Secondary Neurons Research Articles

Suppression of the Excitability of Nociceptive Secondary Sensory Neurons Following Systemic Administration of Astaxanthin in Rats

Although astaxanthin (AST) has demonstrated a modulatory effect on voltage-gated Ca2+ (Cav) channels and excitatory glutamate neuronal transmission in vitro, particularly on the excitability of nociceptive sensory neurons, its action in vivo remains to be determined. This research sought to determine if an acute intravenous administration of AST in rats reduces the excitability of wide-dynamic range (WDR) spinal trigeminal nucleus caudalis (SpVc) neurons in response to nociceptive and non-nociceptive mechanical stimulation in vivo. In anesthetized rats, extracellular single-unit recordings were carried out on SpVc neurons following mechanical stimulation of the orofacial area. The average firing rate of SpVc WDR neurons in response to both gentle and painful mechanical stimuli significantly and dose-dependently decreased after the application of AST (1–5 mM, i.v.), and maximum suppression of discharge frequency for both non-noxious and nociceptive mechanical stimuli occurred within 10 min. These suppressive effects persisted for about 20 min. These results suggest that acute intravenous AST administration suppresses the SpVc nociceptive transmission, possibly by inhibiting Cav channels and excitatory glutamate neuronal transmission, implicating AST as a potential therapeutic agent for the treatment of trigeminal nociceptive pain without side effects.

Transfer with microbiota from lean donors prevents excessive weight gain and restores gut-brain vagal signaling in obese rats maintained on a high fat diet.

The collection of microorganisms, mainly bacteria, which live in the gastrointestinal (GI) tract are collectible known as the gut microbiota. GI bacteria play an active role in regulation of the host's immune system and metabolism, as well as certain pathophysiological processes. Diet is the main factor modulating GI microbiota composition and recent studies have shown that high fat (HF) diets induce detrimental changes, known as dysbiosis, in the GI bacterial makeup. HF diet induced microbiota dysbiosis has been associated with structural and functional changes in gut-brain vagally mediated signaling system, associated with overeating and obesity. Although HF-driven changes in microbiota composition are sufficient to alter vagal signaling, it is unknown if restoring normal microbiota in obesity can improve gut-brain signaling and metabolic outcomes. In this study, we evaluated the effect of lean gut microbiota transfer in obese, vagally compromised, rats on gut-brain communication, food intake, and body weight. Male Sprague-Dawley rats were maintained on regular chow, or 45% HF diet for nine weeks followed by three weeks of microbiota depletion using an antibiotic cocktail. The animals were then divided into four groups (n=10 each): LF - control group on regular chow, LF-LF - chow fed animals that received antibiotics and microbiota from chow fed animals, HF-LF - HF fed animals that received microbiota from chow fed animals, and HF-HF - HF fed animals that received microbiota from HF fed animals. Animals were gavaged with donor microbiota for three consecutive days on week one and once a week thereafter for three more weeks. HF-LF animals received inulin as a prebiotic to aid the establishment of the lean microbiome. We found that transferring a LF microbiota to HF fed animals (HF-LF) reduced caloric intake during the light phase when compared with HF-HF rats and prevented additional excessive weight gain. We did not observe significant changes in the density of vagal afferents terminating in the brainstem among the groups, however, HF-LF animals displayed an increase in postprandial activation of both primary sensory neurons innervating the GI tract and brainstem secondary neurons. We concluded from these data that normalizing microbiota composition in obese rats improves gut-brain communication and restores normal feeding patterns which was associated with a reduction in weight gain.

Volatile working memory representations crystallize with practice

Working memory, the process through which information is transiently maintained and manipulated over a brief period, is essential for most cognitive functions1–4. However, the mechanisms underlying the generation and evolution of working-memory neuronal representations at the population level over long timescales remain unclear. Here, to identify these mechanisms, we trained head-fixed mice to perform an olfactory delayed-association task in which the mice made decisions depending on the sequential identity of two odours separated by a 5 s delay. Optogenetic inhibition of secondary motor neurons during the late-delay and choice epochs strongly impaired the task performance of the mice. Mesoscopic calcium imaging of large neuronal populations of the secondary motor cortex (M2), retrosplenial cortex (RSA) and primary motor cortex (M1) showed that many late-delay-epoch-selective neurons emerged in M2 as the mice learned the task. Working-memory late-delay decoding accuracy substantially improved in the M2, but not in the M1 or RSA, as the mice became experts. During the early expert phase, working-memory representations during the late-delay epoch drifted across days, while the stimulus and choice representations stabilized. In contrast to single-plane layer 2/3 (L2/3) imaging, simultaneous volumetric calcium imaging of up to 73,307 M2 neurons, which included superficial L5 neurons, also revealed stabilization of late-delay working-memory representations with continued practice. Thus, delay- and choice-related activities that are essential for working-memory performance drift during learning and stabilize only after several days of expert performance.

Promising Effects of Casearins in Tumor-Bearing Mice and Antinociceptive Action against Oncologic Pain: Molecular Docking and In Vivo Findings.

Safer analgesic drugs remain a hard challenge because of cardiovascular and/or gastrointestinal toxicity, mainly. So, this study evaluated in vivo the antiproliferative actions of a fraction with casearins (FC) from Casearia sylvestris leaves against human colorectal carcinomas and antihyperalgesic effects on inflammatory- or opiate-based pain relief and oncologic pain in Sarcoma 180 (S180)-bearing mice. Moreover, docking investigations evaluated the binding among Casearin X and NMDA(N-methyl-D-aspartate)-type glutamate receptors. HCT-116 colorectal carcinoma-xenografted mice were treated with FC for 15 days. Antinociceptive assays included chemically induced algesia and investigated mechanisms by pharmacological blockade. Intraplantar region S180-bearing animals received a single dose of FC and were examined for mechanical allodynia and behavior alterations. AutoDock Vina determined molecular interactions among Cas X and NMDA receptor subunits. FC reduced tumor growth at i.p. (5 and 10 mg/kg) and oral (25 mg/kg/day) doses (31.12-39.27%). FC reduced abdominal pain, as confirmed by formalin and glutamate protocols, whose antinociception activity was blocked by naloxone and L-NAME (neurogenic phase) and naloxone, atropine, and flumazenil (inflammatory phase). Meanwhile, glibenclamide potentiated the FC analgesic effects. FC increased the paw withdrawal threshold without producing changes in exploratory parameters or motor coordination. Cas X generated a more stable complex with active sites of the NMDA receptor GluN2B subunits. FC is a promising antitumor agent against colorectal carcinomas, has peripheral analgesic effects by desensitizing secondary afferent neurons, and inhibits glutamate release from presynaptic neurons and/or their action on cognate receptors. These findings emphasize the use of clerodane diterpenes against cancer-related pain conditions.

Microglia in Neuropathic Pain.

Neuropathic pain (NP) is pain resulting from lesions or disease of the somatosensory system. A cardinal feature of NP is tactile allodynia (a painful response to normally innocuous stimulation). In 2003, a breakthrough strategy for inducing NP was proposed in which microglia of the spinal dorsal horn (SDH) are activated after peripheral nerve injury (PNI) to overexpress P2X4 receptor (P2X4R) and play an important role in inducing tactile allodynia. In 2005, it was reported that stimulation of microglial P2X4Rs evokes the release of brain-derived neurotrophic factor (BDNF), which causes a depolarizing shift of the anion reversal potential (Eanion) of secondary sensory neurons. These findings and other facts suggest the mechanism by which innocuous touch stimuli cause severe pain and the important role of microglia in the mechanism.

Developing gene‐independent approaches for retinal dystrophies

Inherited retinal dystrophies (IRDs) display remarkable genetic heterogeneity, with over 300 retinal disease genes mapped and/or identified. Profound vision loss and blindness, often from birth, is a common occurrence in these clinically heterogeneous disease conditions. The therapeutic options for IRDs are currently very limited but recent advancements in understanding mechanisms of retinal degeneration and technological progress paved the way towards development of novel therapies. Much progress has been made in moving from gene discovery to gene therapy. However, the genetic heterogeneity of IRDs represents a significant challenge for development of gene therapy.Many studies from our group have demonstrated that the survival of cones (responsible for diurnal and high‐resolution vision) depends on the presence of rods (responsible for vision at low light levels) that secrete a diffusible trophic factor identified as rod‐derived cone viability factor (RdCVF). The discovery of RdCVF, of its receptor and mechanism of action, and the demonstration of its mutation‐independent therapeutic potential in several animal models of IRDs paved the way to the development (with the start‐up company Sparing Vision) of RdCVF neuroprotective gene therapy that entered in clinical trial in 2023.IRDs destroy photoreceptors but leave intact and functional a significant number of inner retinal cells. Using viral vectors, light‐sensitive microbial opsins can be expressed in these remaining cells, making possible their conversion into ‘artificial photoreceptors’. Our group in Paris, together with Botond Roska (IOB, Basel) and the biotech company GenSight, demonstrated that this technology, named optogenetics, can restore vision in secondary retinal neurons which can survive years after the degeneration of photoreceptors. Our groups established the foundation for the first‐in‐man clinical trial (NCT03326336) with the photoactivatable optogene ChrimsonR (developed by Ed Boyen at MIT) delivered in the eye via adeno‐associated viral vectors and electronic goggles to intensify the light and provide adaptation. We demonstrated (Nat Med 2021) that optogenetics is a remarkably effective way to restore partial vision and provide useful vision restoration in blind people.Gene‐independent approaches such as neuroprotection and optogenetics would allow for the treatment of a broad spectrum of retinal dystrophies, even at advanced stages of disease, giving hope for vision rescue and vision restoration to approximately 2 million people with IRDs worldwide.

Effect of theanine on the hyperexcitability of trigeminal secondary nociceptive neurons following orofacial inflammation in rats.

The present in vivo study investigated whether systemic administration of theanine attenuates the inflammation-induced hyperexcitability of trigeminal spinal nucleus caudalis (SpVc) neurons associated with hyperalgesia. Complete Freund's adjuvant (CFA) was injected into the whisker pads of 24 rats to induce inflammation, and then mechanical stimulation was applied to the orofacial area to assess the threshold of escape. The mechanical threshold was statistically significantly lower in CFA-inflamed rats compared to uninjected naïve rats, and this lowered threshold returned to control levels after 2 days of theanine administration. The mean discharge frequency of SpVc wide-dynamic range (WDR) neurons to mechanical stimuli in anesthetized CFA-inflamed rats was statistically significantly lower after two days of theanine administration. In addition, the increased mean spontaneous discharge of SpVc WDR neurons in CFA-inflamed rats statistically significantly decreased after theanine administration. Similarly, theanine restored the expanded mean receptive field size in CFA-inflamed rats to control levels. Taken together, these results suggest that administration of theanine attenuates inflammatory hyperalgesia associated with hyperexcitability of nociceptive SpVc WDR neurons. These findings support the potential of theanine as a therapeutic agent in complementary alternative medicine strategies to prevent inflammatory hyperalgesia.

Melanocortin-3 receptor expression in AgRP neurons is required for normal activation of the neurons in response to energy deficiency

The melanocortin-3 receptor (MC3R) is a negative regulator of the central melanocortin circuitry via presynaptic expression on agouti-related protein (AgRP) nerve terminals, from where it regulates GABA release onto secondary MC4R-expressing neurons. However, MC3R knockout (KO) mice also exhibit defective behavioral and neuroendocrine responses to fasting. Here, we demonstrate that MC3R KO mice exhibit defective activation of AgRP neurons in response to fasting, cold exposure, or ghrelin while exhibiting normal inhibition of AgRP neurons by sensory detection of food in the ad libitum-fed state. Using a conditional MC3R KO model, we show that the control of AgRP neuron activation by fasting and ghrelin requires the specific presence of MC3R within AgRP neurons. Thus, MC3R is a crucial player in the responsiveness of the AgRP soma to both hormonal and neuronal signals of energy need.

Cellular and Molecular Mechanisms of Vestibular Ageing.

While age-related auditory deficits and cochlear alterations are well described, those affecting the vestibular sensory organs and more broadly the central vestibular pathways are much less documented. Although there is inter-individual heterogeneity in the phenomenon of vestibular ageing, common tissue alterations, such as losses of sensory hair cells or primary and secondary neurons during the ageing process, can be noted. In this review, we document the cellular and molecular processes that occur during ageing in the peripheral and central vestibular system and relate them to the impact of age-related vestibular deficits based on current knowledge.

Kinesin-5 inhibition improves neural regeneration in experimental autoimmune neuritis

BackgroundAutoimmune neuropathies can result in long-term disability and incomplete recovery, despite adequate first-line therapy. Kinesin-5 inhibition was shown to accelerate neurite outgrowth in different preclinical studies. Here, we evaluated the potential neuro-regenerative effects of the small molecule kinesin-5 inhibitor monastrol in a rodent model of acute autoimmune neuropathies, experimental autoimmune neuritis.MethodsExperimental autoimmune neuritis was induced in Lewis rats with the neurogenic P2-peptide. At the beginning of the recovery phase at day 18, the animals were treated with 1 mg/kg monastrol or sham and observed until day 30 post-immunisation. Electrophysiological and histological analysis for markers of inflammation and remyelination of the sciatic nerve were performed. Neuromuscular junctions of the tibialis anterior muscles were analysed for reinnervation. We further treated human induced pluripotent stem cells-derived secondary motor neurons with monastrol in different concentrations and performed a neurite outgrowth assay.ResultsTreatment with monastrol enhanced functional and histological recovery in experimental autoimmune neuritis. Motor nerve conduction velocity at day 30 in the treated animals was comparable to pre-neuritis values. Monastrol-treated animals showed partially reinnervated or intact neuromuscular junctions. A significant and dose-dependent accelerated neurite outgrowth was observed after kinesin-5 inhibition as a possible mode of action.ConclusionPharmacological kinesin-5 inhibition improves the functional outcome in experimental autoimmune neuritis through accelerated motor neurite outgrowth and histological recovery. This approach could be of interest to improve the outcome of autoimmune neuropathy patients.

Suppression of the excitability of rat nociceptive secondary sensory neurons following local administration of the Phytochemical, (-)-Epigallocatechin-3-gallate

The phytochemical, polyphenolic compound, (-)-epigallocatechin-3-gallate (EGCG), is the main catechin found in green tea. Although a modulatory effect of EGCG on voltage-gated sodium and potassium channels has been reported in excitable tissues, the in vivo effect of EGCG on the excitability of nociceptive sensory neurons remains to be determined. Our aim was to investigate whether local administration of EGCG to rats attenuates the excitability of nociceptive spinal trigeminal nucleus caudalis (SpVc) neurons in response to mechanical stimulation in vivo. Extracellular single unit recordings were made from SpVc neurons in response to orofacial mechanical stimulation of anesthetized rats. The mean firing frequency of SpVc wide-dynamic range neurons following both non-noxious and noxious mechanical stimuli was significantly inhibited by EGCG in a dose-dependent and reversible manner. The mean magnitude of inhibition by EGCG on SpVc neuronal discharge frequency was similar to that of the local anesthetic, 1% lidocaine. Local injection of half-dose of lidocaine replaced the half-dose of EGCG. These results suggest that local injection of EGCG suppresses the excitability of nociceptive SpVc neurons, possibly via the inhibition of voltage-gated sodium channels and opening of voltage-gated potassium channels in the trigeminal ganglion. Therefore, administration of EGCG as a local anesthetic may provide relief from trigeminal nociceptive pain without side effects.

Asynchronous transcription and translation of neurotransmitter-related genes characterize the initial stages of neuronal maturation in Drosophila.

Neuron specification and maturation are essential for proper central nervous system development. However, the precise mechanisms that govern neuronal maturation, essential to shape and maintain neuronal circuitry, remain poorly understood. Here, we analyse early-born secondary neurons in the Drosophila larval brain, revealing that the early maturation of secondary neurons goes through 3 consecutive phases: (1) Immediately after birth, neurons express pan-neuronal markers but do not transcribe terminal differentiation genes; (2) Transcription of terminal differentiation genes, such as neurotransmitter-related genes VGlut, ChAT, or Gad1, starts shortly after neuron birth, but these transcripts are, however, not translated; (3) Translation of neurotransmitter-related genes only begins several hours later in mid-pupa stages in a coordinated manner with animal developmental stage, albeit in an ecdysone-independent manner. These results support a model where temporal regulation of transcription and translation of neurotransmitter-related genes is an important mechanism to coordinate neuron maturation with brain development.

Electronic Retinal Prostheses.

Retinal prostheses are a promising means for restoring sight to patients blinded by photoreceptor atrophy. They introduce visual information by electrical stimulation of the surviving inner retinal neurons. Subretinal implants target the graded-response secondary neurons, primarily the bipolar cells, which then transfer the information to the ganglion cells via the retinal neural network. Therefore, many features of natural retinal signal processing can be preserved in this approach if the inner retinal network is retained. Epiretinal implants stimulate primarily the ganglion cells, and hence should encode the visual information in spiking patterns, which, ideally, should match the target cell types. Currently, subretinal arrays are being developed primarily for restoration of central vision in patients impaired by age-related macular degeneration (AMD), while epiretinal implants-for patients blinded by retinitis pigmentosa, where the inner retina is less preserved. This review describes the concepts and technologies, preclinical characterization of prosthetic vision and clinical outcomes, and provides a glimpse into future developments.

Evolutionary conservation and diversification of auditory neural circuits that process courtship songs in Drosophila

Acoustic communication signals diversify even on short evolutionary time scales. To understand how the auditory system underlying acoustic communication could evolve, we conducted a systematic comparison of the early stages of the auditory neural circuit involved in song information processing between closely-related fruit-fly species. Male Drosophila melanogaster and D. simulans produce different sound signals during mating rituals, known as courtship songs. Female flies from these species selectively increase their receptivity when they hear songs with conspecific temporal patterns. Here, we firstly confirmed interspecific differences in temporal pattern preferences; D. simulans preferred pulse songs with longer intervals than D. melanogaster. Primary and secondary song-relay neurons, JO neurons and AMMC-B1 neurons, shared similar morphology and neurotransmitters between species. The temporal pattern preferences of AMMC-B1 neurons were also relatively similar between species, with slight but significant differences in their band-pass properties. Although the shift direction of the response property matched that of the behavior, these differences are not large enough to explain behavioral differences in song preferences. This study enhances our understanding of the conservation and diversification of the architecture of the early-stage neural circuit which processes acoustic communication signals.

Masseter muscle contraction and cervical muscle sensitization by nerve growth factor cause mechanical hyperalgesia in masticatory muscle with activation of the trigemino-lateral parabrachial nucleus system in female rats.

To establish a new rat model of craniofacial myalgia, and to clarify which central nervous system pathways are activated in the model. Craniofacial myalgia, represented by myogenous temporomandibular disorder and tension-type headache with pericranial tenderness, is more common in female patients. The pain is thought to be a type of multifactorial disorder with several coexisting causes. To our knowledge, there are no models of craniofacial muscle hyperalgesia caused by multiple types of stimuli. We injected nerve growth factor into the trapezius muscle of female and male rats and repeatedly stimulated the masseter muscle (MM) electrically for 10 days. We determined the mechanical head-withdrawal threshold of MM and extent of phosphorylated extracellular signal-related kinase 1/2 (pERK) immunoreactivity in various regions of the lower brainstem. We conducted retrograde tract-tracing to determine the projection of mechanosensitive MM-innervating secondary neurons to the lateral parabrachial nucleus. Finally, we administered morphine in rats to determine whether increases of pERK immunoreactivity were dependent on noxious inputs. In female rats, but not male rats, the mechanical head-withdrawal threshold was decreased significantly from days 9 to 12. The number of pERK-immunoreactive neurons in the brainstem was increased significantly in female rats in the group with both stimuli compared to rats in other groups with a single stimulus. Mechanosensitive MM-innervating neurons in the brainstem projected to the parabrachial nucleus. Morphine administration blocked the increase in the number of pERK-immunoreactive neurons in both the brainstem and parabrachial nucleus. We established a model of craniofacial myalgia by combining trapezius and MM stimuli in female rats. We found mechanical hyperalgesia of the MM and activation of the pain pathway from the brainstem to parabrachial nucleus. The model reflects the characteristics of patients with craniofacial myalgia and might be helpful to clarify the pathogenic mechanisms underlying these disorders.

Mechanisms of pruritus in cholestasis: understanding and treating the itch.

Pruritus in cholestatic liver diseases can be a major burden and dramatically impair the quality of life of those affected. Here, we provide an update on the latest insights into the molecular pathogenesis of and novel therapeutic approaches for cholestasis-associated itch. Endogenous and exogenous small-molecule pruritogen candidates bind to their receptors on unmyelinated itch C-fibres in the skin. Candidate pruritogens in cholestasis include certain lysophospholipids and sulfated progesterone metabolites, among others, whereas total bile acid or bilirubin conjugates seem unlikely to have a dominant role in the pathogenesis of cholestasis-associated pruritus. Transmission of itch signals via primary, secondary and tertiary itch neurons to the postcentral gyrus and activation of scratch responses offer various targets for therapeutic intervention. At present, evidence-based treatment options for pruritus in fibrosing cholangiopathies, such as primary biliary cholangitis and primary sclerosing cholangitis, are the peroxisome proliferator-associated receptor (PPAR) agonist bezafibrate and the pregnane X receptor (PXR) agonist rifampicin. In pruritus of intrahepatic cholestasis of pregnancy, ursodeoxycholic acid is recommended and might be supported in the third trimester by rifampicin if needed. Alternatively, non-absorbable anion exchange resins, such as cholestyramine, can be administered, albeit with poor trial evidence. Liver transplantation for intolerable refractory pruritus has become an extremely rare therapeutic strategy.

K+ channel Kv4.1 is expressed in the nociceptors/secondary nociceptive neurons and participates in pain regulation.

Kv4 channels are key components controlling neuronal excitability at membrane potentials below action potential thresholds. It remains elusive whether Kv4.1 participates in pain regulation. We raised a Kv4.1 antibody to map Kv4.1+ neurons in the superficial dorsal horn of the spinal cord and dorsal root ganglion (DRG) of rats. Behavioural, biochemical and immunohistochemical methods were used to examine whether the activity of Kv4.1+ neurons or Kv4.1 expression level is altered after peripheral nerve injury. In lamina I of the spinal cord, Kv4.1 immunoreactivity (IR) was detected in neurokinin-1 receptor positive (NK1R)+ projection neurons (the secondary nociceptive neurons) and NK1R+ excitatory interneurons. Kv4.1, KChIP2 and DPP10 were co-expressed in these neurons. Peripheral nerve injury evoked by lumbar spinal nerve ligation (SNL) immediately induced phosphorylated extracellular regulated protein kinase (pERK, an indicator of enhanced neuronal activity) in lamina I Kv4.1+ neurons and lamina II Kv4.2/Kv4.3+ neurons of the spinal cord. Furthermore, Kv4.1 appeared in 59.9% of DRG neurons with variable sizes. Kv4.1 mRNA and protein levels in DRG neurons were gradually decreased after SNL. Following intrathecal injection of Kv4.1 antisense oligodeoxynucleotide (ASO) into naive rats, Kv4.1 protein level was reduced in the DRG, and mechanical but not thermal hypersensitivity was induced. Kv4.1 appears in the secondary nociceptive neurons, and peripheral nerve injury increases the activity of these neurons. Kv4.1 expression in DRG neurons (including half of the nociceptors) is gradually reduced after peripheral nerve injury, and knockdown of Kv4.1 in DRG neurons induces pain. Thus, Kv4.1 participates in pain regulation. Based on the expression of Kv4.1 and Kv4.3 in the nociceptors, Kv4.1 in the secondary nociceptive neurons, Kv4.1 in spinal lamina I excitatory interneurons that regulate the activity of the secondary nociceptive neurons, as well as Kv4.2 and Kv4.3 in spinal lamina II excitatory interneurons that also regulate the activity of the secondary nociceptive neurons, developing Kv4 activators or genetic manipulation to increase Kv4 channel activity in these pain-related Kv4+ neurons will be useful in future pain therapeutics.

Cingulate-motor circuits update rule representations for sequential choice decisions

Anterior cingulate cortex mediates the flexible updating of an animal’s choice responses upon rule changes in the environment. However, how anterior cingulate cortex entrains motor cortex to reorganize rule representations and generate required motor outputs remains unclear. Here, we demonstrate that chemogenetic silencing of the terminal projections of cingulate cortical neurons in secondary motor cortex in the rat disrupts choice performance in trials immediately following rule switches, suggesting that these inputs are necessary to update rule representations for choice decisions stored in the motor cortex. Indeed, the silencing of cingulate cortex decreases rule selectivity of secondary motor cortical neurons. Furthermore, optogenetic silencing of cingulate cortical neurons that is temporally targeted to error trials immediately after rule switches exacerbates errors in the following trials. These results suggest that cingulate cortex monitors behavioral errors and updates rule representations in motor cortex, revealing a critical role for cingulate-motor circuits in adaptive choice behaviors.

Yokukansan Inhibits the Development of Morphine Tolerance by Regulating Presynaptic Proteins in DRG Neurons.

Opioids, such as morphine, are used in clinical settings for the management of acute and chronic pain. However, long-term use of morphine leads to antinociceptive tolerance and hypersensitivity. The cellular and molecular mechanisms of morphine tolerance seem to be quite complex, with suggestions including internalization of the μ-opioid receptor (MOR), neuroinflammation with activation of microglia and astrocytes, and changes in synaptic function in the central nervous system. Yokukansan (YKS), a traditional Kampo medicine consisting of seven herbs, has been used to treat emotional instability, neurosis, and insomnia. Interestingly, recent studies have begun to reveal the inhibitory effect of YKS on the development of morphine tolerance. In the present study, we determined the effect of YKS on morphine tolerance formation and its mechanisms in a rat model, focusing on the synapses between primary sensory neurons and spinal dorsal horn secondary neurons. We found that morphine tolerance formation was significantly inhibited by YKS (0.3 or 1.0 g/kg/day) preadministration for 7 days. Repeated administration of morphine (10 mg/kg/day) increased the expression of presynaptic proteins, including synaptotagmin I, in the spinal cord, which was suppressed by YKS. Furthermore, these changes in presynaptic protein expression were more pronounced at isolectin B4 (IB4)-positive excitatory synapses around the lamina II of the dorsal horn. These results suggest that YKS suppresses the development of morphine tolerance by inhibiting the enhancement of presynaptic function of dorsal root ganglia neurons projecting to spinal dorsal horn neurons caused by continuous morphine administration.

Attentional signals projecting centrifugally to the avian retina: A dual contribution to visual search

Among vertebrates, birds have a particularly well-developed retinopetal system, i.e., the centrifugal system projecting from the brain to the retina. Primary, secondary and tertiary neurons of the retinopetal system connect serially in a one-to-one-to-one fashion, thereby assembling several thousand retinopetal modules projecting in parallel to the retina. The signal conveyed by the retinopetal system enhances the visual responses of retinal ganglion cells (RGCs), and each retinopetal module may facilitate the RGCs in a topographically restricted area. We investigated how the retinopetal signal contributes to visual search by first training Japanese quail to search and peck a target stimulus presented on one of three touch-sensor monitors around the bird and then examining the effect of retinopetal signal blockage on the task. Our results indicate that the retinopetal signal improves both stimulus detectability and target discriminability. Thus, this study confirms that the avian retinopetal system functions specifically in attentional facilitation of the retina; this new finding suggests that this system benefits early detection and late selection phases of visual search. The fast and precise target detection facilitated by the retinopetal signal may be transformed into quick and correct orienting to the target through visuomotor transformation mechanisms implemented in the optic tectum.