Neuronal structure and hormones

Modern research has challenged the traditional view of hormones as strictly acting in peripheral tissues. Several studies have shown that, apart from their peripheral effects, hormones bind on specific receptors in the CNS and exhibit significant effects both on the mature brain’s functioning (Moult, 2008) and the developing brain’s sexual differentiation (Schwarz, 2008). Leptin, insulin and sex steroids are hormones with established neuromodulatory effects.

Leptin is mainly associated with bone formation, reproduction and regulation of the hypothalamic-pituitary-adrenal axis, however it also influences other brain areas, including the hippocampus (Moult, 2008). Insulin, apart from its role in glucose metabolism, it has been linked to feeding, synaptic regulation and neuronal survival. In addition, insulin’s central effects have been implicated in the pathogenesis of Alzheimer’s disease (Craft, 1998). Sex steroids regulate sexual behavior and affect cognitive functioning, namely memory and learning (Schwartz, 2008).

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The hippocampus has attracted a large amount of scientific attention due to his key position in the neurophysiology of memory and learning. There seems to be a sexual dimorphism in brain development, mostly affecting the hippocampus, the amygdala and the basal ganglia (Lenroot, 2010). Studies in animal models have shown that the female hippocampus contains larger amount of brain derived neurotrophic factor (BDNF) and it exhibits more active cell proliferation due to the effect of estrogens. In contrast, the male hippocampus appears larger in volume and has a larger granule cell layer and more synapses (Hajszan, 2007).

Given the hippocampal role in several functions, including memory, learning, mood and stress response, these discrepancies may explain the presence of between gender differences in cognition and psychopathology. The hippocampus is the centre of declarative, contextual and spatial memory (Neves, 2008). Neuroscientists agree that memory is probably mediated by neuronal plasticity, a term that describes neurons’ ability to proliferate (neurogenesis) and change the strength of synapses (synaptic remodeling).

Research currently suggests that the molecular mechanism underlying neuroplasticity is a phenomenon called Long-Term Potentiation (LTP), which refers to the long-lasting enhancement of synaptic transmission between two neurons, when they are stimulated simultaneously. LTP is attributed to several mechanisms, including calcium-dependent intracellular pathways, in other words, it increases the efficacy of synaptic transmission thus modifying synaptic strength (Cooke, 2006).

Memory refers to the encoding and retrieval of information, while learning is a cognitive function demonstrated by the individual’s ability to modify his/her behavior according to prior experience. Memory and learning are thus interconnected and constitute the foundations of our ability to adapt to environmental changes. The limbic system is considered to have a central role in the neurobiology of memory and learning (Pennartz, 2009). However, other neuronal structures are also implicated, including the prefrontal cortex and the striatum.

Brain circuits connecting prefrontal areas with the basal ganglia and the hippocampus seem to mediate several aspects of memory formation and learning, such as contextual conditioning and spatial information processing (Pennartz, 2009). Research has revealed significant gender differences in memory and learning tasks. Males predominate at tasks involving spatial memory, while females show an advantage at verbal memory. In addition, the capacity of storing information regarding emotionally stressful events largely depends on circulating sex hormone levels (Andreano, 2009)..

In conclusion, there seems to be a bidirectional conversation between the endocrine and the nervous system. The hippocampus, which holds a vital role in the neurobiology of memory and learning, receives hormonal signals that modify its function. The observed sexual dimorphism of the hippocampus and other brain areas which is due to the effect of sex steroids, is probably related to significant differences in memory and learning skills between men and women. References Moult, P. R. , Harvey, J. (2008). Hormonal regulation of hippocampal dendritic morphology and synaptic plasticity. Cell Adh Migr, 2 (4), 269-75.

Craft, S. , Peskind, E. , Schwartz, M. W. , et al. (1998). Cerebrospinal fluid and plasma insulin levels in Alzheimer’s disease: relationship to severity of dementia and apolipoprotein E genotype. Neurology, 50, 164-8. Schwarz, J. M. & McCarthy, M. M. (2008). Steroid-induced sexual differentiation of the developing brain: multiple pathways, one goal. J Neurochem, 105 (5); 1561-72. Lenroot, R. K. & Giedd, J. N. (2010). Sex differences in the adolescent brain. Brain Cogn. , 72 (1), 46-55. Hajszan, T. , Milner, T. A. & Leranth, C. (2007). Sex Steroids and the Dentate Gyrus. Prog Brain Res, 163, 399-415.

Neves, G. , Cooke, S. F. & Bliss, T. V. (2008). Synaptic plasticity, memory and the hippocampus: a neural network approach to causality. Nat Rev Neurosci, 9 (1), 65-75. Cooke, S. F. & Bliss, T. V. P. (2006). Plasticity in the human central nervous system. Brain, 129, 1659-73. Pennartz, C. M. A. , Berke, J. D. , Graybiel, A. M. , et al. (2009). Cotricostriatal interactions during learning, memory processing and decision making. The Journal of Neuroscience, 29 (41); 12831-38. Andreano, J. M. & Cahill, L. (2009). Sex influences on the neurobiology of learning and memory. Learn Mem, 16 (4), 248-66. .

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