Biographical sketch

Beginning this month (July), we are highlighting the research activities of a number of
leading researchers in the general field of taste aversion learning. The first in this series is
that of Dr. Kathleen Chambers. In the mid seventies, Dr. Chambers and her colleagues
introduced the phenomenon of sexual dimorphism in the extinction of conditioned taste
aversions (with males showing significantly slower extinction than females). Subsequent to
this initial demonstration, her laboratory has provided compelling evidence for the role of
testosterone in this differential extinction and has identified a variety of hormones and
peptides (and manipulations) that mediate and/or modulate testosterone and, in so, doing
impact extinction. Her highlight summarizes her initial findings and her efforts to isolate the
physiological bases for sexual dimorphism in taste aversion learning.

Sexual Dimorphism in the Extinction of Conditioned Taste Aversion
Chambers KC
Department of Psychology
University of Southern California
Los Angeles, CA 90089
[email protected]
When I came to the University of Washington as a graduate student, I entered an environment that
was alive with the excitement of the challenges to the widely accepted laws of learning, initiated by a
maverick of those learning laws, conditioned taste aversions. My major advisor, Robert Bolles, had
just published his review article, Species-Specific Defense Reactions and Avoidance Learning, which
broadened the scope of learned behaviors not following the laws of learning and introduced the
concept of functionalism into the study of learned avoidance behaviors. And, John Garcia, the
architect of the violation of the laws by conditioned food aversions, was a frequent presence. I, and
the rest of the graduate students in this program, got swept up in conditioned taste aversions and
functionalism.
So in retrospect, it is not surprising that I would choose to study rats that were not deprived of food or
water, a practice thought to be essential to motivate rats to learn (Calhoun had just reported that rats
in the wild were rarely deprived), to study females as well as males, a practice rarely followed in
learning studies (sexual dimorphisms are a central part of a functionalistic approach), and to study
conditioned taste aversions. This functionalistic approach led to my discovery of a sexual
dimorphism in extinction of conditioned taste aversion: males extinguish a LiCl-induced conditioned
taste aversion more slowly than females. Had I fluid deprived the strain of rats that I used, I would
not have discovered this dimorphism. The road that I and my colleagues and students have traveled
to understand the hormonal basis of this sexually dimorphic behavior has been most intellectually
engaging.

TESTOSTERONE
Introduction

The sexual dimorphism in extinction is due to the higher circulating levels of testosterone in males
during extinction. Increasing testosterone levels in females also slows extinction, which suggests that
the neural mechanism upon which testosterone acts is present in both sexes. However, females
require a higher dose of testosterone than males to show prolonged extinction. This sexual
dimorphism in sensitivity is due to the action of testosterone on the brains of males during the fetal-
neonatal period of development.
What are androgens doing during extinction? In examining the extinction in females and males, I
have found that once they begin to extinguish, the amount of time it takes them to reach
preconditioning levels (slope of the curve) is the same. The difference is in the amount of time it
takes for females and males to begin to show the gradual increase in the amount of sucrose solution
consumed. This brings to mind two possibilities: (1) there may be a delay in relearning so that a
greater number of sucrose-no illness pairings is needed by the male to revise his learned assumptions
about the consequences of consuming the sucrose, or (2) there is a delay in risk taking so that the
male requires a greater amount of time after the sucrose-illness pairing to test whether the sucrose
still predicts illness.
Fluid Restriction and Testosterone

The sexual dimorphism disappears when rats are fluid restricted primarily because the extinction rate
of males becomes faster. We have suggested that this is due to a decrease in blood testosterone
levels. There is compelling evidence to support this hypothesis in Sprague-Dawley male rats. First,
fluid restriction accelerates extinction in a choice situation in males but not females. Second, serum
testosterone levels are lower in fluid restricted male rats than in nondeprived males and administering
testosterone to fluid restricted males restores extinction rates to those of untreated nonrestricted
males. Third, testosterone and fluid restriction have the same pattern of behavioral effects, that is,
they affect extinction by acting during extinction but not during acquisition.
Fischer 344 Rats
Unlike Sprague-Dawley males, gonadectomy has no effect on extinction in Fisher 344 males. We
think that the difference between Sprague-Dawley and Fischer 344 rats lies in the effect of
testosterone during the fetal-neonatal period on developing neural tissue, either by making these
tissues so sensitive to testosterone that androgens secreted by nontesticular sources are sufficient to
slow extinction or by allowing the neural areas mediating extinction to function without circulating
testosterone.
Additional differences between Sprague-Dawley and Fischer 344 rats are that fluid restriction
accelerates extinction in a choice situation in both female and male Fischer 344 rats, it does not
decrease testosterone levels in males, and only extremely high doses of testosterone can slow
extinction. This suggests that there is a testosterone-independent mechanism that contributes to the
acceleration of extinction in fluid restricted Fischer 344 males. A similar mechanism also may exist
along with the testosterone-dependent mechanism in Sprague-Dawley rats. When circulating levels
of testosterone are controlled via exogenous treatment in gonadectomized Sprague-Dawley males,
testosterone is less effective in prolonging extinction in fluid restricted males than nonrestricted
males even though blood testosterone levels are similar.
Interrelationships: Testosterone and Other Hormones
Although the interrelationships among hormones was an area of interest in the early history of
endocrinology, it has often been ignored in the interpretation of hormonal effects on behavior despite
considerable evidence showing that an alteration of the circulating level of one hormone can change
the secretion of other hormones. Given this, the question of which hormone level alteration is
producing an effect becomes germane. The results from my lab demonstrate the importance of
considering the interrelationship of hormones when exploring the mechanism by which a particular
hormone influences conditioned taste aversions.
Adrenocorticotropin Hormone. ACTH is another hormone that prolongs extinction and like
testosterone, it does so when it is present only during extinction. We found that ACTH acts via an
androgen mechanism; ACTH increases testosterone levels and when this increase is blocked through
gonadectomy, ACTH is no longer able to prolong extinction.

Vasopressin. Studies in other labs have demonstrated that (1) fluid deprivation increases blood
levels of vasopressin and decreases vasopressin content in various neural structures, (2) vasopressin,
from either local production in the testes or from the posterior pituitary, exerts a dose-dependent
inhibition of androgen biosynthesis in the Leydig cells of the testes, (3) the integrity of some of the
neural vasopressinergic systems is dependent on sufficient circulating androgen levels, and (4)
homozygous Brattleboro rats, which have compromised hormonal and neural vasopressinergic
systems but a functioning testicular hormonal system, show rapid extinction. Putting all of these data
together, our working model for the effects of fluid restriction in Sprague-Dawley rats is as follows:
fluid restriction reduces testosterone levels via a vasopressin mechanism, which decreases neural
release of vasopressin, which results in accelerated extinction.

VASOPRESSIN

Our interest in this peptide extends beyond its interrelationship with testosterone. A large number of
studies across a wide range of learning tasks have demonstrated that vasopressin facilitates the
maintenance of learned tasks. Because of this it has been widely regarded as a mnemonic
hormone/neuromodulator. Our research has focused on two problems: (1) the role of vasopressin in
the fast extinction of fluid restricted Fischer 344 males and (2) the effect of acute elevations of
vasopressin on extinction when administered during different phases of the learning process.
Fluid Restricted Fischer 344 Males. We measured vasopressin levels in various neural areas of
fluid restricted and nonrestricted Fischer 344 males during extinction of a LiCl-induced conditioned
taste aversion. We found that the vasopressin levels in the paraventricular nucleus of fluid restricted
males were lower than those of nonrestricted Fischer 344 males and similar to those of nonrestricted
males that had not been conditioned. These results raise the possibility that a testosterone-
independent vasopressinergic system in the paraventricular nucleus plays a critical role in the
differential extinction rate of fluid restricted and nonrestricted males.

Acquisition and Vasopressin. We have found that the effect of vasopressin on extinction is
dependent on whether it is present before or after acquisition or whether the dose is low or high. Low
doses of vasopressin prolong extinction when given just before acquisition but accelerate it when
given shortly after acquisition. High doses of vasopressin can be used to induce a conditioned taste
aversion and when given after acquisition of a LiCl-induced aversion, they act like other
unconditioned stimuli; they strengthen acquisition and delay the onset of extinction. These results
clearly show the mnemonic hypothesis to be inadequate in explaining the effects of vasopressin on
conditioned taste aversions.
ESTRADIOL
The ability of testosterone to prolong extinction is diminished in estradiol-treated gonadectomized
females. This diminished effect is due to the ability of estradiol to accelerate extinction. We have
accumulated a considerable amount of evidence showing that the effects of estradiol are similar to
those of LiCl, which suggests that the effect of estradiol on extinction can be accounted for by its
illness-inducing properties. First, like LiCl, estradiol can be used to induce a conditioned taste
aversion. Second, estradiol and LiCl can serve as mutual preexposure agents. Preexposure to
estradiol before acquisition weakens acquisition and accelerates extinction of conditioned taste
aversions induced by estradiol and LiCl and preexposure to LiCl attenuates acquisition of aversions
induced by LiCl and estradiol. Third, both estradiol and LiCl attenuate acquisition when given
during the post-acquisition/pre-extinction period. Fourth, extinction is prolonged when estradiol is
given immediately after the first extinction test. These results are what one would expect if estradiol
acted as an illness-inducing agent. The presence of estradiol during re-exposures to the conditioned
taste stimulus would be equivalent to repeated acquisition tests and therefore should prolong
extinction. Fifth, we have found recently that like LiCl, estradiol increases c-fos-like-
immunoreactivity (c-FLI), a measure of neural activity, in the central, external, and crescent
subnuclei of the lateral parabrachial. The lateral parabrachial is essential for acquisition of LiCl-
induced conditioned taste aversions and there is a strong correspondence between the amount of c-
FLI expression in these subnuclei and the strength of conditioned taste aversion.
In all of the above studies, supraphysiological doses of estradiol were used. It is not surprising that
such doses, which are reported to induce nausea and vomiting in humans, mimic the effects of LiCl.
But testosterone also has a diminished effect in intact females and other labs have shown that
physiological doses of estradiol can produce conditioned taste aversions and preexposure effects. It
is very unlikely that these effects are illness based. There seem to be an unlimited number of agents
that can be used as unconditioned stimuli to produce conditioned taste aversions. Yet, the stimulus
characteristics that are essential for inducing this learned response remain a mystery. Indeed,
whether all of these agents produce true conditioned taste aversions has been questioned by several
investigators and remains a topic of great interest and debate. Our work with estradiol has led us into
this fray. The question we are pursuing now is whether the effects of supraphysiological and
physiological doses of estradiol induce conditioned taste aversion and preexposure effects via
different neural-chemical pathways. In recognition of the likely possibility that not all conditioned
taste aversions are true aversions, we have begun to use the term conditioned consumption reduction.

CONCLUDING COMMENT

It is an honor to have been invited to write a brief intellectual autobiographical statement about my
research on conditioned taste aversions. The journey has been a fascinating one and the questions
that will occupy my future efforts are likely to be varied and intriguing. One often has clearer
perspective on the work of others than on one’s own work, and there is a dialectical tension between
overstating and understating the significance of one’s efforts. Whatever merits the reader may see in
this work, I believe that it has opened avenues of inquiry that can shed light on the complex influence
of hormones on learning.
COLLABORATORS AND STUDENTS
I extend my deepest gratitude to those mentors and colleagues who have influenced my thinking
about hormones and learning through collaboration or intellectual discourse (Robert Bolles, David
Lavond, Charles Phoenix, Cord Sengstake, and Pamela Westfahl) and to the students who have made
important contributions to this work (Ana Brownson, UnJa Hayes, Houri Hintiryan, Yuan Wang, and
David Yuan).

REFERENCES
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Testosterone: Fischer 344 Rats
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Testosterone: Interrelationships: Adrenocorticotropin Hormone.
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Testosterone: Interrelationships: Vasopressin.
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Vasopressin: Fluid Restricted Fischer 344 Males.
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Vasopressin: Acquisition and Vasopressin.
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Estradiol
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elements: conditioned appetites and satieties. Ann. NY Acad. Sci. 443: 22-41. Chambers, K.C. and Hayes, U.L. (2002) Exposure to estradiol before but not during acquisition of LiCl-induced conditioned taste avoidance accelerates extinction. Horm. Behav. 41: 297-305. Chambers, K. C. and Yuan, D. L. (1990) Blockage of the effects of testosterone on extinction of a conditioned by estradiol: Time of action. Physiol. Behav. 48: 277-281. Chambers, K.C. and Wang, Y. (2004) Role of the lateral parabrachial nucleus in apomorphine- induced conditioned consumption reduction: Cooling lesions and relationship of c-Fos-like-immunoreactivity to strength of conditioning. Behav. Neurosci. 118: 199-213. De Beun, R., Jansen, E., Smeets, M. A. M., Niesing, J., Slangen, J. L. and Van De Poll, N. E. (1991) Estradiol-induced conditioned taste aversion and place aversion in rats: sex- and dose-dependent effects. Physiol. Behav. 50: 995-1000. Earley, C. J. and Leonard, B. E. (1979) Effects of prior exposure on conditioned taste aversion in the rat: Androgen- and estrogen-dependent events. J. Comp. Physiol. Psychol. 93: 793-805. Grigson, P.S. (1997) Conditioned taste aversions and drugs of abuse: A reinterpretation. Behav. Neurosci. Hintiryan, H., Hayes, U.L. and Chambers, K.C. (2005) The role of histamine in estradiol-induced conditioned consumption reduction in female rats. Physiol. Behav. 84:117-128. Hunt, T. and Amit, Z. (1987) Conditioned taste aversions induced by self-administered drugs: Paradox Neurosci. Biobehav. Rev. 11: 107-130. Merwin, A. A. and Doty, R. L. (1994) Early exposure to low levels of estradiol (E2) mitigates E2- induced conditioned taste aversions in prepubertally ovariectomized female rats. Physiol. Behav. 55: 185-187. Parker, L.A. (1995) Rewarding drugs produce taste avoidance, but not taste aversion. Neurosci. Biobehav. Rev. 19:143-151. Yuan, D.L., and Chambers, K.C. (1999) Estradiol accelerates extinction of a conditioned taste aversion in female and male rats. Horm. Behav. 36: 1-16. Yuan, D. and Chambers, K.C. (1999 Estradiol accelerates extinction of LiCl-induced conditioned taste aversions through its illness-associated properties. Horm. Behav. 36: 287-298.

Source: http://w.american.edu/cas/psychology/cta/highlights/Chambers_Highlight.pdf