Evolutionary significance of different neurochemical organisation of the internal and external regions of auditory centres in the reptilian brain: an immunocytochemical and reduced NADPH-diaphorase histochemical study in turtles

A circuit for detection of interaural time differences in the nucleus laminaris of turtles

The physiological hearing range of turtles is approximately 50-1000 Hz, as determined by cochlear microphonics ( Wever and Vernon, 1956a). These low frequencies can constrain sound localization, particularly in red-eared slider turtles, which are freshwater turtles with small heads and isolated middle ears. To determine if these turtles were sensitive to interaural time differences (ITDs), we investigated the connections and physiology of their auditory brainstem nuclei. Tract tracing experiments showed that cranial nerve VIII bifurcated to terminate in the first-order nucleus magnocellularis (NM) and nucleus angularis (NA), and the NM projected bilaterally to the nucleus laminaris (NL). As the NL received inputs from each side, we developed an isolated head preparation to examine responses to binaural auditory stimulation. Magnocellularis and laminaris units responded to frequencies from 100 to 600 Hz, and phase-locked reliably to the auditory stimulus. Responses from the NL were binaural, and sensitive to ITD. Measures of characteristic delay revealed best ITDs around ±200 μs, and NL neurons typically had characteristic phases close to 0, consistent with binaural excitation. Thus, turtles encode ITDs within their physiological range, and their auditory brainstem nuclei have similar connections and cell types to other reptiles.

Distinction in the immunoreactivities of two calcium-binding proteins and neuronal birthdates in the first and higher-order somatosensory thalamic nuclei of mice: Evolutionary implications

Comparative embryonic studies are the most effective way to discern phylogenetic changes. To gain insight into the constitution and evolution of mammalian somatosensory thalamic nuclei, we first studied how calbindin (CB) and parvalbumin (PV) immunoreactivities appear during embryonic development in the first-order relaying somatosensory nuclei, i.e., the ventral posteromedial (VPM) and posterolateral (VPL) nuclei, and their neighboring higher-order modulatory regions, including the ventromedial or ventrolateral nucleus, posterior, and the reticular nucleus. The results indicated that cell bodies that were immunoreactive for CB were found earlier (embryonic day 12 [E12]) in the dorsal thalamus than were cells positive for PV (E14), and the adult somatosensory thalamus was characterized by complementary CB and PV distributions with PV dominance in the first-order relaying nuclei and CB dominance in the higher-order regions. We then labeled proliferating cells with [(3) H]-thymidine from E11 to 19 and found that the onset of neurogenesis began later (E12) in the first-order relaying nuclei than in the higher-order regions (E11). Using double-labeling with [(3) H]-thymidine autoradiography and CB or PV immunohistochemistry, we found that CB neurons were born earlier (E11-12) than PV neurons (E12-13) in the studied areas. Thus, similar to auditory nuclei, the first and the higher-order somatosensory nuclei exhibited significant distinctions in CB/PV immunohistochemistry and birthdates during embryonic development. These data, combined with the results of a cladistic analysis of the thalamic somatosensory nuclei, are discussed from an evolutionary perspective of sensory nuclei.

Disinhibition of the midbrain colliculi unmasks coordinated autonomic, respiratory, and somatomotor responses to auditory and visual stimuli

The midbrain superior and inferior colliculi have critical roles in generating coordinated orienting or defensive behavioral responses to environmental stimuli, and it has been proposed that neurons within the colliculi can also generate appropriate cardiovascular and respiratory responses to support such behavioral responses. We have previously shown that activation of neurons within a circumscribed region in the deep layers of the superior colliculus and in the central and external nuclei of the inferior colliculus can evoke a response characterized by intense and highly synchronized bursts of renal sympathetic nerve activity and phrenic nerve activity. In this study, we tested the hypothesis that, under conditions in which collicular neurons are disinhibited, coordinated cardiovascular, somatomotor, and respiratory responses can be evoked by natural environmental stimuli. In response to natural auditory, visual, or somatosensory stimuli, powerful synchronized increases in sympathetic, respiratory, and somatomotor activity were generated following blockade of GABAA receptors in a specific region in the midbrain colliculi of anesthetized rats, but not under control conditions. Such responses still occurred after removal of most of the forebrain, including the amygdala and hypothalamus, indicating that the essential pathways mediating these coordinated responses were located within the brain stem. The temporal relationships between the different outputs suggest that they are driven by a common population of “command neurons” within the colliculi.

Estradiol-dependent modulation of serotonergic markers in auditory areas of a seasonally breeding songbird

Because no organism lives in an unchanging environment, sensory processes must remain plastic so that in any context, they emphasize the most relevant signals. As the behavioral relevance of sociosexual signals changes along with reproductive state, the perception of those signals is altered by reproductive hormones such as estradiol (E2). We showed previously that in white-throated sparrows, immediate early gene responses in the auditory pathway of females are selective for conspecific male song only when plasma E2 is elevated to breeding-typical levels. In this study, we looked for evidence that E2-dependent modulation of auditory responses is mediated by serotonergic systems. In female nonbreeding white-throated sparrows treated with E2, the density of fibers immunoreactive for serotonin transporter innervating the auditory midbrain and rostral auditory forebrain increased compared with controls. E2 treatment also increased the concentration of the serotonin metabolite 5-HIAA in the caudomedial mesopallium of the auditory forebrain. In a second experiment, females exposed to 30 min of conspecific male song had higher levels of 5-HIAA in the caudomedial nidopallium of the auditory forebrain than birds not exposed to song. Overall, we show that in this seasonal breeder, (a) serotonergic fibers innervate auditory areas; (b) the density of those fibers is higher in females with breeding-typical levels of E2 than in nonbreeding, untreated females; and (c) serotonin is released in the auditory forebrain within minutes in response to conspecific vocalizations. Our results are consistent with the hypothesis that E2 acts via serotonin systems to alter auditory processing.

Sites of origin and developmental dynamics of the neurons in the core and shell regions of torus semicircularis in the Chinese softshell turtle (Pelodiscus sinensis)

To know the embryogenesis of the core and shell regions of the midbrain auditory nucleus, a single dose of [(3)H]-thymidine was injected into the turtle embryos at peak stages of neurogenesis in the shell and core of the torus semicircularis. Following sequential survival times, labeled neurons and the dynamics of cell proliferation were examined. The expression of vimentin (VM), reelin, calbindin, parvalbumin, and substance P were also studied. The results showed that: 1) progenitor cells for the core and shell regions were generated in different sites of the ventricular zone; 2) the length of the cell cycle or S-phase for the shell region were both longer than those for the core region (4.7 and 3.2 hours longer, respectively), suggesting that mitotic activity in the core region is higher than it is in the shell region; 3) the elongated cell bodies of the labeled core and shell cells had close apposition to VM fibers, suggesting that the migration of these cells is guided by VM fibers; 4) the germinal sites of the core and shell constructed by projecting the orientation of radial VM fibers back to the ventricular zone was consistent with those obtained by short and sequential survival [(3)H]-thymidine radiography; and 5) the beginning of positive staining for parvalbumin in the core region was interposed between those for calbindin and substance P in the shell regions. This study contributes to the understanding of how auditory nuclei are organized and how their components developed and evolved.

Germinal sites and migrating routes of cells in the mesencephalic and diencephalic auditory areas in the African clawed frog (Xenopus laevis)

There is a clear core-shell organization in the auditory nuclei of amniotes. However, such organization only exists in the mesencephalic, but not in the diencephalic auditory regions of amphibians. To gain insights into how this core-shell organization developed and evolved, we injected a small dose of [(3)H]-thymidine into tadpoles of Xenopus laevis at peak stages of neurogenesis in the mesencephalic and diencephalic auditory areas. Following different survival times, the germinal sites and migrating routes of cells were examined in the shell (laminar nucleus, Tl; magnocellular nucleus, Tmc) and core (principal nucleus, Tp) regions of the mesencephalic auditory nucleus, torus semicircularis (Ts), as well as in the diencephalic auditory areas (posterior thalamic nucleus, P; central thalamic nucleus, C). Double labeling for [(3)H]-thymidine autoradiography and immunohistochemistry for vimentin was also performed to help determine the routes of cell migration. We found three major results. First, the germinal sites of Tp were intercalated between Tl and Tmc, arising from those of the shell regions. Second, although the germinal sites of Tl, Tmc, and Tp were located in the same brain levels (at rostromedial or caudomedial levels of Ts), neurogenesis in Tl or Tmc started earlier than that in Tp. Finally, the P and C were also generated in different ventricle sites. However, unlike Ts their neurogenesis showed no obvious temporal differences. These data demonstrate that a highly differentiated auditory region, such as Tp in Ts, is lacking in the diencephalon of amphibian. Our data are discussed from the view of the constitution and evolutionary origins of auditory nuclei in vertebrates.

Core-and-belt organisation of the mesencephalic and forebrain auditory centres in turtles: expression of calcium-binding proteins and metabolic activity

The distribution of immunoreactivity to the calcium-binding proteins parvalbumin, calbindin and calretinin and of cytochrome oxidase activity was studied in the mesencephalic (torus semicircularis), thalamic (nucleus reuniens) and telencephalic (ventromedial part of the anterior dorsal ventricular ridge) auditory centres of two chelonian species Emys orbicularis and Testudo horsfieldi. In the torus semicircularis, the central nucleus (core) showed intense parvalbumin immunoreactivity and high cytochrome oxidase activity, whereas the laminar nucleus (belt) showed low cytochrome oxidase activity and dense calbindin/calretinin immunoreactivity. Within the central nucleus, the central and peripheral areas could be distinguished by a higher density of parvalbumin immunoreactivity and cytochrome oxidase activity in the core than in the peripheral area. In the nucleus reuniens, the dorsal and ventromedial (core) regions showed high cytochrome oxidase activity and immunoreactivity to all three calcium-binding proteins, while its ventrolateral part (belt) was weakly immunoreactive and showed lower cytochrome oxidase activity. In the telencephalic auditory centre, on the other hand, no particular region differed in either immunoreactivity or cytochrome oxidase activity. Our findings provide additional arguments in favour of the hypothesis of a core-and-belt organisation of the auditory sensory centres in non-mammalian amniotes though this organisation is less evident in higher order centres. The data are discussed in terms of the evolution of the auditory system in amniotes.

Evolutionary significance of delayed neurogenesis in the core versus shell auditory areas of Mus musculus

Early comparative embryogenesis can reflect the organization and evolutionary origins of brain areas. Neurogenesis in the auditory areas of sauropsids displays a clear core-to-shell distinction, but it remains unclear in mammals. To address this issue, [3H]-thymidine was injected into pregnant mice on consecutive embryonic (E) days (E10-E19) to date neuronal birthdays. Immunohistochemistry for substance P, calbindin, and parvalbumin was conducted to distinguish the core and shell auditory regions. The results showed that: 1) cell generation began at E13 in the external or dorsal nucleus of the inferior colliculus (IC), but it did not start in the caudomedial portion of the central nucleus of IC, and significantly fewer cells were produced in the medial and rostromedial portions of the central nucleus of IC; 2) cells were generated at E11 in the dorsal and medial divisions of the medial geniculate complex (MGd and MGm, respectively), whereas cell generation was absent in the medial and rostromedial portions of the ventral medial geniculate complex (MGv), and fewer cells were produced in the caudomedial portion of MGv; 3) in the telencephalic auditory cortex, cells were produced at E11 or E12 in layer I and the subplate, which receive projections from the MGd and MGm. However, cell generation occurred at E13-E18 in layers II-VI, including the area receiving projections from the MGv. The core-to-shell distinction of neurogenesis is thus present in the mesencephalic to telencephalic auditory areas in the mouse. This distinction of neurogenesis is discussed from an evolutionary perspective.

Comparative analysis of neurogenesis between the core and shell regions of auditory areas in the chick (Gallus gallus domesticus)

Early embryogenesis can reflect constituting organizations and evolutionary origins of brain areas. To determine whether a clear core-versus-shell distinction of neurogenesis that occurs from the auditory midbrain to the telencephalon in the reptile also appears in the bird, a single dose of [(3)H]-thymidine was injected into chick (Gallus gallus domesticus) eggs at some successive embryonic days (E) (from E3 to E10). Towards the end of hatching, [(3)H]-thymidine labeling was examined, and the results were as follows: 1) Neuronal generation in the nucleus intercollicularis (ICo) (shell region) began at E3, whereas neurogenesis began at E4 in the nucleus mesencephalicus lateralis pars dorsalis (MLd) (core region); 2) Neurogenesis initiated at E3 in the nucleus ovoidalis (Ov) shell, but initiated at E4 in the rostral Ov core. In the medial or caudal Ov core, the percentage of heavily-labeled neurons with [(3)H]-thymidine was significantly lower at E3 age group than that in the Ov shell; 3) In field L1 and L3, two flanking regions of the primary telencephalic auditory area (field L2a), neurogenesis started at E5, but started at E6 in field L2a. These data indicate that the onset of embryogenesis began earlier in the auditory shell areas than in the core areas from the midbrain to the telencephalon. These findings provide insight into the organization of auditory nuclei and their evolution in amniotes.

Neurogenic development of the auditory areas of the midbrain and diencephalon in the Xenopus laevis and evolutionary implications

To study whether the core-versus-shell pattern of neurogenesis occurred in the mesencephalic and diencephalic auditory areas of amniotes also appears in the amphibian, [(3)H]-thymidine was injected into tadpoles at serial developmental stages of Xenopus laevis. Towards the end of metamorphism, [(3)H]-thymidine labeling was examined and led to two main observations: 1) neuron generation in the principal nucleus (Tp) started at stage 50, and peaked at stage 53, whereas it began at stage 48.5, and peaked around stage 49 in the other two mesencephalic auditory areas, the laminar nucleus (Tl) and the magnocellular nucleus (Tmc). 2) Neuron generation appeared at stage 40, and peaked around stage 52 in the posterior thalamic nucleus (P) and the central thalamic nucleus (C). Our study revealed that, like the cores of mesencephalic auditory nuclei in amniotes, Tp showed differences from Tl and Tmc in the onset and the peak of neurogenesis. However, such differences did not occur in the P and C. Our neurogenetic data were consistent with anatomical and physiological reports indicating a clear distinction between the mesencephalic, but not the diencephalic auditory areas of the amphibian. Our data are helpful to get insights into the organization of auditory nuclei and its evolution in vertebrates.

Differences in neurogenesis differentiate between core and shell regions of auditory nuclei in the turtle (Pelodiscus sinensis): evolutionary implications

There is a clear core-versus-shell distinction in cytoarchitecture, electrophysiological properties and neural connections in the mesencephalic and diencephalic auditory nuclei of amniotes. Determining whether the embryogenesis of auditory nuclei shows a similar organization is helpful for further understanding the constituent organization and evolution of auditory nuclei. Therefore in the present study, we injected [(3)H]-thymidine into turtle embryos (Pelodiscus sinensis) at various stages of development. Upon hatching, [(3)H]-thymidine labeling was examined in both the core and shell auditory regions in the midbrain, diencephalon and dorsal ventricular ridge. Met-enkephalin and substance P immunohistochemistry was used to distinguish the core and shell regions. In the mesencephalic auditory nucleus, the occurrence of heavily labeled neurons in the nucleus centralis of the torus semicircularis reached its peak at embryonic day 9, one day later than the surrounding shell. In the diencephalic auditory nucleus, the production of heavily labeled neurons in the central region of the reuniens (Re) was highest at embryonic day (E) 8, one day later than that in the shell region of reuniens. In the region of the dorsal ventricular ridge that received inputs from the central region of Re, the appearance of heavily labeled neurons also reached a peak one day later than that in the area receiving inputs from the shell region of reuniens. Thus, there is a core-versus-shell organization of neuronal generation in reptilian auditory areas.

Distinction of neurochemistry between the cores and their shells of auditory nuclei in tetrapod species

The distribution of Met-enkephalin (ENK), substance P (SP) and serotonin (5-HT) differs between the core and shell regions of the mesencephalic and diencephalic auditory nuclei of the turtle [Belekhova et al., 2002]. These neurochemical distinctions are also found in other tetrapods (mammals, birds and amphibians). The distribution of ENK, SP and 5-HT was examined in the core and shell regions of both mesencephalic and diencephalic auditory nuclei, and in the telencephalic auditory areas of Bengalese finches (Lonchura striata) and mice (Mus musculus), as well as in corresponding auditory areas in toads (Bufo bufo). ENK, SP and 5-HT immunoreactive fibers and perikarya were largely absent from the core regions of both mesencephalic and diencephalic auditory nuclei, in comparison with the shell regions of mice and Bengalese finches. In the toad, however, this pattern was observed in the mesencephalic auditory nucleus, but not in the diencephalic auditory areas. ENK and SP immunoreactive perikarya were detected in the telencephalic auditory area of mice, whereas no ENK, SP or 5-HT immunolabeling was observed in the telencephalic auditory area (Field L) of Bengalese finches. These findings are discussed in terms of the evolution of the core-and-shell organization of auditory nuclei of tetrapods.

Immunohistochemistry and neural connectivity of the Ov shell in the songbird and their evolutionary implications

The neuropeptide immunohistochemistry and neural connectivity of areas surrounding the thalamic auditory nucleus (the nucleus ovoidalis [Ov]), as well as the areas to which it is connected, were investigated in a songbird, the Bengalese finch. The results showed that met-enkephalin was present in the Ov shell and most of the areas connected to it, but not in the Ov core. Anterograde and retrograde tracing studies showed that the Ov shell was more widely connected than the Ov core. The Ov shell was mainly connected to: 1). areas flanking the primary telencephalic auditory field (i.e., fields L2b, L1, and L3) and areas surrounding the robust nucleus of the archistriatum (RA); 2). several hypothalamic areas such as the nucleus ventromedialis hypothalami (VMN) and the nucleus anterior medialis hypothalami (AM). Some of these areas connected to the Ov shell are thought to be involved in auditory mediated neurosecretory activities. These results, which are similar to those reported previously in non-songbirds, suggest that the Ov shell and other surrounding areas of auditory and song-control nuclei are conserved in birds. These findings are discussed in terms of the evolution of the core-and-surround organization of auditory and song-control nuclei.

Daily and circadian rhythms of neurotransmitters and related compounds in the hypothalamic suprachiasmatic nuclei of a diurnal vertebrate

By using immunocytochemistry we tested whether neurotransmitters, and enzymes specific to neurotransmitters synthesis are rhythmically expressed in the suprachiasmatic nuclei of the hypothalamus of Ruin lizards Podarcis sicula either kept in light-dark cycles or constant darkness. Within the suprachiasmatic nuclei, prominent 24 h rhythms under 12:12 light-dark cycles were found for vasoactive intestinal polypeptide (VIP) and for tyrosine hydroxylase (TH). Peaks of both VIP and TH fell in the light phase of the cycle. Rhythmic expression of TH persisted under constant temperature and darkness, demonstrating the existence of circadian rhythms of TH in the suprachiasmatic nuclei. No rhythmic expression of neurotransmitters and related compounds was found in the periventricular nuclei, the supraoptic nuclei, and the rest of the hypothalamus. Our data are the first demonstration of rhythmic expression of neurotransmitters and related compounds in the suprachiasmatic nuclei of a non-mammalian vertebrate. The demonstration of a diurnal peak of VIP in a diurnal reptile-vs. nocturnal peak of VIP typical of nocturnal mammals-provides new information for comparative studies on the circadian physiology of the suprachiasmatic nuclei across vertebrate classes and their adaptation strategies to different temporal niches.