Frank L. Moore

Stress-induced inhibition of sexual behavior: Corticosterone inhibits courtship behaviors of a male amphibian (Taricha granulosa)

When male rough-skinned newts (Taricha granulosa) are exposed to presumptive stressors, the incidence of courtship decreases and plasma corticosterone concentration increases. When sexually active males are injected intraperitoneally with corticosterone (1, 5, 10, 15, 20, or 25 micrograms), the incidence of courtship decreases rapidly and in proportion to the dose of corticosterone. Intracerebroventricular infusion of synthetic corticotropin-releasing factor (CRF) elevates plasma corticosterone levels and suppresses courtship. When male newts receive an injection of metyrapone, a drug that interferes with corticosterone synthesis, the inhibitory effects of stress or CRF infusion on courtship are reduced. These results support the hypothesis that, in this amphibian, elevated levels of corticosterone associated with exposure to stressful stimuli inhibit sexual behaviors.

Arginine vasotocin induces sexual behavior of newts by acting on cells in the brain

To investigate whether arginine vasotocin (AVT) acts on target cells in the brain of Taricha granulosa (a urodele amphibian), the behavioral effects of intracerebroventricular (ICV) and intraperitoneal (IP) injections of AVT were compared. Male newts exhibited the greatest sexual activity (amplectic clasping) following an ICV injection of 0.1 microgram AVT. Another study showed that nanogram quantities of AVT, administered ICV, stimulated the behavior. An ICV injection of an antagonist to arginine vasopressin, d(CH2)5Tyr(Me)AVP, or an anti-AVT immune serum significantly inhibited the sexual behavior. Intracranial implants of 17 beta-estradiol (E2) or 5 alpha-dihydrotestosterone (DHT) in castrated males maintained the behavioral response to an injection of AVT. Another study found that an IP injection of DHT or E2 did not increase the incidence of newt sexual behavior during the 8 hours following the injection.

Historical perspective: Hormonal regulation of behaviors in amphibians

This review focuses on research into the hormonal control of behaviors in amphibians that was conducted prior to the 21st century. Most advances in this field come from studies of a limited number of species and investigations into the hormonal mechanisms that regulate reproductive behaviors in male frogs and salamanders. From this earlier research, we highlight five main generalizations or conclusions. (1) Based on studies of vocalization behaviors in anurans, testicular androgens induce developmental changes in cartilage and muscles fibers in the larynx and thereby masculinize peripheral structures that influence the properties of advertisement calls by males. (2) Gonadal steroid hormones act to enhance reproductive behaviors in adult amphibians, but causal relationships are not as well established in amphibians as in birds and mammals. Research into the relationships between testicular androgens and male behaviors, mainly using castration/steroid treatment studies, generally supports the conclusion that androgens are necessary but not sufficient to enhance male behaviors. (3) Prolactin acts synergistically with androgens and induces reproductive development, sexual behaviors, and pheromone production. This interaction between prolactin and gonadal steroids helps to explain why androgens alone sometimes fail to stimulate amphibian behaviors. (4) Vasotocin also plays an important role and enhances specific types of behaviors in amphibians (frog calling, receptivity in female frogs, amplectic clasping in newts, and non-clasping courtship behaviors). Gonadal steroids typically act to maintain behavioral responses to vasotocin. Vasotocin modulates behavioral responses, at least in part, by acting within the brain on sensory pathways that detect sexual stimuli and on motor pathways that control behavioral responses. (5) Corticosterone acts as a potent and rapid suppressor of reproductive behaviors during periods of acute stress. These rapid stress-induced changes in behaviors use non-genomic mechanisms and membrane-associated corticosterone receptors.

Steroid Hormones Use Non-Genomic Mechanisms to Control Brain Functions and Behaviors: A Review of Evidence

Progestins, estrogens, androgens, and corticosteroids are capable of modifying brain functions and behaviors by mechanisms that involve the classic genomic model for steroid action. However, experimental evidence indicates that some responses to steroid hormones use non-classical, non-genomic mechanisms. This paper reviews the evidence that steroids can bind to receptors in the plasma membrane, activate cell signaling pathways, and regulate responses on a time scale of seconds or a few minutes. The existence of these alternative regulatory pathways for steroid hormones should make endocrinologists and neurobiologists change how they think about steroid hormones. It is no longer valid to assume that minute-to-minute changes in steroid concentrations are not regulating biologically important, short-term responses, or that the only steroids with biological functions are the ones that bind with high affinity to intracellular steroid receptors.

Stress-induced inhibition of reproduction: Evidence of suppressed secretion of LH-RH in an amphibian

Male rough-skinned newts (Taricha granulosa) were used to investigate the hormonal responses associated with stress-induced inhibition of reproduction. When male newts were confined for 1 hr, using a procedure that previously had elicited physiological stress responses, androgen concentrations decreased in the plasma and immunoreactive (ir) LH-RH concentrations increased in the infundibulum and rostral hypothalamus. Likewise, when male newts were injected with 25 micrograms of corticosterone, androgen concentrations decreased and hypothalamic irLH-RH concentrations increased. These data, which are from experiments in February, support the hypothesis that in this amphibian, exposure to acute stress or to exogenous corticosterone can suppress plasma androgen titers by inhibiting the release of LH-RH from the hypothalamus. The effects of the confinement procedure and the injection of corticosterone on the concentrations of irLH-RH and androgens were different for newts in September than for newts in February.

The Nervus terminalis in Amphibians: Anatomy, Chemistry and Relationship with the Hypothalamic Gonadotropin-Releasing Hormone System

The nervus terminalis (TN), a component of the olfactory system, is found in most vertebrates. The TN of some fishes and mammals contains neurons immunoreactive (ir) to gonadotropin-releasing hormone (LHRH), and to several other neuropeptides and neurotransmitter systems, but there is little information on TN chemistry in other vertebrate taxa. Using immunocytochemical techniques, we found LHRH-ir neurons in amphibian TNs. In anurans, but not in a urodele, the TN was also found to contain Phe-Met-Arg-Phe-NH2 (FMRFamide) immunoreactivity. LHRH-ir neurons of the TN and those of the septal-hypothalamic system are morphologically homogeneous and form a distinct anatomical continuum in amphibians. Based upon topographical and cytological criteria, we hypothesize that LHRH-ir systems in vertebrates might derive embryonically from the TN.

Comparative neuroanatomy of vasotocin and vasopressin in amphibians and other vertebrates.

This review focuses on the neuroanatomical distribution of vasotocin (VT) and vasopressin (VP) and presents a comparative analysis of brain areas in which VT and VP cell bodies have been reported in fish, amphibians, reptiles, birds and mammals. A comparison of information from previous neuroanatomical studies of VT and VP with findings from a recent study of VT in an amphibian (Taricha granulosa) supports the conclusions that the VT/VP system can be subdivided into identifiable groups of cell bodies, based on neuroanatomical and cell morphology characteristics, and that these cell groups are not necessarily delimited by classical neuroanatomical boundaries. The comparative neuroanatomy of the distribution of VT and VP cell bodies also indicates that the neuroanatomy of the VT/VP system is fairly conserved among vertebrates. The review uses comparative data to present a series of tentative hypotheses about the homology of the VT cell groups and VP cell groups in the different vertebrate taxa.

Multiple forms of gonadotropin-releasing hormone in amphibian brains.

Several forms of gonadotropin-releasing hormone (GnRH)-like molecules were found in brains of both anurans (frogs) and urodeles (salamanders). The presence of the mammalian-like GnRH molecule was confirmed by HPLC and cross-reactivity studies. Small amounts of salmonid-like GnRH molecules in the brains of frogs (Rana pipiens, Hyla regilla) and salamanders (Taricha granulosa, Ambystoma gracile) were detected by comparing the HPLC chromatographic pattern and immunological reactivity of the brain extracts with native trout and synthetic salmon GnRH. This nonmammalian form of GnRH in the amphibian brain is similar and perhaps identical, at least by indirect evidence, to a form of GnRH reported earlier to be in sympathetic ganglion, retina, chromaffin tissue, and tadpole brain. If two of the amphibian GnRH molecules prove to be mammalian and salmon GnRH, then it is likely that two separate genes in amphibians code for the distinct primary structures of the molecules. The most parsimonious interpretation of the presence of both mammalian- and salmon-like GnRH in anurans and urodeles is that a common phylogenetic ancestor also possessed the two forms of GnRH. Thus the mammalian form of GnRH may well have been present in labyrinthodont amphibians. Independent of evolutionary origin, the functions of the different GnRH molecules in amphibians are unknown.

Endocrine control of amphibian sexual behavior: evidence for a neurohormone-androgen interaction.

Abstract The incidence of sexual behavior increased after an injection of arginine-8 vasotocin or arginine-8 vasopressin into intact male newts (Taricha granulosa). Administration of arginine vasopressin to males that were castrated 35 days earlier enhanced sexual behaviors in only those males implanted with androgen.

Evolutionary Precedents for Behavioral Actions of Oxytocin and Vasopressina

It is clear that the behavioral actions of oxytocin and vasopressin in mammals are not newly acquired, but have evolutionary antecedents. Injection studies with fish, amphibians, reptiles, and birds indicate that AVT can activate certain reproductive behaviors. The strongest evidence that AVT acts centrally to control reproductive behaviors comes from research on T. granulosa. In this amphibian, injections of AVT agonists activate courtship behaviors (amplectic clasping) in males and egg-laying behaviors in females, whereas injections of AVT antagonists inhibit the behaviors. Also, in Taricha males, AVT concentrations in specific brain areas are associated with the expression of courtship behaviors. Several conclusions about steroid-peptide interactions can be drawn, based on research with this amphibian. First, gonadal steroid hormones act to maintain the behavioral actions of AVT in both males and females. In Taricha, gonadectomy abolishes and steroid implants restore AVT-induced courtship in males and egg-laying in females. Second, gonadal steroids maintain the behavioral actions of AVT, in part, by modulating AVT receptor numbers on target neurons. In Taricha males and females, gonadectomy reduces AVT receptor concentrations (but not binding affinity) in certain brain areas (amygdala pars lateralis) and not others. Third, the type of gonadal steroid determines whether AVT elicits male-like or female-like reproductive behaviors. Ovariectomized Taricha females respond to AVT injections with egg-laying behaviors when implanted with estradiol and with male-like amplectic clasping when implanted with dihydrotestosterone. Fourth, the masculinization of AVT-induced behaviors in females most likely reflects site-specific actions of androgens on AVT-synthesizing neurons. In Taricha, AVTir concentrations in the optic tectum are sexually dimorphic (higher in males than females) and reach peak levels in males during the breeding season. Fifth, AVT content in specific brain areas increase as a function of performing the behaviors. In Taricha, AVTir concentrations in DPOA, CSF, and ventral infundibulum are higher in males that exhibit courtship behaviors than in males that do not. These conclusions illustrate how steroid-peptide interactions in the control of behaviors entail multiple neuroanatomical sites and neurochemical actions.