The effects of stress on central nervous system concentrations of the opioid peptide, dynorphin

The neurophysiological basis of stress and anxiety - comparing neuronal diversity in the bed nucleus of the stria terminalis (BNST) across species

The bed nucleus of the stria terminalis (BNST), as part of the extended amygdala, has become a region of increasing interest regarding its role in numerous human stress-related psychiatric diseases, including post-traumatic stress disorder and generalized anxiety disorder amongst others. The BNST is a sexually dimorphic and highly complex structure as already evident by its anatomy consisting of 11 to 18 distinct sub-nuclei in rodents. Located in the ventral forebrain, the BNST is anatomically and functionally connected to many other limbic structures, including the amygdala, hypothalamic nuclei, basal ganglia, and hippocampus. Given this extensive connectivity, the BNST is thought to play a central and critical role in the integration of information on hedonic-valence, mood, arousal states, processing emotional information, and in general shape motivated and stress/anxiety-related behavior. Regarding its role in regulating stress and anxiety behavior the anterolateral group of the BNST (BNSTALG) has been extensively studied and contains a wide variety of neurons that differ in their electrophysiological properties, morphology, spatial organization, neuropeptidergic content and input and output synaptic organization which shape their activity and function. In addition to this great diversity, further species-specific differences are evident on multiple levels. For example, classic studies performed in adult rat brain identified three distinct neuron types (Type I-III) based on their electrophysiological properties and ion channel expression. Whilst similar neurons have been identified in other animal species, such as mice and non-human primates such as macaques, cross-species comparisons have revealed intriguing differences such as their comparative prevalence in the BNSTALG as well as their electrophysiological and morphological properties, amongst other differences. Given this tremendous complexity on multiple levels, the comprehensive elucidation of the BNSTALG circuitry and its role in regulating stress/anxiety-related behavior is a major challenge. In the present Review we bring together and highlight the key differences in BNSTALG structure, functional connectivity, the electrophysiological and morphological properties, and neuropeptidergic profiles of BNSTALG neurons between species with the aim to facilitate future studies of this important nucleus in relation to human disease.

Kappa opioid receptors internalization is protective against oxygen-glucose deprivation through β-arrestin activation and Akt-mediated signaling pathway

Hypoxia induces reversible κ-opioid receptor (KOR) internalization similar to the internalization that is induced by KOR agonists. In the current study, we demonstrate that this KOR internalization is a protective mechanism via the β-arrestin specific pathway in an oxygen-glucose deprivation (OGD) model. Mouse neuroblastoma Neuro2A cells were stably transfected with mouse KOR-tdTomato fusion protein (N2A-mKOR-tdT cells). Various concentrations of salvinorin A (SA), a highly selective KOR agonist, were given in the presence and absence of norbinaltorphimine (norBNI), which is a KOR antagonist, or Dyngo-4a (internalization inhibitor) or API-2 (Akt/Protein kinase B signaling inhibitor-2). Various concentrations of SA and RB-64 (22-thiocyanatosalvinorin A, selective for the G protein signaling pathway) were administered both in normoxic and hypoxic conditions. Autophagosomes and ultrastructural components of cells were observed using transmission electron microscopy (TEM). Cell viability, severity of cell injury, and levels of proteins related to the Akt signaling pathway were evaluated using live cell counting (by Cell Counting Kit-8), the lactic acid dehydrogenase (LDH) release rate, and Western blot analysis, respectively. SA promoted cell survival and attenuated OGD-induced cell injury. The Akt signaling pathway is activated by SA. KOR internalization, when blocked by norBNI or Dyngo-4a, increased LDH release and decreased cell viability under OGD. Treatment with SA significantly inhibited autophagy, and the effects of SA on autophagy were reversed by API-2 pretreatment. RB-64 in a low concentration without β-arrestin recruitment did not reduce LDH release and increase cell viability as observed with SA. KOR internalization through β-arrestin activation is a protective mechanism against OGD. The Akt pathway might play a critical role in modulating these protective effects by inhibiting autophagy.

Effects of the Alpha-1 Antagonist Prazosin on KOR Agonist-Induced Reinstatement of Alcohol Seeking

BACKGROUND

Stress is associated with relapse to alcohol seeking during abstinence, but the processes underlying this relationship are poorly understood. Noradrenaline is a key transmitter in stress responses and in stress-induced drug seeking. The alpha-1 adrenoceptor antagonist prazosin has been investigated as a treatment for alcoholism and for chronic stress disorders that are frequently comorbid with alcoholism. In rats, we previously showed that prazosin blocks reinstatement of alcohol seeking induced by footshock and yohimbine stressors and reduces yohimbine-induced brain activation. The role of alpha-1 adrenoceptors in reinstatement induced by other stressors is not known. Our most recent work is on the role of kappa opioid receptors in stress-induced reinstatement of alcohol seeking and have reported that the selective kappa opioid receptor agonist U50,488 induces reinstatement and neuronal activation in stress- and relapse-related brain regions. Here we determine the involvement of alpha-1 receptors in reinstatement and brain activation induced by U50,488.

METHODS

We trained male Long-Evans rats to self-administer alcohol (12% w/v), extinguished alcohol-reinforced responding, and then determined the effects of prazosin (1 mg/kg) on U50,488 (2.5 mg/kg)-induced reinstatement and regional Fos expression.

RESULTS

Prazosin blocked U50,488-induced reinstatement and decreased U50,488-induced Fos expression in the orbitofrontal cortex, nucleus accumbens core, ventral bed nucleus of the stria terminalis, central and basolateral amygdalar nuclei and ventral tegmental area.

CONCLUSIONS

These findings suggest that prazosin may reduce U50,488-induced relapse by inhibiting activity in 1 or more of these brain areas.

Effect of chronic alcohol vapor exposure on reinstatement of alcohol seeking induced by U50,488

Alcohol dependence and stress are associated with relapse to alcohol during abstinence, but the underlying mechanisms are poorly understood. Kappa opioid receptors (KOR) are involved in alcohol reward and in the effects of stress. Previously, in non-dependent rats, we showed that KOR in the bed nucleus of the stria terminalis (BNST) mediate reinstatement of alcohol seeking induced by the selective KOR agonist U50,488. Here, we determine the effects of chronic, intermittent exposure to alcohol vapor on U50,488-induced reinstatement of alcohol seeking. We also study brain mechanisms involved using the neuronal activity marker Fos and phosphorylated p38 MAPK (p-p38), an intracellular messenger implicated in the effects of KOR stimulation. We trained male Long-Evans rats to self-administer alcohol (12% w/v) and exposed them to alcohol vapor (14 h vapor/10 h air) daily for 24 d or to the control condition, extinguished alcohol-reinforced responding and determined the dose response for U50,488-induced reinstatement. We then determined the effects of vapor exposure on U50,488-induced Fos and p-p38 expression. Vapor-exposed rats were more sensitive to U50,488-induced reinstatement. U50,488 increased Fos expression in brain areas involved in stress-induced relapse, and Fos activation in the ventral BNST was greater in vapor exposed rats. Vapor exposed rats had increased basal p-p38 expression in the dorsal BNST, LC and NTS. Our findings suggest that changes in the neuronal responses to KOR stimulation in the ventral BNST may be involved in the increased sensitivity to U50,488 accompanying dependence.

Role of κ-Opioid Receptors in the Bed Nucleus of Stria Terminalis in Reinstatement of Alcohol Seeking

κ-Opioid receptors (KORs) and their endogenous ligand dynorphin are involved in stress-induced alcohol seeking but the mechanisms involved are largely unknown. We previously showed that systemic injections of the KOR agonist U50,488, which induce stress-like aversive states, reinstate alcohol seeking after extinction of the alcohol-reinforced responding. Here, we used the neuronal activity marker Fos and site-specific injections of the KOR antagonist nor-BNI and U50,488 to study brain mechanisms of U50,488-induced reinstatement of alcohol seeking. We trained male Long-Evans rats to self-administer alcohol (12% w/v) for 23-30 days. After extinction of the alcohol-reinforced responding, we tested the effect of U50,488 (0, 1.25, 2.5, and 5 mg/kg) on reinstatement of alcohol seeking. Next, we correlated regional Fos expression with reinstatement induced by the most effective U50,488 dose (5 mg/kg). Based on the correlational Fos results, we determined the effect of bed nucleus of the stria terminalis (BNST) injections of nor-BNI (4 μg/side) on U50,488-induced reinstatement of alcohol seeking, and reinstatement induced by injections of U50,488 (0, 0.3, 1, and 3 μg/side) into the BNST. U50,488-induced reinstatement of alcohol seeking was associated with increased Fos expression in multiple brain areas, including the BNST, where it was significantly correlated with lever pressing. U50,488-induced reinstatement was blocked by BNST nor-BNI injections, and BNST U50,488 injections partially mimicked the drug's systemic effect on reinstatement. Our data indicate that the BNST is a critical site for U50,488-induced reinstatement of alcohol seeking and suggest that KOR/dynorphin mechanisms in this brain area play a key role in stress-induced alcohol seeking.

kappa-Opioid receptor signaling and brain reward function

The dynorphin-like peptides have profound effects on the state of the brain reward system and human and animal behavior. The dynorphin-like peptides affect locomotor activity, food intake, sexual behavior, anxiety-like behavior, and drug intake. Stimulation of kappa-opioid receptors, the endogenous receptor for the dynorphin-like peptides, inhibits dopamine release in the striatum (nucleus accumbens and caudate putamen) and induces a negative mood state in humans and animals. The administration of drugs of abuse increases the release of dopamine in the striatum and mediates the concomitant release of dynorphin-like peptides in this brain region. The reviewed studies suggest that chronic drug intake leads to an upregulation of the brain dynorphin system in the striatum and in particular in the dorsal part of the striatum/caudate putamen. This might inhibit drug-induced dopamine release and provide protection against the neurotoxic effects of high dopamine levels. After the discontinuation of chronic drug intake these neuroadaptations remain unopposed which has been suggested to contribute to the negative emotional state associated with drug withdrawal and increased drug intake. kappa-Opioid receptor agonists have also been shown to inhibit calcium channels. Calcium channel inhibitors have antidepressant-like effects and inhibit the release of norepinephrine. This might explain that in some studies kappa-opioid receptor agonists attenuate nicotine and opioid withdrawal symptomatology. A better understanding of the role of dynorphins in the regulation of brain reward function might contribute to the development of novel treatments for mood disorders and other disorders that stem from a dysregulation of the brain reward system.

Molecular and genetic substrates linking stress and addiction

Drug addiction is one of the top three health concerns in the United States in terms of economic and health care costs. Despite this, there are very few effective treatment options available. Therefore, understanding the causes and molecular mechanisms underlying the transition from casual drug use to compulsive drug addiction could aid in the development of treatment options. Studies in humans and animal models indicate that stress can lead to both vulnerability to develop addiction, and increased drug taking and relapse in addicted individuals. Exposure to stress or drugs of abuse results in long-term adaptations in the brain that are likely to involve persistent alterations in gene expression or activation of transcription factors, such as the cAMP Response Element Binding (CREB) protein. The signaling pathways controlled by CREB have been strongly implicated in drug addiction and stress. Many potential CREB target genes have been identified based on the presence of a CRE element in promoter DNA sequences. These include, but are not limited to CRF, BDNF, and dynorphin. These genes have been associated with initiation or reinstatement of drug reward and are altered in one direction or the other following stress. While many reviews have examined the interactions between stress and addiction, the goal of this review was to focus on specific molecules that play key roles in both stress and addiction and are therefore posed to mediate the interaction between the two. Focus on these molecules could provide us with new targets for pharmacological treatments for addiction.

Dynorphin-containing axons directly innervate noradrenergic neurons in the rat nucleus locus coeruleus

Stress causes increased dynorphin (DYN) expression in limbic brain regions and antagonism of kappa-opioid receptors may offer therapeutic potential for the treatment of depression. A potential site of DYN action relevant to stress and related neuropsychiatric disorders is the locus coeruleus (LC), the primary source of forebrain norepinephrine. Therefore, using immunofluorescence and immunoelectron microscopic analyses, we characterized the cellular substrates for interactions between DYN and tyrosine hydroxylase (TH), a catecholamine synthesizing enzyme in single sections through the rat LC. Light microscopic analysis of DYN immunoreactivity indicated that DYN fibers are distributed within the core and pericoerulear subregions of the LC. Using electron microscopy, immunoperoxidase labeling for DYN was primarily found in axon terminals, although in some cases was diffusely localized to somatodendritic processes. When DYN-containing axons formed synaptic contacts, they typically (89%) exhibited an asymmetric morphology. Almost a third (28%) of the postsynaptic targets of DYN-containing axons contained immunogold labeling for TH. These findings reveal some diversity as to the localization of DYN in the LC within axons that contact both TH and non-TH containing dendrites. However, the present data provide the first ultrastructural evidence that DYN-containing axon terminals directly innervate catecholaminergic LC dendrites. Moreover, DYN axon terminals targeting catecholaminergic LC dendrites via asymmetric synapses are consistent with localization within excitatory type afferents to the LC. Therefore, direct modulation of catacholaminergic LC neurons maybe an important site of action for DYN relevant to stress and stress-related disorders.

A Microdialysis Profile of Dynorphin A1-8 Release in the Rat Nucleus Accumbens Following Alcohol Administration

BACKGROUND

Pharmacological studies have implicated the endogenous opioid system in mediating alcohol intake. Other evidence has shown that alcohol administration can influence endorphinergic and enkephalinergic activity, while very few studies have examined its effect on dynorphinergic systems. The aim of the present study was to investigate the effect of alcohol administration or a mechanical stressor on extracellular levels of dynorphin A(1-8) in the rat nucleus accumbens-a brain region that plays a significant role in the processes underlying reinforcement and stress.

METHODS

Male Sprague-Dawley rats were implanted with a microdialysis probe aimed at the shell region of the nucleus accumbens. Artificial cerebrospinal fluid was pumped at a rate of 1.5 microL/min in awake and freely moving animals and the dialysate was collected at 30-minute intervals. In one experiment, following a baseline period, rats were injected intraperitoneally with either physiological saline or 1 of 3 doses of alcohol, 0.8, 1.6, or 3.2 g ethanol/kg body weight. In a second experiment, following a baseline period, rats were applied a clothespin to the base of their tail for 20 minutes. The levels of dynorphin A(1-8) in the dialysate were analyzed with solid-phase radioimmunoassay.

RESULTS

Relative to saline-treated controls, an alcohol dose of 1.6 and 3.2 g/kg caused a transient increase in the extracellular levels of dynorphin A(1-8) in the first 30 minutes of alcohol administration. However, the effect resulting from the high 3.2 g/kg dose was far more pronounced and more significant than with the moderate dose. There was no effect of tail pinch on dynorphin A(1-8) levels in the nucleus accumbens.

CONCLUSIONS

In this experiment, a very high dose of alcohol was especially capable of stimulating dynorphin A(1-8) release in the nucleus accumbens. Dynorphin release in the accumbens has been previously associated with aversive stimuli and may thus reflect a system underlying the aversive properties of high-dose alcohol administration. However, the lack of effect of tail-pinch stress in the present study suggests that dynorphin A(1-8) is not released in response to all forms of stressful/aversive stimuli.

Corticotropin-releasing factor as well as opioid and dopamine are involved in tail-pinch-induced food intake of rats

Several kinds of stress such as psychological stress, restraint, and foot shock inhibit feeding behavior through corticotropin-releasing factor (CRF). In contrast, a mild tail pinch increases food intake in rats. Although dopamine and opioid are thought to be involved in tail-pinch-induced food intake, it is unknown whether CRF participates in this phenomenon. Therefore, we attempted to clarify this issue using rats. A 30-s tail pinch increased food intake in 30 min after the tail pinch, and this increase was blocked by intraperitoneal injection of CRF receptor type 1 selective antagonist. CRF increased food intake in 30 min after intracerebroventricular injection at a dose of 2 or 10 ng, and this increase was also blocked by CRF receptor type 1 antagonist. Tail-pinch- or CRF-induced food intake was blocked by naloxone, pimozide, and spiperone. These results suggest that CRF, through CRF receptor type 1 as well as opioid and dopaminergic systems, are involved in the mechanism of tail-pinch-induced food intake. The results also suggest that brain CRF has dual effects on food intake, hyperphagia and anorexia, in a stress-dependent manner.

Effects of chronic food restriction on prodynorphin-derived peptides in rat brain regions

Chronic food restriction produces a variety of physiological and behavioral adaptations including a potentiation of the reinforcing effect of food, drugs and lateral hypothalamic electrical stimulation. Previous work in this laboratory has revealed that the lowering of self-stimulation threshold by food restriction is reduced by mu- and kappa-selective opioid antagonists. In the present study, the effect of chronic food restriction on levels of three prodynorphin-derived peptides, namely dynorphin A1-17 (A1-17), dynorphin A1-8 (A1-8) and dynorphin B1-13 (B1-13) were measured in eleven brain regions known to be involved in appetite, taste and reward. Food restriction increased levels of A1-17 in dorsal medial (+19.6%), ventral medial (+24.2%) and medial preoptic (+82.9%) hypothalamic areas. Levels of A1-17 decreased in the central nucleus of the amygdala (-35.1%). Food restriction increased levels of A1-8 in nucleus accumbens (+34.4%), bed nucleus of the stria terminalis (+24.5%) and lateral hypothalamus (+41.9%). Food restriction had no effect on levels of B1-13. A1-17 is highly kappa-preferring and the brain regions in which levels increased all have a high ratio of kappa: mu and delta receptors. A1-8 is less discriminating among opioid receptor types and the brain regions in which levels increased have a low ratio of kappa: mu and delta receptors. The present results suggest that food restriction alters posttranslational processing within the dynorphin A domain of the prodynorphin precursor, possibly leading to a change in the balance between kappa and non-kappa opioid receptor stimulation in specific brain regions.

The effect of selective opioid antagonists on butorphanol-induced feeding

Butorphanol tartrate (BT) potently stimulates food intake in satiated rats. The opioid receptor profile of BT is complex and is dependent upon the assay and animal species studied. In the present study we utilized three selective opioid antagonists; namely beta-funaltrexamine (beta-FNA), naltrindole (NTI) and norbinaltorphimine (nor-BNI), to probe the opioid receptor profile of BT as an orexigenic agent. Intracerebroventricular administration of nor-BNI (kappa) antagonized the feeding effects of BT (8 mg/kg, s.c.) at doses of 1, 10 and 100 nmol at the 1-2 h time point and decreased feeding at all time points for the 10 nmol dose. After 1 h, the 100 nmol dose of nor-BNI decreased BT-induced feeding by about 72%. In contrast, intraventricular injection of only the highest dose of the selective mu opioid antagonist, beta-FNA (50 nmol), decreased BT-induced feeding. Intraventricular administration of the delta opioid agonist, NTI, failed to alter BT-induced feeding at doses as high as 50 nmol. These data suggest that BT is dependent upon the kappa and perhaps the mu opioid receptors to increase food intake in satiated rats.

Effect of delayed weaning on opioid receptor control of swim stress-induced antinociception in the developing rat

1. The opioid type of swim-stress induced antinociception (SIA) is mediated via mu-sites in preweanling rats and predominantly by delta-sites in postweanling animals. We have studied the effect of delay of weaning on the receptor transition of this behaviour in the developing rat. 2. Litters were weaned normally at day 21 or allowed to remain with their mothers until assessment of swim SIA. Animals were stressed by warm water (20 degrees C) swimming for 3 min periods and antinociception assessed by the tail immersion test (50 degrees C). 3. Naloxone (10 mg kg-1) partially reversed swim SIA in both 25 day old weaned and non-weaned rats. 4. Naltrindole (1 mg kg-1) partially reversed swim SIA in 25 day old weaned rats but had no effect in non-weaned animals. Naltrindole (5 mg kg-1) completely abolished swim SIA in weaned rats but was without effect in non-weaned groups. Antinociceptive responses to the mu-agonist, alfentanil (60 micrograms kg-1) were unaffected by naltrindole at 1 mg kg-1 but were partially reversed at 5 mg kg-1. 5. In 30 day old non-weaned rats, naltrindole (5 mg kg-1) abolished the swim SIA. 6. In conclusion, transition from mu to delta-receptor control of swim SIA in rat pups can be delayed by between 5 and 10 days by delay of weaning. The environmental stimulus of weaning can activate opioid receptor subtype operation of biological responses in the developing animal.

Chronic food restriction and weight loss produce opioid facilitation of perifornical hypothalamic self-stimulation

Electrical stimulation frequency thresholds for lateral hypothalamic (LH) self-stimulation were monitored throughout a 3 week period of food restriction and a subsequent 3 week period of re-feeding. Rats with electrodes placed in the perifornical LH were sensitive to this dietary manipulation as evidenced by a high positive correlation between body weight and self-stimulation threshold. Rats with electrodes in the zona incerta/subincertal region or ventral hypothalamus displayed little or no change in threshold. Lateral ventricular injection of naltrexone (200.0 nM) reversed the decline in threshold that was otherwise present during food restriction in rats with perifornical placements. Naltrexone had no effect on thresholds of rats with placements outside the perifornical region. These findings suggest that food restriction and weight loss activate an opioid mechanism that facilitates perifornical LH self-stimulation. The documented association of perifornical LH with the phenomenon of stimulation-induced feeding, and the reciprocal connections between this region and gustatory structures, supports the hypothesis that facilitation of self-stimulation by food restriction is related to the natural phenomenon of positive alliesthesia (i.e. the hunger-dependency of food reward).

The kappa-opioid receptor: evidence for the different subtypes

Classification of drugs acting on the kappa-opioid receptors seems to be difficult, since some of these ligands are also sigma agonists and/or display non-opioid actions as well. Furthermore, certain benzomorphans having kappa-agonistic character, are shown to be mu-antagonists too. Therefore the classification of the kappa-opioid receptor has to be presently restricted to two subclasses that also have physiological meaning. Dynorphin and Met-enkephalin-Arg6-Phe7 are proposed as endogenous peptide ligands for kappa-receptors. Nonpeptide agonists are benzeneacetamides interacting with the kappa1 receptor. Benzomorphans bind to both subtypes of kappa-receptors. No selective nonpeptide ligand for the kappa2 receptor exists as yet. Nor-binaltorphimine, a specific kappa-antagonist also inhibits both kappa-subtypes. Further research for kappa2 selective drugs is necessary for clear distinction between the two kappa-opioid binding sites. Molecular cloning of opioid receptors including their subtypes are expected to provide direct proof of their existence.

Microinjection of opioid antagonists into the substantia nigra reduces stress-induced eating in rats

Stress produced by pinching the tail has been shown to cause satiated animals to eat and to display oral stereotypies. Endogenous opioids and central dopamine systems have been implicated in the mediation of these effects. In order to test the possibility that the substantia nigra (SN) might be involved, the amount of food intake and gnawing produced by mild tail pinch were assessed following bilateral microinjections of opioid antagonists into the SN. Evaluations of nociceptive thresholds were also conducted using tail flick and hot plate tests. Eating induced by tail pinch was reduced by microinjections of the non-selective opioid antagonist naloxone (3, 10, 20 and 30 nmol) and by the mu-selective antagonist Cys2, Tyr3, Orn5, Pen7 Amide (CTOP) (1, 3 and 10 nmol). These effects on eating occurred in the absence of effects on gnawing. kappa- and delta-antagonists (10 nmol) had no effect on eating or gnawing. Naloxone did not alter either tail flick or hot-plate response latencies. The highest dose of CTOP increased response latency on the hot-plate test only. The results are interpreted as suggesting that the SN may be an important central site of action for opioid antagonists in reducing stress-induced eating. The possibility that the SN may be a central site mediating the effects of dopamine on this phenomenon is also discussed.

Beta-funaltrexamine (beta-FNA) decreases deprivation and opioid-induced feeding

We studied the effect of the mu antagonist, beta-funaltrexamine (beta-FNA) on deprivation and opioid-induced feeding. Intracerebroventricular pre-treatment of 20 h deprived rats with 0.1, 1, 10 and 20 nmol of beta-FNA decreased feeding by 24%, 50%, 50% and 38% during the first hour. Central administration of beta-FNA (0.1, 1 and 10 nmol) also decreased feeding induced by the mu opioid agonist, DAMGO by 57%, 60% and 71%. Feeding induced by the delta agonist, DSLET, was decreased by pre-treatment with beta-FNA; but only during the 1-2 h time points, a time when relatively little food was ingested. Intraventricular injection of beta-FNA failed to alter feeding stimulated by the kappa opioid agonist, U-50,488H. These data further substantiate a role for the opioid receptor in deprivation and opioid-induced feeding.

Nor-binaltorphimine decreases deprivation and opioid-induced feeding

We evaluated the effect of the kappa antagonist, nor-binaltorphimine (nor-BNI) on deprivation and opioid-induced feeding in rats. Intracerebroventricular administration of nor-BNI (100 nmol) decreased deprivation-induced feeding for as long as 24 h, albeit in a fairly weak manner (maximum decrease of approximately 28%). Nor-BNI (1, 10 and 100 nmol) decreased feeding induced by the kappa ligand U-50,488H by as much as 85% during the first hour of the study. This kappa antagonist also decreased feeding induced by the delta agonist DSLET and the mu agonist DAMGO. Based on previous studies indicating that nor-BNI is a selective kappa antagonist, we conclude that not only U-50,488H (kappa), but also DSLET (delta) and DAMGO (mu)-induced feeding are dependent upon an active kappa receptor.

Dynorphin A-containing neural elements in the nucleus of the solitary tract of the rat. Light and electron microscopic immunohistochemistry

Distribution of dynorphin A (DyA) immunoreactivity in the nucleus of the solitary tract (NTS) was examined in rats after various surgical transections by light and electron microscopic immunohistochemistry. In colchicine-treated animals DyA immunostained perikarya were seen in each subdivision of the NTS. In intact rats, dense network of immunopositive nerve fibers was localized light microscopically, and synaptic contacts were found between DyA immunopositive structures (axo-axonic, axo-dendritic synapses), electron microscopically. Surgical transections medial, caudal or rostral to the nucleus did not alter the distribution pattern of DyA in the NTS. Lesion immediately lateral to the nucleus resulted in an ipsilateral appearance of immunostained cell bodies. Vagal and glossopharyngeal afferents (including baroreceptor fibers) terminate in the medial and commissural subnucleus of the NTS. Two days after extracranial vagotomy, synaptic contacts between degenerated presynaptic boutons and DyA immunopositive postsynaptic elements were observed in both medial and commissural part of the NTS. These observations provide morphological evidence suggesting that (1) axons of dynorphin A-containing cell bodies form an intrinsic network inside the nucleus; (2) these DyA cells receive direct peripheral inputs through the vagus nerve, and (3) projecting DyA neurons may exist in the NTS, they may innervate medullary, rather than forebrain, higher brainstem or spinal cord neurons.

Multiple neurochemical and behavioral consequences of stressors: implications for depression

Animal models of clinical depression have frequently focused on the contribution of stressors to the induction of behavioral impairments and pharmacological intervention in the amelioration of these disturbances. Stressors provoke various behavioral disturbances and influence the activity of central neurotransmitters implicated in depression. It is our contention that those variables which favor the provocation of amine depletions or prevent the development of a neurochemical adaptation will increase vulnerability to behavioral disturbances. It is essential to consider, however, that marked interindividual and interstrain differences exist in the behavioral and neurochemical response to stressors, and in the effectiveness of antidepressant treatments.

Effects of kappa-opioid receptor agonists and morphine on food intake and urinary output in food-deprived and nondeprived rats

The effects of kappa-opioid receptor agonists, bremazocine, U-50, 488H and tifluadom and of a mu-opioid receptor agonist, morphine, on food intake and urinary output in food-deprived and nondeprived Sprague-Dawley rats was determined. In food-deprived animals, intraperitoneal administration of bremazocine at 0.1 mg/kg increased food intake but at 1.0 and 10.0 mg/kg doses decreased it. Tifluadom (0.1-10.0 mg/kg) had no effect on food intake. U-50,488H at 1.0 mg/kg increased food intake, whereas 10.0 mg/kg dose decreased the food consumption. In nondeprived rats, the kappa-opioid receptor agonists failed to produce any effect on food consumption. In food-deprived rats, all the three kappa-opioid receptor agonists increased the urinary output at the highest dose (10 mg/kg). In nondeprived rats similar effects as in food-deprived rats were observed except bremazocine increased urinary output at all the doses used. These results with kappa-opioid agonists may be related to either the existence of more than one population of kappa-opioid receptors or their differential actions at the opioid receptor types.

Central mu, delta, and kappa opioid binding sites, and brain and pituitary beta-endorphin and met-enkephalin in genetically obese (ob/ob) and lean mice

The equilibrium dissociation constants and maximal binding capacities of 3H-dihydromorphine (DHM), 3H-D-Ala2-D-leu3-enkephalin (DADL), and 3H-dynorphin A(1-8) for their respective mu, delta, and kappa opiate binding sites were studied in brain membrane preparations from lean and genetically obese-hyperglycaemic (Aston ob/ob) mice. The concentration of kappa binding sites was 2.7 fold higher in obese compared with lean mouse brain (231 +/- 44.6 versus 83.8 +/- 10.3 fmoles 3H-dynorphin/mg protein respectively, mean +/- SEM). The concentration of delta binding sites in obese was 1.6 fold that in lean mouse brain (94.5 +/- 8.6 versus 59.5 +/- 6.5 fmoles 3H-DADL/mg protein). In contrast, the concentration of brain mu receptors was 40% lower in obese compared with lean mice (20.8 +/- 2.19 and 34.8 +/- 3.1 fmoles 3H-DHM/mg protein respectively). Binding affinities of delta and kappa sites for their respective ligands were not significantly different in lean v. obese mice. However, for mu sites, lean mouse binding data showed two affinities, one was not significantly different from obese (0.35 nM) the second was lower (1.18 nM) for DHM. Increases of approximately 5 fold and 3 fold in the brain content of beta-endorphin and met-enkephalin respectively, and no differences in brain dynorphin levels, were demonstrated in obese mice compared with lean controls. In obese mice, pituitary beta-endorphin content was 9 fold higher, met-enkephalin 4 fold higher and dynorphin 12 fold higher than in lean mice. The striking differences in opioid binding-site characteristics and in endogenous opioid peptide levels in obese compared with lean mice may contribute to the hyperphagia and, directly or indirectly, to the development of hyperglycaemia and hyperinsulinaemia in obese mice.

Lateral hypothalamic dynorphinergic efferents to the amygdala and brainstem in the rat

Dynorphin is present within perikarya of the lateral hypothalamus (LH) and perifornical nucleus (PeF), and within nerve terminals of the central nucleus of the amygdala, central grey, parabrachial nucleus, and the dorsal vagal complex (nucleus of the solitary tract and dorsal motor nucleus of the vagus). Each of these nuclei receive efferent projections from the LH and PeF. In this study, the possibility that dynorphin cells with LH and PeF innervate each of these nuclei was investigated using a combined retrograde tracing-immunofluorescence technique. As enkephalinergic perikarya have also been localized to LH and PeF, peptide E (an enkephalin precursor fragment) was also studied for comparison. Following injections of fast blue into the central nucleus, parabrachial nucleus, central grey, and dorsal vagal complex, numerous retrogradely-labeled dynorphin-immunoreactive neurons were present within the LH and PeF. In comparison, retrogradely-labeled peptide E-immunoreactive cells were infrequently observed. These results suggest the LH and PeF to be a major source of dynorphin to the forebrain and brainstem.

Cold swim stress-induced changes in the levels of opioid peptides in the rat CNS and peripheral tissues

Endogenous opioid peptides have been implicated in stress-induced analgesia and stress-induced feeding behavior. An earlier study from our laboratory showed that rats subjected to cold swim stress consumed significantly more food compared to controls. The present study describes changes in the levels of various opioid peptides in the central nervous system and periphery due to cold swim stress. Male Sprague-Dawley rats were subjected to cold swim stress (1 degree C for 5 min), then sacrificed by decapitation; brain, pituitary, adrenals and plasma were collected. Tissue extracts were assayed for opioid peptides by RIA. Cold swim stress resulted in analgesia which could be blocked by prior administration of naloxone, as observed by a tail-flick latency test. Cold swim stress caused a 42% decrease in pituitary beta-endorphin, but increased the level of this peptide in the hypothalamus and plasma by 36% and 337%, respectively. Dynorphin level decreased by 62% in the hypothalamus, but was not affected in the pituitary. Levels of Leu-enkephalin and Met-enkephalin decreased in the adrenal gland by 37% and 18%, respectively, but were not significantly affected in the CNS. These results indicate that cold swim stress has a differential effect on the level of CNS and peripheral opioid peptides, and that both central and peripheral opioid peptides may be important in stress-induced analgesia and feeding behavior.

Effects of food deprivation and high fat diet on immunoreactive dynorphin A(1-8) levels in brain regions of Zucker rats

The levels of immunoreactive dynorphin A(1-8) (ir-DYN8) were measured in discrete brain regions of lean Zucker rats subjected to food deprivation for 72 hr and to a high fat diet, and in fatty Zucker rats after food deprivation for 72 hr. Fatty rats showed higher concentrations of ir-DYN8 in the cortex and midbrain, when compared to lean rats fed a stock diet ad lib. Food deprivation increased ir-DYN8 levels in the cortex of lean rats and fatty rats and in the hippocampus of fatty rats, but decreased its content in the striatum of lean rats and in the midbrain of fatty rats. The high fat diet increased ir-DYN8 levels in the cortex and midbrain of lean rats. These results suggest that ir-DYN8 levels in extrahypothalamic structures of Zucker rats could be differentially modified under conditions of hereditary obesity and dietary manipulations.

Ingestive behavior of meat and egg-type chickens: equal sensitivity to naloxone

The efficacy of the opioid antagonist naloxone in attenuating ingestive behavior in stocks of chickens genetically selected for either meat (Rock-Cornish, RC) or egg production (Single-Comb White Leghorn, SCWL) was investigated. Because the stocks differ markedly in growth rate, two experiments were conducted to compare RC and SCWL cockerels at similar body weights and at the same age. Birds were injected intramuscularly with either isotonic saline or naloxone HCl at a dose of 5 mg/kg body weight. Food and water were offered ad libitum 15 min postinjection. In RC and SCWL stocks of similar body weight, naloxone significantly attenuated cumulative food and water intake through 210 min following the return of food and water. When administered to RC and SCWL stocks of the same age, naloxone significantly attenuated cumulative food intake for 300 min and cumulative water intake at 60 and 120 min. The relatively long-term depressions in cumulative food and water intake were attributable to significant decreases in incremental consumption within early time periods. There was no significant difference in the efficacy of naloxone in attenuating ingestive behavior when the stocks were compared at either similar body weights or at the same age. The results demonstrate that genetic selection for meat or egg production has not significantly altered opioid mechanisms regulating food and water intake in the domestic fowl.

Effects of food deprivation and high fat diet on opioid receptor binding in rat brain

The effect of food deprivation for 72 h or a high fat diet on [3H]naloxone binding in the discrete brain regions of male lean Zucker rats was studied. In the midbrain, both treatments increased Bmax for the high-affinity site with no change in Kd. In the cortex, the high fat diet increased Bmax for the high-affinity site. These results suggest that dietary manipulations could produce significant changes in the endogenous opioid system.

Effect of food deprivation on opioid receptor binding in the brain of lean and fatty Zucker rats

The effect of food deprivation on opioid receptor binding was studied in 6 brain regions of lean and fatty Zucker rats; using [3H]dynorphin A. There was no significant difference between lean and fatty rats fed ad libitum in binding parameters for any regions studied. Food deprivation increased Bmax and/or Kd for cortex, midbrain and striatum of lean rats, and the former two regions of fatty rats. These results suggest that food deprivation may influence opioid receptor binding in lean and fatty Zucker rats.

CRF antagonist partially reverses CRF- and stress-induced effects on feeding

Exogenous corticotropin releasing factor (CRF) causes centrally mediated behavioral changes including decreased feeding and increased grooming. These behavioral changes are also seen in response to some stressors. However, the role of endogenous CRF in the behavioral response to stressors has not been investigated fully. We report below our findings on the behavioral effects of alpha-helical CRF (9-41), a recently discovered competitive antagonist of CRF-induced ACTH release. Alpha-helical CRF (9-41) partially reversed the decrement in feeding induced by CRF. Furthermore, the reduction in food intake due to restraint stress was partially reversed by alpha-helical CRF (9-41). These results indicate that changes in endogenous CRF release induced by the restraint stressor may play a role in stress-induced anorexia.

Food deprivation-induced changes in the level of opioid peptides in the pituitary and brain of rat

Following 1-4 days of food-deprivation (FD) male rats were sacrificed. The pituitary and different regions of brain were analyzed for beta-endorphin-like immunoreactivity (beta-EI), dynorphin (dyn) and methionine-enkephalin (ME) content by RIA. Pituitary beta-EI increased by 16, 28 and 43% on days 2, 3 and 4 of FD. In striatum also, beta-EI increased by 140 and 176% on days 2 and 3 of FD. Dyn level in pituitary was not affected but decreased in hypothalamus by 20% and in striatum by 73% on the 4th day of FD. There was a significant decrease (33-55%) in ME levels in striatum, hippocampus and cortex on 4th day of FD. When food-deprived rats were fed for 24 hr, concentration of most of the opioid peptides returned to basal level. These results suggest that FD in rats affects the opioid peptide levels in a differential manner.

Seasonal variation in opioid modulation of feeding in the 13-lined ground squirrel

Opioids are well recognized to modulate ingestive behaviors in a variety of species. To study the potential role of opioids in the alteration of ingestive behaviors that occur prior to hibernation, we have administered opiate agonists and the antagonist, naloxone, to the 13-lined ground squirrel (Spermophilus tridecemlineatus) during periods when they were hyperphagic and hypophagic. Naloxone consistently reduced feeding during both phases of the feeding cycle. Hypophagic animals, however, were 10 times more sensitive to the effects of naloxone. The effect of the opiate agonists (morphine and butorphanol, 1 and 10 mg/kg) on feeding also varied between seasons. The low dose of morphine produced a slight, but significant increase in feeding at one hour in hyperphagic animals, while the high doses tended to decrease feeding. When administered to hypophagic animals, feeding was decreased by both doses of each agonist. Immunoreactive (IR)-dynorphin levels in the cortex, hypothalamus and striatum were higher during the hypophagic phase compared with the hyperphagic phase. These data indicate that the 13-lined ground squirrel possesses an opiate sensitive feeding system which is affected by season.

The effect of food and water deprivation on post-stress analgesia in mice and levels of beta-endorphin and dynorphin in blood plasma and hypothalamus

Pain sensitivity of food and/or water-deprived male mice was tested on a hotplate. The most pronounced analgesia ensued in animals given no food and water, and no food but water ad libitum, the least one in water-deprived mice. The magnitude of the rise in pain threshold depended on the duration of deprivation and was correlated with the increase in the blood plasma beta-endorphin level. In the hypothalamus beta-endorphin level increased after 72-h food deprivation only. The level of dynorphin remained unchanged. Naloxone (10 mg/kg) almost completely reversed food or water-deprivation induced analgesia.

Increase in the threshold of a nociceptive test induced by naloxone in morphine-tolerant rats

The effects of various doses of naloxone (3-1000 micrograms/kg i.v.) on the vocalization threshold elicited by pressure on the paw were evaluated in rats chronically treated with high doses of morphine. In addition to the well known precipitation of withdrawal induced by naloxone, an unexpected dose-related increase in the vocalization threshold was observed.

Satiety, hunger and regional brain content of cholecystokinin/gastrin and met-enkephalin immunoreactivity in sheep

Cholecystokinin (CCK) and met-enkephalin (MEK) related peptides have been shown to alter feeding behavior subsequent to their injection into the peripheral circulation or directly into the brains of several species. To evaluate the potential role of endogenous brain pools of these peptides in feeding, groups of sheep were sacrificed either immediately following a meal (satiated) or after various intervals of food deprivation (hungry). Content of CCK-gastrin immunoreactivity in the anterior hypothalami of satiated sheep was elevated compared to 2, 4, or 24 hours of food deprivation. Content of MEK increased progressively with longer intervals of fasting (4 and 24 hours) in the amygdala and basomedial hypothalamus, whereas olfactory bulb content decreased with a similar time course. The results support a potential role for anterior hypothalamic CCK/gastrin in behaviors of satiety, whereas MEK neurons of limbic/rhinencephalic regions appear to form part of a separate circuit gradually activated by increasing hunger. Results are discussed in terms of potential target regions of the peptides, as well as the regional levels and feeding response of sheep as compared to available data from other species.

Opioids and consummatory behavior

Since the second decade of this century it has been known that opiates can influence ingestive behaviors. Generally, opioid agents enhance feeding and opioid antagonists decrease feeding. The present paper reviews the responsiveness of different animal species to opiates in relation to ingestive behaviors, the opioid receptors involved in such consummatory behaviors, the site of action of opioid modulation of feeding, the role of glucose in opioid induced feeding, and endocrine effects on opioid feeding systems. We emphasize the finding that more than one opioid receptor is involved in the modulation of feeding. A large body of evidence indicates a major role for the dynorphin/alpha-neo-endorphin kappa opioid receptor as one of the receptors involved in feeding modulation. Opioids appear to exert their effect predominantly within the central nervous system, though peripheral effects on taste and gastrointestinal function may play a role in opioid-induced feeding. Although opioid blockade acutely blocks food intake, chronic administration of opiate antagonists to humans and laboratory animals has not proven to be an effective means of decreasing body weight. Chronic opiate administration decreases body weight and autosensitization of beta-endorphin increases body weight. Thus, although it is clear that opioids can effect food intake, it is not clear what effect chronic administration of opioids has no food intake or body weight.

Continuous social defeat induces an increase of endogenous opioids in discrete brain areas of the mongolian gerbil

Dyads of a victor and a loser of mongolian gerbils (Meriones unguiculatus) coexisted for seven days; isolated animals served as a further experimental group. beta-Endorphin, Met-enkephalin and dynorphin were measured in several brain areas and in the anterior and neurointermediate pituitary. beta-Endorphin and Met-enkephalin were increased in the amygdala of defeated as compared to victorious animals. Met-enkephalin in the hypothalamus and in the striatum were lower in isolated than in coexisting gerbils. Coexistence decreased beta-endorphin in the amygdala and in the hypothalamus as compared to isolation. The results provide biochemical evidence for the role of central endogenous opioid-peptide systems in the physiology of victory and defeat. Dynorphin showed no variation with social conflict and social status.

CNS tissue levels of dynorphin-A immunoreactivity and the anorexia associated with sodium chloride imbibition in the rat

Forced imbibition of increasing concentrations of sodium chloride (NaCl) in rats reduced daytime 2-deoxy-D-glucose (2-DG) induced feeding in a concentration dependent manner. Pituitary neurointermediate lobe (NIL) levels of immunoreactive (ir)-dynorphin-A 1-17 and 1-8 were also decreased by the NaCl regimen in a concentration dependent manner. However, there was no significant association between the reduction of NIL dynorphin levels and the suppression of 2-DG induced feeding on a within-animal basis. NaCl imbibition did not affect levels of either ir-dynorphin-A 1-17 or 1-8 in the hypothalamus, cerebral cortex, hippocampus, medulla/pons or anterior pituitary. Neither the acute changes following 2-DG administration, nor the comparison of ir-dynorphin-A 1-8/1-17 ratios appeared useful for the assessment of dynorphin-A turnover. Thus, the present results did not support the hypothesis that anorexia of NaCl treated animals results from the depletion of dynorphin-A.

Peptides and feeding

This report reviews the hypothesis that peptides play a role in appetite modulation, stressing that the available evidence is predominantly pharmacological and thus caution needs to be taken in assigning physiological significance at this time. Two peptide systems have been postulated--a peripheral satiety system, typified by the gastrointestinal hormone cholecystokinin and a central feeding system driven by the opioid peptides and neuropeptide Y. This review also discusses the putative role of peptides in the anorexia of aging, drinking elicited by feeding and as mediators of the autonomic effects seen in association with ventromedial hypothalamic lesions.

Social conflict activates opioid analgesic and ingestive behaviors in male mice

The activation of endogenous opioid mechanisms and their subsequent effects on rodent behavior and physiology has usually been characterized following artificial stress. In this study the more naturalistic stress arising from social conflict between male mice was used to investigate the involvement of opioid systems in post-conflict analgesic and ingestive behaviors. Both the aggressive encounters and the subsequent defeat experience resulted in marked analgesia and the induction of ingestive behaviors. Feeding and drinking responses were analogous to those observed after administrations of either the endogenous opioid peptide, beta-endorphin, or the exogenous opioid agonist morphine. The ingestive behaviors following defeat or central opiate administration were blocked by the opiate antagonist naloxone. The present results support the hypothesis of a direct activation of the endogenous opiate system following social conflict.

The role of the endogenous opiates in zinc deficiency anorexia

Anorexia is a major symptom of zinc deficiency, but the mechanism(s) for this anorexia are poorly defined. Recent studies have suggested an integral role for endogenous opiate peptides in appetite regulation. Dynorphin, a leucine-enkephalin containing opiate peptide, is a potent inducer of spontaneous feeding. In this study we showed that zinc deficient animals were relatively resistant to dynorphin-induced feeding. Measurement of dynorphin levels using a highly sensitive radioimmunoassay showed that zinc deficient animals had lower levels of dynorphin in the hypothalamus than did ad lib fed animals, with weight restricted animals having intermediate values. [3H]-naloxone binding was significantly increased to isolated brain membranes from zinc deficient animals using 1 nM unlabeled naloxone when compared to ad lib fed controls with the weight restricted animals again having intermediate values. These data suggest that abnormalities in endogenous opiate regulation of appetite may well play a role in the anorexia of zinc deficiency. The effects of zinc deficiency on endogenous opiate action appear to include alterations in receptor affinity, a post-receptor defect and alterations in the synthesis and/or release of dynorphin.

Involvement of dynorphin and the kappa opioid receptor in feeding

Dynorphin-(1-17) produces a highly specific increase in food ingestion. Similar enhancement of food ingestion is found with dynorphin fragments (1-10), (1-11), (1-13) and (3-13) but not with (1-8) and (1-9). Dynorphin B (rimorphin) also enhances food intake. The highly specific kappa agonist U-50,488 also enhances food intake as do a number of other kappa-opiate receptor agonists. These studies provided further for the role of a highly specific dynorphin-kappa opioid receptor in the modulation of feeding.

Endogenous opiates: 1982

This article is the fifth installment in an annual series of reviews of successive year's research dealing with the endogenous opiate peptides. Due to the continuing massive increase in the number of studies in this field, it has become impossible to continue comprehensive reviews of all aspects of this work. As a result we have decided that beginning this year the coverage will be abbreviated to emphasize non-analgesic and behavioral work. The specific areas discussed include stress, tolerance and dependence, consummatory responses, alcohol consumption, schizophrenia and emotional disorders, learning and memory, cardiovascular responses, respiratory effects, thermoregulatory effects, neurological deficits and other disorders, activity, and other, miscellaneous behaviors. As in previous years, we have attempted a relatively comprehensive review of the subjects covered only for the previous year and have not made an attempt to evaluate their contributions relative to those of past years.

Endogenous opioid withdrawal in the Jarisch-Herxheimer reaction

Meptazinol, a partial opioid agonist, diminishes the Jarisch-Herxheimer reaction in relapsing fever whereas naloxone, a pure opioid antagonist, is virtually inert. Because endogenous opioid activity is probably increased in the acute phase, the efficacy of meptazinol is unlikely to be due to agonist activity. However, it is possible that in such a severe reaction there may be an exhaustion of endogenous opioids, giving rise to a naturally occurring quasi-morphine-withdrawal syndrome which responds to meptazinol.