Lawrence P. Reagan

The effects of acute and repeated stress on expression of the early immediate gene c‐fos in the basolateral amygdala have previously been reported; however, characterization of which neuronal subpopulations are activated by these stimuli has not been investigated. This question is of considerable relevance, insofar as the basolateral amygdala houses a heterogeneous population of neurons, including those of γ‐aminobutyric acid (GABA)‐ergic and glutamatergic phenotypes that may be subcategorized based on their expression of various calcium‐binding proteins, including parvalbumin, calbindin, calretinin, and the calcium‐sensitive enzyme calcium/calmodulin‐dependent kinase II. Characterization of these subpopulations has revealed unique differences in their physiology, synaptology, and morphology, suggesting that each distinct phenotype may have profound effects on the local circuitry of the amygdala. Therefore, we examined the effects of acute and repeated restraint stress on expression of the immediate early gene c‐fos in neurons containing parvalbumin, calbindin, calretinin, or calcium/calmodulin‐dependent kinase II in the basolateral amygdala. Double‐label immunohistochemistry revealed that acute restraint stress activated a proportion of parvalbumin‐, calbindin‐, or calcium/calmodulin‐dependent kinase II‐positive neurons. Prior exposure to repeated restraint stress markedly attenuated acute‐stress mediated activation of these neuronal populations, although not equally. Expression of c‐Fos protein was not detected in calretinin‐positive neurons in any experimental group. These results demonstrate that distinct neuronal phenotypes in the basolateral amygdala are activated by acute restraint stress and that prior repeated restraint stress differentially affects this response. J. Comp. Neurol. 508:458–472, 2008.

Depressive illness is associated with changes in amygdalar volume, and stressful life events are known to precipitate depressive episodes in this patient population. Stress affects amygdalar synaptic plasticity and several neurotransmitter systems have been implicated in stress‐mediated changes in the brain, including the glutamatergic system. However, the role of the glutamatergic system in stress‐mediated plasticity in the amygdala remains to be determined. Accordingly the current study examined the stress modulation of extracellular glutamate levels in the basolateral nucleus (BLA) and the central nucleus (CeA) of the amygdala byin vivomicrodialysis. Acute stress increased extracellular glutamate levels in the BLA and CeA, although the dynamics of these stress‐mediated changes were dramatically different in these amygdalar nuclei. Tetrodotoxin administration reduced basal, and completely eliminated stress‐mediated increases in glutamate efflux in the amygdala, demonstrating that stress effects are dependent on local axonal depolarization. Moreover, stress‐mediated increases in glutamate efflux in the BLA were inhibited by the antidepressant tianeptine but not by the selective serotonin‐reuptake inhibitor fluoxetine. Collectively, these data demonstrate that stress‐induced modulation of glutamate neurochemistry reflects a fundamental pathological change that may contribute to the aetiology and progression of depressive illness, and suggest that some antidepressants such as tianeptine may elicit their clinical effects by modulation of glutamatergic neurotransmission.

Excitatory amino acids play a key role in stress-induced remodeling of dendrites in the hippocampus as well as in suppression of neurogenesis in the dentate gyrus. The regulation of extracellular glutamate levels has been suggested as a potential mechanism through which repeated stress causes dendritic remodeling of CA3 pyramidal neurons. Accordingly, the current study examined the distribution and regulation of the glia glutamate transporter GLT-1 and the recently identified GLT isoform, GLT-1b, in the hippocampus of rats subjected to chronic restraint stress (CRS). We also examined the ability of the antidepressant tianeptine, which blocks CRS-induced dendritic remodeling, to modulate CRS-mediated changes in GLT-1 and GLT-1b expression. CRS increased GLT-1 mRNA expression in the dentate gyrus and CA3 region of Ammon's horn, increases that were inhibited by tianeptine. CRS more selectively increased GLT-1 protein levels in the subregion where dendritic remodeling is most prominent, namely the CA3 region, increases that were also inhibited by tianeptine administration. In contrast, GLT-1b mRNA expression was not modulated in the hippocampus in any of these groups, but CRS increased GLT-1b protein levels in all hippocampal subfields examined, increases that were unaffected by tianeptine treatment. These results point to the importance of understanding the mechanism for the differential and subregional regulation of GLT-1 isoforms in neuronal and glial compartments in the hippocampus as a basis for understanding the effects of chronic stress on structural plasticity as well as the neuroprotective properties of agents such as tianeptine.

Neuronal nitric oxide synthase (nNOS) plays an important role in synaptic plasticity and learning and memory. Since deficits in long-term potentiation (LTP) and learning are observed in diabetic rats and following stress, we examined the expression of nNOS mRNA and protein in the hippocampus of streptozotocin (STZ) diabetic rats and rats subjected to restraint stress. Stress did not modulate nNOS expression, while nNOS mRNA and protein levels were significantly decreased in the hippocampus of STZ diabetic rats. These results suggest that: (1) decreased expression of nNOS mRNA and protein may contribute to deficits in hippocampal dependent learning and LTP in diabetic rats; and (2) other mechanisms may be involved in stress mediated decreases in hippocampal synaptic plasticity.

The expression and localization of glucose transporter isoforms play essential roles in the glucoregulatory activities of the hippocampus and ultimately contribute to cognitive status in physiological and pathophysiological settings. The recently identified glucose transporter GLUT8 is uniquely expressed in neuronal cell bodies in the rat hippocampus and therefore may contribute to hippocampal glucoregulatory activities. We show here that GLUT8 has a novel intracellular distribution in hippocampal neurons and is translocated to intracellular membranes following glucose challenge. Immunoblot analysis revealed that GLUT8 is expressed in high‐density microsomes (HDM), suggesting that GLUT8 is associated with intracellular organelles under basal conditions. Immunogold electron microscopic analysis confirmed this observation, in that GLUT8 immunogold particles were associated with the rough endoplasmic reticulum (ER) and cytoplasm. Peripheral glucose administration produced a rapid twofold increase in GLUT8 levels in the HDM fraction while decreasing GLUT8 levels in low‐density microsomes. Similarly, peripheral glucose administration significantly increased GLUT8 association with the rough ER in the hippocampus. Conversely, under hyperglycemic/insulinopenic conditions, namely, in streptozotocin (STZ) diabetes, hippocampal GLUT8 protein levels were decreased in the HDM fraction. These results demonstrate that GLUT8 undergoes rapid translocation to the rough ER in the rat hippocampus following peripheral glucose administration, trafficking that is impaired in STZ diabetes, suggesting that insulin serves as a stimulus for GLUT8 translocation in hippocampal neurons. Because glucose is liberated from oligosaccharides during N‐linked glycosylation events in the rough ER, we propose that GLUT8 may serve to transport glucose out of the rough ER into the cytosol and in this manner contribute to glucose homeostasis in hippocampal neurons. J. Comp. Neurol. 452:103–114, 2002.