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

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.

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.

Apoptosis in the rat basal forebrain during development and following lesions of connections

Evidence suggests that neurotrophins are essential for the survival and phenotypic maintenance of cholinergic basal forebrain (BF) neurons. We evaluated the pattern of programmed cell death in the BF of the rat during development and after ablations of the cerebral cortex, a major target area and source of neurotrophins for BF neurons. We identified dying cells using the TUNEL (terminal deoxynucleotidyl-transferase-mediated dUTP-biotin nick end labelling) method and confirmed their apoptotic morphology with electron microscopy. Moreover, we demonstrated the expression of the apoptotic marker active caspase-3 in cells with features of apoptosis. TUNEL(+) cells were present in the developing BF during the first two postnatal weeks. Their frequency peaked at postnatal day (P)1 and at P5. TUNEL used in conjunction with immunofluorescence for neuronal nuclear protein (NeuN) showed that, at both peak stages, the majority of apoptotic cells were neurons. Extensive lesions of the cerebral cortex at different ages (P0, P7 and P14) did not induce significant changes in the frequency of apoptotic BF neurons. However, they resulted in alterations in the morphological phenotype of choline acetyltransferase (ChAT)-immunoreactive neurons in the BF, and a reduction in their number which was inversely proportional to the age at which the lesions were performed. We suggest that: (i) apoptosis is temporally coordinated with the morphological and neurochemical differentiation of BF neurons and the establishment of connections with their target areas; and (ii) cortical ablations do not affect the survival of BF neurons, but they influence the phenotype of cholinergic BF neurons.

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.

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.

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.

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.

Formation of synapses between basal forebrain afferents and cerebral cortex neurons: an electron microscopic study in organotypic slice cultures

Co-cultures of rat basal forebrain and cerebral cortex were maintained from 1 to 5 weeks in vitro with serum-free defined medium. The formation of synaptic connections between basal forebrain afferent fibres and cortical neurons was studied by specific labelling with three staining techniques, including (i) neuronal tract tracing with the fluorescent dye 1,1'-dioctodecyl-3,3,3'3'- tetramethylindocarbocyanine perchlorate, (ii) acetylcholinesterase histochemistry, and (iii) choline acetyltransferase immunocytochemistry. Both basal forebrain and cerebral cortex tissue displayed organotypic characteristics in culture. Cerebral cortex revealed a dense innervation by axonal projections from the basal forebrain. All three labelling techniques produced similar results at the light microscopic level, with densest innervation located in the marginal zone. At the fine structural level, the 1,1'-dioctodecyl-3,3,3'3'-tetramethylindocarbocyanine perchlorate-, acetylcholinesterase- and choline acetyltransferase-stained basal forebrain afferents all revealed a number of synaptic contacts with cortical neurons. The contacts displayed consistent synaptic features, including presynaptic accumulation of small round vesicles, cleft widening, and postsynaptic densities forming symmetric synapses. These morphological characteristics of connections formed in vitro are similar to basal forebrain cholinergic projections to cerebral cortex in normal brain. Based on these results, this tissue culture model appears to be an useful tool for investigations of the development of cholinergic innervation of cerebral cortex.