Cholinergic projections from the basal forebrain to the basolateral amygdaloid complex: a combined retrograde fluorescent and immunohistochemical study

Selectivity and persistent firing responses to social vocalizations in the basolateral amygdala

This study examined responsiveness to acoustic stimuli among neurons of the basolateral amygdala. While recording from single neurons in awake mustached bats (Pteronotus parnellii), we presented a wide range of acoustic stimuli including tonal, noise, and vocal signals. While many neurons displayed phasic or sustained responses locked to effective auditory stimuli, the majority of neurons (n=58) displayed a persistent excitatory discharge that lasted well beyond stimulus duration and filled the interval between successive stimuli. Persistent firing usually began seconds (median value, 5.4 s) after the initiation of a train of repeated stimuli and lasted, in the majority of neurons, for at least 2 min after the end of the stimulus train. Auditory-responsive amygdalar neurons were generally excited by one stimulus or very few stimuli. Most neurons did not respond well to synthetic stimuli including tones, noise bursts or frequency-modulated sweeps, but instead responded only to vocal stimuli (82 of 87 neurons). Furthermore, most neurons were highly selective among vocal stimuli. On average, neurons responded to 1.7 of 15 different syllables or syllable sequences. The largest percentage of neurons responded to a hiss-like rectangular broadband noise burst (rBNB) call associated with aggressive interactions. Responsiveness to effective vocal stimuli was reduced or eliminated when the spectrotemporal features of the stimuli were altered in a subset of neurons. Chemical activation of the medial geniculate body (MG) increased both background and evoked firing. Among 39 histologically localized recording sites, we saw no evidence of topographic organization in terms of temporal response pattern, habituation, or the affect of calls to which neurons responded. Overall, these studies demonstrate that amygdalar neurons in the mustached bat show high selectivity to vocal stimuli, and suggest that persistent firing may be an important feature of amygdalar responses to social vocalizations.

The vertebrate mesolimbic reward system and social behavior network: a comparative synthesis

All animals evaluate the salience of external stimuli and integrate them with internal physiological information into adaptive behavior. Natural and sexual selection impinge on these processes, yet our understanding of behavioral decision-making mechanisms and their evolution is still very limited. Insights from mammals indicate that two neural circuits are of crucial importance in this context: the social behavior network and the mesolimbic reward system. Here we review evidence from neurochemical, tract-tracing, developmental, and functional lesion/stimulation studies that delineates homology relationships for most of the nodes of these two circuits across the five major vertebrate lineages: mammals, birds, reptiles, amphibians, and teleost fish. We provide for the first time a comprehensive comparative analysis of the two neural circuits and conclude that they were already present in early vertebrates. We also propose that these circuits form a larger social decision-making (SDM) network that regulates adaptive behavior. Our synthesis thus provides an important foundation for understanding the evolution of the neural mechanisms underlying reward processing and behavioral regulation.

Stress, neurotransmitters, corticosterone and body-brain integration

Stress can be defined as a brain-body reaction towards stimuli arising from the environment or from internal cues that are interpreted as a disruption of homeostasis. The organization of the response to a stressful situation involves not only the activity of different types of neurotransmitter systems in several areas of the limbic system, but also the response of neurons in these areas to several other chemicals and hormones, chiefly glucocorticoids, released from peripheral organs and glands. Thus, stress is probably the process through which body-brain integration plays a major role. Here we review first the responses to an acute stress in terms of neurotransmitters such as dopamine, acetylcholine, glutamate and GABA in areas of the brain involved in the regulation of stress responses. These areas include the prefrontal cortex, amygdala, hippocampus and nucleus accumbens and the interaction among those areas. Then, we consider the role of glucocorticoids and review some recent data about the interaction of these steroids with several neurotransmitters in those same areas of the brain. Also the actions of other substances (neuromodulators) released from peripheral organs such as the pancreas, liver or gonads (insulin, IGF-1, estrogens) are reviewed. The role of an environmental enrichment on these same responses is also discussed. Finally a section is devoted to put into perspective all these environmental-brain-body-brain interactions during stress and their consequences on aging. It is concluded that the integrative perspective framed in this review is relevant for better understanding of how the organism responds to stressful challenges and how this can be modified through different environmental conditions during the process of aging. This article is part of a Special Issue entitled: Brain Integration.

Muscarinic receptor activation enables persistent firing in pyramidal neurons from superficial layers of dorsal perirhinal cortex

Persistent-firing neurons in the entorhinal cortex (EC) and the lateral nucleus of the amygdala (LA) continue to discharge long after the termination of the original, spike-initiating current. An emerging theory proposes that endogenous persistent firing helps support a transient memory system. This study demonstrated that persistent-firing neurons are also prevalent in rat perirhinal cortex (PR), which lies immediately adjacent to and is reciprocally connected with EC and LA. Several characteristics of persistent-firing neurons in PR were similar to those previously reported in LA and EC. Persistent firing in PR was enabled by the application of carbachol, a nonselective cholinergic agonist, and it was induced by injecting a suprathreshold current or by stimulating suprathreshold excitatory synaptic inputs to the neuron. Once induced, persistent firing lasted for seconds to minutes. Persistent firing could always be terminated by a sufficiently large and prolonged hyperpolarizing current; it was prevented by antagonists of muscarinic cholinergic receptors (mAChRs); and it was blocked by flufenamic acid. The latter has been suggested to inhibit a Ca(2+) -activated nonspecific cation conductance (G(CAN) ) that normally furnishes the sustained depolarization during persistent firing. In many PR neurons, the discharge rate during persistent firing was a graded function of depolarizing and/or hyperpolarizing inputs. Persistent firing was not prevented by blocking fast excitatory and inhibitory synaptic transmission, demonstrating that it can be generated endogenously. We suggest that persistent-firing neurons in PR, EC, LA, and certain other brain regions may cooperate in support of a transient-memory system.

Neuronal localization of M2 muscarinic receptor immunoreactivity in the rat amygdala

Muscarinic cholinergic neurotransmission in the amygdala is critical for memory consolidation in emotional/motivational learning tasks, but little is known about the neuronal distribution of different receptor subtypes. Immunohistochemistry was used in the present investigation to localize the m2 receptor (M2R). Differential patterns of M2R-immunoreactivity (M2R-ir) were observed in the somata and neuropil of the various amygdalar nuclei. Neuropilar M2R-ir was strongest in rostral portions of the basolateral nuclear complex (BLC). M2R-positive (M2R+) somata were seen in low numbers in all nuclei of the amygdala. Most M2R+ neurons associated with the BLC were in the lateral nucleus and external capsule. These cells were nonpyramidal neurons that contained glutamatic acid decarboxylase (GAD), somatostatin (SOM), and neuropeptide Y (NPY), but not parvalbumin (PV), calretinin (CR), or cholecystokinin (CCK). Little or no M2R-ir was observed in GAD+, PV+, CR+, or CCK+ axons in the BLC, but it was seen in some SOM+ axons and many NPY+ axons. M2R-ir was found in a small number of spiny and aspiny neurons of the central nucleus that were mainly located along the lateral and ventral borders of its lateral subdivision. Many of these cells contained SOM and NPY. M2R+ neurons were also seen in the medial nucleus, including a distinct subpopulation of neurons that surrounded its anteroventral subdivision. The latter neurons were negative for all neuronal markers analyzed. The intercalated nuclei (INs) were associated with two types of large M2R+ neurons, spiny and aspiny. The small principal neurons of the INs were M2R-negative. The somata and dendrites of the large spiny neurons, which were actually found in a zone located just outside of the rostral INs, expressed SOM and NPY, but not GAD. These findings indicate that acetylcholine can modulate a variety of discrete neuronal subpopulations in various amygdalar nuclei via M2Rs, especially neurons that express SOM and NPY.

Postsynaptic targets of GABAergic basal forebrain projections to the basolateral amygdala

Recent studies indicate that the basolateral amygdala, like the neocortex and hippocampus, receives GABAergic inputs from the basal forebrain in addition to the well-established cholinergic inputs. Since the neuronal targets of these inputs have yet to be determined, it is difficult to predict the functional significance of this innervation. The present study addressed this question in the rat by employing anterograde tract tracing combined with immunohistochemistry at the light and electron microscopic levels of analysis. Amygdalopetal axons from the basal forebrain mainly targeted the basolateral nucleus (BL) of the amygdala. The morphology of these axons was heterogeneous and included GABAergic axons that contained vesicular GABA transporter protein (VGAT). These axons, designated type 1, exhibited distinctive large axonal varicosities that were typically clustered along the length of the axon. Type 1 axons formed multiple contacts with the cell bodies and dendrites of parvalbumin-containing (PV+) interneurons, but relatively few contacts with calretinin-containing and somatostatin-containing interneurons. At the ultrastructural level of analysis, the large terminals of type 1 axons exhibited numerous mitochondria and were densely packed with synaptic vesicles. Individual terminals formed broad symmetrical synapses with BL PV+ interneurons, and often formed additional symmetrical synapses with BL pyramidal cells. Some solitary type 1 terminals formed symmetrical synapses solely with BL pyramidal cells. These results suggest that GABAergic neurons of the basal forebrain provide indirect disinhibition, as well as direct inhibition, of BL pyramidal neurons. The possible involvement of these circuits in rhythmic oscillations related to emotional learning, attention, and arousal is discussed.

Prefrontal stimulation of GABAA receptors counteracts the corticolimbic hyperactivity produced by NMDA antagonists in the prefrontal cortex of the rat

RATIONALE

The hypofunction of NMDA receptors in the prefrontal cortex (PFC) has been suggested to produce corticolimbic hyperactivity through the reduction of cortical GABA transmission.

OBJECTIVES

The present study investigates the effects of injections of the NMDA antagonist 3-[(R)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid (CPP) into the PFC on (1) the release of dopamine and/or acetylcholine in the amygdala and hippocampus, (2) the levels of corticosterone in the hippocampus and (3) spontaneous motor activity. Also, the stimulation of GABA(A) receptors, by prefrontal injections of muscimol, on the effects produced by NMDA antagonists on these same neurochemical, hormonal and behavioural parameters was evaluated.

METHODS

Male Wistar rats were implanted with guide cannulae to perform bilateral microinjections into the PFC and microdialysis experiments in the amygdala and/or ventral hippocampus, simultaneously. Spontaneous motor activity was monitored in the open field.

RESULTS

Injections of CPP (1 μg/0.5 μl) into the PFC increased dialysate concentrations of dopamine and acetylcholine in the amygdala, acetylcholine and free corticosterone in the hippocampus and also motor activity. Simultaneous injections of muscimol (0.5 μg/0.5 μl) into the PFC counteracted the increases of dopamine and acetylcholine in the amygdala and hippocampus and also significantly reduced the peak increase of corticosterone in the hippocampus. Injections of muscimol (0.05 and 0.5 μg/0.5 μl) reduced the increases of motor activity produced by prefrontal NMDA antagonists.

CONCLUSIONS

These results suggest that the hypofunction of NMDA receptors in the PFC produces corticolimbic hyperactivity through the activation of prefrontal efferent projections to subcortical/limbic areas.

Cholinergic innervation of pyramidal cells and parvalbumin-immunoreactive interneurons in the rat basolateral amygdala

The basolateral nucleus of the amygdala receives an extremely dense cholinergic innervation from the basal forebrain that is critical for memory consolidation. Although previous electron microscopic studies determined some of the postsynaptic targets of cholinergic afferents, the majority of postsynaptic structures were dendritic shafts whose neurons of origin were not identified. To make this determination, the present study analyzed the cholinergic innervation of the anterior subdivision of the basolateral amygdalar nucleus (BLa) of the rat using electron microscopic dual-labeling immunocytochemistry. The vesicular acetylcholine transporter (VAChT) was used as a marker for cholinergic terminals; calcium/calmodulin-dependent protein kinase II (CaMK) was used as a marker for pyramidal cells, the principal neurons of the BLa; and parvalbumin (PV) was used as a marker for the predominant interneuronal subpopulation in this nucleus. VAChT(+) terminals were visualized by using diaminobenzidine as a chromogen, whereas CAMK(+) or PV(+) neurons were visualized with Vector very intense purple (VIP) as a chromogen. Quantitative analyses revealed that the great majority of dendritic shafts receiving cholinergic inputs were CAMK(+) , indicating that they were of pyramidal cell origin. In fact, 89% of the postsynaptic targets of cholinergic terminals in the BLa were pyramidal cells, including perikarya (3%), dendritic shafts (47%), and dendritic spines (39%). PV(+) structures, including perikarya and dendrites, constituted 7% of the postsynaptic targets of cholinergic axon terminals. The cholinergic innervation of both pyramidal cells and PV(+) interneurons may constitute an anatomical substrate for the generation of oscillatory activity involved in memory consolidation by the BLa.

Involvement of the cholinergic system in conditioning and perceptual memory

The cholinergic systems play a pivotal role in learning and memory, and have been the centre of attention when it comes to diseases containing cognitive deficits. It is therefore not surprising, that the cholinergic transmitter system has experienced detailed examination of its role in numerous behavioural situations not least with the perspective that cognition may be rescued with appropriate cholinergic 'boosters'. Here we reviewed the literature on (i) cholinergic lesions, (ii) pharmacological intervention of muscarinic or nicotinic system, or (iii) genetic deletion of selective receptor subtypes with respect to sensory discrimination and conditioning procedures. We consider visual, auditory, olfactory and somatosensory processing first before discussing more complex tasks such as startle responses, latent inhibition, negative patterning, eye blink and fear conditioning, and passive avoidance paradigms. An overarching reoccurring theme is that lesions of the cholinergic projection neurones of the basal forebrain impact negatively on acquisition learning in these paradigms and blockade of muscarinic (and to a lesser extent nicotinic) receptors in the target structures produce similar behavioural deficits. While these pertain mainly to impairments in acquisition learning, some rare cases extend to memory consolidation. Such single case observations warranted replication and more in-depth studies. Intriguingly, receptor blockade or receptor gene knockout repeatedly produced contradictory results (for example in fear conditioning) and combined studies, in which genetically altered mice are pharmacological manipulated, are so far missing. However, they are desperately needed to clarify underlying reasons for these contradictions. Consistently, stimulation of either muscarinic (mainly M(1)) or nicotinic (predominantly α7) receptors was beneficial for learning and memory formation across all paradigms supporting the notion that research into the development and mechanisms of novel and better cholinomimetics may prove useful in the treatment of neurodegenerative or psychiatric disorders with cognitive endophenotypes.

Neuronal localization of m1 muscarinic receptor immunoreactivity in the rat basolateral amygdala

Muscarinic cholinergic neurotransmission in the basolateral nuclear complex (BLC) of the amygdala is critical for memory consolidation in emotional/motivational learning tasks. Although knowledge of the localization of muscarinic receptor subtypes in the BLC would contribute to an understanding of the actions of acetylcholine in mnemonic function, previous receptor binding and in situ hybridization studies lacked the resolution necessary to identify which neurons in the BLC express different receptor subtypes. In the present study immunohistochemistry was used to study the neuronal localization of the m1 receptor. The intensity of m1 immunoreactivity varied in different nuclei of the amygdala, and was most robust in the BLC, and in the adjacent posterolateral cortical nucleus. The density and morphology of labeled neurons in the BLC suggested that the m1+ neuronal population included pyramidal cells, the principal neurons in this amygdalar region. In addition, there was dense punctate m1 immunoreactivity in the neuropil of the BLC. Dual labeling immunofluorescence studies of the BLC using antibodies to cell type specific markers were performed to more definitively determine the phenotype of m1-positive (m1+) neurons. An antibody to calcium/calmodulin protein kinase II (CaMK) was used to label pyramidal cells, whereas an antibody to glutamic acid decarboxylase was used to label interneurons. Virtually all of the intensely labeled m1+ neurons of the BLC were CaMK+ pyramidal cells. These data suggest that the ability of M1 receptor antagonists to impair memory consolidation in the BLC is mainly due to blockade of cholinergic influences on the activity of pyramidal neurons.

Parvalbumin-immunoreactive neurons and GABAergic neurons of the basal forebrain project to the rat basolateral amygdala

The basal forebrain (BF) contains a diffuse array of cholinergic and non-cholinergic neurons that project to the cerebral cortex and basolateral nuclear complex of the amygdala (BLC). Previous studies have shown that the GABAergic subpopulation of non-cholinergic corticopetal BF neurons selectively innervates cortical interneurons. Although several investigations in both rodents and primates have indicated that some BF neurons projecting to the BLC are non-cholinergic, there have been no studies that have attempted to identify the neurochemical phenotype(s) of these neurons. The present study combined Fluorogold retrograde tract tracing with immunohistochemistry for two markers of BF GABAergic neurons, parvalbumin (PV) or glutamic acid decarboxylase (GAD), to determine if a subpopulation of BF GABAergic cells projects to the BLC. Injections of Fluorogold confined to the rat BLC, and centered in the basolateral nucleus, produced extensive retrograde labeling in the ventral pallidum and substantia innominata regions of the BF. Although the great majority of retrogradely labeled neurons were not double-labeled, about 10% of these neurons, located mainly along the ventral aspects of the fundus striati and globus pallidus, exhibited immunoreactivity for PV or GAD. The results of this investigation contradict the long-held belief that there is no extra-amygdalar source of GABAergic inputs to the BLC, and indicate that the cortex-like BLC, in addition to the cortex proper, receives inhibitory inputs from the basal forebrain.

Intra-amygdala injections of CREB antisense impair inhibitory avoidance memory: role of norepinephrine and acetylcholine

Infusions of CREB antisense into the amygdala prior to training impair memory for aversive tasks, suggesting that the antisense may interfere with CRE-mediated gene transcription and protein synthesis important for the formation of new memories within the amygdala. However, the amygdala also appears to modulate memory formation in distributed brain sites, through mechanisms that include the release of norepinephrine and acetylcholine within the amygdala. Thus, CREB antisense injections may affect memory by interfering with mechanisms of modulation, rather than storage, of memory. In the present experiment, rats received bilateral intra-amygdala infusions of CREB antisense (2 nmol/1 microL) 6 h prior to inhibitory avoidance training. In vivo microdialysis samples were collected from the right amygdala before, during, and following training. CREB antisense produced amnesia tested at 48 h after training. In addition, CREB antisense infusions dampened the training-related release of norepinephrine, and to a lesser extent of acetylcholine, in the amygdala. Furthermore, intra-amygdala infusions of the beta-adrenergic receptor agonist clenbuterol administered immediately after training attenuated memory impairments induced by intra-amygdala injections of CREB antisense. These findings suggest that intra-amygdala treatment with CREB antisense may affect processes involved in modulation of memory in part through interference with norepinephrine and acetylcholine neurotransmission in the amygdala.

Fos expression following activation of the ventral pallidum in normal rats and in a model of Parkinson's Disease: implications for limbic system and basal ganglia interactions

The circuit-related consequences of activating the ventral pallidum (VP) are not well known, and lacking in particular is how these effects are altered in various neuropathological states. To help to address these paucities, this study investigated the brain regions affected by VP activation by quantifying neurons that stain for Fos-like immunoreactivity (ir). Fos-ir was assessed after intra-pallidal injections of the excitatory amino acid agonist, NMDA, or the GABA(A) antagonist, bicuculline in normal rats and in those rendered Parkinsonian-like by lesioning dopaminergic neurons with the neurotoxin, 6-OHDA. We hypothesized that activation of the VP will alter the activity state of brain regions associated with both the basal ganglia and limbic system, and that this influence would be modified in the Parkinsonian state. Blocking tonically activated GABA(A) receptors with bicuculline (50 ng/0.5 microl) elevated Fos-ir in the VP to 423% above the contralateral, vehicle-injected side. Likewise, intra-VP NMDA (0.23 microg or 0.45 microg/0.5 microl), dose-dependently increased the number of pallidal neurons expressing Fos-ir by 224 and 526%, respectively. At higher NMDA doses, the density of Fos-ir neurons was not elevated above control levels. This inverted U-shaped profile was mirrored by a VP output structure, the medial subthalamic nucleus (mSTN). The mSTN showed a 289% increase in Fos-ir neurons with intra-VP injections of 0.45 microg NMDA, and this response was halved following intra-VP injections of 0.9 microg NMDA. Of the 12 other brain regions measured, three showed VP NMDA-induced enhancements in Fos-ir: the frontal cortex, entopeduncular nucleus and substantia nigra pars reticulata, all regions associated with the basal ganglia. In a second study, we evaluated the NMDA activation profile in a rat model of Parkinson's Disease (PD) which was created by a unilateral injection of 6-OHDA into the rostral substantia nigra pars compacta. Comparisons of responses to intra-VP NMDA between the hemispheres ipsilateral and contralateral to the lesion revealed that Fos-ir cells in the pedunculopontine nucleus was reduced by 62%, whereas Fos-ir for the basolateral amygdala and STN was reduced by 32 and 42%, respectively. These findings support the concept that the VP can influence both the basal ganglia and the limbic system, and that that the nature of this influence is modified in an animal model of PD. As the VP regulates motivation and cognition, adaptations in this system may contribute to the mood and mnemonic disorders that can accompany PD.

Facilitation of cortico-amygdala synapses by nicotine: activity-dependent modulation of glutamatergic transmission

The basolateral nucleus of the amygdala (BLA) receives cholinergic innervation from the basal forebrain and nicotine, via activation of neuronal nicotinic acetylcholine receptors (nAChRs), can improve performance in amygdala-based learning tasks. We tested the hypothesis that acute and prenatal nicotine exposure modulates cortico-amygdala synaptic transmission. We found that low-dose, single-trial exposures to nicotine can elicit lasting facilitation, the extent of which is dependent on the level of stimulation of the cortical inputs to the BLA. In addition, sustained facilitation is ablated by prenatal exposure to nicotine. This study examined synaptic transmission in 238 patch-clamp recordings from BLA neurons in acute slice from mouse brain. Pharmacological studies in wild-type and nAChR subunit knock-out mice reveal that activation of presynaptic alpha 7, containing (alpha 7*) and non-alpha 7* nAChRs, facilitates glutamatergic transmission in an activity-dependent manner. Without prior stimulation, application of nicotine elicits modest and transient facilitation of glutamatergic postsynaptic currents (PSCs) in about 40% of BLA neurons. With low-frequency stimulation of cortical inputs nicotine elicits robust facilitation of transmission at about 60% of cortico-BLA synapses and synaptic strength remains elevated at about 40% of these connections for >15 min after nicotine washout. Following paired-pulse stimulation nicotine elicits long-lasting facilitation of glutamatergic transmission at about 70% of cortico-BLA connections. Nicotine reduces the threshold for activation of long-term potentiation of cortico-BLA synapses evoked by patterned stimulation. Prenatal exposure to nicotine reduced subsequent modulatory responses to acute nicotine application.

Muscarinic control of graded persistent activity in lateral amygdala neurons

The cholinergic system is crucially involved in several cognitive processes including attention, learning and memory. Muscarinic actions have profound effects on the intrinsic firing pattern of neurons. In principal neurons of the entorhinal cortex (EC), muscarinic receptors activate an intrinsic cation current that causes multiple self-sustained spiking activity, which represents a potential mechanism for transiently sustaining information about novel items. The amygdala appears to be important for experience-dependent learning by emotional arousal, and cholinergic muscarinic influences are essential for the amygdala-mediated modulation of memory. Here we show that principal neurons from the lateral nucleus of the amygdala (LA) can generate intrinsic graded persistent activity that is similar to EC layer V cells. This firing behavior is linked to muscarinic activation of a calcium-sensitive non-specific cation current and can be mimicked by stimulation of cholinergic afferents that originate from the nucleus basalis of Meynert (n. M). Moreover, we demonstrate that the projections from the n. M. are essential and sufficient for the control and modulation of graded firing activity in LA neurons. We found that activation of these cholinergic afferents (i) is required to maintain and to increase firing rates in a graded manner, and (ii) is sufficient for the graded increases of stable discharge rates even without an associated up-regulation of Ca2+. The induction of persistent activity was blocked by flufenamic acid or 2-APB and remained intact after Ca2+-store depletion with thapsigargin. The internal ability of LA neurons to generate graded persistent activity could be essential for amygdala-mediated memory operations.

Distribution of IP3-mediated calcium responses and their role in nuclear signalling in rat basolateral amygdala neurons

Metabotropic receptor activation is important for learning, memory and synaptic plasticity in the amygdala and other brain regions. Synaptic stimulation of metabotropic receptors in basolateral amygdala (BLA) projection neurons evokes a focal rise in free Ca(2+) in the dendrites that propagate as waves into the soma and nucleus. These Ca(2+) waves initiate in the proximal dendrites and show limited propagation centrifugally away from the soma. In other cell types, Ca(2+) waves have been shown to be mediated by either metabotropic glutamate receptor (mGluR) or muscarinic receptor (mAChR) activation. Here we show that mGluRs and mAChRs act cooperatively to release Ca(2+) from inositol 1,4,5-trisphosphate (IP(3))-sensitive intracellular Ca(2+) stores. Whereas action potentials (APs) alone were relatively ineffective in raising nuclear Ca(2+), their pairing with metabotropic receptor activation evoked an IP(3)-receptor-mediated Ca(2+)-induced Ca(2+) release, raising nuclear Ca(2+) into the micromolar range. Metabotropic-receptor-mediated Ca(2+)-store release was highly compartmentalized. When coupled with metabotropic receptor stimulation, large robust Ca(2+) rises and AP-induced amplification were observed in the soma, nucleus and sparsely spiny dendritic segments with metabotropic stimulation. In contrast, no significant amplification of the Ca(2+) transient was detected in spine-dense high-order dendritic segments. Ca(2+) rises evoked by photolytic uncaging of IP(3) showed the same distribution, suggesting that IP(3)-sensitive Ca(2+) stores are preferentially located in the soma and proximal dendrites. This distribution of metabotropic-mediated store release suggests that the neuromodulatory role of metabotropic receptor stimulation in BLA-dependent learning may result from enhanced nuclear signalling.

Synaptologic and fine structural features distinguishing a subset of basal forebrain cholinergic neurons embedded in the dense intrinsic fiber network of the caudal extended amygdala

Cholinergic basal forebrain neurons confined within the intrinsic connections of the extended amygdala in the caudal sublenticular region and anterior amygdaloid area (cSLR/AAA) differ from other basal forebrain cholinergic neurons in several morphological and neurochemical respects. These cSLR/AAA cholinergic neurons have been subjected to additional investigations described in this report. First, fibers traced anterogradely following injections of Phaseolus vulgaris-leucoagglutinin in the central amygdaloid nucleus were shown to contact cSLR/AAA cholinergic neurons and dendrites. Second, these neurons were shown to be contacted by numerous GABAergic boutons with symmetric synaptic specializations. Third, the numbers of synaptic densities of morphologically characterized symmetric contacts on the somata and proximal dendrites of cSLR/AAA cholinergic neurons were shown to significantly exceed those of extra-cSLR/AAA cholinergic neurons. Fourth, fine structural features distinguishing cSLR/AAA cholinergic neurons from other basal forebrain cholinergic neurons were revealed. Specifically, cSLR/AAA cholinergic neurons have less abundant cytoplasm and a less well-organized system of rough endoplasmic reticulum than their counterparts in other parts of the basal forebrain. Thus, morphologically and neurochemically distinct cSLR/AAA cholinergic neurons exhibit robust proximal inhibitory inputs, of which a significant number originate in the extended amygdala, while cholinergic neurons outside this region lack a substrate for strong proximal inhibitory input. The implications of these findings for interaction of fear, anxiety, and attention are considered.

Brain stem afferent connections of the amygdala in the rat with special references to a projection from the parabigeminal nucleus: a fluorescent retrograde tracing study

A recently revealed important function of the amygdala (Am) is that it acts as the brain's "lighthouse", which constantly monitors the environment for stimuli which signal a threat to the organism. The data from patients with extensive lesions of the striate cortex indicate that "unseen" fearful and fear-conditioned faces elicit increased Am responses. Thus, also extrageniculostriate pathways are involved. A multisynaptic pathway from the retina to the Am via the superior colliculus (SC) and the pulvinar was recently suggested. We here present data based on retrograde neuronal labeling following injection of the fluorescent tracer Fluoro-Gold in the rat Am that the parabigeminal nucleus (Pbg) emits a substantial, bilateral projection to the Am. This small cholinergic nucleus (Ch8 group) in the midbrain tegmentum is a subcortical relay visual center that is reciprocally connected with the SC. We suggest the existence of a second extrageniculostriate multisynaptic connection to Am: retina-SC-Pbg-Am, that might be very effective since all tracts listed above are bilateral. In addition, we present hodological details on other brainstem afferent connections of the Am, some of which are only recently described, and some others that still remain equivocal. Following selective injections of Fluoro-Gold in the Am, retrogradely labeled neurons were observed in parasubthalamic nucleus, peripeduncular nucleus, periaqueductal gray, dopaminergic nuclear complex (substantia nigra pars lateralis and pars compacta, paranigral, parabrachial pigmented and interfascicular nuclei, rostral and caudal linear nuclei, retrorubral area), deep mesencephalic nucleus, serotoninergic structures (dorsal, median and pontine raphe nuclei), laterodorsal and pedunculopontine tegmental nuclei (Ch6 and Ch5 groups), parabrachial nuclear complex, locus coeruleus, nucleus incertus, ventrolateral pontine tegmentum (A5 group), dorsomedial medulla (nucleus of the solitary tract, A2 group), ventrolateral medulla (A1/C1 group), and pars caudalis of the spinal trigeminal nucleus. A bilateral labeling of the upper cervical spinal cord was also observed.

Neurosteroids and cholinergic systems: implications for sleep and cognitive processes and potential role of age-related changes

RATIONALE

The neurosteroids pregnenolone sulfate (PREGS), dehydroepiandrosterone sulfate (DHEAS) and allopregnanolone (3alpha,5alpha THPROG) have been implicated as powerful modulators of memory processes and sleep states in young and aged subjects with memory impairment. As these processes depend on the integrity of cholinergic systems, a specific effect of neurosteroids on these systems may account for their effects on sleep and memory.

OBJECTIVE

To review the evidence for a specific and differential effect of neurosteroids on cholinergic systems.

METHODS

We carried out keyword searches in "Medline" to identify articles concerning (1) the effects of neurosteroids on cholinergic systems, sleep and memory processes, and (2) changes in neurosteroid concentrations during aging. Few results are available for humans. Most data concerned rodents.

RESULTS

Peripheral and central administrations of PREGS, DHEAS, and 3alpha,5alpha THPROG modulate the basal forebrain and brainstem projection cholinergic neurons but not striatal cholinergic interneurons. Local administration of neurosteroids to the basal forebrain and brainstem cholinergic neurons alters sleep and memory in rodents. There are a few conflicting reports concerning the effects of aging on neurosteroid concentrations in normal and pathological conditions.

CONCLUSIONS

The specific modulation of basal forebrain and brainstem cholinergic systems by neurosteroids may account for the effects of these compounds on sleep and memory processes. To improve our understanding of the role of neurosteroids in cholinergic systems during normal and pathological aging, we need to determine whether there is specific regionalization of neurosteroids, and we need to investigate the relationship between neurosteroid concentrations in cholinergic nuclei and age-related sleep and memory impairments.

The distribution pattern of pathology and cholinergic deficits in amygdaloid complex in Alzheimer's disease and dementia with Lewy bodies

We studied the distribution pattern of pathology and cholinergic deficits in the subnuclei of the amygdaloid complex (AC) in five patients with Alzheimer's disease (AD), eight with dementia with Lewy bodies (DLB) and five normal controls. In controls, the basal nucleus contained the highest choline acetyltransferase activity; the activity in the lateral and central nuclei and those in the cortical, medial and accessory basal nuclei were comparable. In AD, there was a significant decrease in choline acetyltransferase activity in the accessory basal and lateral nuclei, in DLB a significant decrease was observed in the accessory basal, lateral and cortical nuclei. Compared to controls the hyperphosphorylated tau-pathology burden was significantly higher in the basal, central and medial nuclei in AD and in the central, cortical, lateral and medial nuclei in DLB. The amyloid plaque burden was significantly higher in the accessory basal, basal, lateral and cortical nuclei in AD and in all nuclei in DLB. The alpha-synuclein burden was significantly higher in all nuclei in both AD and DLB. Compared to AD alpha-synuclein burden was higher in all nuclei in DLB. There were no correlations between the distribution pattern of hyperphosphorylated tau-pathology, amyloid plaques and alpha-synuclein-positive structures, and choline acetyltransferase activity, except the lateral nucleus in DLB. In conclusion we found no relationship between the pattern of cholinergic deficits and the distribution pattern of lesions in the AC of patients with AD or DLB. Cholinergic deficits were more prominent in the nuclei of basolateral (BL) group in AD, whereas the nuclei of both BL and corticomedial groups were involved in DLB, which may be due to the involvement of both basal forebrain and brainstem cholinergic nuclei in the latter.

Vesicular glutamate transporter 3 immunoreactivity is present in cholinergic basal forebrain neurons projecting to the basolateral amygdala in rat

The basal forebrain (BF) plays a role in behavioral and cortical arousal, attention, learning, and memory. It has been suggested that cholinergic BF neurons co-release glutamate, and some cholinergic BF neurons have been reported to contain vesicular glutamate transporter 3 (VGLUT3). We examined the distribution and projections of BF cholinergic neurons containing VGLUT3, by using dual-label immunofluorescence for choline acetyltransferase (ChAT) and VGLUT3, in situ hybridization, and retrograde tracing. Neurons immunoreactive (+) or containing mRNAs for both ChAT and VGLUT3 were mainly localized to the ventral pallidum and more caudal BF regions; the co-immunoreactive neurons represented 31% of cholinergic neurons in the ventral pallidum and 5-9% more caudally. Examination of cholinergic axon terminals in known target areas of BF projections indicated that the basolateral amygdaloid nucleus contained numerous terminals co-immunoreactive for ChAT and VGLUT3, whereas sampled areas of the olfactory bulb, neocortex, hippocampus, reticular thalamic nucleus, and interpeduncular nucleus were devoid of double-labeled terminals. The basolateral amygdala is innervated by cholinergic BF neurons lacking low-affinity p75 nerve growth factor receptors; many ChAT+VGLUT3+ BF neurons were immunonegative to this receptor. Twenty-five to 79% of ChAT+VGLUT3+ neurons in different BF regions were retrogradely labeled from the basolateral amygdala, up to 52% (ventral pallidum) of the retrogradely labeled ChAT+ neurons were VGLUT3+, and the largest number of amygdala-projecting ChAT+VGluT3+ neurons was found in the ventral pallidum. These findings indicate that BF cholinergic neurons containing VGLUT3 project to the basolateral amygdala and suggest that these neurons might have the capacity to release both acetylcholine and glutamate.

The evolving theory of basal forebrain functional-anatomical 'macrosystems'

The conceptual basis and continuing development of Alheid and Heimer's [Alheid, G.F., Heimer, L., 1988. New perspectives in basal forebrain organization of special relevance for neuropsychiatric disorders: the striatopallidal, amygdaloid and corticopetal components of substantia innominata. Neuroscience 27, 1-39] theory of basal forebrain organization based on the description of basal forebrain functional-anatomical 'macrosytems' is reviewed. It is posed that the macrosystem theory leads to a hypothesis that different macrosystems cooperate and compete to exert distinct influences on motor and cognitive function. Emergent corollaries include, e.g. that the organization of the outputs of different macrosystems should differ. Consistent with these considerations, extant literature and some unpublished data indicate that the input nuclei of macrosystems are not abundantly interconnected and macrosystems systems have distinct neuroanatomical relationships with basal forebrain and brainstem cholinergic and dopaminergic ascending modulatory systems. Furthermore, macrosystem outputs appear to be directed almost exclusively at the reticular formation or structures intimately associated with it. The relative merits of the theory of functional-anatomical macrosystems are discussed in relation to Swanson's model of cerebral hemisphere control of motivated behavior.

Activation of nicotinic acetylcholine receptors increases the frequency of spontaneous GABAergic IPSCs in rat basolateral amygdala neurons

The basolateral amygdala (BLA) is a critical component of the amygdaloid circuit, which is thought to be involved in fear conditioned responses. Using whole cell patch-clamp recording, we found that activation of nicotinic acetylcholine receptors (nAChRs) leads to an action potential-dependent increase in the frequency of spontaneous GABAergic currents in principal neurons in the BLA. These spontaneous GABAergic currents were abolished by a low-Ca2+/high-Mg2+ bathing solution, suggesting that they are spontaneous inhibitory postsynaptic currents (sIPSCs). Blockade of ionotropic glutamate receptors did not prevent this increased frequency of sIPSCs nor did blockade of alpha7 nAChRs. Among the nAChR agonists tested, cystisine was more effective at increasing the frequency of the sIPSCs than nicotine or 1,1-dimethyl-4-phenyl piperazinium iodide, consistent with a major contribution of beta4 nAChR subunits. The nicotinic antagonist, dihydro-beta-erythroidine, was less effective than d-tubocurarine in blocking the increased sIPSC frequency induced by ACh, suggesting that alpha4-containing nAChR subunits do not play a major role in the ACh-induced increased sIPSC frequency. Although alpha2/3/4/7 and beta2/4 nAChR subunits were found in the BLA by RT-PCR, the agonist and antagonist profiles suggest that the ACh-induced increase in sIPSC frequency involves predominantly alpha3beta4-containing nAChR subunits. Consistent with this, alpha-conotoxin-AuIB, a nAChR antagonist selective for the alpha3beta4 subunit combination, inhibited the ACh-induced increase in the frequency of sIPSCs. The observations suggest that nicotinic activation increases the frequency of sIPSCs in the BLA by acting mainly on alpha3beta4-containing nicotinic receptors on GABAergic neurons and may play an important role in the modulation of synaptic transmission in the amygdala.

Opposing Electrophysiological Actions of 5-HT on Noncholinergic and Cholinergic Neurons in the Rat Ventral Pallidum In Vitro

The ventral pallidum in rat is a basal forebrain structure that contains neurons that project in the limbic striatopallidal circuitry and magnocellular cholinergic corticopetal neurons. Because 5-hydroxytryptamine (5-HT) terminals on dorsal raphe projections form close appositions with these neurons, we made patch-clamp recordings in immature rat brain slices to determine whether they are modulated by postsynaptic 5-HT receptors. Inward currents were predominantly induced by 5-HT in noncholinergic neurons, which were distinguished from cholinergic neurons by immunohistochemical and electrophysiological criteria. The inward current induced by 5-HT was mimicked and occluded when adenylyl cyclase was stimulated with forskolin, and was almost abolished when h-currents in noncholinergic neurons were blocked with cesium. Consistent with 5-HT7 receptor activation of h-curents by cAMP in other brain regions, we found inward currents were mimicked by the mixed 5-HT1/5-HT7 agonists 5-methoxytryptamine, and by 5-carboxamidotryptamine (5-CT), which was more potent than 5-HT. In contrast, 5-HT1 preferring 8-OH-DPAT was a weak partial agonist, and the 5-HT1–selective antagonist pindolol had no effect. However, despite this profile, antagonists that bind at the 5-HT7 receptor only partly reduced the agonist inward current (SB-269970 and clozapine), or had no effect (mianserin and pimozide). We found in cholinergic neurons that 5-HT predominantly induced hyperpolarizing currents, which were carried by potassium channels, and were smaller than currents induced by 8-OH-DPAT and 5-CT. We conclude from this study that ascending 5-HT projections from the dorsal raphe could have direct and opposite effects on the activities of neurons within the limbic striatopallidal and cholinergic corticopetal circuitry in the ventral pallidum.

Blockade of ventral pallidal opioid receptors induces a conditioned place aversion and attenuates acquisition of cocaine place preference in the rat

Peripheral administration of naloxone is known to produce a conditioned place aversion and to block cocaine-induced conditioned place preference. The ventral pallidum receives a dense enkephalinergic projection from the nucleus accumbens and is implicated as a locus mediating the rewarding and reinforcing effects of psychostimulant and opiate drugs. We sought to provide evidence for the involvement of pallidal opioid receptors in modulating affective state using the place-conditioning paradigm. Microinjection of naloxone (0.01-10 microg) into the ventral pallidum once a day for 3 days dose-dependently produced a conditioned place aversion when tested in the drug-free state 24 h after the last naloxone injection. This effect was reproduced using the mu-opioid receptor selective agonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP, 1 microg). Locomotor activity was reduced following injection of the highest dose of naloxone (10 microg) but elevated following CTOP (1 microg). Daily injection of cocaine (10 mg/kg) for 3 days produced a conditioned place preference 24 h later. This effect of cocaine was attenuated by concomitant intra-ventral pallidal injection of naloxone at a dose (0.01 microg) that had no significant aversive property when injected alone. In contrast, the locomotor activation induced by peripheral cocaine injection was unaffected by naloxone injection into the ventral pallidum. The data implicate endogenous opioid peptide systems within the ventral pallidum as regulators of hedonic status.

Discovery of CEP-1347/KT-7515, an inhibitor of the JNK/SAPK pathway for the treatment of neurodegenerative diseases

Apoptosis has been proposed as a mechanism of cell death in Alzheimer's, Huntington's and Parkinson's diseases and the occurrence of apoptosis in these disorders suggests a common mechanism. Events such as oxidative stress, calcium toxicity, mitochondria defects, excitatory toxicity, and deficiency of survival factors are all postulated to play varying roles in the pathogenesis of the diseases. However, the transcription factor c-jun may play a role in the pathology and cell death processes that occur in Alzheimer's disease. Parkinson's disease (PD) is also a progressive disorder involving the specific degeneration and death of dopamine neurons in the nigrostriatal pathway. In Parkinson's disease, dopaminergic neurons in the substantia nigra are hypothesized to undergo cell death by apoptotic processes. The commonality of biochemical events and pathways leading to cell death in these diseases continues to be an area under intense investigation. The current therapy for PD and AD remains targeting replacement of lost transmitter, but the ultimate objective in neurodegenerative therapy is the functional restoration and/or cessation of progression of neuronal loss. This chapter will describe a novel approach for the treatment of neurodegenerative diseases through the development of kinase inhibitors that block the active cell death process at an early transcriptional independent step in the stress activated kinase cascade. In particular, preclinical data will be presented on the c-Jun Amino Kinase pathway inhibitor, CEP-1347/KT-7515, with respect to it's properties that make it a desirable clinical candidate for treatment of various neurodegenerative diseases.

Functional recovery of cholinergic basal forebrain neurons under disease conditions: old problems, new solutions?

Recognition of the involvement of cholinergic neurons in the modulation of cognitive functions and their severe dysfunction in neurodegenerative disorders, such as Alzheimer's disease, initiated immense research efforts aimed at unveiling the anatomical organization and cellular characteristics of the basal forebrain (BFB) cholinergic system. Concomitant with our unfolding knowledge about the structural and functional complexity of the BFB cholinergic projection system, multiple pharmacological strategies were introduced to rescue cholinergic nerve cells from noxious attacks; however, a therapeutic breakthrough is still awaited. In this review, we collected recent findings that significantly contributed to our better understanding of cholinergic functions under disease conditions, and to the design of effective means to restore lost or damaged cholinergic functions. To this end, we first provide a brief survey of the neuroanatomical organization of BFB nuclei with emphasis on major evolutionary differences among mammalian species, in particular rodents and primates, and discuss limitations of the translation of experimental data to human therapeutic applications. Subsequently, we summarize the involvement of cholinergic dysfunction in the pathogenesis of severe neurological conditions, including stroke, traumatic brain injury, virus encephalitis and Alzheimer's disease, and emphasize the critical role of pro-inflammatory cytokines as common mediators of cholinergic neuronal damage. Moreover, we review leading functional concepts on the limited recovery of cholinergic neurons and their impaired plastic re-modeling, as well as on the hampered interplay of the ascending cholinergic and monoaminergic projection systems under neurodegenerative conditions. In addition, recent advances in the dynamic labeling of living cholinergic neurons by fluorochromated antibodies, referred to as in vivo labeling, and novel neuroimaging approaches as potential diagnostic tools of progressive cholinergic decline are surveyed. Finally, the potential of cell replacement strategies using embryonic and adult stem cells, and multipotent neural progenitors, as a means to recover damaged cholinergic functions, is discussed.

Activation of histaminergic H3 receptors in the rat basolateral amygdala improves expression of fear memory and enhances acetylcholine release

The basolateral amygdala (BLA) is involved in learning that certain environmental cues predict threatening events. Several studies have shown that manipulation of neurotransmission within the BLA affects the expression of memory after fear conditioning. We previously demonstrated that blockade of histaminergic H3 receptors decreased spontaneous release of acetylcholine (ACh) from the BLA of freely moving rats, and impaired retention of fear memory. In the present study, we examined the effect of activating H3 receptors within the BLA on both ACh release and expression of fear memory. Using the microdialysis technique in freely moving rats, we found that the histaminergic H3 agonists R-alpha-methylhistamine (RAMH) and immepip, directly administered into the BLA, augmented spontaneous release of ACh in a similar manner. Levels of ACh returned to baseline on perfusion with control medium. Rats receiving intra-BLA, bilateral injections of the H3 agonists at doses similar to those enhancing ACh spontaneous release, immediately after contextual fear conditioning, showed stronger memory for the context-footshock association, as demonstrated by longer freezing assessed at retention testing performed 72 h later. Post-training, bilateral injections of 15 ng oxotremorine also had a similar effect on memory retention, supporting the involvement of the cholinergic system. Thus, our results further support a physiological role for synaptically released histamine, that in addition to affecting cholinergic transmission in the amygdala, modulates consolidation of fear memories

Direct catecholaminergic-cholinergic interactions in the basal forebrain. III. Adrenergic innervation of choline acetyltransferase-containing neurons in the rat

The central adrenergic neurons have been suggested to play a role in the regulation of arousal and in the neuronal control of the cardiovascular system. To provide morphological evidence that these functions could be mediated via the basal forebrain, we performed correlated light and electron microscopic double-immunolabeling experiments using antibodies against phenylethanolamine N-methyltransferase (PNMT) and choline acetyltransferase, the synthesizing enzymes for adrenaline and acetylcholine, respectively. Most adrenergic/cholinergic appositions were located in the horizontal limb of diagonal band of Broca, within the substantia innominata, and in a narrow band bordering the substantia innominata and the globus pallidus. Quantitative analysis indicated that cholinergic neurons of the substantia innominata receive significantly higher numbers of adrenergic appositions than cholinergic cells in the rest of the basal forebrain. In the majority of cases, the ultrastructural analysis revealed axodendritic asymmetric synapses. By comparing the number and distribution of dopamine beta-hydroxylase (DBH)/cholinergic appositions, described earlier, with those of PNMT/cholinergic interactions in the basal forebrain, it can be concluded that a significant proportion of putative DBH/cholinergic contacts may represent adrenergic input. Our results support the hypothesis that the adrenergic/cholinergic link in the basal forebrain may represent a critical component of a central network coordinating autonomic regulation with cortical activation.

GABAergic innervation of alpha type II calcium/calmodulin-dependent protein kinase immunoreactive pyramidal neurons in the rat basolateral amygdala

Although calcium/calmodulin-dependent protein kinase II (CaMK) has been shown to play a critical role in long-term potentiation (LTP) and emotional learning mediated by the basolateral amygdala, little is known about its cellular localization in this region. We have utilized immunohistochemical methods to study the neuronal localization of CaMK, and its relationship to gamma-aminobutyric acid (GABA)-ergic structures, in the rat basolateral amygdala (ABL). Light microscopic observations revealed dense CaMK staining in the ABL. Although the cell bodies and proximal dendrites of virtually every pyramidal cell appeared to be CaMK(+), the cell bodies of small nonpyramidal neurons were always unstained. Dual localization of CaMK and GABA immunoreactivity with confocal immunofluorescence microscopy revealed that CaMK and GABA were found in different neuronal populations in the ABL. CaMK was contained only in pyramidal neurons; GABA was contained only in nonpyramidal cells. At the ultrastructural level, it was found that CaMK was localized to pyramidal cell bodies, thick proximal dendrites, thin distal dendrites, most dendritic spines, axon initial segments, and axon terminals forming asymmetrical synapses. These findings suggest that all portions of labeled pyramidal cells, with the exception of some dendritic spines, can exhibit CaMK immunoreactivity. By using a dual immunoperoxidase/immunogold-silver procedure at the ultrastructural level, GABA(+) axon terminals were seen to innervate all CaMK(+) postsynaptic domains, including cell bodies (22%), thick (>1 microm) dendrites (34%), thin (<1 microm) dendrites (22%), dendritic spines (17%), and axon initial segments (5%). These findings indicate that CaMK is a useful marker for pyramidal neurons in ultrastructural studies of ABL synaptology and that the activity of pyramidal neurons in the ABL is tightly controlled by a high density of GABAergic terminals that target all postsynaptic domains of pyramidal neurons.

Multiple parallel memory systems in the brain of the rat

A theory of multiple parallel memory systems in the brain of the rat is described. Each system consists of a series of interconnected neural structures. The "central structures" of the three systems described are the hippocampus, the matrix compartment of the dorsal striatum (caudate-putamen), and the amygdala. Information, coded as neural signals, flows independently through each system. All systems have access to the same information from situations in which learning occurs, but each system is specialized to represent a different kind of relationship among the elements (stimulus events, responses, reinforcers) of the information that flows through it. The speed and accuracy with which a system forms a coherent representation of a learning situation depend on the correspondence between the specialization of the system and the relationship among the elements of the situation. The coherence of these stored representations determines the degree of control exerted by each system on behavior in the situation. Although they process information independently the systems interact in at least two ways: by simultaneous parallel influence on behavioral output and by directly influencing each other. These interactions can be cooperative (leading to similar behaviors) or competitive (leading to different behaviors). Experimental findings consistent with these ideas, mostly from experiments with rats, are reviewed.

The pallial amygdala of amniote vertebrates: evolution of the concept, evolution of the structure

Embryological studies indicate that the amygdala includes pallial structures, namely the cortical amygdala (olfactory and vomeronasal) and the basolateral complex deep to it. In squamate reptiles, the cortical amygdala includes secondary olfactory (the ventral anterior amygdala) and vomeronasal centres (the nucleus sphericus). In birds, the situation is far less clear, due to the relative underdevelopment of the chemosensory systems. The basolateral amygdala of squamate reptiles includes two ventropallial structures: the posterior dorsal ventricular ridge and the lateral amygdala. Like their mammalian counterparts, these centres give rise to glutamatergic projections to the striatal (centromedial) amygdala and the ventromedial hypothalamus. Using the same criteria, the caudal neostriatum and the ventral intermediate archistriatum may represent the ventral pallial amygdala of birds. The basal nucleus of the mammalian amygdala is a lateropallial territory. In reptiles, the lateral pallium includes the dorsolateral amygdala, which, like the mammalian basal nucleus, projects bilaterally to the striatum/accumbens and receives distinct cholinergic and dopaminergic innervations. In the avian brain, the same embryological, hodological, and histochemical criteria are met by the area temporo-parieto-occipitalis, the caudolateral neostriatum and the dorsal intermediate archistriatum. Therefore, the projections from these structures to the paleostriatum and the lobus paraolfactorius are amygdalostriatal, rather than corticostriatal connections.

Understanding the basic circuitry of the cerebral hemispheres: the case of lizards and its implications in the evolution of the telencephalon

The organization of the cerebral hemispheres of mammals is characterized by corticostriatal glutamatergic projections and striatopallidal GABAergic ones, plus the descending projections of the pallium and subpallium to extratelencephalic targets. The present review of the available neuroanatomical data on the forebrain of lizards suggests that the telencephalon of reptiles also follows this basic pattern of connectivity. In addition, we show that this basic circuitry includes a pallido-cortical projection, therefore forming a cortico-striato-pallido-cortical circuit. The analysis of this circuitry for the medial, dorsal, lateral, and ventral pallial divisions in reptiles and mammals leads to the following conclusions: (1) The medial and dorsal cortices of lizards together appear to be equivalent to the medial pallium of mammals. (2) The projection from the lacertilian dorsal cortex to the striatum proper resembles the subiculo-striatal projection of mammals, rather than the isocortical projection to the caudatus-putamen. (3) Most of the dorsal striatum of reptiles is engaged in the corticostriatal circuit corresponding to the ventral pallium (the anterior dorsal ventricular ridge), and therefore, it is not equivalent to the mammalian caudatus-putamen, which is involved in the circuit of the dorsal pallium. (4) The main and accessory olfactory bulbs also follow this pattern of connections.

Single-cell RT-PCR detects shifts in mRNA expression profiles of basal forebrain neurons during aging

The medial septum and nucleus of the diagonal band (MS/nDB) contain cholinergic and GABAergic neuronal populations that have been identified based on immunohistochemical staining and/or electrophysiological properties. We explored the molecular diversity of MS/nDB neurons using single-cell reverse transcription-polymerase chain reaction (scRT-PCR) to assess gene expression profiles during aging in individual neurons acutely isolated from young (2-4 months) and aged (26-27 months) F344 rats. Neuronal gene expression profiles were characterized by detection of mRNAs for choline acetyltransferase (ChAT, cholinergic) and glutamate decarboxylase (GAD67, GABAergic), as well as mRNAs for calcium binding proteins (CaBPs) calbindin-D28k, calretinin and parvalbumin. Four major neuronal populations were identified: ChAT-positive (ChAT+) cells, GAD-positive (GAD+) cells, ChAT+/GAD+ cells and ChAT negative/GAD negative (ChAT-/GAD-) cells. With age, the percentage of cells expressing ChAT mRNA decreased from 53% in young to 40%, and the expression of GAD67 mRNA was reduced from 56 to 35% of the cells tested. The percentage of cells with detectable levels of both ChAT and GAD67 mRNA was reduced from 24% in young to 9% in aged. Concomitantly, the percentage of ChAT-/GAD- cells increased from 15 to 34% with age. Of the CaBPs, calretinin expression was observed most frequently in this study, and its detection decreased from 33 to 22% of the cells with age. Observations concerning the CaBPs were confirmed using in situ hybridization. These results suggest a shift in the mRNA expression profiles of MS/nDB neuronal populations during aging and exemplify the molecular diversity of cholinergic and GABAergic cells.

Histamine H3 receptor-mediated impairment of contextual fear conditioning and in-vivo inhibition of cholinergic transmission in the rat basolateral amygdala

We investigated the effects of agents acting at histamine receptors on both, spontaneous release of ACh from the basolateral amygdala (BLA) of freely moving rats, and fear conditioning. Extensive evidence suggests that the effects of histamine on cognition might be explained by the modulation of cholinergic systems. Using the microdialysis technique in freely moving rats, we demonstrated that perfusion of the BLA with histaminergic compounds modulates the spontaneous release of ACh. The addition of 100 mm KCl to the perfusion medium strongly stimulated ACh release, whereas, 0.5 microm tetrodotoxin (TTX) inhibited spontaneous ACh release by more than 50%. Histaminergic H3 antagonists (ciproxifan, clobenpropit and thioperamide), directly administered to the BLA, decreased ACh spontaneous release, an effect fully antagonized by the simultaneous perfusion of the BLA with cimetidine, an H2 antagonist. Local administration of cimetidine alone increased ACh spontaneous release slightly, but significantly. Conversely, the administration of H1 antagonists failed to alter ACh spontaneous release. Rats receiving intra-BLA, bilateral injections of the H3 antagonists at doses similar to those inhibiting ACh spontaneous release, immediately after contextual fear conditioning, showed memory consolidation impairment of contextual fear conditioning. Post-training, bilateral injections of 50 microg scopolamine also had an adverse effect on memory retention. These observations provide the first evidence that histamine receptors are involved in the modulation of cholinergic tone in the amygdala and in the consolidation of fear conditioning.

Light and electron microscopic study of cholinergic and noradrenergic elements in the basolateral nucleus of the rat amygdala: evidence for interactions between the two systems

Pharmacological studies have suggested that the cholinergic (ACh) and noradrenergic (NA) systems in the amygdala (AM) play an important role in learning and memory storage and that the two systems interact to modulate memory storage. To obtain anatomical evidence for the interaction, the organization of the ACh and NA fibers in rat AM was investigated by immunocytochemistry for choline acetyltransferase (ChAT) and dopamine-beta-hydroxylase (DBH) in conjunction with light, confocal laser scanning, and electron microscopy (LM, CLSM, and TEM, respectively). LM showed that the ChAT immunoreactivity was densest in the basolateral nucleus (BL), whereas the DBH immunoreactivity was densest in the posterior BL. CLSM demonstrated that the ChAT-immunoreactive profiles in the BL were frequently located in juxtaposition to the DBH-immunoreactive axons. The TEM observations were as follows: The majority of the synapses formed by ChAT-immunoreactive terminals were symmetric, but DBH-immunoreactive axons formed both asymmetric and symmetric synapses. The ChAT-immunoreactive terminals usually established the symmetric synaptic contacts with the DBH-immunoreactive terminals and varicosities. The DBH-immunoreactive terminals formed the asymmetric synapses with the ChAT-immunoreactive dendrites of the intrinsic neurons within the AM. The results provide anatomical substrates for mnemonic functions of the ACh and NA systems and for the interactions between the two systems in the AM.

Striatal and central extended amygdala parts of the interstitial nucleus of the posterior limb of the anterior commissure: evidence from tract-tracing techniques in the rat

The interstitial nucleus of the posterior limb of the anterior commissure (IPAC) lies at the junction of the striatopallidal system and the lateral bed nucleus of the stria terminalis-central amygdaloid nucleus continuum (i.e., the central extended amygdala; EAc). Its efferent connections were investigated in the rat with anterograde (Phaseolus vulgaris leucoagglutinin) and retrograde (Fluoro-Gold and cholera toxin B subunit) tracers and compared with those of the central amygdaloid nucleus. Our anterograde tracing experiments reveal that the projections of the medial IPAC largely reciprocate its afferent connections (Shammah-Lagnado et al. [1999] Neuroscience 94:1097-1123) and are very similar to those of the medial part of the central amygdaloid nucleus. The lateral IPAC, on the other hand, innervates the pallidal complex, substantia nigra and retrorubral field. Local connections are found within medial IPAC and within lateral IPAC, but the two divisions are not interconnected. Our retrograde tracing experiments confirm that IPAC projections to EAc components, parabrachial area, and nucleus of the solitary tract originate chiefly from the medial division, whereas both medial and lateral divisions innervate the retrorubral field. Moreover, in sections processed for choline acetyltransferase, the strong projections from caudal IPACm to the posterior basolateral amygdaloid nucleus and the amygdalopiriform transition area were found to arise chiefly from cholinergic cells. Overall, our results suggest that the medial IPAC is intimately related to the EAc, whereas the lateral IPAC represents a striatal territory.

Projections from the nociceptive area of the central nucleus of the amygdala to the forebrain: a PHA-L study in the rat

The lateral capsular division (CeLC) of the central nucleus (Ce) of the amygdala, in the rat, has been shown to be the main terminal area of a spino(trigemino)-parabrachio-amygdaloid nociceptive pathway [Bernard & Besson (1990) J. Neurophysiol. 63, 473-490; Bernard et al. (1992) J. Neurophysiol. 68, 551-569; Bernard et al. (1993) J. Comp. Neurol. 329, 201-229]. The projections to the forebrain from the CeLC and adjacent regions were studied in the rat by using microinjections of Phaseolus vulgaris leucoagglutinin (PHA-L) restricted in subdivisions of the Ce and the basolateral amygdaloid nucleus anterior (BLA). Our data showed that the entire CeLC projects primarily and extensively to the substantia innominata dorsalis (SId). The terminal labelling is especially dense in the caudal aspect of the SId. The other projections of the CeLC in the forebrain were dramatically less dense. They terminate in the bed nucleus of the stria terminalis (BST) and the posterior hypothalamus (pLH). No (or only scarce) other projections were found in the remaining forebrain areas. The Ce lateral division (CeL) and the Ce medial division (CeM), adjacent to the CeLC, also project to the SId with slightly lower density labelling. However, contrary to the case of the CeLC, both the CeL and the CeM extensively project to the ventrolateral subnucleus of the BST (BSTvl) with a few additional terminals found in other regions of the lateral BST. Only the CeM projects densely to both the interstitial nucleus of the posterior limb of the anterior commissure and the caudal most portion of the pLH. The projections of the BLA are totally different from those of the Ce as they terminate in the dorsal striatum, the accumbens nucleus, the olfactory tubercle, the nucleus of olfactory tract and the rostral pole of the cingulate/frontal cortex. This study demonstrates that the major output of the nociceptive spino(trigemino)-parabrachio-CeLC pathway is to the SId. It is suggested that the CeLC-SId pathway could have an important role in anxiety, aversion and genesis of fear in response to noxious stimuli.

Neural interaction between the basal forebrain and functionally distinct prefrontal cortices in the rhesus monkey

The prefrontal cortex in rhesus monkeys is a heterogeneous region by structure, connections and function. Caudal medial and orbitofrontal cortices receive input from cortical and subcortical structures associated with emotions, autonomic function and long-term memory, while lateral prefrontal cortices are linked with structures associated with working memory. With the aid of neural tracers we investigated whether functionally distinct orbitofrontal, medial and lateral prefrontal cortices have specific or common connections with an ascending modulatory system, the basal forebrain. Ascending projections originated in the diagonal band and the basalis nuclei of the basal forebrain in regions demarcated by choline acetyltransferase. Although the origin of projections from the basal forebrain to lateral, medial and orbitofrontal cortices partially overlapped, projections showed a general topography. The posterior part of the nucleus basalis projected preferentially to lateral prefrontal areas while its rostrally adjacent sectors projected to medial and orbitofrontal cortices. The diagonal band nuclei projected to orbitofrontal and medial prefrontal areas. Cortical and subcortical structures that are interconnected appear to have a similar pattern of connections with the basal forebrain. In comparison to the ascending projections, the descending projections were specific, originating mostly in the posterior (limbic) component of medial and orbitofrontal cortices and terminating in the diagonal band nuclei and in the anterior part of the nucleus basalis. In addition, prefrontal limbic areas projected to two other systems of the basal forebrain, the ventral pallidum and the extended amygdala, delineated with the striatal-related markers dopamine, adenosine 3':5'-monophosphate regulated phosphoprotein of M(r) 32kDa, and the related phosphoprotein Inhibitor-1. These basal forebrain systems project to autonomic nuclei in the hypothalamus and brainstem. We interpret these results to indicate that lateral prefrontal areas, which have a role in working memory, receive input from, but do not issue feedback projections to the basal forebrain. In contrast, orbitofrontal and medial prefrontal areas, which have a role in emotions and long-term memory, have robust bidirectional connections with the basal forebrain. Moreover, orbitofrontal and medial prefrontal cortices target the ventral pallidum and the extended amygdala, through which high-order association areas may activate motor autonomic structures for the expression of emotions.

Effects of olfactory bulbectomy on NMDA receptor density in the rat brain:

Olfactory bulbectomy (OBX) transects the glutamatergic efferents from the olfactory bulbs, and the changes of glutamatergic N-methyl-D-aspartate (NMDA) receptor-mediated function are though to be involved in the behavioral deficits seen in OBX rats. In the present study, irritability scores in OBX male Wistar rats were correlated with discrete regional effects on NMDA receptor function measured using a [3H] MK-801 binding assay. Irritability scores, measured before and for 2 weeks after OBX, showed a gradual increase in irritability after OBX. A reduction of the NMDA receptor density was observed in the cerebral cortex and amygdala 16 days after OBX, but not in the striatum, olfactory tubercle, entorhinal cortex, and hippocampus. These results demonstrate that OBX causes changes in the NMDA receptor system in certain brain regions and suggest that these changes may be responsible for the behavioral deficits of OBX rats.

Cognitive effects of neurotoxic lesions of the nucleus basalis magnocellularis in rats: differential roles for corticopetal versus amygdalopetal projections

The cholinergic hypothesis states that cholinergic neurons of the basal forebrain nucleus basalis magnocellularis (nbm) that project to cortical and amygdalar targets play an important role in memory. Biochemical studies have shown that these target areas are differentially sensitive to different excitotoxins (e.g., ibotenate vs. quisqualate). This observation might explain the finding from many behavioural studies of memory that different excitotoxins affect memory differentially even though they produce about the same level of depletion of cholinergic markers in the cortex and similar cortical electrophysiological effects. Thus, the magnitude of mnemonic impairment might be related to the extent of damage to cholinergic projections to the amygdala more than to the extent of damage to corticopetal cholinergic projections. This explanation might similarly apply to the observation that the immunotoxin 192 IgG-saporin produces mild effects on memory when injected into the nbm. This is because it damages cholinergic neurons projecting to the cortex but not those projecting to the amygdala. Studies comparing the effects on memory of ibotenic acid vs. quisqualic acid lesions of the nbm are reviewed as are studies of the mnemonic effects of 192 IgG-saporin. Results support the cholinergic hypothesis and suggest that amygdalopetal cholinergic neurons of the nbm play an important role in the control of memory.

Multiple output pathways of the basal forebrain: organization, chemical heterogeneity, and roles in vigilance

Studies over the last decade have shown that the basal forebrain (BF) consists of more than its cholinergic neurons. The BF also contains non-cholinergic neurons, including gamma-aminobutyric acid-ergic neurons which co-distribute and co-project with the cholinergic neurons. Both types of neuron project, in variable proportions, to the cerebral cortex, hippocampus, thalamus, amygdala, and olfactory bulb, whereas descending projections to the posterior hypothalamus and brainstem nuclei are predominantly non-cholinergic. Some of the cholinergic and non-cholinergic projection neurons contain neuropeptides such as galanin, nitric oxide synthase, and possibly glutamate. To understand better the function of the BF, the organization of the multiple ascending and descending projections of BF neurons is reviewed along with their neurochemical heterogeneity, and possible functions of individual pathways are discussed. It is proposed that BF neurons belong to multiple systems with distinct cognitive, motivational, emotional, motor, and regulatory functions, and that through these pathways, the BF plays a role in controlling both cognitive and non-cognitive aspects of vigilance.

Muscarinic agonist carbachol depresses excitatory synaptic transmission in the rat basolateral amygdala in vitro

Intracellular recordings in slice preparations of the basolateral amygdala were used to test which excitatory amino acid receptors mediate the excitatory postsynaptic potentials due to stimulation of the external capsule. These recordings were also used to examine the action of muscarinic agonists on the evoked excitatory potentials. Intracellular recordings from amygdaloid pyramidal neurons revealed that carbachol (2-20 microM) suppressed, in a dose-dependent manner, excitatory postsynaptic responses evoked by stimulation of the external capsule (EC). This effect was blocked by atropine. The estimated effective concentration to produce half-maximal response (EC(50)) was 6.2 microM. Synaptic suppression was observed with no changes in the input resistance of the recorded cells, suggesting a presynaptic mechanism. In addition, the results obtained using the paired-pulse protocol provided additional support for a presynaptic action of carbachol. To identify which subtype of cholinergic receptors were involved in the suppression of the EPSP, four partially selective muscarinic receptor antagonists were used at different concentrations: pirenzepine, a compound with a similar high affinity for muscarinic M1 and M4 receptors; gallamine, a noncompetitive antagonist for M2; methoctramine, an antagonist for M2 and M4; and 4-diphenylacetoxy-N-methylpiperidine, a compound with similar high affinity for muscarinic receptors M1 and M3. None of them independently antagonized the suppressive effect of carbachol on the evoked EPSP completely, suggesting that more than one muscarinic receptor subtype is involved in the effect. These experiments provide evidence that in the amygdala muscarinic agonists block the excitatory synaptic response, mediated by glutamic acid, by acting on several types of presynaptic receptors.

Electrophysiological properties of cholinergic and noncholinergic neurons in the ventral pallidal region of the nucleus basalis in rat brain slices

The ventral pallidum is a major source of output for ventral corticobasal ganglia circuits that function in translating motivationally relevant stimuli into adaptive behavioral responses. In this study, whole cell patch-clamp recordings were made from ventral pallidal neurons in brain slices from 6- to 18-day-old rats. Intracellular filling with biocytin was used to correlate the electrophysiological and morphological properties of cholinergic and noncholinergic neurons identified by choline acetyltransferase immunohistochemistry. Most cholinergic neurons had a large whole cell conductance and exhibited marked fast (i.e., anomalous) inward rectification. These cells typically did not fire spontaneously, had a hyperpolarized resting membrane potential, and also exhibited a prominent spike afterhyperpolarization (AHP) and strong spike accommodation. Noncholinergic neurons had a smaller whole cell conductance, and the majority of these cells exhibited marked time-dependent inward rectification that was due to an h-current. This current activated slowly over several hundred milliseconds at potentials more negative than -80 mV. Noncholinergic neurons fired tonically in regular or intermittent patterns, and two-thirds of the cells fired spontaneously. Depolarizing current injection in current clamp did not cause spike accommodation but markedly increased the firing frequency and in some cells also altered the pattern of firing. Spontaneous tetrodotoxin-sensitive GABA(A)-mediated inhibitory postsynaptic currents (IPSCs) were frequently recorded in noncholinergic neurons. These results show that cholinergic pallidal neurons have similar properties to magnocellular cholinergic neurons in other parts of the forebrain, except that they exhibit strong spike accommodation. Noncholinergic ventral pallidal neurons have large h-currents that could have a physiological role in determining the rate or pattern of firing of these cells.

Bombesin microinjection into the basolateral amygdala influences feeding behavior in the rat

It has been demonstrated that the basolateral amygdala (ABL) represents a satiety mechanism. Experimental data indicate that peripheral or central applications of neuropeptide bombesin (BN) and BN-like peptides inhibit feeding. Since the amygdala (AMY) is rich in BN-like immunoreactive elements, the present study was performed to determine whether 10 or 40 ng doses of BN microinjected bilaterally into the ABL could modify solid food intake. Twenty nanograms of BN (10 ng per injection site) in 24-h deprived rats caused transient inhibition of food intake and 80 ng resulted in a significant reduction of food consumption for 1 h. This inhibitory effect of BN on feeding was eliminated by prior BN antagonist treatment. Results of behavioral tests showed that BN microinjections into the ABL specifically reduced food intake without altering behavioral patterns or influencing the body temperature. Present results suggest that BN-like peptides may act as a complex satiety signal in the ABL.