These two techniques showed that the volume changes that had been identified either based on seasonal changes or around the manipulations of testosterone concerned populations of cells expressing specific markers such as alpha 2 adrenergic receptors (Bernard & Ball, 1995;Riterset al., 2002) or receiving a specific innervation by neurotransmitters (e.g., nuclei labeled by dense networks of fibers immunoreactive for tyrosine hydroxylase, the rate limiting enzyme for catecholamines, (Appeltantset al., 2001). within two weeks. Significant volume increases are, however, already observable after one day. The Steroid Receptor Coactivator-1 is usually part of the mechanism mediating these effects. Increases in POM volume reflect changes in cell size or spacing and dendritic branching but are not associated with an increase in neuron number. In contrast, seasonal changes in HVC volume reflect incorporation of newborn neurons in addition to changes in cell size and spacing. These are induced by treatments with exogenous testosterone or its metabolites. Expression of doublecortin, a microtubule-associated protein, is increased by testosterone in HVC but not in the adjacent nidopallium suggesting that neuron production in the subventricular zone, the birthplace of newborn neurons, is not affected. Together these data illustrate the high degree of plasticity that extends into adulthood Isosteviol (NSC 231875) and is characteristic of avian brain structures. Many questions do still remain concerning the regulation and specific function of this plasticity. Keywords:Japanese quail, Songbirds – Sexual behavior, Preoptic area, Song control system, HVC, Doublecortin == Steroids, brain plasticity and behavior == The analysis of the interrelationships among hormones, brain and behavior has now been a vibrant field of study for more than half a century and has identified a variety of brain mechanisms that seem to generalize to most vertebrates (Beach 1981). Behavioral neuroendocrinology, as such an analysis is now commonly termed, has attracted a large number of scientists coming from the ethological tradition who always studied a diversity of animal species, including birds in particular (Marler, 2005). There are numerous advantages to studying birds (Konishiet al., 1989;Wingfield, 2005) including the fact, highly relevant for the present topic, that they often display prominent annual cycles of reproductive activity. In many species living in the temperate zone, gonadal weight varies more than ten fold and sometimes close to one hundred fold between periods of sexual Isosteviol (NSC 231875) quiescence in the winter or the summer molt and during the spring time when reproduction takes place. Concentrations of sex steroids similarly display marked seasonal variations and, as a consequence, changes of very large Gpc4 amplitude are also observed in a Isosteviol (NSC 231875) variety of steroid-dependent responses (Wingfield & Farner, 1993;Dawsonet al., 2001;Ball & Balthazart, 2002;Wingfield & Silverin, 2002). It is only recently that scientists have recognized the presence of significant morphological and physiological brain plasticity in adult homeothermic vertebrates. Physiological plasticity generally refers to enduring changes in synaptic physiology that can occur after different patterns of stimulation. Long-term potentiation (LTP) and long-term depressive disorder (LTD) are two well-known examples of this phenomenon (Bear & Malenka, 1994;Huang, 1998;Malenka & Bear, 2004). Morphological plasticity refers to measurable changes in morphology including cell size, cell number, cell shape and the connectivity among brain nuclei. Although variation in synaptic plasticity has been documented in birds (Wieraszko & Ball, 1991;1993;Ding & Perkel, 2004;Meitzenet al., 2009), this review will focus on morphological plasticity and focus on work completed in our labs in recent years. The lack of substantial behavioral recovery after brain damage produced by accidental trauma or neurodegenerative diseases was, for a long time, taken as evidence for the absence of marked plasticity in the adult central nervous system of many vertebrate species including humans. It has, however, been acknowledged during the last few decades that mature adult neurons in the adult mammalian brain change their size and anatomical associations with adjacent glial cells or reorganize their dendritic tree within hours to days (e.g.,Theodosis & Poulain, 1987;Garcia-Seguraet al., 1988;Theodosis & Poulain, 1992). While neurogenesis was previously thought to be completely absent in adult homoeothermic vertebrates, a limited Isosteviol (NSC 231875) amount of neurogenesis has been identified in specific brain regions of the adult mammalian brain (Altman & Das, 1965;Altman, 1969;Kemperman, 2006;Gould, 2007), possibly including humans (Erikssonet Isosteviol (NSC 231875) al., 1998;Bhardwajet al., 2006). Neurogenesis and neural plasticity are, to an even greater extent, also observed in birds and several aspects of this plasticity seem to.