Effect of adrenalectomy on cocaine facilitation of lateral hypothalamic self-stimulation

Pituitary adenylate cyclase-activating polypeptide induces a depressive-like phenotype in rats

BACKGROUND

Major depressive disorder (MDD) is a chronic, life-threatening psychiatric condition characterized by depressed mood, psychomotor alterations, and a markedly diminished interest or pleasure in most activities known as anhedonia. Available pharmacotherapies have limited success and the need for new strategies is clear. Recent studies attribute a major role to the pituitary adenylate cyclase-activating polypeptide (PACAP) system in mediating the response to stress. PACAP knockout mice display profound alterations in depressive-like behaviors, and genetic association studies have demonstrated that genetic variants of the PACAP gene are associated with MDD. However, the effects of PACAP administration on depressive-like behaviors in rodents have not yet been systematically examined.

OBJECTIVES

The present study investigated the effects of central administration of PACAP in rats on depressive-like behaviors, using well-established animal models that represent some of the endophenotypes of depression.

METHODS

We used intracranial self-stimulation (ICSS) to assess the brain reward function, saccharin preference test to assess anhedonia, social interaction to assess social withdrawal, and forced swim test (FST) to assess behavioral despair.

RESULTS

PACAP raised the current threshold for ICSS, elevation blocked by the PACAP antagonist PACAP(6-38). PACAP reduced the preference for a sweet saccharin solution and reduced the time the rats spent interacting with a novel animal. Interestingly, PACAP administration did not affect immobility in the FST.

CONCLUSIONS

Our results demonstrate a role for the central PACAP/PAC1R system in the regulation of depressive-like behaviors and suggest that hyperactivity of the PACAP/PAC1R system may contribute to the pathophysiology of depression, particularly the associated anhedonic symptomatology and social dysfunction.

Effects of time of feeding on psychostimulant reward, conditioned place preference, metabolic hormone levels, and nucleus accumbens biochemical measures in food-restricted rats

RATIONALE

Chronic food restriction (FR) increases rewarding effects of abused drugs and persistence of a cocaine-conditioned place preference (CPP). When there is a single daily meal, circadian rhythms are correspondingly entrained, and pre- and postprandial periods are accompanied by different circulating levels of metabolic hormones that modulate brain dopamine function.

OBJECTIVES

The present study assessed whether rewarding effects of d-amphetamine, cocaine, and persistence of cocaine-CPP differ between FR subjects tested in the pre- and postprandial periods.

MATERIALS AND METHODS

Rats were stereotaxically implanted with intracerebral microinjection cannulae and an electrode in lateral hypothalamus. Rewarding effects of d-amphetamine and cocaine were assessed using electrical self-stimulation in rats tested 1-4 or 18-21 h after the daily meal. Nonimplanted subjects acquired a cocaine-CPP while ad libitum fed and then were switched to FR and tested for CPP at these same times.

RESULTS

Rewarding effects of intranucleus accumbens (NAc) d-amphetamine, intraventricular cocaine, and persistence of cocaine-CPP did not differ between rats tested 18-21 h food-deprived, when ghrelin and insulin levels were at peak and nadir, respectively, and those tested 1-4 h after feeding. Rats that expressed a persistent CPP had elevated levels of p-ERK1, GluA1, and p-Ser845-GluA1 in NAc core, and the latter correlated with CPP expression.

CONCLUSIONS

Psychostimulant reward and persistence of CPP in FR rats are unaffected by time of testing relative to the daily meal. Further, NAc biochemical responses previously associated with enhanced drug responsiveness in FR rats are associated with persistent CPP expression.

From Malthus to motive: how the HPA axis engineers the phenotype, yoking needs to wants

The hypothalamo-pituitary-adrenal (HPA) axis is the critical mediator of the vertebrate stress response system, responding to environmental stressors by maintaining internal homeostasis and coupling the needs of the body to the wants of the mind. The HPA axis has numerous complex drivers and highly flexible operating characterisitics. Major drivers include two circadian drivers, two extra-hypothalamic networks controlling top-down (psychogenic) and bottom-up (systemic) threats, and two intra-hypothalamic networks coordinating behavioral, autonomic, and neuroendocrine outflows. These various networks jointly and flexibly control HPA axis output of periodic (oscillatory) functions and a range of adventitious systemic or psychological threats, including predictable daily cycles of energy flow, actual metabolic deficits over many time scales, predicted metabolic deficits, and the state-dependent management of post-prandial responses to feeding. Evidence is provided that reparation of metabolic derangement by either food or glucocorticoids results in a metabolic signal that inhibits HPA activity. In short, the HPA axis is intimately involved in managing and remodeling peripheral energy fluxes, which appear to provide an unidentified metabolic inhibitory feedback signal to the HPA axis via glucocorticoids. In a complementary and perhaps a less appreciated role, adrenocortical hormones also act on brain to provide not only feedback, but feedforward control over the HPA axis itself and its various drivers, as well as coordinating behavioral and autonomic outflows, and mounting central incentive and memorial networks that are adaptive in both appetitive and aversive motivational modes. By centrally remodeling the phenotype, the HPA axis provides ballistic and predictive control over motor outflows relevant to the type of stressor. Evidence is examined concerning the global hypothesis that the HPA axis comprehensively induces integrative phenotypic plasticity, thus remodeling the body and its governor, the brain, to yoke the needs of the body to the wants of the mind. Adverse side effects of this yoking under conditions of glucocorticoid excess are discussed.

Interaction between glucocorticoid hormones, stress and psychostimulant drugs

In this review we summarize data obtained from animal studies showing that glucocorticoid hormones have a facilitatory role on behavioural responses to psychostimulant drugs such as locomotor activity, self-administration and relapse. These behavioural effects of glucocorticoids involve an action on the meso-accumbens dopamine system, one of the major systems mediating the addictive properties of drugs of abuse. The effects of glucocorticoids in the nucleus accumbens are site-specific; these hormones modify dopamine transmission in only the shell of this nucleus without modifying it in the core. Studies with corticosteroid receptor antagonists suggest that the dopaminergic effects of these hormones depend mostly on glucocorticoid, not on mineralocorticoid receptors. These data suggest that an increase in glucocorticoid hormones, through an action on mesolimbic dopamine neurons, could increase vulnerability to drug abuse. We also discuss the implications of this finding with respect to the physiological role of glucocorticoids. It is proposed that an increase in glucocorticoids, by activating the reward pathway, could counteract the aversive effects of stress. During chronic stress, repeated increases in glucocorticoids and dopamine would result in sensitization of the reward system. This sensitized state, which can persist after the end of the stress, would render the subject more responsive to drugs of abuse and consequently more vulnerable to the development of addiction.

Augmentation of drug reward by chronic food restriction: behavioral evidence and underlying mechanisms

Chronic food restriction and maintenance of low body weight have long been known to increase the self-administration and motor-activating effects of abused drugs. Using a lateral hypothalamic self-stimulation (LHSS) rate-frequency method, it is shown that chronic food restriction augments the rewarding (i.e., threshold lowering) effect of diverse drugs of abuse. Further, the effect is attributed to increased sensitivity of a neural substrate, rather than a change in drug bioavailability or pharmacokinetics, because it is preserved when drugs are injected directly into the lateral cerebral ventricle (intracerebroventricularly). The food restriction regimen that augments drug reward also increases the induction of c-fos, by intracerebroventricular amphetamine, in limbic forebrain dopamine (DA) terminal areas. The possibility of increased DA receptor function is suggested by findings that rewarding and motor-activating effects of direct DA receptor agonists are augmented by food restriction, and the augmented behavioral effects of amphetamine are reversed by an otherwise subthreshold dose of D-1 antagonist. Initial studies of DA receptor-mediated signal transduction, that are focused on the D-2 receptor, suggest increased functional coupling between receptor and G-protein (i.e., quinpirole-stimulated [(35)S]GTPgammaS binding) in dorsal striatum. Unlike behavioral sensitization induced by intermittent stress or psychostimulant treatment, which persist indefinitely following induction, the augmenting effect of food restriction abates within 1 week of restored ad libitum feeding and weight gain. The possible involvement of endocrine hormones and/or 'feeding-related' neuropeptides, whose levels change dynamically with depletion and repletion of adipose stores, is therefore under investigation. Initial tests have been limited to acute treatments aimed at attenuating the effects of hypoinsulinemia, hypoleptinemia and elevated corticosterone levels in food-restricted rats. None of these treatments has attenuated the behavioral effect of food restriction. While a melanocortin receptor agonist has been found to enhance drug reward, melanocortin receptors do not seem to mediate the augmenting effect of food restriction. Continuing investigations of endocrine adiposity signals, 'feeding-related' neuropeptides and dopaminergic signal transduction may further elucidate the way in which drugs of abuse exploit mechanisms that mediate survival-related behavior, and help explain the high comorbidity of drug abuse and eating disorders.

CRF and urocortin decreased brain stimulation reward in the rat: reversal by a CRF receptor antagonist

The present studies were designed to investigate the effects of corticotropin-releasing factor (CRF) receptor activation and antagonism on intracranial self-stimulation (ICSS) reward using a discrete-trial current-intensity threshold procedure. Bipolar electrodes were implanted in the lateral hypothalamus, and cannula guides were implanted above the lateral ventricle of male Wistar rats. Dose-effect functions were established for the effects on ICSS of the competitive CRF receptor agonist h/rCRF (0-5.0 microg, i.c.v. ), the CRF receptor agonist urocortin (0-5.0 microg, i.c.v.), and the CRF receptor antagonist [D-Phe(12), Nle(21,38), C(alpha) MeLeu(37)] h/rCRF(12-41) (0-5.0 microg, i.c.v.). Administration of h/rCRF or urocortin dose-dependently elevated ICSS thresholds without altering performance measures (latencies to respond to stimulation, extra and time-out responses). CRF was more potent than urocortin in terms of threshold dose-effects on ICSS thresholds compared to vehicle. Despite these apparent potency differences, percent effect sizes on ICSS thresholds were comparable at the highest doses of both peptides. In contrast to the significant threshold elevation effects of CRF and urocortin, the competitive CRF antagonist D-Phe CRF(12-41) had no effect on ICSS thresholds or performance measures. To determine the neuropharmacological specificity of the effect of CRF on brain stimulation reward, D-Phe CRF(12-41) was used to antagonize CRF-induced threshold elevations. Pretreatment with either the 5.0- or 10.0-microg doses of D-Phe CRF(12-41) effectively blocked CRF-induced reward threshold elevations (3.0 microg) without affecting other ICSS performance measures. These results indicate that CRF neurotransmission can modulate ICSS reward processes.

Hypoinsulinemia may mediate the lowering of self-stimulation thresholds by food restriction and streptozotocin-induced diabetes

7 days beyond cessation of insulin treatment) elevation of threshold in ad libitum fed rats and, more transiently, reversed the threshold-lowering effect of food restriction. Acute insulin treatment (3 mU, 15 min prior) also elevated threshold in food-restricted rats. These results are consistent with the hypothesis that insulin modulates sensitivity of a brain reward system and that hypoinsulinemia may be the common factor in food restriction and diabetes that accounts for the enhancement of perifornical LHSS.

Chronic food restriction increases fos-like immunoreactivity (FLI) induced in rat forebrain by intraventricular amphetamine

Chronic food restriction enhances behavioral responsiveness to amphetamine and other abused drugs. Because this effect is evident when drugs are administered intracerebroventricularly (i.c.v.) as well as systemically, it would seem to reflect increased sensitivity of a neural substrate rather than a change in drug disposition. In the present study, c-Fos immunohistochemistry was used to evaluate whether the magnitude and pattern of cellular activation induced by i.c.v. amphetamine is altered by a regimen of food restriction previously shown to potentiate amphetamine reward. In the absence of amphetamine challenge, there was generally no difference in brain Fos-like immunoreactivity (FLI) between ad libitum fed and food-restricted rats. In response to amphetamine (50 microg), both groups displayed increased FLI in caudate-putamen, nucleus accumbens, bed nucleus of the stria terminalis, ventral pallidum, central nucleus of the amygdala, and cingulate cortex. With the exception of cingulate cortex and caudal caudate-putamen, a significantly greater response was observed in brain regions of food-restricted rats. These results indicate that food restriction augments a cellular immediate early gene (IEG) response to acute amphetamine in brain regions known to mediate rewarding and other behavioral effects of psychostimulants. The difference between these results and those produced by sensitizing regimens of psychostimulant exposure are discussed, as are possible endocrine factors that could be involved in the modulatory effect of food restriction on cellular and behavioral responses to amphetamine.

Lack of glucocorticoids attenuates the self-stimulation-induced increase in the in vivo synthesis rate of dopamine but not serotonin in the rat nucleus accumbens

Our previous study demonstrated that intracranial self-stimulation of the medial forebrain bundle can increase the in vivo synthesis turnover rate of dopamine (DA) and serotonin (5-HT) in the nucleus accumbens of adrenal-intact rats. The present study examined using microdialysis whether such increases in DA and 5-HT syntheses are influenced by adrenal hormones, which are also activated following intracranial self-stimulation. A decarboxylase inhibitor, NSD-1015, was perfused through reversed microdialysis which enabled the simultaneous measurement of 3,4-dihydroxyphenylalanine (DOPA) and 5-hydroxytryptophan (5-HTP) as an index of the in vivo turnover rate of DA and 5-HT syntheses. Adrenalectomy (ADX) attenuated significantly the self-stimulation-induced increase in dialysate levels of DOPA but not 5-HTP. Corticosterone (Cort) replacement reversed the attenuation in DOPA levels in adrenalectomized rats. The finding indicates that activation of DA synthesis in vivo in the nucleus accumbens during intracranial self-stimulation is dependent on, whereas that of 5-HT synthesis is independent of glucocorticoid modulation.

Food restriction enhances the central rewarding effect of abused drugs

Chronic food restriction increases the systemic self-administration and locomotor-stimulating effect of abused drugs. However, it is not clear whether these behavioral changes reflect enhanced rewarding potency or a CNS-based modulatory process. The purpose of this study was to determine whether food restriction specifically increases the rewarding potency of drugs, as indexed by their threshold-lowering effect on lateral hypothalamic self-stimulation, and whether any such effect can be attributed to an enhanced central response rather than changes in drug disposition. When drugs were administered systemically, food restriction potentiated the threshold-lowering effect of amphetamine (0.125, 0.25, and 0.5 mg/kg, i.p.), phencyclidine (1.0, 2.0, and 3.0 mg/kg, i.p.), and dizocilpine (MK-801) (0.0125, 0.05, and 0.1 mg/kg, i.p.) but not nicotine (0.15, 0.3, 0.45 mg/kg, s.c.). When amphetamine (25.0, 50.0, and 100.0 microgram) and MK-801 (5.0, 10.0, and 20.0 microgram) were administered via the intracerebroventricular route, food restriction again potentiated the threshold-lowering effects and increased the locomotor-stimulating effects of both drugs. These results indicate that food restriction increases the sensitivity of neural substrates for rewarding and stimulant effects of drugs. In light of work that attributes rewarding effects of MK-801 to blockade of NMDA receptors on medium spiny neurons in nucleus accumbens, the elements affected by food restriction may lie downstream from the mesoaccumbens dopamine neurons whose terminals are the site of amphetamine-rewarding action. Possible metabolic-endocrine triggers of this effect are discussed, as is the likelihood that mechanisms mediating the modulatory effect of food restriction differ from those mediating sensitization by intermittent drug exposure.

A search for the metabolic signal that sensitizes lateral hypothalamic self-stimulation in food-restricted rats

Food deprivation and restriction increase the rewarding potency of food, drugs of abuse, and electrical brain stimulation. Based on evidence that the rewarding effects of these stimuli are mediated by the same neuronal circuitry, lateral hypothalamic self-stimulation (LHSS) was used to investigate the involvement of various metabolic signals in the sensitization of reward. In Experiment 1, glucoprivation with 2-deoxy-d-glucose (150 mg/kg, intraperitoneally (i.p.)) and lipoprivation with nicotinic acid (150 mg/kg, subcutaneously (s.c.)), individually and in combination, failed to affect the LHSS threshold in ad lib.-fed rats. These results suggest that signals associated with acute shortage of metabolic substrate do not sensitize reward. Because numerous responses to more prolonged negative energy balance are mediated by neuropeptide Y (NPY), the effect of exogenous neuropeptide Y upon LHSS was investigated in Experiment 2. Intraventricular infusion of orexigenic neuropeptide Y doses (2.0, 5.0, and 12.5 g), in ad lib.-fed rats, had no effect on LHSS threshold. In Experiment 3, other concomitants of prolonged negative energy balance--high circulating levels of free fatty acids (FFA) and beta-hydroxybutyrate (HDB)-were investigated. Nicotinic acid (250 mg/kg, s.c.), which suppressed serum HDB and FFA levels, had no effect on LHSS in food-restricted or ad lib.-fed rats. Mercaptoacetate (68.4 mg/kg, i.p.), which suppressed serum HDB levels and exacerbated the elevation of FFA levels, also had no effect. Thus, the brain reward system, if modulated by these substances, is not affected by transient, though marked, changes in their levels. To investigate the effect of a sustained increase in levels of FFA and HDB, a "ketogenic" diet was employed. Although this diet produced a fourfold increase in serum HDB levels, it had no effect on LHSS thresholds. Moreover, the failure of mercaptoacetate (68.4 mg/kg, i.p.) to decrease LHSS thresholds in these rats supports the conclusion that acute shortage of metabolic substrate does not sensitize reward. Other possible mechanisms of reward sensitization, including sustained decreases in circulating insulin and leptin and increases in corticosterone, are discussed.

Effect of adrenalectomy on cocaine facilitation of medial prefrontal cortex self-stimulation

Adrenalectomy (ADX) is known to block the acquisition of intravenous cocaine self-administration. A previous study therefore examined whether ADX decreases sensitivity of the 'brain reward system' in general, or its response to cocaine in particular, by measuring thresholds for intracranial self-stimulation with and without concurrent cocaine administration. ADX had no effect on thresholds for lateral hypothalamic self-stimulation (LHSS) and did not alter the cocaine dose-response curve for lowering the LHSS threshold. This result suggested that ADX does not affect sensitivity of the brain reward system. However, medial prefrontal cortex (MPFC) appears to be an important site in the mediation of cocaine reinforcing effects, and MPFC self-stimulation (MPFCSS) is mediated by a neural substrate that is largely independent of that which mediates LHSS. The present study therefore assessed whether ADX diminishes cocaine facilitation of MPFCSS. It was found that the threshold-lowering effect of cocaine (5.0, 10.0 and 20.0 mg/kg, i.p. ) did not differ between ADX rats maintained on 0.7% saline, ADX rats maintained on corticosterone (50 microg/ml) in 0.7% saline, and sham-operated controls. However, there was a trend toward desensitization of MPFCSS, itself, following ADX in the group that did not receive corticosterone supplementation. Based on this observation, and the similar responses of MPFCSS and cocaine self-administration to noncontingent priming stimulation, stress, and NMDA receptor antagonism, it is speculated that acquisition of MPFCSS and cocaine self-administration may be dependent upon a common sensitization process that is regulated by corticosterone.