Neuronal Ensembles in the Infralimbic Cortex Dynamically Process Distinct Aspects of Hedonic Value.

Hedonic processing is critical for guiding appropriate behavior, and the infralimbic cortex (IL) is a key neural substrate associated with this function in rodents and humans. We used deep brain in vivo calcium imaging and taste reactivity in freely behaving male and female Sprague Dawley rats to examine whether the infralimbic cortex is involved in encoding innate versus conditioned hedonic states. In experiment 1, we examined the IL neuronal ensemble responsiveness to intraoral innately rewarding (sucrose) versus aversive (quinine) tastants. Most IL neurons responded to either sucrose only or both sucrose and quinine, with fewer neurons selectively processing quinine. Among neurons that responded to both stimuli, some appear to encode hedonic processing. In experiment 2, we examined how IL neurons process devalued sucrose using conditioned taste aversion (CTA). We found that neurons that responded exclusively to sucrose were disengaged while additional quinine-exclusive neurons were recruited. Moreover, tastant-specific neurons that did not change their neuronal activity after CTA appeared to encode objective hedonic value. However, other neuronal ensembles responded to both tastants and appear to encode distinct aspects of hedonic processing. Specifically, some neurons responded differently to quinine and sucrose and shifted from appetitive-like to aversive-like activity after CTA, thus encoding the subjective hedonic value of the stimulus. Conversely, neurons that responded similarly to both tastants were heightened after CTA. Our findings show dynamic shifts in IL ensembles encoding devalued sucrose and support a role for parallel processing of objective and subjective hedonic value.SIGNIFICANCE STATEMENT Disrupted affective processing contributes to psychiatric disorders including depression, substance use disorder, and schizophrenia. We assessed how the infralimbic cortex, a key neural substrate involved in affect generation and affect regulation, processes innate and learned hedonic states using deep brain in vivo calcium imaging in freely behaving rats. We report that unique infralimbic cortex ensembles encode stimulus subjective and objective hedonic value. Further, our findings support similarities and differences in innate versus learned negative affective states. This study provides insight into the neural mechanisms underlying affect generation and helps to establish a foundation for the development of novel treatment strategies to reduce negative affective states that arise in many psychiatric disorders.

Activation of Infralimbic to Nucleus Accumbens Shell Pathway Suppresses Conditioned Aversion in Male But Not Female Rats.

Hedonic processing plays an integral role in directing appropriate behavior, but disrupted hedonic processing is associated with psychiatric disorders such as depression. The infralimbic cortex (IL) is a key structure in affective processing in rodents and activation of its human homolog, the ventromedial prefrontal cortex, has been implicated in suppressing aversive states. Here, we tested whether optogenetic activation of glutamatergic projections from the IL to the nucleus accumbens shell (NAcSh) suppresses the aversive impact of sucrose devalued using the conditioned taste aversion paradigm in males and female rats. In naive rats, no significant differences in appetitive or aversive taste reactivity (TR) to sucrose was observed indicating that initial sucrose palatability was equivalent across sex. However, we found that optical activation of the IL-NAcSh pathway during intraoral infusion of devalued sucrose inhibited aversive TR in male but not female rats. Interestingly, when allowed to freely ingest water and sucrose in a two-bottle test both males and females with a history of IL-NAcSh stimulation exhibited greater preference for sucrose. Optical pathway activation failed to alter TR to innately bitter quinine in either sex. Finally, both sexes lever pressed to self-stimulate the IL-NAcSh pathway. These results indicate that the IL-NAcSh pathway plays an important role in suppressing learned aversive states selectively in males but spares hedonic processing of innately aversive tastants. Further, pathway activation is reinforcing in both sexes, indicating that suppression of conditioned aversive TR can be dissociable from the effects of unconditioned rewarding properties of IL-NAcSh pathway activation.SIGNIFICANCE STATEMENT Negative emotional states contribute to psychiatric disorders including depression and substance use disorders. In this study, we examined whether brain circuitry previously implicated in suppressing negative emotional states in humans can inhibit learned aversion in male and female rats. We found that optical activation of the infralimbic to nucleus accumbens shell pathway attenuates learned aversive responses in male but not female rats, indicating an important sex difference in the function of this brain pathway. Furthermore, we found that pathway stimulation was reinforcing in both sexes. Collectively, these findings support the role of the infralimbic cortex and its projection to the nucleus accumbens shell in suppressing learned negative emotional states and highlight an important sex-specific function of this pathway.

Amphetamine-induced sensitization of hypertension and lamina terminalis neuroinflammation.

Psychomotor stimulants are prescribed for many medical conditions, including obesity, sleep disorders, and attention-deficit/hyperactivity disorder. However, despite their acknowledged therapeutic utility, these stimulants are frequently abused, and their use can have both short- and long-term negative consequences. Although stimulants such as amphetamines acutely elevate blood pressure, it is unclear whether they cause any long-term effects on cardiovascular function after use has been discontinued. Previous work in our laboratory has demonstrated that physiological and psychosocial stressors will produce sensitization of the hypertensive response, a heightened pressor response to a hypertensinogenic stimulus delivered after stressor exposure. Here, we tested whether pretreatment with amphetamine for 1 wk can sensitize the hypertensive response in rats. We found that repeated amphetamine administration induced and maintained sensitization of the pressor response to angiotensin II following a 7-day delay after amphetamine injections were terminated. We also found that amphetamine pretreatment altered mRNA expression for molecular markers associated with neuroinflammation and renin-angiotensin-aldosterone system (RAAS) activation in the lamina terminalis, a brain region implicated in the control of sympathetic nervous system tone and blood pressure. The results indicated amphetamine upregulated mRNA expression underlying neuroinflammation and, to a lesser degree, message for components of the RAAS in the lamina terminalis. However, we found no changes in mRNA expression in the paraventricular nucleus. These results suggest that a history of stimulant use may predispose individuals to developing hypertension by promoting neuroinflammation and upregulating activity of the RAAS in the lamina terminalis.

Orexin neurons couple neural systems mediating fluid balance with motivation-related circuits.

During extracellular dehydration, neural systems that sense deficits in body fluid homeostasis operate in tandem with those that mediate motivation and reward in order to promote ingestive behaviors that restore fluid balance. We hypothesized that hypothalamic orexin (Ox) neurons act as an interface to couple brain regions sensing and processing information about body fluid status with central nervous system motivation and reward systems. An initial set of anterograde and retrograde tracing experiments suggested that structures along the lamina terminalis (LT), a region of the forebrain that serves to monitor and integrate information reflecting body fluid balance, project to hypothalamic Ox neurons that, in turn, project to dopamine neurons in the ventral tegmental area (VTA). A second set of experiments determined whether Ox neuron activation is associated with extracellular dehydration and the seeking out and consumption of water and saline. An elevation of Fos-like immunoreactivity in Ox neurons was observed in fluid-depleted rats that were allowed to ingest water and sodium. A final experiment was conducted to determine whether Ox release in the VTA promotes thirst and salt appetite. Bilateral microinjection of the Ox Type I receptor antagonist SB-408124 into the VTA prior to acute extracellular dehydration attenuated fluid intake. Together, these studies support the hypothesis that structures along the LT modulate activity in the VTA through actions of orexinergic neurons that have cell bodies in the hypothalamus. This pathway may function to facilitate sustained consumption of fluids necessary for restoration of fluid balance. (PsycINFO Database Record

Opposing Roles of Rapid Dopamine Signaling Across the Rostral-Caudal Axis of the Nucleus Accumbens Shell in Drug-Induced Negative Affect.

BACKGROUND: Negative reinforcement theories of drug addiction posit that addicts use drugs to alleviate negative mood states. In a preclinical model developed in our laboratory, rats exhibit negative affect to a normally rewarding taste cue when it predicts impending but delayed cocaine. The emergence of this state is accompanied by a reduction in dopamine concentration in the rostral nucleus accumbens shell. However, the rostral and caudal regions of the shell have been implicated in promoting opposing appetitive and aversive states, respectively. Here, we tested whether dopamine transmission along the rostral-caudal axis of the shell plays differential roles in the emergence of drug-induced negative affect. METHODS: In TH::Cre rats, the dopaminergic pathways from the ventral tegmental area to the rostral and caudal regions of the shell were optogenetically stimulated during intraoral delivery of a taste cue signaling delayed cocaine. Affective responses to the taste cue were measured using taste reactivity, and optical self-stimulation of the rostral and caudal shells was also examined. RESULTS: Optical stimulation of the rostral shell during tastant infusion prevented the emergence of negative affect, but activation of the caudal shell exacerbated aversive responses. These effects endured in the absence of optical stimulation, and the degree of negative affect in our model predicted self-stimulation responding. CONCLUSIONS: These findings reveal unprecedented, pronounced, and opposing roles of rapid dopamine signaling across the rostral-caudal axis of the nucleus accumbens in the control of drug-induced negative affect, a hallmark of continued drug seeking and use in human addicts.

The roles of sensitization and neuroplasticity in the long-term regulation of blood pressure and hypertension.

After decades of investigation, the causes of essential hypertension remain obscure. The contribution of the nervous system has been excluded by some on the basis that baroreceptor mechanisms maintain blood pressure only over the short term. However, this point of view ignores one of the most powerful contributions of the brain in maintaining biological fitness-specifically, the ability to promote adaptation of behavioral and physiological responses to cope with new challenges and maintain this new capacity through processes involving neuroplasticity. We present a body of recent findings demonstrating that prior, short-term challenges can induce persistent changes in the central nervous system to result in an enhanced blood pressure response to hypertension-eliciting stimuli. This sensitized hypertensinogenic state is maintained in the absence of the inducing stimuli, and it is accompanied by sustained upregulation of components of the brain renin-angiotensin-aldosterone system and other molecular changes recognized to be associated with central nervous system neuroplasticity. Although the heritability of hypertension is high, it is becoming increasingly clear that factors beyond just genes contribute to the etiology of this disease. Life experiences and attendant changes in cellular and molecular components in the neural network controlling sympathetic tone can enhance the hypertensive response to recurrent, sustained, or new stressors. Although the epigenetic mechanisms that allow the brain to be reprogrammed in the face of challenges to cardiovascular homeostasis can be adaptive, this capacity can also be maladaptive under conditions present in different evolutionary eras or ontogenetic periods.

The biopsychology of salt hunger and sodium deficiency.

Sodium is a necessary dietary macromineral that tended to be sparsely distributed in mankind's environment in the past. Evolutionary selection pressure shaped physiological mechanisms including hormonal systems and neural circuits that serve to promote sodium ingestion. Sodium deficiency triggers the activation of these hormonal systems and neural circuits to engage motivational processes that elicit a craving for salty substances and a state of reward when salty foods are consumed. Sodium deficiency also appears to be associated with aversive psychological states including anhedonia, impaired cognition, and fatigue. Under certain circumstances the psychological processes that promote salt intake can become powerful enough to cause "salt gluttony," or salt intake far in excess of physiological need. The present review discusses three aspects of the biopsychology of salt hunger and sodium deficiency: (1) the psychological processes that promote salt intake during sodium deficiency, (2) the effects of sodium deficiency on mood and cognition, and (3) the sensitization of sodium appetite as a possible cause of salt gluttony.

The role of the lateral hypothalamus and orexin in ingestive behavior: a model for the translation of past experience and sensed deficits into motivated behaviors.

The hypothalamus has been recognized for its involvement in both maintaining homeostasis and mediating motivated behaviors. The present article discusses a region of the hypothalamus known as the lateral hypothalamic area (LHA). It is proposed that brain nuclei within the LHA including the dorsal region of the lateral hypothalamus (LHAd) and perifornical area (PeF) provide a link between neural systems that regulate homeostasis and those that mediate appetitive motivated behaviors. Functional and immunohistochemical data indicate that the LHA promotes many motivated behaviors including food intake, water intake, salt intake, and sexual behavior. Anatomical tracing experiments demonstrate that the LHA is positioned to receive inputs from brain areas involved in regulating body fluid and energy homeostasis. Regions within the LHA send dense projections to the ventral tegmental area (VTA), providing a pathway for the LHA to influence dopaminergic systems generally recognized to be involved in motivated behaviors and their reinforcement. Furthermore, the LHA contains neurons that synthesize orexin/hypocretin, a neuropeptide that promotes many appetitive motivated behaviors. The LHA also receives inputs from brain areas involved in reward-related learning and orexin neuron activation can become conditioned to environmental stimuli that are associated with rewards. Therefore, it is hypothesized that the LHA integrates signaling from areas that regulate body fluid and energy balance and reward-related learning. In turn, this information is "fed into" mesolimbic circuitry to influence the performance of motivated behaviors. This hypothesis may foster experiments that will result in an improved understanding of LHA function. An improved understanding of LHA function may aid in treating disorders that are associated with an excess or impairment in the expression of ingestive behavior including obesity, anorexia, impairments in thirst, salt gluttony, and salt deficiency.

Sensitization of sodium appetite: evidence for sustained molecular changes in the lamina terminalis.

Animals with a history of sodium depletions exhibit increases in salt intake, a phenomenon described as the sensitization of sodium appetite. Using a novel experimental design, the present experiments investigated whether putative molecular markers of neural plasticity and changes in the message for components of the brain renin-angiotensin-aldosterone-system (RAAS) accompany the sensitization of sodium appetite. An initial set of experiments examined whether the glutamatergic N-methyl-d-aspartate receptor antagonist MK-801 would attenuate sodium appetite sensitization and prevent changes in mRNA expression associated with sensitization. Rats with repeated sodium depletions exhibited enhanced sodium appetite and mRNA expression for components of the RAAS in areas along the lamina terminalis (LT), a region of the brain that is important for the regulation of body fluid homeostasis, and these effects were significantly attenuated by MK-801 pretreatment. A second set of experiments investigated whether successive sodium depletions would elevate sodium intake and induce a pattern of fos-B staining consistent with the Δfos-B isoform in areas along the LT. The pattern of fos-B staining in the subfornical organ was consistent with the characteristics of Δfos-B expression. Specifically, fos-B/Δfos-B expression was increased 4 days after the last of a series of sodium depletions, fos-B/Δfos-B expression was nearly absent in control rats, and the quantity of fos-B/Δfos-B staining was directly associated with a history of sodium depletions. These findings demonstrate that the sensitization of sodium appetite is associated with sustained molecular alterations in the LT that are indicative of neural plasticity and upregulation of the central RAAS.

Dissociation of thirst and sodium appetite in the furo/cap model of extracellular dehydration and a role for N-methyl-D-aspartate receptors in the sensitization of sodium appetite.

Depletion of extracellular fluids motivates many animals to seek out and ingest water and sodium. Animals with a history of extracellular dehydration display enhanced sodium appetite and, in some cases, thirst. The progressive increase in sodium intake induced by repeated sodium depletions is known as sensitization of sodium appetite. Administration of the diuretic and natriuretic drug, furosemide, along with a low dose of captopril (furo/cap), elicits thirst and a rapid onset of sodium appetite. In the present studies the furo/cap model was used to explore the physiological mechanisms of sensitization of sodium appetite. However, when thirst and sodium appetite were measured concurrently in the furo/cap model, individual rats exhibited sensitization of either thirst or sodium appetite. In subsequent studies, thirst and sodium appetite were dissociated by offering either water prior to sodium or sodium before water. When water and sodium intake were dissociated in time, the furo/cap model reliably produced sensitization of sodium appetite. It is likely that neuroplasticity mediates this sensitization. Glutamatergic N-methyl-d-aspartate receptor (NMDA-R) activation is critical for the development of most forms of neuroplasticity. Therefore, we hypothesized that integrity of NMDA-R function is necessary for the sensitization of sodium appetite. Pharmacological blockade of NMDA-Rs with systemic administration of MK-801 (0.15 mg/kg) prevented the sensitization of fluid intake in general when water and sodium were offered concurrently, and prevented sensitization of sodium intake specifically when water and sodium intake were dissociated. The involvement of NMDA-Rs provides support for the possibility that sensitization of sodium appetite is mediated by neuroplasticity.

Effect of amniotic-fluid ingestion on vaginal-cervical-stimulation-induced Fos expression in female rats during estrus.

Placental Opioid-Enhancing Factor (POEF) is a substance found in amniotic fluid (AF) that, when ingested, potentiates opioid-mediated, but not non-opioid-mediated, hypoalgesia. Vaginal-cervical stimulation (VCS) produces a stimulus-bound, partially opioid-mediated hypoalgesia that previous research has shown to be potentiated by AF ingestion. To understand the mechanism of opioid enhancement by POEF we investigated the pattern of neural activation after a bout of VCS that produced hypoalgesia, with and without co-administration of AF. Specifically, virgin Long-Evans rats showing vaginal estrus were handled briefly (control) or received VCS (75g pressure, 1 min), in a pattern that approximated early parturition rather than copulation, using a spring-loaded glass-rod probe. Rats were given an orogastric infusion (0.25 ml) of either AF or 0.9% saline resulting in four groups (VCS or handling; AF or saline). Rats were perfused 90 min after treatment and tissue was processed by immunohistochemistry for Fos. The number of Fos-immunoreactive cells was counted in structures previously shown to express Fos in response to VCS (the medial preoptic area, MPOA; the ventrolateral portion of the ventromedial hypothalamic nucleus, vlVMH; the arcuate nucleus, ARC). We found that this pattern of VCS did not produce a significant increase in Fos expression in the MPOA and vlVMH unless it was paired with AF. VCS produced a significant increase in Fos in the ARC. The interaction of AF and VCS on Fos expression in the MPOA suggests that POEF may enhance vaginal-cervical sensory input at parturition to facilitate sensitization of the MPOA, and presumably facilitate maternal-behavior onset.