Oscillatory synchrony between head direction cells recorded bilaterally in the anterodorsal thalamic nuclei.

The head direction (HD) circuit is a complex interconnected network of brain regions ranging from the brain stem to the cortex. Recent work found that HD cells corecorded ipsilaterally in the anterodorsal nucleus (ADN) of the thalamus displayed coordinated firing patterns. A high-frequency oscillation pattern (130-160 Hz) was visible in the cross-correlograms of these HD cell pairs. Spectral analysis further found that the power of this oscillation was greatest at 0 ms and decreased at greater lags, and demonstrated that there was greater synchrony between HD cells with similar preferred firing directions. Here, we demonstrate that the same high-frequency synchrony exists in HD cell pairs recorded contralaterally from one another in the bilateral ADN. When we examined the cross-correlograms of HD cells that were corecorded bilaterally, we observed the same high-frequency (~150- to 200-Hz) oscillatory relationship. The strength of this synchrony was similar to the synchrony seen in ipsilateral HD cell pairs, and the degree of synchrony in each cross-correlogram was dependent on the difference in tuning between the two cells. Additionally, the frequency rate of this oscillation appeared to be independent of the firing rates of the two cross-correlated cells. Taken together, these results imply that the left and right thalamic HD network are functionally related despite an absence of direct anatomical projections. However, anatomical tracing has found that each of the lateral mammillary nuclei (LMN) project bilaterally to both of the ADN, suggesting the LMN may be responsible for the functional connectivity observed between the two ADN.NEW & NOTEWORTHY This study used bilateral recording electrodes to examine whether head direction cells recorded simultaneously in both the left and right thalamus show coordinated firing. Cross-correlations of the cells' spike trains revealed a high-frequency oscillatory pattern similar to that seen in cross-correlations between pairs of ipsilateral head direction cells, demonstrating that the bilateral thalamic head direction signals may be part of a single unified network.

Do the anterior and lateral thalamic nuclei make distinct contributions to spatial representation and memory?

The anterior and lateral thalamus has long been considered to play an important role in spatial and mnemonic cognitive functions; however, it remains unclear whether each region makes a unique contribution to spatial information processing. We begin by reviewing evidence from anatomical studies and electrophysiological recordings which suggest that at least one of the functions of the anterior thalamus is to guide spatial orientation in relation to a global or distal spatial framework, while the lateral thalamus serves to guide behavior in relation to a local or proximal framework. We conclude by reviewing experimental work using targeted manipulations (lesion or neuronal silencing) of thalamic nuclei during spatial behavior and single-unit recordings from neuronal representations of space. Our summary of this literature suggests that although the evidence strongly supports a working model of spatial information processing involving the anterior thalamus, research regarding the role of the lateral thalamus is limited and requires further attention. We therefore identify a number of major gaps in this research and suggest avenues of future study that could potentially solidify our understanding of the relative roles of anterior and lateral thalamic regions in spatial representation and memory.

Fornical and nonfornical projections from the rat hippocampal formation to the anterior thalamic nuclei.

The hippocampal formation and anterior thalamic nuclei form part of an interconnected network thought to support memory. A central pathway in this mnemonic network comprises the direct projections from the hippocampal formation to the anterior thalamic nuclei, projections that, in the primate brain, originate in the subicular cortices to reach the anterior thalamic nuclei by way of the fornix. In the rat brain, additional pathways involving the internal capsule have been described, linking the dorsal subiculum to the anteromedial thalamic nucleus, as well as the postsubiculum to the anterodorsal thalamic nucleus. Confirming such pathways is essential in order to appreciate how information is transferred from the hippocampal formation to the anterior thalamus and how it may be disrupted by fornix pathology. Accordingly, in the present study, pathway tracers were injected into the anterior thalamic nuclei and the dorsal subiculum of rats with fornix lesions. Contrary to previous descriptions, projections from the subiculum to the anteromedial thalamic nucleus overwhelmingly relied on the fornix. Dorsal subiculum projections to the majority of the anteroventral nucleus also predominantly relied on the fornix, although postsubicular inputs to the lateral dorsal part of the anteroventral nucleus, as well as to the anterodorsal and laterodorsal thalamic nuclei, largely involved a nonfornical pathway, via the internal capsule.

Sex steroid regulation of kisspeptin circuits.

Kisspeptin cells appear to be the "missing link," bridging the divide between levels of gonadal steroids and feedback control of gonadotropin-releasing hormone (GnRH) secretion. Kisspeptin neurons are important in the generation of both sex steroid negative and estrogen positive feedback signals to GnRH neurons, the former being involved in the tonic regulation of GnRH secretion in males and females and the latter governing the preovulatory GnRH/luteinizing hormone (LH) surge in females. In rodents, kisspeptin-producing cells populate the anteroventral periventricular nucleus (AVPV) and the arcuate nucleus (ARC), and estrogen regulation of kisspeptin has been extensively studied in these regions. Kisspeptin cells in the ARC appear to receive and forward signals applicable to negative feedback regulation of GnRH. In the female rodent AVPV, kisspeptin cells are important for positive feedback regulation of GnRH and the preovulatory LH surge. In sheep and primates, a rostral population of kisspeptin cells is located in the dorsolateral preoptic area (POA) as well as the ARC. Initial studies showed kisspeptin cells in the latter were involved in both the positive and negative feedback regulation of GnRH. Interestingly, further studies now suggest that kisspeptin cells in the ovine POA may also play an important role in generating estrogen positive feedback. This chapter discusses the current consensus knowledge regarding the interaction between sex steroids and kisspeptin neurons in mammals.

Frequency-dependent recruitment of fast amino acid and slow neuropeptide neurotransmitter release controls gonadotropin-releasing hormone neuron excitability.

The anteroventral periventricular nucleus (AVPV) is thought to play a key role in regulating the excitability of gonadotropin-releasing hormone (GnRH) neurons that control fertility. Using an angled, parahorizontal brain slice preparation we have undertaken a series of electrophysiological experiments to examine how the AVPV controls GnRH neurons in adult male and female mice. More than half (59%) of GnRH neurons located in the rostral preoptic area were found to receive monosynaptic inputs from the AVPV in a sex-dependent manner. AVPV stimulation frequencies <1 Hz generated short-latency action potentials in GnRH neurons with GABA and glutamate mediating >90% of the evoked fast synaptic currents. The AVPV GABA input was dominant and found to excite or inhibit GnRH neurons in a cell-dependent manner. Increasing the AVPV stimulation frequency to 5-10 Hz resulted in the appearance of additional poststimulus inhibitory as well as delayed excitatory responses in GnRH neurons that were independent of ionotropic amino acid receptors. The inhibition observed immediately following the end of the stimulation period was mediated partly by GABA(B) receptors, while the delayed activation was mediated by the neuropeptide kisspeptin. The latter response was essentially absent in Gpr54 knock-out mice and abolished by a Gpr54 antagonist. Together, these studies show that AVPV neurons provide direct amino acid and neuropeptidergic inputs to GnRH neurons. Low-frequency activation generates predominant GABA/glutamate release with higher frequency activation recruiting release of kisspeptin. This frequency-dependent release of amino acid and neuropeptide neurotransmitters greatly expands the range of AVPV control of GnRH neuron excitability.

Active and passive movement are encoded equally by head direction cells in the anterodorsal thalamus.

The head direction (HD) system is composed of cells that represent the direction in which the animal's head is facing. Each HD cell responds optimally when the head is pointing in a particular, or preferred, direction. Although vestibular system input is necessary to generate the directional signal, motor/proprioceptive inputs can also influence HD cell responses. Previous studies comparing active and passive movement have reported significant suppression of the HD signal during passive restraint. However, in each of these studies there was considerable variability across cells, and the animal's head was never completely fixed. To address these issues, we developed a passive restraint system that more fully prevented head and body movement. HD cell responses in the anterodorsal thalamus (ADN) were evaluated during active and passive movement with this new system. Contrary to previous reports, HD cell responses were not affected by passive restraint. Both head-fixed and hand-held restraint failed to produce significant inhibition of the active HD cell response. Furthermore, direction-specific firing was maintained regardless of 1) the animal's previous experience with restraint, 2) whether it was tested in the light or dark, or 3) the position of the animal relative to the axis of rotation. The maintenance of a stable directional signal without appropriate motor, proprioceptive, or visual input indicates that vestibular input is necessary and sufficient for the generation of the HD signal. Motor and proprioceptive influences may therefore be important for the control of the preferred firing direction of HD cells, but not the generation of the signal itself.

Intact landmark control and angular path integration by head direction cells in the anterodorsal thalamus after lesions of the medial entorhinal cortex.

The medial entorhinal cortex (MEC) occupies a central position within neural circuits devoted to the representation of spatial location and orientation. The MEC contains cells that fire as a function of the animal's head direction (HD), as well as grid cells that fire in multiple locations in an environment, forming a repeating hexagonal pattern. The MEC receives inputs from widespread areas of the cortical mantle including the ventral visual stream, which processes object recognition information, as well as information about visual landmarks. The role of the MEC in processing the HD signal or landmark information is unclear. We addressed this issue by neurotoxically damaging the MEC and recording HD cells within the anterodorsal thalamus (ADN). Direction-specific activity was present in the ADN of all animals with MEC lesions. Moreover, the discharge characteristics of ADN HD cells were only mildly affected by MEC lesions, with HD cells exhibiting greater anticipation of future HDs. Tests of landmark control revealed that HD cells in lesioned rats were capable of accurately updating their preferred firing directions in relation to a salient visual cue. Furthermore, cells from lesioned animals maintained stable preferred firing directions when locomoting in darkness and demonstrated stable HD cell tuning when locomoting into a novel enclosure, suggesting that MEC lesions did not disrupt the integration of idiothetic cues, or angular path integration, by HD cells. Collectively, these findings suggest that the MEC plays a limited role in the formation and spatial updating of the HD cell signal.

Head direction cell instability in the anterior dorsal thalamus after lesions of the interpeduncular nucleus.

Previous research has identified a population of cells throughout the limbic system that discharge as a function of the animal's head direction (HD). Altering normal motor cues can alter the HD cell responses and disrupt the updating of their preferred firing directions, thus suggesting that motor cues contribute to processing the HD signal. A pathway that conveys motor information may stem from the interpeduncular nucleus (IPN), a brain region that has reciprocal connections with HD cell circuitry. To test this hypothesis, we produced electrolytic or neurotoxic lesions of the IPN and recorded HD cells in the anterior dorsal thalamus (ADN) of rats. Direction-specific firing remained present in the ADN after lesions of the IPN, but measures of HD cell properties showed that cells had reduced peak firing rates, large directional firing ranges, and firing that predicted the animal's future heading more than in intact controls. Furthermore, preferred firing directions were moderately less influenced by rotation of a salient visual landmark. Finally, the preferred directions of cells in lesioned rats exhibited large shifts when the animals foraged for scattered food pellets in a darkened environment and when locomoting from a familiar environment to a novel one. We propose that the IPN contributes motor information about the animal's movements to the HD cell circuitry. Furthermore, these results suggest that the IPN plays a broad role in the discharge properties and stability of direction-specific activity in the HD cell circuit.

Synaptic properties of the mammillary and cortical afferents to the anterodorsal thalamic nucleus in the mouse.

Input to sensory thalamic nuclei can be classified as either driver or modulator, based on whether or not the information conveyed determines basic postsynaptic receptive field properties. Here we demonstrate that this distinction can also be applied to inputs received by nonsensory thalamic areas. Using flavoprotein autofluorescence imaging, we developed two slice preparations that contain the afferents to the anterodorsal thalamic nucleus (AD) from the lateral mammillary body and the cortical afferents arriving through the internal capsule, respectively. We examined the synaptic properties of these inputs and found that the mammillothalamic pathway exhibits paired-pulse depression, lack of a metabotropic glutamate component, and an all-or-none response pattern, which are all signatures of a driver pathway. On the other hand, the cortical input exhibits graded paired-pulse facilitation and the capacity to activate metabotropic glutamatergic responses, all features of a modulatory pathway. Our results extend the notion of driving and modulating inputs to the AD, indicating that it is a first-order relay nucleus and suggesting that these criteria may be general to the whole of thalamus.

[Peculiarities of neuronal sizes of anterior dorsal and reticular nuclei of the thalamus in women].

Profile field areas (PFA) were studied in 100 neurons in the anterior dorsal (ADN) and reticular nuclei (RN) of the thalamus in the left and right cerebral hemispheres of five women (10 hemispheres) aged from 19 to 33 years, using 20 microm-thick sections stained with Nissl's cresyl violet. Morphometric method using Image Scope Color <> program-device complex (Germany) was applied. <> (Wilcoxon matched pairs test) was used for the analysis of the data obtained. Greater variability of neuronal maximal PFA was detected in both nuclei in the right hemisphere. PFA average value was greater in RN than in ADN, and this difference was more significant in the left hemisphere in all women. The variability of average PFA value was greater in RN than in ADN, however in both nuclei it was greater on the right than on the left. Neuronal PFA variability was more expressed than its interhemispheric asymmetry.

Proximal perimeter encoding in the rat rostral thalamus.

Perimeters are an important part of the environment, delimiting its geometry. Here, we investigated how perimeters (vertical walls; vertical drops) affect neuronal responses in the rostral thalamus (the anteromedial and parataenial nuclei in particular). We found neurons whose firing patterns reflected the presence of walls and drops, irrespective of arena shape. Their firing patterns were stable across multiple sleep-wake cycles and were independent of ambient lighting conditions. Thus, rostral thalamic nuclei may participate in spatial representation by encoding the perimeters of environments.

Long-range inhibitory intersection of a retrosplenial thalamocortical circuit by apical tuft-targeting CA1 neurons.

Hippocampus, granular retrosplenial cortex (RSCg), and anterior thalamic nuclei (ATN) interact to mediate diverse cognitive functions. To identify cellular mechanisms underlying hippocampo-thalamo-retrosplenial interactions, we investigated the potential circuit suggested by projections to RSCg layer 1 (L1) from GABAergic CA1 neurons and ATN. We find that CA1→RSCg projections stem from GABAergic neurons with a distinct morphology, electrophysiology, and molecular profile. Their long-range axons inhibit L5 pyramidal neurons in RSCg via potent synapses onto apical tuft dendrites in L1. These inhibitory inputs intercept L1-targeting thalamocortical excitatory inputs from ATN to coregulate RSCg activity. Subicular axons, in contrast, excite proximal dendrites in deeper layers. Short-term plasticity differs at each connection. Chemogenetically abrogating CA1→RSCg or ATN→RSCg connections oppositely affects the encoding of contextual fear memory. Our findings establish retrosplenial-projecting CA1 neurons as a distinct class of long-range dendrite-targeting GABAergic neuron and delineate an unusual cortical circuit specialized for integrating long-range inhibition and thalamocortical excitation.

Single-Cell Recordings to Target the Anterior Nucleus of the Thalamus in Deep Brain Stimulation for Patients with Refractory Epilepsy.

Deep brain stimulation (DBS) of the anterior nucleus of the thalamus (ANT) is a promising treatment for patients with refractory epilepsy. However, therapy response varies and precise positioning of the DBS lead is potentially essential for maximizing therapeutic efficacy. We investigate if single-cell recordings acquired by microelectrode recordings can aid targeting of the ANT during surgery and hypothesize that the neuronal firing properties of the target region relate to clinical outcome. We prospectively included 10 refractory epilepsy patients and performed microelectrode recordings under general anesthesia to identify the change in neuronal signals when approaching and transecting the ANT. The neuronal firing properties of the target region, anatomical locations of microelectrode recordings and active contact positions of the DBS lead along the recorded trajectory were compared between responders and nonresponders to DBS. We obtained 19 sets of recordings from 10 patients (five responders and five nonresponders). Amongst the 403 neurons detected, 365 (90.6%) were classified as bursty. Entry into the ANT was characterized by an increase in firing rate while exit of the ANT was characterized by a decrease in firing rate. Comparing the trajectories of responders to nonresponders, we found differences neither in the neuronal firing properties themselves nor in their locations relative to the position of the active contact. Single-cell firing rate acquired by microelectrode recordings under general anesthesia can thus aid targeting of the ANT during surgery, but is not related to clinical outcome in DBS for patients with refractory epilepsy.

Combined blockade of serotonergic and muscarinic transmission disrupts the anterior thalamic head direction signal.

Head direction (HD) cells have been speculated to be part of a network mediating navigational behavior. Previous work has shown that combined administration of serotonergic and muscarinic antagonists eliminates hippocampal theta activity and produces navigational deficits more severe than blockade of either neurotransmitter system alone. The authors sought to assess this effect on the directional characteristics of HD cells. HD cells were recorded from the anterior dorsal thalamus of Long-Evans rats before and after administration of the serotonergic antagonist methiothepin, the muscarinic antagonist scopolamine, both drugs, or saline. Combined drug administration produced HD cells with preferred directions that drifted within recording sessions. In addition, cells showed shifts in the preferred directions at the start of a session relative to the position of the major landmarks, suggesting that combined drug administration led to deficits in landmark control of the HD system. Single drug exposures to methiothepin or scopolamine did not noticeably affect the directional characteristics of HD cells. This finding that navigation-impairing drugs can disrupt the HD signal provides further evidence that this network plays an important role in navigational behavior.

Morphology and synaptic organization of non-dopaminergic nigral projections to the medio dorsal thalamic nucleus of the rat, a study by anterograde transport of PHA-L.

BACKGROUND: Mediodorsal (MD) thalamic nucleus, which is considered to take place between extra pyramidal and limbic feedback circuit, receives projective fibers from ventrolateral neurons of reticular part of substantia nigra (SNr). In order to better understand the influence and chemical reaction of these fibers upon MD nucleus, the morphology and synaptology of them were examined in the present study. METHODS: Phaseolous vulgaris-leucoagglutin (PHA-L) was injected into substantia nigra pars reticulate. After 3-4 days, the sections of SNr injection site and MD nucleus were prepared. Then, we examined organization, morphology and, synaptology of PHA-L labeled SNr fibers that go to caudal and lateral part of MD thalamic nucleus. RESULTS: At the electron microscopic level, the SNr terminals made synapses predominantly with the medium to small dendrites and far less frequently with soma and large dendrites. These terminals were packed with polymorphic synaptic vesicles and formed symmetrical synapses; furthermore, it has been already recognized that cortico straital fibers from sensory-motor cortex go to region of the SNr that give rise to the nigrothalamic fibers. CONCLUSION: This data suggest that upon the synaptic organization, morphology and chemical nature of GABAergic, SNr fibers may have different inhibitory influence on MD neurons regulating the thalamic output from MD to cerebral cortex in the control of limbic and extra pyramidal feedback system.

Collateral Projections Innervate the Mammillary Bodies and Retrosplenial Cortex: A New Category of Hippocampal Cells.

To understand the hippocampus, it is necessary to understand the subiculum. Unlike other hippocampal subfields, the subiculum projects to almost all distal hippocampal targets, highlighting its critical importance for external networks. The present studies, in male rats and mice, reveal a new category of dorsal subiculum neurons that innervate both the mammillary bodies (MBs) and the retrosplenial cortex (RSP). These bifurcating neurons comprise almost half of the hippocampal cells that project to RSP. The termination of these numerous collateral projections was visualized within the medial mammillary nucleus and the granular RSP (area 29). These collateral projections included subiculum efferents that cross to the contralateral MBs. Within the granular RSP, the collateral projections form a particularly dense plexus in deep Layer II and Layer III. This retrosplenial termination site colocalized with markers for VGluT2 and neurotensin. While efferents from the hippocampal CA fields standardly collateralize, subiculum projections often have only one target site. Consequently, the many collateral projections involving the RSP and the MBs present a relatively unusual pattern for the subiculum, which presumably relates to how both targets have complementary roles in spatial processing. Furthermore, along with the anterior thalamic nuclei, the MBs and RSP are key members of a memory circuit, which is usually described as both starting and finishing in the hippocampus. The present findings reveal how the hippocampus simultaneously engages different parts of this circuit, so forcing an important revision of this network.

Differential activation of anterior and midline thalamic nuclei following retrieval of aversively motivated learning tasks.

Two thalamic nuclear groups, the anterior thalamic nuclei (ATN) and midline and intralaminar thalamic complex (MITC) have connections to the prefrontal cortex, amygdala, hippocampus and accumbens that are important for learning and memory. However, the anatomical proximity between the ATN and MITC makes it difficult to reveal their roles in memory retrieval of aversive conditioned behavior. To address the issue, we explored the activation of the ATN and MITC, as represented by the expression of the immediate early gene c-fos, following either the retrieval of a conditioned taste aversion (CTA) induced by taste-LiCl pairing (visceral aversion) or of inhibitory avoidance (IA) induced by context-foot shock pairing (somatic aversion) in rats. The anterodorsal (AD) nucleus in the ATN was activated by foot shock and the recall of IA, but not by i.p. injection of LiCl or the recall of CTA. No significant elevation was observed in the other ATN following these treatments. Among nuclei of the MITC, the paraventricular thalamic nucleus (PVT) was activated by the delivery of shock or LiCl and by the recall of both CTA and IA, while the mediodorsal thalamus (MD) and central medial and intermediate thalamus (CM/IMD) were not. The innately aversive taste of quinine did not elevate c-fos expression in either the ATN or MITC. These results suggest that the PVT in the MITC is involved in the processing and retrieval of both taste-malaise and context-shock association tasks, while the AD in the ATN is involved in those of context-shock association only. The difference of the activity between the ATN and MITC demonstrates their functional and anatomical heterogeneity in neural substrates for aversive learning tasks.

Degradation of head direction cell activity during inverted locomotion.

Head direction (HD) cells in the rat limbic system carry information about the direction the head is pointing in the horizontal plane. Most previous studies of HD functioning have used animals locomoting in an upright position or ascending/descending a vertical wall. In the present study, we recorded HD cell activity from the anterodorsal thalamic nucleus while the animal was locomoting in an upside-down orientation. Rats performed a shuttle-box task requiring them to climb a vertical wall and locomote across the ceiling of the apparatus while inverted to reach an adjoining wall before ascending into the reward compartment. The apparatus was oriented toward the preferred direction of the recorded cell, or the 180 degrees opposite direction. When the animal was traversing the vertical walls of the apparatus, the HD cells remained directionally tuned as if the walls were an extension of the floor. When the animal was locomoting inverted on the ceiling, however, cells showed a dramatic change in activity. Nearly one-half (47%) of the recorded cells exhibited no directional specificity during inverted locomotion, despite showing robust directional tuning on the walls before and after inversion. The remaining cells showed significantly degraded measures of directional tuning and random shifts of the preferred direction relative to the floor condition while the animal was inverted. It has previously been suggested that the HD system uses head angular velocity signals from the vestibular system to maintain a consistent representation of allocentric direction. These findings suggest that being in an inverted position causes a distortion of the vestibular signal controlling the HD system.

Rapid spatial reorientation and head direction cells.

It is surprising how quickly we can find our bearings when suddenly confronted with a familiar environment, for instance when the lights are turned on in a dark room. Subjectively, this appears to occur almost instantaneously, yet the neural processes permitting this rapid reorientation are unknown. A likely candidate is the head direction (HD) cell system. These limbic neurons found in several brain regions, including the thalamus and the hippocampus, discharge selectively when the head of an animal is oriented in a particular ("preferred") direction. This neuronal activity is independent of position and ongoing behavior and is thus likely to constitute a physiological basis for the sense of direction. Remarkably, although the HD cell system has properties resembling those of a compass, it is independent of geomagnetic fields. Rather, the preferred directions of the HD cells are strongly anchored to visual cues in the environment. Here, we bring evidence for the first time that a fundamental component of the capacity to rapidly reorient in a familiar environment may be brought about by updating of HD cell responses as rapidly as 80 msec after changes in the visual scene. Continuous attractor networks have been used successfully to model HD cell ensemble dynamics. The present results suggest that after large rotations of the surrounding landmarks, activity in such networks may be propagated in abrupt jumps rather than in a gradually progressive manner.

Spatial working memory and the brainstem cholinergic innervation to the anterior thalamus.

The anteroventral thalamic nucleus (AV) has a role in spatial memory, but the influence of the prominent brainstem cholinergic projection to this region is unknown. Here, spatial memory in a 12-arm radial maze was examined after 0.15 microl bilateral AV infusions of scopolamine. In part one, rats visited six arms singly (the phase 1 arms) and, after a 10 min delay, were allowed free choice to both phase 1 arms and the remaining six baited arms (phase 2 arms). Scopolamine (10 microg) administered during the delay increased errors to both phase 1 and phase 2 arms, whereas PBS infusions increased phase 1 arm errors only. The PBS effect was the result of inserting the internal cannulas alone and not the infusion. The same dose of scopolamine (10 microg) infused before maze testing (part two: no phase 1 arms, no delay) also impaired spatial memory over and above the effects of both PBS and no-infusion, which did not differ markedly. Part two also showed that choice latency and choice strategies were unaffected by PBS and scopolamine. Cannulation and infusion procedures in both parts one and two did not produce any negative carryover effects across multiple control (no internal cannula) sessions, and a trypan blue manipulation indicated that infusions were restricted to the AV region. This study provides the first direct evidence that the brainstem cholinergic innervation to the limbic thalamus influences learning and memory, which may have important implications for human neurological conditions such as alcohol-related disorders and schizophrenia.