Serotonergic lesions decrease mu- and delta-opiate receptor binding in discrete areas of the hypothalamus and in the midbrain central gray

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

A wealth of evidence indicates that the activation of 5-HT1A and 5-HT2A receptors in the dorsal periaqueductal grey matter (dPAG) inhibits escape, a panic-related defensive behaviour. Results that were previously obtained with the elevated T-maze test of anxiety/panic suggest that 5-HT1A and μ-opioid receptors in this midbrain area work together to regulate this response. To investigate the generality of this finding, we assessed whether the same cooperative mechanism is engaged when escape is evoked by a different aversive stimulus electrical stimulation of the dPAG. Administration of the μ-receptor blocker CTOP into the dPAG did not change the escape threshold, but microinjection of the μ-receptor agonist DAMGO (0.3 and 0.5 nmol) or the 5-HT1A receptor agonist 8-OHDPAT (1.6 nmol) increased this index, indicating a panicolytic-like effect. Pretreatment with CTOP antagonised the anti-escape effect of 8-OHDPAT. Additionally, combined administration of subeffective doses of DAMGO and 8-OHDPAT increased the escape threshold, indicating drug synergism. Therefore, regardless of the aversive nature of the stimulus, μ-opioid and 5-HT1A receptors cooperatively act to regulate escape behaviour. A better comprehension of this mechanism might allow for new therapeutic strategies for panic disorder.

Cooperative regulation of anxiety and panic-related defensive behaviors in the rat periaqueductal grey matter by 5-HT1A and μ-receptors

Previous results with the elevated T-maze (ETM) test indicate that the antipanic action of serotonin (5-HT) in the dorsal periaqueductal grey (dPAG) depends on the activation endogenous opioid peptides. The aim of the present work was to investigate the interaction between opioid- and serotonin-mediated neurotransmission in the modulation of defensive responses in rats submitted to the ETM. The obtained results showed that intra-dPAG administration of morphine significantly increased escape latency, a panicolytic-like effect that was blocked by pre-treatment with intra-dPAG injection of either naloxone or the 5-HT1A antagonist N-[2-[4-(2-methoxyphenyl)-1 piperazinyl] ethyl] -N- 2- pyridinyl-ciclohexanecarboxamide maleate (WAY-100635). In addition, previous administration of naloxone antagonized both the anti-escape and the anti-avoidance (anxiolytic-like) effect of the 5-HT1A agonist (±)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT), but did not affect the anti-escape effect of the 5-HT2A agonist (±)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI). Moreover, the combination of sub-effective doses of locally administered 5-HT and morphine significantly impaired ETM escape performance. Finally, the µ-antagonist D-PHE-CYS-TYR-D-TRP-ORN-THR-PEN (CTOP) blocked the anti-avoidance as well as the anti-escape effect of 8-OHDPAT, and the association of sub-effective doses of the µ-opioid receptor agonist [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin acetate salt (DAMGO) and of 8-OHDPAT had anti-escape and anti-avoidance effects in the ETM. These results suggest a synergic interaction between the 5-HT1A and the µ-opioid receptor at post-synaptic level on neurons of the dPAG that regulate proximal defense, theoretically related to panic attacks.

Vulnerability to stressor-induced disturbances in self-stimulation from the dorsal and ventral A10 area: differential effects of intraventricular D-Ala2-Met5-enkephalinamide, D-Ala2, N-Me-Phe4, Gly-Ol5-enkephalin, and D-Pen2, D-Pen5-enkephalin administration

D-Ala2-Met5-enkephalinamide (DALA) (1.0 microg/microl) was administered intraventricularly to mice responding for electrical stimulation from the dorsal or ventral aspects of the VTA immediately prior to footshock (Experiment 1). Predictably, footshock reduced self-stimulation from the dorsal but not the ventral VTA immediately, 24, and 168 h following the stressor. Intraventricular DALA administration effected a partial attenuation of stressor-induced self-stimulation reductions from the dorsal VTA immediately and 24 h poststressor. Deficits appeared among DALA-Shocked mice responding for brain stimulation from the ventral VTA during comparable test intervals. The long-term depressant influence of footshock on self-stimulation from the dorsal VTA was abolished among DALA-treated mice and DALA-associated reductions in self-stimulation from the ventral A10 region among stressed mice were not evident 1 week later. Administration of D-Ala2, N-Me-Phe4, Gly-Ol5-enkephalin (DAGO) (0.01 microg/microl) or D-Pen2, D-Pen5-enkephalin (DPDPE) (1.0 microg/microl) intraventricularly prior to footshock effected an immediate and a delayed antagonism, respectively, of the stressor on self-stimulation from the dorsal VTA, which persisted for 1 week. Prophylactic administration of 0.001 microg/microl DAGO or 0.01 microg/microl DPDPE prior to the stressor failed to influence self-stimulation from the ventral VTA (Experiment 2). Administration of 0.01 microg/microl DAGO or 1.0 microg/microl DPDPE among mice responding for brain stimulation from the dorsal VTA following footshock produced a weak therapeutic effect immediately poststressor, but effected protracted amelioration of footshock-induced reductions of self-stimulation from the dorsal VTA (Experiment 3). Taken together, mu, delta, and mu-delta activation influenced self-stimulation differentially from the dorsal and ventral VTA according to the temporal order of opioid peptide challenge relative to stressor imposition. These data are discussed with respect to stressors, motivational alterations, and the putative modulating influence of endogenous enkephalin activity in subareas of the VTA.

Estrogen-induced alteration of mu-opioid receptor immunoreactivity in the medial preoptic nucleus and medial amygdala

The mu-opioid receptor (mu-OR), like most G-protein-coupled receptors, is rapidly internalized after agonist binding. Although opioid peptides induce internalization in vivo, there are no studies that demonstrate mu-OR internalization in response to natural stimuli. In this study, we used laser-scanning microscopy to demonstrate that estrogen treatment induces the translocation of mu-OR immunoreactivity (mu-ORi) from the membrane to an internal location in steroid-sensitive cell groups of the limbic system and hypothalamus. Estrogen-induced internalization was prevented by the opioid antagonist naltrexone, suggesting that translocation was largely dependent on release of endogenous agonists. Estrogen treatment also altered the pattern of mu-ORi at the bright-field light microscopic level. In the absence of stimulation, the majority of immunoreactivity is diffuse, with few definable mu-OR+ cell bodies or processes. After stimulation, the density of distinct processes filled with mu-ORi was significantly increased. We interpreted the increase in the number of mu-OR+ processes as indicating increased levels of internalization. Using this increase in the density of mu-OR+ fibers, we showed that treatment of ovariectomized rats with estradiol benzoate induced a rapid and reversible increase in the number of fibers. Significant internalization was noted within 30 min and lasted for >24 hr after estrogen treatment in the medial preoptic nucleus, the principal part of the bed nucleus, and the posterodorsal medial amygdala. Naltrexone prevented the increase of mu-OR+ processes. These data imply that estrogen treatment stimulates the release of endogenous opioids that activate mu-OR in the limbic system and hypothalamus providing a "neurochemical signature" of steroid activation of these circuits.

Neuropeptide FF receptors control morphine-induced analgesia in the parafascicular nucleus and the dorsal raphe nucleus

The ability of (1DMe)Y8Fa (D.Tyr-Leu-(NMe)Phe-Gln-Pro-Gln-Arg-Phe-NH2), a selective neuropeptide FF analog resistant to enzymatic degradation, to control morphine-induced analgesia was investigated in rat after microinfusion into the dorsal raphe nucleus and the nucleus parafascicularis of the thalamus. Infusion of (1DMe)Y8Fa (2.5 nmol) in the nucleus raphe dorsalis did not modify the animal response in the tail-immersion test but significantly reversed analgesia induced by coinjected morphine (27 nmol). Similarly, (1DMe)Y8Fa (5 nmol) inhibited morphine effects in the hot-plate test after co-injection into the parafascicular nucleus. Furthermore, (1DMe)Y8Fa injected into the parafascicular nucleus attenuated analgesia induced by morphine injected into the nucleus raphe dorsalis and similarly, the neuropeptide FF analog in the nucleus raphe dorsalis decreased the effects of 27 nmol morphine injected in the parafascicular nucleus. The density of neuropeptide FF receptors did not decrease in the nucleus raphe dorsalis after lesion of serotonergic neurons by 5,7-dihydroxytryptamine. However, after this lesion, (1DMe)Y8Fa injected in the nucleus raphe dorsalis was no longer able to modify analgesic effects of morphine in hot-plate and tail-immersion tests. Similarly, the serotonin (5-HT) depletion induced by a systemic administration of para-chlorophenylalanine did not modify morphine analgesia microinjected into the nucleus raphe dorsalis and the parafascicular nucleus but blocked the ability of (1DMe)Y8Fa to reverse morphine effects in both nuclei. These data show that neuropeptide FF exerts anti-opioid effects directly into both the nucleus raphe dorsalis and the parafascicular nucleus and acts also at distance on opioid functions. Furthermore, anti-opioid effects of neuropeptide FF require functional serotonergic neurons although neuropeptide FF receptors are not carried on these neurons.

Postsynaptic and extrasynaptic localization of kappa-opioid receptor in selected brain areas of young rat and chick using an anti-receptor monoclonal antibody

kappa-opioid receptors were visualized by light and electron microscopical immunohistochemistry in young rat and chick brains, using a monoclonal antibody KA8 (IgG1, kappa) raised against a kappa-opioid receptor preparation from frog brain, which recognizes selectively the kappa-type receptor with preference for the kappa-2 subtype. The most pronounced kappa-opioid receptor-like immunoreactivity was observed in the hypothalamic nuclei of the rat brain and in the chick optic tectum, in regions where the functional significance of kappa-opioid receptors is well documented. Both neurons and glia were stained, the former on both somata and dendrites. At the ultrastructural level, the receptor-like immunoreactivity was similar in both species. Immunoprecipitate decorated the inner surface of the plasma membrane of glial cells, neuronal somata and dendrites, in a discontinuous arrangement. In the cytoplasm, labelling was associated with ribosomes, polyribosomes and rough endoplasmic reticulum membranes but not with Golgi cisternae. In the neuropil, the immunoprecipitate was observed along the dendritic microtubules and was also associated with postsynaptic sites. Nuclei and axons were devoid of label and immunoreactivity was never visible presynaptically. Our findings indicate that the antibody used in the present study marks various forms of the kappa-opioid receptor protein including those synthesised in ribosomes, transported along dendritic microtubules and incorporated into postsynaptic and non-synaptic membranes. The antibody also recognizes glial opioid receptors. The observed subcellular distribution appears to be conserved in phylogenetically distant species.