Ampakines reduce methamphetamine-driven rotation and activate neocortex in a regionally selective fashion.

It has been proposed that glutamatergic and dopaminergic systems are functionally opposed in their regulation of striatal output. The present study tested the effects of drugs that enhance AMPA-receptor-mediated glutamatergic transmission (ampakines) for their effects on dopamine-related alterations in cortical activity and locomotor behavior. Rats with unilateral 6-hydroxydopamine lesions of the ascending nigro-striatal dopamine system were sensitized to methamphetamine and then tested for methamphetamine-induced circling behavior in the presence and absence of ampakines CX546 and CX614. Both ampakines produced rapid, dose-dependent reductions in circling that were evident within 15 min and sustained through 1 h of behavioral testing. In situ hybridization maps of c-fos mRNA expression showed that in the intact hemisphere, ampakine cotreatment markedly increased c-fos expression in parietal, sensori-motor neocortex above that found in rats treated with methamphetamine alone. Ampakine cotreatment did not augment c-fos expression in frontal, sensori-motor cortex or striatum. Still larger ampakine-elicited effects were obtained in parietal cortex of the dopamine-depleted hemisphere where labeling densities were increased by approximately 60% above values found in methamphetamine-alone rats. With these effects, the hemispheric asymmetry of cortical activation was less pronounced in the ampakine-cotreatment group as compared with the methamphetamine-alone group. These results indicate that positive modulation of AMPA-type glutamate receptors 1) can offset behavioral disturbances arising from sensitized dopamine receptors and 2) increases aggregate neuronal activity in a regionally selective manner that is probably dependent upon behavioral demands.

Mapping brain networks engaged by, and changed by, learning.

Major goals of research into the neurobiology of learning and memory are to identify (1) brain areas/circuitries that subserve different mnemonic functions and (2) chemistries that encode the memory trace. The discovery that activity modulates neuronal gene expression provided techniques attendant to the first goal and candidates for cellular changes pertinent to the second. Studies in our laboratories have exploited activity-regulated changes in c-fos gene expression to map regions engaged in two-odor discrimination learning, with particular interest in neuronal groups in hippocampus and amygdala. The results of these studies demonstrate that the subdivisions of hippocampus and amygdala do not act in concert across behaviors but are differentially activated depending on task demands. In hippocampus, preferential activation of field CA3 was uniquely associated with initial learning of an odor pair, whereas predominant activation of CA1 occurred with exploration of a novel field and with overtrained responding to odors. The reappearance of precisely the same balance of subfield activation within disparate behavioral contexts was taken to suggest that the hippocampus has basic modes of function that recur in different circumstances and make rather generalized contributions to behavior. Within the amygdala, the basolateral division was most prominently active during task acquisition but not during performance of the well-learned discrimination. Indeed, the amygdala appeared to play the dominant role relative to hippocampus in the early stages of associating positive and negative valences with discriminative cues. These results demonstrate that the balance of neuronal activity both within and between limbic structures changes across sequential stages of odor learning in a fashion that is likely to define behavioral output.

Amyloid beta protein is internalized selectively by hippocampal field CA1 and causes neurons to accumulate amyloidogenic carboxyterminal fragments of the amyloid precursor protein.

A critical issue concerning Alzheimer's disease is its selectivity, which leads to cellular degeneration in certain brain areas but not in others, and whether this pathogenic selectivity involves products of the amyloid precursor protein (APP). Here, we show that the amyloid beta protein Abeta1-42 is accumulated gradually and is retained intact by field CA1, but not by other subdivisions, of organotypic hippocampal slice cultures. In contrast, the slightly shorter Abeta1-40 peptide was not sequestered selectively. Sequestration of Abeta1-42 was followed by the build-up of carboxyterminal fragments of the endogenous precursor protein that were identified by immunoprecipitation. Unlike the peptide uptake, this induction appeared to be stochastic at the cellular level. In addition, the APP fragments were distributed more broadly within the CA1 pyramidal neurons than the sequestered Abeta1-42, and they appeared to be localized to synaptic terminals in the molecular layer of the dentate gyrus and in the stratum lacunosum-moleculare of the subfield CA3. Concentrations of synaptophysin, a presynaptic marker, decreased as the number of neurons producing amyloidogenic species increased. These results indicate that exogenous Abeta1-42 sets into motion a sequence that involves 1) selective uptake of the peptide by vulnerable cells at risk in Alzheimer's disease, 2) markedly enhanced production of amyloidogenic precursor material, and 3) slow deterioration of central synapses.

Comparison of the effects of an ampakine with those of methamphetamine on aggregate neuronal activity in cortex versus striatum.

The present study used in situ hybridization to c-fos mRNA to compare the effects of an 'ampakine' (a positive modulator of AMPA type glutamate receptors) with those of methamphetamine on the balance of aggregate neuronal activity in the cortex versus striatum. Methamphetamine (n = 11) induced a marked increase in c-fos mRNA in the dorsomedial quadrant of the striatum and a 21% smaller, but still reliable, increase in the ventrolateral quadrant. The drug also elevated c-fos mRNA levels in the ventral and medial segments of the orbitofrontal cortex but had no detectable effects in motor and somatosensory neocortices. The ampakine (n = 11) caused a near inverse pattern of changes; i.e. a sizable increase in somatosensory labeling and a significant decrease in striatal labeling with statistically insignificant effects in motor and orbitofrontal cortex. Within-rat cortical and striatal values were correlated in both the vehicle (n = 11) and ampakine groups, and appropriate comparisons established that the ampakine caused 27-55% increases in the ratio of cortical to striatal labeling. These results are in accord with the idea that facilitation of glutamatergic transmission has 'network level' effects that are opposite in nature to those resulting from enhanced dopaminergic transmission. The potential relevance of ampakines alone or in conjunction with dopamine antagonists for the treatment of schizophrenia is discussed.

Differential patterns of c-fos mRNA expression in amygdala during successive stages of odor discrimination learning.

Expression of the activity-dependent gene c-fos was used to assess relative levels of neuronal activation in the amygdala and related structures of rats at different stages of odor discrimination learning. In situ hybridization was used to evaluate c-fos mRNA content within the amygdalar subdivisions, the bed nucleus of the stria terminalis, and the hippocampus. After initial exploration of the test apparatus, c-fos mRNA levels were increased in the medial and, to lesser extent, basolateral subdivisions and remained low in the central division. The balance of amygdala to hippocampal labeling favored hippocampus. Rats engaged in familiar nose-poke responses had comparably elevated labeling in the medial and basolateral divisions and low labeling densities in the central division. The ratio of hippocampal to amygdala labeling was at control levels. Rats required to switch from ad libitum responding to cued responding to odors had high basolateral to medial labeling ratios. This was in marked contrast to the medial dominance found in control and exploration rats. Hybridization was substantially more dense in basolateral amygdala than in hippocampal CA1; this imbalance was unique to the group required to form first associations between odors and rewards. Rats performing an overtrained odor discrimination had the least differentiation between amygdalar subdivisions of any behavioral group. The hippocampus-to-amygdala labeling ratio favored hippocampus and was nearly identical to the ratio in exploration rats. These results demonstrate that the balance of activity within the between limbic structures shifts according to behavioral demands. It is suggested that the balances reflect the availability of pertinent afferent cues, interactions between hippocampus and the extended amygdala, and relative levels of activity in the diffuse projections to the limbic system.

Regional patterns of c-fos mRNA expression in rat hippocampus following exploration of a novel environment versus performance of a well-learned discrimination.

Previous studies using c-fos cRNA in situ hybridization demonstrated a differential involvement of hippocampal subfields CA1 and CA3 in the acquisition of an olfactory discrimination (Hess et al., 1995). The present experiments employed the same method to examine changes in neuronal activity associated with two related behaviors: (1) initial exploration of the training apparatus and (2) performance of a well-learned odor discrimination. Rats in the two groups had similar labeling patterns within hippocampus indicating increased expression in all three major subfields with the greatest effect being in CA1. This pattern of "CA1 dominance" was notably different from that produced during early stages of two-odor discrimination learning in prior experiments. Hippocampal labeling in exploration and performance rats differed in that (1) hybridization was greater in CA1, CA3, and dentate gyrus in the former group and (2) a tendency for labeled cells to occur in clusters was more evident in exploration animals. Levels of c-fos mRNA in olfactory and visual structures were not predictive of expression patterns within hippocampus although labeling in piriform cortex and dentate gyrus was correlated in rats performing a well-practiced discrimination. Moreover, the pattern of hybridization in olfactory bulb was found to be behaviorally dependent. These results, together with those from previous studies, indicate that hippocampus has multiple patterns of regional activation but that one of these is common to very different behavioral circumstances. It is hypothesized that this common pattern emerges whenever the animal responds to distant cues using species-specific or well-learned behaviors and involves coordinated temporal convergence of sensory and septal/brainstem inputs.

Changes in c-fos mRNA expression in rat brain during odor discrimination learning: differential involvement of hippocampal subfields CA1 and CA3.

Levels of c-fos mRNA were measured with in situ hybridization to test for behaviorally dependent changes in neuronal activity in three subdivisions of hippocampus and in components of the olfactory and visual systems. In rats that performed a well-learned nose-poke response for water reward, c-fos mRNA levels were broadly increased, relative to values in home cage-control rats, in visual cortex, superior colliculus, olfactory bulb, and, to comparable levels, regions CA3 and CA1 of hippocampus; hybridization was not increased in the dentate gyrus. In rats first trained on the nose-poke behavior and then required to discriminate between two odors for water reward, the increase in c-fos mRNA was generally not as great and was more regionally differentiated. Thus, in olfactory bulb, hybridization was more greatly elevated in lateral than medial fields, thereby exhibiting regional activation corresponding to the topographic representation of the predominant odor sampled in the discrimination task. In hippocampus of odor-discrimination rats, c-fos mRNA levels were far greater in the region CA3 than region CA1, but remained at cage control values in stratum granulosum. Interestingly, c-fos mRNA levels in each hippocampal subdivision were highly correlated with levels in other regions (e.g., visual cortex) for home cage controls but not for rats in the two behavioral groups. Thus, c-fos mRNA levels in cage-control rats appeared to be regulated by some generalized factor acting throughout much of the brain (e.g., arousal), while odor-discrimination performance changed the pattern of expression within hippocampus, and allowed for a differentiated response by olfactory regions to emerge. These findings suggest that hippocampus possesses multiple modes of functioning and makes contributions to behavior that vary according to task demands.