MscL is a mechanosensitive channel in bacteria that responds directly to membrane tension by opening a large conductance pore. To determine functionally important residues within this molecule, we have randomly mutagenized mscL, expressed the genes in living bacteria, and screened for gain-of-function mutants with hampered growth. Expression of these genes caused leakage of cytoplasmic solutes on little or no hypo-osmotic stress. In excised patches, the mutant channels gated at membrane tensions that are less than that required for the gating of the wild-type MscL. Hence, the data suggest that the slowed or no-growth phenotype is caused by solute loss because of inappropriate gating of the channel. Most of the mutations mapped to the first transmembrane domain. When this domain is modeled as an ␣-helix, the most severe mutations are substitutions of smaller amino acids (three glycines and one valine) on one facet, suggesting an important role for this structure in MS channel gating.

Although clinical experience suggests that brain injury in the aged is associated with a poor prognosis, little research has examined this phenomenon at a cellular or molecular level. Unilateral 6-hydroxydopamine lesions of the nigrostriatal system were produced in 6-, 15- or 24-month-old rats. In the deafferented neostriatum, the time-dependent induction of glial fibrillary acidic protein (GFAP) was larger and persisted longer in the aged rats. The response of middle-aged rats was intermediate. In contrast, no induction of S-100 or glutamine synthetase was observed in any age group. In a second series of rats with stab wounds in the neostriatum, there were substantially larger GFAP inductions than after deafferentation, but fewer effects of age. However, in both lesion paradigms, GFAP staining increased in the contralateral striatum of old rats, but not in young rats. These data support and extend our earlier work describing larger GFAP RNA inductions after fornix transections in aged mouse hippocampus. The consistency of this exaggerated glial reactivity in the aged brain after modest injury suggests the following: 1) aged astrocytes are more sensitive to gliotrophic factors released by terminal degeneration, 2) larger quantities of such factors are produced after injury, 3) clearance of these factors is delayed in old rodents, and/or 4) aged astrocytes are less able to terminate GFAP inductions after activation. Given the potential role of inflammatory reactions as pathogenic mechanisms in Alzheimer's dementia, these data suggest that age-related glial hypersensitivity may independently increase the risk for some degenerative diseases.

Aging disrupts the expression of synaptic plasticity in many central nervous system (CNS) structures including the striatum. We found age differences in paired-pulse plasticity to persist at excitatory striatal synapses following block of gamma aminobutyric acid (GABA)A and GABA(B) receptors, a property that was independent of the number of afferents activated. High Mg2+/low Ca2+ artificial cerebral spinal fluid (ACSF) reduced release probability and consequently the size of the evoked excitatory post-synaptic potential (EPSP). High Mg2+/low Ca2+ ACSF also increased the expression of paired-pulse facilitation and eliminated the age difference seen previously in normal ACSF. These data suggest that age differences in paired-pulse plasticity reflect an alteration in release probability at excitatory striatal synapses. In support of this hypothesis, we found age differences in another presynaptic form of plasticity referred to as synaptic augmentation. Examination of the synaptic depression that developed during the conditioning tetanus also revealed an age-related increase in synaptic depression. These data indicate that age-related changes in facilitation may be due in part to a reduction in the readily releasable pool of synaptic vesicles. Dendritic structure (spine density and dendritic length) was correlated with short-term synaptic plasticity, but these relationships depended upon the variance associated with age (hierarchical regression). Post-hoc within-age group regressions demonstrated relationship between spine density and paired-pulse plasticity. No other age-specific correlations were found. These findings imply an age-dependent association between altered dendritic morphology and changes in synaptic plasticity.

Changes in calcium (Ca2+) homeostasis have been proposed to contribute to the aging process. Paired-pulse facilitation, a form of synaptic enhancement that relies upon an accumulation of Ca2+ in the presynaptic terminal, was used to examine the effect of aging at the corticostriatal synapse. Intracellular recordings in striatal neurons from young rats demonstrated a consistent enhancement in the second of two synaptic responses evoked by stimulation of the corpus callosum. In contrast, neurons from aged rats showed a consistent depression of the second synaptic response at identical pairing intervals. These differences were not explained by an age-dependent increase in synaptic depression and demonstrate an alteration in the Ca(2+)-mediated process of presynaptic facilitation.

Although clinical experience suggests that brain injury in the aged is associated with a poor prognosis, little research has examined this phenomenon at a cellular or molecular level. Unilateral 6-hydroxydopamine lesions of the nigrostriatal system were produced in 6-, 15- or 24-month-old rats. In the deafferented neostriatum, the time-dependent induction of glial fibrillary acidic protein (GFAP) was larger and persisted longer in the aged rats. The response of middle-aged rats was intermediate. In contrast, no induction of S-100 or glutamine synthetase was observed in any age group. In a second series of rats with stab wounds in the neostriatum, there were substantially larger GFAP inductions than after deafferentation, but fewer effects of age. However, in both lesion paradigms, GFAP staining increased in the contralateral striatum of old rats, but not in young rats. These data support and extend our earlier work describing larger GFAP RNA inductions after fornix transections in aged mouse hippocampus. The consistency of this exaggerated glial reactivity in the aged brain after modest injury suggests the following: 1) aged astrocytes are more sensitive to gliotrophic factors released by terminal degeneration, 2) larger quantities of such factors are produced after injury, 3) clearance of these factors is delayed in old rodents, and/or 4) aged astrocytes are less able to terminate GFAP inductions after activation. Given the potential role of inflammatory reactions as pathogenic mechanisms in Alzheimer's dementia, these data suggest that age-related glial hypersensitivity may independently increase the risk for some degenerative diseases.