Cocaine- and amphetamine-regulated transcript peptide (CART) in the central nucleus of amygdala potentiates behavioral and hormonal responses of the rat exposed to its predator

Cocaine and amphetamine regulated transcript (CART) mediates sex differences in binge drinking through central taste circuits

The neuropeptide cocaine- and amphetamine-regulated transcript (CART) has been implicated in alcohol consumption and reward behaviours, yet mechanisms mediating these effects have yet to be identified. Using a transgenic CART knockout (KO) mouse line we uncovered a sexually dimorphic effect of CART in binge drinking, with male CART KO mice increasing intake, whilst female CART KO mice decreased their alcohol intake compared to controls. Female CART KO mice show greater sensitivity to bitter solutions that can be overshadowed through addition of a sweetener, implicating taste as a factor. Further we identify that this is not driven through peripherally circulating sex hormones, but the central nucleus of the amygdala (CeA) is a locus where CART contributes to the regulation of alcohol consumption, with CeA CART neutralisation specifically reducing plain alcohol, but not sweetened alcohol consumption in female mice. These findings may have implications for the development of sex-specific treatment options for alcohol use disorders through targeting the CART system.

Cocaine and amphetamine regulated transcript (CART) signalling in the central nucleus of the amygdala modulates stress-induced alcohol seeking

The central nucleus of the amygdala (CeA) is a key hub of the neural circuitry regulating alcohol and stress interactions. However, the exact neuronal populations that govern this interaction are not well defined. Here we examined the role of the neuropeptide cocaine and amphetamine regulated transcript (CART) within the CeA in stress-induced alcohol seeking. We found that CART-containing neurons are predominantly expressed in the capsular/lateral division of the CeA and are a subpopulation of protein kinase Cδ (PKCδ) cells, distinct from corticotrophin releasing factor (CRF)-expressing cells. Both stress (yohimbine) and stress-induced alcohol seeking activated CART cells within the CeA, while neutralisation of endogenous CeA CART signalling (via antibody administration) attenuated stress-induced alcohol, but not sucrose seeking. Further, blocking CART signalling within the CeA did not alter the motivation to obtain and consume alcohol but did attenuate stressor-induced anxiety-like behaviour during abstinence from alcohol. Together, these data identify CeA CART cells as a subpopulation of PKCδ cells that influence stress × alcohol interactions and mediate stress-induced alcohol seeking behaviours.

Cocaine- and amphetamine-regulated transcript (CART): A multifaceted neuropeptide

Over the last 35 years, the continuous discovery of novel neuropeptides has been the key to the better understanding of how the central nervous system has integrated with neuronal signals and behavioral responses. Cocaine and amphetamine-regulated transcript (CART) was discovered in 1995 in the rat striatum but later was found to be highly expressed in the hypothalamus. The widespread distribution of CART peptide in the brain complicated the understanding of the role played by this neurotransmitter. The main objective of the current compact review is to piece together the fragments of available information about origin, expression, distribution, projection, and function of CART peptides. Accumulative evidence suggests CART as a neurotransmitter and neuroprotective agent that is mainly involved in regulation of feeding, addiction, stress, anxiety, innate fear, neurological disease, neuropathic pain, depression, osteoporosis, insulin secretion, learning, memory, reproduction, vision, sleep, thirst and body temperature. In spite of the vast number of studies about the CART, the overall pictures about the CART functions are sketchy. First, there is a lack of information about cloned receptor, specific agonist and antagonist. Second, CART peptides are detected in discrete sets of neurons that can modulate countless activities and third; CART peptides exist in several fragments due to post-translational processing. For these reasons the overall picture about the CART peptides are sketchy and confounding.

CART neuropeptide modulates the extended amygdalar CeA-vBNST circuit to gate expression of innate fear

Innate fear is critical for the survival of animals and is under tight homeostatic control. Deregulation of innate fear processing is thought to underlie pathological phenotypes including, phobias and panic disorders. Although central processing of conditioned fear has been extensively studied, the circuitry and regulatory mechanisms subserving innate fear remain relatively poorly defined. In this study, we identify cocaine- and amphetamine-regulated transcript (CART) neuropeptide signaling in the central amygdala (CeA) - ventral bed nucleus of stria terminalis (vBNST) axis as a key modulator of innate fear expression. 2,4,5-trimethyl-3-thiazoline (TMT), a component of fox faeces, induces a freezing response whose intensity is regulated by the extent of CART-signaling in the CeA neurons. Abrogation of CART activity in the CeA attenuates the freezing response and reduces activation of vBNST neurons. Conversely, ectopically elevated CART signaling in the CeA potentiates the fear response concomitant with enhanced vBNST activation. We show that local levels of CART signaling modulate the activation of CeA neurons by NMDA receptor-mediated glutamatergic inputs, in turn, regulating activity in the vBNST. This study identifies the extended amygdalar CeA-vBNST circuit as a CART modulated axis encoding innate fear. CART signaling regulates the glutamatergic excitatory drive in the CeA-vBNST circuit, in turn, gating the expression of the freezing response to TMT.

Differences in inhibitory avoidance, cortisol and brain gene expression in TL and AB zebrafish

Recently, we established an inhibitory avoidance paradigm in Tupfel Long-Fin (TL) zebrafish. Here, we compared task performance of TL fish and fish from the AB strain; another widely used strain and shown to differ genetically and behaviourally from TL fish. Whole-body cortisol and telencephalic gene expression related to stress, anxiety and fear were measured before and 2 h post-task. Inhibitory avoidance was assessed in a 3-day paradigm: fish learn to avoid swimming from a white to a black compartment where a 3V-shock is given: day 1 (first shock), day 2 (second shock) and day 3 (no shock, sampling). Tupfel Long-Fin fish rapidly learned to avoid the black compartment and showed an increase in avoidance-related spatial behaviour in the white compartment across days. In contrast, AB fish showed no inhibitory avoidance learning. AB fish had higher basal cortisol levels and expression levels of stress-axis related genes than TL fish. Tupfel Long-Fin fish showed post-task learning-related changes in cortisol and gene expression levels, but these responses were not seen in AB fish. We conclude that AB fish show higher cortisol levels and no inhibitory avoidance than TL fish. The differential learning responses of these Danio strains may unmask genetically defined risks for stress-related disorders.

The effects of environmental enrichment and age-related differences on inhibitory avoidance in zebrafish (Danio rerio Hamilton)

The inhibitory avoidance paradigm allows the study of mechanisms underlying learning and memory formation in zebrafish (Danio rerio Hamilton). For zebrafish, the physiology and behavior associated with this paradigm are as yet poorly understood. We therefore assessed the effects of environmental enrichment and fish age on inhibitory avoidance learning. Fish raised in an environmentally enriched tank showed decreased anxiety-like behavior and increased exploration. Enrichment greatly reduced inhibitory avoidance in 6-month (6M)- and 12-month (12 M)-old fish. Following inhibitory avoidance, telencephalic mRNA levels of proliferating cell nuclear antigen (pcna), neurogenic differentiation (neurod), cocaine- and amphetamine-regulated transcript 4 (cart4), and cannabinoid receptor 1 (cnr1) were lower in enriched-housed fish, while the ratios of mineralocorticoid receptor (nr3c2)/glucocorticoid receptor α [nr3c1(α)] and glucocorticoid receptor β [nr3c1(β)]/glucocorticoid receptor α [nr3c1(α)] were higher. This was observed for 6M-old fish only, not for 24-month (24 M) old fish. Instead, 24 M-old fish showed delayed inhibitory avoidance, no effects of enrichment, and reduced expression of neuroplasticity genes. Overall, our data show strong differences in inhibitory avoidance behavior between zebrafish of different ages and a clear reduction in avoidance behavior following housing under environmental enrichment.

Identification of the CART neuropeptide circuitry processing TMT-induced predator stress

Abundance of cocaine- and amphetamine-regulated transcript (CART) neuropeptide in the limbic areas like the olfactory system, central nucleus of amygdala (CeA), ventral bed nucleus of stria terminalis (vBNST) and the hypothalamus suggests involvement of the peptide in emotive processing. We examined the role of CART in mediating fear, a strong emotion with profound survival value. Rats, exposed to 2,4,5-trimethyl-3-thiazoline (TMT), a predator related cue extracted from fox feces, showed significant increase in freezing, escape and risk assessment behavior, whereas grooming was reduced. Neuronal activity was up-regulated in the CeA and vBNST in terms of increased immunoreactivity in CART elements and c-Fos expression. Increased expression of both the markers was also seen in some discrete magnocellular as well as parvicellular subdivisions of the paraventricular nucleus (PVN). However, CART containing mitral cells in the main or accessory olfactory bulb did not respond. CART antibody was stereotaxically injected bilaterally into the CeA to locally immunoneutralize endogenous CART. On exposure to TMT, these rats showed reduced freezing, risk assessment and escape behavior while grooming was restored to normal value. We suggest that the CART signaling in the CeA and vBNST, but not in the olfactory system, might be an important component of the innate fear processing, and expression of stereotypic behavior, while CART in the PVN subdivisions might mediate the neuroendocrine response to predator stress.

Differences in the structure of drinking, cart expression and dopamine turnover between polydipsic and non polydipsic rats in the quinpirole model of psychotic polydipsia

RATIONALE

Dopaminergic D2/D3 agonist quinpirole (QNP) elicits nonregulatory drinking in rats, a model of psychotic polydipsia. Why only a fraction of QNP-treated rats responds to the treatment becoming polydipsic is still unclear.

OBJECTIVES

To unveil possible factors contributing to such variability, we analyzed drinking microstructure in saline and QNP-treated rats, the hypothalamic expression of the cocaine and amphetamine regulated transcript (CART), and the monoaminergic turnover in selected brain areas.

METHODS

Rats were daily treated with saline or QNP 0.5 mg/kg, and their 5-h water intake was measured for five consecutive days. The number of bouts and episodes of licking, and their duration, were also measured. Brain CART expression was measured by in situ hybridization and monoamines turnover by HPLC analysis of tissue extracts. Based on the amount of water ingested during the 5-h session, QNP-treated rats were post hoc grouped in polydipsic (PD) and in nonpolydipsic (NPD) rats, and the results compared accordingly.

RESULTS

The number of drinking bouts and episodes increased in PD rats, while NPD rats behaved as the controls. CART expression decreased in the arcuate nucleus of the hypothalamus of the PD rats. In contrast, both PD and NPD rats showed a reduction of DA turnover in both ventral tegmental area (VTA) and nucleus accumbens (NAcc). No difference was detected in the turnover of 5HT and NA.

CONCLUSIONS

Microstructure analysis confirms that QNP acts on the appetitive component of drinking behavior, making it compulsive. CART expression reduction in response to dopaminergic hyperstimulation might sustain excessive drinking in PD rats.

Unpredictable chronic stress decreases inhibitory avoidance learning in Tuebingen long-fin zebrafish: stronger effects in the resting phase than in the active phase

Zebrafish (Danio rerio Hamilton) are increasingly used as a model to study the effects of chronic stress on brain and behaviour. In rodents, unpredictable chronic stress (UCS) has a stronger effect on physiology and behaviour during the active phase than during the resting phase. Here, we applied UCS during the daytime (active phase) for 7 and 14 days or during the night-time (resting phase) for 7 nights in an in-house-reared Tuebingen long-fin (TLF) zebrafish strain. Following UCS, inhibitory avoidance learning was assessed using a 3 day protocol where fish learn to avoid swimming from a white to a black compartment where they will receive a 3 V shock. Latencies of entering the black compartment were recorded before training (day 1; first shock) and after training on day 2 (second shock) and day 3 (no shock, tissue sampling). Fish whole-body cortisol content and expression levels of genes related to stress, fear and anxiety in the telencephalon were quantified. Following 14 days of UCS during the day, inhibitory avoidance learning decreased (lower latencies on days 2 and 3); minor effects were found following 7 days of UCS. Following 7 nights of UCS, inhibitory avoidance learning decreased (lower latency on day 3). Whole-body cortisol levels showed a steady increase compared with controls (100%) from 7 days of UCS (139%), to 14 days of UCS (174%) to 7 nights of UCS (231%), suggestive of an increasing stress load. Only in the 7 nights of UCS group did expression levels of corticoid receptor genes (mr, grα, grβ) and of bdnf increase. These changes are discussed as adaptive mechanisms to maintain neuronal integrity and prevent overload, and as being indicative of a state of high stress load. Overall, our data suggest that stressors during the resting phase have a stronger impact than during the active phase. Our data warrant further studies on the effect of UCS on stress axis-related genes, especially grβ; in mammals this receptor has been implicated in glucocorticoid resistance and depression.

Inhibitory avoidance learning in zebrafish (Danio rerio): effects of shock intensity and unraveling differences in task performance

The zebrafish (Danio rerio) is increasingly used as a model in neurobehavioral and neuroendocrine studies. The inhibitory avoidance paradigm has been proposed as tool to study mechanisms underlying learning and memory in zebrafish. In this paradigm subjects receive a shock after entering the black compartment of a black-white box. On the next day, latency to enter the black compartment is assessed; higher latencies are indicative of increased avoidance learning. Here, we aimed to understand the effects of different shock intensities (0, 1, 3, and 9 V) and to unravel variation in inhibitory avoidance learning in an in-house reared Tuebingen Long-Fin zebrafish (D. rerio) strain. While median latencies had increased in the 1, 3, and 9 V groups, no increase in median latency was found in the 0 V group. In addition, higher shock intensities resulted in a higher number of avoiders (latency ≥180 s) over nonavoiders (latency <60 s). Both changes are indicative of increased avoidance learning. We assessed whole-body cortisol content and the expression levels of genes relevant to stress, anxiety, fear, and learning 2 h after testing. Shock intensity was associated with whole-body cortisol content and the expression of glucocorticoid receptor alpha [nr3c1(alpha)], cocaine- and amphetamine-regulated transcript (cart4), and mineralocorticoid receptor (nr3c2), while avoidance behavior was associated with whole-body cortisol content only. The inhibitory avoidance paradigm in combination with measuring whole-body cortisol content and gene expression is suitable to unravel (genetic) mechanisms of fear avoidance learning. Our data further show differences in brain-behavior relationships underlying fear avoidance learning and memory in zebrafish. These findings serve as starting point for further unraveling differences in brain-behavior relationships underlying (fear avoidance) learning and memory in zebrafish.

CART in the brain of vertebrates: circuits, functions and evolution

Cocaine- and amphetamine-regulated transcript peptide (CART) with its wide distribution in the brain of mammals has been the focus of considerable research in recent years. Last two decades have witnessed a steady rise in the information on the genes that encode this neuropeptide and regulation of its transcription and translation. CART is highly enriched in the hypothalamic nuclei and its relevance to energy homeostasis and neuroendocrine control has been understood in great details. However, the occurrence of this peptide in a range of diverse circuitries for sensory, motor, vegetative, limbic and higher cortical areas has been confounding. Evidence that CART peptide may have role in addiction, pain, reward, learning and memory, cognition, sleep, reproduction and development, modulation of behavior and regulation of autonomic nervous system are accumulating, but an integration has been missing. A steady stream of papers has been pointing at the therapeutic potentials of CART. The current review is an attempt at piecing together the fragments of available information, and seeks meaning out of the CART elements in their anatomical niche. We try to put together the CART containing neuronal circuitries that have been conclusively demonstrated as well as those which have been proposed, but need confirmation. With a view to finding out the evolutionary antecedents, we visit the CART systems in sub-mammalian vertebrates and seek the answer why the system is shaped the way it is. We enquire into the conservation of the CART system and appreciate its functional diversity across the phyla.