Quantitative electron microscopic analysis of postnatal development of zinc-positive nerve endings in the rat amygdala using Timm's sulphide silver technique

Ecologically relevant neurobehavioral assessment of the development of threat learning

As altricial infants gradually transition to adults, their proximate environment changes. In three short weeks, pups transition from a small world with the caregiver and siblings to a complex milieu rich in dangers as their environment expands. Such contrasting environments require different learning abilities and lead to distinct responses throughout development. Here, we will review some of the learned fear conditioned responses to threats in rats during their ontogeny, including behavioral and physiological measures that permit the assessment of learning and its supporting neurobiology from infancy through adulthood. In adulthood, odor-shock conditioning produces robust fear learning to the odor that depends upon the amygdala and related circuitry. Paradoxically, this conditioning in young pups fails to support fear learning and supports approach learning to the odor previously paired with shock. This approach learning is mediated by the infant attachment network that does not include the amygdala. During the age range when pups transition from the infant to the adult circuit (10-15 d old), pups have access to both networks: odor-shock conditioning in maternal presence uses the attachment circuit but the adult amygdala-dependent circuit when alone. However, throughout development (as young as 5 d old) the attachment associated learning can be overridden and amygdala-dependent fear learning supported, if the mother expresses fear in the presence of the pup. This social modulation of the fear permits the expression of defense reactions in life threatening situations informed by the caregiver but prevents the learning of the caregiver itself as a threat.

Morphology and dendritic maturation of developing principal neurons in the rat basolateral amygdala

The basolateral nucleus of the amygdala (BLA) assigns emotional valence to sensory stimuli, and many amygdala-dependent behaviors undergo marked development during postnatal life. We recently showed principal neurons in the rat BLA undergo dramatic changes to their electrophysiological properties during the first postnatal month, but no study to date has thoroughly characterized changes to morphology or gene expression that may underlie the functional development of this neuronal population. We addressed this knowledge gap with reconstructions of biocytin-filled principal neurons in the rat BLA at postnatal days 7 (P7), 14, 21, 28, and 60. BLA principal neurons underwent a number of morphological changes, including a twofold increase in soma volume from P7 to P21. Dendritic arbors expanded significantly during the first postnatal month and achieved a mature distribution around P28, in terms of total dendritic length and distance from soma. The number of primary dendrites and branch points were consistent with age, but branch points were found farther from the soma in older animals. Dendrites of BLA principal neurons at P7 had few spines, and spine density increased nearly fivefold by P21. Given the concurrent increase in dendritic material, P60 neurons had approximately 17 times as many total spines as P7 neurons. Together, these developmental transitions in BLA principal neuron morphology help explain a number of concomitant electrophysiological changes during a critical period in amygdala development.

The development and neurobiology of infant attachment and fear

Survival of altricial infants depends on attachment to the caregiver - a process that requires infants to identify, learn, remember, and approach their attachment figure. Here we review the neurobiology of attachment in infant rats where learning about the caregiver is supported by a specialized attachment neural circuitry to promote the infant-caregiver relationship. Specifically, the attachment circuit relies on infants acquiring learned preferences to the maternal odor, and this behavior is supported by the hyperfunctioning locus coeruleus and generous amounts of norepinephrine to produce experience-induced changes in the olfactory bulb and anterior piriform cortex. Infants also possess a reduced ability to acquire learned aversions or fear, and this behavior is facilitated through attenuated amygdala plasticity to block fear learning. Presumably, this attachment circuitry constrains the infant animal to express only learned preferences regardless of the quality of care received. As pups mature, and begin to travel in and out of the nest, the specialized attachment learning becomes contextually confined to when pups are with the mother. Thus, when outside the nest, these older pups show learning more typical of adult learning, presumably to prepare for independent life outside the nest. The quality of attachment can alter this circuitry, with early life stress prematurely terminating the pups' access to the attachment system through premature functional activation of the amygdala. Overall, the attachment circuit appears to have a dual function: to keep pups close to the caregiver but also to shape pups' behavior to match the environment and define long-term emotion and cognition.

Neurobiology of infant attachment

A strong attachment to the caregiver is critical for survival in altricial species, including humans. While some behavioral aspects of attachment have been characterized, its neurobiology has only recently received attention. Using a mammalian imprinting model, we are assessing the neural circuitry that enables infant rats to attach quickly to a caregiver, thus enhancing survival in the nest. Specifically, the hyper-functioning noradrenergic locus coeruleus (LC) enables pups to learn rapid, robust preference for the caregiver. Conversely, a hypo-functional amygdala appears to prevent the infant from learning aversions to the caregiver. Adult LC and amygdala functional emergence correlates with sensitive period termination. This study suggests the neonatal brain is not an immature version of the adult brain but is uniquely designed to optimize attachment to the caregiver. Although human attachment may not rely on identical circuitry, the work reviewed here suggests a new conceptual framework in which to explore human attachments, particularly attachments to abusive caregivers.

Distribution of synaptic zinc in the macaque monkey amygdala

We have mapped the macaque amygdala for the distribution of synaptic zinc (Zn), a co-factor of glutamate implicated in plasticity, as well as in several excitotoxic and other pathophysiological conditions. In brief, we found that the amygdala is Zn enriched in all nuclear groups (i.e., basolateral and cortical groups, as well as central and medial nuclei) but with marked differences in density. By comparing parallel tissue series histologically reacted for Zn and parvalbumin (PV), we further found that regions high in Zn are typically low in PV neuropil. In the basolateral group, there is a particularly distinct dorsoventral gradation such that Zn levels are most dense ventrally, i.e., in the paralaminar nucleus, the ventral division of the lateral nucleus, and the parvicellular divisions of both the basal nucleus and the accessory basal nucleus. PV levels are least dense in these same regions. For the central and medial nuclei, there is a slight mediolateral gradient, with Zn levels being higher medially. PV is low overall in these nuclei. Electron microscopic results confirmed that Zn is contained in synaptic boutons. These form asymmetrical, presumably excitatory, synapses, and the postsynaptic targets are mainly spines of projection neurons. The inhomogeneous distribution of Zn in the monkey amygdala may be related to different types or degrees of plasticity among the amygdaloid subnuclei. The complementary distribution with PV parallels that of several other substances and is interesting in the context of subnuclear vulnerability for human neuronal disease, such as seizure and Alzheimer's disease.

Developing a sense of safety: the neurobiology of neonatal attachment

Clinical data suggests a strong negative impact of traumatic attachments on adult mental illness, presumably through organizing brain development. To further explore this clinical issue, a mammalian model of imprinting was developed to characterize the neural basis of attachment in both healthy and traumatic attachments. The altricial neonatal rat must learn the mother's odor for nipple attachment, huddling, and orienting to the mother, all of which are required for pup survival. While it appears maladaptive to depend upon learning for attachment, the unique learning system of neonatal pups greatly enhances odor-preference learning and attachment while pups are confined to the nest. This heightened learning is expressed behaviorally as an enhanced ability to acquire learned odor preferences and a decreased ability to acquire learned odor aversions. Specifically, both odor-milk and odor-shock (0.5 mA) conditioning result in odor-preference acquisition. It appears as though there are at least three brain structures underlying the neonatal rat's sensitive period for heightened odor learning: (1) odor learning is encoded in the olfactory bulb; (2) the hyperfunctioning noradrenergic locus coeruleus (LC) appears to support preference conditioning through release of NE; and (3) the hypofunctioning amygdala appears to underlie pups' difficulty in learning odor aversions. Overall, this suggests that the CNS of altricial infants is specialized for optimizing attachments to their caregiver.

Developing a sense of safety: the neurobiology of neonatal attachment

Clinical data suggests a strong negative impact of traumatic attachments on adult mental illness, presumably through organizing brain development. To further explore this clinical issue, a mammalian model of imprinting was developed to characterize the neural basis of attachment in both healthy and traumatic attachments. The altricial neonatal rat must learn the mother's odor for nipple attachment, huddling, and orienting to the mother, all of which are required for pup survival. While it appears maladaptive to depend upon learning for attachment, the unique learning system of neonatal pups greatly enhances odor-preference learning and attachment while pups are confined to the nest. This heightened learning is expressed behaviorally as an enhanced ability to acquire learned odor preferences and a decreased ability to acquire learned odor aversions. Specifically, both odor-milk and odor-shock (0.5 mA) conditioning result in odor-preference acquisition. It appears as though there are at least three brain structures underlying the neonatal rat's sensitive period for heightened odor learning: (1) odor learning is encoded in the olfactory bulb; (2) the hyperfunctioning noradrenergic locus coeruleus (LC) appears to support preference conditioning through release of NE; and (3) the hypofunctioning amygdala appears to underlie pups' difficulty in learning odor aversions. Overall, this suggests that the CNS of altricial infants is specialized for optimizing attachments to their caregiver.

Postnatal development of zinc-rich terminal fields in the brain of the rat

The appearance and distribution of zinc-rich terminal fields in the rat forebrain was analyzed at 12 stages of postnatal development using the selenium method. Zinc stain was detected in neonates in piriform, cingulate, and motor cortices, septal area, and hippocampal formation. In the neocortex, a laminar pattern appeared progressively following an inside-out gradient: layer VI at postnatal day 0 (P0), layer V at P1, layers Va and Vb at P5, layer II-III at P9, and layer IV at P12. In the hippocampal formation the layered pattern in the dentate molecular layer appeared at P1-P3, and in the hilus and mossy fibers the stain was observed at P5. Patches in the caudate-putamen were sharply delimited at P1-P3. At these ages, staining was observed in the amygdaloid complex. In the thalamic and hypothalamic nuclei, stain appeared at P5-P7. Thus, a general increase in vesicular zinc over different telencephalic areas was determined until P15-P21, which was followed by a slight decrease thereafter (at P41). The increased stain in zinc-rich terminal fields is consistent with the development of telencephalic circuits. The rise in zinc might be relevant for the establishment and maturation of these circuits. On the other hand, the decrease in staining for zinc at later stages might be due to methodological problems but it might also reflect pruning of supernumerary connections and programmed cell death affecting zinc-rich circuits.

Cytochemical techniques for zinc and heavy metals localization in nerve cells

Zinc is one of the most abundant oligoelements in the living cell. It appears tightly bound to metallothioneins, loosely bound to some metalloproteins and nucleic acids, or even as free ion. Small amounts of zinc ions (in the nanomolar range) regulate a plentitude of enzymatic proteins, receptors, and transcription factors; thus, cells need accurate homeostasis of zinc ions. Some neurons have developed mechanisms to accumulate zinc in specific membrane compartments ("vesicular zinc") which can be revealed using histochemical techniques. This article is a short report on the different direct-indirect experimental approaches for zinc and heavy metal detection in neurons. Substances giving a bright color or emitting fluorescence when in contact with divalent metal ions are currently used to detect them inside cells; their use leads to the so called "direct" methods. The fixation and precipitation of metal ions as insoluble salt precipitates, their maintenance along the histological process, and their demonstration after autometallographic development are essential steps for other methods, the so-called "indirect methods" (Timm and Danscher Neo-Timm methods).

Unique Characteristics of Neonatal Classical Conditioning: The Role of the Amygdala and Locus Coeruleus

The central nervous system of altricial infants is specialized for optimizing attachments to their caregiver. During the first postnatal days, infant rats show a sensitive period for learning and are particularly susceptible to learning an attraction to their mother's odor. Classical conditioning appears to underlie this learning that is expressed behaviorally as an increased ability to acquire odor preferences and a decreased ability to acquire odor aversions. Specifically, in neonatal rats, pairing an odor with moderately painful shock (0.5mA) or milk produces a subsequent relative preference for that odor. The neural circuitry supporting the increased ability to acquire odor preferences appears to be the heightened functioning of the noradrenergic pontine nucleus locus coeruleus. Indeed, norepinephrine from the locus coeruleus appears to be both necessary and sufficient for learning during the sensitive period. On the other hand, the decreased ability to acquire odor aversions seems to be due to the lack of participation of the amygdala in at least some aversive learning situations. The site of plasticity in the pup's brain appears to be limited to the olfactory bulb. This neonatal sensitive period for learning ends around postnatal day 9-10, at which time pups make the transition from crawling to walking and classical conditioning becomes "adultlike." The neonatal behavioral and neural induced changes are retained into adulthood where it modifies sexual behavior.

The neuronal organization of the supracapsular part of the stria terminalis in the rat: the dorsal component of the extended amygdala

In the present normal anatomical light and electron microscopic study in the rat, histochemical (Nissl, Timm, Golgi) or immunocytochemical (microtubule-associated protein type 2, glutamate decarboxylase, glutamate receptor subunit 1, synaptophysin) stains were used to analyse neurons embedded within the stria terminalis and their associated neuropil. These cells are closely related to the bed nucleus of the stria terminalis and the centromedial amygdala, and have been termed the "supracapsular part of the bed nucleus of the stria terminalis". The largest part of this neuronal complex is located in the ventrolateral part of the stria, where it appears as a round or oval "lateral pocket" in virtually any type of light microscopic preparation because of its collection of neuronal cell bodies and dense neuropil, in addition to a lacework of unmyelinated axons. A much smaller but still distinct "medial pocket" is located in the medial corner of the stria. The large lateral subdivision of the supracapsular stria terminalis is directly continuous with the lateral bed nucleus of the stria terminalis and extends to the central amygdaloid nucleus, containing a column of neurons that is only broken up into cell clusters at the most caudal levels of the stria as it drops vertically toward the amygdala. The considerably smaller medial subdivision appears, in turn, to be directly continuous with the medial part of the bed nucleus of the stria terminalis. The medial column tapers off more rapidly than the lateral part, so that as the middle levels are approached, only small interrupted clusters of cells are seen. Solitary neurons can also be found in practically every part of the stria terminalis except among the ventrally located axons of the commissural component. Most of the neurons are small to medium in size, as viewed in transverse sections of the stria, but larger neurons are also encountered. In sections parallel to the stria, many neurons are fusiform in appearance. The dendrites are often aligned in a longitudinal fashion; many of the dendrites related to the cells in the lateral pocket are moderately to densely spined, whereas those in the medial pocket are more sparsely spined. The neuropil in both the lateral and medial pockets is characterized by boutons, bundles of unmyelinated axons, and dendrites. Based on their vesicle content, the boutons are divided into three major types: (A) round or slightly oval, agranular vesicles of uniform size; (B) pleomorphic, agranular vesicles, many of which are flattened; and (C) pleomorphic agranular vesicles, some of which are considerably larger than the ones in type B boutons. Type A boutons establish contacts with both dendritic spines and shafts, whereas types B and C usually contact dendritic shafts and sometimes somata. These synaptic components are similar to those described earlier for the central and medial amygdaloid nuclei. Overall, our results support the contention advanced in 1923 by Johnston [J. comp. Neurol. 35, 337481] that the cells accompanying the stria terminalis are interconnecting columns of a macrostructure encompassing the bed nucleus of the stria terminalis and centromedial amygdala. More recently, it has been appreciated that columns of neurons below the globus pallidus also belong to this macrostructure [Alheid G. F. et al. (1995) In The Rat Nervous System, 2nd edn, pp. 495 578, Academic, San Diego; de Olmos J. S. et al. (1985) In The Rat Nervous System, pp. 223-334, Academic, Sydney], which has been named the "extended amygdala".

Zinc-positive presynaptic boutons of the rabbit hippocampus during early postnatal development

The evolution of vesicular zinc-containing boutons in the developing rabbit hippocampus has been studied during early postnatal life using the selenite-Danscher histochemical method. By P3, large immature mossy fiber boutons with labeled synaptic vesicles were seen in the hilus of the dentate gyrus and in the stratum lucidum of the CA3-CA4 hippocampal areas. After P5, smaller boutons with labeled vesicles were identified in the stratum oriens and stratum radiatum of all hippocampal areas, and even transiently in the stratum lacunosum of P6 animals. Vesicular zinc-containing boutons increased in number and underwent ultrastructural maturation; light microscope densitometric-volumetric measurements were used to quantify their presence in every hippocampal lamina. Electron microscope stereologic analysis permitted accurate estimation of the actual numbers along early postnatal development. Three main phases of zinc-positive bouton growth were detected during the first postnatal month. The first phase, starting at P5, is characterised by an abrupt rise in vesicular zinc content which at P8-P9 begins to decrease. The second phase is characterised by a consistent rise in vesicular zinc content from P10 to P12 to a level which is maintained until P18; this steady period is the result of partial and sequential elimination of zinc-positive boutons in some areas (i.e. oriens of CA3 by P11, radiatum of CA3 by P13, and radiatum of CA1 and lucidum of CA3 by P15) while they continue to increase in other areas. The final phase is a continuous increase to almost adult levels.

Postnatal development of zinc-containing cells and neuropil in the hippocampal region of the mouse

The present study describes the postnatal development of zinc-containing boutons and their neurons of origin in the hippocampal region of the mouse. Ages investigated for the development of zinc-containing neuropil were postnatal days 0 (P0), P3, P7, P11, P15, P21, and P28. For zinc-containing cell bodies P7, P15, P21, and P28 were studied. In the area dentata, zinc-containing neuropil appeared first by P3 adjacent to the suprapyramidal limb of the granule cell layer and extended later toward the infrapyramidal limb. By P15, inter- and intralaminar gradients corresponded to those seen in adult animals. The appearance of labeled granule cells followed closely, although temporally delayed, the pattern of granule cell neurogenesis. All granule cells were labeled by P28. In the hippocampus proper, zinc-containing neuropil was seen by P0, but staining of the incipient mossy fiber zone was first visible by P3. Staining pattern and intensity developed gradually until they reached their mature appearance by P15. The distribution of labeled cells was identical to that seen in mature animals by P7 in CA3, but first by P21 in CA1. In the subiculum, neuropil staining first appeared proximally by P7, included all of this area by P11, and appeared mature by P21. A few labeled cells were seen in the proximal subiculum at all ages at which labeled cells were present in CA1. Labeled cells which extended further distally became first visible by P21. Their number and labeling intensity reached mature levels by P28. In the presubiculum, retrosplenial area 29e, and parasubiculum, neuropil staining first appeared by P3. The retrosplenial area 29e could be distinguished by P11. This area and the presubiculum reached their adult appearance by P21. This occurred first by P28 in the parasubiculum due to the late maturation of the parasubiculum a. Labeled cells were first seen by P7 in layer III of the presubiculum and by P15 in the retrosplenial area 29e and the parasubiculum. Cell labeling appeared mature by the same times as the neuropil staining. In the entorhinal areas a very light neuropil stain was apparent in the deeper layers by P0. A distinct rise in staining intensity was first observed by P7 in layers I-III. Thereafter, mature characteristics developed gradually and were attained by P21. Cell labeling was not seen in the medial entorhinal area. A few labeled cells were apparent by P7 in the lateral entorhinal area. After a slight increase by P15, numerous labeled cells were found in layer II and layer VI by P21. Their distribution and labeling intensity appeared mature by P28. Zinc-containing cells appear to represent cells formed late in the course of neurogenesis in all areas aside from the lateral entorhinal area. As far as intrinsic connections are concerned, it is the development of projections from this subset of neurons which is monitored in this study. We suggest that the appearance of zinc may contribute via its different effects on N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors to the end of a developmental phase that is permissive to changes in synaptic efficacy. Species differences and alternative functions of zinc are considered.

Hypoxia protects against the neurotoxicity of kainic acid

A normobar hypoxia (9% oxygen) of 8 h reduces the neurotoxicity of a subcutaneous injection of 10 mg/kg kainic acid given one week later. Both seizures and degenerative changes, including cell death of hippocampal and cortical neurons are markedly decreased by hypoxia. It is also shown that hypoxia also markedly reduced the extensive depletion of zinc from mossy fiber terminals normally induced by kainic acid. This suggests that a protective mechanism induced by hypoxia may affect the glutamatergic transmission in these synapses and prevent excessive synaptic excitation. The possible involvement of adenosine and/or GABA in this protective mechanism is discussed.

Histochemical localization of synaptic zinc in the developing cat visual cortex

The terminal boutons of many neurons in the telencephalon are known to contain a vesicle-bound, chelatable pool of zinc (Zn2+) that can be selectively visualized with histochemical procedures. In this paper, the normal laminar, areal, and ultrastructural distribution of histochemically reactive zinc in the visual cortex of the adult cat as well as its development from birth are described. In the adult cat visual cortex, intense zinc staining was found in layers I, II, III, and V, with layer VI staining only lightly. The primary geniculostriate input zone, layer IV, was conspicuously distinguished by the relative absence of zinc. This distinct pattern was restricted only to areas 17 and 18 and differentiated them from adjacent cortical area 19 laterally and the subadjacent cingulate cortex. The earliest zinc-positive staining in visual cortical areas 17 and 18 was first apparent by postnatal day 2 (P2) and was characterized by staining of a thin layer at the bottom of the cortical plate. By P10, and continuing through P20, synaptic zinc formed a trilaminar pattern of dense staining in areas 17 and 18, which included the top of layer I, and layers III and V. The laminar pattern of synaptic zinc in visual cortex appeared mature by P30, except that the distribution of zinc in layer IV was not uniform. This was most apparent around P50 in tangential sections through layer IV from opened and flattened cortex, where columnar patches of increased zinc staining were apparent in area 17. These columns were approximately 400 microns in diameter, with a centre-to-centre spacing of approximately 900 microns. The distribution of synaptic zinc apparently reflects the process of synaptic maturity of the cat visual cortex and appears to demarcate a particular form of columnar organization in visual cortex.