It is now obvious that the CNS is capable of undergoing a variety of plastic changes
at all stages of development. Although the magnitude and distribution of these changes
may be more dramatic in the immature animal, the adult brain retains a remarkable
capacity for undergoing morphological and functional modifications. Throughout development,
as well as in the postpubertal animal, gonadal steroids exert an important influence
over the architecture of specific sex steroid-responsive areas, resulting in sexual
dimorphisms at both morphological and physiological levels. We are only now beginning
to gain insight into the mechanisms involved in gonadal steroid-induced synaptic changes.
The number of synaptic inputs to specific neuronal populations is sexually dimorphic
and this can be modulated by changes in the sex steroid environment. These modifications
can be correlated with other morphological changes, such as glial cell activation,
that are occurring simultaneously in the same anatomical area. Indeed, the close physical
relationship between glial cells and neuronal synaptic contacts makes them an ideal
candidate for participating in this process. Interestingly, not only can the morphology
and immunoreactivity of glial cells be modulated by gonadal steroids, but a close
negative correlation between the number of synapses and the amount of glial ensheathing
of a neuron has been demonstrated, suggesting an active participation of these cells
in this process. Glia have sex steroid receptors, are capable of producing and metabolizing
steroids, and can produce other neuronal trophic factors in response to sex steroids.
Hence, their role in gonadal steroid-induced synaptic plasticity is becoming more
apparent. In addition, there is recent evidence that this process may involve certain
cell surface molecules, such as the N-CAMs, since a specific isoform of this molecule,
previously referred to as the embryonic form, is found in those areas of the brain
which maintain the capacity to undergo synaptic remodelling. However, there is much
work to be done in order to fully understand this phenomenon and before bringing it
into a clinical setting in hopes of treating neurodegenerative diseases or injuries
to the nervous system.