Cytoarchitecture and neurocytology of rabbit cingulate cortex

Altered pain sensitivity in 5×familial Alzheimer disease mice is associated with dendritic spine loss in anterior cingulate cortex pyramidal neurons

ABSTRACT

Chronic pain is highly prevalent. Individuals with cognitive disorders such as Alzheimer disease are a susceptible population in which pain is frequently difficult to diagnosis. It is still unclear whether the pathological changes in patients with Alzheimer disease will affect pain processing. Here, we leverage animal behavior, neural activity recording, optogenetics, chemogenetics, and Alzheimer disease modeling to examine the contribution of the anterior cingulate cortex (ACC) neurons to pain response. The 5× familial Alzheimer disease mice show alleviated mechanical allodynia which can be regained by the genetic activation of ACC excitatory neurons. Furthermore, the lower peak neuronal excitation, delayed response initiation, as well as the dendritic spine reduction of ACC pyramidal neurons in 5×familial Alzheimer disease mice can be mimicked by Rac1 or actin polymerization inhibitor in wild-type (WT) mice. These findings indicate that abnormal of pain sensitivity in Alzheimer disease modeling mice is closely related to the variation of neuronal activity and dendritic spine loss in ACC pyramidal neurons, suggesting the crucial role of dendritic spine density in pain processing.

The retrosplenial cortex of Carollia perspicillata, Seba's short-tailed fruit bat

Retrosplenial cortex (RSC) is a brain region involved in critical cognitive functions including memory, planning, and spatial navigation and is commonly affected in neurodegenerative diseases. Subregions of RSC are typically described as Brodmann areas 29 and 30, which are defined by cytoarchitectural features. Using immunofluorescence, we studied the distributions of neurons immunoreactive for NeuN, latexin, and calcium binding proteins (calbindin, calretinin, and parvalbumin) in RSC of Carollia perspicillata, Seba's short-tailed fruit bat. We observed that latexin was specifically present in areas 29a and 29b but not 29c and 30. We further identified distribution patterns of calcium binding proteins that group areas 29a and 29b separately from areas 29c and 30. We conclude first that latexin is a useful marker to classify subregions of RSC and second that these subregions contain distinct patterns of neuronal immunoreactivity for calcium binding proteins. Given the long lifespan of Carollia, bat RSC may be a useful model in studying age-related neurodegeneration.

The Paracingulate Sulcus Is a Unique Feature of the Medial Frontal Cortex Shared by Great Apes and Humans

Primate brains display a wide range of variation in size and cerebral gyrification, leading to the appearance of novel sulci in particular groups of species. We investigated sulcal organization in the medial frontal cortex of great apes, with a particular focus on the paracingulate sulcus (PCGS). Until recently, the presence of the PCGS was thought to be a structural feature unique to the human brain. However, upon closer examination, the PCGS has been observed as a variable feature that also may appear in chimpanzee brains. To understand the evolutionary origins of the sulcal anatomy in the medial frontal cortex of apes, we examined high-resolution MRI scans for the presence or absence of the PCGS and, when present, measured its length in a sample of ape brains (chimpanzees, bonobos, gorillas, orangutans, gibbons, and siamangs). We found that the PCGS is variable in its appearance among these species, being present in 23 to 50% of great ape individuals depending on the species, but not present in gibbons or siamangs. We did not find population level hemispheric lateralization patterns or sex differences in PCGS presence across species, and we did not detect a relationship between cerebral volume and PCGS occurrence or length. Our data suggest that the PCGS is a common sulcal variant present in great apes and humans due to a shared evolutionary ancestry.

Distribution of neuronal structures immunoreactive for parvalbumin in the midcingulate cortex of the rabbit

The rabbit midcingulate cortex that enclosed four cortical areas was immunohistochemically studied using a calcium-binding protein, parvalbumin, as a neurochemical marker. The distribution of parvalbumin-immunopositive somata and fibres was similar across all four areas, where they were present mainly in layers 2/3 and 5. However, there were a slightly greater number of the immunopositive structures in the two ventral areas than the two dorsal areas. Similarity in the distribution of parvalbumin-immunopositive structures across the four areas suggests that neurons expressing parvalbumin may be involved in similar functions across the constituent areas of the rabbit midcingulate cortex.

Activation of the Intrinsic Pain Inhibitory Circuit from the Midcingulate Cg2 to Zona Incerta Alleviates Neuropathic Pain

Neuropathic pain is one of the most common and notorious neurological diseases. The changes in cerebral structures after nerve injury and the corresponding contributions to neuropathic pain are not well understood. Here we found that the majority of glutamatergic neurons in the area 2 of midcingulate cortex (MCC Cg2^Glu) were inhibited by painful stimulation in male mice. Optogenetic manipulation revealed that these neurons were tonically involved in the inhibitory modulation of multimodal nociception. We further identified the projections to GABAergic neurons in the zona incerta (ZI^GABA) mediated the pain inhibitory role. However, MCC Cg2^Glu became hypoactive after nerve injury. Although a brief activation of the MCC Cg2^Glu to ZI^GABA circuit was able to relieve the aversiveness associated with spontaneous ongoing pain, consecutive activation of the circuit was required to alleviate neuropathic allodynia. In contrast, glutamatergic neurons in the area 1 of MCC played opposite roles in pain modulation. They became hyperactive after nerve injury and only consecutive inhibition of their activity relieved allodynia. These results demonstrate that MCC Cg2^Glu constitute a component of intrinsic pain inhibitory circuitry and their hypoactivity underlies neuropathic pain. We propose that selective and persistent activation of the MCC Cg2^Glu to ZI^GABA circuit may serve as a potential therapeutic strategy for this disease.SIGNIFICANCE STATEMENT Glutamatergic neurons in the area 2 of midcingulate cortex (MCC Cg2^Glu) are tonically involved in the intrinsic pain inhibition via projecting to GABAergic neurons in the zona incerta. They are hypoactive after nerve injury. Selective activation of the circuit compensates the reduction of its analgesic strength and relieves neuropathic pain. Therefore, MCC Cg2^Glu and the related analgesic circuit may serve as therapeutic targets for neuropathic pain. In contrast, MCC Cg1^Glu have an opposite role in pain modulation and become hyperactive after nerve injury. The present study provides novel evidence for the concept that neuropathic pain is associated with the dysfunction of endogenous pain modulatory system and new perspective on the treatment of neuropathic pain.

Distribution of calretinin immunopositive somata and fibers in the rabbit midcingulate cortex

The midcingulate cortex (MCC; area 24’) resides in the mid-rostrocaudal part of the cingulate gyrus, and it plays important roles in nociceptive, cognitive and skeletomotor functions. The MCC has recently been shown to consist of four cortical areas (areas a24a’, a24b’, p24a’ and p24b’) in the rabbit, based on immunohistochemistry. To further characterize the organization of these areas, here we immunohistochemically identified structures immunopositive (+) for calretinin (CR) as a marker of a subpopulation of inhibitory neurons. CR+ somata were identified as multipolar and bipolar neurons. The multipolar neurons were predominant throughout the MCC. CR+ somata were present mainly in layer (L) 2/3 and L6, and CR+ fibers occurred mainly in L1, L2/3 and L6. However, there were differences in the distribution of CR+ structures in each area. CR+ somata tended to be most densely distributed in area a24a’, followed by area p24a’, area a24b’ and area p24b’. CR+ fibers were most densely distributed in area p24a’, followed by area p24b’, area a24a’ and area a24b’. In addition, only areas p24a’ and p24b’ enclosed patchy CR+ fibers and terminals in deep L2/3. These results show the distinct distribution of CR+ structures in each area of the MCC in the rabbit, suggesting that CR+ neurons may contribute to information processing for cognitive functions in somewhat different manners in each area of the MCC.

Brain anatomy of the 4-day-old European rabbit

The European rabbit (Oryctolagus cuniculus) is a widely used model in fundamental, medical and veterinary neurosciences. Besides investigations in adults, rabbit pups are relevant to study perinatal neurodevelopment and early behaviour. To date, the rabbit is also the only species in which a pheromone - the mammary pheromone (MP) - emitted by lactating females and active on neonatal adaptation has been described. The MP is crucial since it contributes directly to nipple localisation and oral seizing in neonates, i.e. to their sucking success. It may also be one of the non-photic cues arising from the mother, which stimulates synchronisation of the circadian system during pre-visual developmental stages. Finally, the MP promotes neonatal odour associative and appetitive conditioning in a remarkably rapid and efficient way. For these different reasons, the rabbit offers a currently unique opportunity to determine pheromonal-induced brain processing supporting adaptation early in life. Therefore, it is of interest to create a reference work of the newborn rabbit pup brain, which may constitute a tool for future multi-disciplinary and multi-approach research in this model, and allow comparisons related to the neuroethological basis of social and feeding behaviour among newborns of various species. Here, in line with existing experimental studies, and based on original observations, we propose a functional anatomical description of brain sections in 4-day-old rabbits with a particular focus on seven brain regions which appear important for neonatal perception of sensory signals emitted by the mother, circadian adaptation to the short and single daily nursing of the mother in the nest, and expression of specific motor actions involved in nipple localisation and milk intake. These brain regions involve olfactory circuits, limbic-related areas important in reward, motivation, learning and memory formation, homeostatic areas engaged in food anticipation, and regions implicated in circadian rhythm and arousal, as well as in motricity.

A nociceptive stress model of adolescent physical abuse induces contextual fear and cingulate nociceptive neuroplasticities

Adolescent physical abuse impairs emotional development and evokes cingulate pathologies, but its neuronal and circuit substrates are unknown. Conditioning adolescent rabbits with noxious colorectal distension for only 2 h over 3 weeks simulated the human child abuse in amplitude, frequency, and duration. Thermal withdrawal thresholds were unchanged suggesting that sensitized spinal mechanisms may not be operable. Unchanged weight, stools, colorectal histology, and no evidence of abdominal pain argue against tissue injury or irritable bowel syndrome. Contextual fear was amplified as they avoided the site of their abuse. Conditioning impacted anterior cingulate and anterior midcingulate (ACC, aMCC) neuron excitability: (1) more neurons responded to cutaneous and visceral (VNox) noxious stimuli than controls engaging latent nociception (present but not manifest in controls). (2) Rear paw stimulation increased responses over forepaws with shorter onsets and longer durations, while forepaw responses were of higher amplitude. (3) There were more VNox responses with two excitatory phases and longer durations. (4) Some had unique three-phase excitatory responses. (5) Long-duration VNox stimuli did not inhibit neurons as in controls, suggesting the release of an inhibitory circuit. (6) aMCC changes in cutaneous but not visceral nociception confirmed its role in cutaneous nociception. For the first time, we report neuroplasticities that may be evoked by adolescent physical abuse and reflect psychogenic pain: i.e., no ongoing peripheral pain and altered ACC nociception. These limbic responses may be a cognitive trace of abuse and may shed light on impaired human emotional development and sexual function.