Collateral sprouting of somatostatin-immunoreactive axons after partial deafferentation of the central nucleus of the rat amygdala

Synaptic Plasticity in the Central Nucleus of the Amygdala

In recent years, the amygdala has emerged as a critical site of plasticity for the acquisition of various forms of Pavlovian learning, either aversive or appetitive. In most of these models, the critical site of plasticity has been localized to the basolateral complex of the amygdala (BLA). In contrast, the central nucleus of the amygdala has emerged as a passive relay of potentiated BLA outputs toward downstream effectors. At odds with this view, however, recent studies suggest that the central nucleus may also be a site of plasticity and play an active role in some forms of Pavlovian learning. The present review summarizes the evidence supporting this possibility.

Lesions of the Cerebral Cortex and Caudate-Putamen Enhance GABA Function in the Rat Superior Colliculus

Unilateral lesions of the rat frontal cortex were made either alone or in combination with the caudate-putamen in order to examine (a) their morphological influence on the substantia nigra and (b) their neurochemical influence on GABA function in the superior colliculus. One to two months following the combined lesion, neuronal somata in the ipsilateral pars reticulata of the substantia nigra were clearly hypertrophied (+ 30%). Morphological changes in the substantia nigra were not evident contralaterally or in animals bearing only cortical lesions. One to two months following cortex-only lesions, no significant alterations in tectal GABA concentration were observed. However, the combined lesion induced elevations of GABA within both the medial and lateral sectors of the intermediate and deep layers of the superior colliculus. This effect was restricted to the ipsilateral side and was most pronounced in lateral sectors. The vast majority of GABA released from superfused control tectal slices by a depolarizing stimulus (35 mM KCl) was calcium-dependent. Such evoked GABA release from ipsilateral tectal slices was significantly reduced (- 25%) by unilateral lesions of the substantia nigra, a structure that is known to provide GABA-containing inputs to the tectum. In contrast, cortical lesions alone significantly enhanced the evoked tectal GABA release (+ 66%), although their influence was again confined to the ipsilateral side. Combined lesions of the cerebral cortex and caudate-putamen significantly enhanced the evoked GABA release from tectal slices in both hemispheres but the changes were most marked ipsilaterally (+ 147%). It is suggested that the hypertrophy of GABA-containing nigrotectal somata seen after removal of corticostriatal, corticotectal and in particular GABA-containing striatonigral fibres may reflect concomitant increases in GABA synthesis within and/or sprouting of nigrotectal terminals.

The intrinsic organization of the central extended amygdala

The central component of the extended amygdala (CEA) comprises the central amygdaloid nucleus (Ce), the dorsal substantia innominata (SI), and the bed nucleus of the stria terminalis (BNST). Anatomical studies have suggested the presence of an intrinsic system of GABAergic neurons that not only connects homologous subareas of the Ce, SI, and BNST but that also acts as an interface between sensory afferents and brain stem-projecting neurons. CEA outputs, with a few exceptions, arise from separate populations of neurons, but all, including GABAergic neurons themselves, are heavily innervated by GABAergic terminals. GABAergic neurons may serve to integrate output activity of the CEA, though GABAergic neurons form a heterogeneous population whose differential intrinsic connections appear related to their peptide content. Afferents from the dysgranular insular cortex and lateral parabrachial complex preferentially innervate GABAergic neurons, suggesting these neurons may also integrate afferent activity. Afferents from the basolateral amygdala (BL) appear to innervate both output neurons and intrinsic GABAergic neurons. Evidence will be presented to show that BL afferents form synaptic complexes with cortical, GABAergic, and TH-immunoreactive terminal boutons on GABAergic dendritic spines. These complexes may be a key element in control of CEA output activity.

Substantia innominata: a notion which impedes clinical-anatomical correlations in neuropsychiatric disorders

Comparative neuroanatomical investigations in primates and non-primates have helped disentangle the anatomy of the basal forebrain region known as the substantia innominata. The most striking aspect of this region is its subdivision into two major parts. This reflects the fundamental organizational scheme for this portion of the forebrain. According to this scheme, two major subcortical telencephalic structures, i.e. the striatopallidal complex and extended amygdala, form large diagonally oriented bands. The rostroventral extension of the pallidum accounts for a large part of the rostral subcommissural substantia innominata, while the sublenticular substantia innominata is primarily occupied by elements of the extended amygdala. Also dispersed across this region is the basal nucleus of Meynert, which is part of a more or less continuous collection of cholinergic and non-cholinergic corticopetal and thalamopetal cells, which stretches from the septum diagonal band rostrally to the caudal globus pallidus. The basal nucleus of Meynert is especially prominent in the primate, where it is sometimes inappropriately applied as a synonym for the substantia innominata, thereby tacitly ignoring the remaining components. In most mammals, the extended amygdala presents itself as a ring of neurons encircling the internal capsule and basal ganglia. The extended amygdala may be further subdivided, i.e. into the central extended amygdala (related to the central amygdaloid nucleus) and the medial extended amygdala (related to the medial amygdaloid nucleus), which generally form separate corridors both in the sublenticular region and along the supracapsular course of the stria terminalis. The extended amygdala is directly continuous with the caudomedial shell of the accumbens, and to some extent appears to merge with it. Together the accumbens shell and extended amygdala form an extensive forebrain continuum, which establishes specific neuronal circuits with the medial prefrontal-orbitofrontal cortex and medial temporal lobe. This continuum is particularly characterized by a prominent system of long intrinsic association fibers, and a variety of highly differentiated downstream projections to the hypothalamus and brainstem. The various components of the extended amygdala, together with the shell of the accumbens, are ideally structured to generate endocrine, autonomic and somatomotor aspects of emotional and motivational states. Behavioral observations support this proposition and demonstrate the relevance of these structures to a variety of functions, ranging from the various elements of the reproductive cycle to drug-seeking behavior. The neurochemical and connectional features common to the accumbens shell and the extended amygdala are especially relevant to understanding the etiology and treatment of neuropsychiatric disorders. This is discussed in general terms, and also in specific relation to the neurodevelopmental theory of schizophrenia and to the neurosurgical treatment of neuropsychiatric disorders.

Neuroactive substances in the developing dorsomedial telencephalon of the pigeon (Columba livia): differential distribution and time course of maturation

The avian hippocampal formation has previously been shown to contain many of the same neurotransmitters and related enzymes that are found in mammals. In order to determine whether the relatively delayed development of the mammalian hippocampus is typical of other vertebrates, we investigated the maturation of a variety of neuroactive substances in the hippocampal formation of the homing pigeon. The distribution of two transmitter-related enzymes, choline acetyltransferase (ChAT) and tyrosine hydroxylase (TH), the neurotransmitter GABA, and four neuropeptides (substance P, enkephalin, neuropeptide Y, and somatostatin) was studied by immunohistochemistry in the developing hippocampal complex. The pattern and/or the time course of changes in the distribution of immunoreactivity varied among the different neuroactive substances examined. Immunoreactivity to ChAT and TH was found exclusively in fibers and terminal-like processes, whereas GABA and peptide immunoreactivity was seen in cells and neuropil. Quantitative differences in the density, number, and size of stained cells were assessed by a computer-assisted image analyzer. For the majority of the substances, developmental patterns in the distribution of immunoreactivity differ between the hippocampus proper and the area parahippocampalis, the two major areas that together make up the avian hippocampal complex. The adult pattern of immunoreactivity was generally attained by 3 weeks after hatching. For many of the neuroactive substances found in cell bodies, there was a gradual decrease in the density of immunoreactive cells with a concomitant increase in the density of immunoreactive neuropil. The actual number of stained cells usually increased to a peak at 9 days posthatching and then declined until 3 weeks posthatching, when the adult value was reached. These results are discussed in relation to the advantages that the pigeon hippocampal complex may provide in the study of developmental processes. Parallels with the distribution of the same neuroactive substances in the mammalian hippocampus are used to suggest possible functional similarities between the avian and mammalian hippocampal regions.

Maturation of somatostatin immunoreactivity in the pigeon retina: morphological characterization and quantitative analysis

In addition to a modulatory function, somatostatin (SS) is likely to exert a morphogenetic and/or trophic role in the developing nervous system. In this study, a mouse monoclonal antibody directed to SS was used to investigate the posthatching development of SS-immunoreactivity (SS-ir) in the pigeon retina to provide a basis for a better understanding of the role of this peptide in retinal maturation. In the adult, SS-ir was observed in amacrine cells located in the inner nuclear layer (INL) of the entire retina. Two cell types were recognized according to their morphology. They showed a differential density distribution. Cell type indicated as "adult 1" (AD1) was characterized by pear-shaped cell bodies with single primary processes directed to the inner plexiform layer (IPL) and was mostly present in the red field. In contrast, cell type indicated as "adult 2" (AD2) was characterized by round-shaped somata with 1-3 primary processes and was highly represented in the fovea and the dorsal periphery. Posthatching maturation of the pigeon retina was characterized by drastic changes in the pattern of SS-ir. Over the first days posthatching, SS-ir was observed in sparsely distributed somata mostly located in the ganglion cell layer (GCL). This cell type indicated as "hatch" (H) was characterized by dense granular staining and became extremely rare at 7 days. Over the same period, growing SS-positive axons displaying enlarged growth cones were found in the optic tract (TrO). These observations suggest the possibility that ganglion cells transiently expressing SS are present at early stages of posthatching development. Of the two types of SS-containing cells observed in the adult, the first to be recognized morphologically was cell type AD1 which appeared at 2 days after hatching in the INL. These cells were virtually adult-like in morphology by 7 days. In contrast, cell type AD2 was not apparent until 7 days posthatching. The density (defined as number of cells/mm2 of retinal tissue) and the total number of SS-containing cells changed during posthatching maturation. In particular, the adult number of cell type AD1 was reached at about 10 days, while the number of cell type AD2 was reached at about 3 weeks posthatching. At this stage, both cell types also displayed their mature density distribution. The present findings suggest a temporal relationship between the maturation of SS-ir and developmental events which include the onset of light-driven activity and the maturation of retinal acuity.(ABSTRACT TRUNCATED AT 400 WORDS)

Hypertrophy of rat sensory ganglion neurons following intestinal obstruction

BACKGROUND

Neuroplastic changes following ileum hypertrophy have been reported in intrinsic enteric neurons. The hypothesis in the present study was that intestinal hypertrophy induces neuronal changes in dorsal root ganglia (DRG).

METHODS

Under sodium pentobarbital anesthesia, partial obstruction was produced in the rat by tying a plastic ring around the terminal loop of ileum. Fast Blue (FB) (Sigma, St. Louis, MO) was injected into the obstructed ileum wall, and the rat was perfused after 8 days. DRG were immunostained and examined to identify and measure sizes of perikarya containing FB and/or calcitonin gene-related peptide (CGRP) or FB and/or substance P (SP).

RESULTS

Of the DRG neurons that projected to the ileum in control or obstructed animals, approximately 50% were CGRP-immunoreactive (IR) and 30% were SP-IR (colchicine pretreatment was not used). Neurons that projected to the obstructed ileum were increased in size compared with neurons in nonobstructed controls. Some of these neurons were CGRP-IR or SP-IR; some were large FB-labeled neurons that were not SP-IR or CGRP-IR.

CONCLUSIONS

The morphology of sensory autonomic neurons in adult animals is influenced by dynamic interactions with the targets they innervate, whether directly or transneuronally.

Regulation of brain-derived neurotrophic factor messenger RNA and protein at the cellular level in pentylenetetrazol-induced epileptic seizures

We have examined the effects of pentylenetetrazol-induced epileptic seizures on brain-derived neurotrophic factor messenger RNA and protein and on the messenger RNA of its receptor in the rat. Pentylenetrazol, which acts at the picrotoxin recognition site of the GABAA receptor, was injected intraperitoneally and induced seizures by decreasing the inhibitory GABAergic activity. The effects of a single acute convulsive dose (50 mg/kg) of pentylenetetrazol were analysed at different time points by in situ hybridization or immunohistochemistry. Kindling was induced by daily subconvulsive injections (30 mg/kg) of pentylenetetrazol. At different time points during the kindling process, the messenger RNAs of brain-derived neurotrophic factor and trkB and the protein levels of brain-derived neurotrophic factor were analysed. We showed that brain-derived neurotrophic factor messenger RNA dramatically increased in neurons of the granule cell layer, piriform cortex and amygdala 3 h but not 6 h after an acute high dose of pentylenetetrazol, while brain-derived neurotrophic factor-like immunoreactivity was decreased in the granule cell layer and neurons of the hilus. The trkB messenger RNA was similarly increased 3 h and 6 h after the injection and returned to control levels after 24 h. The first change during the kindling development was seen after the first severe seizure: brain-derived neurotrophic factor messenger RNA was markedly increased in the piriform cortex and amygdala but not in the hippocampus. In fully kindled rats, which had several severe seizures, brain-derived neurotrophic factor messenger RNA and trkB messenger RNA were unaffected 3 h and 24 h after the last pentylenetetrazol injection. However, brain-derived neurotrophic factor-like immunoreactivity was markedly increased in the hippocampal formation 3 h, 24 h and three days after the last pentylenetetrazol injection, and still increased after 10 days. These results suggest that brain-derived neurotrophic factor may be involved in protection mechanisms after damage during seizures and in sprouting responses. The piriform cortex/amygdala seems to be an area of origin for the kindling development.

The development of somatostatin immunoreactive neurons in cat visual cortical areas

The development of somatostatin immunoreactive (SOM-ir) neurons in cat striate and extrastriate cortex was studied to determine whether temporal changes in the morphology, distribution and density of SOM-ir neurons during development would provide clues to the emergence of specific cortical areas. The visual cortical areas examined included areas 17-19 and 7, posteromedial lateral suprasylvian, posterolateral lateral suprasylvian cortex and splenial visual area. We observed that the pattern of SOM-ir neurons in the cortical plate reflects the maturation of the cortical plate. At 1 week of age, SOM-ir neurons were only found in layers V and VI of the developing cortex; by 2 weeks of age, SOM-ir neurons were found in layer IV; and by 3 weeks of age, SOM-ir neurons were located in all layers of the cortex except layer I. SOM-ir neurons in the subplate were much more numerous under lateral cortical areas than under medial areas. This difference decreased over the first 2 postnatal weeks and by the 14th day after birth (P14), the distribution and numbers of SOM-ir neurons in the subplate/white matter had reached the adult pattern. The timing of exuberant SOM expression in the subplate suggests a function in the formation of visual corticocortical connections which begin to develop during the first postnatal week in the kitten.