Transsynaptic impulse activity regulates postsynaptic density molecules in developing and adult rat superior cervical ganglion

Adaptive properties and heterogeneity of dopamine D(2) receptors - pharmacological implications

In this review, we focus on the marked adaptability of dopamine D(2) receptors to varying agonist levels and we discuss the extent to which this phenomenon can account for the heterogeneity of these receptors in regard to function and pharmacological responsiveness. We emphasize the significance of a distinction between synaptic and extrasynaptic receptors in this context. For example, the application of this dichotomy appears to shed new light on the various subgroups of antipsychotic drugs and the mechanisms underlying their different profiles.

Pituitary adenylate cyclase-activating polypeptide and vasoactive intestinal peptide inhibit dendritic growth in cultured sympathetic neurons

Pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP) are related neuropeptides that are released by the preganglionic sympathetic axons. These peptides have previously been implicated in the regulation of sympathetic neurotransmitter metabolism and cell survival in postganglionic sympathetic neurons. In this study we consider the possibility that PACAP and VIP also affect the morphological development of these neurons. Postganglionic rat sympathetic neurons formed extensive dendritic arbors after exposure to bone morphogenetic protein-7 (BMP-7) in vitro. PACAP and VIP reduced BMP-7-induced dendritic growth by approximately 70-90%, and this suppression was maintained for 3 weeks. However, neither PACAP nor VIP affected axonal growth or cell survival. The actions of PACAP and VIP appear to be mediated by PAC1 receptors because their effects were suppressed by an antagonist that binds to PAC1 and VPAC2 receptors (PACAP6-38), but not by an antagonist that binds to the VPAC1 and VPAC2 receptors. Moreover, exposure to PACAP and VIP caused phosphorylation and nuclear translocation of cAMP response element-binding protein, and agents that increase the intracellular concentration of cAMP mimicked the PACAP-induced inhibition of dendritic growth. These data suggest that peptides released by preganglionic nerves modulate dendritic growth in sympathetic neurons by a cAMP-dependent mechanism.

Localization of brain-derived neurotrophic factor and TrkB receptors to postsynaptic densities of adult rat cerebral cortex

Although neurotrophins are critical for neuronal survival and differentiation, recent studies suggest that they also regulate synaptic plasticity. Brain-derived neurotrophic factor (BDNF) rapidly increases synaptic transmission in hippocampal neurons, and enhances long-term potentiation (LTP), a cellular and molecular model of learning and memory. Loci and precise mechanisms of BDNF action remain to be defined: evidence supports both pre- and postsynaptic sites of action. To help elucidate the synaptic mechanisms of BDNF action, we used antisera directed against the extracellular and intracellular domains of trkB receptors, anti-trkBout and anti-trkBin, respectively, to localize the receptors in relation to synapses. Synaptic localization of BDNF was examined in parallel using anti-BDNF antisera. By light microscopy, trkBin and trkBout immunoreactivities were localized to hippocampal neurons and all layers of the overlying visual cortex. Immunoelectron microscopic analysis of the cerebral cortex revealed that trkB and BDNF localize discretely to postsynaptic densities (PSD) of axo-spinous asymmetric synaptic junctions, that are the morphological correlates of excitatory, glutamatergic synapses. TrkB immunoreactivity was also detected in the nucleoplasm by light and electron microscopy. Western blot analysis indicated that both anti-trkBout and anti-trkBin antisera react with a protein band in the PSD corresponding to the molecular weight expected for trkB; however, molecular species distinct from that for trkB were recognized in the nuclear fraction by both anti-trkBin and anti-trkBout antisera, indicating that the nuclear immunoreactivity, seen by immunocytochemistry, reflects cross-reactivity with proteins closely related to, but distinct from, trkB. The PSD localization of both BDNF and trkB supports the contention that this receptor/ligand pair participates in postsynaptic plasticity.

Functional trkB neurotrophin receptors are intrinsic components of the adult brain postsynaptic density

Neurotrophins have long been thought to act as target-derived factors that regulate the survival and differentiation of afferent neurons. Recently, brain-derived neurotrophic factor (BDNF) was shown to elicit rapid increases in synaptic activity of cultured hippocampal neurons by enhancing responsiveness to excitatory input. These findings suggest a postsynaptic localization of neurotrophin receptors. In this study, we examined the expression of trkB, a high-affinity receptor for BDNF, in the postsynaptic density (PSD), a proteinaceous specialization of the postsynaptic membrane. Western blot analyses with antibodies to trkB revealed localization to the PSD in adult rat cerebral cortex and hippocampus. Only the full-length, active form of trkB was detected in PSD samples. BDNF treatment of the adult cortical PSD resulted in a 5-fold increase in trkB autophosphorylation, supporting the contention that the PSD contains functional trkB. Truncated trkB, which does not contain the tyrosine kinase signaling domain, though present in membrane fractions, was undetectable in the PSD. The presence of trkB in the PSD is consistent with a role for neurotrophins in the regulation of synaptic activity via direct postsynaptic mechanisms.

The effects of extracellular matrix and osteogenic protein-1 on the morphological differentiation of rat sympathetic neurons

The growth patterns of axons and dendrites differ with respect to their number, length, branching, and spatial orientation; therefore, it is likely that these processes differ in their growth requirements. To examine this hypothesis, we have been analyzing the responses of cultured rat sympathetic neurons to three types of stimuli: large structural proteins of the extracellular matrix, matrix-associated growth factors, and neurotrophins. Purified structural proteins such as laminin and collagen IV have been found to promote only axonal growth; whereas the matrix associated growth factor, osteogenic protein-1, selectively stimulates dendritic growth. In contrast, nerve growth factor modulates the growth of both types of processes. These data suggest that process-specific interactions with the extracellular environment may be critical determinants of cell shape in neurons. Perinatal rat sympathetic neurons grown in culture in the absence of serum or glial cells extend a single process which is axonal in nature. Exposure to osteogenic protein-1 causes the formation of additional processes which express the morphological, cytoskeletal, and ultrastructural characteristics of dendrites. Consistent with observations on the regulation of dendritic growth in sympathetic neurons in situ, the dendrite-promoting activity of osteogenic protein-1 is independent of synaptic or electrical activity, but is modulated by nerve growth factor. In the presence of optimal concentrations of osteogenic protein-1 and nerve growth factor, the size of the dendritic arbor extended by cultured sympathetic neurons approximates that seen in situ at comparable developmental stages. Osteogenic protein-1 does not promote dendritic growth in cultured neurons obtained from embryonic ciliary, dorsal root, trigeminal or nodose ganglia, suggesting that its morphogenetic effects are cell selective. Since mRNA for osteogenic protein-1 is expressed in mature as well as embryonic target tissues of the sympathetic nervous system, we also examined the effects of osteogenic protein-1 on cultures of sympathetic neurons derived from adult rats. Consistent with results obtained with perinatal neurons, osteogenic protein-1 selectively promoted dendritic growth in adult neurons. These data suggest that this matrix-associated growth factor could play a role not only in the morphogenesis of the developing nervous system, but also in the maintenance and remodeling of dendritic structures in the mature animal.

Changes in glutamate immunoreactivity in the somatic sensory cortex of adult monkeys induced by nerve cuts

Antibodies to glutamate (Glu) were used to study the effects of reduced afferent input on excitatory neurons in the somatic sensory cortex of adult monkeys. In each monkey, immunocytochemical staining was compared to thionin and cytochrome oxidase (CO) staining in adjacent sections. In the cervical spinal cord, dorsal column nuclei, ventroposterior thalamus, and primary somatic sensory cortex (SI), Glu immunoreactivity (Glu-ir) was analogous to that described in normal animals; regions with reduced or absent Glu-ir were never observed and no appreciable differences were noted between the experimental and normal side. There were also no differences in CO or thionin-stained sections from the affected hemisphere. In the insuloparietal operculum, sections in the hemisphere contralateral to the nerve cut showed that most cortical fields had a normal pattern of Glu-ir (pattern a), some exhibited a reduction of Glu-ir (pattern b), and that in the central portion of the upper bank of the central sulcus, which corresponds to the general location of the hand representation of the second somatic sensory cortex (SII), Glu-ir had virtually disappeared (pattern c). Adjacent sections processed for CO or stained with thionin showed that in the regions corresponding to those characterized by pattern c, CO was slightly decreased and that glial cells had increased in number. In the regions of SII characterized by pattern c, small intensely stained glial cells displayed Glu-ir. These findings indicate that Glu-ir is regulated by afferent activity and suggest that changes in Glu levels in neurons as well as in glial cells may trigger the biochemical processes underlying the functional and structural changes occurring during a slow phase of reorganizational plasticity in the cerebral cortex of adult monkeys.

Deficiency of brain synaptic dystrophin in human Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is characterized by a defect in dystrophin, a high molecular weight protein that is located predominantly in muscle, but which has been detected in brain. Brain dystrophin has been localized to the synapse, in the postsynaptic density (PSD), and is absent in the mdx mouse, an animal model of human DMD. To define the potential pathogenic role of dystrophin deficiency in cognitive impairment, we examined the protein in human DMD brain. The 427-kd dystrophin was absent in the PSD from DMD brain, but was normally expressed in the brain from an age-matched control subject. Our findings indicate that dystrophin is deficient in human DMD cortical synapses and provide a potential pathogenic mechanism for cognitive impairment.

Effects of axotomy, deafferentation, and reinnervation on sympathetic ganglionic synapses: a comparative study

The main physiological and morphological features of the synapses in the superior cervical ganglia of mammals and the last two abdominal ganglia of the frog sympathetic chain are summarized. The effects of axotomy on structure and function of ganglionic synapses are then reviewed, as well as various changes in neuronal metabolism in mammals and in the frog, in which the parallel between electrophysiological and morphological data leads to the conclusion that a certain amount of synaptic transmission occurs at "simple contacts." The effects of deafferentation on synaptic transmission and ultrastructure in the mammalian ganglia are reviewed: most synapses disappear, but a number of postsynaptic thickenings remain unchanged. Moreover, intrinsic synapses persist after total deafferentation and their number is strongly increased if axotomy is added to deafferentation. In the frog ganglia, the physiological and morphological evolution of synaptic areas is comparable to that of mammals, but no intrinsic synapses are observed. The reinnervation of deafferented sympathetic ganglia by foreign nerves, motor or sensory, is reported in mammals, with different degrees of efficiency. In the frog, the reinnervation of sympathetic ganglia with somatic motor nerve fibers is obtained in only 20% of the operated animals. The possible reasons for the high specificity of ganglionic connections in the frog are discussed.

Haloperidol-induced morphological alterations are associated with changes in calcium/calmodulin kinase II activity and glutamate immunoreactivity

Administration of haloperidol for 2 weeks causes an increase within the caudate nucleus of asymmetrical synapses associated with a discontinuous or perforated, postsynaptic density (PSD) [Meshul et al. (1992), Psychopharmacology, 106:45-52; Meshul et al. (1992), Neuropsychopharmacology, 7:285-293]. Coadministration of the N-methyl-D-aspartate noncompetitive antagonist, MK-801, with haloperidol blocked the increase in striatal synapses containing a perforated PSD [Meshul et al. (1994), Brain Res., 648:181-195]. Examination of the caudate using immuno-gold electron microscopy revealed the vast majority (90%) of asymmetrical synapses were labelled with a glutamate antibody [Meshul et al. (1994), Brain Res., 648:181-195]. The purpose of this study was to determine if there were any changes in the density of glutamate immunoreactivity within presynaptic terminals of asymmetric synapses within the striatum following treatment with haloperidol for 1 month that would correlate with the previously observed increase in synapses with perforated PSDs. We also determined the activity of striatal calcium/calmodulin kinase II (CaMK II), an enzyme known to be localized within the synaptic region, after administration of haloperidol. We report here that haloperidol causes an increase in the activity of CaMK II and a decrease in the density of immuno-gold labelling for glutamate within the nerve terminals of asymmetrical synapses containing a perforated or nonperforated PSD. These results are consistent with the hypothesis that the haloperidol-induced increase in activity of CaMK II and the increase in glutamate release, as suggested by the decrease in presynaptic glutamate immunoreactivity, may ultimately lead to an increase in the number of synapses displaying a perforated PSD. These results support the speculation that the haloperidol-induced increase in synapses containing a perforated PSD may be associated with enhanced activity at excitatory synapses.

Cellular localization and laminar distribution of NMDAR1 mRNA in the rat cerebral cortex

N-methyl-D-aspartate (NMDA) receptors, which play a critical role in many cortical functions, are composed of a heteromeric assembly of different subunits: of these, the NMDA receptor subunit 1 (NMDAR1) is a constant component of, and thus an excellent marker for, NMDA receptors. In this study, we have investigated the cellular localization and laminar distribution of NMDAR1 mRNA in the cerebral cortex of adult rats by in situ hybridization histochemistry with a 35S-labeled cRNA probe. Specificity and background levels were determined in adjacent sections incubated with a 35S-labeled sense RNA. In sections incubated with the antisense RNA probe, specific hybridization signal was observed in a large number of cells. Some cells, however, did not appear to contain NMDAR1 mRNA. The vast majority of these unlabeled cells were small, suggesting that they are astrocytes or other small nonneuronal cells. Double-labeling studies with in situ hybridization histochemistry and immunocytochemistry with antibodies to glial fibrillary acidic protein (GFAP) showed that about 95.7% of the GFAP-positive cells did not express NMDAR1 mRNA, indicating that virtually all astrocytes do not contain this transcript. A semiquantitative evaluation of cortical neurons, defined as those cells larger than the GFAP-positive astrocytes, revealed that about 80% were associated with silver grains. The number of silver grains associated with every neuron was determined from sections exposed for 15 days, the background level was subtracted, and all labeled neurons were grouped into five groups: A (< or = 10 grains), B (11-20 grains), C (21-30 grains), D (31-40 grains), and E (> 40 grains). The number of neurons belonging to each group was then evaluated according to their occurrence in each cortical layer. In layer I all labeled neurons were in group A, whereas in layers II-III and V-VI positive neurons were in group A-E. In layer IV most neurons were in groups A and B, whereas only a few were in group E. These observations indicate that 1) virtually all cortical cells containing NMDAR1 mRNA in adult rats are neurons; 2) about 80% of all cortical neurons express NMDAR1 mRNA; and 3) labeled neurons can be divided into several groups on the basis of NMDAR1 mRNA levels expressed, which presumably reflect the number of NMDA receptors. The existence of neurons with a different number of receptors may be a critical factor for determining the physiological effect of NMDA receptor activation.(ABSTRACT TRUNCATED AT 400 WORDS)

Detection of dystrophin in the postsynaptic density of rat brain and deficiency in a mouse model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a common, lethal, chromosome X-linked inherited disease. Moderate cognitive impairment is a feature of DMD, but the underlying mechanisms are unknown. DMD is characterized by a defect in a protein, dystrophin, that is located predominantly in muscle but has been detected in brain. We sought to directly localize dystrophin within the complex synaptic structure of the cerebral cortex by focusing on the postsynaptic density (PSD), which appears to be central to synaptic function. We report that a specific anti-dystrophin antibody (anti 6-10) recognizes three distinct proteins in the purified PSD: the 400-kDa dystrophin and two previously unidentified dystrophin-related proteins of 120 and 110 kDa. These proteins exhibited differential regional expression in PSDs from cerebral cortex, cerebellum, and olfactory bulb. In the cortical PSD, the 400-kDa dystrophin was predominant, whereas the 120-kDa protein was the major species in cerebellum and olfactory bulb PSDs. The three proteins were differentially expressed in the PSD during cortical development: the 400-kDa protein exhibited a selective 9-fold increase during postnatal days 7 to 10, suggesting a normal physiological role in synaptic maturation. The PSD from the mdx mouse, a model of human DMD, contained no detectable 400-kDa dystrophin but expressed the two dystrophin-related proteins. Our results indicate that brain dystrophins are localized to the PSD, potentially as three isoforms, and raise the possibility that cognitive abnormalities in DMD are attributable to synaptic dysfunction associated with deficits in brain dystrophin molecules.

On the identity of the major postsynaptic density protein

Increasing evidence suggests that the postsynaptic density (PSD) plays a critical role in synaptic communication and plasticity. The major PSD protein (mPSDp), a calcium/calmodulin-dependent protein kinase, appears to be central to PSD function. The mPSDp has long been considered identical to the alpha subunit of the soluble calmodulin kinase II (alpha-CKII). However, mPSDp and alpha-CKII do differ in solubility and antigenicity, raising the possibility that the two proteins are distinct. To further define the relationship between the two proteins, we purified the mPSDp to homogeneity from adult rat cerebral cortex and compared the proteins. In contrast to alpha-CKII, the purified mPSDp was insoluble in high concentrations of salt, various detergents, chelators of divalent cations, and the strong denaturant guanidine hydrochloride. The pI value of the mPSDp was 6.2, whereas that of alpha-CKII was 6.7-7.2. The purified mPSDp bound calmodulin in the presence of Ca2+ and was autophosphorylated in a Ca2+/calmodulin-dependent manner. Polyclonal antiserum raised against mPSDp (anti-mPSDp) recognized purified mPSDp or mPSDp in synaptic membrane, indicating immunologic specificity among the synaptic proteins. Anti-mPSDp did not recognize alpha-CKII, whereas anti-alpha-CKII antibodies reacted only weakly with mPSDp, suggesting that the proteins are distinct but structurally similar. Moreover, sequence analysis of protease V8-digested polypeptides revealed that there was at least an 8-amino acid sequence, MLKVPNIS, that is not present in alpha-CKII. Finally, HPLC analysis of V8-digested fragments of mPSDp and alpha-CKII in parallel revealed dissimilar peptide patterns. Thus our observations suggest that mPSDp and alpha-CKII are similar but not identical. The unique physicochemical and structural properties of the mPSDp may provide insights into molecular mechanisms mediating synaptic plasticity.

Visual Experience Specifically Regulates Synaptic Molecules in Rat Visual Cortex

To study environmental modulation of synaptic molecular structure, the major postsynaptic density protein (mPSDp) from rat visual cortex was monitored. This membrane component, a Ca2+/calmodulin-dependent protein kinase subunit, was measured during normal postnatal development and after visual deprivation. Total synaptic membrane (SM) protein was used as an index of synapses as a whole.

During the first 2 postnatal months, total SM protein in the visual cortex increased 32–fold. In contrast, the mPSDp increased 455–fold, indicating that different molecular components of the cortical synapse develop differentially. Exposure to complete darkness during the first 2 postnatal weeks prevented normal development of total SM protein in visual cortex, values reaching only 66% of normal. Moreover, environmental lighting preferentially modulated the mPSDp, which attained only 34% of the normal value after dark rearing. Thus, visual deprivation selectively inhibited the normal development of specific synaptic components. Moreover, experience-dependent modulation was area specific. In contrast to the marked effect in visual cortex, light deprivation did not alter synapses in the nonvisual parietal and prefrontal cortices. Finally, the modulation of visual cortex mPSDp was stage specific, since visual experience did not alter the synaptic protein in adults. Our results suggest that early visual experience selectively and specifically modifies molecular synaptic components in the visual cortex.

Depolarization regulates expression of a synaptic vesicle protein in rat superior cervical ganglia in vitro

The role of membrane depolarization in the regulation of expression of a neuron specific protein was evaluated by culturing superior cervical ganglia from neonatal rats in defined medium and manipulating neuronal activity by depolarizing agents. P65 is an integral membrane protein of synaptic vesicles and can be used as a marker for general neuronal maturation. P65 antigen levels were quantified by indirect radioimmunoassay, using monoclonal antibodies. The expression of p65 in ganglion explants increased by 40-100% when the cultures were treated with the depolarizing agents, veratridine or high potassium. The veratridine effect could be blocked by simultaneous treatment with the sodium channel blocker, tetrodotoxin (TTX). The rise in p65 was not evident until 36 h after depolarizing treatment had begun and reached peak levels after 48 h, with no further increases observed with sustained treatment. After removal of the depolarizing treatment, p65 levels returned to control values after 24 h. P65 joins a growing number of molecules whose expression is regulated by membrane depolarization.