Biomedical Engineering Reference
In-Depth Information
identify the specifi c neural circuits that are activated by production of the behavior.
This fi rst step can be achieved with activity-dependent genes, which serve as molec-
ular markers of neural activity to map functional domains and cells of the brain.
This “behavioral molecular brain-mapping approach” has been successively used to
identify and characterize neural systems involved in perceiving and producing
behaviors (Jarvis and Nottebohm
1997
; Jarvis et al.
2000
; Mello et al.
1992
).
Transcription of the early neural activity-induced genes, known as immediate early
genes (IEGs), in the postsynaptic cell is initiated by presynaptic action potential
fi ring and subsequent neurotransmitter and/or neuromodulator release, followed by
binding to postsynaptic receptors (Clayton
2000
; Worley et al.
1987
). After binding,
extracellular Ca
+
infl ux or release of Ca
+
from intracellular stores activates a signal
transduction cascade, including several kinds of protein kinases, such as protein
kinase A, mitogen-activated protein kinases, and calcium- and calmodulin-
dependent kinases. These kinases regulate nuclear gene expression via phosphory-
lation of targeted specifi c transcription factors, including cAMP response
element-binding protein (CREB)/activating transcription factor (ATF) family and
serum response factors (SRFs). These transcription factors bind to promoter regions
of IEGs and initiate the mRNA transcription of IEGs at several sites on chromo-
somes. This IEG transcription response occurs within a few minutes after neuron
activation, because induction of IEG mRNAs does not require de novo protein syn-
thesis (Guzowski et al.
1999
).
The initial IEGs discovered were transcription factors that regulate other genes
(Cole et al.
1989
; Greenberg et al.
1986
). Subsequent molecular studies discovered
other types of IEGs, encoding a diverse range of functional proteins, including regu-
latory transcription factors, structural proteins, signal transduction proteins, growth
factors, and enzymes (Loebrich and Nedivi
2009
; Saffen et al.
1988
; Wada et al.
2006
). Thus, IEGs fall into two subcategories, the inducible transcription factors
(ITFs) and all other molecules as inducible direct effectors (IDEs). There are at least
two popularly studied ITFs, c-fos and egr1 (also named zif268, NGFI, Krox-24, or
zenk), and one late effector, Arc [activity-regulated cytoskeleton-associated gene
(Steward et al.
1998
)] in vertebrates. Along with another direct effector we have
studied, called dual specifi city phosphatase 1 (dusp1), the combination of these four
genes can be used to ensure identifying activation in all neuron types in the verte-
brate forebrain (Horita et al.
2010
). In invertebrates, especially in honeybees, some
IEGs identifi ed, such as
kakusei
, are induced in the mushroom body of reorienting
bees and foragers (Kiya et al.
2007
). Whether a transcription factor or direct effec-
tor, mRNA of most IEGs is usually induced and accumulated in the cellular cyto-
plasm at maximal level up to 30 min during repeated production of animal's
behavior repeated sensory stimulation.
To detect behaviorally regulated (motor or sensory) mRNA expression of IEGs
within cells and tissues in vivo, in situ hybridization is a powerful method. It allows
single-cell resolution in tissue sections after an animal has performed behavior and
processed a sensory stimulus. In situ hybridization is also a useful approach to
