Experimental dissociation of neural circuits underlying conditioned avoidance and hypophagic responses to lithium chloride.

We previously reported that noradrenergic (NA) neurons in the nucleus of the solitary tract (NST) are necessary for exogenous CCK octapeptide to inhibit food intake in rats. To determine whether NST NA neurons also are necessary for lithium chloride (LiCl) to inhibit food intake and/or to support conditioned avoidance behavior, saporin toxin conjugated to an antibody against dopamine beta hydroxylase (DSAP) was microinjected bilaterally into the NST to ablate resident NA neurons. DSAP and sham control rats subsequently were tested for the ability of LiCl (0.15M, 2% body wt) to inhibit food intake and to support conditioned flavor avoidance (CFA). LiCl-induced hypophagia was significantly blunted in DSAP rats, and those with the most extensive loss of NST NA neurons demonstrated the most attenuated LiCl-induced hypophagia. Conversely, LiCl supported a robust CFA that was of similar magnitude in sham control and DSAP rats, including rats with the most extensive NA lesions. A terminal c-Fos study revealed intact LiCl-induced c-Fos expression in the lateral parabrachial nucleus and central amygdala in DSAP rats, despite significant loss of NST NA neurons and attenuated c-Fos activation of corticotropin-releasing hormone-positive neurons in the paraventricular nucleus of the hypothalamus (PVN). Thus, NST NA neurons contribute significantly to LiCl-induced hypophagia and recruitment of stress-responsive PVN neurons but appear to be unnecessary for CFA learning and expression. These findings support the view that distinct central nervous system circuits underlie LiCl-induced inhibition of food intake and conditioned avoidance behavior in rats.