Blockade of mesolimbic dopamine transmission dramatically increases sensitivity to the rewarding effects of nicotine in the ventral tegmental area

Nicotine produces rewarding and aversive motivational effects in humans and other animal species. Here, we report that the mammalian ventral tegmental area (VTA) represents a critical neural substrate for the mediation of both the rewarding and aversive properties of nicotine. We demonstrate that direct infusions of nicotine into the VTA can produce both rewarding and aversive motivational effects. While the rewarding effects of higher doses of nicotine were not attenuated by dopamine (DA) receptor blockade, blockade of mesolimbic DA signalling with either systemic or intra-nucleus accumbens (NAc) neuroleptic pretreatment potentiated the sensitivity to nicotine's rewarding properties over a three-order-of-magnitude dose range. Furthermore, the behavioural effects of lower doses of intra-VTA nicotine were reversed, switching the motivational valence of nicotine from aversive to rewarding. Our results suggest that blockade of mesolimbic DA signalling induced by neuroleptic medications may block selectively the aversive properties of nicotine, thus increasing the vulnerability to nicotine's rewarding and addictive properties by inducing a unique, drug-vulnerable phenotype.

A mutation in the AMPA-type glutamate receptor, glr-1, blocks olfactory associative and nonassociative learning in Caenorhabditis elegans

The alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-type ionotropic glutamate receptor mediates fast excitatory neurotransmission in the vertebrate brain and is important for synaptic plasticity and the initial induction of long-term potentiation (LTP). This study found that the putative Caenorhabditis elegans AMPA receptor gene, glr-1, plays a significant role in experience-dependent behavior in C. elegans. glr-1 mutants are deficient in an olfactory associative learning task, in which diacetyl (DA) is paired with acetic acid solution. glr-1 mutant nematodes are also impaired in nonassociative learning (habituation) with the same DA stimulus. The C. elegans learning mutants, lrn-1 and lrn-2, are impaired in chemosensory associative learning yet have no deficits in habituation. The results suggest that although associative and nonassociative learning can be genetically dissociated (lrn-1 and lrn-2), they also share some common molecular processes, including glr-1-mediated neurotransmission.

Mouse strain differences in opiate reward learning are explained by differences in anxiety, not reward or learning

Gene-targeting techniques to produce null mutations provide a powerful method for evaluating the contribution of particular candidate genes involved in motivation. The embryonic stem cell lines in which homologous recombination is undertaken are derived from 129 mice, but because of the impoverished performance of 129 mice on a number of behavioral tasks, mice chimeric for the mutation are often bred with a C57BL/6 mouse strain. Thus, an examination of both parental strains is important in the study of the knock-out mice. Although the C57BL/6 behavioral phenotype is well documented, details of the 129 phenotype have not been the focus of study until recently. We investigated opiate motivation in both 129/SvJ and C57BL/6J mouse strains to determine whether, and under what circumstances, the 129/SvJ mouse exhibited motivated behavior toward opiates. 129/SvJ mice required both drug and contextual cues to demonstrate morphine conditioned place preferences on test day, whereas C57BL/6J mice required only contextual cues to express opiate place conditioning. Pentobarbital and diazepam but not saline, cocaine, or naloxone could substitute for morphine on test day in 129/SvJ mice, demonstrating that morphine indeed has rewarding motivational valence in the 129/SvJ mouse strain. This critical, interoceptive cue in 129/SvJ mice on test day may be the anxiolytic properties of the effective drugs. Therefore, some deficits observed in 129 mice and mice harboring this genetic background may be attributed to high levels of anxiety during the retrieval period rather than to sensory, learning, or motivational deficits.

A mutation in the AMPA-type glutamate receptor, glr-1, blocks olfactory associative and nonassociative learning in Caenorhabditis elegans

The alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-type ionotropic glutamate receptor mediates fast excitatory neurotransmission in the vertebrate brain and is important for synaptic plasticity and the initial induction of long-term potentiation (LTP). This study found that the putative Caenorhabditis elegans AMPA receptor gene, glr-1, plays a significant role in experience-dependent behavior in C. elegans. glr-1 mutants are deficient in an olfactory associative learning task, in which diacetyl (DA) is paired with acetic acid solution. glr-1 mutant nematodes are also impaired in nonassociative learning (habituation) with the same DA stimulus. The C. elegans learning mutants, lrn-1 and lrn-2, are impaired in chemosensory associative learning yet have no deficits in habituation. The results suggest that although associative and nonassociative learning can be genetically dissociated (lrn-1 and lrn-2), they also share some common molecular processes, including glr-1-mediated neurotransmission.

Regulation of distinct attractive and aversive mechanisms mediating benzaldehyde chemotaxis in Caenorhabditis elegans

Olfactory-mediated chemotaxis in nematodes provides a relatively simple system to study biological mechanisms of information processing. Analysis of the kinetics of chemotaxis in response to 100% benzaldehyde revealed an initial attractive response that is followed by a strong aversion to the odorant. We show that this behavior is mediated by two genetically separable attraction- and aversion-mediating response pathways. The attraction initially dominates behavior but with prolonged exposure habituation leads to a behavioral change, such that the odorant becomes repulsive. This olfactory habituation is susceptible to dishabituation, thereby re-establishing the attractive response to the odorant. Re-examination of the putative olfactory adaptation mutant adp-1(ky20) revealed that the phenotype observed in this line is due to a supersensitivity to a dishabituating stimulus, rather than a defect in the adaptation to odorants per se. A modified benzaldehyde chemotaxis assay was developed and used for the isolation of a mutant with a specific defect in habituation kinetics, expressed as a persistence of the attractive response.

Combined hippocampal and amygdala lesions block learning of a response-independent form of occasion setting

This study compared rats with dorsal striatal, ventrolateral prefrontal cortical, and combined lesions of the hippocampus and amygdala to sham controls on a conditional discrimination task in which contextual cues modulated a taste aversion. All groups were able to acquire this occasion setting task. The 2nd experiment functionally minimized the stimulus-response component of the paradigm, creating a "tasteless" form of occasion setting. Rats with pretraining lesions of the hippocampus and amygdala were impaired compared with shams on the acquisition of this tasteless occasion setting task. Rats with posttraining combined lesions did not retain the ability to perform the tasteless occasion setting task learned preoperatively. Rats with selective lesions of either the hippocampus or the amygdala alone were not impaired in the acquisition of the tasteless occasion setting task. The findings suggest that this occasion setting task may be learned by several redundant neural systems.

Direct neural fate specification from embryonic stem cells: a primitive mammalian neural stem cell stage acquired through a default mechanism

Little is known about how neural stem cells are formed initially during development. We investigated whether a default mechanism of neural specification could regulate acquisition of neural stem cell identity directly from embryonic stem (ES) cells. ES cells cultured in defined, low-density conditions readily acquire a neural identity. We characterize a novel primitive neural stem cell as a component of neural lineage specification that is negatively regulated by TGFbeta-related signaling. Primitive neural stem cells have distinct growth factor requirements, express neural precursor markers, generate neurons and glia in vitro, and have neural and non-neural lineage potential in vivo. These results are consistent with a default mechanism for neural fate specification and support a model whereby definitive neural stem cell formation is preceded by a primitive neural stem cell stage during neural lineage commitment.

GABA(A) receptors in the ventral tegmental area control bidirectional reward signalling between dopaminergic and non-dopaminergic neural motivational systems

In the midbrain ventral tegmental area (VTA), both dopaminergic and nondopaminergic neural substrates mediate various behavioural reward phenomena. VTA GABAergic neurons are anatomically positioned to influence the activity of both the mesolimbic dopamine system and nondopamine efferents from the VTA. In order to examine the possible functional role of VTA GABA(A) receptors in neural reward processes, we performed discrete, bilateral microinjections of the GABA(A) receptor agonist, muscimol, or the GABA(A) receptor antagonist, bicuculline, into the VTA. Using a fully counterbalanced, unbiased conditioned place-preference paradigm, we demonstrate that activation of VTA GABA(A) receptors, with the GABA(A) receptor agonist muscimol (5--50 ng/microL), or inhibition of VTA GABA(A) receptors, with the GABA(A) receptor antagonist bicuculline (5--50 ng/microL), both produce robust rewarding effects. Furthermore, these rewarding effects can be pharmacologically dissociated: blockade of dopamine receptors with a dopamine receptor antagonist, alpha-flupenthixol (0.8 mg/kg; i.p.), or concurrent activation of VTA GABA(B) receptors with a GABA(B) receptor agonist, baclofen (70 ng/microL), blocked the rewarding properties of the GABA(A) receptor agonist, but had no effect on the rewarding properties of the GABA(A) receptor antagonist. These results suggest that, within the VTA, a single GABA(A) receptor substrate controls bidirectional reward signalling between dopaminergic and nondopaminergic brain reward systems.

The D2 receptor is critical in mediating opiate motivation only in opiate-dependent and withdrawn mice

According to the dual systems model for opiate reward, dopamine mediates opiate motivation when an animal is in a deprived motivational state (i.e. opiate-dependent and in withdrawal) and not when an animal is in a nondeprived state (i.e. previously drug-naive). To determine the role of the D2 dopamine receptor subtype in mediating opiate motivation, we examined the behaviour of N5 congenic D2 receptor knockout mice and their wild-type siblings in opiate-naive and opiate-dependent and withdrawn place conditioning paradigms. Opiate-naive D2 receptor knockout mice demonstrated acquisition of morphine-conditioned place preference but failed to acquire place preference when conditioned in the deprived state. We propose that D2 receptor function is critical in mediating the motivational effects of opiates only when the animal is in an opiate-dependent and withdrawn motivational state. These findings also underscore the important influence of the genetic background to a given phenotype, as evidenced by the observation that increasing the allelic contribution from the 129/SvJ strain abolishes morphine place preference in C57BL/6 wild-type mice.

A new 'spin' on neural stem cells?

The existence of neural stem cells in the adult brain was essentially denied until the last decade. Within the past ten years, considerable progress has been made in examining the fundamental properties of neural stem cells. Most recently there has been much interest in the identification and precise location of the adult neural stem cells in vivo. Studies examining the localization of neural stem cells are controversial and suggest two distinct locations within the adult brain: the ependymal layer lining the ventricles, and the subependymal layer immediately adjacent to the ependyma.

A behavioral and genetic dissection of two forms of olfactory plasticity in Caenorhabditis elegans: adaptation and habituation

Continuous presentation of an olfactory stimulus causes a decrement of the chemotaxis response in the nematode Caenorhabditis elegans. However, the differences between the learning process of habituation (a readily reversible decrease in behavioral response) and other types of olfactory plasticity such as adaptation (a decrement in response due to sensory fatigue, which cannot be dishabituated) have not been addressed. The volatile odorant diacetyl (DA) was used within a single paradigm to assess the distinct processes of olfactory adaptation and habituation. Preexposing and testing worms to 100% DA vapors caused a chemotaxis decrement that was not reversible despite the presentation of potentially dishabituating stimuli. This DA adaptation was abolished in worms with an odr-10 mutation (encoding a high-affinity DA receptor on the AWA neuron), even though naive chemotaxis remained unaffected. Conversely, DA adaptation remained intact in odr-1 mutants (defective in AWC neuron-mediated olfactory behavior), even though naive chemotaxis to DA decreased. Surprisingly, exposure to vapors of intermediate concentrations of DA (0.01% and 25%) did not cause worms to exhibit any response decrement. In contrast to preexposure to high DA concentrations, preexposure to low DA concentrations (0.001%) produced habituation of the chemotaxis response (a dishabituating stimulus could reverse the response decrement back to baseline levels). The distinct behavioral effects produced by DA preexposure highlight a concentration-dependent dissociation between two decremental olfactory processes: adaptation at high DA concentrations versus habituation at low DA concentrations.

Retinal stem cells in the adult mammalian eye

The mature mammalian retina is thought to lack regenerative capacity. Here, we report the identification of a stem cell in the adult mouse eye, which represents a possible substrate for retinal regeneration. Single pigmented ciliary margin cells clonally proliferate in vitro to form sphere colonies of cells that can differentiate into retinal-specific cell types, including rod photoreceptors, bipolar neurons, and Müller glia. Adult retinal stem cells are localized to the pigmented ciliary margin and not to the central and peripheral retinal pigmented epithelium, indicating that these cells may be homologous to those found in the eye germinal zone of other nonmammalian vertebrates.

Why stem cells?

Stem cells are viewed from the perspectives of their function, evolution, development, and cause. Counterintuitively, most stem cells may arise late in development, to act principally in tissue renewal, thus ensuring an organism's long-term survival. Surprisingly, recent reports suggest that tissue-specific adult stem cells have the potential to contribute to replenishment of multiple adult tissues.

A cell-survival factor (N-acetyl-L-cysteine) alters the in vivo fate of constitutively proliferating subependymal cells in the adult forebrain

The adult mouse brain contains a population of constitutively proliferating subependymal cells that surround the lateral ventricle and are the direct progeny of the neural stem cell. Constitutively proliferating cells divide rapidly; 6 days after labeling, 60% of their progeny undergo cell death, 25% migrate to the olfactory bulbs, and 15% continue to proliferate within the subependyma. We have intraventricularly infused a cell survival factor N-acetyl-L-cysteine (NAC), which is known to have survival effects without concomitant proliferative effects on cells in vitro, and examined the resulting fate of cells spared from the normally occurring cell death. NAC infusion for 5 days results in a five-fold increase in the number of retrovirally labeled subependymal cells compared to saline-infused controls. The increase in the number of subependymal cells is directly proportional to the amount of time during which NAC is present and is not due to increased proliferation. While NAC is able to keep all the normally dying progeny alive, the cells spared from death remain confined to the subependyma lining the lateral ventricles and do not migrate to the olfactory bulbs (one normal fate of constitutively proliferating progeny) or into the surrounding brain parenchyma. When animals survive for an additional 6 days following NAC infusion, the number of retrovirally labeled subependymal cells returns to control values, indicating that the continued presence of NAC is necessary for cell survival. These data suggest that preventing cell death is not sufficient to keep all of the progeny of these cells in a proliferative mode.

Migrational analysis of the constitutively proliferating subependyma population in adult mouse forebrain

Initial experiments to evaluate the in vivo fate(s) of constitutively proliferating subependymal cells determined that, following in vivo labeling of this population by infection with a retrovirus containing a beta-galactosidase reporter gene, there was a progressive and eventually complete loss of histochemically beta-galactosidase-positive cells within the lateral ventricle subependyma with increasing survival times of up to 28 days after retroviral infection. Subsequent experiments were designed to ascertain the potential contributions of: (i) the migration of subependymal cells away from the forebrain lateral ventricles; and (ii) the down-regulation of the retroviral reporter gene expression. Retroviral lineage tracing experiments demonstrate that a major in vivo fate for constitutively proliferating subependymal cells is their rostral migration away from the walls of the lateral ventricle to the olfactory bulb. Although down-regulation of retroviral reporter gene expression does not contribute to the loss of detection of beta-galactosidase-labeled cells from the lateral ventricle subependyma, it does result in an underestimation of the absolute number of retrovirally labeled cells in the olfactory bulb at longer survival times. Furthermore, a temporal decrease in the double labeling of beta-galactosidase-labeled cells with [3H]thymidine was observed, indicating that only a subpopulation of the migratory subependymal-derived cells continue to actively proliferate en route to the olfactory bulb. These two events may contribute to the lack of a significant increase in the total number of retrovirally labeled subependymal cells during rostral migration. Evidence from separately published studies suggests that cell death is also an important regulator of the size of the constitutively proliferating subependymal population. In summary, in vivo studies utilizing retroviral reporter gene labeling demonstrate that constitutively proliferating subependymal cells born in the lateral ventricle migrate rostrally to the olfactory bulb. Loss of proliferation potential and retroviral reporter gene down-regulation contribute to the lack of any significant increase in the total number of labeled cells recovered in the olfactory bulb.

Olfactory associative learning in Caenorhabditis elegans is impaired in lrn-1 and lrn-2 mutants

The C. elegans mutants, lrn-1 and lrn-2, are impaired in associative learning using conditioned taste cues. Both mutants are defective in associative learning about appetitive and aversive events, indicating that lrn-1 and lrn-2 exert effects across motivational boundaries. In a new olfactory associative learning paradigm, in which wild type worms learn to avoid a previously attractive diacetyl odor after it has been paired with an aversive acetic acid solution, lrn-1 and lrn-2 are impaired. Although defective in associative learning using a conditioned olfactory cue, nonassociative learning (habituation and dishabituation) using this same olfactory cue is unaffected. The discovery that lrn-1 and lrn-2 are defective in associative learning with both taste and olfactory cues may suggest that associative learning in different sensory modalities converges on a common genetic pathway in C. elegans that is subserved by lrn-1 and lrn-2.

Adult mammalian forebrain ependymal and subependymal cells demonstrate proliferative potential, but only subependymal cells have neural stem cell characteristics

The adult derivatives of the embryonic forebrain germinal zones consist of two morphologically distinct cell layers surrounding the lateral ventricles: the ependyma and the subependyma. Cell cycle analyses have revealed that at least two proliferating populations exist in this region, one that is constitutively proliferating and one that is relatively quiescent and thought to include the endogenous adult neural stem cells. Earlier studies demonstrated that specific dissection of the region surrounding the lateral ventricles was necessary for the in vitro isolation of multipotent, self-renewing neural stem cells. However, in these studies, the ependymal layer was not physically separated from the subependymal layer to identify the specific adult laminar localization of the neural stem cells around the lateral ventricles. To determine which cellular compartment in the adult forebrain contained the neural stem cells, we isolated and cultured the ependyma separately from the subependyma and tested for the presence of neural stem cells using the in vitro neurosphere assay. We demonstrate that the ependymal cells can proliferate in vitro to form sphere-like structures. However, the ependymal cells generating spheres do not have the ability to self-renew (proliferate to form secondary spheres after dissociation) nor to produce neurons, but rather only seem to generate glial fibrillary acidic protein-positive ependymal cells when plated under differentiation conditions in culture. On the other hand, a subpopulation of subependymal cells do possess the self-renewing and multipotential characteristics of neural stem cells. Therefore, the adult forebrain neural stem cell resides within the subependymal compartment.

Olfactory associative learning in Caenorhabditis elegans is impaired in lrn-1 and lrn-2 mutants

The C. elegans mutants, lrn-1 and lrn-2, are impaired in associative learning using conditioned taste cues. Both mutants are defective in associative learning about appetitive and aversive events, indicating that lrn-1 and lrn-2 exert effects across motivational boundaries. In a new olfactory associative learning paradigm, in which wild type worms learn to avoid a previously attractive diacetyl odor after it has been paired with an aversive acetic acid solution, lrn-1 and lrn-2 are impaired. Although defective in associative learning using a conditioned olfactory cue, nonassociative learning (habituation and dishabituation) using this same olfactory cue is unaffected. The discovery that lrn-1 and lrn-2 are defective in associative learning with both taste and olfactory cues may suggest that associative learning in different sensory modalities converges on a common genetic pathway in C. elegans that is subserved by lrn-1 and lrn-2.

Distinct neural stem cells proliferate in response to EGF and FGF in the developing mouse telencephalon

Multipotent, self-renewing neural stem cells reside in the embryonic mouse telencephalic germinal zone. Using an in vitro neurosphere assay for neural stem cell proliferation, we demonstrate that FGF-responsive neural stem cells are present as early as E8.5 in the anterior neural plate, but EGF-responsive neural stem cells emerge later in development in a temporally and spatially specific manner. By separately blocking EGF and FGF2 signaling, we also show that EGF alone and FGF2 alone can independently elicit neural stem cell proliferation and at relatively high cell densities separate cell nonautonomous effects can substantially enhance the mitogen-induced proliferation. At lower cell densities, neural stem cell proliferation is additive in the presence of EGF and FGF2 combined, revealing two different stem cell populations. However, both FGF-responsive and EGF-responsive neural stem cells retain their self-renewal and multilineage potential, regardless of growth factor conditions. These results support a model in which separate, lineage-related EGF- and FGF-responsive neural stem cells are present in the embryonic telencephalic germinal zone.

CNS stem cells: where's the biology (a.k.a. beef)?

Central nervous system (CNS) stem cells have become the subject of many laboratories' efforts, presentations, and publications. Yet, in the stem cell world, CNS cells are viewed with skepticism. This is likely due to a dearth of biology (in vivo function) to accompany a flurry of phenomenological and restorative neurology studies. In this article, we compare and contrast the biological knowledge of adult forebrain epidermal growth factor-responsive neural stem cells that has emerged from our laboratories with that of hematopoietic stem cells, using two recent papers in the latter field as specific examples. A comparison of stem cell location, lineage, and repopulation suggests that our understanding of CNS stem cell biology is immature. We conclude that a greater focus on in vivo biology will enhance our knowledge and understanding of CNS stem cells.

Striatal cholinergic interneurons: birthdates predict compartmental localization

The striatal patch and matrix compartment neurons are born at different times during rat development. The majority of the early born neurons preferentially end up in the patch compartment, while the majority of the later born neurons end up in the matrix compartment. Although the cholinergic interneurons are all born early in neurogenesis (between embryonic day E12 and E17), and we would therefore expect them to be located mainly in the patches, they are relatively homogeneously distributed in the adult, with a preference for the matrix area just outside the patches (the intermediate zone). To ask if birthdate can predict the compartmental localization of cholinergic neurons in the striatum, we marked new postmitotic neurons in the embryo with a maternal injection of bromodeoxyuridine (BrdU) on E13, E15 or E17 and labeled the patch compartment with an injection of the retrograde tracer True Blue into the substantia nigra on postnatal day (P) 1. The pups were sacrificed at P40 and the tissue was processed for BrdU, choline acetyltransferase, and True Blue triple labeling. Cholinergic neurons that became postmitotic at E13, had a higher chance of ending up in the patch compartment compared to either the intermediate zone or the rest of the matrix compartment. On the other hand cholinergic neurons that became postmitotic at E17 had a higher chance of ending up in the matrix compartment (including the intermediate zone). We conclude that birthdate can predict compartmental localization, with the cholinergic neurons in the intermediate zone following the same pattern as the cholinergic neurons in the rest of the matrix compartment. Cholinergic neurons show the same relative birthdate/compartment relationship as do other striatal neurons, although the absolute birthdates of cholinergic neurons are shifted earlier in neurogenesis.

In vivo clonal analyses reveal the properties of endogenous neural stem cell proliferation in the adult mammalian forebrain

The adult mammalian forebrain contains a population of multipotential neural stem cells in the subependyma of the lateral ventricles whose progeny are the constitutively proliferating cells, which divide actively throughout life. The adult mammalian brain is ideal for examining the kinetics of the stem cells due to their strict spatial localization and the limited and discrete type of progeny generated (constitutively proliferating cells). Clonal lineage analyses 6 days after retrovirus infection revealed that under baseline conditions 60% of the constitutively proliferating cells undergo cell death, 25% migrate to the olfactory bulb and 15% remain confined to the lateral ventricle subependyma (where they reside for approximately 15 days). Analysis of single cell clones 31 days after retroviral infection revealed that the stem cell divides asymmetrically to self-renew and give rise to constitutively proliferating cells. Following repopulation of the depleted subependyma the average clone size is 2.8 times larger than control, yet the absolute number of cells migrating to the olfactory bulb is maintained and the stem cell retains its asymmetric mode of division. The number of neural stem cells in the adult forebrain 33 days after repopulation of the subependyma was estimated using bromodeoxyuridine labeling of subepenydmal cells. There were calculated to be 1200–1300 cells between the rostral corpus callosum and rostral anterior commissure; these data support a lineage model similar to those based on stem cell behavior in other tissue types.

A two-separate-motivational-systems hypothesis of opioid addiction

There has been a long debate as to whether opioids are sought for withdrawal relief or for their ability to serve as incentives in their own right. We suggest that independent motivational systems mediate the rewarding effects of opioids in the nondependent state and in the physically dependent/withdrawal state. In the opioid-dependent state and the presence of opioid withdrawal, the rewarding effects of withdrawal relief inhibit or mask the acute rewarding effects initially exerted in the nondependent state, but the acute rewarding effects are unmasked after the alleviation of withdrawal.

Clonal heterogeneity in the early embryonic rodent cortical germinal zone and the separation of subventricular from ventricular zone lineages

The cell cycle kinetics of individual clones of cells in the embryonic cortex were studied to determine the amount of heterogeneity among cortical progenitors. This kinetic heterogeneity may be the first sign of heterogeneous differentiation in the proliferating cortical germinal zone. Retroviral lineage tracing of clonal progeny in the embryonic day 16 (E16) rat cortex (48 hr after the retroviral infection and [3H]thymidine labeling of proliferating cells) permitted a description of the formation of the preplate in the medial cortex and the formation of the subventricular zone (SVZ) in the lateral cortex. Forty percent of the retrovirally tagged clones were double-labeled with a pulse of [3H]thymidine, corresponding to the 40% of ventricular zone cells in S-phase at the time of [3H]thymidine injection. Most of clones had cell numbers with powers of 2 (2, 4, and 8 cells), suggesting synchronous modes of division. Nevertheless, 15% of the retrovirally tagged clones that were double-labeled with [3H]thymidine at the time of [3H]thymidine injection showed asynchronous mode of division. Clonally related cells in the ventricular zone showed considerable variability in cell cycle times: 56% of the clones (8-cell clones) were composed of faster cycling cells with cell cycle times of 12 hr, and 25% of the clones (4-cell clones) represented slower cycling cells with cell cycle times of 16 hr. The clones migrating outside the ventricular zone differed in size and spatial distribution in the lateral versus medial cortex. In the lateral cortex, half the migrating clones were large proliferating 8-cell clones with all their members contained within the forming SVZ. In the medial cortex, the majority of the migrating clones were 2-cell and 4-cell clones. Given that the medial cortex matures later than the lateral neocortex and that no SVZ has formed in the rat medial cortex by E16, we suggest that the majority of cells that leave the medial cortical VZ by E16 are cells destined to form the neuronal populations of the preplate. The early embryonic cortical ventricular zone includes a mosaic of specialized progenitor cells.

Transforming growth factor-alpha null and senescent mice show decreased neural progenitor cell proliferation in the forebrain subependyma

The adult mammalian forebrain subependyma contains neural stem cells and their progeny, the constitutively proliferating progenitor cells. Using bromodeoxyuridine labeling to detect mitotically active cells, we demonstrate that the endogenous expression of transforming growth factor-alpha (TGFalpha) is necessary for the full proliferation of progenitor cells localized to the dorsolateral corner of the subependyma and the full production of the neuronal progenitors that migrate to the olfactory bulbs. Proliferation of these progenitor cells also is diminished with age (in 23- to 25-months-old compared with 2- to 4-months-old mice), likely because of a lengthening of the cell cycle. Senescence or the absence of endogenous TGFalpha does not affect the numbers of neural stem cells isolated in vitro in the presence of epidermal growth factor. These results suggest that endogenous TGFalpha and the effects of senescence may regulate the proliferation of progenitor cells in the adult subependyma, but that the number of neural stem cells is maintained throughout life.

Cold shock before associative conditioning blocks memory retrieval, but cold shock after conditioning blocks memory retention in Caenorhabditis elegans

The effects of cooling on associative learning and memory processes in Caenorhabditis elegans were investigated by giving the worms cold shock at various times before or after conditioning. A pretraining cold shock in the 30 min immediately before conditioning and a posttraining cold shock in the 30 min immediately after conditioning both disrupted learning and memory processes tested a short time after conditioning. However, if tested 3 hr after conditioning, worms given a pretraining cold shock demonstrated learned preferences, whereas worms given a posttraining cold shock still had memory deficits. These results suggest that the effects of cold shock on associative learning and memory can be dissociated into effects on memory retrieval and memory retention.

Mutations that prevent associative learning in C. elegans

The nematode Caenorhabditis elegans offers a promising system for the reductionist study of learning and memory. In this article, classical conditioning in C. elegans is demonstrated with a variety of associative learning assays. These assays allowed for the isolation and behavioral characterization of 2 mutant C. elegans lines impaired in associative learning. Both lines show no short-term or long-term associative conditioning; however, they appear relatively normal in tests of nonassociative learning and sensorimotor function. In combination with the well-described genetics and neuroanatomy of C. elegans, the isolation of mutants selectively, yet completely, blocked in associative learning provides the basis for an effective characterization of the cellular and molecular aspects of associative learning.

Cold shock before associative conditioning blocks memory retrieval, but cold shock after conditioning blocks memory retention in Caenorhabditis elegans

The effects of cooling on associative learning and memory processes in Caenorhabditis elegans were investigated by giving the worms cold shock at various times before or after conditioning. A pretraining cold shock in the 30 min immediately before conditioning and a posttraining cold shock in the 30 min immediately after conditioning both disrupted learning and memory processes tested a short time after conditioning. However, if tested 3 hr after conditioning, worms given a pretraining cold shock demonstrated learned preferences, whereas worms given a posttraining cold shock still had memory deficits. These results suggest that the effects of cold shock on associative learning and memory can be dissociated into effects on memory retrieval and memory retention.

Lesions of the lateral parabrachial nucleus block the aversive motivational effects of both morphine and morphine withdrawal but spare morphine's discriminative properties

This study examined if the aversive properties of morphine, the aversive properties of morphine withdrawal, and the discriminative properties of morphine are mediated by common neurobiological substrates. Lesions of the lateral parabrachial nucleus, which blocked the aversive properties of morphine in the conditioned taste aversion paradigm, also blocked the acquisition of conditioned place aversions to environments paired with the aversive properties of morphine withdrawal in morphine-dependent rats. When morphine and saline were used as cues in a discrimination task, however, both sham-operated and lesioned rats were able to solve the task.

Mutations that prevent associative learning in C. elegans

The nematode Caenorhabditis elegans offers a promising system for the reductionist study of learning and memory. In this article, classical conditioning in C. elegans is demonstrated with a variety of associative learning assays. These assays allowed for the isolation and behavioral characterization of 2 mutant C. elegans lines impaired in associative learning. Both lines show no short-term or long-term associative conditioning; however, they appear relatively normal in tests of nonassociative learning and sensorimotor function. In combination with the well-described genetics and neuroanatomy of C. elegans, the isolation of mutants selectively, yet completely, blocked in associative learning provides the basis for an effective characterization of the cellular and molecular aspects of associative learning.

Neurobiological constraints on behavioral models of motivation

The application of neurobiological tools to behavioral questions has produced a number of working models of the mechanisms mediating the rewarding and aversive properties of stimuli. The authors review and compare three models that differ in the nature and number of the processes identified. The dopamine hypothesis, a single system model, posits that the neurotransmitter dopamine plays a fundamental role in mediating the rewarding properties of all classes of stimuli. In contrast, both nondeprived/deprived and saliency attribution models claim that separate systems make independent contributions to reward. The former identifies the psychological boundary defined by the two systems as being between states of nondeprivation (e.g. food sated) and deprivation (e.g. hunger). The latter identifies a boundary between liking and wanting systems. Neurobiological dissociations provide tests of and explanatory power for behavioral theories of goal-directed behavior.

Deprivation state switches the neurobiological substrates mediating opiate reward in the ventral tegmental area

The population of mesolimbic dopaminergic neurons is believed to be a primary site at which opiates produce their rewarding effects. Using an unbiased, counterbalanced place conditioning paradigm, we reexamined the contribution made by these cells to the rewarding properties of morphine. Rats were conditioned such that distinct environments were paired with an intra-ventral tegmental area (VTA) microinfusion of either 500 ng per 0.5 microl per side morphine or 0. 5 microl per side sterile saline. Furthermore, rats were conditioned either previously drug-naive or while in a motivational state of opiate dependence and withdrawal. We report that pretreatment with the broad-spectrum dopamine antagonist alpha-flupentixol blocked the acquisition of conditioned place preferences for environments paired with morphine microinjections directly into the VTA in opiate-dependent and withdrawn, but not in previously drug-naive, rats. Lesions of the tegmental pedunculopontine nucleus (TPP) produced exactly the opposite pattern of results. TPP lesions blocked the acquisition of conditioned place preferences for environments paired with VTA morphine microinjections in previously drug-naive, but not in opiate-dependent and withdrawn, rats. These data double-dissociate two independent reward substrates within the VTA itself and suggest that deprivation state selects which of these two substrates will be active. Furthermore, these findings are the first to demonstrate a nondopaminergic substrate for reward within the VTA itself.

Lesions of the lateral parabrachial nucleus block the aversive motivational effects of both morphine and morphine withdrawal but spare morphine's discriminative properties

This study examined if the aversive properties of morphine, the aversive properties of morphine withdrawal, and the discriminative properties of morphine are mediated by common neurobiological substrates. Lesions of the lateral parabrachial nucleus, which blocked the aversive properties of morphine in the conditioned taste aversion paradigm, also blocked the acquisition of conditioned place aversions to environments paired with the aversive properties of morphine withdrawal in morphine-dependent rats. When morphine and saline were used as cues in a discrimination task, however, both sham-operated and lesioned rats were able to solve the task.

Clonidine antagonizes the aversive effects of opiate withdrawal and the rewarding effects of morphine only in opiate withdrawn rats

The researchers asked whether clonidine, an alpha 2-noradrenergic agonist, would block selectively the motivational effects of opiate withdrawal and whether clonidine's effects would respect the boundary between nondeprived and deprived motivational states. In a place conditioning paradigm, clonidine (0.05 mg/kg ip) blocked the rewarding effects of morphine in opiate-withdrawn rats (as well as the aversive properties of withdrawal itself), but did not affect morphine place preferences (2 and 20 mg/kg) in drug-naive rats. Furthermore, clonidine blocked the acquisition of morphine (15 mg/kg), but not LiCl (15 mg/kg), conditioned taste aversions in water-deprived rats. The results suggest that the motivational system activated in deprived animals includes dopaminergic and noradrenergic components that are in series with each other.

Is there a neural stem cell in the mammalian forebrain?

Neural precursor cells have been of interest historically as the building blocks of the embryonic CNS and, most recently, as substrates for restorative neurological approaches. The majority of previous in vitro studies of the regulation of neural-cell proliferation by polypeptide growth factors, and in vivo studies of neural lineage, argue for the presence of precursors with limited proliferative or lineage potential in the mammalian CNS. This is in contrast to renewable tissues, such as the blood or immune system, skin epithelium and epithelium of the small intestinal crypts, which contain specialized, self-renewing cells known as stem cells. However, recent in vitro and in vivo studies from our and other laboratories lead us to conclude that neural stem cells, with self-renewal and multilineage potential, are present in the embryonic through to adult mammalian forebrain.

Separate neural substrates mediate the motivating and discriminative properties of morphine

A previous study (T. V. Jaeger & D. van der Kooy, 1993) has implicated a visceral and taste region (parabrachial nucleus), but not mesolimbic dopamine terminal fields (nucleus accumbens), as a substrate for opiate discriminative effects. The authors now show that (a) morphine's discriminative effects in the parabrachial nucleus (PBN) require the activation of opiate receptors; (b) in rats trained to discriminate morphine from saline, infusions of morphine into the ventral tegmental area (VTA) do not generalize to the systemic training condition; (c) infusions of morphine into the PBN, but not the VTA, serve as a stimulus for the acquisition of discrimination learning; and (d) morphine applied to the VTA, but not the PBN, is motivating. The data show that the motivating and discriminative effects of morphine are processed separately by the brain. Further, discriminative drug effects are neither necessary nor sufficient for opiate motivational effects.

Early postnatal lesions of the substantia nigra produce massive shrinkage of the rat striatum, disruption of patch neuron distribution, but no loss of patch neurons

Unilateral lesions of the substantia nigra in the first postnatal week cause a massive shrinkage (> 50%) of the ipsilateral rat striatum, due primarily to neuronal cell death. Striatal patch neurons (marked by retrograde labeling from the substantia nigra prior to the lesions) selectively survive the striatal cell death caused by the lesions. However, after the lesions these early retrogradely labeled patch neurons are distributed diffusely through the remaining striatum, in contrast to their normal patchy distribution throughout the postnatal period. Thus, many striatal neurons (but not the patch and matrix neurons with early axonal projections to the substantia nigra) are critically dependent upon interactions with the substantia nigra for their survival during the early postnatal period, and in the absence of these missing striatal and substantia nigra neurons, the remaining striatal patch neurons are no longer distributed in a patchy fashion.

In vivo growth factor expansion of endogenous subependymal neural precursor cell populations in the adult mouse brain

The lateral ventricle subependyma in the adult mammalian forebrain contains both neural stem and progenitor cells. This study describes the in situ modulation of these subependymal neural precursor populations after intraventricular administration of exogenous growth factors. In vivo infusion of epidermal growth factor (EGF) into adult mouse forebrain for 6 consecutive days resulted in a dramatic increase in the proliferation and total number of subependymal cells and induced their migration away from the lateral ventricle walls into adjacent parenchyma. Immediately after EGF infusion, immunohistochemical characterization of the EGF-expanded cell population demonstrated that >95% of these cells were EGF receptor- and nestin-positive, whereas only 0.9% and 0.2% labeled for astrocytic and neuronal markers, respectively. Seven weeks after EGF withdrawal, 25% of the cells induced to proliferate after 6d of EGF were still detectable; 28% of these cells had differentiated into new astrocytes and 3% into new neurons in the cortex, striatum, and septum. Newly generated oligodendrocytes were also observed. These in vivo results (1) confirm the existence of EGF-responsive subependymal neural precursor cells in the adult mouse forebrain and (2) suggest that EGF acts directly as a proliferation, survival, and migration factor for subependymal precursor cells to expand these populations and promote the movement of these cells into normal brain parenchyma. Thus, in situ modulation of endogenous forebrain precursor cells represents a novel model for studying neural development in the adult mammalian brain and may provide insights that will achieve adult replacement of neurons and glia lost to disease or trauma.

Clonidine antagonizes the aversive effects of opiate withdrawal and the rewarding effects of morphine only in opiate withdrawn rats

The researchers asked whether clonidine, an alpha 2-noradrenergic agonist, would block selectively the motivational effects of opiate withdrawal and whether clonidine's effects would respect the boundary between nondeprived and deprived motivational states. In a place conditioning paradigm, clonidine (0.05 mg/kg ip) blocked the rewarding effects of morphine in opiate-withdrawn rats (as well as the aversive properties of withdrawal itself), but did not affect morphine place preferences (2 and 20 mg/kg) in drug-naive rats. Furthermore, clonidine blocked the acquisition of morphine (15 mg/kg), but not LiCl (15 mg/kg), conditioned taste aversions in water-deprived rats. The results suggest that the motivational system activated in deprived animals includes dopaminergic and noradrenergic components that are in series with each other.

Steel mutant mice are deficient in hippocampal learning but not long-term potentiation

Mice carrying mutations in either the dominant white-spotting (W) or Steel (Sl) loci exhibit deficits in melanogenesis, gametogenesis, and hematopoiesis. W encodes the Kit receptor tyrosine kinase, while Sl encodes the Kit ligand, Steel factor, and the receptor-ligand pair are contiguously expressed at anatomical sites expected from the phenotypes of W and Sl mice. The c-kit and Steel genes are also both highly expressed in the adult murine hippocampus: Steel is expressed in dentate gyrus neurons whose mossy fiber axons synapse with the c-kit expressing CA3 pyramidal neurons. We report here that Sl/Sld mutant mice have a specific deficit in spatial learning. These mutant mice are also deficient in baseline synaptic transmission between the dentate gyrus and CA3 but show normal long-term potentiation in this pathway. These observations demonstrate a role for Steel factor/Kit signaling in the adult nervous system and suggest that a severe deficit in hippocampal-dependent learning need not be associated with reduced hippocampal long-term potentiation.

Separate neural substrates mediate the motivating and discriminative properties of morphine

A previous study (T. V. Jaeger & D. van der Kooy, 1993) has implicated a visceral and taste region (parabrachial nucleus), but not mesolimbic dopamine terminal fields (nucleus accumbens), as a substrate for opiate discriminative effects. The authors now show that (a) morphine's discriminative effects in the parabrachial nucleus (PBN) require the activation of opiate receptors; (b) in rats trained to discriminate morphine from saline, infusions of morphine into the ventral tegmental area (VTA) do not generalize to the systemic training condition; (c) infusions of morphine into the PBN, but not the VTA, serve as a stimulus for the acquisition of discrimination learning; and (d) morphine applied to the VTA, but not the PBN, is motivating. The data show that the motivating and discriminative effects of morphine are processed separately by the brain. Further, discriminative drug effects are neither necessary nor sufficient for opiate motivational effects.

Variability and partial synchrony of the cell cycle in the germinal zone of the early embryonic cerebral cortex

Cell cycle parameters were estimated using the cumulative 3H-thymidine S-phase labeling and percentage of labeled mitoses methods in the embryonic day 14 and 15 germinal zone of the rat cerebral cortex. The shortest cell cycle time was seen in the dorsal neocortex and the longest in the lateral neocortex and fimbria (the latter also had a low growth fraction). No differences were observed in cell cycle times between the cells in the ventricular and subventricular zone in the same neocortical region. The results suggest gradients of lengthening cell cycle times extending ventrolaterally and ventromedially from the dorsomedial neocortex. Although a majority of proliferating cells in individual cortical regions seem to belong to one population in terms of cell kinetics, several pieces of evidence suggest some heterogeneity: the asymmetric shapes of the percentages of labeled mitoses curves, the small population of noncycling neuroepithelial cells in the neocortex and mesocortex, and small population of cells that become pyknotic. Groups of DNA-synthesizing nuclei that were ectopically located in the inner half of the ventricular zone also indicate the existence of different subpopulations of neuroepithelial cells. In addition, after a pulse injection of 3H-thymidine the germinal zone is characterized by alternating clusters of heavily and lightly labeled cell nuclei that may reflect the simultaneous passage of a cluster of cells through the same portion of S-phase. We suggest that partial cell cycle synchrony within groups of ventricular cells may explain the presence of these iterative cell kinetic patterns in the developing cortex.

Tegmental pedunculopontine nucleus lesions do not block cocaine reward

Previous studies have implicated the tegmental pedunculopontine (TPP) nucleus in mediating the rewarding effects of opiates, food, and amphetamine, provided that animals are not in aversive motivational states induced by food--or drug--withdrawal. We wondered if bilateral TPP lesions could block the reinforcing effects of systemic cocaine in a place conditioning paradigm. Both lesioned and sham animals acquired cocaine place preferences. TPP-lesioned animals subsequently failed to acquire place preferences when conditioned with morphine, replicating previous data with TPP lesions. It is possible that our cocaine place conditioning protocol induced aversions during drug withdrawal, thus explaining the inability of TPP lesions to block conditioning. We looked for place aversions by conditioning animals at various times postinjection of cocaine. At no time point following drug withdrawal from cocaine were significant conditioned aversions observed. Cocaine's systemic motivational effects are mediated by a substrate separate from the TPP substrate underlying the rewarding effects of opiates, food, and amphetamine.

Mechanics of motility: distinct dynein binding domains on alpha- and beta-tubulin

Microtubules (MTs) interact with force-generating proteins to generate a variety of intracellular movements, including intracellular particle transport, ciliary-flagellar beating, and chromosome-spindle movements during mitosis-meiosis. Relatively little is known about the mechanics of these motor-MT interactions, in part because the motor binding domains of the MT and the corresponding MT binding domains of the motor have not been well characterized. Using a flagellar motility assay, we report that the MT subunits, alpha- and beta-tubulin, each contain a dynein binding domain located near the C-termini of their respective tubulin subunits. Blocking either alpha- or beta-tubulin binding domains of dynein attenuates motility in demembranated sea urchin sperm up to 50%. Interestingly, blocking both alpha- and beta-tubulin binding domains on dynein produces much greater decreases in motility. These data suggest that flagellar dynein binds to both subunits of the MT polymer, alpha- and beta-tublin. In addition, the two subunits appear to contribute equivalent, but functionally separate, roles to flagellar motility.

The mouse mutation reeler causes increased adhesion within a subpopulation of early postmitotic cortical neurons

Early postmitotic cortical neurons are mostly corticofugal projection neurons that take up positions in deep cortical laminae. Later postmitotic neurons are preferentially localized to superficial cortical laminae. In reeler mutant mice it appears that cortical laminar positions with respect to birthdate are reversed (Caviness, 1982). In a reanalysis of reeler lamination we found that early postmitotic cortical neurons labeled by embryonic day (E) 11-13 injections of a birthdate marker, or by early postnatal day (PND) 2 retrograde labeling through their output projections, appear to take up positions both in the superficial and deep cortex. Neurons born on E11 and E12 are more likely to be situated superficially in the reeler cortex and neurons born on E13 are more likely to be situated in the deep reeler cortex. Many corticofugal projection neurons in the deep (but not superficial) reeler cortex either die or retract their axons before PND 21. We hypothesize that the earliest postmitotic (E11-E12) of the early postmitotic reeler cortical neurons are overly adhesive and act as a barrier to later postmitotic migrating neurons. In vitro cortical aggregation cultures confirmed that early postmitotic (E12) reeler neurons are more adhesive than early postmitotic (E12) wild-type neurons or late postmitotic (E16) reeler or wild-type cortical neurons. We suggest that the moderate wild-type preferential adhesion of early postmitotic cortical neurons to each other helps deep and superficially fated lineages to form cortical laminae.

Pattern formation in the mammalian forebrain: striatal patch and matrix neurons intermix prior to compartment formation

The striatum of the mammalian forebrain is divided into two compartments: the patches and the matrix. Neurons of the patch compartment in the rat striatum become postmitotic earlier in neurogenesis than neurons of the matrix compartment. The selective adhesion of patch neurons to one another has been suggested previously to be an important developmental mechanism of striatal compartmentation. We asked if the selective adhesion of patch neurons is expressed before or after the migration of the majority of the matrix neurons into the striatum. Patch neurons were labelled in vivo by a fluorescent retrograde tracer injected into the substantia nigra on embryonic day 19, which almost exclusively labelled patch neurons. Matrix neurons were labelled with a maternal injection of bromodeoxyuridine at embryonic day 18. When animals were killed at embryonic day 20, the majority of the retrogradely labelled patch neurons were intermixed with the bromodeoxyuridine-labelled matrix neurons, although there appeared to be clustering of some of the patch neurons. However, by postnatal day 2 there was a complete segregation of the clusters of the retrogradely labelled patch neurons from the bromodeoxyuridine-labelled matrix neurons in the striatum. This process was modelled in vitro. The patch and matrix compartments were labelled in vivo at embryonic day 13 and 18 respectively, with different birthdate markers ([3H]thymidine or bromodeoxyuridine). At embryonic day 20 the striatal tissue was removed, dissociated and reaggregated in suspension cultures. After 1 day in vitro, labelled patch and matrix neurons were randomly intermixed within the reaggregates. Examination of the cultures at 2.5 and 4 days in vitro revealed clumping of the labelled patch neurons towards the centres of the reaggregates. Over this same period, the labelled matrix neurons did not clump and were dispersed towards the periphery of the reaggregates. The results suggest that patch neuron adhesiveness may appear relatively soon after these neurons become postmitotic, but that this adhesiveness is unable to overcome the initial force produced by the massive migration of matrix neurons into the striatum. We hypothesize that a migratory phase of embryonic striatal development exists, when fated patch and matrix neurons intermix. After this migratory phase, patch neuron adhesiveness can produce the mature segregation of the striatal compartments.

Neuroleptics block high- but not low-dose heroin place preferences: further evidence for a two-system model of motivation

The researchers studied whether 2 separate motivational systems in the brain underlie the rewarding effects of morphine. The brainstem tegmental pedunculopontine nucleus (TPP) is involved in mediating the motivational effects of opiates in nondeprived (drug-naive) rats, whereas dopamine transmission is necessary in mediating the motivational effects of opiates in deprived rats (opiate withdrawal). The results show that heroin's motivational properties obey the same boundary between a nondeprived and a deprived motivational state. Bilateral ibotenic acid lesions of the TPP blocked the acquisition of a place preference for an environment paired with 0.05 mg/kg heroin (a dose that induces no withdrawal aversion) but had no effect on place preference for an environment paired with 0.5 mg/kg heroin (a dose that does induce withdrawal aversion). Dopamine antagonist pretreatment produced the opposite pattern of results.

Tegmental pedunculopontine lesions in rats decrease saccharin's rewarding effects but not its memory-improving effect

The tegmental pedunculopontine nucleus (TPP) of the brainstem has been identified as a critical substrate for both opiate and food reward in nondeprived rats. In this study of rats, TPP lesions blocked saccharin-conditioned place preferences, in both the presence and the absence of water deprivation. TPP lesions also attenuated the unconditioned intake of saccharin and water over several hours after recovery from food and water deprivation. TPP lesions did not block saccharin preferences over water in short-duration tests. The researchers propose that the absence of a lesion effect may reflect previously conditioned discriminations. TPP lesions had no effect on the ability of posttrial presentations of a 3.2% saccharin solution to improve lithium-chloride-conditioned taste aversions. TPP lesions dissociate 2 behavioral processes elicited by saccharin: One mediates unconditioned-reward-conditioned-reinforcing effects, and another mediates the memory-improving effect.