Are the cannabinoids involved in bony fish reproduction?

Acute administration of a cannabinoid CB1 receptor antagonist impairs stress-induced antinociception in fish

This study evaluated the influence of the pre-treatment with AM251 (a cannabinoid type I receptor (CB1) selective antagonist) on the stress-induced antinociception promoted by restraint in the fish Leporinus macrocephalus. The application of 3 and 5 min of restraint stress promoted an inhibition of the behavioural response to the subcutaneous injection of 3% formaldehyde (increase in locomotor activity), suggesting the activation of an antinociceptive system. The acute intraperitoneal administration of AM251 (3 mg·kg(-1)) impaired this antinociceptive response induced by 3 and 5 min of restraint stress. The fish treated with AM251 before the application of restraint stress presented an increase in locomotor activity after the subcutaneous injection of formaldehyde, similar to fish not exposed to restraint, suggesting that the stress-induced antinociception promoted by restraint in fish is probably mediated by cannabinoid CB1 receptors. The results presented in this paper suggest the participation of the endocannabinoid system in nociception modulation in fish, supporting the hypothesis that an endogenous antinociceptive system activated by restraint stress is present in fish and that the modulation of antinociception by the CB1 receptor is evolutionary well-conserved across vertebrates.

Endocannabinoids are Involved in Male Vertebrate Reproduction: Regulatory Mechanisms at Central and Gonadal Level

Endocannabinoids (eCBs) are natural lipids regulating a large array of physiological functions and behaviors in vertebrates. The eCB system is highly conserved in evolution and comprises several specific receptors (type-1 and type-2 cannabinoid receptors), their endogenous ligands (e.g., anandamide and 2-arachidonoylglycerol), and a number of biosynthetic and degradative enzymes. In the last few years, eCBs have been described as critical signals in the control of male and female reproduction at multiple levels: centrally, by targeting hypothalamic gonadotropin-releasing-hormone-secreting neurons and pituitary, and locally, with direct effects on the gonads. These functions are supported by the extensive localization of cannabinoid receptors and eCB metabolic enzymes at different levels of the hypothalamic-pituitary-gonadal axis in mammals, as well as bonyfish and amphibians. In vivo and in vitro studies indicate that eCBs centrally regulate gonadal functions by modulating the gonadotropin-releasing hormone-gonadotropin-steroid network through direct and indirect mechanisms. Several proofs of local eCB regulation have been found in the testis and male genital tracts, since eCBs control Sertoli and Leydig cells activity, germ cell progression, as well as the acquisition of sperm functions. A comparative approach usually is a key step in the study of physiological events leading to the building of a general model. Thus, in this review, we summarize the action of eCBs at different levels of the male reproductive axis, with special emphasis, where appropriate, on data from non-mammalian vertebrates.

Cannabinoid receptors are widely expressed in goldfish: molecular cloning of a CB2-like receptor and evaluation of CB1 and CB2 mRNA expression profiles in different organs

Cannabinoids, the bioactive constituents of Cannabis sativa, and endocannabinoids, among which the most important are anandamide and 2-arachidonoylglycerol, control various biological processes by binding to specific G protein-coupled receptors, namely CB1 and CB2 cannabinoid receptors. While a vast amount of information on the mammalian endocannabinoid system does exist, few data have been reported on bony fish. In the goldfish, Carassius auratus, the CB1 receptor has been cloned and its distribution has been analyzed in the retina, brain and gonads, while CB2 had not yet been isolated. In the present paper, we cloned the goldfish CB2 receptor and show that it presents a quite high degree of amino acid identity with zebrafish Danio rerio CB2A and CB2B receptors, while the percentage of identity is lower with the puffer fish Fugu rubripes CB2, as also confirmed by the phylogenetic analysis. The sequence identity becomes much lower when comparing the goldfish and the mammalian CB2 sequences; as for other species, goldfish CB2 and CB1 amino acid sequences share moderate levels of identity. Western-blotting analysis shows the CB2 receptor as two major bands of about 53 and 40 kDa and other faint bands with apparent molecular masses around 70, 57 and 55 kDa. Since the distribution of a receptor could give information on its physiological role, we evaluated and compared CB1 and CB2 mRNA expression in different goldfish organs by means of qReal-Time PCR. Our results show that both CB1 and CB2 receptors are widely expressed in the goldfish, displaying some tissue specificities, thus opening the way for further functional studies on bony fish and other nonmammalian vertebrates.

Role of the endocannabinoid system in the central regulation of nonmammalian vertebrate reproduction

The endocannabinoid system (ECS) has a well-documented pivotal role in the control of mammalian reproductive functions, by acting at multiple levels, that is, central (CNS) and local (gonads) levels. Since studies performed in animal models other than mammals might provide further insight into the biology of these signalling molecules, in the present paper we review the comparative data pointing toward the endocannabinoid involvement in the reproductive control of non-mammalian vertebrates, focussing in particular on the central regulation of teleost and amphibian reproduction. The morphofunctional distribution of brain cannabinoid receptors will be discussed in relation to other crucial signalling molecules involved in the control of reproductive functions, such as GnRH, dopamine, aromatase, and pituitary gonadotropins.

The evolution and comparative neurobiology of endocannabinoid signalling

CB(1)- and CB(2)-type cannabinoid receptors mediate effects of the endocannabinoids 2-arachidonoylglycerol (2-AG) and anandamide in mammals. In canonical endocannabinoid-mediated synaptic plasticity, 2-AG is generated postsynaptically by diacylglycerol lipase alpha and acts via presynaptic CB(1)-type cannabinoid receptors to inhibit neurotransmitter release. Electrophysiological studies on lampreys indicate that this retrograde signalling mechanism occurs throughout the vertebrates, whereas system-level studies point to conserved roles for endocannabinoid signalling in neural mechanisms of learning and control of locomotor activity and feeding. CB(1)/CB(2)-type receptors originated in a common ancestor of extant chordates, and in the sea squirt Ciona intestinalis a CB(1)/CB(2)-type receptor is targeted to axons, indicative of an ancient role for cannabinoid receptors as axonal regulators of neuronal signalling. Although CB(1)/CB(2)-type receptors are unique to chordates, enzymes involved in biosynthesis/inactivation of endocannabinoids occur throughout the animal kingdom. Accordingly, non-CB(1)/CB(2)-mediated mechanisms of endocannabinoid signalling have been postulated. For example, there is evidence that 2-AG mediates retrograde signalling at synapses in the nervous system of the leech Hirudo medicinalis by activating presynaptic transient receptor potential vanilloid-type ion channels. Thus, postsynaptic synthesis of 2-AG or anandamide may be a phylogenetically widespread phenomenon, and a variety of proteins may have evolved as presynaptic (or postsynaptic) receptors for endocannabinoids.

Xenoestrogens elicit a modulation of endocannabinoid system and estrogen receptors in 4NP treated goldfish, Carassius auratus

Based on pharmacological, behavioral and neuroanatomical studies, the endocannabinoids appear to be pivotal in some important neuroendocrine regulations of both vertebrates and invertebrates. Interestingly, a well developed endocannabinoid system was recently demonstrated by us in different bonyfish brain areas which control reproduction, energy balance and stress. Fish in particular are very sensitive to different types of stressors which can heavily affect their reproductive activity and negatively reverberate on aquaculture. Since recent new data have been reported on endocrine disruptors (EDs) impact on zebrafish receptor CB1 expression, in the present research we have investigated the response of the endocannabinoid system to acute treatment with an environmental stressor such as the xenoestrogen nonylphenol (4NP) in the brain and peripheral tissues of the goldfish Carassius auratus. First of all the estrogenic effects induced by 4NP were demonstrated by a dose-dependent increase of plasma levels and gene expression of the biomarker vitellogenin, then changes in cannabinoid receptors and anandamide degradative enzyme, the fatty acid amide hydrolase (FAAH), were analysed by means of Real Time PCR. As the exposure to EDs may lead to an activation of estrogen receptors and affects the Aromatase (AROB) transcription, changes in mRNA levels for ER subtypes and AROB were also evaluated. Our results confirm in goldfish the effect of 4NP on ERα and ERβ1 receptors and point out a different sensitivity of CB1 and CB2 for this compound, suggesting distinct roles of these cannabinoid receptors in some adaptive processes to contrast stress induced by xenoestrogen exposure.

A novel role for the endocannabinoid system during zebrafish development

The aim of this study was to increase our understanding of the physiological functions controlled by the endocannabinoid system during embryogenesis. Using genomic and proteomic methodologies applied to zebrafish, we proved, for the first time in an oviparous species, that the cannabinoid receptor CB1 is not a maternal factor. The analysis of different developmental stages showed that the zygotic expression of CB1 occurs from the 3 somites stage while CB1 protein becomes evident during hatching time, indicating an involvement in the hatching process. This result was supported by the data regarding embryo exposure to the CB1 antagonist, AM251, consisting in a 75% decrease in hatching rate. In addition, as previously described for mammals, we observed a role of CB1 in the motility behavior in zebrafish larvae.

Partial cloning of CB1 cDNA and CB1 mRNA changes in stress responses in the Solea solea

Endogenous cannabinoids, through the CB1 receptor, are involved in the control of several functions including stress responses. The aim of this study was to investigate the presence of cannabinoid receptor CB1 in the sole ovary by partial cloning of brain CB1 cDNA; in a stress paradigm of disturbance by handling, which consisted in catching, netting and hand-sorting, changes of CB1 mRNA were related with those of proopiomelanocortin (POMC) mRNA; the trend and timing of stress responses and adaptation were monitored by measuring plasma cortisol levels. We characterized two forms of CB1-like receptor, termed CB1A and CB1B. The two sole CB1 (both 799bp) share 76% identity in their cDNAs, and the deduced amino acid sequences are 80% identical. The handling stress induced a sustained increase in plasma cortisol levels 1h after the handling began and decreased to low levels 12h after initiation of handling, showing the same trend of ovarian POMC mRNA expression. In addition, while CB1A mRNA did not show any significant changes during handling stress, significantly lower levels of CB1B mRNA were found in stressed fish 1h after the beginning of handling, with CB1 expression increased 24h after stress induction, both in the ovary and brain. It can be concluded that endocannabinoid system is involved in the modulation of adaptive responses to environmental conditions.

Endocannabinoids affect the reproductive functions in teleosts and amphibians

Following the discovery in the brain of the bonyfish Fugu rubripes of two genes encoding for type 1 cannabinoid receptors (CB1A and CB1B), investigations on the phylogeny of these receptors have indicated that the cannabinergic system is highly conserved. Among the multiple functions modulated by cannabinoids/endocannabinoids through the CB1 receptors one of the more investigated is the mammalian reproduction. Therefore, since studies performed in animal models other than mammals might provide further insight into the biology of these signalling molecules, the major aim of the present paper was to review the comparative data pointing toward the endocannabinoid involvement in the reproductive control of non-mammalian vertebrates, namely bonyfish and amphibians. The expression and distribution of CB1 receptors were investigated in the CNS and gonads of two teleosts, Pelvicachromis pulcher and Carassius auratus as well as in the anuran amphibians Xenopus laevis and Rana esculenta. In general the large diffusion of neurons targeted by cannabinoids in both fish and amphibian forebrain indicate endocannabinoids as pivotal local messengers in several neural circuits involved in either sensory integrative activities, like the olfactory processes (in amphibians) and food response (in bonyfish), or neuroendocrine machinery (in both). By using immunohistochemistry for CB1 and GnRH-I, the codistribution of the two signalling molecules was found in the fish basal telencephalon and preoptic area, which are key centers for gonadotropic regulation in all vertebrates. A similar topographical codistribution was observed also in the septum of the telencephalon in Rana esculenta and Xenopus laevis. Interestingly, the double standard immunofluorescence on the same brain section, aided with a laser confocal microscope, showed that in anurans a subset of GnRH-I neurons exhibited also the CB1 immunostaining. The fact that CB1-LI-IR was found indeed in the FSH gonadotrophs of the Xenopus pituitary gland and CB1 receptors together with the fatty acid amide hydrolase, the degradative enzyme of the endocannabinoid anandamide, were demonstrated in both bonyfish and frog gonads, strongly suggests that endocannabinoids are involved in central and peripheral gonadotropic functions of teleosts and amphibians.

Potentiation of electrical and chemical synaptic transmission mediated by endocannabinoids

Endocannabinoids are well established as inhibitors of chemical synaptic transmission via presynaptic activation of the cannabinoid type 1 receptor (CB1R). Contrasting this notion, we show that dendritic release of endocannabinoids mediates potentiation of synaptic transmission at mixed (electrical and chemical) synaptic contacts on the goldfish Mauthner cell. Remarkably, the observed enhancement was not restricted to the glutamatergic component of the synaptic response but also included a parallel increase in electrical transmission. This effect involved the activation of CB1 receptors and was indirectly mediated via the release of dopamine from nearby varicosities, which in turn led to potentiation of the synaptic response via a cAMP-dependent protein kinase-mediated postsynaptic mechanism. Thus, endocannabinoid release can potentiate synaptic transmission, and its functional roles include the regulation of gap junction-mediated electrical synapses. Similar interactions between endocannabinoid and dopaminergic systems may be widespread and potentially relevant for the motor and rewarding effects of cannabis derivatives.

Changes of gonadal CB1 cannabinoid receptor mRNA in the gilthead seabream, Sparus aurata, during sex reversal

Two cannabinoid receptor-like genes (CB1-like), named CB1A and CB1B, have been isolated in teleost fish, specifically in the puffer fish, Fugu rubripes. However, information on the physiological roles, such as the control of reproduction and development in fish is still scarce. Therefore, the aim of the present study was to investigate the presence of CB1-like mRNA in the gonads of a marine teleost species, the gilthead seabream, Sparus aurata, a hermaphrodite species in which the gonadal tissues first develop as testes, and then as functional ovary. We isolated an 890 bp fragment (GenBank accession number ); that corresponded to the open reading frame of the teleost CB1 receptor gene, encoding for the central portion of the protein, which was aligned with the other bony fish sequence. Using "in situ" hybridization, CB1-like mRNA was localized in both mature and sex-reversing gonads, and relative changes in CB1-like expression levels were detected through semi-quantitative RT-PCR. In the mature testis and in the testicular part of the sex-reversing gonad, CB1 expression levels were found to be much higher compared to the ovarian portion. This suggests that the CB1 signaling is likely involved in the process of testicular regression of the S. aurata, but its actual role has yet to be determined.

Endocannabinoid system in Xenopus laevis development: CB1 receptor dynamics

This study investigates for the first time the dynamics of endocannabinoid system appearance during low vertebrate Xenopus laevis development. We observed that the CB1 gene started to be expressed during the organogenesis period (+/- 1 dpf, st. 28) and expression persisted throughout the three further stages analyzed. Attention was focused on the localization of the CB1 messenger that was found both at the central level (in romboencephalon and in olfactory placods) and at the peripheral level (in the gastrointestinal tract) at +/- 3 dpf (st. 41), +/- 4 dpf (st. 46) and +/- 12 dpf (st. 49). We also considered the synthesis of CB1 protein that occurred from st. 41 onwards and, from this stage, we tested the receptor functionality in response to anandamide using cytosensor microphysiometry. CB1 functionality increased with development at both central and peripheral level. These data provide sufficient evidence to encourage further analysis on endocannabinoid physiological roles during embryonic and larval X. laevis growth.