The mirror neuron system and the consequences of its dysfunction

The discovery of premotor and parietal cells known as mirror neurons in the macaque brain that fire not only when the animal is in action, but also when it observes others carrying out the same actions provides a plausible neurophysiological mechanism for a variety of important social behaviours, from imitation to empathy. Recent data also show that dysfunction of the mirror neuron system in humans might be a core deficit in autism, a socially isolating condition. Here, we review the neurophysiology of the mirror neuron system and its role in social cognition and discuss the clinical implications of mirror neuron dysfunction.     

Occurrence and possible consequences of multipolar mitoses in primary cultures of adult rat hepatocytes

Proliferating primary cultures of adult rat hepatocytes are characterized by the occurrence of multipolar mitoses, and chromosome loss resulting in the formation of micronuclei at telophase. The percentage of multipolar mitotic figures was determined to be 12.76 ± 7.9%, 80% of which were tripolar. Multipolar mitotic stages showed a high incidence of chromosome loss, increasing from meta- (61.7 ± 16.6%) to telophase (72.1 ± 19.3%). Regular bipolar mitotic figures on the other hand also showed chromosome loss, however, to a lesser degree and decreasing from meta- (49.5 ± 10.4%) to telophase (34.9 ± 7.9%). The incidence of chromosome loss even in regular mitotic figures is very high compared to other cells and appears to depend on another special feature of hepatocytes: they remain flat and well attached during mitosis, so that shearing forces could be responsible for the separation of chromosomes from the mitotic spindle. Additionally this morphology creates a situation allowing for a maximal interaction of mitotic spindles of binucleated cells, leading to the high rate of multipolar mitoses observed. Both multipolar mitoses and chromosome loss could also explain the consecutive detachment of hepatocytes reported for proliferating primary cultures, since the aneuploid daughter cells generated can be expected to be nonviable in most cases and eventually detach. © 1993 Wiley-Liss, Inc.     

Long-term functioning of enucleate sieve elements: possible mechanisms of damage avoidance and damage repair

Abstract. The sieve-tube elements of long-lived arborescent monocotyledons which lack secondary thickening remain functional for many decades despite lacking a nucleus. A minimal requirement for transport by mass flow powered by the Munch mechanism is maintenance of semi-permeability of the plasmalemma of the sieve tube elements; loading and re-loading could be deputed to nucleate cells symplastically linked to the sieve elements. An additional requirement in the long-term relates to replacement of components damaged by mechanical, chemical or radiation intrusions. Minimizing the damage from radiation and chemical agents can be related to a number of commonly observed features of sieve tubes. Damage from O₂ and radicals derived there from is minimized in these essentially aerobic cells by (1) the absence of intercellular gas spaces in the phloem combined with the lower O₂ solubility and diffusivity in concentrated disaccharide (or sugar alcohol) solutions, (2) the absence of photosynthetic machinery which could generate singlet oxygen, and (3) the presence of at least some components of scavenging mechanisms (glutathione, peroxidase, abscorbate). Non-enzymic glycosylation (exacerbated by O2) of proteins is minimized by the low concentrations of reducing sugars in sieve-tubes. Ultraviolet damage is minimized by UV-absorbing materials between the plant surface and the sieve tubes, including the selerenchymatous cap of fibres on the vascular bundles. The extent to which repair involves symplastic import of polypeptides from nucleate companion cells, thus breaching the −800-Da limit on symplastic transport, is unclear, but it could occur in fully differentiated companion cell-sieve element associations without necessarily upsetting development.     

Predator-induced behavioral defense and its ecological consequences for two calanoid copepods

Summary We used an automated technique for the observation and quantification of zooplankton swimming behavior to study the behavioral responses of two congeneric, herbivorous, freshwater copepod prey to a copepod predator (Limnocalanus macrurus). One prey, Diaptomus sicilis, often co-occurs with Limnocalanus, while previous studies indicated that the zoogeographic distribution of the second prey, Diaptomus oregonensis, was independent of the predator. We found that in the presence of Limnocalanus, D. sicilis swims more slowly and with less hopping and jumping than D. oregonensis. Diaptomus sicilis is also attacked and consumed by the predator Limnocalanus macrurus less frequently than D. oregonensis. We suggest that the faster, noisier swimming of D. oregonensis increases its vulnerability to Limnocalanus. The behavioral defenses to both prey are induced by the presence of the predator, and may represent two different anti-predator strategies, crypsis and avoidance for D. sicilis and D. oregonensis respectively. In a zoogeographical analysis D. oregonensis occurs at densities below D. sicilis in lakes where Limnocalanus is at elevated abundances, while in low-predator lakes the opposite is true. This distribution pattern supports our experimental results, and suggests that D. sicilis is adapted to survive with Limnocalanus, while D. oregonensis is not.     

Synthesis of glycoproteins in a single identified neuron of Aplysia californica

Incorporation of L-[³H]fucose into glycoproteins was studied in R2, the giant neuron in the abdominal ganglion of Aplysia. [³H]fucose injected directly into the cell body of R2 was readily incorporated into glycoproteins which, as shown by autoradiography, were confined almost entirely to the injected neuron. Within 4 h after injection, 67% of the radioactivity in R2 had been incorporated into glycoproteins; at least 95% of these could be sedimented by centrifugation at 105,000 g, suggesting that they are associated with membranes. Extraction of the particulate fraction with sodium dodecyl sulfate (SDS), followed by gel filtration on Sephadex G-200 and polyacrylamide gel electrophoresis in SDS revealed the presence of only five major radioactive glycoprotein components which ranged in apparent molecular weight from 100,000 to 200,000 daltons. Similar results were obtained after intrasomatic injection of [³H]N-acetylgalactosamine. Mild acid hydrolysis of particulate fractions released all of the radioactivity in the form of fucose. When ganglia were incubated in the presence of [³H]fucose, radioactivity was preferentially incorporated into glial cells and connective tissue. In contrast to the relatively simple electrophoretic patterns obtained from cells injected with [³H]fucose, gel profiles of particulate fractions labeled with [¹⁴C]valine were much more complex.     

Proteolytic processing of a peptide precursor in Aplysia neuron R14

The large neurons of the mollusc Aplysia are useful for studying the biogenesis of neuropeptides in single cells. Neuron R14 in the abdominal ganglion synthesizes large quantities of a 10-kDa neuropeptide precursor. The amino acid sequence of this precursor has been defined by analysis of the nucleotide sequence of a cDNA clone. We labeled proteins in vivo by microinjection of radioactive amino acids into individual R14 neurons. The labeled peptides were fractionated by high performance liquid chromatography and subjected to Edman degradation, thus enabling us to determine post-translational processing sites. Cleavage of the signal sequence was observed and at two internal sites. Cleavage at these internal sites occurs at basic amino acids and results in three products, a 2.9-, a 4.9-, and a 1.4-kDa peptide. These studies of protein processing serve as a basis for further investigations of the biogenesis and physiological activities of the neuropeptides.     

Sensitivity of a peptide-activated neuron in Aplysia to serotonin and cyclic AMP-relevant agents

Serotonin depolarized Aplysia buccal motoneuron B16. The response could be obtained in high-magnesium/low-calcium medium, indicating a direct effect on B16 and supporting a putative monosynaptic input to B16 from the serotonergic metacerebral neurons. Similar depolarizing effects in high-magnesium/low-calcium medium were obtained in response to 8-bromo cyclic AMP, isobutylmethylxanthine, theophylline and forskolin. Tolbutamide, a putative inhibitor of cyclic AMP-dependent protein kinase, blocked or reversed responses of B16 to egg-laying hormone containing extracts and to serotonin. Serotonin and forskolin significantly increased the cyclic AMP content of buccal ganglia, whereas egg-laying hormone-containing extracts did not.     

Patterned jaw movements and the motor neuron activity during rejection of seaweed in Aplysia kurodai

Japanese Aplysia kurodai feeds well on Ulva. In the present experiments we collected several species of seaweed at a location with many animals and initially explored the preference behavior for them. The animals rejected Grateloupia, Pachydictyon, Gelidium and Laurencia with rhythmic jaw and radula movements (active rejection). The animals sometimes bit off a piece of them (biting-off response). Recording activity of muscles contributing to jaw-opening and jaw-closing in freely moving animals showed that the onset of the jaw-closing activity, which always started later than the jaw-opening activity during each cycle of ingestion of Ulva, was advanced toward that of the jaw-opening activity during each cycle of the active rejection. Semi-intact experiments also showed that application of Pachydictyon or Gelidium extract to the lip region advanced the firing onset of the jaw-closing motor neurons at the radula-retraction phase. Video analysis showed that during the Ulva response the jaws opened at the radula-protraction phase and remained half-open at the earlier radula-retraction phase, while the jaws opened similarly at the radula-protraction phase but immediately closed at the radula-retraction phase during the Pachydictyon or Gelidium response. Accepted: 8 October 1997     

Cell polarity: an examination of its behavioral expression and its consequences for polymorphonuclear leukocyte chemotaxis

Locomoting polymorphonuclear leukocytes (PMNs) exhibit a morphological polarity. We demonstrate that they also exhibit a behavioral polarity in their responsiveness to chemotactic factor stimulation. This is demonstrated by (a) the pattern of their locomotion in a homogeneous concentration of chemotactic factors, (b) their responses to increases in the homogeneous concentration of chemotactic factors, and (c) their responses to changes in the direction of a chemotactic gradient. The behavioral polarity is not a function of the rate of locomotion of the particular stimulant used to orient the cells, but may reflect an asymmetric distribution of chemotactic receptors or the motile machinery. The polar behavior affects the chemotactic ability of PMNs. The data are discussed in relation to possible mechanisms of sensing a chemotactic gradient.     

Age-dependent behavioral changes and physiological changes in identified neurons in Aplysia californica

The gill withdrawal reflex (GWR) to direct gill stimulation was studied in sexually mature Aplysia and in those older by at least two months. The GWR threshold in old Aplysia was five- to sevenfold higher than that in mature animals. In the habituation paradigm, the GWR amplitude decremented rapidly to zero in old animals whereas in mature animals it persisted for at least ten trials. The GWR could not be dishabituated in old animals. The GWR is an age-dependent behavior in that parieto-visceral ganglion suppression of the GWR appears to increase with age. Also the electrophysiological properties of two neurons in the parieto-visceral ganglion were compared in the two age groups: L₇ a neuron which dishabituates the GWR in mature and not in old animals; and R₂ which manifests cytological changes with age. In old animals L₇′s input resistance was lower, the time constant was increased, and the size of the psp evoked by gill stimulation was smaller than those of mature L₇s. Similar membrane changes with age were measured in R₂. Soma size of L₇ was approximately the same in the two age groups as was that of R₂. The physiological parameters of neurons of known function continue to change during postmetamorphic life of Aplysia.     

Central actions of R15, a putative peptidergic neuron in Aplysia

We report that the bursting pacemaker neuron R15 has central actions on other identified neurons in the abdominal ganglion of Aplysia california. The follower cells are located on the dorsal surface of the left lower quadrant of the ganglion and include members of the LC cell cluster. A spontaneous burst of impulses in R15 produces a slow, graded, excitatory potential of up to 8 mV in follower cells. The response begins about 2-3 s after the first impulse in an R15 burst, and reaches its peak at about 4-6 s (corresponding approximately to the end of the R15 burst). In some preparations a biphasic response was seen composed of the early depolarization followed by a slower excitatory or inhibitory phase. All the responses were blocked when R15 was hyperpolarized to prevent spiking. The magnitude of the response was reduced in a graded fashion by prematurely terminating the R15 burst with hyperpolarizing current and was increased when depolarizing current was injected into R15 during a burst. Central actions of R15 were observed in only 28% of our preparations, and their presence may depend on unknown physiological factors. The effects are likely to be mediated by R15 neuropeptides. The accessibility of both R15 and its targets in this preparation should facilitate further analysis of this interaction.     

Metabolism and axonal transport of polyamines in a single identified neuron of Aplysia californica

Abstract: The metabolism of polyamines was investigated by injecting purified [³H]putrescine directly into the soma of the giant neuron R2 of Aplysia . Injected putrescine was rapidly metabolized to spermidine, spermine, and several catabolites, including GABA and monoacetylputrescine. Identification of these products was by comparison with the authentic compound using ion exchange chromatography. When R2 was injected with amounts of [³H]putrescine determined so that the intracellular content of labeled precursor was less than 6 × 10-⁶ M , metabolism was rapid and occurred via pathways similar to those in mammalian tissues. At concentrations of labeled precursor greater than 2 × 10⁻⁴ M , relatively little putrescine was converted to product. By 4 h after injection, putrescine and its labeled products appeared in R2's axon, where additional metabolism occurred. These results indicated that the enzymes involved in polyamine interconversion are not restricted to R2's cell body, and this suggestion was corroborated by finding ornithine decarboxylase and S -adenosylmethionine decarboxylase activities in Aplysia nerves. The distribution of the polyamines along R2's axon was compared with that of ³H-glycoproteins, with the finding that while the acid-soluble polyamines move by diffusion, labeled polyamines associated with protein are rapidly transported.     

Development and modulation of endogenous bursting in identified neuron R15 of juvenile Aplysia

Evidence from a variety of both vertebrate and invertebrate preparations has demonstrated that modulation of the intrinsic firing patterns of individual neurons can have a dramatic effect on the functional output of a neural circuit. Although the mechanisms underlying the production and modulation of intrinsic firing patterns have been extensively studied in adult nervous systems, relatively little is known about how these two features of intrinsically active neurons develop. To address these issues, we have examined the development of endogenous bursting and its modulation by neuropeptides in the identified cell R15 of juvenile Aplysia. Confirming Ohmori (1981), we found that the mature parabolic bursting pattern of R15 is absent in early juvenile stages and develops only gradually over the last stage of juvenile development. We have then analyzed the modulatory effects of extracts made from the neurosecretory bag cells of Aplysia on the immature firing pattern of juvenile R15 cells. In the adult, neuroactive peptides released from the bag cells are known to intensify bursting. In juveniles, we have found that bag cell extract (BCE) can induce bursting prematurely as well as intensify immature bursts, whereas control extracts have no effect on the firing pattern of R15. These results show that the ionic currents necessary for the generation of endogenous bursting in R15 are present and can be modulated before the normal developmental expression of the burst pattern.     

Immune and behavioral consequences of microglial reactivity in the aged brain

Bidirectional communication between the immune system and the brain is essential for mounting the appropriate immunological, physiological, and behavioral responses to immune activation. Aging, however, may impair this important bi-directional interaction. In support of this notion, peripheral infection in the elderly is associated with an increased frequency of behavioral and cognitive complications. Recent findings in animal models of aging and neurodegenerative disease indicate that microglia, innate immune cells of the brain, become primed or reactive. Understanding age- and disease-associated alterations in microglia is important because glia (microglia and astrocytes) play an integral role in propagating inflammatory signals that are initiated in the periphery. In this capacity, brain glia produce inflammatory cytokines that target neuronal substrates and elicit a sickness-behavior syndrome that is normally beneficial to the host organism. Increased reactivity of microglia sets the stage for an exaggerated neuroinflammatory cytokine response following activation of the peripheral innate immune system, which may underlie subsequent long-lasting behavioral and cognitive deficits. In support of this premise, recent findings indicate that stimulation of the peripheral immune system in aged rodents causes exaggerated neuroinflammation that is paralleled by cognitive impairment, prolonged sickness, and depressive-like complications. Therefore, the purpose of this review is to discuss the new evidence that age-associated priming of microglia could play a pathophysiological role in exaggerated behavioral and cognitive sequelae to peripheral infection.     

Ecological and behavioral consequences of digestion in frugivorous animals

The postingestional effect of seed size and mass and nutrient composition on fruit profitability are reviewed. It is emphasized that profitability results from the interaction between fruit characteristics and the physiological and morphological traits of frugivores. The processes by which frugivores regurgitate or defecate seeds and the differential processing of the nutrient rich pulp and ballast in fruit are strongly dependent on the interaction between frugivore gut morphology and seed size. Euphonias that lack a functional gizzard defecate seeds, whereas tanagers that have a delete a functional gizzards regurgitate seeds. Some frugivores separate seeds from pulp and exocarp in the gizzard. It is hypothesized that the gizzard plays an important role in determining the postigestional fate of different pulp components. Although fruit nutrient content is often invoked as a determinant of frugivore feeding choices, studies that rely on proximal nutrient analysis, have often failed to find clear nutrient composition-preference correlations. It is argued that a partial reason for this failure is that proximal nutrient analysis ignores the complexities of fruit digestion. The ecological and physiological correlates of lipid and sugar assimilation are used to identify the limitations of traditional proximal nutrient analyses in fruit-frugivore studies. We suggest that recognizing the intricacies of the digestive characteristics of frugivores may reveal a much richer patterning in the interaction of frugivores with plants than has been previously hypothesized.     

Synthetic egg-laying hormone of Aplysia: quantitative studies of induction of egg laying in Stylocheilus and of the activation of Aplysia buccal neuron B16

1. Synthetic Aplysia egg-laying hormone (ELH-lysine-amide) elicited egg laying in Stylocheilus at a threshold dose of 0.5 microgram per recipient, estimated to be a concentration in the circulation of Stylocheilus of approximately 70 nM. 2. Threshold level and size of egg mass produced by ELH-lysine-amide and bag cell extracts (containing biological ELH) were not significantly different. Latency to lay of recipients of 0.5-4.0 micrograms ELH-lysine-amide (30 +/- 1 min) was significantly longer (P less than 0.05) than for bag cell extract recipients (21 +/- 1 min). 3. ELH-lysine-amide depolarized and activated action potentials in Aplysia buccal neuron B16 in high magnesium, low calcium medium. 4. The lowest concentration of ELH-lysine-amide to activate a supra-threshold response of left and right B16 neurons ranged from 250 nM to 1 microM. 5. Threshold levels for responses to synthetic ELH-lysine-amide and to biological ELH were approximately the same in both egg-laying assay and electrophysiological assay, indicating the likely identity of synthetic and biological ELH. However, the shorter egg-laying latency with bag cell extract suggests that there may be additional factors in the extract that facilitate egg laying.