Opiate receptors modulate estrogen-induced cholecystokinin and tachykinin but not enkephalin messenger RNA levels in the limbic system and hypothalamus

Neural circuit mechanisms of the cholecystokinin (CCK) neuropeptide system in addiction

Given historical focus on the roles for cholecystokinin (CCK) as a peripheral hormone controlling gastrointestinal processes and a brainstem peptide regulating food intake, the study of CCK as a limbic neuromodulator coordinating reward-seeking and emotional behavior remains underappreciated. Furthermore, localization of CCK to specialized interneurons throughout the hippocampus and cortex relegated CCK to being examined primarily as a static cell type marker rather than a dynamic functional neuromodulator. Yet, over three decades of literature have been generated by efforts to delineate the central mechanisms of addiction-related behaviors mediated by the CCK system across the striatum, amygdala, hypothalamus, and midbrain. Here, we cover fundamental findings that implicate CCK neuron activity and CCK receptor signaling in modulating drug intake and drug-seeking (focusing on psychostimulants, opioids, and alcohol). In doing so, we highlight the few studies that indicate sex differences in CCK expression and corresponding drug effects, emphasizing the importance of examining hormonal influences and sex as a biological variable in translating basic science discoveries to effective treatments for substance use disorders in human patients. Finally, we point toward understudied subcortical sources of endogenous CCK and describe how continued neurotechnology advancements can be leveraged to modernize understanding of the neural circuit mechanisms underlying CCK release and signaling in addiction-relevant behaviors.

Estradiol Membrane-Initiated Signaling and Female Reproduction

The discoveries of rapid, membrane-initiated steroid actions and central nervous system steroidogenesis have changed our understanding of the neuroendocrinology of reproduction. Classical nuclear actions of estradiol and progesterone steroids affecting transcription are essential. However, with the discoveries of membrane-associated steroid receptors, it is becoming clear that estradiol and progesterone have neurotransmitter-like actions activating intracellular events. Ultimately, membrane-initiated actions can influence transcription. Estradiol membrane-initiated signaling (EMS) modulates female sexual receptivity and estrogen feedback regulating the luteinizing hormone (LH) surge. In the arcuate nucleus, EMS activates a lordosis-regulating circuit that extends to the medial preoptic nucleus and subsequently to the ventromedial nucleus (VMH)--the output from the limbic and hypothalamic regions. Here, we discuss how EMS leads to an active inhibition of lordosis behavior. To stimulate ovulation, EMS facilitates astrocyte synthesis of progesterone (neuroP) in the hypothalamus. Regulation of GnRH release driving the LH surge is dependent on estradiol-sensitive kisspeptin (Kiss1) expression in the rostral periventricular nucleus of the third ventricle (RP3V). NeuroP activation of the LH surge depends on Kiss1, but the specifics of signaling have not been well elucidated. RP3V Kiss1 neurons appear to integrate estradiol and progesterone information which feeds back onto GnRH neurons to stimulate the LH surge. In a second population of Kiss1 neurons, estradiol suppresses the surge but maintains tonic LH release, another critical component of the estrous cycle. Together, evidence suggests that regulation of reproduction involves membrane action of steroids, some of which are synthesized in the brain.

Substance P immunoreactivity exhibits frequent colocalization with kisspeptin and neurokinin B in the human infundibular region

Neurons synthesizing neurokinin B (NKB) and kisspeptin (KP) in the hypothalamic arcuate nucleus represent important upstream regulators of pulsatile gonadotropin-releasing hormone (GnRH) neurosecretion. In search of neuropeptides co-expressed in analogous neurons of the human infundibular nucleus (Inf), we have carried out immunohistochemical studies of the tachykinin peptide Substance P (SP) in autopsy samples from men (21-78 years) and postmenopausal (53-83 years) women. Significantly higher numbers of SP-immunoreactive (IR) neurons and darker labeling were observed in the Inf of postmenopausal women than in age-matched men. Triple-immunofluorescent studies localized SP immunoreactivity to considerable subsets of KP-IR and NKB-IR axons and perikarya in the infundibular region. In postmenopausal women, 25.1% of NKB-IR and 30.6% of KP-IR perikarya contained SP and 16.5% of all immunolabeled cell bodies were triple-labeled. Triple-, double- and single-labeled SP-IR axons innervated densely the portal capillaries of the infundibular stalk. In quadruple-labeled sections, these axons formed occasional contacts with GnRH-IR axons. Presence of SP in NKB and KP neurons increases the functional complexity of the putative pulse generator network. First, it is possible that SP modulates the effects of KP and NKB in axo-somatic and axo-dendritic afferents to GnRH neurons. Intrinsic SP may also affect the activity and/or neuropeptide release of NKB and KP neurons via autocrine/paracrine actions. In the infundibular stalk, SP may influence the KP and NKB secretory output via additional autocrine/paracrine mechanisms or regulate GnRH neurosecretion directly. Finally, possible co-release of SP with KP and NKB into the portal circulation could underlie further actions on adenohypophysial gonadotrophs.

Site-specific estrogen and progestin regulation of orphanin FQ/nociceptin and nociceptin opioid receptor mRNA expression in the female rat limbic hypothalamic system

The distributions of orphanin FQ (OFQ/N; also known as nociceptin) and its cognate receptor, opioid receptor-like receptor-1 (NOP), overlap steroid-responsive regions throughout reproductive circuits of the limbic system and hypothalamus. For example, in the ventromedial nucleus of the hypothalamus (VMH), OFQ/N facilitates lordosis in female rats through estrogen and progesterone regulation of nociceptin activity. We studied estrogen and progesterone regulation of OFQ/N and NOP mRNA expression in limbic-hypothalamic reproductive circuits. Ovariectomized rats were treated with 17beta-estradiol-benzoate (2 microg) and 26 hours later with oil or progesterone (500 microg) and were killed 30 hours after initial treatment. Alternate brain sections were processed for OFQ/N or NOP mRNA in situ hybridization. High levels of hybridization for NOP and OFQ/N and overlapping distributions were observed throughout the limbic hypothalamic reproductive circuits; however, in VMH, only NOP expression was observed. Estrogen treatment increased NOP mRNA expression in anteroventral periventricular nucleus (AVPV), median preoptic nucleus, and VMH. Subsequent progesterone treatment did not alter estrogen-induced expression of NOP mRNA in VMH or median preoptic nucleus but reduced expression in the AVPV. OFQ/N mRNA levels were also regulated by steroids. In the caudal part of the posterodorsal medial amygdala, estrogen increased OFQ/N mRNA levels, and progesterone did not alter this increase, whereas, in the medial part of the medial preoptic nucleus, estrogen and progesterone were needed to increase OFQ/N mRNA levels. Steroid regulation of OFQ/N and NOP in the medial preoptic nucleus and VMH is consistent with emerging data indicating that this opioid system regulates female reproduction.

Medial preoptic area δ-opioid receptors inhibit lordosis

Endogenous opioid peptides that activate the delta-opioid receptor (DOR) are thought to facilitate female receptive behavior. This facilitation of lordosis has been demonstrated by intracerebroventricular infusions and injection of DOR-active ligands into the ventromedial hypothalamic nucleus, an area with robust DOR binding. However, DOR binding is distributed throughout the hypothalamus, and the role of DOR in other areas of the hypothalamus has not been examined. In the current study, we demonstrated DOR immunoreactivity in the medial preoptic area (MPO), in particular medial preoptic nucleus (MPN) of the preoptic area. DOR immunoreactive processes were sparsely distributed in the medial and lateral parts of the MPN. Larger DOR immunoreactive fibers were localized in the ventrolateral aspect of the lateral MPN. The MPN is involved in the modulation of female sexual receptivity and the distribution of DOR in this area suggested to us that DOR may regulate lordosis. Ovariectomized rats with unilateral cannulae aimed at the MPN were given 5microg 17beta-estradiol benzoate (EB), once every 4 days and tested for lordosis. [D-Pen(2), D-Pen(5)]-enkephalin (DPDPE), a DOR agonist, microinfused into the MPO, 52-54h after EB-priming, inhibited lordosis when compared with the aCSF (vehicle) control (P <== 0.05). The inhibitory effects of DPDPE were reversed by microinjection of naltrindole, a DOR antagonist (P <== 0.05). Interestingly, the DOR inhibition of lordosis is similar to the micro-opioid receptor inhibition of lordosis in the MPN. These results indicate that DOR in the MPO, particularly in the MPNm, plays an important role in the regulation of lordosis.

Oestrogen modulates cholecystokinin: opioid interactions in the nervous system

Responses of the nervous system to introceptive and extroceptive inputs depend upon the state of the brain. Oestrogen has the ability to modulate brain state and dramatically alter interactions among neural circuits to influence an organism's responses to given stimuli. Cholecystokinin (CCK) and endogenous opioid peptides (EOP) have a wide and parallel distribution in the nervous system. Their reciprocal interactions regulate a diverse physiology including reproduction, cortical function and nociception. The actions of CCK and EOP are diametrically opposed, in many regions. For example, when opioids inhibit reproductive behaviour or nociception, CCK facilitates. Because oestrogen is a powerful regulator of the expression of CCK and EOP, we examined whether oestrogen-state also modulated the interactions of these neuropeptides. In this paper we present new data and review previous work that demonstrates oestrogen modulation of functional CCK-opioid interactions that regulate reproductive behaviour, cortical function and nociception.

Sex differences in delta opioid receptor immunoreactivity in rat medial amygdala

Although gonadal hormones can modulate the opioid system in several limbic structures, it remains unclear if there are sex differences in amygdalar opioid systems. This study compared enkephalin immunoreactivity, delta opioid receptor immunoreactivity (DORir), and mu opioid receptor immunoreactivity in the amygdala of male and proestrous female rats. A striking sex difference in DORir was observed in the posterodorsal region of the medial nucleus of the amygdala, with males showing much greater DORir than females. No other sex differences were seen in amygdalar regions. Although the functional significance of this sexually dimorphic staining for DORir in medial amygdala is unknown, it may contribute to sex differences in reproductive functions, social behaviors, and/or stress responses.

Progesterone blockade of estrogen activation of mu-opioid receptors regulates reproductive behavior

The mu-opioid receptor (MOR), a G-protein-coupled receptor, is internalized after endogenous agonist binding. Although receptor activation and internalization are separate events, internalization is a good assay for activation because endogenous opioid peptides all induce internalization. Estrogen treatment of ovariectomized rats induces MOR internalization, providing a neurochemical signature of estrogen activation of the medial preoptic nucleus. MOR activation appears to be the mechanism via which estrogen acts in the medial preoptic area to prevent the display of female reproductive behavior during the first 20-24 hr after estrogen treatment. Naltrexone, an alkaloid universal opioid receptor antagonist, prevented MOR internalization, suggesting that estrogen induces the release of endogenous opioid peptides that in turn activate the MOR. Enkephalins and beta-endorphin are nonselective endogenous MOR ligands. The most selective endogenous MOR ligands are the endomorphins. Infusions of selective MOR agonists, H-Tyr-d-Ala-Gly-N-Met-Phe-glycinol-enkephalin (DAMGO) or endomorphin-1, into the medial preoptic nucleus attenuated lordosis, and their effects were blocked with the MOR antagonist H-d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP). Infusion of endomorphin-1 internalized MOR. To determine whether progestin also acts via the MOR system to facilitate reproductive behavior, ovariectomized rats were primed with 17beta-estradiol and progesterone. Progestin facilitation of lordosis was correlated with a reduction of estrogen-induced MOR internalization. Progestin reversed estrogen-induced MOR internalization, suggesting that progesterone blocked estrogen-induced endogenous opioid release, relieving estrogen inhibition and facilitating lordosis. These results indicate a central role of MOR in the mediation of sex steroid activation of the CNS to regulate female reproductive behavior.

Estrogen and endogenous opioids regulate CCK in reproductive circuits

This review focuses on the interaction of estrogen with the cholecystokinin (CCK) and endogenous opioid peptide systems in the medial preoptic nucleus, and how these interactions result in alterations of a stereotypic female reproductive behavior--lordosis. The medial preoptic nucleus is an integral part of a circuit controlling lordosis that extends from the limbic system through the hypothalamus. Estrogen alters the integration of sensory information in the circuit that results in the display of sexually receptive behavior. Estrogen determines the activity of CCK and endogenous opioid peptide systems through regulation of expression, release and interaction with specific receptors. Studies of each system individually have indicated that they are pivotal to the expression of lordosis. Recent studies demonstrate an estrogen-dependent interaction between endogenous opioid and CCK systems that control reproductive behavior.

Preproenkephalin mRNA levels are regulated by acute stress and estrogen stimulation

Enkephalins facilitate female reproductive behavior. Within the limbic system and hypothalamus, estrogen induced the expression of preproenkephalin (PPE) mRNA. Estrogen injection caused a biphasic increase in the PPE mRNA levels within the ventromedial hypothalamic nucleus and posterodorsal medial amygdala. The first peak of PPE mRNA levels occurred within an hour, and the second 24 to 48 h after subcutaneous injection of estrogen. The present studies indicated that the rapid first peak of PPE mRNA expression was stress induced, whereas the second peak was estrogen induced. In the posterodorsal medial amygdala but not in the ventromedial hypothalamic nucleus, the antiestrogen, tamoxifen, did not inhibit the first peak, but blocked the second peak of PPE mRNA expression. Subcutaneous oil injection induced a 1-h peak of PPE mRNA levels but not a 24-h peak. Peak levels of plasma corticosterone were coincident with peak PPE mRNA levels. Adrenalectomy plus a constant, low level of corticosterone eliminated the injection-induced increase of corticosterone levels and the subsequent increase in PPE mRNA expression in the ventromedial hypothalamic nucleus and posterodorsal medial amygdala. The present results indicate that both stress steroids and estrogen positively regulate PPE mRNA levels in the ventromedial hypothalamic nucleus and posterodorsal medial amygdala. These results are consistent with the hypothesis that acute, mild stress may contribute to the activation of circuits that facilitate reproductive behavior in the female.

The medial extended amygdala in male reproductive behavior. A node in the mammalian social behavior network

Hormonal and chemosensory signals regulate social behaviors in a wide variety of mammals. In the male Syrian hamster, these signals are integrated in nuclei of the medial extended amygdala, where olfactory and vomeronasal system transmission is modulated by populations of androgen- and estrogen-sensitive neurons. Evidence from behavioral changes following lesions and from immediate early gene expression supports the hypothesis that the medial extended amygdala and medial preoptic area belong to a circuit that functions selectively in male sexual behavior. However, accumulated behavioral, neuroanatomical, and neuroendocrine data in hamsters, other rodents, and other mammals indicate that this circuit is embedded in a larger integrated network that controls not only male mating behavior, but female sexual behavior, parental behavior, and various forms of aggression. In this context, perhaps an individual animal's social responses can be more easily understood as a repertoire of closely interrelated, hormone-regulated behaviors, shaped by development and experience and modulated acutely by the environmental signals and the hormonal milieu of the brain.

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.

Interaction of thyroxine and estrogen on the expression of estrogen receptor alpha, cholecystokinin, and preproenkephalin messenger ribonucleic acid in the limbic-hypothalamic circuit

To study thyroid hormone and estrogen interactions in the central nervous system (CNS), the expression of estrogen sensitive genes was examined within the limbic-hypothalamic circuit. Estrogen up-regulates the expression of reproductively relevant neuropeptide messenger RNAs (mRNAs) encoding cholecystokinin (CCK) and enkephalin, peptides that stimulate lordosis. Estrogen down-regulates the expression of the estrogen receptor alpha (ER alpha) mRNA in the nuclei of the circuit. We examined the possibility that thyroid hormone treatment would block the estrogen modulation of these messages. Estradiol benzoate (EB), EB + thyroxine (T4), T4, or oil were administered to ovariectomized, adult female rats for 10 days. Isotopic in situ hybridization histochemistry revealed that within the limbic-hypothalamic nuclei, levels of CCK and preproenkephalin (PPE) mRNA levels were significantly higher in EB and EB + T4-treated animals compared with T4 or oil-treated animals. ER alpha mRNA levels were low in EB treated animals, elevated in T4 or oil-treated animals and further elevated in EB + T4-treated animals. In summary, T4 treatment had neither an independent nor an antagonistic effect on estrogen induced expression of CCK or PPE mRNA in the circuit. However, T4 did prevent the normal estrogenic decrease of ER alpha mRNA levels in the nuclei of the limbic-hypothalamic circuit.

Estrogen modulation of opioid and cholecystokinin systems in the limbic-hypothalamic circuit

The display of lordosis behavior has been correlated with the estrogen-induced expression of cholecystokinin (CCK) and enkephalin within the limbic-hypothalamic circuit. These neuropeptides have opposing effects on lordosis; for example, in the medial preoptic nucleus, CCK facilitates and opiates inhibit lordosis. Antisense oligodeoxynucleotide blockade of receptor expression indicated that CCK modulates lordosis in the medial preoptic nucleus through the CCK(A)-receptor. Sequence-specific antibodies directed against delta- and mu-opiate receptor proteins labeled fibers in the medial preoptic nucleus. Estrogen treatment of ovariectomized rats or etorphine (a nonselective opiate agonist) treatment altered the appearance of the immunoreactivity from a diffuse pattern to one of distinctly stained mu-opiate receptor immunoreactive cells and varicose fibers in the medial preoptic nucleus. Such a pattern of staining reflects an internalization of mu-opiate receptors following agonist stimulation. This type of internalization has been used as an indication of synaptic activity. The distribution of receptor internalization surrounds the distribution of CCK cells in the medial preoptic nucleus, suggesting that endogenous opioid peptides may modulate estrogen-induced CCK mRNA expression. Interestingly, nonselective and delta-opiate receptor selective antagonists potentiated the estrogen-induced CCK mRNA expression in the medial preoptic nucleus. Together, these results suggest that endogenous opioid peptides may modulate the estrogenic upregulation of CCK mRNA expression and demonstrate an important level of regulation of gene expression in which synaptic activity modifies hormonal input.