CART peptides: regulators of body weight, reward and other functions.

Over the past decade or so, CART (cocaine- and amphetamine-regulated transcript) peptides have emerged as major neurotransmitters and hormones. CART peptides are widely distributed in the CNS and are involved in regulating many processes, including food intake and the maintenance of body weight, reward and endocrine functions. Recent studies have produced a wealth of information about the location, regulation, processing and functions of CART peptides, but additional studies aimed at elucidating the physiological effects of the peptides and at characterizing the CART receptor(s) are needed to take advantage of possible therapeutic applications.

Beta-adrenergic-receptor localization by light microscopic autoradiography.

beta-Receptors were identified in rat brain by a light microscopic autoradiographic technique. The procedure involved binding 3H-labeled dihydroalprenolol to beta-receptors in intact slide-mounted tissue sections and generating autoradiograms by the apposition of emulsion-coated cover slips, Biochemical analysis of the binding indicated that these conditions provided a high degree of selective labeling of beta-receptors. High densities of receptors were found in superficial layers of the cerebral cortex, throughout the caudate-putamen, in the periventricular nucleus of the thalamus, in the molecular layer of the cerebellum, and in other areas. These results are in agreement with other electrophysiological and histochemical data. This radiohistochemical approach should be an important addition to other methods for mapping functional catecholamine neuronal pathways and sites of hormonal action.

Cocaine receptors on dopamine transporters are related to self-administration of cocaine.

Although cocaine binds to several sites in the brain, the biochemical receptor mechanism or mechanisms associated with its dependence producing properties are unknown. It is shown here that the potencies of cocaine-like drugs in self-administration studies correlate with their potencies in inhibiting [3H]mazindol binding to the dopamine transporters in the rat striatum, but not with their potencies in binding to a large number of other presynaptic and postsynaptic binding sites. Thus, the cocaine receptor related to substance abuse is proposed to be the one associated with dopamine uptake inhibition.

Opioid receptors undergo axonal flow.

Previous studies have indicated the presence of opiate receptors on axons of the rat vagus nerve and on other small diameter fibers. In examinations of the effect of ligation on the distribution of receptors in the vagus nerve by in vitro labeling light microscopic autoradiography, a large buildup of receptors was found proximal to the ligature. This result indicates an axonal flow of receptors.

Adenosine receptors: autoradiographic evidence for their location on axon terminals of excitatory neurons.

Adenosine receptors were made visible on light microscopy by autoradiography with tritiated cyclohexyladenosine. In the cerebellum, adenosine receptors were absent in Weaver mice, which lack granule cells, and were displaced in Reeler mice, which have displacements of granule cells. Thus, adenosine receptors appear to be located on the axon terminals of excitatory granule cells in the cerebellum. Removal of one eye of a rat depleted adenosine receptors in the contralateral superior colliculus, suggesting that the receptors occur on axon terminals of excitatory projections from retinal ganglion cells. The presence of adenosine receptors on excitatory axon terminals may explain synaptic inhibition by adenosine and the behavioral effects of xanthines.

Corticotropin-releasing factor receptors in rat forebrain: autoradiographic identification.

Corticotropin-releasing factor (CRF) receptors were identified in rat forebrain by autoradiography with an iodine-125-labeled analog of ovine CRF substituted with norleucine and tyrosine at amino acid residues 21 and 32, respectively. High-affinity receptors for CRF were found in discrete areas of rat forebrain, including laminae I and IV of the neocortex, the external layer of the medium eminence, the lateral nucleus of the amygdala, and the striatum. These results are consistent with earlier findings on the immunohistochemical distribution of CRF and suggest that endogenous CRF has a physiological role in regulating activity of the central nervous system.

Normalization of spiroperidol binding in the denervated rat striatum by homologous grafts of substantia nigra.

Transplantation of embryonic substantia nigra into the adult rat brain decreases the motor asymmetry that is produced by dopamine receptor supersensitivity after a unilateral lesion of the substantia nigra. The authors report that this effect of transplantation is specific to grafts of substantia nigra. They also report that, in conjunction with the decrease in motor asymmetry, these grafts cause postsynaptic dopaminergic binding sites to return to normal density as measured by tritiated spiroperidol autoradiography. Thus, in animals with brain lesions, grafts of substantia nigra produce a long-term alteration in the functional status of host brain cell receptors that is associated with a reduction in the behavioral deficit.

Imaging dopamine receptors in the human brain by positron tomography.

Neurotransmitter receptors may be involved in a number of neuropsychiatric disease states. The ligand 3-N-[11C]methylspiperone, which preferentially binds to dopamine receptors in vivo, was used to image the receptors by positron emission tomography scanning in baboons and in humans. This technique holds promise for noninvasive clinical studies of dopamine receptors in humans.

Positron emission tomography reveals elevated D2 dopamine receptors in drug-naive schizophrenics.

In postmortem studies of patients with schizophrenia, D2 dopamine receptors in the basal ganglia have been observed to be more numerous than in patients with no history of neurological or psychiatric disease. Because most patients with schizophrenia are treated with neuroleptic drugs that block D2 dopamine receptors in the caudate nucleus, it has been suggested that this increase in the number of receptors is a result of adaptation to these drugs rather than a biochemical abnormality intrinsic to schizophrenia. With positron emission tomography (PET), the D2 dopamine receptor density in the caudate nucleus of living human beings was measured in normal volunteers and in two groups of patients with schizophrenia--one group that had never been treated with neuroleptics and another group that had been treated with these drugs. D2 dopamine receptor densities in the caudate nucleus were higher in both groups of patients than in the normal volunteers. Thus, schizophrenia itself is associated with an increase in brain D2 dopamine receptor density.

Leptin activates hypothalamic CART neurons projecting to the spinal cord.

The adipocyte-derived hormone leptin decreases body weight in part by activating the sympathetic nervous system, resulting in increased thermogenesis and energy expenditure. We investigated hypothalamic pathways underlying leptin's effects on stimulating the sympathetic nervous system. We found that leptin activates neurons in the retrochiasmatic area (RCA) and lateral arcuate nucleus (Arc) that innervate the thoracic spinal cord and also contain cocaine- and amphetamine-regulated transcript (CART). We also found that most CART-containing neurons in the RCA and Arc of the hypothalamus also contain proopiomelanocortin (POMC) mRNA. The finding that leptin activates CART/POMC neurons innervating sympathetic preganglionic neurons in the thoracic spinal cord suggests that this pathway may contribute to the increased thermogenesis and energy expenditure and decreased body weight observed following leptin administration.

Cocaine administration increases the fraction of CART cells in the rat nucleus accumbens that co-immunostain for c-Fos.

In order to further test whether or not psychostimulant drugs activate CART peptide-containing cells in the nucleus accumbens, we examined the fraction of CART positive cells that co-immunostained for c-Fos after administration of saline or cocaine (10 and 25 mg/kg i.p.). There was about a 45% increase in the fraction of cells that stained for both CART and c-Fos after administration of cocaine, but there was no change in the fraction after administration of saline. Moreover, the increase was not found 24h after injection and is therefore reversible. These results support the notion that psychostimulant drugs activate CART cells in the nucleus accumbens, even under conditions where it is difficult to show a change in CART levels.

Unique responses of midbrain CART neurons in macaques to ovarian steroids.

CART (cocaine and amphetamine regulated transcript) is a neuropeptide involved in the control of several physiological processes, such as response to psychostimulants, food intake, depressive diseases and neuroprotection. It is robustly expressed in the brain, mainly in regions that control emotional and stress responses and it is regulated by estrogen in the hypothalamus. There is a distinct population of CART neurons located in the vicinity of the Edinger-Westphal nucleus of the midbrain that also colocalize urocortin-1. The aims of this study were 1) to determine the distribution of CART immunoreactive neurons in the monkey midbrain, 2) to examine the effects of estrogen (E) and progesterone (P) on midbrain CART mRNA and peptide expression and 3) to determine whether midbrain CART neurons contain steroid receptors. Adult female rhesus monkeys (Macaca mulatta) were spayed and either treated with placebo (OVX), estrogen alone (E), progesterone alone (P) or E+P. Animals were prepared (a) for RNA extraction followed by microarray analysis and quantitative (q) RT-PCR (n=3/group); (b) for immunohistochemical analysis of CART and CART+tryptophan hydroxylase (TPH), CART+estrogen receptors (ER) or CART+progesterone receptors (n=5/group) and (c) for Western blots (n=3/group). Both E- and E+P-administration decreased CART gene expression on the microarray and with qRT-PCR. Stereological analysis of CART immunostaining at five levels of the Edinger-Westphal nucleus indicated little effect of E or E+P administration on the area of CART immunostaining. However, P administration increased CART-immunopositive area in comparison to the OVX control group with Student's t-test, but not with ANOVA. CART 55-102 detection on Western blot was unchanged by hormone administration. ERbeta and PR were detected in CART neurons and CART fibers appeared to innervate TPH-positive serotonin neurons in the dorsal raphe. In summary, E decreased CART mRNA, but this effect did not translate to the protein level. Moreover, P administration alone had a variable effect on CART mRNA, but it caused an increase in CART immunostaining. Together, the data suggest that CART neurons in the midbrain have a unique steroid response, which may be mediated by nuclear receptors, neuroactive steroids or interneurons.

Frequency of maternal licking and grooming correlates negatively with vulnerability to cocaine and alcohol use in rats.

Because licking and grooming behavior of dams with pups can influence some behaviors of pups when they are adults, we tested if licking and grooming scores in a maternal separation protocol correlated with cocaine or ethanol self-administration in the pups as adults. The protocol produced litters that were separated from dams for 0 (MS0), 15 (MS15) or 180 (MS180) min, and a nonhandled (NH) group as well. Self-administration of both drugs as shown in earlier studies was lowest in the MS15 group, highest in the NH group and intermediate in the other groups. Licking and grooming scores correlated negatively with drug intake and suggests that maternal care of pups can influence drug use in pups when they are adults.

CART deficiency increases body weight but does not alter bone strength.

The regulation of bone metabolism mediated by leptin is a complex process that is not clearly understood. Recent studies suggest that CART (cocaine-amphetamine related transcript) is a significant neuronal co-factor when combined with leptin. CART deficiency is thought to result in low trabecular bone mass, but since leptin exerts contrasting effects on trabecular and cortical bone it is possible that cortical bone may not respond to the absence of CART signaling in the same manner as trabecular bone. We tested the hypothesis that CART deficiency decreases cortical bone mass, density, and strength by examining femora of adult wild-type mice (CART(+/+)) and CART-deficient mice (CART(-/-)). DEXA densitometry (PIXImus system) was used to measure whole-bone mineral content (BMC) and mineral density (BMD) from right femora, and pQCT used to calculate densitometric and geometric parameters from the femur midshaft. Femora were also tested in three-point bending, and sections of the tibia analyzed histologically to determine bone marrow adipocyte density (N.At./M.Ar) and endocortical osteoclast number (N.Oc/B.Pm). The control mice weighed less than the mice lacking CART (P<0.001), but mechanical testing data showed no differences (p>0.05) in ultimate force, energy to fracture, stiffness, or intrinsic properties such as ultimate stress, ultimate strain, or modulus. CART-deficient mice did not differ from normal controls in whole-femur BMC (p=0.09), BMD (p=0.19), midshaft cortical bone thickness (p=0.67), midshaft cortical bone area (p=0.59) or N.Oc/B.Pm (p=0.94), although CART deficiency was associated with a three-fold increase in bone marrow adipocyte density (p<0.001). Our data suggest that while the central, neuroendocrine regulation of bone mass via CART signaling may have effects on trabecular mass, absence of CART expression does not significantly alter cortical bone geometry, density, or strength.

Maternal separation alters drug intake patterns in adulthood in rats.

Maternal separation/handling (MS/H) is an animal model of early life stress that causes profound neurochemical and behavioral alterations in pups that persist into adulthood. Many recent studies have used the MS/H model to study changes in drug effects in adulthood that are linked to behavioral treatments and stressors in the perinatal period. The drug effects focused on in this review are the reinforcing properties of the abused drugs, cocaine and alcohol. A striking finding is that variations in maternal separation and handling cause changes in ethanol and cocaine self-administration. Further, these changes indicate that various manipulations in the perinatal period can have long lasting effects of interest to biochemical pharmacologists. This article will review recent studies on ethanol and cocaine self-administration using the MS/H model and the neurochemical alterations that may play a role in the effects of MS/H on ethanol and cocaine self-administration. Studying the MS/H model can provide important clues into the vulnerability to drug abuse and perhaps identify a crucial window of opportunity for therapeutic intervention.

CART-peptide immunoreactivity in enteric nerves in patients with Hirschsprung's disease.

AIMS: Cocaine- and amphetamine-regulated transcript (CART)-peptide is found in the brain and participates in the control of feeding behavior. It is also expressed in the peripheral nervous system and is suggested to have neuromodulatory and/or neurotrophic effects in rat intestine. The aims of this study were to investigate the presence of CART-peptide in the normal ganglionic as well as aganglionic intestine from patients with Hirschsprung's disease and the peptide's possible coexistence with other neurotransmitters. METHODS: Intestinal specimens from nine patients with Hirschsprung's disease were examined using immunohistochemistry. A double immunostaining technique was used in order to elucidate the presence of CART-peptide in NOS and VIP-containing enteric neurons. RESULTS: In ganglionic intestine, CART-peptide was found in numerous nerve fibers, predominantly within the smooth muscle layers and in myenteric nerve cell bodies. A high degree of co-localization of CART with NOS and VIP was seen. Only very few CART immunoreactive nerve fibers and no nerve cell bodies were found in the aganglionic intestine. CONCLUSIONS: This is the first report on the presence of CART-peptide in the human intestine. In the ganglionic intestine CART was detected mainly in myenteric neurons, while only very few CART-IR nerve fibers were found in the aganglionic intestine. This, together with the coexistence of CART with NOS and VIP, indicates an intrinsic origin of the CART-containing neurons and suggests that CART may influence NO and VIP-induced effects.

CART peptides: novel addiction- and feeding-related neuropeptides.

CART peptides are novel, putative brain-gut neurotransmitters and co-transmitters that probably have a role in drug abuse, the control of feeding behavior, sensory processing, stress and development. They are abundant, processed and apparently released. Exogenously applied peptides cause inhibition of feeding and have neurotrophic properties. Although the precise sequences, relative abundance and efficacy of all CART peptides are currently being determined, small molecules that are active at putative CART receptors could have substantial therapeutic promise.

The dopamine hypothesis of the reinforcing properties of cocaine.

A variety of evidence suggests a 'dopamine hypothesis' for the reinforcing properties of cocaine. This hypothesis proposes that cocaine binds at the dopamine transporter and mainly inhibits neurotransmitter re-uptake; the resulting potentiation of dopaminergic neurotransmission in mesolimbocortical pathways ultimately causes reinforcement. This model suggests potential medications for treatment of cocaine abuse and dependence. Some, but not all, pharmacological data in humans support the hypothesis and additional experimentation is needed.

Maternal separation alters serotonergic transporter densities and serotonergic 1A receptors in rat brain.

RATIONALE: The basic mechanisms underlying the association between early life maternal separation and adulthood psychiatric disorders are largely unknown. One possible candidate is the central serotonergic system, which is also abnormal in psychiatric illnesses. Neuroadaptational changes in serotonergic transporter and serotonergic 1A receptors may underlie links between early life stress and adulthood psychiatric disorders. OBJECTIVE: The aim of this study was to investigate the consequences of a rat model of maternal separation on serotonergic transporter and serotonergic 1A receptor densities and function in adult rat forebrain. METHODS: Rat pups were separated from dams from postnatal day 2 to postnatal day 14, each day, for zero time, 15 min and 180 min to determine the time-course of effects. A non-handled group was added to control for the effects of handling by an experimenter compared with the animal facility-reared group. Quantitative [(125)I]3beta-(4-iodophenyl)tropan-2beta-carboxylic acid methyl ester and [(125)I]-mPPI autoradiography was used to determine serotonergic transporter and serotonergic 1A densities, respectively. Adult rats were challenged with saline or serotonergic 1A agonist (+) 8-hydroxy-2-(di-n-propylamino)tetralin, 0.4 mg/kg, s.c.) and plasma adrenocorticotropic hormone and corticosterone were determined. RESULTS: serotonergic transporter and serotonergic 1A densities were significantly lower in the non-handled group in the paraventricular, arcuate, dorsomedial and ventromedial nuclei of the hypothalamus. The non-handled group also displayed lower serotonergic transporter and serotonergic 1A densities in the basolateral anterior, basolateral ventral and basomedial amygdaloid nuclei. Serotonergic transporter densities were also decreased in the CA3 area of the hippocampus in the non-handled group. In contrast, the maternal separation 15 min group displayed the highest serotonergic transporter and serotonergic 1A densities in the basomedial nucleus of amygdala, basolateral anterior nucleus of amygdala, basolateral ventral nucleus of amygdala and basomedial nucleus of amygdala amygdaloid nuclei. CONCLUSIONS: Early life maternal separation and the extent of handling can alter adult brain serotonergic transporter and serotonergic 1A levels and function in the forebrain. Alterations in these serotonergic systems by early rearing conditions might increase vulnerability for behavioral disorders in adulthood.

Preferential increases in nucleus accumbens dopamine after systemic cocaine administration are caused by unique characteristics of dopamine neurotransmission.

In vivo voltammetry was used to investigate the preferential increase of extracellular dopamine in the nucleus accumbens relative to the caudate-putamen after systemic cocaine administration. In the first part of this study, cocaine (40 mg/kg, i.p.) was compared with two other blockers of dopamine uptake, nomifensine (10 mg/kg, i.p.) and 3beta-(p-chlorophenyl)tropan-2beta-carboxylic acid p-isothiocyanatophenylmethyl ester hydrochloride (RTI-76; 100 nmol, i.c.v.), to assess whether the inhibitory mechanism of cocaine differed in the two regions. All three drugs robustly increased electrically evoked levels of dopamine, and cocaine elevated dopamine signals to a greater extent in the nucleus accumbens. However, kinetic analysis of the evoked dopamine signals indicated that cocaine and nomifensine increased the K(m) for dopamine uptake whereas the dominant effect of RTI-76 was a decrease in V(max). Under the present in vivo conditions, therefore, cocaine is a competitive inhibitor of dopamine uptake in both the nucleus accumbens and caudate-putamen. Whether the preferential effect of cocaine was mediated by regional differences in the presynaptic control of extracellular DA that are described by rates for DA uptake and release was examined next by a correlation analysis. The lower rates for dopamine release and uptake measured in the nucleus accumbens were found to underlie the preferential increase in extracellular dopamine after cocaine. This relationship explains the paradox that cocaine more effectively increases accumbal dopamine despite identical effects on the dopamine transporter in the two regions. The mechanism proposed for the preferential actions of cocaine may also mediate the differential effects of psychostimulant in extrastriatal regions and other uptake inhibitors in the striatum.