Neuropeptidergic organization of the suprachiasmatic nucleus in the blind mole rat (Spalax ehrenbergi)

MEDETOMIDINE-KETAMINE-BUPRENORPHINE ANESTHESIA OF THE SOLITARY SUBTERRANEAN CAPE DUNE MOLE-RAT (BATHYERGUS SUILLUS) AND THE CAPE MOLE-RAT (GEORYCHUS CAPENSIS)

A prospective, descriptive study was conducted to evaluate the safety and efficacy of a field-ready anesthetic drug combination of medetomidine-ketamine-buprenorphine for data logger implantation surgery or recheck in free-ranging Cape dune (Bathyergus suillus: n = 41) and Cape (Georychus capensis: n = 37) mole-rats. All anesthesia data were reported as mean (±standard deviation). Medetomidine-ketamine-buprenorphine doses were 0.1 (±0.03), 10.6 (±2.8), and 0.06 (±0.03) mg/kg, respectively, for Cape dune mole-rats; and 0.2 (±0.03), 19.4 (±4.0), and 0.14 (±0.03) mg/kg, respectively, for Cape mole-rats. Induction was calm and took 2.00 (range: 1.00-6.00) min for the Cape dune and 1.75 (range 1.25 to 8.16) min for Cape mole-rats. A surgical plane of anesthesia was achieved in most Cape dune mole-rats (92%) and Cape mole-rats (90%). The remainder required supplementation with a single intramuscular injection of ketamine (3-9 mg/kg) during surgery. Heart and respiratory rates were 149 (±37) beats and 24 (±8) breaths per minute, respectively, for Cape dune mole-rats and 179 (±40) beats and 25 (±10) and breaths per minute, respectively for Cape mole-rats. Surgical time for mole-rats ranged from 25 to 38 min. Recovery was calm and took 8.50 (range: 2.00-19.00) min for Cape dune mole-rats and 9.75 (range: 2.00-34.00) min for Cape mole-rats to recover. For recovery, atipamezole was administered intramuscularly at 0.5 (±0.15) mg/kg for Cape dune mole-rats and 1 (±0.15) mg/kg for Cape mole-rats. All mole-rats were returned to their original burrows within 48 h of recovery. The medetomidine-ketamine-buprenorphine combination induced a predictable, safe anesthesia in Cape dune and Cape mole-rats suitable for short intraabdominal surgery. This combination is suited to in situ studies where the use of a formal surgery or laboratory is not feasible.

ANESTHETIC EFFECT OF DEXMEDETOMIDINE-KETAMINEMIDAZOLAM COMBINATION ADMINISTERED INTRAMUSCULARLY TO ZOO-HOUSED NAKED MOLE-RATS (HETEROCEPHALUS GLABER)

In this study, adult intact male and female (n = 10) naked mole-rats (Heterocephalus glaber) were anesthetized using a combination of dexmedetomidine (0.06 mg/kg intramuscularly [IM]), ketamine (20 mg/kg IM), and midazolam (1.0 mg/kg IM). Atipamezole (1.0 mg/kg IM) and flumazenil (0.1 mg/kg IM) were administered 40 min after induction. Induction and recovery times were monitored and recorded. Vital parameters, including heart rate, respiratory rate, and SpO₂, and reflexes were monitored every 5 min during the anesthetic period. Anesthetic induction was smooth and rapid. All monitored reflexes were lost within a median time of 60 sec (interquartile range, 15 sec). Heart rate and respiratory rate were significantly decreased from baseline, whereas there was no difference in SpO₂ over the anesthetic period. The mean time to recovery was 15 ± 7 min (mean ± SD). One animal was found dead 40 min after apparent recovery, which is suspected to be an anesthetic-related death. Based on these findings, dexmedetomidine-ketamine-midazolam anesthesia is an effective anesthetic protocol in naked mole rats that provides a consistent anesthetic plane but should be used with caution in animals with underlying conditions.

Anesthetic Effects of Intramuscular Alfaxalone-Ketamine in Naked Mole Rats (Heterocephalus glaber)

In this study, adult intact male and female (n = 10) naked mole rats (Heterocephalus glaber) were anesthetized by using a combination of ketamine (20 mg/kg IM), and alfaxalone (4.0 mg/kg IM). Induction and recovery times were recorded. Vital parameters, including heart rate, respiratory rate, and reflexes, were monitored every 5 min during the anesthetic period. Anesthetic induction was smooth and rapid. Induction time was significantly longer in male rats (median, 325 s; range, 180 to 385 s) than in females (median, 145 s; range, 118 to 180 s). In addition, overall duration of loss of righting reflex was shorter in male mole rats (median, 50 min; range, 36 to 65 min) than females (median, 70 min; range, 60 to 85 min). Males largely had intact withdrawal reflexes, whereas females showed variable loss of both forelimb and hindlimb withdrawal reflexes. Neither recovery time (mean ± 1 SD, 16 ± 13 min) nor vital parameters differed between sexes. None of animals showed any anesthesia-related adverse responses. According to these findings, intramuscular AK is a safe and effective protocol that provides brief, light anesthesia in male naked mole rats and deeper anesthesia in females. We recommend adding analgesics when this AK protocol is used for pain-inducing or invasive procedures, and further studies evaluating higher doses and different combinations are indicated.

Comparison of intramuscular administration of alfaxalone-ketamine-dexmedetomidine and alfaxalone-butorphanol-midazolam in naked mole-rats (Heterocephalus glaber)

OBJECTIVE

To compare anesthetic effects of alfaxalone-ketamine-dexmedetomidine (AKD) and alfaxalone-butorphanol-midazolam (ABM) in naked mole-rats (Heterocephalus glaber).

ANIMALS

20 naked mole-rats.

PROCEDURES

Naked mole-rats received AKD (alfaxalone, 2 mg/kg; ketamine, 20 mg/kg; and dexmedetomidine, 0.02 mg/kg; n = 10) or ABM (alfaxalone, 2 mg/kg; butorphanol, 2 mg/kg; and midazolam, 1 mg/kg; 9) IM; 1 animal was removed from the study. Atipamezole (I mg/kg) and flumazenil (0.1 mg/kg) were administered 40 minutes after anesthetic induction (defined as loss of the righting reflex) with AKD and ABM, respectively. Heart rate, respiratory rate, oxygen saturation, and reflexes were recorded every 5 minutes.

RESULTS

The ABM group had significantly longer median times for induction and recovery than the AKD group. Administration of ABM resulted in significantly lower respiratory rates than administration of AKD from time of anesthetic induction to 10 minutes after induction. Respiratory rate significantly decreased in the AKD group from I0 minutes after induction through the end of the anesthetic period but did not change over time in the ABM group. Males had higher respiratory rates in both groups. Loss of the righting reflex was still evident 40 minutes after induction in both groups. In the AKD group, all tested reflexes were absent from I0 to 40 minutes after induction; the ABM group had variable reflexes that recovered within individual animals over time.

CONCLUSIONS AND CLINICAL RELEVANCE

Both AKD and ABM provided effective immobilization in naked mole-rats, but AKD appeared to provide more consistent and deeper anesthesia, compared with administration of ABM.

ES. The Suprachiasmatic Nucleus and the Intergeniculate Leaflet of the Flat-Faced Fruit-Eating Bat (Artibeus planirostris): Retinal Projections and Neurochemical Anatomy

In mammals, the suprachiasmatic nucleus (SCN) and the intergeniculate leaflet (IGL) are the main components of the circadian timing system. The SCN, classically known as the master circadian clock, generates rhythms and synchronizes them to environmental cues. The IGL is a key structure that modulates SCN activity. Strategies on the use of time by animals can provide important clues about how some species are adapted to competitive process in nature. Few studies have provided information about temporal niche in bats with special attention on the neural substrate underlies circadian rhythms. The aim of this study was to investigate these circadian centers with respect to their cytoarchitecture, chemical content and retinal projections in the flat-faced fruit-eating bat (Artibeus planirostris), a chiropteran endemic to South America. Unlike other species of phyllostomid bats, the flat-faced fruit-eating bat's peak of activity occurs 5 h after sunset. This raises several questions about the structure and function of the SCN and IGL in this species. We carried out a mapping of the retinal projections and cytoarchitectural study of the nuclei using qualitative and quantitative approaches. Based on relative optical density findings, the SCN and IGL of the flat-faced fruit-eating bat receive bilaterally symmetric retinal innervation. The SCN contains vasopressin (VP) and vasoactive intestinal polypeptide (VIP) neurons with neuropeptide Y (NPY), serotonin (5-HT) and glutamic acid decarboxylase (GAD) immunopositive fibers/terminals and is marked by intense glial fibrillary acidic protein (GFAP) immunoreactivity. The IGL contains NPY perikarya as well as GAD and 5-HT immunopositive terminals and is characterized by dense GFAP immunostaining. In addition, stereological tools were combined with Nissl stained sections to estimate the volumes of the circadian centers. Taken together, the present results in the flat-faced fruit-eating bat reveal some differences compared to other bat species which might explain the divergence in the hourly activity among bats in order to reduce the competitive potential and resource partitioning in nature.

Chemical restraint of African mole-rats (Fukomys sp.) with a combination of ketamine and xylazine

OBJECTIVE

To establish an accurate anaesthetic dose for chemical restraint of African mole-rats using ketamine and xylazine.

STUDY DESIGN

Prospective nonrandomized laboratory study.

ANIMALS

Sixteen adult Ansell's mole-rats (Fukomys anselli) and eight giant mole-rats (F. mechowii).

METHODS

Fukomys anselli of different ages, sexes and reproductive status were systematically anaesthetized starting with an intramuscular injection of ketamine (2.5 mg kg(-1) ) and increasing the doses in steps of 0.5 mg kg(-1) until loss of the righting reflex (LRR) was observed. Xylazine was added to a constant dose of ketamine, starting at 0.5 mg kg(-1) that was increased by 0.5 mg kg(-1) in further trials. Once an effective combination was established and evaluated in F. anselli, it was also tested in F. mechowii. Heart and respiratory rates and rectal temperatures were measured during anaesthesia. anova for repeated measures and Student's t-test were used to compare means.

RESULTS

Chemical restraint was accomplished at a dose of 6 mg kg(-1) ketamine combined with 2.5 mg kg(-1) xylazine. LRR lasted on average mean 56 ± SD 19 minutes (F. anselli) and 140 ± 41 minutes (F. mechowii). Loss of pedal withdrawal reflex (LPR) lasted for 20 ± 15 minutes (F. anselli) and for 29 ± 2 minutes (F. mechowii), respectively. All animals recovered satisfactorily. Heart and respiratory rates were stable during anaesthesia, but rectal temperature fell significantly in F. mechowii after losing the righting reflex (LRR) from T1 (32.6 ± 0.6 °C) to T3 (30.4 ± 0.9 °C).

CONCLUSIONS AND CLINICAL RELEVANCE

African mole-rats (Bathyergidae) live in closed burrow systems under particular conditions (hypercapnia, hypoxia, stable temperature, humidity, darkness) and show several physiological adaptations. Injectable anaesthetics in the dose rates used in other rodents are not appropriate for use in these subterranean species. Here, a reliable protocol for chemical restraint is provided.

Physiology of circadian entrainment

Mammalian circadian rhythms are controlled by endogenous biological oscillators, including a master clock located in the hypothalamic suprachiasmatic nuclei (SCN). Since the period of this oscillation is of approximately 24 h, to keep synchrony with the environment, circadian rhythms need to be entrained daily by means of Zeitgeber ("time giver") signals, such as the light-dark cycle. Recent advances in the neurophysiology and molecular biology of circadian rhythmicity allow a better understanding of synchronization. In this review we cover several aspects of the mechanisms for photic entrainment of mammalian circadian rhythms, including retinal sensitivity to light by means of novel photopigments as well as circadian variations in the retina that contribute to the regulation of retinal physiology. Downstream from the retina, we examine retinohypothalamic communication through neurotransmitter (glutamate, aspartate, pituitary adenylate cyclase-activating polypeptide) interaction with SCN receptors and the resulting signal transduction pathways in suprachiasmatic neurons, as well as putative neuron-glia interactions. Finally, we describe and analyze clock gene expression and its importance in entrainment mechanisms, as well as circadian disorders or retinal diseases related to entrainment deficits, including experimental and clinical treatments.

Photosensitivity to different light intensities in blind and sighted rodents

Photoperiod is an important cue regulating biological rhythms in mammals, including ‘blind’ subterranean and sighted fossorial rodent species. These species may respond differentially to changes in light quality according to their retinal complexity. The effects of increasing light intensity on daily rhythms of urine excretion and urinary output of 6-sulfatoxymelatonin levels were compared in ‘blind’ mole rats Spalax ehrenbergi and sighted social voles, Microtus socialis. Our results show that the threshold irradiance required to entrain rhythms of voles is three magnitudes greater than that for mole rats. The results suggest that mole rats have an operational photoreceptive pathway with a lower threshold irradiance than voles. Such a low threshold reflects the remarkable capability of this ‘blind’ species to utilize light signals even under challenging light conditions.

Computational modeling of synchronization process of the circadian timing system of mammals

This paper presents a model for the circadian temporization system of mammals which associates the synchronization dynamics of coupling oscillators to a set of equations able to reproduce the synaptic characteristics of somatodendritic membrane of neurons. The circadian timing system is organized in a way to receive information from the external and internal environments, and its function is the timing organization of physiological and behavioral processes in a circadian pattern. Circadian timing system in mammals is constituted by a group of structures which includes the suprachiasmatic nucleus, the intergeniculate leaflet and the pineal gland. In suprachiasmatic nucleus are found neuron groups working as a biological pacemaker-the so-called biological master clock. By means of numerical simulations using the Kuramoto model, we simulated the dynamics behavior of the biological pacemaker. For this we used a set of 1,000 coupled oscillators with long-range coupling, which were distributed on a 10 x 10 x 10 spherical lattice, and a new method to estimate the order parameter, which characterizes the degree of synchronization of oscillator system. Our model has been able to produce frequency responses in accordance with physiological patterns, and to reproduce two fundamental characteristics of biological rhythms: the endogenous generation and synchronization to the light-dark cycle.

Distribution of vasopressin in the brain of the eusocial naked mole-rat

Naked mole-rats are eusocial rodents that live in large subterranean colonies in which one queen breeds with one to three males. All other animals are nonbreeding subordinates. The external features of male and female subordinates, including their genitalia, are remarkably monomorphic, as is their behavior. Because vasopressin (VP) is associated with social behaviors and sex differences in other species, its distribution in naked mole-rats was of interest. We used immunohistochemistry to examine VP in the brains of subordinate and breeding naked mole-rats of both sexes. As in other mammals, VP-immunoreactive (-ir) somata were found in the paraventricular (PVN) and supraoptic nuclei (SON) and VP-ir projections from these nuclei ran through the internal and external zone of the median eminence. However, naked mole-rats had very few VP-ir cells in the bed nucleus of the stria terminalis (BST) and none in the suprachiasmatic nucleus (SCN); the extensive network of fine-caliber VP-ir fibers usually seen in projection sites of the BST and SCN were also absent. Equally unexpected was the abundance of large-caliber VP-ir fibers in the dorsomedial septum. VP immunoreactivity was generally similar in all groups, with the exception of VP-ir cell number in the dorsomedial hypothalamus (DMH). Breeders had a population of labeled cells in the DMH that was absent, or nearly absent, in subordinates. Future studies on the function of VP in these areas are needed to determine how the atypical distribution of VP immunoreactivity relates to eusociality and the unusual physiology of naked mole-rats.

The circadian visual system, 2005

The primary mammalian circadian clock resides in the suprachiasmatic nucleus (SCN), a recipient of dense retinohypothalamic innervation. In its most basic form, the circadian rhythm system is part of the greater visual system. A secondary component of the circadian visual system is the retinorecipient intergeniculate leaflet (IGL) which has connections to many parts of the brain, including efferents converging on targets of the SCN. The IGL also provides a major input to the SCN, with a third major SCN afferent projection arriving from the median raphe nucleus. The last decade has seen a blossoming of research into the anatomy and function of the visual, geniculohypothalamic and midbrain serotonergic systems modulating circadian rhythmicity in a variety of species. There has also been a substantial and simultaneous elaboration of knowledge about the intrinsic structure of the SCN. Many of the developments have been driven by molecular biological investigation of the circadian clock and the molecular tools are enabling novel understanding of regional function within the SCN. The present discussion is an extension of the material covered by the 1994 review, "The Circadian Visual System."

The use of a novel and simple method of revealing neural fibers to show the regression of the lateral geniculate nucleus in the naked mole-rat (Heterocephalus glaber)

The lateral geniculate nucleus (LG) is an important subcortical nucleus in the visual system. It receives primary projections from the retina and relays these to central visual structures. Although there are studies on the retina and visual cortex of animals with regressed vision, little is known about the LG in such animals. The strictly subterranean naked mole-rat (Heterocephalus glaber) has markedly reduced visual acuity with concomitant pronounced changes in the visual cortex. We used a novel method to reveal myelinated neural fibers in a histological study assessing if the LG shows regressive changes commensurate with the level of reliance on vision by this rodent. Myelin detection here relies on significant differences in visible light reflection between neural fibers and the gray matter. Moreover, this simple method does not interfere with further staining for additional analyses. This method reveals that the contribution of the LG to brain volume in the naked mole-rat is less than a third of that of the rat. This shows that the retinogeniculocortical system in the naked mole-rat is considerably smaller than that of rodents that rely heavily on their visual system, but is nevertheless less regressed than that of the extensively studied blind mole-rat; this may facilitate limited responses to visual stimuli.

Subcortical visual system of the African mole-rat Cryptomys anselli: to see or not to see?

Abstract We studied the retinal projections, the distribution of cytochrome oxidase activity and the cyto- and myeloarchitecture of the subcortical visual system in the subterranean Ansell's mole-rat Cryptomys anselli. The optic nerve contained 1500 myelinated and a similar number of unmyelinated fibres. The retina projected to all the visual structures described in surface-dwelling sighted rodents. The suprachiasmatic nucleus was large and received bilateral retinal input. All other visual nuclei were reduced in size, were cytoarchitecturally poorly developed and received almost exclusively contralateral retinal projections. The dorsal and ventral lateral geniculate nuclei were moderately reduced and heavily innervated. The intergeniculate leaflet could be identified between these two nuclei. Pretectal nuclei were also relatively well-developed. The nucleus of the optic tract, the olivary pretectal nucleus, and the anterior and posterior pretectal nuclei were innervated by the retina. By contrast, the superficial, retinorecipient layers of the superior colliculus showed extreme reduction. The strata zonale, griseum superficiale and opticum were collapsed to a single layer 40 micro m thick. The accessory optic system was vestigial. These findings indicate that the functional subsystems involved in photoperiod perception, form and brightness discrimination, and movement analysis are anatomically rather well developed, whereas those involved in coordination of visuomotor reflexes are severely reduced. Thus, the visual system of C. anselli is much better developed than that of the blind mole-rat Spalax ehrenbergi. We suggest that Cryptomys anselli has retained basic visual capabilities.

Organization of the circadian system in the subterranean mole rat, Cryptomys hottentotus (Bathyergidae)

The mole rat, Cryptomys hottentotus (Bathyergidae) is a gregarious subterranean rodent, which shows no entrainment to ambient light-dark cycles. The locomotor activity of individuals or of a whole colony, which shows no circadian rhythmicity. Since the lack of both synchronization to light-dark cycle and an endogenous rhythm of locomotor activity could be related to the organization of the circadian system, we have investigated the neuropeptidergic features of the SCN and IGL, and have used pseudorabies viral tracing methods to identify the visual and circadian pathways in this species. The precise topographic distribution of certain neuropeptide populations in the SCN differs from typical rodent pattern of organization and may be correlated with the apparent absence of light entrainment of activity and lack of endogenous rhythmicity. The IGL is highly reduced in size. This structure can nevertheless be identified by the presence of NPY and CALB positive neurons, as well as by a dense network of SP fibers. Viral tracing using intraocular injection of the PRV-Becker and PRV-Bartha strains, leads to differential infection of neurons in circadian and visual structures. With the Bartha strain, infected neurons are principally observed in the SCN, whereas the Becker strain leads primarily to infection of the dLGN and shows an anatomical regression of visual structures. Transsynaptic retrograde infection of the retina contralateral to the injected eye reveals a morphologically homogeneous population, which resemble to retinohypothalamic ganglion cells described in other mammals.

A switch from diurnal to nocturnal activity in S. ehrenbergi is accompanied by an uncoupling of light input and the circadian clock

The subterranean mole rat Spalax ehrenbergi superspecies represents an extreme example of adaptive visual and neuronal reorganization. Despite its total visual blindness, its daily activity rhythm is entrainable to light-dark cycles, indicating that it can confer light information to the clock. Although most individuals are active during the light phase under laboratory conditions (diurnal animals), some individuals switch their activity period to the night (nocturnal animals). Similar to other rodents, the Spalax circadian clock is driven by a set of clock genes, including the period (sPer) genes. In this work, we show that diurnal mole rats express the Per genes sPer1 and sPer2 with a peak during the light period. Light can synchronize sPer gene expression to an altered light-dark cycle and thereby reset the clock. In contrast, nocturnal Spalax express sPer2 in the dark period and sPer1 in a biphasic manner, with a light-dependent maximum during the day and a second light-independent maximum during the night. Although sPer1 expression remains light inducible, this is not sufficient to reset the molecular clockwork. Hence, the strict coupling of light, Per expression, and the circadian clock is lost. This indicates that Spalax can dissociate the light-driven resetting pathway from the central clock oscillator.

The circadian photopigment melanopsin is expressed in the blind subterranean mole rat, Spalax

Despite severe degeneration of its eyes, the blind subterranean mole rat, Spalax, is able to adjust circadian rhythms to the environmental light/dark cycle due to a conserved retinohypothalamic tract (RHT). The photopigment mediating the circadian photoreception and it cellular localisation is unknown in the Spalax retina. Here we show, using in situ hybridization and immunohistochemistry, that melanopsin, a recently identified opsin, is expressed in retinal ganglion cells which also co-store PACAP, a neurotransmitter of the RHT. The melanopsin-component of retinal ganglion cells in the Spalax retina is well conserved resulting in a relatively higher density of melanopsin positive cells per area compared to the rat. The results show that the Spalax, as sighted animals expresses melanopsin in ganglion cells projecting to the circadian clock supporting a role of melanopsin as a circadian photopigment.