Basal forebrain neurons undergo somatal and dendritic remodeling during postnatal development: a single-section Golgi and choline acetyltransferase analysis

Adolescent binge ethanol-induced loss of basal forebrain cholinergic neurons and neuroimmune activation are prevented by exercise and indomethacin

Basal forebrain cholinergic neurons mature in adolescence coinciding with development of adult cognitive function. Preclinical studies using the rodent model of adolescent intermittent ethanol (AIE; 5.0 g/kg, i.g., 2-days on/2-days off from postnatal day [P]25 to P55) reveal persistent increases of brain neuroimmune genes that are associated with cognitive dysfunction. Adolescent intermittent ethanol exposure also reduces basal forebrain expression of choline acetyltransferase (ChAT), an enzyme critical for acetylcholine synthesis in cholinergic neurons similar to findings in the post-mortem human alcoholic basal forebrain. We report here that AIE decreases basal forebrain ChAT+IR neurons in both adult female and male Wistar rats following early or late adolescent ethanol exposure. In addition, we find reductions in ChAT+IR somal size as well as the expression of the high-affinity nerve growth factor (NGF) receptor tropomyosin receptor kinase A (TrkA) and the low-affinity NGF receptor p75NTR, both of which are expressed on cholinergic neurons. The decrease in cholinergic neuron marker expression was accompanied by increased phosphorylation of NF-κB p65 (pNF-κB p65) consistent with increased neuroimmune signaling. Voluntary wheel running from P24 to P80 prevented AIE-induced cholinergic neuron shrinkage and loss of cholinergic neuron markers (i.e., ChAT, TrkA, and p75NTR) as well as the increase of pNF-κB p65 in the adult basal forebrain. Administration of the anti-inflammatory drug indomethacin (4.0 mg/kg, i.p prior to each ethanol exposure) during AIE also prevented the loss of basal forebrain cholinergic markers and the concomitant increase of pNF-κB p65. In contrast, treatment with the proinflammatory immune activator lipopolysaccharide (1.0 mg/kg, i.p. on P70) caused a loss of cholinergic neuron markers that was paralleled by increased pNF-κB p65 in the basal forebrain. These novel findings are consistent with AIE causing lasting activation of the neuroimmune system that contributes to the persistent loss of basal forebrain cholinergic neurons in adulthood.

Role of Nerve Growth Factor in Plasticity of Forebrain Cholinergic Neurons

Neuronal plastic rearrangements during the development and functioning of neurons are largely regulated by trophic factors, including nerve growth factor (NGF). NGF is also involved in the pathogenesis of Alzheimer's disease. In the brain, NGF is produced in structures innervated by basal forebrain cholinergic neurons and retrogradely transported along the axons to the bodies of cholinergic neurons. NGF is essential for normal development and functioning of the basal forebrain; it affects formation of the dendritic tree and modulates the activities of choline acetyltransferase and acetylcholinesterase in basal forebrain neurons. The trophic effect of NGF is mediated through its interactions with TrkA and p75 receptors. Experimental and clinical studies have shown that brain levels of NGF are altered in various pathologies. However, the therapeutic use of NGF is limited by its poor ability to penetrate the blood-brain barrier, adverse side effects that are due to the pleiotropic action of this factor, and the possibility of immune response to NGF. For this reason, the development of gene therapy methods for treating NGF deficit-associated pathologies is of particular interest. Another approach is creation of low molecular weight NGF mimetics that would interact with the corresponding receptors and display high biological activity but be free of the unfavorable effects of NGF.

Adolescent Alcohol Exposure Persistently Impacts Adult Neurobiology and Behavior

Adolescence is a developmental period when physical and cognitive abilities are optimized, when social skills are consolidated, and when sexuality, adolescent behaviors, and frontal cortical functions mature to adult levels. Adolescents also have unique responses to alcohol compared with adults, being less sensitive to ethanol sedative-motor responses that most likely contribute to binge drinking and blackouts. Population studies find that an early age of drinking onset correlates with increased lifetime risks for the development of alcohol dependence, violence, and injuries. Brain synapses, myelination, and neural circuits mature in adolescence to adult levels in parallel with increased reflection on the consequence of actions and reduced impulsivity and thrill seeking. Alcohol binge drinking could alter human development, but variations in genetics, peer groups, family structure, early life experiences, and the emergence of psychopathology in humans confound studies. As adolescence is common to mammalian species, preclinical models of binge drinking provide insight into the direct impact of alcohol on adolescent development. This review relates human findings to basic science studies, particularly the preclinical studies of the Neurobiology of Adolescent Drinking in Adulthood (NADIA) Consortium. These studies focus on persistent adult changes in neurobiology and behavior following adolescent intermittent ethanol (AIE), a model of underage drinking. NADIA studies and others find that AIE results in the following: increases in adult alcohol drinking, disinhibition, and social anxiety; altered adult synapses, cognition, and sleep; reduced adult neurogenesis, cholinergic, and serotonergic neurons; and increased neuroimmune gene expression and epigenetic modifiers of gene expression. Many of these effects are specific to adolescents and not found in parallel adult studies. AIE can cause a persistence of adolescent-like synaptic physiology, behavior, and sensitivity to alcohol into adulthood. Together, these findings support the hypothesis that adolescent binge drinking leads to long-lasting changes in the adult brain that increase risks of adult psychopathology, particularly for alcohol dependence.

The effects of caffeine on sleep and maturational markers in the rat

Adolescence is a critical period for brain maturation during which a massive reorganization of cortical connectivity takes place. In humans, slow wave activity (<4.5 Hz) during NREM sleep was proposed to reflect cortical maturation which relies on use-dependent processes. A stimulant like caffeine, whose consumption has recently increased especially in adolescents, is known to affect sleep wake regulation.

The goal of this study was to establish a rat model allowing to assess the relationship between cortical maturation and sleep and to further investigate how these parameters are affected by caffeine consumption. To do so, we assessed sleep and markers of maturation by electrophysiological recordings, behavioral and structural readouts in the juvenile rat. Our results show that sleep slow wave activity follows a similar inverted U-shape trajectory as already known in humans. Caffeine treatment exerted short-term stimulating effects and altered the trajectory of slow wave activity. Moreover, caffeine affected behavioral and structural markers of maturation. Thus, caffeine consumption during a critical developmental period shows long lasting effects on sleep and brain maturation.

Supplemental choline during the periweaning period protects against trace conditioning impairments attributable to post-training ethanol exposure in adolescent rats

Supplemental choline during early stages of development can result in long-lasting improvements to memory function. In addition, pre- or postnatal choline has been shown to be protective against some of the adverse effects of early alcohol exposure. The present experiment examined whether supplemental choline given to rats would protect against the effects of posttraining alcohol administration on trace fear conditioning. Posttraining alcohol exposure in adolescent rats results in poor performance in this hippocampus-dependent task, although delay conditioning is unaffected. Here, rats were given an s.c. injection of either saline or choline chloride daily on postnatal days (PD) 15-26. On PD 30 subjects were trained in a trace fear conditioning procedure. For the next 3 days animals were administered 2.5 g/kg ethanol or water control, and conditional stimulus (CS)-elicited freezing was measured on PD 34. Results indicated that posttraining alcohol disrupted the expression of trace conditioning and that supplemental choline on PD 15-26 was protective against this effect. That is, choline-treated animals subsequently given posttraining ethanol performed as well as animals not given ethanol. These results indicate that supplemental choline given during the periweaning period protects against ethanol-induced impairments in a hippocampus-dependent learning task. Findings contribute to the growing literature showing improvements in learning and memory in subjects given extra dietary choline during critical periods of brain development.

Postnatal development of limb motor innervation in the opossum Monodelphis domestica: immunohistochemical localization of acetylcholine

The development of limb motor innervation was studied in the opossum Monodelphis domestica, a marsupial born with immature mobile forelimbs and immobile hindlimbs. Choline acetyltransferase (ChAT), the synthesis enzyme of acetylcholine, was evidenced on sections of the spinal enlargements, and the protein that transports acetylcholine (VAChT) on limb sections. In newborn, ChAT immunolabeling occurred in small, undifferentiated neurons of the ventral horn, presumably motoneurons, and intermediate and dorsal gray matter, and in the presumptive white matter, all less abundant at lumbosacral than brachial levels. Scant immunolabeling for VAChT marked small terminal-looking profiles, presumably growth cones or immature neuromuscular junctions, decreasing proximodistally in each limb and being less abundant in hindlimbs than forelimbs; it was absent distally in the foot where no muscle tissue was formed. ChAT labeling disappeared from the white matter within 1 week while cholinergic neurons increased in number and size. Motoneurons segregated in a medial and lateral group by 4-5 weeks. VAChT-labeled profiles increased in number and size and they flattened along a proximodistal gradient within each limb, but later in the hindlimbs than in the forelimbs. Labeling appeared in distal foot muscle at 1 week. The density, size, and shape of terminals became comparable in all segments of a given limb by 3-4 weeks. Their number and size increased, and by 8 weeks, they clustered in 3 or 4 along muscle fibers. Thus, limb motor innervation develops largely postnatally in the opossum, along rostrocaudal and proximodistal gradients. Its timecourse is compared to the development of motor behaviors.

Pontine cholinergic neurons depend on three neuroprotection systems to resist nitrosative stress

Brainstem cholinergic populations survive in neurodegenerative disease, while basal forebrain cholinergic neurons degenerate. We have postulated that variable resistance to oxidative stress may in part explain this. Rat primary cultures were used to study the effects of several nitrosative/oxidative stressors on brainstem (upper pons, containing pedunculopontine and lateraldorsal tegmental nuclei; BS) cholinergic neurons, comparing them with medial septal (MS), and striatal cholinergic neurons. BS cholinergic neurons were significantly more resistant to S-nitro-N-acetyl-d,l-penicillamine (SNAP), sodium nitroprusside (SNP), and hydrogen peroxide than were MS cholinergic neurons, which in turn were more resistant than striatal cholinergic neurons. Pharmacological analyses using specific inhibitors of neuroprotective systems also revealed differences between these three cholinergic populations with respect to their vulnerability to SNAP. Toxicity of SNAP to BS neurons was exacerbated by blocking NF-kappaB activation with SN50 or ERK1/2 activation by PD98059, or by inhibition of phosphoinositide-3 kinase (PI3K) activity by LY294002. In contrast, SNAP toxicity to MS neurons was augmented only by SN50, and SNAP toxicity to striatal cholinergic neurons was not increased by any of these three pharmacological agents. In neuron-enriched primary cultures, BS cholinergic neurons remained resistant to SNAP while MS cholinergic neurons remained vulnerable to this agent. Immunohistochemical experiments demonstrated nitric oxide (NO)-induced increases in nuclear levels of phospho-epitopes for ERK1/2 and Akt, and of the p65 subunit of NF-kappaB, within BS cholinergic neurons. These data indicate that the relative resistance of BS cholinergic neurons to toxic levels of nitric oxide involves three intrinsic neuroprotective pathways that control transcriptional and anti-apoptotic cellular functions.

Metabolic imprinting of choline by its availability during gestation: implications for memory and attentional processing across the lifespan

A growing body of research supports the view that choline is an essential nutrient during early development that has long-lasting effects on memory and attentional processes throughout the lifespan. This review describes the known effects of alterations in dietary choline availability both in adulthood and during early development. Although modest effects of choline on cognitive processes have been reported when choline is administered to adult animals, we have found that the perinatal period is a critical time for cholinergic organization of brain function. Choline supplementation during this period increases memory capacity and precision of the young adult and appears to prevent age-related memory and attentional decline. Deprivation of choline during early development leads to compromised cognitive function and increased decline with age. We propose that this organizational effect of choline availability may be due to relatively permanent alterations in the functioning of the cholinergic synapse, which we have called 'metabolic imprinting'.

Delayed NGF infusion fails to reverse axotomy-induced degeneration of basal forebrain cholinergic neurons in adult p75(LNTR)-deficient mice

The p75 low-affinity neurotrophin receptor (p75(LNTR)) appears to have various functions that include enhancing nerve growth factor (NGF)-mediated survival by increasing TrkA (high-affinity NGF receptor) efficiency, and mediating apoptosis by acting as a ligand-regulated pro-apoptotic receptor. Here, we investigated the role of p75(LNTR) for adult cholinergic basal forebrain neurons by comparing neuronal responses to injury in control and p75(LNTR)-deficient mice. In both types of mice, approximately 70% of the cholinergic neurons in the ipsilateral medial septum had lost their markers choline acetyltransferase and tyrosine kinase A by 28 days following unilateral transection of the dorsal septohippocampal pathway (fimbria fornix). A 7-day delayed infusion of NGF that started 28 days after the injury resulted in reversal of choline acetyltransferase expression and cell atrophy in control, but not in p75(LNTR)-deficient, mice. This lack of response to delayed NGF treatment in p75(LNTR)-deficient mice was most likely not due to cell death, as all of the septohippocampal neurons, labeled with Fluorogold before the lesion, were present at 28 days post-lesion, similar to control mice. p75(LNTR)-deficient cholinergic neurons can respond to NGF as they were protected by NGF infusions that started immediately after the injury. These observations, the fact that lesioned p75(LNTR)-deficient neurons atrophy faster, and that non-lesioned neurons hypertrophy in response to NGF in control but not in p75(LNTR)-deficient mice, suggest that p75(LNTR) is needed for tyrosine kinase A and NGF signaling efficiency.In conclusion, during adulthood p75(LNTR) appears to play a beneficial role in the response of cholinergic neurons to injury, consistent with the proposed role of p75(LNTR) in the enhancement of TrkA signaling and the transport of neurotrophins by these neurons.

Postnatal development of cholinergic system in mouse basal forebrain: acetylcholinesterase histochemistry and choline-acetyltransferase immunoreactivity

The distribution of acetylcholinesterase histochemistry and choline-O-acetyltransferase immunohistochemistry in the basal forebrain was studied in newborn mice (P0) and until 60 days of postnatal life (P60). A weak acetylcholinesterase activity was found at P0 and P2 in the anterior and intermediate parts of the basal forebrain, and higher in the posterior region. The intensity of labeling, neuronal size and dendritic growth seems to increase progressively in all regions of basal forebrain from P4 to P10. The AChE+ cell count shows that in the anterior portion of the magnocellular basal nucleus the number of cells does not vary significantly from birth to the second month of postnatal life. However, in the intermediate and posterior portions of the nucleus the mean number of labeled cells increases significantly from birth to the end of the second week of postnatal life (P13). The choline-acetyltransferase immunoreactivity appears only detectable at the end of the first week (P6) as a slight immunoreaction, which increases progressively in intensity at P8, and at P10 seems to attain the same intensity of labeling found at P60. These results seem to indicate that the acetylcholinesterase could have a non-classic cholinergic role in the first stages of postnatal development, acting as a growth and cellular differentiation factor.

BDNF is needed for postnatal maturation of basal forebrain and neostriatum cholinergic neurons in vivo

Neurotrophins regulate survival, neurite outgrowth, and phenotypic maturation of developing neurons. Brain-derived neurotrophic factor (BDNF) can promote the survival of developing cholinergic forebrain neurons in vitro and reduce their degeneration following injury in adult rats. We investigated the role of endogenous BDNF during postnatal development of these cholinergic neurons by analyzing homozygous BDNF-deficient (-/-) mice and their littermates (+/+, +/-). At P6, the number of choline acetyltransferase- (ChAT) positive neurons in the medial septum was approximately 23% lower in BDNF-/- mice, although their brain and body weight was normal. At P15, control (+/+) littermates had approximately 45% more and approximately 45% larger ChAT-positive neurons and a much denser cholinergic hippocampal innervation than at P6, indicative of maturation of the septohippocampal system. In BDNF-/- mice, the number, size, and ChAT-immunostaining intensity of the cholinergic neurons remained the same between P6 and P15 (few mice survive longer). BDNF-/- mice had about three times more TUNEL-labeled (a marker of apoptosis) cells in the medial septum at P6, consistent with (but not proof of) the possibility that the cholinergic neurons were dying. The cholinergic hippocampal innervation in BDNF-/- mice expanded to a lesser extent than in controls and had reduced levels of acetylcholinesterase staining at P15. The developmental deficits were largely similar in the neostriatum of BDNF-/- mice. These findings suggest that BDNF is critical for postnatal development and maturation of cholinergic forebrain neurons.

P75 nerve growth factor receptor is important for retrograde transport of neurotrophins in adult cholinergic basal forebrain neurons

The role of the p75 nerve growth factor receptor in the retrograde transport of neurotrophins in the adult CNS was investigated by comparing the transport of 125I-labeled neurotrophins by normal and p75 nerve growth factor receptor-deficient cholinergic septohippocampal neurons. In control mice, nerve growth factor was selectively transported from the hippocampal formation to the cholinergic neurons in the septum. Nerve growth factor labeling was found in three to four times as many septal cholinergic neuronal cell bodies than labeling for neurotrophin-3 or neurotrophin-4/5, and transported brain-derived neurotrophic factor was barely detectable. Cells were considered as labeled when the number of grains per cell exceeded five times background. In p75 nerve growth factor receptor-deficient mice, the number of cholinergic neurons labeled with each of the neurotrophins was reduced by 85-95%. Retrograde labeling of septohippocampal neurons with Fluorogold was not obviously reduced in p75 nerve growth factor receptor-deficient mice, suggesting that general transport mechanisms were not impaired. Despite the reduced neurotrophin transport, cholinergic neurons of p75 nerve growth factor receptor-deficient mice were larger than controls and had an apparently normal density of immunostaining for choline acetyltransferase. Since nerve growth factor is reportedly involved in size regulation and choline acetyltransferase expression, this raises the possibility that the retrograde transport itself is not essential for these events. Thus, p75 nerve growth factor receptor plays an important, although not exclusive, role in the transport of neurotrophins by cholinergic basal forebrain neurons, and retrograde transport of nerve growth factor may not be needed for regulating certain cellular processes.

p75(NGFR) and cholinergic neurons in the developing forebrain: a re-examination

The low-affinity nerve growth factor receptor (p75(NGFR)) apparently can mediate apoptosis in a variety of cells in vitro and in vivo. Previously, our laboratory suggested that p75(NGFR) induced apoptosis in a subpopulation of cholinergic forebrain neurons during postnatal development, i.e., the number of choline acetyltransferase (ChAT)-positive neurons in a control strain of mice decreased whereas it remained higher in p75(NGFR)-deficient (-/-) mice. Discrepancies with subsequent data sets in our laboratory caused us to thoroughly re-analyze the fate of these cholinergic medial septum and neostriatal neurons in new sets of p75(NGFR) -/- and two DNA control strains of mice during development. Between postnatal day (P)6 and P15 the number of ChAT-positive neurons detected in the medial septum of 129/Sv mice and Balb/c mice increased by approximately 64% and approximately 62%, respectively. This increase is contrary to previous reports from our laboratory and indicative of normal postnatal development (including an increase in ChAT-enzyme) of the cholinergic forebrain neurons. In p75(NGFR) -/- mice the number of ChAT-positive neurons in the medial septum remained constant between P6 and P15 and was approximately 31% and approximately 56% higher at P6 than 129/Sv and Balb/c mice, respectively. At P15 and adulthood, p75(NGFR) -/- mice had similar numbers of cholinergic neurons as control mice. In the developing neostriatum, the number of ChAT-positive neurons increased by approximately 56% between P6 and P15 and did not differ between p75(NGFR) -/- and control mice at any time. Analyses for apoptotic DNA fragmentation (TUNEL labeling) at P8 revealed no differences between p75(NGFR) -/- and control mice in 12 forebrain regions, including the septum and neostriatum. At all times, all mice had similar levels of acetylcholinesterase-positive cholinergic innervation of the molecular layer in the dorsal dentate gyrus. These findings suggest that the p75(NGFR) does not necessarily mediate apoptosis in medial septum or neostriatal cholinergic neurons during the postnatal time period. The discrepant results of the previous study are most likely due to a less rigorous application of criteria for data acquisition, including anatomical boundaries that define the nucleus.

Choline availability to the developing rat fetus alters adult hippocampal long-term potentiation

Supplementation with choline during pregnancy in rats causes a long-lasting improvement of visuospatial memory of the offspring. To determine if the behavioral effects of choline are related to physiological changes in hippocampus, the effect of perinatal choline supplementation or deficiency on long-term potentiation (LTP) was examined in hippocampal slices of 6-8 and 12-14 month old rats born to dams consuming a control, choline-supplemented, or a choline-free diet during pregnancy. Stimulating and recording electrodes were placed in stratum radiatum of area CA1 to record extracellular population excitatory postsynaptic potentials (pEPSPs). To induce LTP, a theta-like stimulus train was generated. The amplitude of the stimulus pulses was set at either 10% or 50% of the stimulus intensity which had induced the maximal pEPSP slope on the input/output curve. We found that at both ages, a significantly smaller percentage of slices from perinatally choline-deficient rats displayed LTP after 10% stimulus intensity (compared with control and choline-supplemented rats), and a significantly larger percentage of slices from choline-supplemented rats displayed LTP at 50% stimulus intensity (compared with control and choline-deficient rats). Results reveal that alterations in the availability of dietary choline during discrete periods of development lead to changes in hippocampal electrophysiology that last well into adulthood. These changes in LTP threshold may underlie the observed enhancement of visuospatial memory seen after prenatal choline supplementation and point to the importance of choline intake during pregnancy for development of brain and memory function.

Choline supplementation during prenatal development reduces proactive interference in spatial memory

Previous research has demonstrated that increasing dietary choline during early development can have long-lasting effects on cholinergic (Ch) function that are correlated with improvement of spatial memory ability in rats. The present study is designed to further our understanding of these organizational changes in brain and behavior by examining the effects of spaced vs. massed trials. A third of the rats (n=10) were supplemented with choline chloride prenatally by adding it to the drinking water of their dams. Another third were made deficient of choline during early development by removing choline from the dams diet. The remaining rats served as untreated controls. Postnatally, the offspring were maintained on a choline-sufficient diet and at 120 days of age they began 12-arm radial maze training. The maze data revealed two major effects of early choline availability: (1) Both choline-supplemented and choline-deficient rats performed more accurately than control littermates when trials were spaced. These differences in spatial ability did not appear to be a function of differential response or cue-use strategies. (2) Choline-supplemented rats showed little proactive interference when trials were massed; whereas control rats demonstrated moderate levels and choline-deficient rats exhibited high levels of proactive interference as a function of massed trials. These data suggest that the behavioral consequences of early dietary availability of choline may involve the modification of the discriminative abilities used to attend to stimuli that demarcate the end of one trial and the start of another as well as the capacity for remembering the locations that have been visited during a trial.

Developmental and aging aspects of the cholinergic innervation of the olfactory bulb

The olfactory bulb is a limbic paleocortex which receives monosynaptic sensory afferents from the olfactory mucosa, and a strong direct cholinergic input from the basal forebrain. This review focuses on the rat olfactory bulb as a suitable model to study cholinergic involvements in cortical processing, during development, adulthood and aging. Anatomical and biochemical data show that cholinergic influences upon the bulbar neuronal network are exerted through several types of target cells and receptors (muscarinic and nicotinic). Functional data indicate that cholinergic afferents to the olfactory bulb are involved in local events related to olfactory learning. Neurodegenerative disorders such as Alzheimer's disease involve early olfactory deficits and typical histopathological lesions in the olfactory bulb. In summary, with its exclusively extrinsic cholinergic innervation and direct sensory input, the rat olfactory bulb offers the opportunity to study the cellular and molecular mechanisms of cholinergic influences on cortical processing, in both normal and pathological conditions.

Calbindin-D28k- and astroglial protein-immunoreactivities, and ultrastructural differentiation in the prenatal rat cerebral cortex and hippocampus are affected by maternal adrenalectomy

Maternal adrenal steroid hormones have been proven to be crucial for lung and adrenal prenatal maturation. These hormones mediate the effects of prenatal stress crossing the placenta and influencing the development of the hypothalamus-pituitary-adrenal axis of fetuses. In the present study, we have compared the prenatal development of fetuses from adrenalectomized mothers (ADX group) and from sham-operated mothers. We have used immunohistochemistry for calcium binding-protein Calbindin-D28k, astroglial proteins vimentin and glial fibrillary acidic protein (GFAP), and the ultrastructural differentiation of the cerebral cortex and hippocampus to measure putative differences. The ontogeny of the Calbindin-D28k immunoreactivity was delayed, as transient Calbindin-positive neuronal populations in the ADX group disappeared later during development as compared to that of control animals both in cerebral cortex and hippocampus; cell counts revealed that ADX animals had a significantly higher number of Calbindin-positive cells than controls in the cerebral cortex, while that number was lower in ADX fetuses' hippocampus. Cerebral cortex of ADX animals also had a scattered distribution of stained cells compared with controls, while the hippocampi of the ADX animals had an impaired migration of marginal zone interneurons. No GFAP immunoreactivity was found in the studied prenatal stages. Instead, vimentin-immunoreactivity appeared more profusely distributed throughout the cerebral cortex, in the ADX group than in control animals. At the ultrastructural level, no remarkable differences were found before E20, when a higher undifferentiation in the ADX group, in both cerebral cortex and hippocampus, was evident. The results show for the first time the vulnerability of the prenatal rat brain to maternal adrenalectomy and the necessity of maternal glucocorticoids for encephalic development.

Hypertrophy of basal forebrain neurons and enhanced visuospatial memory in perinatally choline-supplemented rats

The effects of choline supplementation during two time-frames of early development on radial-arm maze performance and the morphology of basal forebrain neurons immunoreactive for the P75 neurotrophin receptor (NTR) in male and female Sprague-Dawley rats were examined. In the first experiment, rats were supplemented with choline chloride from conception until weaning. At 80 days of age, subjects were trained once a day on a 12-arm radial maze for 30 days. Compared to control littermates, supplemented rats made fewer working and reference memory errors; however, the memory enhancing effects of choline supplementation were greater in males than females. A morphometric analysis of NTR-immunoreactive cell bodies at three levels through the medial septum/diagonal band (MS/DBv) of these rats revealed that perinatal choline supplementation caused the somata of cells in the MS/DBv to be larger by 8-15%. In a second experiment, choline supplementation was restricted to embryonic days 12-17. A developmental profile of NTR immunoreactive cell bodies in the MS/DBv of 0-, 8-, 16-, 30- and 90-day old male and female rats again revealed that cell bodies were larger in choline-supplemented rats than controls. As in the behavioral studies, the effect of choline supplementation was greater in male than female rats. These data are consistent with the hypothesis that supplementation with choline chloride during early development leads to an increase in the size of cell bodies of NTR-immunoreactive cells in the basal forebrain and that this change may contribute to long-term improvement in spatial memory.

The serotonin innervation of the basal forebrain shows a transient phase during development

The serotonergic innervation of the adult and developing basal forebrain nuclei of the rat was studied with immunocytochemical techniques at the light and electron microscopic levels. A substantial number of relatively thick serotonergic fibers with few varicosities and random orientation were observed at the time of birth. During the subsequent weeks, the serotonergic fibers increased in number and became thinner with many varicosities. They were also re-oriented, and around the end of the third postnatal week they exhibited the pattern of distribution and density seen in the adult. Electron microscopic analysis revealed that serotonin varicosities formed symmetrical or asymmetrical synapses mainly with dendritic shafts throughout postnatal life. Stereological extrapolation from single sections to the whole volume of varicosities showed that the percentage of serotonin varicosities engaged in synaptic junctions varied according to age. The proportion of labelled varicosities forming synapses increased from birth (21.3%) to the end of the second postnatal week (42.5%), then declined markedly in the following week (17.1%) before increasing again to an adult value of 46%. These findings suggest that the formation of synaptic connections by serotonin axons in the basal forebrain shows two distinct phases in postnatal development: exuberant synapses present in the first two weeks of life may be related to the involvement of serotonin in the maturation of this area, whereas synapses formed later in development may affect the functional state of basal forebrain projections to the neocortex and hippocampus. Thus, at these late stages of development and in the adult, serotonin may influence the activity of these forebrain structures both directly and indirectly.

Developmental profiles of various cholinergic markers in the rat main olfactory bulb using quantitative autoradiography

The existence of possible relationships among the developmental profile of various cholinergic markers in the main olfactory bulb (OB) was assessed by using in vitro quantitative autoradiography. Muscarinic receptors were visualized with [3H]pirenzepine (muscarinic M1-like sites) and [3H]AF-DX 384 (muscarinic M2-like sites); nicotinic receptors by using [3H]cytisine (nicotinic 42-like subtype) and [125I] alpha-bungarotoxin (nicotinic 7-like subtype); cholinergic nerve terminals by using [3H]vesamicol (vesicular acetylcholine transport sites) and [3H]hemicholinium-3 (high-affinity choline uptake sites). These various cholinergic markers exhibited their lowest levels at birth and reached adult values by the end of the 4-5 postnatal weeks. However, the density of presynaptic cholinergic markers and nicotinic receptors at postnatal day 2 represented a large proportion of the levels observed in adulthood, and displays a transient overexpression around postnatal day 20. In contrast, the postnatal development of cholinergic muscarinic M1-like and M2-like receptors is apparently regulated independently of the presynaptic cholinergic markers and nicotinic receptors. Two neurochemically and anatomically separate olfactory glomeruli subsets were observed in the posterior OB of the developing rat. These atypical glomeruli expressed large amounts of [3H]vesamicol-and [3H]hemicholinium binding sites without significant amounts of muscarinic M1, M2, or nicotinic alpha 4 beta 2 receptor binding sites. A significant density of [125I] alpha-bungarotoxin binding sites could be detected only at early postnatal ages. A few olfactory glomeruli specifically restricted to the dorsal posterior OB expressed a high density of [3H]cytisine binding sites but lacked significant binding of the two presynaptic cholinergic markers used here, suggesting their noncholinergic but cholinoceptive nature.

Effects of neonatal lesions of the basal forebrain cholinergic system by 192 immunoglobulin G-saporin: biochemical, behavioural and morphological characterization

Selective removal of the basal forebrain cholinergic neurons by the immunotoxin 192 immunoglobulin G-saporin has offered a new powerful tool for the study of the relationships between cholinergic dysfunction and cognitive impairments. In the present study the morphological and functional consequences of selective lesions of the basal forebrain cholinergic system during early postnatal development have been investigated following bilateral intraventricular injections of 192 immunoglobulin G-saporin to immature (four-day-old) rats. Administration of increasing doses (0.2-0.8 microgram) of the immunotoxin produced dose-dependent loss of cholinergic neurons in the septal/diagonal band area (up to 72-86%) and in the nucleus basalis magnocellularis (up to 91-93%), paralleled by marked reductions in choline acetyltransferase activity in the hippocampus and several cortical regions (73-84%). The parvalbumin-positive neurons in the septal/diagonal band area and the calbindin-positive Purkinje cells in the cerebellum were unaffected at all dose levels. Brain dopamine or noradrenaline levels were unaffected or increased by the immunotoxin treatment. At the optimal dose, 0.4 microgram, the toxin conjugate produced maximal cholinergic depletion without significant mortality. Higher doses (0.8, 1.2 and 1.6 micrograms) of toxin, on the other hand, proved to be lethal for most or all of the injected animals. When tested at three and eight months after the optimal dose, in spite of persisting cholinergic depletion, the noenatally lesioned animals showed no impairment in the water maze task or in locomotor activity and exploration as compared to normal controls, probably reflecting partial sparing of the cholinergic neurons by the neonatal immunotoxic lesion (above all in the vertical and horizontal limbs of the diagonal band area), and/or a greater degree of plasticity in the developing as compared to the mature cholinergic system. The place navigational performance of the neonatally lesioned animals in the water maze task was abolished by central muscarinic cholinergic receptor blockade (by atropine) or by a second immunotoxic lesion, which eliminated virtually all residual cholinergic neurons in the septal/diagonal band area and the nucleus basalis. Administration of 192 immunoglobulin G-saporin to similarly trained, but previously normal adult rats, produced similar cholinergic depletions but much less severe place navigation deficits, suggesting that preoperative training on the task may reduce the functional consequences of a subsequent cholinergic lesion. The results thus support the view that the basal forebrain cholinergic system may be implicated in the acquisition rather than retention of spatial memory in the water maze task.

Development of basal forebrain projections to visual cortex: DiI studies in rat

We performed experiments using retrograde and anterograde labeling with DiI to examine the development of basal forebrain (BFB) projections to the visual cortex in postnatal rats. DiI placed in occipital cortex led to retrograde labeling of BFB neurons as early as postnatal day 0 (P0); labeled cells were found mainly in the diagonal band complex but also in the medial septum, globus pallidus, and substantia innominata. The retrogradely labeled BFB cells displayed remarkably well-developed dendritic arbors, even in younger animals, and showed increases in soma size, dendritic arbors, and dendritic spines over the first 2 postnatal weeks. DiI placements in the diagonal band led to anterogradely labeled axons in cortex. At early ages (P0-P1), labeled axons were largely confined to white matter. With increasing age, greater numbers of labeled axons were seen in the white matter and in deep cortical layers, and labeled axons extended into superficial layers. The leading edge of labeled fibers reached layer V of visual cortex by P2 and layer IV by P4 and were found throughout the cortical layers by P6. Numbers and densities of labeled axons in visual cortex were greater in older animals, at least through P14. The time of ingrowth of labeled BFB axons into visual cortex indicates that these afferents grow into particular cortical layers after those layers have differentiated from the cortical plate. These data indicate that basal forebrain projections arrive in occipital cortex after cortical lamination is well underway and after the entry of primary thalamocortical projections.

Transient dendritic appendages on differentiating septohippocampal neurons are not the sites of synaptogenesis

The factors which determine the final shape and synaptic connections of a neuronal phenotype are largely unknown. In adult animals, a large number of projection neurons, e.g. cortical pyramidal neurons, bear spines which, in the case of pyramidal cells, are postsynaptic elements of mainly asymmetric synapses. In contrast, mature septohippocampal neurons do not bear spines. During maturation, however, septohippocampal projection neurons develop a variety of dendritic appendages. Because the appearance of these processes falls into the period of synaptogenesis, it has been hypothesized that these transient appendages may be the site of synaptogenesis. Here we have investigated whether these transient dendritic appendages are the site of initial synaptic contacts of septohippocampal neurons. Septohippocampal projection neurons in late embryonic and early postnatal rats were identified by retrograde tracing with the carbocyanine dye DiI or biocytin. Subsequently, selected cells were processed for electron microscopy. Serial thin sections through identified dendritic appendages did not reveal synaptic contacts with presynaptic boutons but immature to mature synapses were always found on dendritic shafts or somata. Often, synapses are located close to the appendages. These data indicate that the transient appendages are not the place where ingrowing afferent fibers make their synapses. The available information about transient dendritic appendages suggests, that they may be involved in short-term contacts with ingrowing axons, without being themselves the final site of the synaptic contact.

Distribution of basic fibroblast growth factor in the developing rat brain

Previous studies in vitro indicate that basic fibroblast growth factor participates in the survival, proliferation and differentiation of immature neural cells, predicting that it may have the same types of roles in vivo. In order to evaluate a possible role of basic fibroblast growth factor in neural development, we have examined its localization in the rodent brain at critical stages of development. We characterized basic fibroblast growth factor immunoreactivity at embryonic days 13 and 18, and postnatal days 1, 4, 6, 10, 20 and 90. Our results showed that basic fibroblast growth factor was transiently expressed by different cellular phenotypes throughout development. At embryonic day 13, basic fibroblast growth factor immunoreactivity was sparsely distributed in various cell phenotypes. At embryonic day 18, the primitive cerebral cortex showed basic fibroblast growth factor immunoreactivity within its emerging laminar structure, including the cortical plate and subplate regions. At postnatal day 1, basic fibroblast growth factor immunoreactivity was mostly concentrated in the hippocampal subfields cornu Ammon 1, cornu Ammon 2 and cornu Ammon 3, and neurons of the medical septum and the vertical limb of the diagonal band nuclei. At postnatal days 4-6, astrocyte-like cells showed basic fibroblast growth factor immunoreactivity for the first time during development. At this stage, basic fibroblast growth factor in the hippocampus was mostly shown within subfields cornu Ammon 2 and cornu Ammon 1. In the medical septum, just a few neuronal profiles were weakly stained, and basic fibroblast growth factor positive astrocytes appeared to accumulate around these basic fibroblast growth factor-stained neurons. At postnatal day 20, the adult pattern of basic fibroblast growth factor immunoreactivity was fully established. Astrocytes throughout the brain expressed basic fibroblast growth factor, and neuronal basic fibroblast growth factor was restricted to particular populations such as cingulate cortex and hippocampus. The cornu Ammon 2 subfield was the main neuronal location for basic fibroblast growth factor in the mature hippocampus. Our results showed that the cellular location of basic fibroblast growth factor changes during development, suggesting that basic fibroblast growth factor has multiple and evolving roles during histogenesis and differentiation of the CNS.

Differential induction of Pax genes by NGF and BDNF in cerebellar primary cultures

The Pax genes encode sequence-specific DNA binding transcription factors that are expressed in embryonic development of the nervous system. Primary neuronal cell cultures derived from the cerebellar cortex of embryonic day 14, newborn and 7-d old mice, were used to investigate the cell-type specific expression patterns of three members of the murine paired box containing gene family (Pax gene family), in vitro. Cell types which express Pax-2, Pax-3, and Pax-6 RNA in primary cultures correspond to those found in regions of the cerebellum which show RNA signals in sections of the developing mouse brain. To find mechanisms regulating Pax gene expression during cerebellar development, the differential regulation of Pax-2, Pax-3, and Pax-6 by NGF and BDNF, two structurally related neurotrophins, was studied in such primary cultures. Pax-2 and Pax-6 RNA increased slightly by 1 h and remained elevated throughout a 24-h treatment with BDNF and NGF. Pax-3 RNA was not detected in newborn cultures, but underwent a rapid (1 h) and transient (2 h) induction upon treatment with either BDNF or NGF. No response was seen with EGF or FGF. Cycloheximide treatment amplified Pax-3 induction and prolonged the signal. Thus, Pax-3 induction resembles that of the immediate-early gene c-fos, which transduces growth factor signals during the development of particular neuronal/glial cell types. The changes in Pax expression were inductive rather than trophic.

Late developmental changes of the innervation densities of the myelinated fibres and the outer hair cell efferent fibres in the rat cochlea

The baso-apical distributions of the myelinated nerve fibres (representative for the inner hair cell afferent fibres) and the outer hair cell efferent fibres were studied during postnatal development of the rat cochlea. The myelinated fibres were counted in the primary osseos spiral lamina from semi-thin sections. The outer hair cell efferent fibres were counted in the tunnel of Corti by means of ultra-thin sections. The developmental changes of the myelinated fibres were investigated between 8 and 60 days after birth (DAB); those of the outer hair cell efferent fibres between 20 and 30 DAB. Between 12 DAB (onset of hearing) and 20 DAB the baso-apical distribution of the myelinated fibres does not change. Striking maturational changes occur later after the onset of hearing, between 20 and 30 DAB. The innervation density of the myelinated fibres increases in the lower middle region of the cochlea. In this region a maximum of innervation density appears. The efferent fibres to the outer hair cells show at 20 DAB a maximum of innervation density in the middle of the cochlea but between 20 and 30 DAB, the fibre density decreases in this region. During the same period the maximum of innervation density shifts towards the base. The change in the innervation densities of the myelinated fibres and the outer hair cell efferent fibres occurs late in development, after the onset of hearing, and after the organ of Corti shows an adult-like appearance.

Lesions of the nucleus basalis magnocellularis in immature rats: short- and long-term biochemical and behavioral changes

Short- and long-term effects of unilateral lesions of the nucleus basalis magnocellularis (NBM) on cortical choline acetyltransferase (ChAT) activity and passive avoidance conditioned responses were examined in immature rats. The lesions were made by stereotaxic injection of quisqualic acid on postnatal days 14 (P14), 17 (P17), and 21 (P21). A marked loss of ChAT activity was found 7 days after surgery in all age groups of lesioned rats. Unoperated P14 rats were unable to perform the passive avoidance conditioned responses. Acquisition began on P17. Lesions made on P17 and P21 strongly impaired the acquisition and retention of the task, evaluated 7 days postoperation. No biochemical but a partial behavioral recovery was observed 3 months after surgery in rats lesioned on P14. On the contrary, despite a persistent decrease in cortical ChAT activity, rats lesioned on P21 were able to acquire and retain the passive avoidance conditioned response. These results indicate that destruction of NBM cholinergic neurons shortly after birth is not compensated for by the developmental plasticity of the residual neurons but results in permanent cholinergic hypofunction. They also demonstrate that cholinergic NBM neurons play an important role in the acquisition and retention of a passive avoidance task; nevertheless, a behavioral recovery may take place 3 months after the lesion, even in the presence of a persistent cholinergic hypofunction.

Morphology of intracellularly labeled interneurons in the dentate gyrus of the immature rat

Although many aspects of the morphological development of interneurons in the dentate gyrus have been described, the full extent of their dendrites and local axon projections in immature rodents has not been examined. Here intracellular labeling was used to assess the branching patterns of interneurons in the dentate gyrus of rat pups between 7 and 9 days of age. Labeled neurons were located within or just below the granule cell layer, and most were classified as GABAergic basket neurons on the basis of their dendritic morphologies. All labeled interneurons exhibited immature characteristics. Spines were present on cell bodies and dendrites, and growth cones were visible on some dendrites and axons. In spite of these immature features, the dendrites and axon arbors of the labeled neurons were extensive. Many apical dendrites reached the top of the molecular layer, and a number of basal dendrites extended to the CA3 pyramidal cell layer of the hippocampus. Elaborate axon plexuses were present within the dentate gyrus itself, and axon collaterals of several neurons extended beyond the dentate gyrus to branch within regions CA3 and CA1 of the hippocampus. These results indicate that the dendrites and axon collaterals of dentate interneurons are extensive at a time when the principal neurons, the granule cells, are still proliferating. These data are consistent with the idea that GABAergic interneurons may influence granule cell development in the dentate gyrus, as well as pyramidal cell maturation in the hippocampus proper.

The development of neurons in the nuclei of the horizontal and vertical limb of the diagonal band of Broca of the rat: a qualitative and quantitative analysis of Golgi preparations

We have studied the morphological alterations of neurons in the nuclei of the horizontal (NHL) and vertical (NVL) limbs of the diagonal band of Broca of rats from late embryonic life to maturity using the Golgi-Stensaas and Golgi-Cox methods. During late embryonic life and in the first postnatal days, the two nuclei of the diagonal band of Broca were found to be located near the ventral surface of the brain. Shortly thereafter, neurons in the NHL and NVL gradually take up the positions which they normally occupy in adulthood. At this stage neurons were small with round or elongated somata and 1-3 primary dendrites that only occasionally bore spines and very seldom showed varicosities, features commonly shown by neurons at later postnatal ages. At birth, cells showing varying soma shapes and dendritic morphology were present, and by postnatal day 4 (P4) the three forms of neurons previously described in adult rats (Dinopoulos et al., J. Comp. Neurol., 272 (1988) 461-474) were readily distinguished. During the second postnatal week, the size of cell somata as well as the number, size and extent of dendritic branching underwent considerable increases in both nuclei and at P14 neurons showed features typical of their adult counterparts. In addition they showed a dramatic increase in the number of spines which was followed during the next 10 days by a substantial decrease. Overall, the dendritic geometry of neurons in the NHL and NVL did not change significantly after P14, although their cell bodies continued to increase in size until the middle of the fourth and fifth postnatal weeks respectively. These findings suggest that neurons in the nuclei of the diagonal band of Broca show continuous growth from embryonic life to the end of the second postnatal week when they acquire morphological features comparable to the adult. Thereafter they exhibit only minor morphological alterations with the exception of extensive spine elimination which is pronounced during the third postnatal week and continues until adulthood.

Development of the basal forebrain cholinergic system: phenotype expression prior to target innervation

We measured choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) activities in the rat to determine the time course of development, maturity, and senescence of ChAT activity. Tissue was obtained from Sprague-Dawley rats ranging in age from embryonic day 14 through 23 months. Seven regions were examined, including the magnocellular preoptic/substantia innominata region, frontal cortex, medial septal region, hippocampus, diagnoal band, and medial and lateral striatum. ChAT and AChE activities were first detected as early as E18 in the medial septum, diagonal band and magnocellular preoptic area, all regions of cholinergic cell bodies. Enzyme activity subsequently developed in terminal fields of these cholinergic perikarya (hippocampus and frontal cortex) as well as in the striatum. For all regions, enzyme activity rose during the first four postnatal weeks. This increase in enzyme activity was transient and, in most instances, decreases were observed between postnatal days 30 and 60. Most dramatic were the decreases in enzyme activity in the magnocellular preoptic/substantia innominata and diagonal band regions. Age-related declines also occurred in the frontal cortex, hippocampus, magnocellular preoptic/substantia innominata region, and the striatum. Cholinergic systems undergo dynamic changes especially during development and adulthood.

Postnatal development of cholinergic markers in the rat olfactory bulb: a histochemical and immunocytochemical study

The present study has defined the developmental time course and the distribution patterns of neuronal fibers and cell bodies displaying acetylcholinesterase (AChE) activity or choline acetyltransferase (ChAT) immunoreactivity in the rat olfactory bulb. The results indicate that the deployment of centrifugal AChE-containing fibers is essentially postnatal. The subset of atypical glomeruli, including the modified glomerular complex, is innervated as early as the first postnatal day while the normal ones are not reached by this type of afferent before postnatal day 6. The comparison of AChE labelling with ChAT immunoreactivity strongly supports the assumption that AChE-containing fibers represent mainly, if not exclusively, the cholinergic bulbopetal innervation emanating from the basal forebrain. A quantitative study has confirmed that the density of labelled fibers increases gradually in the postnatal period and spreads heterogeneously among the bulbar layers. The selective precocious innervation of atypical glomeruli is in favor of their involvement in the early processing of olfactory information in the olfactory bulb. Acetylcholinesterase is also expressed within a subset of ChAT-negative interneurons of the developing olfactory bulb. The number of neurons expressing AChE increases from birth to postnatal day 15 and then decreases to reach the adult value on about postnatal day 30. This neuronal population could constitute a cholinoceptive subset mediating the effects of cholinergic afferents on the bulbar neuronal network.

Postnatal development of cholinergic neurons in the rat: I. Forebrain

The postnatal development of cholinergic projection and local-circuit neurons in the rat forebrain was examined by use of choline acetyltransferase (ChAT) immunohistochemistry and acetylcholinesterase (AChE) histochemistry. Although regional nuances were apparent, a general trend emerged in which cholinergic projection neurons in the basal nuclear complex (i.e., medial septal nucleus, vertical and horizontal diagonal band nuclei, magnocellular preoptic field, substantia innominata, nucleus basalis, and nucleus of the ansa lenticularis) demonstrated ChAT-like immunoreactivity earlier in postnatal development than intrinsically organized cholinergic cells in the caudate-putamen nucleus and nucleus accumbens, although this disparity was less apparent for local circuit neurons in the olfactory tubercle and Islands of Calleja complex. Ontologic gradients of enzyme expression also existed in some regions. A lateral to medial progression of ChAT and AChE appearance was observed as a function of increasing postnatal age in the nucleus accumbens and rostral caudate-putamen nucleus. By comparison, a rostrocaudal gradient of expression of ChAT-like immunoreactivity was apparent within the basal nuclear complex. Moderate to intense ChAT positivity, for example, appeared first in the medial septal nucleus. Furthermore, compared to more caudal regions, a greater proportion of AChE-positive neurons in rostral aspects of the basal forebrain expressed ChAT immunoreactivity on postnatal day 1, a difference that was no longer present by postnatal day 5. Cholinergic neurons in all forebrain regions also underwent an initial stage of progressive soma and proximal-dendrite hypertrophy, which peaked during the third postnatal week, followed by a period of cell-body and dendritic shrinkage that persisted into the fifth postnatal week when adult configurations were reached. These soma and dendritic size increases and decreases were not correlated with the magnitude of postnatal ChAT expression, which increased progressively until adult levels were attained approximately by the third to fifth weeks after birth. Expression of AChE in putative cholinergic neurons appeared to precede that of ChAT, especially in the caudate-putamen complex. Staining intensity of AChE also incremented earlier than that of ChAT.

Thyroid hormone causes sexually distinct neurochemical and morphological alterations in rat septal-diagonal band neurons

Sex differences were investigated in cholinergic neurons of the septal-diagonal band region of adult rats subjected to neonatal treatment with 3,3',5-triiodo-L-thyronine (T3). Neonatal hyperthyroidism resulted in a 44% increase in specific activity of choline acetyltransferase (ChAT; EC 2.3.1.6) in adult male rat septal-diagonal band region, whereas no change in ChAT activity could be detected in either dorsal or ventral hippocampus. An increase in muscarinic cholinergic receptors, as measured by [3H]quinuclidinyl benzilate [( 3H]QNB) binding, was discovered in both septum-diagonal band and dorsal hippocampus of the T3-treated male rats. Immunohistochemistry in the septal-diagonal band region indicated a more intense staining in the neonatally T3-treated adult male rats than in controls, with larger and more abundant ChAT-positive and nerve growth factor receptor (NGF-R)-positive varicosities. ChAT immunocytochemistry showed a substantial decrease in cell body area in the medial septum and in the vertical limb of the diagonal band of T3-treated male rats, while cell density increased twofold. Female littermates subjected to the same treatment showed no changes in any of the biochemical or immunohistochemical cholinergic markers. Only in the medial septum was morphology significantly altered in the female T3-treated rats in that ChAT-positive cell body area increased. These results indicate a marked sexual variation in the septal-diagonal band region with respect to the sensitivity of postnatally developing cholinergic neurons to the actions of excess thyroid hormone.

The hippocampal formation: morphological changes induced by thyroid, gonadal and adrenal hormones

The hippocampal formation is of considerable interest due to its proposed role in a number of important functions, including learning and memory processes. Manipulations of thyroid, gonadal and adrenal hormones have been shown to influence hippocampal physiology as well as learning and memory. The cellular events which underlie these hormone-induced functional changes are largely unexplored. However, studies suggest that hormonal manipulations during development and in adulthood result in dramatic morphological changes within the hippocampal formation. Because neuronal physiology has been suggested to depend upon neuronal morphology, we have been determining the morphologic sensitivity of hippocampal neurons to thyroid and steroid hormones in an effort to elucidate possible structural mechanisms to account for differences in hippocampal function. In this review, hormone-induced structural changes in the developing and adult hippocampal formation are discussed, with particular emphasis on their functional relevance. Sex differences, as well as the developmental effects of thyroid hormone and glucocorticoids, are described. Moreover, the effects of ovarian steroids, thyroid hormone and glucocorticoids on neuronal morphology in the hippocampal formation of the adult rat are reviewed. These hormone-induced structural changes may account, at least in part, for previously reported hormone-induced changes in hippocampal function.

Dendritic growth and regression in rat dentate granule cells during late postnatal development

The goal of this study was to determine whether dendritic regression occurs in granule neurons of the rat dentate gyrus during late postnatal development. In vitro hippocampal slices were prepared from rats between the ages of 14 and 60 days, and granule neurons in one portion of the suprapyramidal blade were labeled by intracellular injection of horseradish peroxidase. The dendrites of filled neurons were analyzed in both two and three dimensions directly from 400 microns thick whole-mounts. Results showed that the molecular layer expanded by approximately 50% between days 14 and 60. At every age examined, granule cell dendrites reached the top of the molecular layer, suggesting that dendrites continued to grow during this time period. In contrast, the number of dendritic segments per neuron decreased from an average of 36 to 28. Three-dimensional measurements showed that total dendritic length and surface area per granule cell did not change, suggesting that the overall dendritic tree size of granule neurons may be regulated during late postnatal development in the rodent.

Developing forebrain astrocytes are sensitive to thyroid hormone

Previous studies have shown that developing neurons of the basal forebrain and hippocampus are sensitive to thyroid hormone (Gould and Butcher: J. Neurosci., 9:3347-3358, 1989; Rami et al: Neuroscience, 19:1217-1226, 1986). In order to determine whether or not thyroid hormone influences the development of astrocytes in brain regions where neurons are affected, we performed vimentin and glial fibrillary acidic protein (GFAP) immunocytochemical and single-section Golgi-impregnation analyses on the basal forebrain and hippocampus of control and neonatally thyroid hormone treated rats. For purposes of comparison, glial cells of the pontomesencephalotegmental (PMT) region, a region where developing neurons are not morphologically affected by thyroid hormone imbalances (Gould and Butcher, op. cit.), were also examined. Neonatal thyroid hormone treatment resulted in a premature disappearance of vimentin-immunoreactive radial glia in the basal forebrain and hippocampus. In addition, a premature appearance of GFAP-immunoreactive astrocytes with mature morphological characteristics was observed in the basal forebrain and hippocampus of thyroid hormone treated animals. Quantitative analyses revealed significant increases in the density of GFAP-immunostained astrocytes and in the cross-sectional cell body area and the number of primary processes in Golgi-impregnated astrocytes of the basal forebrain and hippocampus of animals treated neonatally with thyroid hormone. In contrast, no changes in any of these parameters were observed in glial cells of the PMT region with neonatal thyroid hormone treatment.