Hippocampal convergence during anticipatory midbrain activation promotes subsequent memory formation.

The hippocampus has been a focus of memory research since H.M's surgery abolished his ability to form new memories, yet its mechanistic role in memory remains debated. Here, we identify a candidate memory mechanism: an anticipatory hippocampal "convergence state", observed while awaiting valuable information, and which predicts subsequent learning. During fMRI, participants viewed trivia questions eliciting high or low curiosity, followed seconds later by its answer. We reasoned that encoding success requires a confluence of conditions, so that hippocampal states more conducive to memory formation should converge in state space. To operationalize convergence of neural states, we quantified the typicality of multivoxel patterns in the medial temporal lobes during anticipation and encoding of trivia answers. We found that the typicality of anticipatory hippocampal patterns increased during high curiosity. Crucially, anticipatory hippocampal pattern typicality increased with dopaminergic midbrain activation and uniquely accounted for the association between midbrain activation and subsequent recall. We propose that hippocampal convergence states may complete a cascade from motivation and midbrain activation to memory enhancement, and may be a general predictor of memory formation.

Fos response of the tail of the ventral tegmental area to food restriction entails a prediction error processing.

The tail of the ventral tegmental area (tVTA) or rostromedial tegmental nucleus (RMTg) receives lateral habenula inputs and projects heavily to midbrain dopamine neurons. Midbrain dopamine and lateral habenula neurons participate in learning processes predicting the outcomes of actions, placing the tVTA in a critical location into prediction error pathways. tVTA GABA neurons show electrophysiological inhibition or activation after reward and aversive stimuli, respectively, and their predictive cues. tVTA molecular recruitment, however, is not elicited by all aversive stimuli. Indeed, precipitated opioid withdrawal, repeated footshocks or food restriction raise tVTA Fos expression, whereas various other unpleasant, stressful or painful stimuli does not elicit that molecular response. However, the basis of that difference remains unknown. In the present study, we tried to disentangle whether the tVTA c-Fos induction observed after food restriction was due to the aversive state of food restriction or to procedure-related reward prediction error. To this end, male Sprague-Dawley rats were food-restricted for 7-8 days. During this period, animals were handled and weighed every day before feeding. On the test day, rats underwent several behavioral procedures to explore the impact of food restriction and food-predictive cue exposure on tVTA c-Fos expression. We showed that food restriction per se was not able to recruit c-Fos in the tVTA. On the contrary, the food-predicting cues induced c-Fos locally in the absence of feeding, whereas the food-predicting cues followed by feeding evoked lower c-Fos expression. Overall, our results support the proposed involvement of the tVTA in reward prediction error.

Promotive effects of tetrahydroxystilbene glucoside on the differentiation of neural stem cells from the mesencephalon into dopaminergic neurons.

Parkinson's disease (PD) is a common neurodegenerative disorder characterized by the loss of midbrain dopaminergic (DA) neurons. Neural stem cells (NSCs) are the most promising cells for cell-replacement therapy for PD. However, the poor differentiation and maturation of DA neurons and decreased cell survival after transplantation are a challenge. Tetrahydroxystilbene glucoside (2,3,5,4'-tetrahydroxystilbene-2-O-glucoside; TSG), an active component of the popular traditional Chinese medicinal plant Polygonum multiflorum Thunb, possesses multiple pharmacological actions. In this study, we determined whether TSG can induce neural stem cell (NSCs) differentiation into neurons, especially DA neurons, and the possible involvement of Wnt/ß-catenin signaling pathways. Results revealed that NSCs differentiated primarily into astrocytes when cultured in 2 % serum-containing medium. However, TSG treatment during NSC differentiation in vitro increased the number of Tuj-1-positive neurons, as well as the proportion of tyrosine hydroxylase(TH)-positive cells and dopamine- transporter- positive neurons, a late marker of mature DA neurons. We also found that TSG enhanced the expression of nuclear receptor related factor 1, a transcription factor specific for the development and maintenance of midbrain DA neurons in inducing NSC differentiation into TH -immunoreactive DA neurons. Moreover, TSG upregulated the expression of Wnt/ß-catenin signaling molecules (Wnt1, Wnt3a, Wnt5a, and ß-catenin). However, these promoting effects were significantly inhibited by the application of IWR1, a Wnt signaling-specific blocker in culture. Our findings suggested that TSG may have potential in inducing the DA neuronal differentiation of mouse NSCs mediated by triggering the Wnt/ß-catenin signaling pathway. These results indicated the possible role for TSG in the transplantation of NSCs for PD.

Increased Macrophages and C1qA, C3, C4 Transcripts in the Midbrain of People With Schizophrenia.

Increased cytokine and inflammatory-related transcripts are found in the ventral midbrain, a dopamine neuron-rich region associated with schizophrenia symptoms. In fact, half of schizophrenia cases can be defined as having a "high inflammatory/immune biotype." Recent studies implicate both complement and macrophages in cortical neuroinflammation in schizophrenia. Our aim was to determine whether measures of transcripts related to phagocytosis/macrophages (CD163, CD64, and FN1), or related to macrophage adhesion [intercellular adhesion molecule 1 (ICAM1)], or whether CD163+ cell density, as well as protein and/or gene expression of complement pathway activators (C1qA) and mediators (C3 or C4), are increased in the midbrain in schizophrenia, especially in those with a high inflammatory biotype. We investigated whether complement mRNA levels correlate with macrophage and/or microglia and/or astrocyte markers. We found CD163+ cells around blood vessels and in the parenchyma and increases in ICAM1, CD163, CD64, and FN1 mRNAs as well as increases in all complement transcripts in the midbrain of schizophrenia cases with high inflammation. While we found positive correlations between complement transcripts (C1qA and C3) and microglia or astrocyte markers across diagnostic and inflammatory subgroups, the only unique strong positive correlation was between CD163 and C1qA mRNAs in schizophrenia cases with high inflammation. Our study is the first to suggest that more circulating macrophages may be attracted to the midbrain in schizophrenia, and that increased macrophages are linked to increased complement pathway activation in tissue and may contribute to dopamine dysregulation in schizophrenia. Single-cell transcriptomic studies and mechanistic preclinical studies are required to test these possibilities.

Neonatal Nicotine Exposure Primes Midbrain Neurons to a Dopaminergic Phenotype and Increases Adult Drug Consumption.

BACKGROUND: Nicotine intake induces addiction through neuroplasticity of the reward circuitry, altering the activity of dopaminergic neurons of the ventral tegmental area. Prior work demonstrated that altered circuit activity can change neurotransmitter expression in the developing and adult brain. Here we investigated the effects of neonatal nicotine exposure on the dopaminergic system and nicotine consumption in adulthood. METHODS: Male and female mice were used for two-bottle-choice test, progressive ratio breakpoint test, immunohistochemistry, RNAscope, quantitative polymerase chain reaction, calcium imaging, and DREADD (designer receptor exclusively activated by designer drugs)-mediated chemogenic activation/inhibition experiments. RESULTS: Neonatal nicotine exposure potentiates drug preference in adult mice, induces alterations in calcium spike activity of midbrain neurons, and increases the number of dopamine-expressing neurons in the ventral tegmental area. Specifically, glutamatergic neurons are first primed to express transcription factor Nurr1, then acquire the dopaminergic phenotype following nicotine re-exposure in adulthood. Enhanced neuronal activity combined with Nurr1 expression is both necessary and sufficient for the nicotine-mediated neurotransmitter plasticity to occur. CONCLUSIONS: Our findings illuminate a new mechanism of neuroplasticity by which early nicotine exposure primes the reward system to display increased susceptibility to drug consumption in adulthood.

Expression of aggressiveness modulates mesencephalic c-fos activation during a social interaction test in Japanese quail (Coturnix japonica).

It is well known that during a social conflict, interactions are dependent on the animal's propensity to behave aggressively as well as the behavior of the opponent. However, discriminating between these two confounding factors was difficult. Recently, a Social Interaction (SI) test using photocastrated males as non-aggressive stimuli was proposed as a useful tool to evaluate aggressiveness. The avian Intercollicular- Griseum centralis complex (comparable to mammalian periaqueductal gray) has been reported as a crucial node in the descending pathways that organize behavioral and autonomic aspects of defensive responses and aggressiveness. Herein, using the SI test, we evaluated whether mesencephalic areas are activated (expressed c-fos) when photostimulated adult males are confronted with non-responsive (non-aggressive) opponents. Furthermore, we also examined whether mesencephalic activation is related to male performance during the SI test (i.e., aggressive vs. non-aggressive males) in birds reared in enriched or in standard environments. Five mesencephalic areas at two anatomic levels (intermediate and rostral) and locomotion during SI testing were studied. Aggressive males showed increased c-fos expression in all areas studied, and moved at faster speeds in comparison to their non-aggressive and control counterparts. Non-aggressive males and the test controls showed similar c-fos labeling. In general, rearing condition did not appear to influence c-fos expression nor behavior during the SI test. Findings suggest that mesencephalic activation is involved when males are actively expressing aggressive behaviors. This overall phenomenon is shown regardless of both the environmental stimuli provided during the birds´ rearing and the potentially stressful stimuli during the SI trial.

Auditory sensitivity exhibits sexual dimorphism and seasonal plasticity in music frogs.

Seasonal changes in the structure and function of the vertebrate brain have been described in many species, particularly in seasonal breeders. However, it remains unclear whether sexual dimorphism varies between breeding seasons for specific brain regions. Auditory event-related potential (ERP) changes were evaluated in the Emei music frog (Babina daunchina) to assess sexual dimorphism and seasonal variations in auditory sensitivity. An acoustic playback experiment using an oddball paradigm design was conducted, in which two conspecific call types were used as deviant stimuli with synthesized white noise as standard stimulus. ERP components were analyzed for the telencephalon and mesencephalon of both sexes in the non-reproductive and reproductive states. Results show that auditory sensitivity is modulated by reproductive status, implying that seasonal plasticity is involved in auditory perception. Moreover, the amplitude of the N1 ERP component (mean amplitudes during the interval occurring 30-130 ms after stimulus onset) is higher in females for the telencephalon and higher in males for the mesencephalon, regardless of reproductive status and acoustic stimulus type. These results show that auditory ERP responses for specific brain regions exhibit sexual dimorphism in the absence of exogenous sexual stimulation during both the two reproductive states in the music frog.

Tyrosine hydroxylase down-regulation after loss of Abelson helper integration site 1 (AHI1) promotes depression via the circadian clock pathway in mice.

Abelson helper integration site 1 (AHI1) is associated with several neuropsychiatric and brain developmental disorders, such as schizophrenia, depression, autism, and Joubert syndrome. Ahi1 deficiency in mice leads to behaviors typical of depression. However, the mechanisms by which AHI1 regulates behavior remain to be elucidated. Here, we found that down-regulation of expression of the rate-limiting enzyme in dopamine biosynthesis, tyrosine hydroxylase (TH), in the midbrains of Ahi1-knockout (KO) mice is responsible for Ahi1-deficiency-mediated depressive symptoms. We also found that Rev-Erbα, a TH transcriptional repressor and circadian regulator, is up-regulated in the Ahi1-KO mouse midbrains and Ahi1-knockdown Neuro-2a cells. Moreover, brain and muscle Arnt-like protein 1 (BMAL1), the Rev-Erbα transcriptional regulator, is also increased in the Ahi1-KO mouse midbrains and Ahi1-knockdown cells. Our results further revealed that AHI1 decreases BMAL1/Rev-Erbα expression by interacting with and repressing retinoic acid receptor-related orphan receptor α, a nuclear receptor and transcriptional regulator of circadian genes. Of note, Bmal1 deficiency reversed the reduction in TH expression induced by Ahi1 deficiency. Moreover, microinfusion of the Rev-Erbα inhibitor SR8278 into the ventral midbrain of Ahi1-KO mice significantly increased TH expression in the ventral tegmental area and improved their depressive symptoms. These findings provide a mechanistic explanation for a link between AHI1-related behaviors and the circadian clock pathway, indicating an involvement of circadian regulatory proteins in AHI1-regulated mood and behavior.

A GABAergic cell type in the lateral habenula links hypothalamic homeostatic and midbrain motivation circuits with sex steroid signaling.

The lateral habenula (LHb) has a key role in integrating a variety of neural circuits associated with reward and aversive behaviors. There is limited information about how the different cell types and neuronal circuits within the LHb coordinate physiological and motivational states. Here, we report a cell type in the medial division of the LHb (LHbM) in male rats that is distinguished by: (1) a molecular signature for GABAergic neurotransmission (Slc32a1/VGAT) and estrogen receptor (Esr1/ERα) expression, at both mRNA and protein levels, as well as the mRNA for vesicular glutamate transporter Slc17a6/VGLUT2, which we term the GABAergic estrogen-receptive neuron (GERN); (2) its axonal projection patterns, identified by in vivo juxtacellular labeling, to both local LHb and to midbrain modulatory systems; and (3) its somatic expression of receptors for vasopressin, serotonin and dopamine, and mRNA for orexin receptor 2. This cell type is anatomically located to receive afferents from midbrain reward (dopamine and serotonin) and hypothalamic water and energy homeostasis (vasopressin and orexin) circuits. These afferents shared the expression of estrogen synthase (aromatase) and VGLUT2, both in their somata and axon terminals. We demonstrate dynamic changes in LHbM VGAT+ cell density, dependent upon gonadal functional status, that closely correlate with motivational behavior in response to predator and forced swim stressors. The findings suggest that the homeostasis and reward-related glutamatergic convergent projecting pathways to LHbMC employ a localized neurosteroid signaling mechanism via axonal expression of aromatase, to act as a switch for GERN excitation/inhibition output prevalence, influencing depressive or motivated behavior.

Comparative review of adult midbrain and striatum neurogenesis with classical neurogenesis.

Parkinson's Disease (PD) motor symptoms are caused by loss of dopamine (DA) neurons in the substantia nigra pars compacta (SNc) of the midbrain. Dopamine cell replacement therapy (DA CRT), either by cell transplantation or endogenous repair, has been a potential treatment to replace dead cells and improve PD motor symptoms. Adult midbrain and striatum have been studied for many years to find evidence of neurogenesis. Although the literature is controversial, recent research has revived the possibility of neurogenesis here. This paper aims to review the process of neurogenesis (by focusing on gene expression patterns) in the adult midbrain/striatum and compare it with classical neurogenesis that occurs in developing midbrain, Sub Ventricular Zone (SVZ) and Sub Granular Zone (SGZ) of the adult brain.

Role of estrogen receptor beta in neural differentiation of mouse embryonic stem cells.

The ability to propagate mature cells and tissue from pluripotent stem cells offers enormous promise for treating many diseases, including neurodegenerative diseases. Before such cells can be used successfully in neurodegenerative diseases without causing unwanted cell growth and migration, genes regulating growth and migration of neural stem cells need to be well characterized. Estrogen receptor beta (ERß) is essential for migration of neurons and glial cells in the developing mouse brain. To examine whether ERß influences differentiation of mouse embryonic stem cells (mESC) into neural lineages, we compared control and ERß knockout (BERKO) mESCs at defined stages of neural development and examined the effects of an ERß-selective ligand (LY3201) with a combination of global and targeted gene-expression profiling and the expression of key pluripotency markers. We found that ERß was induced in embryoid bodies (EBs) and neural precursor cells (NPCs) during development. Proliferation was higher in BERKO NPCs and was inhibited by LY3201. Neurogenesis was reduced in BERKO ES cells, and oligodendrogliogenesis was enhanced. BERKO EBs expressed higher levels of key ectodermal and neural progenitor markers and lower levels of markers for mesoderm and endoderm lineages. ERß-regulated factors are involved in cell adhesion, axon guidance, and signaling of Notch and GABA receptor pathways, as well as factors important for the differentiation of neuronal precursors into dopaminergic neurons (Engrailed 1) and for the oligodendrocyte fate acquisition (Olig2). Our data suggest that ERß is an important component for differentiation into midbrain neurons as well as for preventing precocious oligodendrogliogenesis.

TNF superfamily member APRIL enhances midbrain dopaminergic axon growth and contributes to the nigrostriatal projection in vivo.

We have studied the role of the tumor necrosis factor superfamily member APRIL in the development of embryonic mouse midbrain dopaminergic neurons in vitro and in vivo. In culture, soluble APRIL enhanced axon growth during a window of development between E12 and E14 when nigrostriatal axons are growing to their targets in the striatum in vivo. April transcripts were detected in both the striatum and midbrain during this period and at later stages. The axon growth-enhancing effect of APRIL was similar to that of glial cell-derived neurotrophic factor (GDNF), but in contrast to GDNF, APRIL did not promote the survival of midbrain dopaminergic neurons. The effect of APRIL on axon growth was prevented by function-blocking antibodies to one of its receptors, BCMA (TNFRSF13A), but not by function-blocking antibodies to the other APRIL receptor, TACI (TNFRSF13B), suggesting that the effects of APRIL on axon growth are mediated by BCMA. In vivo, there was a significant reduction in the density of midbrain dopaminergic projections to the striatum in April-/- embryos compared with wild type littermates at E14. These findings demonstrate that APRIL is a physiologically relevant factor for the nigrostriatal projection. Given the importance of the degeneration of dopaminergic nigrostriatal connections in the pathogenesis and progression of Parkinson's disease, our findings contribute to our understanding of the factors that establish nigrostriatal integrity.

Menthol Enhances Nicotine Reward-Related Behavior by Potentiating Nicotine-Induced Changes in nAChR Function, nAChR Upregulation, and DA Neuron Excitability.

Understanding why the quit rate among smokers of menthol cigarettes is lower than non-menthol smokers requires identifying the neurons that are altered by nicotine, menthol, and acetylcholine. Dopaminergic (DA) neurons in the ventral tegmental area (VTA) mediate the positive reinforcing effects of nicotine. Using mouse models, we show that menthol enhances nicotine-induced changes in nicotinic acetylcholine receptors (nAChRs) expressed on midbrain DA neurons. Menthol plus nicotine upregulates nAChR number and function on midbrain DA neurons more than nicotine alone. Menthol also enhances nicotine-induced changes in DA neuron excitability. In a conditioned place preference (CPP) assay, we observed that menthol plus nicotine produces greater reward-related behavior than nicotine alone. Our results connect changes in midbrain DA neurons to menthol-induced enhancements of nicotine reward-related behavior and may help explain how smokers of menthol cigarettes exhibit reduced cessation rates.

A novel BK channel-targeted peptide suppresses sound evoked activity in the mouse inferior colliculus.

Large conductance calcium-activated (BK) channels are broadly expressed in neurons and muscle where they modulate cellular activity. Decades of research support an interest in pharmaceutical applications for modulating BK channel function. Here we report a novel BK channel-targeted peptide with functional activity in vitro and in vivo. This 9-amino acid peptide, LS3, has a unique action, suppressing channel gating rather than blocking the pore of heterologously expressed human BK channels. With an IC50 in the high picomolar range, the apparent affinity is higher than known high affinity BK channel toxins. LS3 suppresses locomotor activity via a BK channel-specific mechanism in wild-type or BK channel-humanized Caenorhabditis elegans. Topical application on the dural surface of the auditory midbrain in mouse suppresses sound evoked neural activity, similar to a well-characterized pore blocker of the BK channel. Moreover, this novel ion channel-targeted peptide rapidly crosses the BBB after systemic delivery to modulate auditory processing. Thus, a potent BK channel peptide modulator is open to neurological applications, such as preventing audiogenic seizures that originate in the auditory midbrain.

Neurotropin® inhibits calpain activity upregulated by specific alternation of rhythm in temperature in the mesencephalon of rats.

AIMS: Neurotropin® (NTP), an analgesic for chronic pain, has antihyperalgesic effects in specific alternation of rhythm in temperature (SART)-stressed rats. Previous studies have shown that SART stress induces hyperalgesia, as well as post-translational modification of proteins (including substrates for calpain, a calcium-dependent cysteine protease) in the mesencephalon of rats. To better understand the mechanism of action of NTP, we investigated whether SART stress activates calpain in the mesencephalon of rats and whether NTP inhibits this activation. MAIN METHODS: Wistar rats were exposed to SART stress for 5days. NTP (200NU/kg/day) was administered intraperitoneally every day from the onset of SART stress. The mechanical pain threshold was measured using the Randall-Selitto test on the 6th day. Thereafter, the rat mesencephalon was immediately collected and calpain activity was examined using western blot analysis with a calpain cleavage site-specific antibody. KEY FINDINGS: SART stress induced hyperalgesia and increased the calpain activity in the mesencephalon of rats. In contrast, NTP treatment attenuated the hyperalgesia and prevented the increase in calpain activity in the mesencephalon of SART-stressed rats. Interestingly, a negative correlation was identified between calpain activity and mechanical pain threshold in SART-stressed rats treated with or without NTP. Furthermore, NTP inhibited calpain activity on mammalian uncoordinated-18 in rat mesencephalon homogenate and Ac-LLY-AFC as substrates in an in vitro cell-free system. SIGNIFICANCE: Our data demonstrate that NTP treatment prevents SART stress-induced calpain activation in the mesencephalon of rats and suggests that NTP-mediated antihyperalgesia is associated with an inhibition of calpain activity in the mesencephalon.

Differential brain responses to gradual intragastric nutrient infusion and gastric balloon distension: A role for gut peptides?

BACKGROUND: Rapid gastric balloon distension to discomfort threshold activates the "pain neuromatrix" and deactivates exteroceptive sensory and "default mode network" regions. However, little is known about brain mechanisms underlying tolerance of meal-induced gastric distension. We aimed to directly compare brain responses to gradual balloon distension and intragastric nutrient infusion and to explore the role of differential gut peptide release in these responses. MATERIALS AND METHODS: Brain responses to balloon- and nutrient-induced distension (to individually titrated pain or maximal satiation threshold) were measured in 15 healthy volunteers using H215O-PET on 2 separate days in counterbalanced order. The effects of increasing gastric distension and plasma levels of ghrelin and peptide YY3-36 (PYY3-36) on neural activity were assessed. RESULTS: Balloon distension progressively activated pain-responsive regions and deactivated exteroceptive sensory and "default mode network" areas. During nutrient infusion, "pain neuromatrix" regions and the orbitofrontal cortex were progressively deactivated, while the midbrain was activated. Plasma levels of PYY3-36 and ghrelin increased and decreased, respectively, during nutrient infusion only; decreasing ghrelin levels correlated with increasing midbrain activity. CONCLUSION: Different brain responses to gastric balloon distension and intragastric nutrient infusion are associated with nutrient-induced gut-brain signals, particularly to the midbrain, where these signals may interfere with both descending pain modulatory and mesolimbic reward processes. Deactivation of the "pain neuromatrix" during nutrient infusion may constitute the neurophysiological mechanism underlying the tolerance of normal meal volumes in health without induction of (painful) symptoms. Nutrient-induced deactivation of the orbitofrontal cortex may represent a key interoceptive meal termination signal.

Contralateral migration of oculomotor neurons is regulated by Slit/Robo signaling.

BACKGROUND: Oculomotor neurons develop initially like typical motor neurons, projecting axons out of the ventral midbrain to their ipsilateral targets, the extraocular muscles. However, in all vertebrates, after the oculomotor nerve (nIII) has reached the extraocular muscle primordia, the cell bodies that innervate the superior rectus migrate to join the contralateral nucleus. This motor neuron migration represents a unique strategy to form a contralateral motor projection. Whether migration is guided by diffusible cues remains unknown. METHODS: We examined the role of Slit chemorepellent signals in contralateral oculomotor migration by analyzing mutant mouse embryos. RESULTS: We found that the ventral midbrain expresses high levels of both Slit1 and 2, and that oculomotor neurons express the repellent Slit receptors Robo1 and Robo2. Therefore, Slit signals are in a position to influence the migration of oculomotor neurons. In Slit 1/2 or Robo1/2 double mutant embryos, motor neuron cell bodies migrated into the ventral midbrain on E10.5, three days prior to normal migration. These early migrating neurons had leading projections into and across the floor plate. In contrast to the double mutants, embryos which were mutant for single Slit or Robo genes did not have premature migration or outgrowth on E10.5, demonstrating a cooperative requirement of Slit1 and 2, as well as Robo1 and 2. To test how Slit/Robo midline repulsion is modulated, we found that the normal migration did not require the receptors Robo3 and CXCR4, or the chemoattractant, Netrin 1. The signal to initiate contralateral migration is likely autonomous to the midbrain because oculomotor neurons migrate in embryos that lack either nerve outgrowth or extraocular muscles, or in cultured midbrains that lacked peripheral tissue. CONCLUSION: Overall, our results demonstrate that a migratory subset of motor neurons respond to floor plate-derived Slit repulsion to properly control the timing of contralateral migration.

Effect of electrolytic lesions of the dorsal diencephalic conduction system on the distribution of Fos-like immunoreactivity induced by rewarding electrical stimulation.

The dorsal diencephalic conduction system (DDC) is an important pathway of the brain reward circuitry, linking together forebrain and midbrain structures. The present work was aimed at describing the effect of a DDC lesion on the distribution of Fos-like immunoreactivity (FLIR) following intracranial self-stimulation (ICSS) of the lateral hypothalamus (LH). Rats were implanted with monopolar electrodes and divided into three groups; the first two groups were trained to self-stimulate at the LH, whereas the third group received no stimulation and served as a control. Among the two groups that were trained for ICSS, one of them received a lesion at the DDC and was tested for ICSS on the subsequent 5days. On the last day of testing, control rats were placed in operant chambers without receiving any stimulation, and the remaining rats were allowed to receive the stimulation for 1h. All rats were then processed for FLIR. As previously shown, a lesion at the DDC resulted in significant attenuations of the rewarding effectiveness of LH stimulation. Results also show a higher FLIR in several reward-related areas following LH stimulation, especially in the hemisphere ipsilateral to the stimulation electrode. Compared to non-lesioned rats, lesioned animals had lower FLIR in certain brain regions, suggesting that those regions that were activated by the rewarding stimulation may be functionally interconnected with the DDC.

Behavioral and Physiological Effects of a Novel Kappa-Opioid Receptor-Based DREADD in Rats.

In the past decade, novel methods using engineered receptors have enabled researchers to manipulate neuronal activity with increased spatial and temporal specificity. One widely used chemogenetic method in mice and rats is the DREADD (designer receptors exclusively activated by designer drugs) system in which a mutated muscarinic G protein-coupled receptor is activated by an otherwise inert synthetic ligand, clozapine-N-oxide (CNO). Recently, the Roth laboratory developed a novel inhibitory DREADD in which a mutated kappa-opioid receptor (KORD) is activated by the pharmacologically inert drug salvinorin B (SalB; Vardy et al, 2015). They demonstrated the feasibility of using KORD to study brain circuits involved in motivated behavior in mice. Here, we used behavioral, electrophysiological, and neuroanatomical methods to demonstrate the feasibility of using the novel KORD to study brain circuits involved in motivated behavior in rats. In Exp. 1, we show that SalB dose-dependently decreased spontaneous and cocaine-induced locomotor activity in rats expressing KORD to midbrain (ventral tegmental area/substantia nigra). In Exp. 2, we show that SalB completely inhibited tonic firing in KORD-expressing putative dopamine neurons in midbrain. In Exp. 3, we used a 'retro-DREADD' dual-virus approach to restrict expression of KORD in ventral subiculum neurons that project to nucleus accumbens shell. We show that KORD activation selectively decreased novel context-induced Fos expression in this projection. Our results indicate that the novel KORD is a promising tool to selectively inactivate brain areas and neural circuits in rat studies of motivated behavior.

Electrosensory Midbrain Neurons Display Feature Invariant Responses to Natural Communication Stimuli.

Neurons that respond selectively but in an invariant manner to a given feature of natural stimuli have been observed across species and systems. Such responses emerge in higher brain areas, thereby suggesting that they occur by integrating afferent input. However, the mechanisms by which such integration occurs are poorly understood. Here we show that midbrain electrosensory neurons can respond selectively and in an invariant manner to heterogeneity in behaviorally relevant stimulus waveforms. Such invariant responses were not seen in hindbrain electrosensory neurons providing afferent input to these midbrain neurons, suggesting that response invariance results from nonlinear integration of such input. To test this hypothesis, we built a model based on the Hodgkin-Huxley formalism that received realistic afferent input. We found that multiple combinations of parameter values could give rise to invariant responses matching those seen experimentally. Our model thus shows that there are multiple solutions towards achieving invariant responses and reveals how subthreshold membrane conductances help promote robust and invariant firing in response to heterogeneous stimulus waveforms associated with behaviorally relevant stimuli. We discuss the implications of our findings for the electrosensory and other systems.