups were also impaired at matching-to-place in the water maze and showed little improvement with practice. Nevertheless, once the lesion groups had been trained on a task, they were not affected differentially either by an increase of proactive interference or by retention intervals of up to 30 min. Although both mammillary body and mammillothalamic tract lesions resulted in similar impairments, the mammillothalamic tract group was the more affected when acquiring new spatial information. Together, these results suggest that mammillary body damage causes an encoding deficit when learning new spatial tasks, resulting in a suboptimal mode of performance, which may reflect a loss of directional heading information.

Lesions of the posterior insular cortex impair water maze performance in the rat

Performance of rats in the Morris water maze was measured after small excitotoxic lesions were produced bilaterally in two areas in the insular cortex, 0.3 and 2.3 mm posterior to bregma in the upper bank of the rhinal sulcus. The rats were trained and tested in two successive days to find an underwater platform. Compared to intact animals groups with lesions at AP-0.3 and AP-2.3 were impaired as measured by latency. These animals also did not prefer the correct quadrant while searching for the platform. There were no differences between the groups in percentage of time spent in periphery and speed of swimming. The results do not support the hypothesis that the destruction of an autonomic center at AP-0.3 will impair performance while the destruction of a more posterior part of the insular cortex will not, and are interpreted as further evidence for multifactor sensitivity of the water maze task.

Cognitive maps beyond the hippocampus

We present a conceptual framework for the role of the hippocampus and its afferent and efferent structures in rodent navigation. Our proposal is compatible with the behavioral, neurophysiological, anatomical, and neuropharmacological literature, and suggests a number of practical experiments that could support or refute it. We begin with a review of place cells and how the place code for an environment might be aligned with sensory cues and updated by self-motion information. The existence of place fields in the dark suggests that location information is maintained by path integration, which requires an internal representation of direction of motion. This leads to a consideration of the organization of the rodent head direction system, and thence into a discussion of the computational structure and anatomical locus of the path integrator. If the place code is used in navigation, there must be a mechanism for selecting an action based on this information. We review evidence that the nucleus accumbens subserves this function. From there, we move to interactions between the hippocampal system and the environment, emphasizing mechanisms for learning novel environments and for aligning the various subsystems upon re-entry into familiar environments. We conclude with a discussion of the relationship between navigation and declarative memory.

Local norepinephrine depletion and learning-related neuronal activity in cingulate cortex and anterior thalamus of rabbits

Multi-unit neuronal activity was recorded in posterior cingulate cortex (area 29) and the anterior ventral (AV) thalamic nucleus during discriminative instrumental avoidance learning wherein a response (stepping in an activity wheel) to a 0.5-s tone (CS+) prevented a foot-shock 5 s after CS+ onset. Presentations of a different tone (CS-) on 50% of the conditioning trials in an irregular sequence with the CS+ did not predict shock and thus required no response. Two groups of rabbits received intracranial micro-injections of 6-hydroxydopamine (6-OHDA) to locally deplete the NE in area 29 or the AV nucleus. Vehicle was injected in the non-depleted area in each group and a third group received vehicle injections in both areas. Dopamine neurons in subjects that received 6-OHDA were protected by pre-treatment with GBR-12909. Neuronal data were collected during two pre-training sessions in response to the tones only and when the tones and shock were presented unpaired. Thalamically depleted rabbits made more, and cortically depleted rabbits made fewer, avoidance responses than controls during the early stages of behavioral acquisition, and cortically depleted rabbits made fewer responses than controls and thalamically depleted rabbits during extinction testing administered after the completion of acquisition. One effect of NE depletion on neuronal activity was entirely local: elimination of neuronal sensitization effects (enhanced discharges elicited by tones during the unpaired tone-shock pre-training treatment relative to pre-training with tones only). Other neuronal effects of NE depletion were system-wide, i.e., they occurred whether the depletion was cortical or thalamic.(ABSTRACT TRUNCATED AT 250 WORDS)