David M. Armstrong

Adrenaline neurons in the rostral ventrolateral medulla innervate thoracic spinal cord: A combined immunocytochemical and retrograde transport demonstration

Adrenaline-containing neurons in the C1 group of the ventrolateral rostral medullary reticular formation which project to the thoracic spinal cord were identified by a combined retrograde transport immunocytochemical technique. No other medullary catecholamine neurons, including the A1 and A2 noradrenaline cells, project to thoracic spinal cord. These data, taken with results of other studies of spinal catecholamine innervation, suggest a segmental segregation of projections to spinal cord by dopaminergic, noradrenergic, and adrenergic neurons.

Astrocytes are important for sprouting in the septohippocampal circuit

Damage to the fimbria-fornix, and separately to the perforant path, leads to distinct and dramatic time-dependent increases in glial fibrillary acidic protein immunoreactivity (GFAP-IR) in specific areas of the hippocampal formation. Specifically, fimbria-fornix lesions resulted in an increase in the GFAP-IR in the pyramidal and oriens area of the CA3 as well as the inner molecular layer of the dentate gyrus. In addition, in the septum ipsilateral to the lesion, there was a rapid and robust increase in GFAP-IR in the dorsal lateral quadrant of the septum, but not in the medial region. Only after 30 days did the GFAP-IR reach the medial septum. Following perforant path lesions, there was a selective increase in GFAP-IR in the outer molecular layer of the dentate gyrus. Most of these changes were transient and had disappeared by 30 days postlesion. We speculate that the increase in GFAP-IR in these target areas is a necessary requirement for the sprouting responses that are observed. This hypothesis is supported by the fact that astrocytes secrete NGF in vitro and that NGF activity increases in these target areas following these same lesions. A mechanism for the selective activation of the astrocytes through the initial activation of microglia and secretion of interleukin-1 is postulated.

Mapping the development of the rat brain by GAP-43 immunocytochemistry

Growth-associated protein-43 (GAP-43) is a phosphoprotein of the nerve terminal membrane which has been linked to the development and restructuring of axonal connections. Using a monospecific antibody prepared in sheep against purified GAP-43, we examined the temporal and spatial changes in the distribution of this protein from embryonic stage day 13 (E13) to adulthood. At stages in which neurons are still dividing and migrating, levels of GAP-43 are extremely low, as is seen in the cortical plate throughout the embryonic period. With the onset of process outgrowth, intense GAP-43 immunoreactivity appears along the length of axons: by E13, such staining is already strong in the brainstem, where it continues up through the first postnatal week and then disappears. In the neocortex, intense fiber staining first appears several days later but ends at the same time as in the brainstem. At the end of the period of intense axonal staining there is a brief interval in which high levels of GAP-43 immunostaining are seen in the neuropil. In regions of the brain in which specific developmental events have been characterized anatomically and physiologically, the period of dense neuropil staining coincides with the formation of axonal end-arbors, the beginning of synaptogenesis, and the time at which synaptic organization can be modified by the impingent pattern of activity (i.e. the critical period). Over the next few days, staining in neuropil declines sharply in most regions except for certain structures in the rostral neuraxis which may be sites of ongoing synaptic remodeling.

An immunohistochemical quantification of fibrous astrocytes in the aging human cerebral cortex

In order to determine whether cortical fibrous astrocytes increase with age, we studied 25 patients ranging in age from 24 to 100 years with no clinical or pathological evidence of dementia or other cerebral disorder. Paraffin sections of mid-frontal cortex were obtained and stained with the avidin-biotin immunolabeling procedure for glial intermediate filament protein. The resulting immunolabeled fibrous astrocytes were then counted in the molecular and cellular (cortical laminae 2-6) layers. Populations of fibrous astrocytes in both layers varied widely among individuals, and in the molecular layer their numbers were not significantly correlated with advancing age. In the cellular layer, however, despite widely ranging cell counts among individuals within the same decades of life, there was a significant linear increase with age. Our data suggest that the increase occurs or accelerates significantly after age 70, but the case numbers preclude reaching such a conclusion with statistical confidence. However, when the patients are divided into those less than 70 and those older, fibrous astrocytes in the cellular layer are shown to be significantly increased in the latter group compared to the former.

Electric activity in the neocortex of freely moving young and aged rats

Electroencephalographic activity of the neocortex was evaluated in young (5-7 months) and aged (26-28 months) rats. All animals in the aged group showed behavioral impairment in a spatial task (water maze). A neocortical electroencephalogram was derived simultaneously from 16 different neocortical locations and was subjected to spectral analysis. The frequency of occurrence and duration of high-voltage spindles was determined in two sessions, each involving a total of 30 min alert immobility. Changes in spectral characteristics and high-voltage spindles in response to scopolamine administration were also evaluated. The power of high-frequency activity (8-20 Hz) was significantly reduced in the aged subjects. This was greatest in the temporo-occipital regions, while no significant changes were seen in the mediofrontal region. Scopolamine resulted in a large power increase in all frequency bands, but the increase in the higher-frequency range (8-20 Hz) was significantly less in the aged group. The incidence of high-voltage spindles was 6 times higher and their total duration was 9 times longer in aged rats, with virtually no overlap with the young group. In young rats, scopolamine increased the incidence and total duration of high-voltage spindles, while it decreased both parameters in the aged subjects. Cholinergic neurons in the nucleus basalis appeared shrunken in the aged animals. These findings demonstrate that reliable electroencephalographic changes are present in the neocortex of the aged rat, and that some of the physiological alterations may be due to the pathological changes in the cholinergic nucleus basalis.

Cholinergic neurons from the dorsolateral pons project to the medial pons: A WGA-HRP and choline acetyltransferase immunohistochemical study

In this study we determined that cholinergic neurons from the lateral dorsal tegmental (LDT) and peribrachial pontine region (PPG) innervate the medial pontine reticular formation (medial PRF), a region involved in the generation of REM sleep. Wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) was injected into the medial PRF and the brainstem tissue was processed using a combined retrograde transport/immunocytochemical procedure. Results showed that 10-15% of choline acetyltransferase (ChAT) immunoreactive neurons in the LDT and PPG project to the medial PRF. It is hypothesized that these neurons play an important role in the generation of the REM sleep state.

Biochemical and molecular studies of NMDA receptor subunits NR1/2A/2B in hippocampal subregions throughout progression of Alzheimer's disease pathology

Alzheimers disease (AD) is characterized by loss of specific cell populations within selective subregions of the hippocampus. Excitotoxicity, mediated via ionotropic glutamate receptors, may play a crucial role in this selective neuronal vulnerability. We investigated whether alterations in NMDA receptor subunits occurred during AD progression. Employing biochemical and in situ hybridization techniques in subjects with a broad range of AD pathology, protein levels, and mRNA expression of NR1/2A/2B subunits were assayed. With increasing AD neuropathology, protein levels and mRNA expression for NR1/2B subunits were significantly reduced, while the NR2A subunit mRNA expression and protein levels were unchanged. Cellular analysis of neuronal mRNA expression revealed a significant increase in the NR2A subunit in subjects with moderate neurofibrillary tangle neuropathology. This investigation supports the hypothesis that alterations occur in the expression of specific NMDA receptor subunits with increasing AD pathologic severity, which is hypothesized to contribute to the vulnerability of these neurons.

GABAA receptors in aging and Alzheimer’s disease

In this article we present a comprehensive review of relevant research and reports on the GABAA receptor in the aged and Alzheimer’s disease (AD) brain. In comparison to glutamatergic and cholinergic systems, the GABAergic system is relatively spared in AD, but the precise mechanisms underlying differential vulnerability are not well understood. Using several methods, investigations demonstrate that despite resistance of the GABAergic system to neurodegeneration, particular subunits of the GABAA receptor are altered with age and AD, which can induce compensatory increases in GABAA receptor subunits within surrounding cells. We conclude that although aging‐ and disease‐related changes in GABAA receptor subunits may be modest, the mechanisms that compensate for these changes may alter the pharmacokinetic and physiological properties of the receptor. It is therefore crucial to understand the subunit composition of individual GABAA receptors in the diseased brain when developing therapeutics that act at these receptors.

Basal forebrain cell loss following fimbria/fornix transection

Following fimbria/fornix transection, cells in the medial septum appear to undergo retrograde degeneration as shown by Nissl and acetylcholine esterase (AChE) staining. Recent studies using immunocytochemical techniques have also demonstrated loss of choline acetyltransferase (ChAT) and nerve growth factor receptor (NGFr) labeling of neurons in this region. Whether the apparent loss of ChAT- and NGFr-positive neurons is the result of the actual death of these neurons, or is instead a loss of ChAT enzyme or NGFr expression below levels detectable by immunocytochemical methods, remains an unresolved issue. In order to address this question, rhodamine-labeled fluorescent latex microspheres were injected into the hippocampus where they retrogradely transported to the cell bodies of the medial septum. Five days later these animals received either unilateral or bilateral fimbria/fornix lesions and were allowed to survive an additional 4 weeks. Compared to unlesioned control animals, unilaterally lesioned animals showed a 91% loss of fluorescently labeled cells and bilaterally lesioned animals showed a 93% loss. The inability to detect the fluorescent microspheres in the medial septum suggests that the majority of medial septal cells die after fimbria/fornix transection. ChAT and NGFr immunohistochemical staining were also performed. Cells stained for ChAT were reduced in number by 92% in animals with unilateral lesions and by 75% in animals with bilateral lesions, while NGFr-stained cells were reduced in number by 75% in animals with unilateral lesions and by 68% in animals with bilateral lesions.(ABSTRACT TRUNCATED AT 250 WORDS)

GABA(A) receptors in aging and Alzheimer's disease.

In this article we present a comprehensive review of relevant research and reports on the GABAA receptor in the aged and Alzheimer’s disease (AD) brain. In comparison to glutamatergic and cholinergic systems, the GABAergic system is relatively spared in AD, but the precise mechanisms underlying differential vulnerability are not well understood. Using several methods, investigations demonstrate that despite resistance of the GABAergic system to neurodegeneration, particular subunits of the GABAA receptor are altered with age and AD, which can induce compensatory increases in GABAA receptor subunits within surrounding cells. We conclude that although aging‐ and disease‐related changes in GABAA receptor subunits may be modest, the mechanisms that compensate for these changes may alter the pharmacokinetic and physiological properties of the receptor. It is therefore crucial to understand the subunit composition of individual GABAA receptors in the diseased brain when developing therapeutics that act at these receptors.