Age, experience and the changing brain

Reproductive experience modulates dopamine-related behavioral responses

Reproductive experience (RE), i.e., mating, pregnancy, parturition and lactation, has long-term physiological effects. It reduces the basal levels of circulating prolactin in parous women, decreases the intensity of nocturnal and diurnal prolactin surges in multigravid rats during early pregnancy, as well as the hormonal and neurochemical responses to the dopamine receptor antagonists metoclopramide and haloperidol. In the present study, we evaluated the possible influences of RE on some dopaminergic-related behaviors: (1) acute responses to a new environment represented by an open-field arena plus injection stress; (2) modulation of behavior after a short-term withdrawal subsequent to 7 days amphetamine (AMPH) pretreatment; (3) stereotypy elicited by AMPH and apomorphine (APO); and (4) APO-induced hypothermia. In the 3-min open-field test, there was a decrease in locomotor activity as a function of RE. Behavioral depression was mild and AMPH pretreatment revealed RE alterations. APO-induced stereotyped behavior was slightly more intense in primiparous animals, although no significant differences were found in AMPH-induced stereotyped behavior. No differences were observed between intact and ovariectomized primiparous and nulliparous animals in APO-induced hypothermia. Our data suggest that RE modifies some DA-related behavioral responses. The physiological relevance of the phenomenon is discussed.

Experience-associated structural events, subependymal cellular proliferative activity, and functional recovery after injury to the central nervous system

Considerable structural plasticity is possible in the damaged neocortex and connected brain areas, and the potential for significant functional recovery remains even during the chronic phases of the recovery process. In this article, the authors review the literature on use-dependent morphologic events, focusing on the direct interaction of behavioral experience and structural changes associated with plasticity and degeneration. Experience-associated neural changes have the potential to either hinder or enhance functional recovery; therefore, issues concerning the nature, timing, and intensity of behavior-based intervention strategies are addressed.

Morphology of layer III pyramidal neurons is altered following induction of LTP in sensorimotor cortex of the freely moving rat

The organization of specific cortical connections can be altered by sensory and motor experience. These changes are believed to result from activity-dependent changes in synaptic connectivity, similar to those induced in the hippocampus by high-frequency stimulation in long-term potentiation (LTP) experiments. If similar mechanisms are involved, then neocortical LTP induction may induce some of the same morphological changes that are seen following learning. We induced LTP in the contralateral sensorimotor cortex by repeated, daily tetanization of the corpus callosum in chronically implanted, freely moving rats. Anatomical results showed that the LTP induction was associated with alterations in dendrite morphology and increased spine density. These changes are qualitatively and quantitatively similar to those commonly observed in studies in which rats are housed in complex environments. The similarity of results following exposure to complex environments and after LTP induction in the neocortex may indicate a reliance on the same cellular mechanisms in both situations.

Effects of prenatal alcohol exposure on the hippocampus: spatial behavior, electrophysiology, and neuroanatomy

Prenatal exposure to alcohol can result in fetal alcohol syndrome (FAS), characterized by growth retardation, facial dysmorphologies, and a host of neurobehavioral impairments. Neurobehavioral effects in FAS, and in alcohol-related neurodevelopmental disorder, include poor learning and memory, attentional deficits, and motor dysfunction. Many of these behavioral deficits can be modeled in rodents. This paper reviews the literature suggesting that many fetal alcohol effects result, at least in part, from teratogenic effects of alcohol on the hippocampus. Neurobehavioral studies show that animals exposed prenatally to alcohol are impaired in many of the same spatial learning and memory tasks sensitive to hippocampal damage, including T-mazes, the Morris water maze, and the radial arm maze. Direct evidence for hippocampal involvement is provided by neuroanatomical studies of the hippocampus documenting reduced numbers of neurons, lower dendritic spine density on pyramidal neurons, and decreased morphological plasticity after environmental enrichment in rats exposed prenatally to alcohol. Electrophysiological studies also demonstrate changes in synaptic activity in in vitro hippocampal brain slices isolated from prenatal alcohol-exposed animals. Considered together, these observations demonstrate that prenatal exposure to alcohol can result in abnormal hippocampal development and function. Such studies provide a better understanding of neurological deficits associated with FAS in humans, and may also contribute to the development of strategies to ameliorate the effects of prenatal alcohol exposure on behavior.

The expression of TAPA (CD81) correlates with the reactive response of astrocytes in the developing rat CNS

During the development of the brain, astrocytes acquire the ability to become reactive and form a scar. This change in the astrocytes occurs at approximately the same time that there is a decrease in the regenerative capacity of the CNS. Previous work from our laboratory had revealed that TAPA (Target of Anti-Proliferative Antibody, also known as CD81) is associated with reactive gliosis and the glial scar. TAPA is a member of the tetraspan family of proteins that appears to be associated with the regulation of cellular behavior. In order to define the role of TAPA in relation to the developmentally regulated CNS response to injury, we examined the levels of TAPA and GFAP immunoreactivity in rat pups that received a penetrating cerebral cortical injury. All of the animals injured at postnatal day 9 (PND 9), PND 18, or as adults, exhibited reactive gliosis scar formation when they were sacrificed 10 days after the cortical injury. Of the nine animals injured at PND 2, only three displayed reactive gliosis and scar formation. The remaining six rat pups had either a modest gliotic response or no detectable gliosis. The level of TAPA at the site of injury mimicked the reactive gliosis as defined by GFAP immunoreactivity. In all of the rats with a glial scar, there was a dramatic upregulation of TAPA that is spatially restricted to the reactive astrocytes. These results suggest that the upregulation of TAPA is an integral component of glial scar formation.

Morphine alters the structure of neurons in the nucleus accumbens and neocortex of rats

Rats were given repeated injections of 10 mg/kg of morphine and were then left undisturbed for 24-25 days before their brains were processed for Golgi-Cox staining. Prior exposure to morphine decreased the complexity of dendritic branching and the number of dendritic spines on medium spiny neurons in the shell of the nucleus accumbens and on pyramidal cells in the prefrontal and parietal cortex. It is suggested that some of the long-term behavioral consequences of repeated exposure to morphine may be due to its ability to reorganize patterns of synaptic connectivity in the forebrain.

Sex Differences in Mathematical Abilities: Commentary on the Math-Fact Retrieval Hypothesis

The hypothesis that a male advantage in speed of math-fact retrieval underlies the sex difference, favoring males, in mathematical abilities is unique and provocative. However, the hypothesis does not provide an explanation for the male advantage in mathematical domains, such as geometry, that do not require arithmetic, nor does it accommodate the sex differences in social and occupational interests that contribute to the sex difference in mathematical achievement. An alternative hypothesis focusing on the sex difference in three-dimensional spatial cognition is favored over the math-fact retrieval hypothesis. Copyright 1999 Academic Press.

Alterations in the morphology of dendrites and dendritic spines in the nucleus accumbens and prefrontal cortex following repeated treatment with amphetamine or cocaine

Repeated treatment with psychostimulant drugs produces changes in brain and behaviour that far outlast their initial neuropharmacological actions. The nature of persistent drug-induced neurobehavioural adaptations is of interest because they are thought to contribute to the development of dependence and addiction, and other forms of psychopathology, e.g. amphetamine psychosis. There are many reports that psychostimulants produce biochemical adaptations in brain monoamine systems, especially dopamine systems. The purpose of the present study was to determine if they might also alter the morphology of neurons in brain regions that receive monoaminergic innervation. Rats were given repeated injections of either amphetamine or cocaine, or, to control for general motor activity, allowed access to a running wheel. They were then left undisturbed for 24-25 days before their brains were processed for Golgi-Cox staining. Treatment with either amphetamine or cocaine (but not wheel running experience) increased the number of dendritic branches and the density of dendritic spines on medium spiny neurons in the shell of the nucleus accumbens, and on apical dendrites of layer V pyramidal cells in the prefrontal cortex. Cocaine also increased dendritic branching and spine density on the basilar dendrites of pyramidal cells. In addition, both drugs doubled the incidence of branched spines on medium spiny neurons. It is suggested that some of the persistent neurobehavioural consequences of repeated exposure to psychostimulant drugs may be due to their ability to reorganize patterns of synaptic connectivity in the nucleus accumbens and prefrontal cortex.

Environmental influences on cognitive decline in aged rats

Two experiments are reported in which young and old rats, housed in an impoverished (IE), enriched (EE), or standard (SE), environment, were tested on a series of complex, blind-alley mazes. In Experiment 1, 3-months exposure to IE exacerbated age differences in maze performance, relative to the differences between young and old rats in EE and SE. Old rats in the EE and SE conditions did not differ from each other. In Experiment 2, rats were raised for an additional 3 months in either IE or EE before further maze testing. The main findings were that the maze performance of old rats, transferred from IE to EE, improved significantly, whereas the performance of old rats, transferred from SE or EE to IE, declined. These results indicated that the deleterious effects of an impoverished environment on learning and memory are, at least partly, reversible, and that experience in a stimulating environment can protect old rats from the adverse effects of relocation to a deprived environment. Taken together, the results highlight the impact of environmental influences on cognitive function in old age, and emphasize the need to consider nonbiological factors in understanding the process of cognitive aging.

A method for vibratome sectioning of Golgi-Cox stained whole rat brain

A method for impregnating the whole rat brain with Golgi-Cox stain and sectioning with the vibratome is described. The method is simple, inexpensive and provides good resolution of dendrites and spines.