The mushroom bodies (MBs), prominent paired neuropils of the insect brain, are multimodal sensory processing centers. In the MBs of Papilio xuthus, a flower-foraging swallowtail butterfly with sophisticated color vision, the inner zone and the rim of the outer zone of the primary calyx receive prominent visual input. These two visual zones are spatially segregated from the outer zone that receives olfactory input. Sensory information is transmitted to the MB output sites (the lobes) via the pedunculus, but it remains unknown how the concentric modality organization of the calyx is transformed and represented in the spheroidal lobe neuropils in P. xuthus. To address this question, we investigated the architecture of the Papilio MBs in detail. Immunofluorescent staining and tracer injections into the MBs revealed the branching patterns of Kenyon cells (KCs), the MB intrinsic neurons, and several output pathways to other neuropils of the central brain. We newly defined the α/β, α'/β', and γ lobes of the Papilio MBs following the nomenclature system commonly used in other insects. KCs conveying visual information extend axon-like fibers to the α/β lobe (a large ventral division), while KCs conveying olfactory information send fibers to the α'/β' lobe located between the α/β and γ lobes. These parallel sensory pathways in the MBs of P. xuthus were summarized as a three-dimensional reconstruction of the MB subdivisions. Our study facilitates future physiological and functional studies of multisensory processing in the swallowtail butterfly brain.
A quantitative comparison of the absolute and relative volumes of different brain areas is useful for predicting the sensory capabilities and behavior of large pelagic teleosts, which are difficult to study in the field or in vivo. However, the size of pelagic teleost brain regions has only been approximated using the idealized ellipsoid method, which is susceptible to over- or underestimation, as it assumes the shape of brain regions to be an idealized ellipsoid or half-ellipsoid. This study examines the gross morphology and volumes of different sensory brain areas of southern bluefin tuna Thunnus maccoyii using magnetic resonance imaging (MRI). The results show that the optic tectum (568 ± 11 mm3) has a larger absolute volume compared to the olfactory bulb (50 ± 5 mm3), eminentia granularis (62 ± 9 mm3), and cristae cerebelli (47 ± 3 mm3), suggesting the significance of vision for T. maccoyii. The full segmentation of a T. maccoyii brain allowed the quantification of the integration areas, which reveals that the corpus cerebelli (1299 mm3) occupies the largest proportion (35%) of total brain volume, whereas the optic tectum only occupies 15% of total brain volume. The corpus cerebelli also exhibits a rostro-caudal elongation with multiple horizontal sulci, which resemble the corpus cerebelli of some species of sharks. The results reveal that the brain of T. maccoyii is dominated by the locomotive area of the corpus cerebelli and highlight the benefits of using MRI when performing quantitative analyses on the brain volumes of large pelagic teleosts.
The perirhinal and parahippocampal cortices are two prominent structures of the medial temporal lobe that play essential roles in memory and perceptual processes. In humans, major changes in memory capacities occur within the first 7 years of life, but the neurobiological substrates underlying these changes have long been hypothetical. Previous studies have shown that distinct regions, layers, and cells of the hippocampal formation, including the entorhinal cortex, exhibit different profiles of structural and molecular development. Here, to further understand the postnatal maturation of the medial temporal lobe, we implemented stereological techniques to characterize the structural development of the perirhinal and parahippocampal cortices in macaque monkeys. We found distinct, age-related differences in volume, neuronal soma size, and neuron number in different layers and subdivisions. Volumetric data indicated a late maturation of areas 36r and 36c compared to areas 35, TF, and TH. There was also an earlier maturation of the superficial layers compared to the deep layers in areas 36r and 36c. We observed a transient increase in neuronal soma size at 6 months of age in several subdivisions. Additionally, we found a decrease in neuron numbers in both the perirhinal and parahippocampal cortices, but particularly in area 35 and layer III of area TF between birth and 6 months. These findings are consistent with the differential maturation of the rostral and caudal entorhinal cortex, which are interconnected with the perirhinal and parahippocampal cortices, respectively. Altogether, they support the theory that the differential maturation of distinct hippocampal circuits underlies the emergence of specific "hippocampus-dependent" memory processes.
The subgenual organ complex is an elaborate mechanosensory complex in the insect leg containing chordotonal organs. In stick and leaf insects (Phasmatodea), it includes the subgenual organ and the distal organ. This study documents the neuroanatomy and functional morphology of the subgenual organ complex in the stick insect Bacillus rossius (Bacillidae: Bacillinae) by axonal tracing and micro-computed tomography. It also considers the first report on the subgenual organ complex in stick insects that reported a relatively simple organization of the sensory organ and its nerves on the basis of histological sections. Our findings show the neuroanatomy and nerve pattern of the subgenual organ complex in B. rossius with a subgenual organ and a distal organ. The subgenual organ is placed in the hemolymph channel. The distal organ is also located in the hemolymph channel, and it has several attachment elements, linking it to the cuticle of the tibia, the tibial tracheae, and the subgenual organ. The connections to the tibia may form an input pathway for vibrations transmitted over the cuticle, whereas the position in the hemolymph channel and the connection to the subgenual organ indicate a mechanical activation by vibrations transmitted via the hemolymph. Overall, the axonal tracing preparations document neuroanatomical details for B. rossius and resolve the numbers of sensilla in the sensory organs, the length of the distal organ, and confirm a single nerve branch for the subgenual organ. The data provide support for a consistent organization of the subgenual organ complex within stick insects.
To navigate its environment, an animal extracts salient information from sounds using temporal and intensity cues. In birds, the nucleus laminaris (NL) detects the submillisecond differences in the arrival time of sound to the two ears, the interaural time differences (ITDs), to localize sounds. This ability is facilitated by inhibitory long-range projection neurons from the ipsilateral superior olivary nucleus (SON) that enable NL neurons to remain sensitive to ITDs across a large range of sound intensities. It is well known that the excitatory inputs to NL, from nucleus magnocellularis (NM), innervate a narrow isofrequency band along the ITD axis. However, the organization of the inhibitory input from the SON remains largely unknown. We analyzed the innervation pattern of individual axons from SON neurons within the chicken NL. SON axonal arborizations vary greatly in size and topographic organization. On average, an inhibitory SON neuron innervates one-third of both the tonotopic and ITD axes, markedly larger target regions than do the excitatory inputs from NM. Unlike the excitatory axons that are confined to one dendritic lamina (separating inputs from the two ears), most SON cells innervate both laminae to similar extents, as well as the somata of the NL neurons. In addition, we found that some NL-projecting SON neurons also send collateral axons to NM or the nucleus angularis. The pattern of synapses along SON axons suggests that the inhibitory activity of individual NL neurons is shaped by many SON neurons. A single SON neuron contributes only a small proportion of the inhibition on each NL neuron. This broad innervation pattern of SON neurons is well-suited to control the overall activity of NL, supporting accurate ITD detection in a broad range of sound environments.
The primate anterior entorhinal cortex (EC) receives rich projections from the amygdala and from multimodal association areas, including the medial prefrontal, anterior cingulate, and orbitofrontal cortices. Axon terminations from these structures on the anterior EC facilitate processing of the emotional aspects of stimuli and events. The EC projects to hippocampus, which is associated with episodic memory. Processing in the anterior EC is modulated by inhibitory neurons, which in primates express the calcium-binding proteins (CBPs): calretinin (CR), or calbindin (CB) or parvalbumin (PV), which collectively account for most inhibitory neurons in the primate cortex. Here, stereological analysis of these neurochemical classes of inhibitory neurons in the anterior half of EC in humans revealed similar patterns as in rhesus monkeys. In both primate species, the densest neuronal subpopulation of presumed inhibitory neurons expressed CR, followed by CB, and lastly by PV. In both species CR neurons were most prevalent in layers I and II, CB neurons in layers II and III and PV neurons in the middle-deep layers. Moreover, the medial and lateral sectors of the anterior EC had different densities of neurons expressing these CBPs. Further analysis revealed that in the human anterior EC, virtually all PV neurons expressed the GABAergic marker GAD67/GAD1 (glutamate decarboxylase 67/glutamate decarboxylase 1), whereas only two-thirds of CB neurons and only one-third of CR neurons colocalized with GAD67/GAD1. In the entire neuronal population of the anterior half of human EC estimated by stereology, 10% expressed GAD67/GAD1, comparable to the collective population of CBP-positive neurons that colocalized with GAD67/GAD1. These findings reveal that the medial and lateral sectors of anterior EC have distinct inhibitory microenvironments, which likely affect the processing of input and output of hippocampus.
The trigeminal nerve consists of three major branches or rami (r) in jawed vertebrates: the ophthalmic (rV1), maxillary (rV2), and mandibular (rV3), with rV2 and rV3 innervating the upper and lower jaws, respectively. This three-branch configuration is conserved in the lamprey, a representative of the cyclostomes, the sole survivor of jawless vertebrates. While the first branch is widely accepted to be homologous to that of jawed vertebrate rV1, the homology of the other two remains unclear, so they are called rV1, rV2/3A, and rV2/3B. In this study, we investigate evolutionary modifications of the trigeminal nerve by comparing its organization in lamprey and jawed vertebrate embryos. By detailed neuronal tracing analysis of rV2/3B, which innervates the velum, we demonstrate that it consists solely of motor components. Conversely, we identify both sensory and motor fibers in rV2/3A. These results make a striking contrast to the trigeminal nerve of the jawed vertebrates, as their rV2 is purely sensory, but rV3 is a mixed nerve. Furthermore, through comparative analyses of sensory projection inside the brain, we clarify that the principal sensory nucleus of the trigeminal nerve (PrV) of anamniotes (shark, catfish, and Xenopus) lacks a somatotopic organization, suggesting that somatotopy evolved in at least the last common ancestor of amniotes. Last, by gene expression analysis of Drg11, a marker for the PrV and the spinal trigeminal nucleus (SpV), we show that a PrV-equivalent region is not present in the lamprey. These findings indicate significant structural changes in the trigeminal nerve associated with jaw acquisition, including shifts in innervation territories, the disappearance of the branch innervating the velum, and the emergence of novel brain nuclei.
We examined the distribution of axons throughout the spinal cord of the rat that were either immunoreactive for the adrenaline-synthesizing enzyme, phenylethanolamine N-methyltransferase (PNMT), or derived from medullary C1 neurons, one of the three groups of neurons in the brain that synthesize PNMT. We observed that PMNT-immunoreactive axons, as well as C1 axons labelled with GFP from viral transduction, innervate most, but not all, sympathetic preganglionic neurons in the thoracolumbar spinal cord. GFP-positive C1 axons provided innervation to sympathetic preganglionic neurons that expressed cocaine and amphetamine regulated transcript, an accepted marker of sympathetic vasomotor neurons. In addition, we observed axons from PNMT-containing and C1 neurons caudal to the distribution of sympathetic preganglionic neurons in the sacral spinal cord where they closely apposed parasympathetic preganglionic neurons retrogradely labelled from the major pelvic ganglion. We also found close appositions from PNMT-immunoreactive or GFP-labelled C1 axons on choline acetyltransferase-stained parasympathetic preganglionic neurons activated by the micturition reflex, thus providing clear evidence of a non-cardiovascular target for RVLM C1 neurons. Furthermore, we observed a few PNMT-positive and GFP-positive C1 axons making close appositions with somatic motor neurons in Onuf's nucleus in the sacral cord and in the ventral horn at more rostral levels. These data provide a comprehensive map of the distribution of adrenergic inputs to the spinal cord and identify parasympathetic preganglionic neurons, including those involved in the micturition reflex, as well as sympathetic preganglionic neurons as the major targets for these inputs.
Octopuses have an incredibly rich behavioral repertoire, exhibiting complex motor acts that require the coordination of eight highly flexible arms, each with hundreds of suckers. These movements are controlled by an axial nerve cord (ANC), analogous to the spinal cord, situated in the center of the arm musculature. The ANC has a cell body layer which forms a U-shape around its neuropil and is capped aborally, or opposite the sucker, by the cerebrobrachial tract (CBT), a massive fiber bundle known to interconnect the arms and the brain. In vertebrate spinal cords, in addition to the major fiber tracts that interconnect the brain and spinal cord, there are spinospinal connectives that coordinate complex motor behaviors across the appendages. Here, we asked with tract-tracing and immunohistochemistry, whether an octopus arm's ANC might also have intrinsic longitudinal connections for coordinated arm and sucker movements. We found that the ANC neuropil is enriched in longitudinal fibers. These fibers form distinct tracts, two within the oral (sucker-side) neuropil and two in the aboral (brachial-side) neuropil. In addition, the CBT itself demonstrates four major subtracts, and DiI labeling and dextran tracing suggest that (1) the CBT also carries arm-intrinsic longitudinal connections and (2) the CBT and the neuropil tracts can be subcategorized into those that primarily connect with the sucker and those that serve the arm musculature. We also examined the organization of fiber tracts in the ANC of the arms and tentacles of two species of squid, establishing that an aboral, extra-neuropil tract is a shared feature across all cephalopod species studied. In addition, the squids have an oral longitudinal tract, though its positioning and size varied with species and appendage. In sum, these findings describe the neural substrate for coordinating motor behaviors along the length of a cephalopod appendage.
The efficient processing of visual information relies on a mature visual cortex, characterized by hierarchically organized areas and a broad diversity of inhibitory and excitatory neurons. A tightly regulated excitation-inhibition (E/I) balance is essential for the optimal processing of visual inputs. A known regulator of the cortical E/I balance is the endocannabinoid (ECB) system, which relies on the cannabinoid type-1 receptor (CB1R) to perform its functions. To better understand the embedding of the CB1R in the neuronal networks of the visual cortex in adolescence and adulthood, we characterized the distribution of the receptor protein and its mRNA (cnr1) across layers, areas, and interneuron subtypes. We describe a specific laminar distribution of CB1R in the mature visual cortex along the full extent of the rostrocaudal brain axis. Moreover, cnr1 is expressed in the three main nonoverlapping subtypes of interneurons and is predominantly enriched in the 5ht3ar subtypes. Comparison of adolescent and adult visual cortex revealed a higher number of cnr1+ reelin interneurons in layer 1 and a lower number of cnr1+ somatostatin interneurons in layer 4 of the primary visual cortex (V1) in adolescence compared with adulthood. Overall, our findings confirm a distinct distribution of the receptor in V1 compared with higher-order visual areas based on a lower CB1R expression in layer 4, a broad cnr1 expression across cortical interneurons in key locations of top-down modulation, and a still immature ECB system in adolescence, making it potentially vulnerable to exogenous cannabinoids during this life period.
The neurotransmitters vasotocin, oxytocin, dopamine, and serotonin are widely involved in vertebrate social behavior, and changes in their abundance and distribution in the brain have been linked to the evolution of complex sociality. Reptiles provide an excellent system in which to investigate the neural mechanisms of social living. Using immunohistochemistry, we compare distributions of these transmitters in two skinks differing primarily in social ecology: the family-living Liopholis whitii and the solitary Eulpamrus quoyii. We describe patterns of immunopositive signal for both cell bodies and fibers across the entire brain (excluding the olfactory bulbs). In both species, vasotocin and oxytocin were found in the preoptic area, paraventricular nucleus, supraoptic nucleus, dorsomedial hypothalamus, and supraoptic decussation, as well as surrounding the lateral forebrain bundle. Tyrosine hydroxylase (a marker for dopamine) was found in the paraventricular organ nucleus, substantia nigra, and ventral tegmental area, and serotonin was found in the raphe nuclei and superior reticular field. We found novel oxytocin cell groups in the dorsomedial hypothalamus and cerebellum of L. whitii, and novel serotonin signal in the red nucleus of E. quoyii. Immunopositive signals found only in L. whitii also include vasotocin in the ventral tegmental area, tyrosine hydroxylase in the interpeduncular nucleus, and serotonin in the suprachiasmatic nucleus. The qualitatively greater abundance of these transmitters in the family-living L. whitii suggests that these molecules may have played an important role in the evolution of social behavior in these skinks and provides a foundation for broader comparisons across the social skinks.
The ascending sensory projections from the dorsal thalamus to the pallium are one of the most distinctive features of the amniote brain. In mammals, these projections can be broadly classified into two main types: (1) those arising from "core" neurons, mostly located in the central region of sensory nuclei, which relay topographic inputs to the intermediate layers of primary sensory cortices, forming a reciprocal circuit with the reticular thalamic nucleus (RTN); and (2) those originating from "matrix" neurons, located in the periphery of sensory nuclei, which project to superficial layers of primary and higher order cortices. To date, both the reciprocal core-RTN circuitry and the matrix projections have only been described in mammals. In this study, we re-examined the organization of thalamic projections to the visual dorsal ventricular ridge (DVR) in pigeons using neural tracing techniques. In addition to the well-known projection from the thalamic nucleus rotundus (Rt) to the ventral layer of the visual DVR (entopallium), we described a reciprocal projection pattern between the Rt and the RTN. Moreover, we identified a second ascending thalamopallial projection, arising from neurons surrounding the Rt and terminating primarily in the intermediate layer of the visual DVR. These neurons form a continuous "matrix" of cells within the dorsal thalamus, which as a group also give rise to axons targeting several associative pallial areas. Furthermore, we found that this "matrix" region receives descending inputs from the motor arcopallium. Collectively, these findings indicate that the avian thalamopallial system is more complex than previously recognized and thus more comparable to its mammalian counterpart.
The mesodiencephalic junction (MDJ) is located in a caudorostrally elongated, column-shaped region encompassing the medial accessory oculomotor nucleus and perirubral area. It relays cerebral projections to the rostral subnuclei of the inferior olive (IO), which, in turn, send climbing fibers to the cerebellar hemisphere and receive nucleo-olivary projections from the cerebellar nuclei (CN), forming a parallel, loop-shaped, modular circuitry. However, the topographical organization of the MDJ-IO projection, which may contribute to functional localization within the cerebellar hemisphere, has not been fully elucidated. Here, we analyzed the distribution of labeled neurons in the MDJ in relation to those in the CN by injecting a Lumafluor retrograde tracer into various sites within the rostral IO subnuclei of mice. Injections mainly targeting the lateral or medial parts of the rostral IO subnuclei labeled neurons predominantly in the ventral or dorsal CN, respectively (referred to as v- and d-CN-predominant injection cases). Correspondingly, v-CN-predominant injection cases labeled neurons in the rostromedial, centroventral, and caudolateral regions of the MDJ, collectively termed the "sheathing subarea," whereas d-CN-predominant injection cases labeled neurons primarily in the central "shaft subarea" of the MDJ. Considering the previously reported lobule-related topography among the IO, cerebellar cortex, and CN, the sheathing and shaft subareas of the MDJ are likely to project preferentially to crus I and to other neighboring cerebellar hemispheric lobules, respectively. We speculate that the sheathing subarea of the MDJ is more strongly involved in non-somatosensorimotor cognitive functions than the shaft subarea.
The endocranial anatomy of oviraptorosaurian theropods, and their inferred sensory adaptations, has played a key role in understanding the origin of bird-like senses among avian ancestors. However, within oviraptorosaurs, sampling has thus far been limited, and a major gap exists in our understanding of the endocranial anatomy of an enigmatic subclade, Caenagnathidae. Here, we describe the inner ear morphology of three caenagnathid oviraptorosaurs, spanning a wide range of stratigraphy and body size. Our data show that the incipiently bird-like inner ears of caenagnathids were relatively similar both over the course of >10 million years and over multiple orders of magnitude of body mass. Overall, the caenagnathids sampled here show similarities to other maniraptoran theropods and particularly other oviraptorosaurs, but with some key differences that possibly speak to behavioral differences in these groups. In particular, a reduced cochlear duct in the most complete labyrinth suggests restricted hearing sensitivity in caenagnathids, which is at odds with previous suggestions that they were more predatory than oviraptorids. Our results highlight the independent acquisition of some bird-like features along the avian stem and show that late-diverging members of these clades may exhibit divergent features that do not characterize the diversity within the entire group.
The central complex comprises an assemblage of midline-spanning neuropils in the brain of insects that play a key role in goal-directed orientation and navigation vector calculation. The central complex consists of layers of tangential input neurons that contact topographically organized columnar neurons which provide outputs to the right and left brain hemispheres. Its anatomical organization and functional role are regarded as highly conserved across insects. In addition to classical neurotransmitters, a wide range of neuropeptides have been detected in the central complex including peptides of the tachykinin family. Because the cellular identity of tachykinin-containing neurons in the central complex has not been determined in most cases, we used antisera against tachykinin I and II from the migratory locust, termed Lom-TKs, to identify the immunolabeled neurons in hexapods ranging from flightless two-pronged bristletails to flies. The data show that LomTK-related peptides are present in the central complex of all studied species except crickets. In most species one or several types of columnar neurons were immunolabeled, sometimes together with certain subsystems of tangential input neurons. The types of immunolabeled columnar neurons, however, are distinctly different between species from different orders, in some cases even between insects within the same order, and comprise cell types that innervate either the upper or lower division of the central body. This high degree of evolutionary divergence of tachykinin-positive neurons, even in closely related groups, may be related to species-specific differences in navigational requirements and calls for caution with respect to homologizing neurons across clades.
While spiders are known for their tactile and vibratory senses, their sense of smell has not been well researched at all. Only relatively few behavioral studies have been published showing that spiders can perceive pheromones and may be able to smell prey or predators. Less is known about the nature of their olfactory neurons and sense organs (sensilla), and nothing is known about how and where olfactory information is processed in the spider's central nervous system. This comparative anatomical study is the first to describe presumed chemosensory glomeruli in the central nervous system of spiders. Such glomeruli are the anatomical hallmark of olfactory processing in other taxa. This study finds presumed olfactory glomeruli residing ventrally in each neuromere of the legs and pedipalps (feelers) of all spider families examined. Unlike olfactory glomeruli in most insects or vertebrates, in spiders, these presumed chemosensory glomeruli are often elongated rather than spherical. Anterograde tracing of neurons originating from chemosensory sensilla on the legs shows that these presumed chemosensory glomeruli are supplied by primary chemosensory afferents, at least some of which may supply more than one glomerulus. In addition, a previously described central structure, referred to as the Blumenthal neuropil and presumed to receive hygro- and thermo-sensory input, is here shown to also comprise glomerular components. One or more of these central Blumenthal glomeruli are particularly large in male spiders. In analogy to insect macroglomeruli, this suggests that these glomeruli might be involved in processing pheromone information. These findings are discussed with reference to insect and vertebrate olfactory information processing.
Projections from widefield vertical (WFV) cells of the superior colliculus to the pulvinar nucleus form a conserved pathway for motion detection across mammalian species. While subtypes of tectopulvinar cells have been identified in the ground squirrel, it is unclear whether subtypes can be identified in mice. Using viral tracing, immunohistochemistry, and confocal and electron microscopy in Tac1, Ntsr1-GN209, and C57BL/6J mice, we attempted to characterize WFV cells based on their morphology, pulvinar projection patterns, expression of substance P, parvalbumin, or hyperpolarization-activated cation channels (HCN1), and their retinal innervation. We found that the Ntsr1-GN209 line labels a subset of tectopulvinar cells that lack parvalbumin but express HCN1. Furthermore, comparison of previous ultrastructural datasets revealed that Ntsr1-GN209 WFV cells innervate the pulvinar with synaptic terminals that are smaller than the overall population of tectopulvinar terminals. However, using retrograde, anterograde, and transsynaptic viral tracing, we found that in all three mouse lines, WFV cells could not be subdivided based on their dendritic arbors or projections to the lateral pulvinar (Pl) or caudal medial pulvinar (Pcm). We also found that a subset of WFV cells receive ipsilateral retinal input, but these cells did not differentially innervate the Pl or Pcm. Finally, we found that the retina provides one-third of the synaptic input to Ntsr1-GN209 WFV cells and primarily innervates their small distal dendrites. Together, our data indicate that although the Ntsr1-GN209 line labels a subset, the overall population of mouse WFV cells cannot be subdivided based on features identified in the ground squirrel.
The neurotransmitter serotonin modulates a variety of behavioral and physiological responses in the brain. Serotonergic neurons from the dorsal raphe nuclei send a dense projection to the auditory system, including the inferior colliculus (IC), the midbrain hub of the central auditory system. In the IC, serotonin alters how neurons respond to complex sounds, and it has been implicated in the generation or perception of tinnitus. However, the distribution of serotonin receptors and the identity of neurons that express serotonin receptors in the IC remain unclear. Here, we hypothesized that IC GABAergic and glutamatergic neurons differentially express serotonin receptors. To test this hypothesis, we performed in situ hybridization in IC brain slices of male and female mice using probes for Vgat (GABAergic neuron marker) and Vglut2 (glutamatergic neuron marker), along with probes for six subtypes of metabotropic serotonin receptors: 5-HT1A and 5-HT1B (Htr1a and Htr1b, inhibitory, Gi/o G protein receptors), 5-HT2A, 5-HT2B, and 5-HT2C (Htr2a, Htr2b, and Htr2c excitatory, Gq11 G protein receptors), and 5-HT7 (excitatory, Gs G protein receptors). Our data show that glutamatergic IC neurons primarily express inhibitory serotonin receptors. In contrast, a larger proportion of GABAergic neurons express excitatory serotonin receptors. Our data suggest that serotonin likely exerts an inhibitory net effect on IC neuronal circuits. These findings contribute to our understanding of how serotonin signaling influences auditory processing. The differential expression of serotonin receptors may help shape the balance of excitation and inhibition in the auditory midbrain, affecting sound processing.
The length of the spinal cord differs from that of the vertebral column in vertebrates, and their segmental positional relationship exhibits significant variability and complexity. However, research on the spinal cord structure of the Korean water deer and Siberian roe deer remains unexplored. This study aimed to investigate the anatomical structure of spinal cord segments and their association with vertebrae in these species, as well as to compare the findings with those from previously studied species. Morphometric analyses of the spinal cords and vertebrae were conducted using various measurement parameters. Notably, the cervical enlargement extended from C6 to T1, and the lumbar enlargement from L4 to S1, with significant decreases in spinal segment length observed in these regions. Transitions were noted around these enlargements, including shifts from caudal to cranial in spinal segments relative to vertebral segments and from acute to more obtuse trapezoidal shapes in dorsal rootlets. We propose that reduced growth rates at enlargements determine the positional relationship between the spinal cord and vertebrae. Since this study provides the first anatomical characterization of the spinal cord in these two deer species, our findings advance our understanding of spinal cord structure in Cervidae. Furthermore, through comparative analysis with previous studies, these findings elucidate the segmental positional relationship between the spinal cord and vertebral column, providing a foundation for further morphological and embryological studies across various species.
Although dopamine receptors (DARs) have been identified in various vertebrate neural circuits, their expression in the central auditory system remains poorly characterized. Reproductive-state changes in catecholamine innervation of central auditory nuclei in the plainfin midshipman fish (Porichthys notatus) highlight a unique, ethologically relevant role for catecholamines, including dopamine, in modulating auditory function to enhance reproductive success. Dopamine's effects on these systems are mediated via its receptors; thus, the goal of the present study is to characterize excitatory dopamine D1Aa and inhibitory dopamine D2a receptor transcript expression throughout the central auditory system of the plainfin midshipman. Fluorescence in situ hybridization-immunohistochemistry (FISH-IHC) revealed robust D1Aa and D2a expression in forebrain auditory-recipient centers that receive catecholaminergic input: postcommissural and ventral nuclei of the ventral telencephalon, anterior tuberal nucleus, central posterior nucleus of the thalamus, and parvocellular preoptic nuclei. Large dopamine neurons in the periventricular posterior tuberculum, which are, in part, responsible for the reproductive-state changes in central and peripheral catecholamine innervation only express D2a, whereas large noradrenergic neurons in the locus coeruleus express both D1Aa and D2a. The midbrain torus semicircularis, periaqueductal gray, hindbrain octavolateralis efferent nucleus, and descending/secondary octaval nuclei also express both receptor types. D2a expression predominates over D1Aa, and we identify a subpopulation of cells throughout the auditory system that co-express both receptors. The robust distribution of inhibitory and excitatory DAR expression in the central auditory system, coupled with co-expression in a subset of cells, provides strong neuroanatomical evidence of dopamine's complex role in modulating auditory sensitivity and processing.