GABAB receptors accentuate neural excitation contrast in rat insular cortex

KangPiLao decoction modulates cognitive and emotional disorders in rats with central fatigue through the GABA/Glu pathway

Background: Central fatigue (CF) is a subjective sense of tiredness associated with cognitive and memory disorders, accompanied by reduced physical endurance and negative emotions, such as anxiety and depression. Disease progression and prognosis with regards to CF have been unfavorable and possibly contribute to dementia, schizophrenia, and other diseases. Additionally, effective treatments for CF are lacking. KangPiLao decoction (KPLD) has been widely applied in clinical treatment and is composed of six Chinese herbal medicines, some of which have confirmed anti-fatigue effects. While glutamic acid (Glu) is the main excitatory transmitter in the central nervous system (CNS), gamma-aminobutyric acid (GABA) is the major inhibitory transmitter. Both are involved in emotional, cognitive, and memory functions. This research was designed to explore how KPLD regulates cognitive and emotional disorders in rats with CF and to identify the relationship between the regulatory effect and the GABA/Glu pathway.

Methods: The compounds comprising KPLD were analyzed using high-performance liquid chromatography-mass spectrometry. Sixty Wistar rats were randomly divided into six groups. The modified multiple platform method was used to induce CF. Cognitive, emotional, and fatigue states were evaluated by performing behavioral tests (Morris water maze [MWM], open-field test [OFT], and grip strength test). Histomorphology, western blotting, immunohistochemistry, and RT-qPCR were performed to investigate protein and mRNA expression levels in the hippocampus and prefrontal cortexes involved in the GABA/Glu pathway.

Results: Rats with CF exhibited impaired spatial cognition and increased negative emotions in the MWM and OFT. KPLD enabled the improvement of these symptoms, especially in the high-concentration group. Western blotting and RT-qPCR demonstrated that the expression of GABAARα1, GABAARγ2, GABABR1, and GAD67 in rats with CF was higher, whereas GAT-1 and NMDAR2B were lower in the hippocampus and prefrontal cortex. KPLD decreased the expression of GABAARα1, GABABR1, GABAARγ2, and GAD67 in the hippocampus and prefrontal cortex and enhanced the expression of NR2B in the prefrontal cortex.

Conclusion: KPLD significantly improved cognitive and emotional disorders in rats with CF by regulating the GABA/Glu pathway. Overall, KPLD may be a promising candidate for developing a drug for treating CF.

Olfactory signals from the main olfactory bulb converge with taste information from the chorda tympani nerve in the agranular insular cortex of rats

Gustation and olfaction are integrated into flavor, which contribute to detection and identification of foods. We focused on the insular cortex (IC), as a possible center of flavor integration, because the IC has been reported to receive olfactory in addition to gustatory inputs. In the present report, we tested the hypothesis that these two chemosensory signals are integrated in the IC. We examined the spatiotemporal dynamics of cortical responses induced by stimulating the chorda tympani nerve (CT) and the main olfactory bulb (mOB) in male Sprague-Dawley rats by in vivo optical imaging with a voltage-sensitive dye (VSD). CT stimulation elicited responses in the rostral part of the dysgranular IC (DI), while responses to mOB stimulation were observed in the agranular IC (AI) as well as in the piriform cortex (PC). To characterize the temporal specificity of these responses, we performed combined mOB and CT stimulation with three different timings: simultaneous stimulation and the stimulation of the mOB 150 ms before or after CT stimulation. Simultaneous stimulation increased the signal amplitude in AI additively. These results indicate that the AI and DI contribute to the convergence of gustatory and olfactory information. Of them the DI predominantly processes the taste information, whereas the AI is more sensitive to the olfactory signal.

Suppression of Superficial Microglial Activation by Spinal Cord Stimulation Attenuates Neuropathic Pain Following Sciatic Nerve Injury in Rats

We evaluated the mechanisms underlying the spinal cord stimulation (SCS)-induced analgesic effect on neuropathic pain following spared nerve injury (SNI). On day 3 after SNI, SCS was performed for 6 h by using electrodes paraspinally placed on the L4-S1 spinal cord. The effects of SCS and intraperitoneal minocycline administration on plantar mechanical sensitivity, microglial activation, and neuronal excitability in the L4 dorsal horn were assessed on day 3 after SNI. The somatosensory cortical responses to electrical stimulation of the hind paw on day 3 following SNI were examined by using in vivo optical imaging with a voltage-sensitive dye. On day 3 after SNI, plantar mechanical hypersensitivity and enhanced microglial activation were suppressed by minocycline or SCS, and L4 dorsal horn nociceptive neuronal hyperexcitability was suppressed by SCS. In vivo optical imaging also revealed that electrical stimulation of the hind paw-activated areas in the somatosensory cortex was decreased by SCS. The present findings suggest that SCS could suppress plantar SNI-induced neuropathic pain via inhibition of microglial activation in the L4 dorsal horn, which is involved in spinal neuronal hyperexcitability. SCS is likely to be a potential alternative and complementary medicine therapy to alleviate neuropathic pain following nerve injury.

How to Manage Self-Poisoning With Baclofen in Alcohol Use Disorder? Current Updates

Specialists in addiction medicine continue to debate whether baclofen is still indicated to treat alcohol use disorders in view of conflicting results as to its efficacy. This review summarizes current knowledge on self-poisoning with baclofen focusing of alcohol-use disorder in order to provide an overview of the reliable scientific knowledge on management of such an intoxication. Moreover, as alcohol-dependent patients experience many psychiatric co-morbidities, the risk in suicide attempt using baclofen seems real. Numerous studies have suggested that patients given daily-doses of baclofen higher than 80 mg/day are more likely to attempt suicides than others. Following an ingestion of a large amount of baclofen, central nervous system depression is usually observed. Seizures require the patient to be admitted in intensive care unit and should be treated like other toxicological seizures. Cardiac complications include prolonged QTc interval, degree heart block, premature atrial contractions, and supraventricular tachycardia, hypotension and bradycardia. In cases of intoxication, the elimination half-life of baclofen may last between 12 and 36 h post-overdose and renal failure is known to delay its clearance. Rarely measured in clinical practice, the toxic level of baclofen blood level ranges from 1.1 to 3.5 mg/l, and coma or fatal intoxication are observed from 6 to 9.6 mg/l. Baclofen withdrawal has been observed but making the diagnosis of withdrawal in case of suspected self-poisoning is difficult as baclofen intoxication and baclofen withdrawal share many clinical signs. Admission to hospital to manage of suicide attempt with baclofen is mandatory and should not be limited to baclofen alone. It needs to include other aspects of the overall care of patients with alcohol disorders (psychological and psychosocial interventions, management of comorbid mental conditions and physical complications).

Histamine H3 Heteroreceptors Suppress Glutamatergic and GABAergic Synaptic Transmission in the Rat Insular Cortex

Histamine H₃ receptors are autoreceptors that regulate histamine release from histaminergic neuronal terminals. The cerebral cortex, including the insular cortex (IC), expresses abundant H₃ receptors; however, the functions and mechanisms of H₃ receptors remain unknown. The aim of this study was to elucidate the functional roles of H₃ in synaptic transmission in layer V of the rat IC. Unitary excitatory and inhibitory postsynaptic currents (uEPSCs and uIPSCs) were obtained through paired whole-cell patch-clamp recording in cerebrocortical slice preparations. The H₃ receptor agonist, R-α-methylhistamine (RAMH), reduced the uEPSC amplitude obtained from pyramidal cell to pyramidal cell or GABAergic interneuron connections. Similarly, RAMH reduced the uIPSC amplitude in GABAergic interneuron to pyramidal cell connections. RAMH-induced decreases in both the uEPSC and uIPSC amplitudes were accompanied by increases in the failure rate and paired-pulse ratio. JNJ 5207852 dihydrochloride or thioperamide, H₃ receptor antagonists, inhibited RAMH-induced suppression of uEPSCs and uIPSCs. Unexpectedly, thioperamide alone increased the uIPSC amplitude, suggesting that thioperamide was likely to act as an inverse agonist. Miniature EPSC or IPSC recordings support the hypothesis that the activation of H₃ receptors suppresses the release of glutamate and GABA from presynaptic terminals. The colocalization of H₃ receptors and glutamate decarboxylase or vesicular glutamate transport protein 1 in presynaptic axon terminals was confirmed through double pre-embedding microscopy, using a combination of pre-embedding immunogold and immunoperoxidase techniques. The suppressive regulation of H₃ heteroreceptors on synaptic transmission might mediate the regulation of sensory information processes, such as gustation and visceral sensation, in the IC.

Baclofen systemic toxicity: Experimental histopathological and biochemical study

OBJECTIVE

The present study was designed to highlight the toxic impact of baclofen on both biochemical and histopathological aspects in rats' liver, gastric, lung, kidney, and brain tissues.

METHODS

The study was performed on 30 healthy adult male albino rats divided into four groups with 5 rats in each control group, and 10 rats in either experimental groups (two experimental and two control groups). Five rats (negative control) were kept in a quite non-stressful environment, provided with food ad libitum and free access to water. Normal saline (1 ml) was given orally as placebo in the positive control group ( n = 5). Experimental group III, baclofen acute toxicity group (10 rats): Each animal received a single dose of lethal dose (LD₅₀) of baclofen orally by gavage. It equals 145 mg/kg body weight. The rats were observed for acute toxicity manifestations as well as for LD₅₀ deaths. Group IV, (baclofen-dependent group, 10 rats): Each animal received baclofen (1/10th LD₅₀) in gradually increasing doses for 1 month.

RESULTS

The levels of blood urea nitrogen, creatinine kinase, alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase, γ-glutamyl transpeptidase, cardiac troponin I, and prothrombin time in both baclofen-treated groups showed significant elevation when compared to controls. There were brain, lung, gastric, hepatic, and renal histopathological changes in baclofen-treated rats whose severity varied between the two experimental groups.

CONCLUSION AND RECOMMENDATION

Baclofen toxicity is an under diagnosed emergency. Physicians should consider baclofen toxicity in users having hepatorenal dysfunction, presenting with altered mental status, bradycardia, and hypotension.

Spatiotemporal Profiles of Proprioception Processed by the Masseter Muscle Spindles in Rat Cerebral Cortex: An Optical Imaging Study

Muscle spindles in the jaw-closing muscles, which are innervated by trigeminal mesencephalic neurons (MesV neurons), control the strength of occlusion and the position of the mandible. The mechanisms underlying cortical processing of proprioceptive information are critical to understanding how sensory information from the masticatory muscles regulates orofacial motor function. However, these mechanisms are mostly unknown. The present study aimed to identify the regions that process proprioception of the jaw-closing muscles using in vivo optical imaging with a voltage-sensitive dye in rats under urethane anesthesia. First, jaw opening that was produced by mechanically pulling down the mandible evoked an optical response, which reflects neural excitation, in two cortical regions: the most rostroventral part of the primary somatosensory cortex (S1) and the border between the ventral part of the secondary somatosensory cortex (S2) and the insular oral region (IOR). The kinetics of the optical signal, including the latency, amplitude, rise time, decay time and half duration, in the S1 region for the response with the largest amplitude were comparable to those in the region with the largest response in S2/IOR. Second, we visualized the regions responding to electrical stimulation of the masseter nerve, which activates both motor efferent fibers and somatosensory afferent fibers, including those that transmit nociceptive and proprioceptive information. Masseter nerve stimulation initially excited the rostral part of the S2/IOR region, and an adjacent region responded to jaw opening. The caudal part of the region showing the maximum response overlapped with the region responding to jaw opening, whereas the rostral part overlapped with the region responding to electrical stimulation of the maxillary and mandibular molar pulps. These findings suggest that proprioception of the masseter is processed in S1 and S2/IOR. Other sensory information, such as nociception, is processed in a region that is adjacent to these pulpal regions and is located in the rostral part of S2/IOR, which receives nociceptive inputs from the molar pulps. The spatial proximity of these regions may be associated with the mechanisms by which masseter muscle pain is incorrectly perceived as dental pain.

Distinct Excitation to Pulpal Stimuli between Somatosensory and Insular Cortices

Somatosensory information from the dental pulp is processed in the primary (S1) and secondary somatosensory cortex (S2) and in the insular oral region (IOR). Stimulation of maxillary incisor and molar initially induces excitation in S2/IOR, rostrodorsal to the mandibular incisor and molar pulp-responding regions. Although S1 and S2/IOR play their own roles in nociceptive information processing, the anatomical and physiological differences in the temporal activation kinetics, dependency on stimulation intensity, and additive or summative effects of simultaneous pulpal stimulation are still unknown. This information contributes not only to understanding topographical organization but also to speculating about the roles of S1 and S2/IOR in clinical aspects of pain regulation. In vivo optical imaging enables investigation of the spatiotemporal profiles of cortical excitation with high resolution. We determined the distinct features of optical responses to nociceptive stimulation of dental pulps between S1 and S2/IOR. In comparison to S1, optical signals in S2/IOR showed a larger amplitude with a shorter rise time and a longer decay time responding to maxillary molar pulp stimulation. The latency of excitation in S2/IOR was shorter than in S1. S2/IOR exhibited a lower threshold to evoke optical responses than S1, and the peak amplitude was larger in S2/IOR than in S1. Unexpectedly, the topography of S1 that responded to maxillary and mandibular incisor and molar pulps overlapped with the most ventral sites in S1 that was densely stained with cytochrome oxidase. An additive effect was observed in both S1 and S2/IOR after simultaneous stimulation of bilateral maxillary molar pulps but not after contralateral maxillary and mandibular molar pulp stimulation. These findings suggest that S2/IOR is more sensitive for detecting dental pulp sensation and codes stimulation intensity more precisely than S1. In addition, contra- and ipsilateral dental pulp nociception converges onto spatially closed sites in S1 and S2/IOR.

The topology of connections between rat prefrontal and temporal cortices

Understanding the structural organization of the prefrontal cortex (PFC) is an important step toward determining its functional organization. Here we investigated the organization of PFC using different neuronal tracers. We injected retrograde (Fluoro-Gold, 100 nl) and anterograde [Biotinylated dextran amine (BDA) or Fluoro-Ruby, 100 nl] tracers into sites within PFC subdivisions (prelimbic, ventral orbital, ventrolateral orbital, dorsolateral orbital) along a coronal axis within PFC. At each injection site one injection was made of the anterograde tracer and one injection was made of the retrograde tracer. The projection locations of retrogradely labeled neurons and anterogradely labeled axon terminals were then analyzed in the temporal cortex: area Te, entorhinal and perirhinal cortex. We found evidence for an ordering of both the anterograde (anterior-posterior, dorsal-ventral, and medial-lateral axes: p < 0.001) and retrograde (anterior-posterior, dorsal-ventral, and medial-lateral axes: p < 0.001) connections of PFC. We observed that anterograde and retrograde labeling in ipsilateral temporal cortex (i.e., PFC inputs and outputs) often occurred reciprocally (i.e., the same brain region, such as area 35d in perirhinal cortex, contained anterograde and retrograde labeling). However, often the same specific columnar temporal cortex regions contained only either labeling of retrograde or anterograde tracer, indicating that PFC inputs and outputs are frequently non-matched.

Spatiotemporal profiles of dental pulp nociception in rat cerebral cortex: an optical imaging study

Somatosensation is topographically organized in the primary (S1) and secondary somatosensory cortex (S2), which contributes to identify the region receiving sensory inputs. However, it is still unknown how somatosensory inputs from the oral region, especially nociceptive inputs from the teeth, are processed in the somatosensory cortex. We performed in vivo optical imaging and identified the precise cortical regions responding to electrical stimulation of the maxillary and mandibular dental pulp in rats. Electrical stimulation of the mandibular incisor pulp evoked neural excitation in two areas: the most rostroventral part of S1, and the ventral part of S2 caudal to the middle cerebral artery. Maxillary incisor pulp stimulation initially evoked responses only in the ventral part of S2, although later maximum responses were also observed in S1 similar to mandibular incisor stimulation responses. The maxillary and mandibular molar pulp-responding regions were located in the most ventral S2, a part of which was histologically classified as the insular oral region (IOR). In terms of the initial responses, maxillary incisor and molar stimulation induced excitation in the S2/IOR rostral to the mandibular dental pulp-responding region. Contrary to the spatially segregated initial responses, the maximum excitatory areas responding to both incisors and molars in the mandible and maxilla overlapped in S1 and the S2/IOR. Multielectrode extracellular recording supported the characteristic localization of S2/IOR neurons responding to mandibular and maxillary molar pulp stimulation. The discrete and overlapped spatial profiles of initial and maximum responses, respectively, may characterize nociceptive information processing of dental pain in the cortex.

Cytoarchitecture-Dependent Decrease in Propagation Velocity of Cortical Spreading Depression in the Rat Insular Cortex Revealed by Optical Imaging

Cortical spreading depression (SD) is a self-propagating wave of depolarization accompanied by a substantial disturbance of the ionic distribution between the intra- and extracellular compartments. Glial cells, including astrocytes, play critical roles in maintenance of the extracellular environment, including ionic distribution. Therefore, SD propagation in the cerebral cortex may depend on the density of astrocytes. The present study aimed to examine the profile of SD propagation in the insular cortex (IC), which is located between the neocortex and paleocortex and is where the density of astrocytes gradually changes. The velocity of SD propagation in the neocortex, including the somatosensory, motor, and granular insular cortices (5.7 mm/min), was higher than that (2.8 mm/min) in the paleocortex (agranular insular and piriform cortices). Around thick vessels, including the middle cerebral artery, SD propagation was frequently delayed and sometimes disappeared. Immunohistological analysis of glial fibrillary acidic protein (GFAP) demonstrated the sparse distribution of astrocytes in the somatosensory cortex and the IC dorsal to the rhinal fissure, whereas the ventral IC showed a higher density of astrocytes. These results suggest that cortical cytoarchitectonic features, which possibly involve the distribution of astrocytes, are crucial for regulating the velocity of SD propagation in the cerebral cortex.

The topology of connections between rat prefrontal, motor and sensory cortices

The connections of prefrontal cortex (PFC) were investigated in the rat brain to determine the order and location of input and output connections to motor and somatosensory cortex. Retrograde (100 nl Fluoro-Gold) and anterograde (100 nl Biotinylated Dextran Amines, BDA; Fluorescein and Texas Red) neuronanatomical tracers were injected into the subdivisions of the PFC (prelimbic, ventral orbital, ventrolateral orbital, dorsolateral orbital) and their projections studied. We found clear evidence for organized input projections from the motor and somatosensory cortices to the PFC, with distinct areas of motor and cingulate cortex projecting in an ordered arrangement to the subdivisions of PFC. As injection location of retrograde tracer was moved from medial to lateral in PFC, we observed an ordered arrangement of projections occurring in sensory-motor cortex. There was a significant effect of retrograde injection location on the position of labelled cells occurring in sensory-motor cortex (dorsoventral, anterior-posterior and mediolateral axes p < 0.001). The arrangement of output projections from PFC also displayed a significant ordered projection to sensory-motor cortex (dorsoventral p < 0.001, anterior-posterior p = 0.002 and mediolateral axes p < 0.001). Statistical analysis also showed that the locations of input and output labels vary with respect to one another (in the dorsal-ventral and medial-lateral axes, p < 0.001). Taken together, the findings show that regions of PFC display an ordered arrangement of connections with sensory-motor cortex, with clear laminar organization of input connections. These results also show that input and output connections to PFC are not located in exactly the same sites and reveal a circuit between sensory-motor and PFC.

Anatomical and electrophysiological mechanisms for asymmetrical excitatory propagation in the rat insular cortex: in vivo optical imaging and whole-cell patch-clamp studies

The insular cortex (IC) integrates limbic information from the amygdala and hypothalamic nucleus to multimodal sensory inputs, including visceral, gustatory, and somatosensory information. However, the functional framework of excitation in the IC is still unknown. We performed optical imaging and single pyramidal neuronal staining using a whole-cell patch-clamp technique in urethane-anesthetized rats to elucidate the precise anatomical and physiological features of IC pyramidal neurons, which regulate cortical information processing via their horizontal connections. Optical imaging revealed that electrical stimulation of the granular (GI) or dysgranular (DI) IC elicited characteristic excitatory propagations along the rostrocaudal axis parallel to the rhinal fissure, with a preference toward the rostral direction. Spatial patterns of the dendrites and axons of layer II/III pyramidal cells in the DI/GI support these functional features of excitation; for example, rostrocaudal axonal arbors tend to extend with a rostral directional preference. The mean length of the axons from the soma to the farthest site rostrally was ∼50% longer than that of the caudal length. Pyramidal cells in the DI/GI exhibited spontaneous membrane oscillation in the UP and DOWN states. Similarly to the evoked signals obtained by optical imaging, repetitive electrical stimulation of the caudal IC ∼1 mm away from the recorded cells (five pulses at 50 Hz) induced the summation of evoked excitatory postsynaptic potentials during the DOWN state and profound inhibitory postsynaptic potentials during the UP state. Clarification of the excitation feature with its cellular basis provides new clues about the functional mechanisms of the asymmetric propagation of neural activities in the IC.

Neural processing of gustatory information in insular circuits

The insular cortex is the primary cortical site devoted to taste processing. A large body of evidence is available for how insular neurons respond to gustatory stimulation in both anesthetized and behaving animals. Most of the reports describe broadly tuned neurons that are involved in processing the chemosensory, physiological and psychological aspects of gustatory experience. However little is known about how these neural responses map onto insular circuits. Particularly mysterious is the functional role of the three subdivisions of the insular cortex: the granular, the dysgranular and the agranular insular cortices. In this article we review data on the organization of the local and long-distance circuits in the three subdivisions. The functional significance of these results is discussed in light of the latest electrophysiological data. A view of the insular cortex as a functionally integrated system devoted to processing gustatory, multimodal, cognitive and affective information is proposed.