Interaction between μ-opioid and 5-HT1A receptors in the regulation of panic-related defensive responses in the rat dorsal periaqueductal grey

Changes in hypothalamic mu-opioid receptor expression following acute olanzapine treatment in female rats: Implications for feeding behavior

Advances have been made in recent years in using opioid receptor antagonists as an adjunct therapy to psychotropic medication to reduce debilitating weight gain and metabolic adverse effects associated with in particular second generation antipsychotics. However, it is unknown whether second generation antipsychotics produce a change in opioid receptor expression in the brain. The present study investigated early changes in opioid receptor expression in the female rat hypothalamus, a master controller of hunger and metabolic regulation, after acute treatment with olanzapine, a commonly used second generation antipsychotic. Using quantitative spatial in situ hybridization and receptor autoradiography, expression levels of the three opioid receptors; kappa, mu and delta, were determined at mRNA and protein level, respectively, in the five hypothalamic areas: paraventricular nucleus, arcuate nucleus, ventromedial nucleus, dorsomedial nucleus and lateral hypothalamus. After 48 h of olanzapine treatment at clinically relevant plasma concentration weight gain and food intake changes, and increased plasma glucose were observed in female rats. Olanzapine treatment also led to a significant increase in mu opioid receptor availability in the arcuate nucleus, which contains both satiety and hunger controlling neurons. No other areas showed any opioid receptor expressional changes with olanzapine treatment on neither at mRNA nor protein level. Technical difficulties made it impossible to analyze mRNA levels in the lateral hypothalamus and overall binding of delta opioid receptors. Thus, the present study provided insights in to how olanzapine at clinically relevant plasma levels already at an early stage modulated the opioid system in the hypothalamus.

Defensive and Emotional Behavior Modulation by Serotonin in the Periaqueductal Gray

Serotonin 5-hydroxytryptamine (5-HT) is a key neurotransmitter for the modulation and/or regulation of numerous physiological processes and psychiatric disorders (e.g., behaviors related to anxiety, pain, aggressiveness, etc.). The periaqueductal gray matter (PAG) is considered an integrating center for active and passive defensive behaviors, and electrical stimulation of this area has been shown to evoke behavioral responses of panic, fight-flight, freezing, among others. The serotonergic activity in PAG is influenced by the activation of other brain areas such as the medial hypothalamus, paraventricular nucleus of the hypothalamus, amygdala, dorsal raphe nucleus, and ventrolateral orbital cortex. In addition, activation of other receptors within PAG (i.e., CB1, Oxytocin, µ-opioid receptor (MOR), and γ-aminobutyric acid (GABA_A)) promotes serotonin release. Therefore, this review aims to document evidence suggesting that the PAG-evoked behavioral responses of anxiety, panic, fear, analgesia, and aggression are influenced by the activation of 5-HT_1A and 5-HT_2A/C receptors and their participation in the treatment of various mental disorders.

Antipanic-like effect of esketamine and buprenorphine in rats exposed to acute hypoxia

The antidepressant effect of ketamine has been widely acknowledged and the use of one of its enantiomers, S-ketamine (esketamine), has recently been approved for the clinical management of treatment-resistant depression. As with ketamine, the non-selective opioid receptor-interacting drug buprenorphine is reported to have antidepressant and anxiolytic properties in humans and rodents. Given the fact that antidepressant drugs are also first line treatment for panic disorder, it is surprising that the potential panicolytic effect of these compounds has been scarcely (ketamine), or not yet (buprenorphine) investigated. We here evaluated the effects of ketamine (the racemic mixture), esketamine, and buprenorphine in male Wistar rats submitted to a panicogenic challenge: acute exposure to hypoxia (7% O₂). We observed that esketamine (20 mg/kg), but not ketamine, decreased the number of escape attempts made during hypoxia, and this effect could be observed even 7 days after the drug administration. A panicolytic-like effect was also observed with MK801, which like esketamine, antagonizes NMDA glutamate receptors. Buprenorphine (0.3 mg/kg) also impaired hypoxia-induced escape, an effect blocked by the non-selective opioid receptor antagonist naloxone, indicating an interaction with classical ligand sites, such as µ and kappa receptors, but not with nociception/orphanin FQ receptors. Altogether, the results suggest that esketamine and buprenorphine cause rapid-onset panicolytic-like effects, and may be alternatives for treating panic disorder, particularly in patients who are refractory to standard pharmacological treatment.

Mu-opioid and CB1 cannabinoid receptors of the dorsal periaqueductal gray interplay in the regulation of fear response, but not antinociception

Evidence indicates that periaqueductal gray matter (PAG) plays an important role in defensive responses and pain control. The activation of cannabinoid type-1 (CB1) or mu-opioid (MOR) receptors in the dorsal region of this structure (dPAG) inhibits fear and facilitates antinociception induced by different aversive stimuli. However, it is still unknown whether these two receptors work cooperatively in order to achieve these inhibitory actions. This study investigated the involvement and a likely interplay between CB1 and MOR receptors localized into the dPAG on the regulation of fear-like defensive responses and antinociception (evaluated in tail-flick test) evoked by dPAG chemical stimulation with N-methyl-d-aspartate (NMDA). Before the administration of NMDA, animals were first intra-dPAG injected with the CB1 agonist ACEA (0.5 pmol), or with the MOR agonist DAMGO (0.5 pmol) in combination with the respective antagonists AM251 (CB1 antagonist, 100 pmol) or CTOP (MOR antagonist, 1 nmol). To investigate the interplay between these receptors, microinjection of CTOP was combined with ACEA, or microinjection of AM251 was combined with DAMGO. Our results showed that both the intra-PAG treatments with ACEA or DAMGO inhibited NMDA-induced freezing expression, whereas only the treatment with DAMGO increased antinociception induced with NMDA, which are completely blocked by its respective antagonists. Interestingly, the inhibitory effects of ACEA or DAMGO on freezing was blocked by CTOP and AM251, respectively, indicating a functional interaction between these two receptors in the mediation of defensive behaviors. However, this cooperative interaction was not observed during the NMDA-induced antinociception. Our findings indicate that there is a cooperative action between the MOR and CB1 receptors within the dPAG and it is involved in the mediation of NMDA-induced defensive responses. Additionally, the MORs into the dPAG are involved in the modulation of the antinociceptive effects that follow a fear-like defense-reaction induced by dPAG chemical stimulation with NMDA.

The Blockade of µ1- and κ-Opioid Receptors in the Inferior Colliculus Decreases the Expression of Panic Attack-Like Behaviours Induced by Chemical Stimulation of the Dorsal Midbrain

BACKGROUND

Gamma-aminobutyric acid (GABA)ergic and opioid systems play a crucial role in the neural modulation of innate fear organised by the inferior colliculus (IC). In addition, the IC is rich in GABAergic fibres and opioid neurons, which are also connected to other mesencephalic structures, such as the superior colliculus and the substantia nigra. However, the contribution of distinct opioid receptors (ORs) in the IC during the elaboration and expression of innate fear and panic-like responses is unclear. The purpose of the present work was to investigate a possible integrated action exerted by ORs and the GABAA receptor-mediated system in the IC on panic-like responses.

METHODS

The effect of the blockade of either µ1- or κ-ORs in the IC was evaluated in the unconditioned fear-induced responses elicited by GABAA antagonism with bicuculline. Microinjections of naloxonazine, a µ1-OR antagonist, or nor-binaltorphimine (nor-BNI), a κ-OR antagonist, were made into the IC, followed by intramesencephalic administration of the GABAA-receptor antagonist bicuculline. The defensive behaviours elicited by the treatments in the IC were quantitatively analysed, recording the number of escapes expressed as running (crossing), jumps, and rotations, over a 30-min period in a circular arena. The exploratory behaviour of rearing was also recorded.

RESULTS

GABAA-receptor blockade with bicuculline in the IC increased defensive behaviours. However, pretreatment of the IC with higher doses (5 µg) of naloxonazine or nor-BNI followed by bicuculline resulted in a significant decrease in unconditioned fear-induced responses.

CONCLUSIONS

These findings suggest a role played by µ1- and κ-OR-containing connexions and GABAA receptor-mediated neurotransmission on the organisation of panic attack-related responses elaborated by the IC neurons.

Effects of the adjunctive treatment of antidepressants with opiorphin on a panic-like defensive response in rats

BACKGROUND

Antidepressants are the first-choice for pharmacological treatment of panic disorder. However, they present disadvantages, such as delayed therapeutic effect, many side effects and a considerable rate of non-responders. These shortcomings prompt the development of new therapeutic strategies. Among these are the adjunctive use of enkephalinase inhibitors, such as opiorphin, which supposedly acts by increasing the availability of brain enkephalins and other endogenous opioids.

AIMS

We here evaluated whether opiorphin in the dorsal periaqueductal grey matter (dPAG), a key panic-related area, accelerates and/or facilitates the antipanic-like effect of fluoxetine or imipramine. We also verified whether the panicolytic effect of imipramine depends on activation of μ-opioid receptors (MORs).

METHODS

Male Wistar rats were submitted to the escape task of the elevated T-maze, an index of panic attack, after treatment with imipramine (3, 7 or 21 days) or fluoxetine (3, 7, 14 or 21 days), combined with an intra-dPAG injection of opiorphin.

RESULTS

Opiorphin facilitated and accelerated the panicolytic-like effect caused by imipramine, but not with fluoxetine. The antipanic-like effect caused by chronic imipramine did not depend on MOR activation in the dPAG.

CONCLUSION

Combined treatment of antidepressant drugs with opiorphin for hastening or potentiating the effects of the former compounds may not be generally effective, with the results varying depending on the type/class of these panicolytic drugs.

A serotonergic deficit in the dorsal periaqueductal gray matter may underpin enhanced panic-like behavior in diabetic rats

It is known that diabetic (DBT) animals present dysregulation on the serotonergic system in several brain areas associated with anxiety-like responses. The aim of this study was to investigate the involvement of 5-HT1A receptors on dorsal periaqueductal gray (dPAG) in the behavioral response related to panic disorder in type-1 DBT animals. For this, the escape response by electric stimulation (ES) of dPAG in DBT and normoglycemic (NGL) animals was assessed. Both NGL and DBT animals were exposed to an open-field test (OFT) 28 days after DBT confirmation. The current threshold to induce escape behavior in DBT animals was reduced compared with NGL animals. No impairment in locomotor activity was observed when DBT animals were compared with NGL animals. An intra-dPAG injection of the 5-HT1A receptor agonist (±)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) increased the [INCREMENT] threshold in both DBT and NGL, suggesting a panicolytic-like effect. DBT animals presented a more pronounced panicolytic-like response compared with NGL as a higher [INCREMENT] threshold was observed after 8-OH-DPAT treatment, which could be a consequence of the increased expression of the 5-HT1A receptor in the dPAG from DBT animals. Our results are in line with the proposal that a deficiency in serotonergic modulation of the dPAG is involved in triggering the panic attack and the 5-HT1A receptors might be essential for the panicolytic-like response.

B2-kinin receptors in the dorsal periaqueductal gray are implicated in the panicolytic-like effect of opiorphin

Reported results have shown that the pentapeptide opiorphin inhibits oligopeptidases that degrade brain neuropeptides, and has analgesic and antidepressant effects in experimental animals, without either tolerance or dependency after chronic administration. In a previous study we showed that opiorphin has a panicolytic-like effect in the dorsal periaqueductal gray (dPAG) electrical stimulation test (EST), mediated by the μ-opioid receptor (MOR). This study further analyzes the mechanism of opiorphin panicolytic action, using the EST and drug injection inside the dPAG. The obtained results showed that blockade of the 5-HT_1A receptors with WAY-100635 did not change the escape-impairing effect of opiorphin, and combined injection of sub-effective doses of opiorphin and the 5-HT_1A-agonist 8-OH-DPAT did not have a significant anti-escape effect. In contrast, the anti-escape effect of opiorphin was antagonized by pretreatment with the kinin B2 receptor blocker HOE-140, and association of sub-effective doses of opiorphin and bradykinin caused a significant anti-escape effect. The anti-escape effect of bradykinin was not affected by previous administration of WAY-100635. Therefore, the anti-escape effect of opiorphin in the dPAG seems to be mediated by endogenous bradykinin, acting on kinin B2 receptors, which previous results have shown to interact synergistically with MOR in the dPAG to restrain escape in two animal models of panic. Chemical compounds: Opiorphin (PubChem CID: 25195667); WAY100635 maleate salt (PubChem CID: 11957721); 8-OH-DPAT hydrobromide (PubChem CID: 6917794); Bradykinin (PubChem CID: 439201); HOE-140 (Icatibant) (PubChem CID: 6918173).

Using InVivoStat to perform the statistical analysis of experiments

The need to improve reproducibility and reliability of animal experiments has led some journals to increase the stringency of the criteria that must be satisfied before manuscripts can be considered suitable for publication. In this article we give advice on experimental design, including minimum group sizes, calculating statistical power and avoiding pseudo-replication, which can improve reproducibility. We also give advice on normalisation, transformations, the gateway analysis of variance strategy and the use of p-values and confidence intervals. Applying all these statistical procedures correctly will strengthen the validity of the conclusions. We discuss how InVivoStat, a free-to-use statistical software package, which was designed for life scientists, especially animal researchers, can be used to help with these principles.

μ-Opioid and 5-HT1A receptors in the dorsomedial hypothalamus interact for the regulation of panic-related defensive responses

The dorsomedial hypothalamus (DMH) and the dorsal periaqueductal gray (DPAG) have been implicated in the genesis and regulation of panic-related defensive behaviors, such as escape. Previous results point to an interaction between serotonergic and opioidergic systems within the DPAG to inhibit escape, involving µ-opioid and 5-HT1A receptors (5-HT1AR). In the present study we explore this interaction in the DMH, using escape elicited by electrical stimulation of this area as a panic attack index. The obtained results show that intra-DMH administration of the non-selective opioid receptor antagonist naloxone (0.5 nmol) prevented the panicolytic-like effect of a local injection of serotonin (20 nmol). Pretreatment with the selective μ-opioid receptor (MOR) antagonist CTOP (1 nmol) blocked the panicolytic-like effect of the 5-HT1AR agonist 8-OHDPAT (8 nmol). Intra-DMH injection of the selective MOR agonist DAMGO (0.3 nmol) also inhibited escape behavior, and a previous injection of the 5-HT1AR antagonist WAY-100635 (0.37 nmol) counteracted this panicolytic-like effect. These results offer the first evidence that serotonergic and opioidergic systems work together within the DMH to inhibit panic-like behavior through an interaction between µ-opioid and 5-HT1A receptors, as previously described in the DPAG.

Panicolytic-like action of bradykinin in the dorsal periaqueductal gray through μ-opioid and B2-kinin receptors

A wealth of evidence has shown that opioid and kinin systems may control proximal defense in the dorsal periaqueductal gray matter (dPAG), a critical panic-associated area. Studies with drugs that interfere with serotonin-mediated neurotransmission suggest that the μ-opioid receptor (MOR) synergistically interacts with the 5-HT_1A receptor in the dPAG to inhibit escape, a panic-related behavior. A similar inhibitory effect has also been reported after local administration of bradykinin (BK), which is blocked by the non-selective opioid receptor antagonist naloxone. The latter evidence, points to an interaction between BK and opioids in the dPAG. We further explored the existence of this interaction through the dPAG electrical stimulation model of panic. We also investigated whether intra-dPAG injection of captopril, an inhibitor of the angiotensin-converting enzyme (ACE) that also degrades BK, causes a panicolytic-like effect. Our results showed that intra-dPAG injection of BK inhibited escape performance in a dose-dependent way, and this panicolytic-like effect was blocked by the BK type 2 receptor (B2R) antagonist HOE-140, and by the selective MOR antagonist CTOP. Conversely, the panicolytic-like effect caused by local administration of the selective MOR agonist DAMGO was antagonized by pre-treatment with either CTOP or HOE-140, indicating cross-antagonism between MOR and B2R. Finally, intra-dPAG injection of captopril also impaired escape in a dose-dependent way, and this panicolytic-like effect was blocked by pretreatment with HOE-140, suggesting mediation by endogenous BK. The panicolytic-like effect of captopril indicates that the use of ACE inhibitors in the clinical management of panic disorder may be worth exploring.

Participation of dorsal periaqueductal gray 5-HT1A receptors in the panicolytic-like effect of the κ-opioid receptor antagonist Nor-BNI

Panic patients may have abnormalities in serotonergic and opioidergic neurotransmission. The dorsal periaqueductal gray (dPAG) plays an important role in organizing proximal defense, related to panic attacks. The 5-HT_1A receptor (5-HT_1A-R) is involved in regulating escape behavior that is organized in the dPAG. Activation of κ-opioid receptor (KOR) in this region causes anxiogenic effects. In this study, we investigated the involvement of KOR in regulating escape behavior, using systemic and intra-dPAG injection of the KOR antagonist Nor-BNI. As panic models, we used the elevated T-maze (ETM) and the dPAG electrical stimulation test (EST). We also evaluated whether activation of the 5-HT_1A-R or the μ-opioid receptor (MOR) in the dPAG contributes to the Nor-BNI effects. The results showed that systemic administration of Nor-BNI, either subcutaneously (2.0 and 4.0mg/kg) or intraperitoneally (2.0mg/kg), impaired escape in the EST, indicating a panicolytic-like effect. Intra-dPAG injection of this antagonist (6.8nmol) caused the same effect in the EST and in the ETM. Association of ineffective doses of Nor-BNI and the 5-HT_1A-R agonist 8-OH-DPAT caused panicolytic-like effect in these two tests. Previous administration of the 5-HT_1A-R antagonist WAY-100635, but not of the MOR antagonist CTOP, blocked the panicolytic-like effect of Nor-BNI. These results indicate that KOR enhances proximal defense in the dPAG through 5-HT_1A-R modulation, independently of MOR. Because former results indicate that the 5-HT_1A-R is involved in the antipanic action of antidepressants, KOR antagonists may be useful as adjunctive or alternative drug treatment of panic disorder.

Endogenous opiates and behavior: 2014

This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants). This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants).