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Adult Neural Plasticity in Naked Mole-Rats: Implications of Fossoriality, Longevity and Sociality on the Brain's Capacity for Change. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2021; 1319:105-135. [PMID: 34424514 DOI: 10.1007/978-3-030-65943-1_4] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/09/2023]
Abstract
Naked mole-rats (Heterocephalus glaber) are small African rodents that have many unique behavioral and physiological adaptations well-suited for testing hypotheses about mammalian neural plasticity. In this chapter, we focus on three features of naked mole-rat biology and how they impact neural plasticity in this species: (1) their fossorial lifestyle, (2) their extreme longevity with a lack of demonstrable senescence, and (3) their unusual social structure. Critically, each of these features requires some degree of biological flexibility. First, their fossorial habitat situates them in an environment with characteristics to which the central nervous system is particularly sensitive (e.g., oxygen content, photoperiod, spatial complexity). Second, their long lifespan requires adaptations to combat senescence and declines in neural functioning. Finally, their extreme reproductive skew and sustained ability for release from reproductive suppression indicates remarkable neural sensitivity to the sociosexual environment that is distinct from chronological age. These three features of naked mole-rat life are not mutually exclusive, but they do each offer unique considerations for the possibilities, constraints, and mechanisms associated with adult neural plasticity.
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Androgen receptors and muscle: a key mechanism underlying life history trade-offs. J Comp Physiol A Neuroethol Sens Neural Behav Physiol 2017; 204:51-60. [DOI: 10.1007/s00359-017-1222-4] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/16/2017] [Revised: 10/05/2017] [Accepted: 10/08/2017] [Indexed: 12/18/2022]
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Kerver HN, Wade J. Seasonal and sexual dimorphisms in expression of androgen receptor and its coactivators in brain and peripheral copulatory tissues of the green anole. Gen Comp Endocrinol 2013; 193:56-67. [PMID: 23892016 DOI: 10.1016/j.ygcen.2013.07.006] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 03/04/2013] [Revised: 06/14/2013] [Accepted: 07/08/2013] [Indexed: 12/22/2022]
Abstract
Green anoles are seasonally breeding lizards, with an annual rise in testosterone (T) being the primary activator of male sexual behaviors. Responsiveness to T is decreased in the non-breeding season (NBS) compared to breeding season (BS) on a variety of levels, including displays of reproductive behavior and the morphology and biochemistry of associated tissues. To evaluate the possibility that seasonal changes in responsiveness to T are regulated by androgen receptors (AR) and/or two of its coactivators, CREB binding protein (CBP) and steroid receptor coactivator-1 (SRC-1), we tested whether they differ in expression across season in brains of both sexes and in peripheral copulatory tissues of males (hemipenis and retractor penis magnus muscle). AR mRNA was increased in the brains of males compared to females and in copulatory muscle in the BS compared to NBS. In the hemipenis, transcriptional activity appeared generally diminished in the NBS. T-treatment increased AR mRNA in the copulatory muscle and AR protein in the hemipenis, the latter to a greater extent in the BS than the NBS. T also decreased SRC-1 protein in hemipenis. Interpretations are complicated, in part because levels of mRNA and protein expression were not correlated and multiple sizes of the AR and CBP proteins were detected, with some tissue specificity. However, the results are consistent with the idea that differences in receptor and coactivator expression at central and peripheral levels may play roles in regulating sex and seasonal differences in the motivation or physical ability to engage in sexual behavior.
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Affiliation(s)
- Halie N Kerver
- Neuroscience Program, Michigan State University, East Lansing, MI 48824-1101, United States.
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Neal JK, Wade J. Courtship and copulation in the adult male green anole: effects of season, hormone and female contact on reproductive behavior and morphology. Behav Brain Res 2006; 177:177-85. [PMID: 17174414 PMCID: PMC2892282 DOI: 10.1016/j.bbr.2006.11.035] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/25/2006] [Revised: 11/20/2006] [Accepted: 11/21/2006] [Indexed: 12/26/2022]
Abstract
Interactions among reproductive season, testosterone (T) and female presence were investigated on the structure and function of forebrain and neuromuscular systems controlling courtship and copulation in the green anole lizard. Under breeding (BS) or non-breeding (NBS) environmental conditions, male green anoles were implanted with either T or blank capsules and exposed to one of three female stimulus conditions: physical, visual or no female contact. T and at least visual exposure to females increased courtship displays (extension of a throat fan, or dewlap), and these effects were greater during the BS than NBS. T also facilitated copulation, and did so to a greater extent in the BS. The hormone increased soma size in the preoptic area (POA) and amygdala (AMY), and in the AMY the effects were greater in the BS than NBS. Cross-sectional areas of copulatory organs and associated muscle fibers were enhanced by T, and more so in the BS than NBS. However, no effects on morphology of dewlap motoneurons or muscles or copulatory motoneurons were detected. Thus, (1) changes in behavior and neural and/or muscular morphology are not always parallel and (2) differences in responsiveness to T exist across seasons and among tissues.
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Affiliation(s)
- Jennifer K Neal
- Michigan State University, Neuroscience Program, East Lansing, MI 48824-1101, United States.
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Seney M, Goldman BD, Forger NG. Breeding status affects motoneuron number and muscle size in naked mole-rats: recruitment of perineal motoneurons? ACTA ACUST UNITED AC 2006; 66:1354-64. [PMID: 16967507 DOI: 10.1002/neu.20314] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022]
Abstract
Naked mole-rats live in large colonies and exhibit a strict reproductive hierarchy. Each colony has one breeding female and one to three breeding males; all other individuals are nonreproductive subordinates. Subordinates show a remarkable lack of sex differences in behavior and anatomy, but can become reproductive if removed from the colony. We recently reported that the striated perineal muscles and their innervating motoneurons, which are sexually dimorphic in all other mammals examined to date, are not dimorphic in subordinate naked mole-rats. Here we asked whether sexual differentiation of this neuromuscular system occurs when a subordinate becomes a breeder. The size and number of cells within Onuf's nucleus (homologue of the rat spinal nucleus of the bulbocavernosus) as well as perineal muscle volume were examined in subordinate and breeding naked mole-rats of both sexes. Sex differences in perineal motoneurons were not observed, regardless of social status. To our surprise, however, counts of motoneurons in Onuf's nucleus were increased approximately 30% in breeders of both sexes. This was accompanied by a reciprocal decrease in cells in Onuf's nucleus that were characterized by small soma size, and lacked a clear nucleus or nucleolus. Although not exhibiting typical motoneuron morphology, some of these small cells were positive for the motoneuron marker, SMI-32. The neuronal changes correlate with increased perineal muscle volumes in breeders. We propose that small, relatively undifferentiated cells are recruited to the pool of large Onuf's nucleus motoneurons when subordinate naked mole-rats become breeders.
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Affiliation(s)
- Marianne Seney
- Department of Psychology and Center for Neuroendocrine Studies, University of Massachusetts, Amherst, Massachusetts 01003, USA.
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Hazelett DJ, Weeks JC. Segment-specific muscle degeneration is triggered directly by a steroid hormone during insect metamorphosis. ACTA ACUST UNITED AC 2005; 62:164-77. [PMID: 15452849 DOI: 10.1002/neu.20077] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
During metamorphosis of the hawkmoth, Manduca sexta, some larval muscles degenerate while others are respecified for new functions. In larvae, accessory planta retractor muscles (APRMs) are present in abdominal segments 1 to 6 (A1 to A6). APRMs serve as proleg retractors in A3 to A6 and body wall muscles in A1 and A2. At pupation, all APRMs degenerate except those in A2 and A3, which are respecified to circulate hemolymph in pupae. The motoneurons that innervate APRMs, the APRs, likewise undergo segment-specific programmed cell death (PCD), as a direct, cell-autonomous response to the prepupal peak of ecdysteroids. The segment-specific patterns of APR and APRM death differ. The present study tested the hypothesis that APRM death is a direct, cell-autonomous response to the prepupal peak of ecdysteroids. Prevention of the prepupal peak prevented APRM degeneration, and replacement of the peak by infusion of 20-hydroxyecdysone restored the correct segment-specific pattern of APRM degeneration. Surgical denervation of APRMs did not perturb their segment-specific degeneration at pupation, indicating that signals from APRs are not required for the muscles' segment-specific responses to ecdysteroids. The possibility that instructive signals originate from APRMs' epidermal attachment points was tested by treating the epidermis with a juvenile hormone analog to prevent pupal development. This manipulation likewise did not alter APRM fate. We conclude that both the muscles and motoneurons in this motor system respond directly and cell-autonomously to prepupal ecdysteroids to produce a segment-specific pattern of PCD that is matched to the functional requirements of the pupal body.
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Affiliation(s)
- Dennis J Hazelett
- Institute of Neuroscience, 1254 University of Oregon, Eugene, Oregon 97403-1254, USA
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Morris JA, Jordan CL, Breedlove SM. Sexual differentiation of the vertebrate nervous system. Nat Neurosci 2004; 7:1034-9. [PMID: 15452574 DOI: 10.1038/nn1325] [Citation(s) in RCA: 446] [Impact Index Per Article: 22.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/12/2004] [Accepted: 08/13/2004] [Indexed: 11/09/2022]
Abstract
Understanding the mechanisms that give rise to sex differences in the behavior of nonhuman animals may contribute to the understanding of sex differences in humans. In vertebrate model systems, a single factor-the steroid hormone testosterone-accounts for most, and perhaps all, of the known sex differences in neural structure and behavior. Here we review some of the events triggered by testosterone that masculinize the developing and adult nervous system, promote male behaviors and suppress female behaviors. Testosterone often sculpts the developing nervous system by inhibiting or exacerbating cell death and/or by modulating the formation and elimination of synapses. Experience, too, can interact with testosterone to enhance or diminish its effects on the central nervous system. However, more work is needed to uncover the particular cells and specific genes on which testosterone acts to initiate these events.
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Affiliation(s)
- John A Morris
- Neuroscience Program, Michigan State University, East Lansing, Michigan 48824, USA
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Devaud JM, Ferrús A. Molecular genetics of activity-dependent structural changes at the synapse. J Neurogenet 2004; 17:271-93. [PMID: 15204080 DOI: 10.1080/01677060390441840] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
Affiliation(s)
- Jean Marc Devaud
- Laboratoire de Génomique Fonctionelle, CNRS, UPR-2580, Montpellier, France
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Holmes MM, Wade J. Seasonal plasticity in the copulatory neuromuscular system of green anole lizards: a role for testosterone in muscle but not motoneuron morphology. ACTA ACUST UNITED AC 2004; 60:1-11. [PMID: 15188267 DOI: 10.1002/neu.10334] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
The copulatory system of green anoles is highly sexually dimorphic. Males possess bilateral copulatory organs called hemipenes, each independently controlled by two muscles: the transversus penis (TPN) and retractor penis magnus (RPM). The TPN everts the hemipene through the cloaca and the RPM retracts it. Adult females do not possess hemipenes or either of these two muscles. The spinal nucleus projecting to the TPN and RPM contains more and larger motoneurons in males than females. Because anoles breed seasonally, two experiments were designed to test whether adult copulatory morphology varies with environmental condition, and if so, whether the effect is mediated by testicular androgens. Three groups of adult males were used in each experiment: males from breeding environmental conditions with reproductive testes (BS); males in breeding conditions with regressed testes (BS-X); and males in nonbreeding conditions with regressed testes (NBS). Experiment 1 compared gonadally intact males and Experiment 2 compared castrated males treated with either testosterone (T) or an empty implant. In both experiments, copulatory and control motoneurons appeared smaller in NBS males, but T did not affect their size. In contrast, while hemipene and RPM muscle fiber size were not plastic across season in gonadally intact males, T in castrated males significantly increased both measures under BS and BS-X, but not NBS, conditions. These results demonstrate that neuron soma size might change on a general level and environmental cues can mediate T-induced changes in peripheral structures, suggesting that plasticity across copulatory system components is regulated by different mechanisms.
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Monks DA, O'Bryant EL, Jordan CL. Androgen receptor immunoreactivity in skeletal muscle: enrichment at the neuromuscular junction. J Comp Neurol 2004; 473:59-72. [PMID: 15067718 DOI: 10.1002/cne.20088] [Citation(s) in RCA: 84] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
Abstract
Potential cellular targets of androgen action within skeletal muscle of the rat were determined by comparing the cellular distribution of androgen receptor (AR)-positive nuclei in the highly androgen-responsive levator ani (LA) muscle with that of the relatively androgen-unresponsive extensor digitorum longus (EDL) muscle. We found that androgen responsiveness correlates with AR expression in muscle fibers and not in fibroblasts. Results indicate that a much higher percentage of myonuclei in the LA are AR(+) than in the EDL (74% vs. 7%), correlating with differences in androgen responsiveness. Both muscles contain an equivalent proportion of AR(+) fibroblasts (approximately 62%). AR(+) nuclei were not observed in terminal Schwann cells in either muscle. These results suggest that ARs within LA muscle fibers mediate the androgen-dependent survival and growth of the LA muscle and its motoneurons. We also observed an unexpected enrichment of AR(+) myonuclei and fibroblasts proximate to neuromuscular junctions, suggesting that ARs at muscle synapses may selectively regulate synapse-specific genes important for the survival and growth of motoneurons. Although castration reduced the proportion of AR(+) fibroblasts in both muscles, the proportion of AR(+) myonuclei was reduced only in the LA. As expected, testosterone treatment prevented these effects of castration but, unexpectedly, increased the proportion of AR(+) myonuclei in the EDL to above normal. These results suggest that how AR expression in skeletal muscle is influenced by androgens depends not only on the particular muscle but on the particular cell type within that muscle.
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Affiliation(s)
- Douglas Ashley Monks
- Neuroscience Program and Department of Psychology, Michigan State University, East Lansing, Michigan 48824-1101, USA
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Siegford JM, Hadi Mansouri S, Ulibarri C. Normal ontogeny of perineal muscles and testosterone levels in Mongolian gerbils; Response to testosterone in developing females. ACTA ACUST UNITED AC 2003; 275:997-1008. [PMID: 14533174 DOI: 10.1002/ar.a.10118] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
The spinal nucleus of the bulbocavernosus (SNB) of Mongolian gerbils (Meriones unguiculatus) becomes sexually dimorphic during postnatal life, rather than prenatally as in rats. We therefore examined the early postnatal ontogeny of Mongolian gerbils, focusing on growth, serum testosterone (T) levels, and the sexually dimorphic perineal musculature innervated by the SNB. Serum T levels were higher in males than in females from birth through adulthood, with several early postnatal peaks and a large increase in T occurring during puberty in males. The SNB target muscles-the bulbocavernosus (BC) and levator ani (LA)-were present in both sexes on postnatal day 1 (PND1). Cross-sectional areas of BC fibers in males increased with age, and concurrently the myofibers of the BC became more fully developed and organized. In PND10 female pups, the BC muscle was virtually absent, while the LA muscle remained (although it was reduced in size). Postnatal treatment of female gerbils with androgen caused the BC muscle to remain and the LA muscle to become larger by PND10. Sexual dimorphism of the SNB develops differently in gerbils compared to other species, although its target muscles appear to respond to androgen in a manner similar to that in rats.
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Affiliation(s)
- Janice M Siegford
- Department of Veterinary, College of Veterinary Medicine, Washington State University, Pullman, Washington, USA.
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Weeks JC. Thinking globally, acting locally: steroid hormone regulation of the dendritic architecture, synaptic connectivity and death of an individual neuron. Prog Neurobiol 2003; 70:421-42. [PMID: 14511700 DOI: 10.1016/s0301-0082(03)00102-3] [Citation(s) in RCA: 48] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
Steroid hormones act via evolutionarily conserved nuclear receptors to regulate neuronal phenotype during development, maturity and disease. Steroid hormones exert 'global' effects in organisms to produce coordinated physiological responses whereas, at the 'local' level, individual neurons can respond to a steroidal signal in highly specific ways. This review focuses on two phenomena-the loss of dendritic processes and the programmed cell death (PCD) of neurons-that can be regulated by steroid hormones (e.g. during sexual differentiation in vertebrates). In insects such as the moth, Manduca sexta, and fruit fly, Drosophila melanogaster, ecdysteroids orchestrate a reorganization of neural circuits during metamorphosis. In Manduca, accessory planta retractor (APR) motoneurons undergo dendritic loss at the end of larval life in response to a rise in 20-hydroxyecdysone (20E). Dendritic regression is associated with a decrease in the strength of monosynaptic inputs, a decrease in the number of contacts from pre-synaptic neurons, and the loss of a behavior mediated by these synapses. The APRs in different abdominal segments undergo segment-specific PCD at pupation and adult emergence that is triggered directly and cell-autonomously by a genomic action of 20E, as demonstrated in cell culture. The post-emergence death of APRs provides a model for steroid-mediated neuroprotection. APR death occurs by autophagy, not apoptosis, and involves caspase activation and the aggregation and ultracondensation of mitochondria. Manduca genes involved in segmental identity, 20E signaling and PCD are being sought by suppressive subtractive hybridization (SSH) and cDNA microarrays. Experiments utilizing Drosophila as a complementary system have been initiated. These insect model systems contribute toward understanding the causes and functional consequences of dendritic loss and neurodegeneration in human neurological disorders.
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Affiliation(s)
- Janis C Weeks
- Institute of Neuroscience, 1254 University of Oregon, Eugene, OR 97403-1254, USA.
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