Conditioned taste reactivity in rats after phenylpropanolamine, d-amphetamine or lithium chloride

Developmental 17-OHPC exposure disrupts behavior regulated by the mesocorticolimbic dopaminergic system in rats

The synthetic progestin, 17α-hydroxyprogesterone caproate (17-OHPC), is administered to pregnant individuals with the intention of reducing preterm birth. Although there is evidence that 17-OHPC is likely transferred from mother to fetus, there is little information regarding the potential effects of 17-OHPC administration on behavioral and neural development in offspring. Neonatal 17-OHPC exposure disrupts the development of the mesocorticolimbic dopaminergic pathway and associated behaviors in rats. 17-OHPC exposure altered dopaminergic innervation of prelimbic medial prefrontal cortex (mPFC) in neonates and adolescents and altered performance in measures of decision-making, set-shifting, and reversal-learning tasks. The present study tested the effects of developmental 17-OHPC exposure on numerous cognitive behaviors mediated by the mesocorticolimbic dopaminergic system, such as decision-making in a delay discounting task, latent inhibition following conditioned taste aversion (CTA), and spatial memory in the Morris Water Maze (MWM). The present work also aimed to further investigate response omissions in rats exposed to 17-OHPC during development and the potential role of dopamine D2 receptor in altering omissions in a delay discounting task. 17-OHPC exposure rendered rats less sensitive to an Eticlopride-induced increase in omissions in a delay discounting task when compared to controls. Quinpirole flattened the discount curve in both groups but did not significantly affect omissions in 17-OHPC-exposed or control rats. Following CTA, sucrose-pre-exposed 17-OHPC-exposed rats demonstrated decreased latent inhibition when compared to controls. In Morris Water Maze testing, 17-OHPC-exposed rats did not differ from controls after the first day of testing or during probe testing. These results suggest that exposure to 17-OHPC altered aspects of decision-making and latent inhibition in adult male rats, without affecting performance in a measure of spatial learning and memory. Further, the insensitivity of 17-OHPC-exposed males to an Eticlopride-induced increase in omissions suggests a dysfunction in the D2 receptor following exposure to this clinically used synthetic progestin.

The Functional and Neurobiological Properties of Bad Taste

The gustatory system serves as a critical line of defense against ingesting harmful substances. Technological advances have fostered the characterization of peripheral receptors and have created opportunities for more selective manipulations of the nervous system, yet the neurobiological mechanisms underlying taste-based avoidance and aversion remain poorly understood. One conceptual obstacle stems from a lack of recognition that taste signals subserve several behavioral and physiological functions which likely engage partially segregated neural circuits. Moreover, although the gustatory system evolved to respond expediently to broad classes of biologically relevant chemicals, innate repertoires are often not in register with the actual consequences of a food. The mammalian brain exhibits tremendous flexibility; responses to taste can be modified in a specific manner according to bodily needs and the learned consequences of ingestion. Therefore, experimental strategies that distinguish between the functional properties of various taste-guided behaviors and link them to specific neural circuits need to be applied. Given the close relationship between the gustatory and visceroceptive systems, a full reckoning of the neural architecture of bad taste requires an understanding of how these respective sensory signals are integrated in the brain.

Conditioned taste aversions: From poisons to pain to drugs of abuse

Learning what to eat and what not to eat is fundamental to our well-being, quality of life, and survival. In particular, the acquisition of conditioned taste aversions (CTAs) protects all animals (including humans) against ingesting foods that contain poisons or toxins. Counterintuitively, CTAs can also develop in situations in which we know with absolute certainty that the food did not cause the subsequent aversive systemic effect. Recent nonhuman animal research, analyzing palatability shifts, has indicated that a wider range of stimuli than has been traditionally acknowledged can induce CTAs. This article integrates these new findings with a reappraisal of some known characteristics of CTA and presents a novel conceptual analysis that is broader and more comprehensive than previous accounts of CTA learning.

Conditioned taste aversion, drugs of abuse and palatability

We consider conditioned taste aversion to involve a learned reduction in the palatability of a taste (and hence in amount consumed) based on the association that develops when a taste experience is followed by gastrointestinal malaise. The present article evaluates the well-established finding that drugs of abuse, at doses that are otherwise considered rewarding and self-administered, cause intake suppression. Our recent work using lick pattern analysis shows that drugs of abuse also cause a palatability downshift and, therefore, support conditioned taste aversion learning.

Reduced palatability in pain-induced conditioned taste aversions

The current study investigated whether internal pain-inducing agents can modulate palatability of a tastant in the same way as illness-inducing agents (e.g., lithium chloride). Similar to traditional conditioned taste aversion (CTA) experiments, during conditioning the rats were exposed to a saccharin solution followed by intraperitoneal injections of either gallamine (Experiment 1) or hypertonic sodium chloride (NaCl; Experiments 1 and 2). In addition to the total amount consumed, the time of each lick was recorded for lick pattern analysis. The results showed that both gallamine and hypertonic NaCl caused suppression in saccharin intake. Importantly, both lick cluster size and initial lick rate (the measures of taste palatability) were reduced as well. This pattern of results suggests that these pain-inducing agents reduce the hedonic value of the associated tastant and thus CTA is acquired. The current finding serves as evidence supporting the view that CTA is a broadly tuned mechanism that can be triggered by changes in internal body states following consummatory experience.

Taste neophobia and palatability: the pleasure of drinking

Taste neophobia is manifested behaviorally as lower intake of a novel, potentially dangerous tastant relative to the same tastant when it is perceived as safe and familiar. To further characterize this phenomenon, microstructural analysis of lick patterns was used to track the transition from novel to familiar for three tastants: saccharin, quinine and Polycose. The results revealed that in addition to an increase in the amount consumed (for saccharin and quinine but not Polycose), cluster size (an index of palatability) became larger as familiarity with the benign tastants increased. The current finding suggests that the pleasure of drinking increases as the novel, potentially dangerous tastant becomes accepted as safe.

Reduced palatability in drug-induced taste aversion: I. Variations in the initial value of the conditioned stimulus

Like illness-inducing agents (e.g., lithium chloride), drugs of abuse also suppress intake of a taste solution. To explore the nature of this drug-induced intake reduction, in the current study three aqueous stimuli with different initial values served as the conditioned stimuli (CSs) that were paired with a standard dose of amphetamine in a voluntary intake procedure and lick patterns were analyzed. Consistent with earlier studies, amphetamine significantly reduced intake of all three CSs (quinine, sodium chloride, and orange odor). In contrast to studies that analyze orofacial responses, we found that lick cluster size was significantly lowered by amphetamine, indicating that the psychoactive drug induced a conditioned reduction in taste palatability.

Reduced palatability in drug-induced taste aversion: II. Aversive and rewarding unconditioned stimuli

Drugs of abuse are known to reduce intake of a taste conditioned stimulus (conditional stimulus, CS), a behavioral response sometimes seen as paradoxical because the same drugs also serve as rewards in other behavioral procedures. In the present study we compared patterns of intake and palatability (assessed using microstructural analysis of licking) for a standard saccharin CS paired with the following: lithium chloride, morphine, amphetamine, or sucrose. We found that morphine and amphetamine, like lithium-induced illness, each suppressed CS intake and caused a reduction in saccharin palatability. Sucrose, a rewarding stimulus, did not reduce the palatability of the saccharin CS. We interpret these finds as evidence that drugs of abuse induce conditioned taste aversions.

An assessment of the involvement of paraventricular hypothalamic alpha 2-adrenoceptors in phenylpropanolamine anorexia

Systemic injection of phenylpropanolamine (PPA), an alpha 1-adrenergic receptor agonist with some activity at alpha 2-adrenergic receptors, suppresses food intake in rats. However, only limited information is available as to the effect of intracranial PPA injections on food and water intake. In Experiment 1, microinjection of PPA (80-240 nM) into the hypothalamic paraventricular nucleus (PVN) induced a dose-dependent suppression of feeding (ED50 = 181 nM) but was without significant effect on water intake. Experiment 2 evaluated the effect of systemic PPA on paraventricular hypothalamic norepinephrine (NE) levels. Rats were treated with either vehicle or 20 mg/kg (IP) PPA prior to a 100-min period in which extracellular NE within the PVN was monitored via an indwelling microdialysis probe. Systemic injection of PPA suppressed extracellular NE level within PVN by approximately 70%, an action consistent with stimulation by PPA of a presynaptic alpha 2-adrenergic autoreceptor. Experiment 3 evaluated whether the alpha 2-adrenergic activity of PPA contributes to its feeding-suppressive action. Unlike prior results using the alpha 1-antagonist benoxathian, PVN microinjection of the alpha 2-antagonist rauwolscine in Experiment 3 of the present study failed to block systemically induced PPA anorexia. These results further support the contention that PVN alpha 1-adrenergic receptors suppress feeding and suggest that PPA's alpha 2-adrenergic effects do not modulate the anorexic action of PPA.