Journal of Neuroscience, Vol 14, 7261-7271, Copyright © 1994 by Society for Neuroscience
Opioids modulate stress-induced proenkephalin gene expression in the hypothalamus of transgenic mice: a model of endogenous opioid gene regulation by exogenous opioids
D Borsook, O Falkowski, H Rosen, M Comb and SE Hyman
Laboratory of Molecular and Developmental Neuroscience, Massachusetts General Hospital, Boston 02114.
Stressful stimuli strongly induce proenkephalin gene expression within the
paraventricular nucleus (PVN) of the hypothalamus. A human
proenkephalin-beta-galactosidase fusion gene has previously been shown to
give correct phenotypic expression and appropriate stress regulation within
the hypothalamus of transgenic mice; this model provides high sensitivity,
cellular resolution, and ready quantification of levels of proenkephalin
gene expression. Here we describe use of this transgenic model to study
modulation of stress-regulated gene expression in the PVN by opiates. Acute
or subacute morphine administration prior to a hypertonic saline stress
produced marked superinduction of transgene expression compared with
hypertonic saline stress alone. In contrast, chronic morphine
administration decreased basal expression of the transgene, and inhibited
stress-induced expression of the transgene. The endogenous proenkephalin
mRNA was induced in parallel with the transgene as demonstrated by in situ
hybridization; the immediate-early gene c-fos was also regulated in
parallel with the transgene. These data suggest that acute or subacute
morphine administration sensitizes proenkephalin neurons within the PVN and
other regions of the hypothalamus to stress and that chronic morphine
administration desensitizes this response. Because the molecular mechanisms
regulating the expression of the transgene are well understood, this model
provides a useful tool for investigating cellular and molecular effects of
opioids on the hypothalamus.