Neural associations of the substantia innominata in the rat: afferent connections

Age-related changes in basal forebrain afferent activation in response to food paired stimuli

The basal forebrain contains a phenotypically-diverse assembly of neurons, including those using acetylcholine as their neurotransmitter. This basal forebrain cholinergic system projects to the entire neocortical mantle as well as subcortical limbic structures that include the hippocampus and amygdala. Basal forebrain pathology, including cholinergic dysfunction, is thought to underlie the cognitive impairments associated with several age-related neurodegenerative conditions, including Alzheimer's disease. Basal forebrain dysfunction may stem, in part, from a failure of normal afferent regulation of cholinergic and other neurons in this area. However, little is understood regarding how aging, alone, affects the functional regulation of basal forebrain afferents in the context of motivated behavior. Here, we used neuronal tract-tracing combined with motivationally salient stimuli in an aged rodent model to examine how aging alters activity in basal forebrain inputs arising from several cortical, limbic and brainstem structures. Young rats showed greater stimulus-associated activation of basal forebrain inputs arising from prelimbic cortex, nucleus accumbens and the ventral tegmental area compared with aged rats. Aged rats also showed increased latency to respond to palatable food presentation compared to young animals. Changes in activation of intrinsic basal forebrain cell populations or afferents were also observed as a function of age or experimental condition. These data further demonstrate that age-related changes in basal forebrain activation and related behavioral and cognitive functions reflect a failure of afferent regulation of this important brain region.

Target-specific projections of amygdala somatostatin-expressing neurons to the hypothalamus and brainstem

Somatostatin neurons in the central nucleus of the amygdala (CeA/Sst) can be parsed into subpopulations that project either to the nucleus of the solitary tract (NST) or parabrachial nucleus (PBN). We have shown recently that inhibition of CeA/Sst-to-NST neurons increased the ingestion of a normally aversive taste stimulus, quinine HCl (QHCl). Because the CeA innervates other forebrain areas such as the lateral hypothalamus (LH) that also sends axonal projections to the NST, the effects on QHCl intake could be, in part, the result of CeA modulation of LH-to-NST neurons. To address these issues, the present study investigated whether CeA/Sst-to-NST neurons are distinct from CeA/Sst-to-LH neurons. For comparison purposes, additional experiments assessed divergent innervation of the LH by CeA/Sst-to-PBN neurons. In Sst-cre mice, two different retrograde transported flox viruses were injected into the NST and the ipsilateral LH or PBN and ipsilateral LH. The results showed that 90% or more of retrograde-labeled CeA/Sst neurons project either to the LH, NST, or PBN. Separate populations of CeA/Sst neurons projecting to these different regions suggest a highly heterogeneous population in terms of synaptic target and likely function.

Efferent projections of CGRP/Calca-expressing parabrachial neurons in mice

The parabrachial nucleus (PB) is composed of glutamatergic neurons at the midbrain-hindbrain junction. These neurons form many subpopulations, one of which expresses Calca, which encodes the neuropeptide calcitonin gene-related peptide (CGRP). This Calca-expressing subpopulation has been implicated in a variety of homeostatic functions, but the overall distribution of Calca-expressing neurons in this region remains unclear. Also, while previous studies in rats and mice have identified output projections from CGRP-immunoreactive or Calca-expressing neurons, we lack a comprehensive understanding of their efferent projections. We began by identifying neurons with Calca mRNA and CGRP immunoreactivity in and around the PB, including populations in the locus coeruleus and motor trigeminal nucleus. Calca-expressing neurons in the PB prominently express the mu opioid receptor (Oprm1) and are distinct from neighboring neurons that express Foxp2 and Pdyn. Next, we used Cre-dependent anterograde tracing with synaptophysin-mCherry to map the efferent projections of these neurons. Calca-expressing PB neurons heavily target subregions of the amygdala, bed nucleus of the stria terminalis, basal forebrain, thalamic intralaminar and ventral posterior parvicellular nuclei, and hindbrain, in different patterns depending on the injection site location within the PB region. Retrograde axonal tracing revealed that the previously unreported hindbrain projections arise from a rostral-ventral subset of CGRP/Calca neurons. Finally, we show that these efferent projections of Calca-expressing neurons are distinct from those of neighboring PB neurons that express Pdyn. This information provides a detailed neuroanatomical framework for interpreting experimental work involving CGRP/Calca-expressing neurons and opioid action in the PB region.

Anatomical and neurochemical organization of the serotonergic system in the mammalian brain and in particular the involvement of the dorsal raphe nucleus in relation to neurological diseases

The brainstem is a neglected brain area in neurodegenerative diseases, including Alzheimer's and Parkinson's disease, frontotemporal lobar degeneration and autonomic dysfunction. In Depression, several observations have been made in relation to changes in one particular the Dorsal Raphe Nucleus (DRN) which also points toward as key area in various age-related and neurodevelopmental diseases. The DRN is further thought to be related to stress regulated processes and cognitive events. It is involved in neurodegeneration, e.g., amyloid plaques, neurofibrillary tangles, and impaired synaptic transmission in Alzheimer's disease as shown in our autopsy findings. The DRN is a phylogenetically old brain area, with projections that reach out to a large number of regions and nuclei of the central nervous system, particularly in the forebrain. These ascending projections contain multiple neurotransmitters. One of the main reasons for the past and current interest in the DRN is its involvement in depression, and its main transmitter serotonin. The DRN also points toward the increased importance and focus of the brainstem as key area in various age-related and neurodevelopmental diseases. This review describes the morphology, ascending projections and the complex neurotransmitter nature of the DRN, stressing its role as a key research target into the neural bases of depression.

Neural Networks With Motivation

Animals rely on internal motivational states to make decisions. The role of motivational salience in decision making is in early stages of mathematical understanding. Here, we propose a reinforcement learning framework that relies on neural networks to learn optimal ongoing behavior for dynamically changing motivation values. First, we show that neural networks implementing Q-learning with motivational salience can navigate in environment with dynamic rewards without adjustments in synaptic strengths when the needs of an agent shift. In this setting, our networks may display elements of addictive behaviors. Second, we use a similar framework in hierarchical manager-agent system to implement a reinforcement learning algorithm with motivation that both infers motivational states and behaves. Finally, we show that, when trained in the Pavlovian conditioning setting, the responses of the neurons in our model resemble previously published neuronal recordings in the ventral pallidum, a basal ganglia structure involved in motivated behaviors. We conclude that motivation allows Q-learning networks to quickly adapt their behavior to conditions when expected reward is modulated by agent's dynamic needs. Our approach addresses the algorithmic rationale of motivation and makes a step toward better interpretability of behavioral data via inference of motivational dynamics in the brain.

Endogenous opioid peptides in the descending pain modulatory circuit

The opioid epidemic has led to a serious examination of the use of opioids for the treatment of pain. Opioid drugs are effective due to the expression of opioid receptors throughout the body. These receptors respond to endogenous opioid peptides that are expressed as polypeptide hormones that are processed by proteolytic cleavage. Endogenous opioids are expressed throughout the peripheral and central nervous system and regulate many different neuronal circuits and functions. One of the key functions of endogenous opioid peptides is to modulate our responses to pain. This review will focus on the descending pain modulatory circuit which consists of the ventrolateral periaqueductal gray (PAG) projections to the rostral ventromedial medulla (RVM). RVM projections modulate incoming nociceptive afferents at the level of the spinal cord. Stimulation within either the PAG or RVM results in analgesia and this circuit has been studied in detail in terms of the actions of exogenous opioids, such as morphine and fentanyl. Further emphasis on understanding the complex regulation of endogenous opioids will help to make rational decisions with regard to the use of opioids for pain. We also include a discussion of the actions of endogenous opioids in the amygdala, an upstream brain structure that has reciprocal connections to the PAG that contribute to the brain's response to pain.

The anterior insular cortex unilaterally controls feeding in response to aversive visceral stimuli in mice

Reduced food intake is common to many pathological conditions, such as infection and toxin exposure. However, cortical circuits that mediate feeding responses to these threats are less investigated. The anterior insular cortex (aIC) is a core region that integrates interoceptive states and emotional awareness and consequently guides behavioral responses. Here, we demonstrate that the right-side aIC CamKII⁺ (aIC^CamKII) neurons in mice are activated by aversive visceral signals. Hyperactivation of the right-side aIC^CamKII neurons attenuates food consumption, while inhibition of these neurons increases feeding and reverses aversive stimuli-induced anorexia and weight loss. Similar manipulation at the left-side aIC does not cause significant behavioral changes. Furthermore, virus tracing reveals that aIC^CamKII neurons project directly to the vGluT2⁺ neurons in the lateral hypothalamus (LH), and the right-side aIC^CamKII-to-LH pathway mediates feeding suppression. Our studies uncover a circuit from the cortex to the hypothalamus that senses aversive visceral signals and controls feeding behavior.

Involvement of ventral pallidal vasopressin in the sex-specific regulation of sociosexual motivation in rats

The ventral pallidum (VP) is a critical node of the mesocorticolimbic reward circuit and is known to modulate social behaviors in rodents. Arginine vasopressin (AVP) signaling via the V1A receptor (V1AR) within the VP is necessary for the expression of socially motivated affiliative behaviors in monogamous voles. However, whether the VP-AVP system regulates socially motivated behaviors in non-monogamous species remains unknown. Here, we determined the extent of AVP fiber innervation in the VP as well as the involvement of the VP-AVP system in sociosexual motivation in adult male and female rats. We found that males have nearly twice the density of AVP-immunoreactive (AVP-ir) fibers in the VP compared to females, suggesting the possibility that males experience enhanced AVP signaling in the VP. We further found that this sex difference in VP-AVP-ir fiber density likely arises from an observed sex difference (males > females) in the percentage of VP-projecting AVP-ir cell bodies located in the bed nucleus of the stria terminalis and medial amygdala. To determine the behavioral implications of this sex difference, we next blocked AVP signaling in the VP by antagonizing VP-V1ARs in male and female rats and tested their preference to investigate an unfamiliar male rat or unfamiliar estrus female rat confined to corrals located on opposite ends of a three-chamber apparatus. Under vehicle conditions, males showed a significantly greater innate preference to investigate an opposite sex over same sex conspecific than estrus females. Interestingly, VP-V1AR antagonism significantly reduced males' opposite sex preference, while enhancing estrus females' opposite sex preference. Importantly, all subjects reliably discriminated between male and female stimulus rats regardless of drug treatment, demonstrating a change in motivational state rather than a perceptual impairment induced by VP-V1AR blockade. These results provide a novel functional link between a sex difference in ventral pallidal AVP fiber density and the sex-specific regulation of a sexually motivated behavior necessary for reproductive success.

Basal forebrain subcortical projections

The basal forebrain (BF) contains at least three distinct populations of neurons (cholinergic, glutamatergic, and GABA-ergic) across its different regions (medial septum, diagonal band, magnocellular preoptic area, and substantia innominata). Much attention has focused on the BF's ascending projections to cortex, but less is known about descending projections to subcortical regions. Given the neurochemical and anatomical heterogeneity of the BF, we used conditional anterograde tracing to map the patterns of subcortical projections from multiple BF regions and neurochemical cell types using mice that express Cre recombinase only in cholinergic, glutamatergic, or GABAergic neurons. We confirmed that different BF regions innervate distinct subcortical targets, with more subcortical projections arising from neurons in the caudal and lateral BF (substantia innominata and magnocellular preoptic area). Additionally, glutamatergic and GABAergic BF neurons have distinct patterns of descending projections, while cholinergic descending projections are sparse. Considering the intensity of glutamatergic and GABAergic descending projections, the BF likely acts through subcortical targets to promote arousal, motivation, and other behaviors.

Relative contributions and mapping of ventral tegmental area dopamine and GABA neurons by projection target in the rat

The ventral tegmental area (VTA) is a heterogeneous midbrain structure that contains dopamine (DA), GABA, and glutamate neurons that project to many different brain regions. Here, we combined retrograde tracing with immunocytochemistry against tyrosine hydroxylase (TH) or glutamate decarboxylase (GAD) to systematically compare the proportion of dopaminergic and GABAergic VTA projections to 10 target nuclei: anterior cingulate, prelimbic, and infralimbic cortex; nucleus accumbens core, medial shell, and lateral shell; anterior and posterior basolateral amygdala; ventral pallidum; and periaqueductal gray. Overall, the non-dopaminergic component predominated VTA efferents, accounting for more than 50% of all projecting neurons to each region except the nucleus accumbens core. In addition, GABA neurons contributed no more than 20% to each projection, with the exception of the projection to the ventrolateral periaqueductal gray, where the GABAergic contribution approached 50%. Therefore, there is likely a significant glutamatergic component to many of the VTA's projections. We also found that VTA cell bodies retrogradely labeled from the various target brain regions had distinct distribution patterns within the VTA, including in the locations of DA and GABA neurons. Despite this patterned organization, VTA neurons comprising these different projections were intermingled and never limited to any one subregion. These anatomical results are consistent with the idea that VTA neurons participate in multiple distinct, parallel circuits that differentially contribute to motivation and reward. While attention has largely focused on VTA DA neurons, a better understanding of VTA subpopulations, especially the contribution of non-DA neurons to projections, will be critical for future work.

Ventral pallidal encoding of reward-seeking behavior depends on the underlying associative structure

Despite its being historically conceptualized as a motor expression site, emerging evidence suggests the ventral pallidum (VP) plays a more active role in integrating information to generate motivation. Here, we investigated whether rat VP cue responses would encode and contribute similarly to the vigor of reward-seeking behaviors trained under Pavlovian versus instrumental contingencies, when these behavioral responses consist of superficially similar locomotor response patterns but may reflect distinct underlying decision-making processes. We find that cue-elicited activity in many VP neurons predicts the latency of instrumental reward seeking, but not of Pavlovian response latency. Further, disruption of VP signaling increases the latency of instrumental but not Pavlovian reward seeking. This suggests that VP encoding of and contributions to response vigor are specific to the ability of incentive cues to invigorate reward-seeking behaviors upon which reward delivery is contingent.

Sounds or other cues associated with receiving a reward can have a powerful effect on an individual’s behavior or emotions. For example, the sound of an ice cream truck might cause salivation and motivate an individual to stand in a long line. Cues may prompt specific actions necessary to receive a reward, for example, approaching the ice cream truck and paying to get an ice cream. This is called instrumental conditioning. Some cues predict reward delivery, without requiring a specific action. This is called Pavlovian conditioning. Pavlovian cues can still prompt actions, such as approaching the truck, even though the action is not required. But exactly what happens in the brain to generate these actions during the two types of learning, is unclear.

Learning more about these reward-driven brain mechanisms might help scientists to develop better treatments for people with addiction or other conditions that involve compulsive reward-seeking behavior. Currently, scientists do not know enough about how the brain triggers this kind of behavior or how these processes lead to relapse in individuals who have been abstinent. Basic studies on the brain mechanisms that trigger reward-seeking behavior are needed.

Now, Richard et al. show that a greater activity in neurons, or brain cells, in a part of the brain called the ventral pallidum predicts a faster response to a reward cue. In the experiments, some rats were trained to approach a certain location when they heard a particular sound in order to receive sugar water, a form of instrumental conditioning. Another group of rats underwent Pavlovian training and learned to expect sugar water every time they heard sound even if they did nothing. Both groups learned to approach the sugar water location when they heard the cue, despite the different training requirements.

Richard et al. measured the activity of neurons in the ventral pallidum when the rats in the two groups heard the reward-associated sound. The experiments showed that the amount of activity in the brain cells in this area predicted whether a rat would approach the sugar-water delivery area and how quickly they would approach the reward after hearing the cue. The predictions were most reliable for rats that had to do something to get the sugar water. When Richard et al. reduced the activity in these cells they found the rats took longer to approach the reward source, but only when this action was required to receive sugar water. The experiments show that the ventral pallidum may provide the motivation to undertake reward-seeking behavior.

Supramammillary glutamate neurons are a key node of the arousal system

Basic and clinical observations suggest that the caudal hypothalamus comprises a key node of the ascending arousal system, but the cell types underlying this are not fully understood. Here we report that glutamate-releasing neurons of the supramammillary region (SuM^vglut2) produce sustained behavioral and EEG arousal when chemogenetically activated. This effect is nearly abolished following selective genetic disruption of glutamate release from SuM^vglut2 neurons. Inhibition of SuM^vglut2 neurons decreases and fragments wake, also suppressing theta and gamma frequency EEG activity. SuM^vglut2 neurons include a subpopulation containing both glutamate and GABA (SuM^vgat/vglut2) and another also expressing nitric oxide synthase (SuM^Nos1/Vglut2). Activation of SuM^vgat/vglut2 neurons produces minimal wake and optogenetic stimulation of SuM^vgat/vglut2 terminals elicits monosynaptic release of both glutamate and GABA onto dentate granule cells. Activation of SuM^Nos1/Vglut2 neurons potently drives wakefulness, whereas inhibition reduces REM sleep theta activity. These results identify SuM^vglut2 neurons as a key node of the wake−sleep regulatory system.

Supramammillary nucleus (SuM) neurons have been studied in the context of REM sleep but their possible role in mediating wakefulness is not known. Here the authors elucidate the distinct functional contributions of three subpopulations in the SuM on electrographical and behavioral arousal in mice using genetically targeted approaches.

Slow-wave sleep is controlled by a subset of nucleus accumbens core neurons in mice

Sleep control is ascribed to a two-process model, a widely accepted concept that posits homoeostatic drive and a circadian process as the major sleep-regulating factors. Cognitive and emotional factors also influence sleep–wake behaviour; however, the precise circuit mechanisms underlying their effects on sleep control are unknown. Previous studies suggest that adenosine has a role affecting behavioural arousal in the nucleus accumbens (NAc), a brain area critical for reinforcement and reward. Here, we show that chemogenetic or optogenetic activation of excitatory adenosine A_2A receptor-expressing indirect pathway neurons in the core region of the NAc strongly induces slow-wave sleep. Chemogenetic inhibition of the NAc indirect pathway neurons prevents the sleep induction, but does not affect the homoeostatic sleep rebound. In addition, motivational stimuli inhibit the activity of ventral pallidum-projecting NAc indirect pathway neurons and suppress sleep. Our findings reveal a prominent contribution of this indirect pathway to sleep control associated with motivation.

In addition to circadian and homoeostatic drives, motivational levels influence sleep−wake cycles. Here the authors demonstrate that adenosine receptor-expressing neurons in the nucleus accumbens core that project to the ventral pallidum are inhibited by motivational stimuli and are causally involved in the control of slow-wave sleep.

Intrinsic Circuits in the Lateral Central Amygdala

Network activity in the lateral central amygdala (CeL) plays a crucial role in fear learning and emotional processing. However, the local circuits of the CeL are not fully understood and have only recently begun to be explored in detail. Here, we characterized the intrinsic circuits in the CeL using paired whole-call patch-clamp recordings, immunohistochemistry, and optogenetics in C57BL/6J wild-type and somatostatin-cre (SOM-Cre) mice. Our results revealed that throughout the rostrocaudal extent of the CeL, neurons form inhibitory connections at a rate of ∼29% with an average amplitude of 20 ± 3 pA (at -40 mV). Inhibitory input from a single neuron is sufficient to halt firing in the postsynaptic neuron. Post hoc immunostaining for protein kinase Cδ (PKCδ) in wild-type mice and paired recordings in SOM-Cre mice demonstrated that the most common local connections were PKCδ(-) → PKCδ(-) and SOM(+) → SOM(+). Finally, by optogenetically activating either SOM(+) or SOM(-) neurons, we found that almost all neurons in the CeL were innervated by these neuronal populations and that connections between like neurons were stronger than those between different neuronal types. These findings reveal a complex network of connections within the CeL and provide the foundations for future behavior-specific circuit analysis of this complex network.

Parasubthalamic and calbindin nuclei in the posterior lateral hypothalamus are the major hypothalamic targets for projections from the central and anterior basomedial nuclei of the amygdala

The parasubthalamic nucleus (PSTN) and the ventrally adjacent calbindin nucleus (CbN) form a nuclear complex in the posterior lateral hypothalamic area (LHA), recently characterized as connected with the central nucleus of the amygdala (CEA). The aim of the present work is to analyze in detail the projections from the amygdala into the PSTN/CbN, also focusing on pathways into the LHA. After fluorogold injections into the PSTN/CbN, the medial part of the CEA (CEAm) appears to be the main supplier of projections from the CEA. Other amygdalar nuclei contribute to the innervation of the PSTN/CbN complex, including the anterior part of the basomedial nucleus (BMAa). Injections of the anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHAL), into the CEAm and BMAa revealed that projections from the CEAm follow two pathways into the LHA: a dorsal pathway formed by axons that also innervate the paraventricular hypothalamic nucleus, the anterior perifornical LHA and the PSTN, and a ventral pathway that runs laterally adjacent to the ventrolateral hypothalamic tract (vlt) and ends in the CbN. By contrast, the BMAa and other telencephalic structures, such as the fundus striatum project to the CbN via the ventral pathway. Confirming the microscopic observation, a semi-quantitative analysis of the density of these projections showed that the PSTN and the CbN are the major hypothalamic targets for the projections from the CEAm and the BMAa, respectively. PSTN and CbN receive these projections through distinct dorsal and ventral routes in the LHA. The ventral pathway forms a differentiated tract, named here the ventrolateral amygdalo-hypothalamic tract (vlah), that is distinct from, but runs adjacent to, the vlt. Both the vlt and the vlah had been previously described as forming an olfactory path into the LHA. These results help to better characterize the CbN within the PSTN/CbN complex and are discussed in terms of the functional organization of the network involving the PSTN and the CbN as well as the CEA and the BMAa.

Connectivity between the central nucleus of the amygdala and the bed nucleus of the stria terminalis in the non-human primate: neuronal tract tracing and developmental neuroimaging studies

The lateral division of the bed nucleus of the stria terminalis (BSTL) and central nucleus of the amygdala (Ce) form the two poles of the 'central extended amygdala', a theorized subcortical macrostructure important in threat-related processing. Our previous work in nonhuman primates, and humans, demonstrating strong resting fMRI connectivity between the Ce and BSTL regions, provides evidence for the integrated activity of these structures. To further understand the anatomical substrates that underlie this coordinated function, and to investigate the integrity of the central extended amygdala early in life, we examined the intrinsic connectivity between the Ce and BSTL in non-human primates using ex vivo neuronal tract tracing, and in vivo diffusion-weighted imaging and resting fMRI techniques. The tracing studies revealed that BSTL receives strong input from Ce; however, the reciprocal pathway is less robust, implying that the primate Ce is a major modulator of BSTL function. The sublenticular extended amygdala (SLEAc) is strongly and reciprocally connected to both Ce and BSTL, potentially allowing the SLEAc to modulate information flow between the two structures. Longitudinal early-life structural imaging in a separate cohort of monkeys revealed that extended amygdala white matter pathways are in place as early as 3 weeks of age. Interestingly, resting functional connectivity between Ce and BSTL regions increases in coherence from 3 to 7 weeks of age. Taken together, these findings demonstrate a time period during which information flow between Ce and BSTL undergoes postnatal developmental changes likely via direct Ce → BSTL and/or Ce ↔ SLEAc ↔ BSTL projections.

Cholinergic regulation of fear learning and extinction

Cholinergic activation regulates cognitive function, particularly long-term memory consolidation. This Review presents an overview of the anatomical, neurochemical, and pharmacological evidence supporting the cholinergic regulation of Pavlovian contextual and cue-conditioned fear learning and extinction. Basal forebrain cholinergic neurons provide inputs to neocortical regions and subcortical limbic structures such as the hippocampus and amygdala. Pharmacological manipulations of muscarinic and nicotinic receptors support the role of cholinergic processes in the amygdala, hippocampus, and prefrontal cortex in modulating the learning and extinction of contexts or cues associated with threat. Additional evidence from lesion studies and analysis of in vivo acetylcholine release with microdialysis similarly support a critical role of cholinergic neurotransmission in corticoamygdalar or corticohippocampal circuits during acquisition of fear extinction. Although a few studies have suggested a complex role of cholinergic neurotransmission in the cellular plasticity essential for extinction learning, more work is required to elucidate the exact cholinergic mechanisms and physiological role of muscarinic and nicotinic receptors in these fear circuits. Such studies are important for elucidating the role of cholinergic neurotransmission in disorders such as posttraumatic stress disorder that involve deficits in extinction learning as well as for developing novel therapeutic approaches for such disorders. © 2016 Wiley Periodicals, Inc.

Cell type-specific long-range connections of basal forebrain circuit

The basal forebrain (BF) plays key roles in multiple brain functions, including sleep-wake regulation, attention, and learning/memory, but the long-range connections mediating these functions remain poorly characterized. Here we performed whole-brain mapping of both inputs and outputs of four BF cell types - cholinergic, glutamatergic, and parvalbumin-positive (PV+) and somatostatin-positive (SOM+) GABAergic neurons - in the mouse brain. Using rabies virus -mediated monosynaptic retrograde tracing to label the inputs and adeno-associated virus to trace axonal projections, we identified numerous brain areas connected to the BF. The inputs to different cell types were qualitatively similar, but the output projections showed marked differences. The connections to glutamatergic and SOM+ neurons were strongly reciprocal, while those to cholinergic and PV+ neurons were more unidirectional. These results reveal the long-range wiring diagram of the BF circuit with highly convergent inputs and divergent outputs and point to both functional commonality and specialization of different BF cell types.

Ventral Pallidum Neurons Encode Incentive Value and Promote Cue-Elicited Instrumental Actions

The ventral pallidum (VP) is posited to contribute to reward seeking by conveying upstream signals from the nucleus accumbens (NAc). Yet, very little is known about how VP neuron responses contribute to behavioral responses to incentive cues. Here, we recorded activity of VP neurons in a cue-driven reward-seeking task previously shown to require neural activity in the NAc. We find that VP neurons encode both learned cue value and subsequent reward seeking and that activity in VP neurons is required for robust cue-elicited reward seeking. Surprisingly, the onset of VP neuron responses occurs at a shorter latency than cue-elicited responses in NAc neurons. This suggests that this VP encoding is not a passive response to signals generated in the NAc and that VP neurons integrate sensory and motivation-related information received directly from other mesocorticolimbic inputs.

Toward a systems-oriented approach to the role of the extended amygdala in adaptive responding

Research into the structure and function of the basal forebrain macrostructure called the extended amygdala (EA) has recently seen considerable growth. This paper reviews that work, with the objectives of identifying underlying themes and developing a common goal towards which investigators of EA function might work. The paper begins with a brief review of the structure and the ontological and phylogenetic origins of the EA. It continues with a review of research into the role of the EA in both aversive and appetitive states, noting that these two seemingly disparate avenues of research converge on the concept of reinforcement - either negative or positive - of adaptive responding. These reviews lead to a proposal as to where the EA may fit in the organization of the basal forebrain, and an invitation to investigators to place their findings in a unifying conceptual framework of the EA as a collection of neural ensembles that mediate adaptive responding.

Arborization patterns of amygdalopetal axons from the rat ventral pallidum

We previously analyzed the arborization patterns of rat ventral pallidal (VP) axons that coursed caudally to innervate the thalamus and brainstem (Tripathi et al. in Brain Struct Funct 218:1133-1157, 2013). Here, we have reconstructed 16 previously undetected axons from the same tracer deposits that follow a more lateral trajectory. Virtually all 16 axons emanating from the different VP compartments collateralized in the extended amygdala system (EAS) and amygdaloid complex. The most frequent targets of axons from the lateral and medial (VPm) VP compartments were the rostral sublenticular extended amygdala, the extended amygdala (EA), the central nucleus of the amygdala and the posterior part of the basolateral amygdaloid nucleus. In contrast, axons from the rostral extension of the VP preferentially innervated the anterior amygdaloid area, the magnocellular preoptic nucleus, and the anterior part of the basomedial amygdaloid nucleus. We additionally found and reconstructed a single corticopetal axon arising from the VPm. The new results show that both direct and indirect projections from the basolateral complex and EAS to the ventral striatopallidal system are reciprocated by VP projections, and suggest that the systems can be activated simultaneously. The results additionally suggest that the amygdaloid complex and cortex are innervated separately from the VP. Finally, the combination of new and previous data indicate that approximately 84 % of VP axons (88/105) participate in basal ganglia circuits, 15 % (16/105) target the amygdaloid complex, and less than 1 % innervate the cortex.

Chronic wheel running-induced reduction of extinction and reinstatement of methamphetamine seeking in methamphetamine dependent rats is associated with reduced number of periaqueductal gray dopamine neurons

Exercise (physical activity) has been proposed as a treatment for drug addiction. In rodents, voluntary wheel running reduces cocaine and nicotine seeking during extinction, and reinstatement of cocaine seeking triggered by drug-cues. The purpose of this study was to examine the effects of chronic wheel running during withdrawal and protracted abstinence on extinction and reinstatement of methamphetamine seeking in methamphetamine dependent rats, and to determine a potential neurobiological correlate underlying the effects. Rats were given extended access to methamphetamine (0.05 mg/kg, 6 h/day) for 22 sessions. Rats were withdrawn and were given access to running wheels (wheel runners) or no wheels (sedentary) for 3 weeks after which they experienced extinction and reinstatement of methamphetamine seeking. Extended access to methamphetamine self-administration produced escalation in methamphetamine intake. Methamphetamine experience reduced running output, and conversely, access to wheel running during withdrawal reduced responding during extinction and, context- and cue-induced reinstatement of methamphetamine seeking. Immunohistochemical analysis of brain tissue demonstrated that wheel running during withdrawal did not regulate markers of methamphetamine neurotoxicity (neurogenesis, neuronal nitric oxide synthase, vesicular monoamine transporter-2) and cellular activation (c-Fos) in brain regions involved in relapse to drug seeking. However, reduced methamphetamine seeking was associated with running-induced reduction (and normalization) of the number of tyrosine hydroxylase immunoreactive neurons in the periaqueductal gray (PAG). The present study provides evidence that dopamine neurons of the PAG region show adaptive biochemical changes during methamphetamine seeking in methamphetamine dependent rats and wheel running abolishes these effects. Given that the PAG dopamine neurons project onto the structures of the extended amygdala, the present findings also suggest that wheel running may be preventing certain allostatic changes in the brain reward and stress systems contributing to the negative reinforcement and perpetuation of the addiction cycle.

Advances in the neurobiological bases for food 'liking' versus 'wanting'

The neural basis of food sensory pleasure has become an increasingly studied topic in neuroscience and psychology. Progress has been aided by the discovery of localized brain subregions called hedonic hotspots in the early 2000s, which are able to causally amplify positive affective reactions to palatable tastes ('liking') in response to particular neurochemical or neurobiological stimulations. Those hedonic mechanisms are at least partly distinct from larger mesocorticolimbic circuitry that generates the incentive motivation to eat ('wanting'). In this review, we aim to describe findings on these brain hedonic hotspots, especially in the nucleus accumbens and ventral pallidum, and discuss their role in generating food pleasure and appetite.

The EEG as an index of neuromodulator balance in memory and mental illness

There is a strong correlation between signature EEG frequency patterns and the relative levels of distinct neuromodulators. These associations become particularly evident during the sleep-wake cycle. The monoamine-acetylcholine balance hypothesis is a theory of neurophysiological markers of the EEG and a detailed description of the findings that support this proposal are presented in this paper. According to this model alpha rhythm reflects the relative predominance of cholinergic muscarinic signals and delta rhythm that of monoaminergic receptor effects. Both high voltage synchronized rhythms are likely mediated by inhibitory Gαi/o-mediated transduction of inhibitory interneurons. Cognitively, alpha and delta EEG measures are proposed to indicate automatic and flexible strategies, respectively. Sleep is associated with marked changes in relative neuromodulator levels corresponding to EEG markers of distinct stages. Sleep studies on memory consolidation present some of the strongest evidence yet for the respective roles of monoaminergic and cholinergic projections in declarative and non-declarative memory processes, a key theoretical premise for understanding the data. Affective dysregulation is reflected in altered EEG patterns during sleep.

Amygdala projections to the lateral bed nucleus of the stria terminalis in the macaque: comparison with ventral striatal afferents

The lateral bed nucleus of the stria terminalis (BSTL) is involved in mediating anxiety-related behaviors to sustained aversive stimuli. The BSTL forms part of the central extended amygdala, a continuum composed of the BSTL, the amygdala central nucleus, and cell columns running between the two. The central subdivision (BSTLcn) and the juxtacapsular subdivision (BSTLJ) are two BSTL regions that lie above the anterior commissure, near the ventral striatum. The amygdala, a heterogeneous structure that encodes emotional salience, projects to both the BSTL and ventral striatum. We placed small injections of retrograde tracers into the BSTL, focusing on the BSTLcn and BSTLJ, and analyzed the distribution of labeled cells in amygdala subregions. We compared this to the pattern of labeled cells following injections into the ventral striatum. All retrograde results were confirmed by anterograde studies. We found that the BSTLcn receives stronger amygdala inputs relative to the BSTLJ. Furthermore, the BSTLcn is defined by inputs from the corticoamygdaloid transition area and central nucleus, while the BSTLJ receives inputs mainly from the magnocellular accessory basal and basal nucleus. In the ventral striatum, the dorsomedial shell receives inputs that are similar, but not identical, to inputs to the BSTLcn. In contrast, amygdala projections to the ventral shell/core are similar to projections to the BSTLJ. These findings indicate that the BSTLcn and BSTLJ receive distinct amygdala afferent inputs and that the dorsomedial shell is a transition zone with the BSTLcn, while the ventral shell/core are transition zones with the BSTLJ.

A cholinergic hypothesis of the unconscious in affective disorders

The interactions between distinct pharmacological systems are proposed as a key dynamic in the formation of unconscious memories underlying rumination and mood disorder, but also reflect the plastic capacity of neural networks that can aid recovery. An inverse and reciprocal relationship is postulated between cholinergic and monoaminergic receptor subtypes. M1-type muscarinic receptor transduction facilitates encoding of unconscious, prepotent behavioral repertoires at the core of affective disorders and ADHD. Behavioral adaptation to new contingencies is mediated by the classic prototype receptor: 5-HT1A (Gi/o) and its modulation of M1-plasticity. Reversal of learning is dependent on increased phasic activation of midbrain monoaminergic nuclei and is a function of hippocampal theta. Acquired hippocampal dysfunction due to abnormal activation of the hypothalamic-pituitary-adrenal (HPA) axis predicts deficits in hippocampal-dependent memory and executive function and further impairments to cognitive inhibition. Encoding of explicit memories is mediated by Gq/11 and Gs signaling of monoamines only. A role is proposed for the phasic activation of the basal forebrain cholinergic nucleus by cortical projections from the complex consisting of the insula and claustrum. Although controversial, recent studies suggest a common ontogenetic origin of the two structures and a functional coupling. Lesions of the region result in loss of motivational behavior and familiarity based judgements. A major hypothesis of the paper is that these lost faculties result indirectly, from reduced cholinergic tone.

Brain circuitry mediating arousal from obstructive sleep apnea

Obstructive sleep apnea (OSA) is a disorder of repetitive sleep disruption caused by reduced or blocked respiratory airflow. Although an anatomically compromised airway accounts for the major predisposition to OSA, a patient's arousal threshold and factors related to the central control of breathing (ventilatory control stability) are also important. Arousal from sleep (defined by EEG desynchronization) may be the only mechanism that allows airway re-opening following an obstructive event. However, in many cases arousal is unnecessary and even worsens the severity of OSA. Mechanisms for arousal are poorly understood. However, accumulating data are elucidating the relevant neural pathways and neurotransmitters. For example, serotonin is critically required, but its site of action is unknown. Important neural substrates for arousal have been recently identified in the parabrachial complex (PB), a visceral sensory nucleus in the rostral pons. Moreover, glutamatergic signaling from the PB contributes to arousal caused by hypercapnia, one of the arousal-promoting stimuli in OSA. A major current focus of OSA research is to find means to maintain airway patency during sleep, without sleep interruption.

Corticotropin-releasing factor-related peptides, serotonergic systems, and emotional behavior

Corticotropin-releasing factor (CRF) is a 41-amino acid neuropeptide that is involved in stress-related physiology and behavior, including control of the hypothalamic-pituitary-adrenal (HPA) axis. Members of the CRF family of neuropeptides, including urocortin 1 (UCN 1), UCN 2, and UCN 3, bind to the G protein-coupled receptors, CRF type 1 (CRF₁) and CRF₂ receptors. In addition, CRF binding protein (CRFBP) binds both CRF and UCN 1 and can modulate their activities. There are multiple mechanisms through which CRF-related peptides may influence emotional behavior, one of which is through altering the activity of brainstem neuromodulatory systems, including serotonergic systems. CRF and CRF-related peptides act within the dorsal raphe nucleus (DR), the major source for serotonin (5-HT) in the brain, to alter the neuronal activity of specific subsets of serotonergic neurons and to influence stress-related behavior. CRF-containing axonal fibers innervate the DR in a topographically organized manner, which may contribute to the ability of CRF to alter the activity of specific subsets of serotonergic neurons. CRF and CRF-related peptides can either increase or decrease serotonergic neuronal firing rates and serotonin release, depending on their concentrations and on the specific CRF receptor subtype(s) involved. This review aims to describe the interactions between CRF-related peptides and serotonergic systems, the consequences for stress-related behavior, and implications for vulnerability to anxiety and affective disorders.

Subpopulations of somatostatin-immunoreactive non-pyramidal neurons in the amygdala and adjacent external capsule project to the basal forebrain: evidence for the existence of GABAergic projection neurons in the cortical nuclei and basolateral nuclear complex

The hippocampus and amygdala are key structures of the limbic system whose connections include reciprocal interactions with the basal forebrain (BF). The hippocampus receives both cholinergic and GABAergic afferents from the medial septal area of the BF. Hippocampal projections back to the medial septal area arise from non-pyramidal GABAergic neurons that express somatostatin (SOM), calbindin (CB), and neuropeptide Y (NPY). Recent experiments in our lab have demonstrated that the basolateral amygdala, like the hippocampus, receives both cholinergic and GABAergic afferents from the BF. These projections arise from neurons in the substantia innominata (SI) and ventral pallidum (VP). It remained to be determined, however, whether the amygdala has projections back to the BF that arise from GABAergic non-pyramidal neurons. This question was investigated in the present study by combining Fluorogold (FG) retrograde tract tracing with immunohistochemistry for GABAergic non-pyramidal cell markers, including SOM, CB, NPY, parvalbumin, calretinin, and glutamic acid decarboxylase (GAD). FG injections into the BF produced a diffuse array of retrogradely labeled neurons in many nuclei of the amygdala. The great majority of amygdalar FG+ neurons did not express non-pyramidal cell markers. However, a subpopulation of non-pyramidal SOM+ neurons, termed “long-range non-pyramidal neurons” (LRNP neurons), in the external capsule, basolateral amygdala, and cortical and medial amygdalar nuclei were FG+. About one-third of the SOM+ LRNP neurons were CB+ or NPY+, and one-half were GAD+. It remains to be determined if these inhibitory amygdalar projections to the BF, like those from the hippocampus, are important for regulating synchronous oscillations in the amygdalar-BF network.

Cholinergic modulation of cognitive processing: insights drawn from computational models

Acetylcholine plays an important role in cognitive function, as shown by pharmacological manipulations that impact working memory, attention, episodic memory, and spatial memory function. Acetylcholine also shows striking modulatory influences on the cellular physiology of hippocampal and cortical neurons. Modeling of neural circuits provides a framework for understanding how the cognitive functions may arise from the influence of acetylcholine on neural and network dynamics. We review the influences of cholinergic manipulations on behavioral performance in working memory, attention, episodic memory, and spatial memory tasks, the physiological effects of acetylcholine on neural and circuit dynamics, and the computational models that provide insight into the functional relationships between the physiology and behavior. Specifically, we discuss the important role of acetylcholine in governing mechanisms of active maintenance in working memory tasks and in regulating network dynamics important for effective processing of stimuli in attention and episodic memory tasks. We also propose that theta rhythm plays a crucial role as an intermediary between the physiological influences of acetylcholine and behavior in episodic and spatial memory tasks. We conclude with a synthesis of the existing modeling work and highlight future directions that are likely to be rewarding given the existing state of the literature for both empiricists and modelers.

The avian subpallium: new insights into structural and functional subdivisions occupying the lateral subpallial wall and their embryological origins

The subpallial region of the avian telencephalon contains neural systems whose functions are critical to the survival of individual vertebrates and their species. The subpallial neural structures can be grouped into five major functional systems, namely the dorsal somatomotor basal ganglia; ventral viscerolimbic basal ganglia; subpallial extended amygdala including the central and medial extended amygdala and bed nuclei of the stria terminalis; basal telencephalic cholinergic and non-cholinergic corticopetal systems; and septum. The paper provides an overview of the major developmental, neuroanatomical and functional characteristics of the first four of these neural systems, all of which belong to the lateral telencephalic wall. The review particularly focuses on new findings that have emerged since the identity, extent and terminology for the regions were considered by the Avian Brain Nomenclature Forum. New terminology is introduced as appropriate based on the new findings. The paper also addresses regional similarities and differences between birds and mammals, and notes areas where gaps in knowledge occur for birds.

Cholinergic control of visual categorization in macaques

Acetylcholine (ACh) is a neurotransmitter acting via muscarinic and nicotinic receptors that is implicated in several cognitive functions and impairments, such as Alzheimer’s disease. It is believed to especially affect the acquisition of new information, which is particularly important when behavior needs to be adapted to new situations and to novel sensory events. Categorization, the process of assigning stimuli to a category, is a cognitive function that also involves information acquisition. The role of ACh on categorization has not been previously studied. We have examined the effects of scopolamine, an antagonist of muscarinic ACh receptors, on visual categorization in macaque monkeys using familiar and novel stimuli. When the peripheral effects of scopolamine on the parasympathetic nervous system were controlled for, categorization performance was disrupted following systemic injections of scopolamine. This impairment was observed only when the stimuli that needed to be categorized had not been seen before. In other words, the monkeys were not impaired by the central action of scopolamine in categorizing a set of familiar stimuli (stimuli which they had categorized successfully in previous sessions). Categorization performance also deteriorated as the stimulus became less salient by an increase in the level of visual noise. However, scopolamine did not cause additional performance disruptions for difficult categorization judgments at lower coherence levels. Scopolamine, therefore, specifically affects the assignment of new exemplars to established cognitive categories, presumably by impairing the processing of novel information. Since we did not find an effect of scopolamine in the categorization of familiar stimuli, scopolamine had no significant central action on other cognitive functions such as perception, attention, memory, or executive control within the context of our categorization task.

Neuroanatomical projections of the species-specific tyrosine hydroxylase-immunoreactive cells of the male prairie vole bed nucleus of the stria terminalis and medial amygdala

The principal nucleus of the bed nucleus of the stria terminalis (BSTpr) and posterodorsal part of the medial amygdalar nucleus (MEApd) are densely interconnected sites transmitting olfactory information to brain areas mediating sociosexual behaviors. In male prairie voles (Microtus ochrogaster), the BSTpr and MEApd contain hundreds of cells densely immunoreactive for tyrosine hydroxylase (TH). Such tremendous numbers of TH-immunoreactive (TH-ir) cells do not exist in other rodents examined, and studies from our laboratory suggest these cells may be part of a unique chemical network necessary for monogamous behaviors in prairie voles. To obtain information about how these TH-ir cells communicate with other sites involved in social behaviors, we first used biotinylated dextran amine (BDA) to determine sites that receive BSTpr efferents and also contain TH-ir fibers. Only in the medial preoptic area (MPO) and MEApd did we find considerable comingling of BDA-containing and TH-ir fibers. To examine if these sites receive input specifically from BSTpr TH-ir cells, the retrograde tracer Fluorogold was infused into the MPO or MEApd. Almost 80% of TH-ir projections to the MPO originated from the BSTpr or MEApd, involving about 40% of all TH-ir cells in these sites. In contrast, the MEApd received almost no input from TH-ir cells in the BSTpr, and received it primarily from the ventral tegmental area. Retrograde tracing from the BSTpr itself revealed substantial input from MEApd TH-ir cells. Thus, the male prairie vole brain contains a species-specific TH-ir network involving the BSTpr, MEApd, and MPO. By connecting brain sites involved in olfaction, sociality and motivation, this network may be essential for monogamous behaviors in this species.

Reassessment of the structural basis of the ascending arousal system

The "ascending reticular activating system" theory proposed that neurons in the upper brainstem reticular formation projected to forebrain targets that promoted wakefulness. More recent formulations have emphasized that most neurons at the pontomesencephalic junction that participate in these pathways are actually in monoaminergic and cholinergic cell groups. However, cell-specific lesions of these cell groups have never been able to reproduce the deep coma seen after acute paramedian midbrain lesions that transect ascending axons at the caudal midbrain level. To determine whether the cortical afferents from the thalamus or the basal forebrain were more important in maintaining arousal, we first placed large cell-body-specific lesions in these targets. Surprisingly, extensive thalamic lesions had little effect on electroencephalographic (EEG) or behavioral measures of wakefulness or on c-Fos expression by cortical neurons during wakefulness. In contrast, animals with large basal forebrain lesions were behaviorally unresponsive and had a monotonous sub-1-Hz EEG, and little cortical c-Fos expression during continuous gentle handling. We then retrogradely labeled inputs to the basal forebrain from the upper brainstem, and found a substantial input from glutamatergic neurons in the parabrachial nucleus and adjacent precoeruleus area. Cell-specific lesions of the parabrachial-precoeruleus complex produced behavioral unresponsiveness, a monotonous sub-1-Hz cortical EEG, and loss of cortical c-Fos expression during gentle handling. These experiments indicate that in rats the reticulo-thalamo-cortical pathway may play a very limited role in behavioral or electrocortical arousal, whereas the projection from the parabrachial nucleus and precoeruleus region, relayed by the basal forebrain to the cerebral cortex, may be critical for this process.

Functional topography of midbrain and pontine serotonergic systems: implications for synaptic regulation of serotonergic circuits

RATIONALE

Dysfunction of serotonergic systems is thought to play an important role in a number of neurological and psychiatric disorders. Recent studies suggest that there is anatomical and functional diversity among serotonergic systems innervating forebrain systems involved in the control of physiologic and behavioral responses, including the control of emotional states.

OBJECTIVE

Here, we highlight the methods that have been used to investigate the heterogeneity of serotonergic systems and review the evidence for the unique anatomical, hodological, and functional properties of topographically organized subpopulations of serotonergic neurons in the midbrain and pontine raphe complex.

CONCLUSION

The emerging understanding of the topographically organized synaptic regulation of brainstem serotonergic systems, the topography of the efferent projections of these systems, and their functional properties, should enable identification of novel therapeutic approaches to treatment of neurological and psychiatric conditions that are associated with dysregulation of serotonergic systems.

Sleep state switching

We take for granted the ability to fall asleep or to snap out of sleep into wakefulness, but these changes in behavioral state require specific switching mechanisms in the brain that allow well-defined state transitions. In this review, we examine the basic circuitry underlying the regulation of sleep and wakefulness and discuss a theoretical framework wherein the interactions between reciprocal neuronal circuits enable relatively rapid and complete state transitions. We also review how homeostatic, circadian, and allostatic drives help regulate sleep state switching and discuss how breakdown of the switching mechanism may contribute to sleep disorders such as narcolepsy.

Astrocytes in the amygdala

The amygdala has received considerable attention because of its established role in specific behaviors and disorders such as anxiety, depression, and autism. Studies have revealed that the amygdala is a complex and dynamic brain region that is highly connected with other areas of the brain. Previous works have focused on neurons, demonstrating that the amygdala in rodents is highly plastic and sexually dimorphic. However, our more recent work explores sex differences in nonneuronal cells, joining a rich literature concerning glia in the amygdala. Prior investigation of glia in the amygdala can generally be divided into disease-related and hormone-related categories, with both areas of research producing interesting findings concerning glia in this important brain region. Despite a wide range of research topics, the collected findings make it clear that glia in the amygdala are sensitive and plastic cells that respond and develop in a highly region specific manner.

Activation of the basal forebrain by the orexin/hypocretin neurones

The orexin neurones play an essential role in driving arousal and in maintaining normal wakefulness. Lack of orexin neurotransmission produces a chronic state of hypoarousal characterized by excessive sleepiness, frequent transitions between wake and sleep, and episodes of cataplexy. A growing body of research now suggests that the basal forebrain (BF) may be a key site through which the orexin-producing neurones promote arousal. Here we review anatomical, pharmacological and electrophysiological studies on how the orexin neurones may promote arousal by exciting cortically projecting neurones of the BF. Orexin fibres synapse on BF cholinergic neurones and orexin-A is released in the BF during waking. Local application of orexins excites BF cholinergic neurones, induces cortical release of acetylcholine and promotes wakefulness. The orexin neurones also contain and probably co-release the inhibitory neuropeptide dynorphin. We found that orexin-A and dynorphin have specific effects on different classes of BF neurones that project to the cortex. Cholinergic neurones were directly excited by orexin-A, but did not respond to dynorphin. Non-cholinergic BF neurones that project to the cortex seem to comprise at least two populations with some directly excited by orexin-A that may represent wake-active, GABAergic neurones, whereas others did not respond to orexin-A but were inhibited by dynorphin and may be sleep-active, GABAergic neurones. This evidence suggests that the BF is a key site through which orexins activate the cortex and promote behavioural arousal. In addition, orexins and dynorphin may act synergistically in the BF to promote arousal and improve cognitive performance.

Distinct patterns of neuronal inputs and outputs of the juxtaparaventricular and suprafornical regions of the lateral hypothalamic area in the male rat

We have analyzed at high resolution the neuroanatomical connections of the juxtaparaventricular region of the lateral hypothalamic area (LHAjp); as a control and in comparison to this, we also performed a preliminary analysis of a nearby LHA region that is dorsal to the fornix, namely the LHA suprafornical region (LHAs). The connections of these LHA regions were revealed with a coinjection tract-tracing technique involving a retrograde (cholera toxin B subunit) and anterograde (Phaseolus vulgaris leucoagglutinin) tracer. The LHAjp and LHAs together connect with almost every major division of the cerebrum and cerebrospinal trunk, but their connection profiles are markedly different and distinct. In simple terms, the connections of the LHAjp indicate a possible primary role in the modulation of defensive behavior; for the LHAs, a role in the modulation of ingestive behavior is suggested. However, the relation of the LHAjp and LHAs to potential modulation of these behaviors, as indicated by their neuroanatomical connections, appears to be highly integrative as it includes each of the major functional divisions of the nervous system that together determine behavior, i.e., cognitive, state, sensory, and motor. Furthermore, although a primary role is indicated for each region with respect to a particular mode of behavior, intermode modulation of behavior is also indicated. In summary, the extrinsic connections of the LHAjp and LHAs (so far as we have described them) suggest that these regions have a profoundly integrative role in which they may participate in the orchestrated modulation of elaborate behavioral repertoires.

Regulation of cortical acetylcholine release: insights from in vivo microdialysis studies

Acetylcholine release links the activity of presynaptic neurons with their postsynaptic targets and thus represents the intercellular correlate of cholinergic neurotransmission. Here, we review the regulation and functional significance of acetylcholine release in the mammalian cerebral cortex, with a particular emphasis on information derived from in vivo microdialysis studies over the past three decades. This information is integrated with anatomical and behavioral data to derive conclusions regarding the role of cortical cholinergic transmission in normal behavioral and how its dysregulation may contribute to cognitive correlates of several neuropsychiatric conditions. Some unresolved issues regarding the regulation and significance of cortical acetylcholine release and the promise of new methodology for advancing our knowledge in this area are also briefly discussed.

Intrinsic neuronal plasticity in the juxtacapsular nucleus of the bed nuclei of the stria terminalis (jcBNST)

The juxtacapsular nucleus of the anterior division of the BNST (jcBNST) receives robust glutamatergic projections from the basolateral nucleus of the amygdala (BLA), the postpiriform transition area, and the insular cortex as well as dopamine (DA) inputs from the midbrain. In turn the jcBNST sends GABAergic projections to the medial division of the central nucleus of the amygdala (CEAm) as well as other brain regions. We recently described a form of long-term potentiation of the intrinsic excitability (LTP-IE) of neurons of the juxtacapsular nucleus of BNST (jcBNST) in response to high-frequency stimulation (HFS) of the stria terminalis that was impaired during protracted withdrawal from alcohol, cocaine, and heroin and in rats chronically treated with corticotropin-releasing factor (CRF) intracerebroventricularly. Here we show that DAergic neurotransmission is required for the induction of LTP-IE of jcBNST neurons through dopamine (DA) D1 receptors. Thus, activation of the central CRF stress system and altered DAergic neurotransmission during protracted withdrawal from alcohol and drugs of abuse may contribute to the disruption of LTP-IE in the jcBNST. Impairment of this form of intrinsic neuronal plasticity in the jcBNST could result in inadequate neuronal integration and reduced inhibition of the CEA, contributing to the negative affective state that characterizes protracted abstinence in post-dependent individuals. These results provide a novel neurobiological target for vulnerability to alcohol and drug dependence.

The basolateral amygdala modulates specific sensory memory representations in the cerebral cortex

Stress hormones released by an experience can modulate memory strength via the basolateral amygdala, which in turn acts on sites of memory storage such as the cerebral cortex [McGaugh, J. L. (2004). The amygdala modulates the consolidation of memories of emotionally arousing experiences. Annual Review of Neuroscience, 27, 1-28]. Stimuli that acquire behavioral importance gain increased representation in the cortex. For example, learning shifts the tuning of neurons in the primary auditory cortex (A1) to the frequency of a conditioned stimulus (CS), and the greater the level of CS importance, the larger the area of representational gain [Weinberger, N. M. (2007). Associative representational plasticity in the auditory cortex: A synthesis of two disciplines. Learning & Memory, 14(1-2), 1-16]. The two lines of research suggest that BLA strengthening of memory might be accomplished in part by increasing the representation of an environmental stimulus. The present study investigated whether stimulation of the BLA can affect cortical memory representations. In male Sprague-Dawley rats studied under urethane general anesthesia, frequency receptive fields were obtained from A1 before and up to 75min after the pairing of a tone with BLA stimulation (BLAstm: 100 trials, 400ms, 100Hz, 400microA [+/-16.54]). Tone started before and continued after BLAstm. Group BLA/1.0 (n=16) had a 1s CS-BLAstm interval while Group BLA/1.6 (n=5) has a 1.6s interval. The BLA/1.0 group did develop specific tuning shifts toward and to the CS, which could change frequency tuning by as much as two octaves. Moreover, its shifts increased over time and were enduring, lasting 75min. However, group BLA/1.6 did not develop tuning shifts, indicating that precise CS-BLAstm timing is important in the anesthetized animal. Further, training in the BLA/1.0 paradigm but stimulating outside of the BLA did not produce tuning shifts. These findings demonstrate that the BLA is capable of exerting highly specific, enduring, learning-related modifications of stimulus representation in the cerebral cortex. These findings suggest that the ability of the BLA to alter specific cortical representations may underlie, at least in part, the modulatory influence of BLA activity on strengthening long-term memory.

Sex differences and laterality in astrocyte number and complexity in the adult rat medial amygdala

The posterodorsal portion of the medial amygdala (MePD) is sexually dimorphic in several rodent species. In several other brain nuclei, astrocytes change morphology in response to steroid hormones. We visualized MePD astrocytes using glial-fibrillary acidic protein (GFAP) immunocytochemistry. We compared the number and process complexity of MePD astrocytes in adult wildtype male and female rats and testicular feminized mutant (TFM) male rats that lack functional androgen receptors (ARs) to determine whether MePD astrocytes are sexually differentiated and whether ARs have a role. Unbiased stereological methods revealed laterality and sex differences in MePD astrocyte number and complexity. The right MePD contained more astrocytes than the left in all three genotypes, and the number of astrocytes was also sexually differentiated in the right MePD, with males having more astrocytes than females. In contrast, the left MePD contained more complex astrocytes than did the right MePD in all three genotypes, and males had more complex astrocytes than females in this hemisphere. TFM males were comparable to wildtype females, having fewer astrocytes on the right and simpler astrocytes on the left than do wildtype males. Taken together, these results demonstrate that astrocytes are sexually dimorphic in the adult MePD and that the nature of the sex difference is hemisphere-dependent: a sex difference in astrocyte number in the right MePD and a sex difference in astrocyte complexity in the left MePD. Moreover, functional ARs appear to be critical in establishing these sex differences in MePD astrocyte morphology.