STRESS AND HIPPOCAMPAL PLASTICITY

Animal research has established that distress-inducing experience can have deleterious effects on brain structure and function.

“Animal research has clearly established that distress-inducing experience can have deleterious effects on brain structure and function”

The hippocampus and PFC may have greater capacity for change or plasticity following stress, with many of the effects being reversible following the termination of stress.

“There is a growing body of evidence to suggest that the hippocampus and PFC may have greater capacity for change or plasticity following stress with many of the effects being reversible following the termination of stress (McEwen, 1999, Vyas et al., 2004, Liston et al., 2009).”

Reduced dendritic complexity largely agrees with changes in other brain regions.

“Reduced dendritic complexity largely agrees with changes in other brain regions [151, 152] and is associated with reduced excitatory input but increased intrinsic excitability [88, 89], which may reflect homeostatic self-tuning [153].”

Chronic restraint stress causes retraction and simplification of dendrites in the CA3 region of the hippocampus.

“Chronic restraint stress causes retraction and simplification of dendrites in the CA3 region of the hippocampus (McEwen, 1999); Sousa et al., 2000).”

Excitatory amino acids released by the mossy fiber pathway play a key role in the remodeling of the CA3 region of the hippocampus, and regulation of glutamate release by adrenal steroids may play an important role.

“Excitatory amino acids released by the mossy fiber pathway play a key role in the remodeling of the CA3 region of the hippocampus, and regulation of glutamate release by adrenal steroids may play an important role (McEwen, 1999).”

New neurons continue to be produced in the dentate gyrus throughout adult life, and CA3 pyramidal cells undergo a reversible remodeling of their dendrites in conditions such as hibernation and chronic stress.

“there is also adaptive structural plasticity, in that new neurons continue to be produced in the dentate gyrus throughout adult life, and CA3 pyramidal cells undergo a reversible remodeling of their dendrites in conditions such as hibernation and chronic stress (Popov et al., 1992; Popov and Bocharova, 1992; McEwen, 1999).”

Chronic stress for 21 days or longer impairs hippocampal-dependent cognitive function.

“chronic stress for 21 days or longer impairs hippocampal-dependent cognitive function (McEwen, 1999)”

Repeated stress has been linked with neuronal atrophy in stress-reactive areas including the hippocampus, and hence neurons may also be a significant source of HMGB-1.

“Repeated stress has been linked with neuronal atrophy in stress-reactive areas including the hippocampus (McEwen, 1999), and hence neurons may also be a significant source of HMGB-1.”

Chronic restraint stress causes retraction and simplification of dendrites in the CA3 region of the hippocampus.

“Chronic restraint stress causes retraction and simplification of dendrites in the CA3 region of the hippocampus.58,69”

Adrenal steroids are important mediators of remodeling of hippocampal neurons during repeated stress, and exogenous adrenal steroids can cause remodeling in the absence of an external stressor.

“Regarding the mechanism of structural remodeling, adrenal steroids are important mediators of remodeling of hippocampal neurons during repeated stress, and exogenous adrenal steroids can also cause remodeling in the absence of an external stressor.”

Excitatory amino acids released by the mossy fiber pathway play a key role in the remodeling of the CA3 region of the hippocampus, and regulation of glutamate release by adrenal steroids may play an important role.

“Excitatory amino acids released by the mossy fiber pathway play a key role in the remodeling of the CA3 region of the hippocampus, and regulation of glutamate release by adrenal steroids may play an important role.58”

One granule neuron innervates on average 12 CA3 neurons, and each CA3 neuron innervates on average 50 other CA3 neurons via axon collaterals, as well as 25 inhibitory cells via other axon collaterals, resulting in a 600-fold amplification of excitation and a 300-fold amplification of inhibition.

“One granule neuron innervates, on the average, 12 CA3 neurons, and each CA3 neuron innervates, on the average, 50 other CA3 neurons via axon collaterals, as well as 25 inhibitory cells via other axon collaterals. The net result is a 600-fold amplification of excitation, as well as a 300-fold amplification of inhibition, that provides some degree of control of the system.58”

New neurons continue to be produced in the dentate gyrus throughout adult life, and CA3 pyramidal cells undergo reversible remodeling of their dendrites in conditions such as hibernation and chronic stress.

“However, because the DG-CA3 system is so delicately balanced in its function and vulnerability to damage, there is also adaptive structural plasticity, in that new neurons continue to be produced in the dentate gyrus throughout adult life, and CA3 pyramidal cells undergo a reversible remodeling of their dendrites in conditions such as hibernation and chronic stress.58,60,61”

Chronic stress for 21 days or longer impairs hippocampal-dependent cognitive function and enhances amygdala-dependent unlearned fear and fear conditioning.

“Moreover, chronic stress for 21 days or longer impairs hippocampal-dependent cognitive function58 and enhances amygdala-dependent unlearned fear and fear conditioning,100 which are consistent with the opposite effects of stress on hippocampal and amygdala structure.”

Chronic stress causes dendritic shrinkage not only in CA3 and dentate gyrus neurons of hippocampus but also in medial amygdala and medial prefrontal cortex, while dendrites in basolateral amygdala and orbitofrontal cortex expand with chronic stress.

“Chronic stress causes dendritic shrinkage not only in CA3 and dentate gyrus neurons of hippocampus but also in medial amygdala and mPFC, while dendrites in BlA and orbitofrontal cortex expand with chronic stress.”

Excitatory amino acids, particularly glutamate, play a key role in structural as well as functional changes in the brain; glutamate is the major excitatory transmitter, while excess glutamate causes damage and inflammation.

“Excitatory amino acids, particularly glutamate, play a key role in structural as well as functional changes in the brain since glutamate is the major excitatory transmitter, while, at the same time, excess glutamate causes damage and inflammation.”