Different kinds of axon terminals forming symmetric synapses with the cell bodies and initial axon segments of layer II/III pyramidal cells. III. Origins and frequency of occurrence of the terminals

NLRP3-Mediated Piezo1 Upregulation in ACC Inhibitory Parvalbumin-Expressing Interneurons Is Involved in Pain Processing after Peripheral Nerve Injury

The anterior cingulate cortex (ACC) is particularly critical for pain information processing. Peripheral nerve injury triggers neuronal hyper-excitability in the ACC and mediates descending facilitation to the spinal dorsal horn. The mechanically gated ion channel Piezo1 is involved in the transmission of pain information in the peripheral nervous system. However, the pain-processing role of Piezo1 in the brain is unknown. In this work, we found that spared (sciatic) nerve injury (SNI) increased Piezo1 protein levels in inhibitory parvalbumin (PV)-expressing interneurons (PV-INs) but not in glutaminergic CaMKⅡ+ neurons, in the bilateral ACC. A reduction in the number of PV-INs but not in the number of CaMKⅡ+ neurons and a significant reduction in inhibitory synaptic terminals was observed in the SNI chronic pain model. Further, observation of morphological changes in the microglia in the ACC showed their activated amoeba-like transformation, with a reduction in process length and an increase in cell body area. Combined with the encapsulation of Piezo1-positive neurons by Iba1+ microglia, the loss of PV-INs after SNI might result from phagocytosis by the microglia. In cellular experiments, administration of recombinant rat TNF-α (rrTNF) to the BV2 cell culture or ACC neuron primary culture elevated the protein levels of Piezo1 and NOD-like receptor (NLR) family pyrin domain containing 3 (NLRP3). The administration of the NLRP3 inhibitor MCC950 in these cells blocked the rrTNF-induced expression of caspase-1 and interleukin-1β (key downstream factors of the activated NLRP3 inflammasome) in vitro and reversed the SNI-induced Piezo1 overexpression in the ACC and alleviated SNI-induced allodynia in vivo. These results suggest that NLRP3 may be the key factor in causing Piezo1 upregulation in SNI, promoting an imbalance between ACC excitation and inhibition by inducing the microglial phagocytosis of PV-INs and, thereby, facilitating spinal pain transmission.

Microglial displacement of inhibitory synapses provides neuroprotection in the adult brain

Microglia actively survey the brain microenvironment and play essential roles in sculpting synaptic connections during brain development. While microglial functions in the adult brain are less clear, activated microglia can closely appose neuronal cell bodies and displace axosomatic presynaptic terminals. Microglia-mediated stripping of presynaptic terminals is considered neuroprotective, but the cellular and molecular mechanisms are poorly defined. Using 3D electron microscopy, we demonstrate that activated microglia displace inhibitory presynaptic terminals from cortical neurons in adult mice. Electrophysiological recordings further establish that the reduction in inhibitory GABAergic synapses increased synchronized firing of cortical neurons in γ-frequency band. Increased neuronal activity results in the calcium-mediated activation of CaM kinase IV, phosphorylation of CREB, increased expression of antiapoptotic and neurotrophic molecules and reduced apoptosis of cortical neurons following injury. These results indicate that activated microglia can protect the adult brain by migrating to inhibitory synapses and displacing them from cortical neurons.

Microglia play essential roles in sculpting synaptic connections during brain development but their role in the adult brain is less clear. Here the authors show that activated microglia can prophylactically protect the adult rodent brain from injury by migrating to and displacing inhibitory synapses from cortical neurons.

Dopaminergic innervation of pyramidal cells in the rat basolateral amygdala

Dopaminergic (DA) inputs to the basolateral nuclear complex of the amygdala (BLC) are critical for several important functions, including reward-related learning, drug-stimulus learning, and fear conditioning. Despite the importance of the DA projection to the BLC, very little is known about which neuronal subpopulations are innervated. The present study utilized dual-labeling immunohistochemistry at the electron microscopic level to examine DA inputs to pyramidal cells in the anterior basolateral amygdalar nucleus (BLa) in the rat. DA axon terminals and BLa pyramidal cells were labeled using antibodies to tyrosine hydroxylase (TH) and calcium/calmodulin-dependent protein kinase II (CaMK), respectively. Serial section reconstructions of TH-positive (TH+) terminals were performed to determine the extent to which these axon terminals formed synapses versus non-synaptic appositions in the BLa. Our results demonstrate that at least 77% of TH+ terminals form synapses in the BLa, and that 90% of these synapses are with pyramidal cells. The distal dendritic compartment received the great majority of these synaptic contacts, with CaMK+ distal dendrites and spines receiving one-third and one-half, respectively, of all synaptic inputs to pyramidal cells. Many spines receiving innervation from TH+ terminals also received asymmetrical synaptic inputs from putative excitatory terminals. In addition, TH+ terminals often formed non-synaptic appositions with axon terminals, most of which were putatively excitatory in that they were CaMK+ and/or made asymmetrical synapses. Thus, using CaMK as a marker, the present study demonstrates that pyramidal cells, especially their distal dendritic compartments, are the primary targets of dopaminergic inputs to the basolateral amygdala.

Serotonin-immunoreactive axon terminals innervate pyramidal cells and interneurons in the rat basolateral amygdala

The basolateral nuclear complex of the amygdala (BLC) receives a dense serotonergic innervation that appears to play a critical role in the regulation of mood and anxiety. However, little is known about how serotonergic inputs interface with different neuronal subpopulations in this region. To address this question, dual-labeling immunohistochemical techniques were used at the light and electron microscopic levels to examine inputs from serotonin-immunoreactive (5-HT+) terminals to different neuronal subpopulations in the rat BLC. Pyramidal cells were labeled by using antibodies to calcium/calmodulin-dependent protein kinase II, whereas different interneuronal subpopulations were labeled by using antibodies to a variety of interneuronal markers including parvalbumin (PV), vasoactive intestinal peptide (VIP), calretinin, calbindin, cholecystokinin, and somatostatin. The BLC exhibited a dense innervation by thin 5-HT+ axons. Electron microscopic examination of the anterior basolateral nucleus (BLa) revealed that 5-HT+ axon terminals contained clusters of small synaptic vesicles and a smaller number of larger dense-core vesicles. Serial section reconstruction of 5-HT+ terminals demonstrated that 76% of these terminals formed synaptic junctions. The great majority of these synapses were symmetrical. The main targets of 5-HT+ terminals were spines and distal dendrites of pyramidal cells. However, in light microscopic preparations it was common to observe apparent contacts between 5-HT+ terminals and all subpopulations of BLC interneurons. Electron microscopic analysis of the BLa in sections dual-labeled for 5-HT/PV and 5-HT/VIP revealed that many of these contacts were synapses. These findings suggest that serotonergic axon terminals differentially innervate several neuronal subpopulations in the BLC.

Axo-axonic structures in the medial prefrontal cortex of the rat: reduction by prenatal exposure to cocaine

The cognitive deficits associated with prenatal exposure to cocaine have been hypothesized to be the results of changes in the anatomy and function of the frontal cortex. In this study, pregnant dams were treated with cocaine (3 mg/kg i.v. twice a day) and the resulting adolescent (postnatal day, approximately 45) male offspring were killed for immunocytochemical determination of the total linear measure, number, location, and lengths of inhibitory GABA transporter-1 immunoreactive axo-axonic structures commonly called "candles" or "cartridges" in the medial prefrontal cortex. These inhibitory structures are the axon terminals of GABAergic cells that impinge on the initial axon segments of excitatory pyramidal neurons. We report that prenatal cocaine exposure decreased the number of these inhibitory candles. The greatest reduction of candles was observed in the ventral prelimbic cortex. Additionally, there was a subtle difference in the pattern of distribution of candles, namely the depth of the initial candle in the ventral portions of the prefrontal cortex was greater in rats exposed to prenatal cocaine. However, there was no overt change in the number of cells that were immunoreactive for the calcium-binding protein parvalbumin, an indicator of a subset of GABAergic interneurons that includes axo-axonic chandelier cells. We conclude that exposure to cocaine in utero disrupts the development of the axo-axonic cells in the prefrontal cortex and this disruption could contribute to the cognitive deficits reported with prenatal cocaine exposure.

Neurochemical features and synaptic connections of large physiologically-identified GABAergic cells in the rat frontal cortex

Physiological and morphological properties of large non-pyramidal cells immunoreactive for cholecystokinin, parvalbumin or somatostatin were investigated in vitro in the frontal cortex of 18-22-day-old rats. These three peptides were expressed in separate populations including large cells. Cholecystokinin cells and parvalbumin cells made boutons apposed to other cell bodies, but differed in their firing patterns in response to depolarizing current pulses. Parvalbumin cells belonged to fast-spiking cells. Parvalbumin fast-spiking cells also included chandelier cells. In contrast, cholecystokinin cells were found to be regular-spiking non-pyramidal cells or burst-spiking non-pyramidal cells with bursting activity from hyperpolarized potentials (two or more spikes on slow depolarizing humps). Large somatostatin cells belonged to the regular-spiking non-pyramidal category and featured wide or ascending axonal arbors (wide arbor cells and Martinotti cells) which did not seem to be apposed to the somata so frequently as large cholecystokinin and parvalbumin cells. For electron microscopic observations, another population of eight immunohistochemically-uncharacterized non-pyramidal cells were selected: (i) five fast spiking cells including one chandelier cell which are supposed to contain parvalbumin, and (ii) three large regular-spiking non-pyramidal cells with terminals apposed to somata, which are not considered to include somatostatin cells, but some of which may belong to cholecystokinin cells. The fast-spiking cells other than a chandelier cell and the large regular-spiking non-pyramidal cells made GABA-positive synapses on somata (4% and 12% of the synapses in two small to medium fast-spiking cells, 22% and 35% of the synapses in two large fast-spiking cells, and 10%, 18% and 37% of the synapses in three large regular-spiking non-pyramidal cells). A few terminals of the fast-spiking and regular-spiking non-pyramidal cells innervated GABAergic cells. About 30% of the fast-spiking cell terminals innervated spines, but few of the regular-spiking non-pyramidal cell terminals did. A fast-spiking chandelier cell made GABA-positive synapses on GABA-negative axon initial segments. These results suggest that large GABAergic cells are heterogeneous in neuroactive substances, firing patterns and synaptic connections, and that cortical cells receive heterogeneous GABAergic somatic inputs.

Neuritic differentiation and synaptogenesis in serum-free neuronal cultures of the rat cerebral cortex

To better understand the dynamics of the cellular processes involved in early neocortical development, we studied the neuritic differentiation and synaptogenesis of dispersed neurons grown in serum-free cultures under a wide variety of culture conditions. Microtubule-associated protein (MAP2), phosphorylated neurofilament (SMI 31) and synaptophysin immunocytochemistry was complemented with time-lapse studies. During the first week in vitro dissociated cortical neurons developed from roundish cells without processes to neurons with axons and differentiated dendrites, going through five distinct phases. The sequence of these phases was unaltered in a wide range of culturing methods, but the timing of the steps varied among cultures started with different cell densities. Synaptic terminals were first observed after 3-4 days in vitro, coincident with the beginning of dendritic differentiation. Synaptogenesis progressed at least until the end of the third week in vitro, despite a decline in cell density during the second week in vitro. The process of cellular differentiation of cerebral cortical neurons in vitro resembled the development of these cells in the intact tissue, suggesting that organized cell migration is not a prerequisite for the differentiation of single cortical neurons.

Synchronous development of pyramidal neuron dendritic spines and parvalbumin-immunoreactive chandelier neuron axon terminals in layer III of monkey prefrontal cortex

Postnatal development of the primate cerebral cortex involves an initial proliferation and the subsequent attrition of cortical synapses. Although these maturational changes in synaptic density have been observed across the cortical mantle, little is known about the precise time course of developmental refinements in synaptic inputs to specific populations of cortical neurons. We examined the postnatal development of two markers of excitatory and inhibitory inputs to a subpopulation of layer III pyramidal neurons in area 9 and 46 of rhesus monkey prefrontal cortex. These neurons are of particular interest because they play a major role in the flow of information both within and between cortical regions. Quantitative reconstructions of Golgi-impregnated mid-layer III pyramidal neurons revealed substantial developmental changes in the relative density of dendritic spines, the major site of excitatory inputs to these neurons. Relative spine density on both the apical and basilar dendritic trees increased by 50% during the first two postnatal months, remained at a plateau through 1.5 years of age, and then decreased over the peripubertal age range until stable adult levels were achieved. As a measure of the postnatal changes in inhibitory input to the axon initial segment of these pyramidal neurons, we determined the density of parvalbumin-immunoreactive axon terminals belonging to the chandelier class of local circuit neurons. The density of these distinctive axon terminals (cartridges) exhibited a temporal pattern of change that exactly paralleled the changes in dendritic spine density. These results suggest that subpopulations of cortical neurons may be regulated by dynamic interactions between excitatory and inhibitory inputs during development and, in concert with other data, they emphasize the cellular specificity of postnatal refinements in cortical circuitry.

A comparison of synapses onto the somata of intrinsically bursting and regular spiking neurons in layer V of rat SmI cortex

Regular spiking (RS) and intrinsically bursting (IB) neurons show distinct differences in their inhibitory responses. Under various conditions, the synaptic responses of RS cells display marked inhibitory postsynaptic potentials (IPSPs), whereas the responses of most IB cells do not (Silva et al: Soc Neurosci Abstr 14:883, 1988; Chagnac-Amitai and Connors: J Neurophysiol 61:747, 62:1149, 1989; Connors and Gutnick: TINS 13:99, 1990). This investigation is designed to determine if differences in the inhibitory responses of RS versus IB cells are reflected in differences in the concentration of inhibitory synapses onto their somata. RS and IB neurons in rat somatosensory cortex were identified by using intracellular recording and labeling, examined with the light microscope, and then serial thin-sectioned prior to examination with the electron microscope. Axonal terminals presynaptic to their somata and proximal dendrites were identified and classified according to criteria described by Peters and coworkers (Peters et al: J Neurocytol 19:584, 1990; Peters and Harriman: J Neurocytol 19:154, 1990; 21:679, 1992). The locations of these boutons were displayed on the surfaces of 3-D reconstructions of the somata and proximal dendrites. The reconstructions were produced directly from the serial thin sections by using a novel, electron microscopic, image-processing computer resource. Our analysis showed no significant difference in the types and concentration of boutons presynaptic to the cell bodies and proximal dendrites of intrinsically bursting versus regular spiking neurons. We conclude that the differences observed in the inhibitory responses of intrinsically bursting versus regular spiking neurons cannot be explained by differences in the concentrations of synapses onto their somata.

Map of the synapses formed with the dendrites of spiny stellate neurons of cat visual cortex

The synaptic input of six spiny stellate neurons in sublamina 4A of cat area 17 was assessed by electron microscopy. The neurons were physiologically characterized and filled with horseradish peroxidase in vivo. After processing the neurons were reconstructed at the light microscopic level using computer-assisted methods and analyzed quantitatively. The extensive branching of the dendritic tree about 50 microns from the soma meant that the distal branches constituted five times the length of proximal dendrite. Proximal and distal portions of a single dendrite from each neuron were examined in series of ultrathin sections (1,456 sections) in the electron microscope. The majority (79%) of the 263 synapses examined were asymmetric; the remainder (21%) were symmetric. Symmetric synapses formed 35% of synapses sampled on proximal dendrites and were usually located on the shaft. They formed only 4% of synapses sampled on distal dendrites. Spines accounted for less than half of the total asymmetric synapses (45%); the remainder were on shafts. Symmetric synapses formed with four of 92 spines. Nine spines formed no synapses. Spiny stellate neurons in cat visual cortex appear to differ considerably from pyramidal neurons in having a significant asymmetric (excitatory) synaptic input to the dendritic shaft.

Polyneuronal innervation of spiny stellate neurons in cat visual cortex

Our hypothesis was that spiny stellate neurons in layer 4 of cat visual cortex receive polyneuronal innervation. We characterised the synapses of four likely sources of innervation by three simple criteria: the type of synapse, the target (spine, dendritic shaft), and the area of the presynaptic bouton. The layer 6 pyramids had the smallest boutons and formed asymmetric synapses mainly with the dendritic shaft. The thalamic afferents had the largest boutons and formed asymmetric synapses mainly with spines. The spiny stellates had medium-sized boutons and formed asymmetric synapses mainly with spines. We used these to make a "template" to match against the boutons forming synapses with the spiny stellate dendrite. Of the asymmetric synapses, 45% could have come from layer 6 pyramidal neurons, 28% from spiny stellate neurons, and 6% from thalamic afferents. The remaining 21% of asymmetric synapses could not be accounted for without assuming some additional selectivity of the presynaptic axons. Additional asymmetric synapses may come from a variety of sources, including other cortical neurons and subcortical nuclei such as the claustrum. Of the symmetric synapses, 84% could have been provided by clutch cells, which form large boutons. The remainder, formed by small boutons, probably come from other smooth neurons in layer 4, e.g., neurogliaform and bitufted neurons. Our analysis supports the hypothesis that the spiny stellate receives polyneuronal innervation, perhaps from all the sources of boutons in layer 4. Although layer 4 is the major recipient of thalamic afferents, our results show that they form only a few percent of the synapses of layer 4 spiny stellate neurons.

Effects of peripheral axotomy on presynaptic axon terminals with GABA-like immunoreactivity

The facial nerve was unilaterally crushed at its exit from the stylomastoid foramen in three 3-month old male rats. After 10 days survival, before the regenerating axons had reinnervated their target muscles, the facial nucleus was examined to determine central patterns of response in material prepared to demonstrate the presence of GABA-like immunoreactivity with postembedding procedures using gold-labeled secondary antibody. The uninjured nucleus served as a control. In both control and injured nuclei, the GABAergic terminals synapse with all parts of the motor neurons, except the axon, and exhibit diverse morphologies. GABAergic axon terminals vary in their size and in the electron density of their axoplasm and the majority of the terminals contain pleomorphic vesicle profiles that display a range in their packing density and size. In both control and injured facial nuclei, only approximately 40% of the axon terminal profiles with pleomorphic vesicles exhibit GABA immunoreactivity. A morphometric analysis of the synaptic vesicle profiles in the GABA-positive terminals reveals that following axotomy there is no change in the mean number of synaptic vesicle profiles per GABAergic terminal profile. However, the mean size of the synaptic vesicle profiles in these terminals shows an axotomy-induced 50% increase, without change in the shapes of the enlarged vesicle profiles. Also, the numerical density of gold particles associated with the GABA-positive terminals is consistently greater in the injured than the control axon terminals. In the control animals quantitative analysis of the relative distribution of all axon terminal profiles in the neuropil categorized by the shape of their vesicle profiles as round, pleomorphic, or flat is 57:37:6. Ten days after axotomy the ratio of these categories in the injured nucleus has shifted to 35:60:5. This study demonstrates that the functional state of a postsynaptic target can influence the morphology of vesicle profiles in presynaptic elements as well as patterns of its afferent input.

Quantitative aspects of the GABA circuitry in the primary visual cortex of the adult rat

The number and size of synaptic contacts made by GABA-immunoreactive axonal boutons were estimated in each layer of the primary visual cortex (area Oc1M) of adult rats by using the dissector method. Immunoreactivity for GABA was detected with the postembedding immunogold technique on ultrathin sections. Targets of GABA synaptic contacts were also identified to predict the sites of GABA influence in the rat visual cortex. For the total cortical depth, 82 million out of an overall population of 666 million synaptic contacts per mm3 of tissue (or 1 in 8 contacts, 12%) were GABA. Layer IV averaged 62% more GABA contacts per unit volume than did any other cortical layer. Consequently, these represented a larger proportion (1 in 6, 17%) of the overall population of layer IV synaptic contacts. This higher number of GABA contacts was not due to a greater density of GABA boutons, but to an increased number of contacts made by each layer IV GABA bouton (mean of 1.4 contacts per bouton compared to 1.1 in other cortical layers). The total area occupied by the contacts on an average GABA bouton was similar in all layers; the higher number of contacts per GABA bouton in layer IV being compensated for by their smaller size. This observed constancy in the area of synaptic contacts suggests the presence of one or more regulatory mechanisms maintaining optimal numbers of the different macromolecules forming the synaptic contacts. The increased density of GABA contacts in layer IV compared to other cortical layers was due to their greater number targeting distal regions of the dendritic tree. Since layer IV receives the vast majority of thalamocortical terminals and since these axons preferentially target dendritic spines, the specific arrangement of GABA synaptic contacts in this layer could be designed to exert a precise inhibition near the site of the thalamic input and thus serve as the structural basis for the strong GABA-related hyperpolarization that followed the excitatory response after physiological stimulations of the thalamocortical pathway.