Medial Division Research Articles

Separate sets of neurons of the central nucleus of the amygdala project to the substantia innominata and the caudal pontine reticular nucleus in the rat

The central nucleus of the amygdala (CeA) is generally regarded as a control nucleus of subcortical target systems. Due to its widespread projections to different brain areas it is able to modulate emotional behavior of the organism. However, it is still not clear whether single neurons of the CeA project to different areas or to one target area. Injections of the retrograde tracers Fluorogold and True Blue into target regions of the central nucleus of the amygdala, i.e., the substantia innominata (SI) and the caudal pontine reticular nucleus (PNC), revealed overlapping but otherwise distinct neuronal populations within mainly the medial division of the CeA. From our study we conclude that SI and PNC receive input from different subsets of amygdala neurons.

The Hypothalamic-Pituitary-Adrenal Axis in the Neuroendocrine Regulation of Food Intake and Obesity: The Role of Corticotropin Releasing Hormone

The aim of this paper is to review the present knowledge on the role of the hypothalamic-pituitary-adrenal axis in the control of food intake and the pathogenesis of obesity and to discuss, on the basis of available literature, the interactions between other neurosystems and this hormonal axis. Food intake is influenced by a system of physiologic signals and behavioral controls consisting of positive and negative sensory feedback mechanisms. It is regulated by a complex neuroendocrine system consisting of peripheral signals (cortisol, leptin) in constant interplay with central neurosystems such as the cocaine-amfetamine-regulated transcript system. In these neurosystems, corticotropin-releasing hormone, pro-opiomelanocortin, melanin-concentrating hormone and neuropeptide Y are actively involved. The corticotropin-releasing hormone system is widely distributed throughout the brain, but it is particularly abundant in the medial parvocellular division of the paraventricular nucleus. Within the brain corticotropin-releasing hormone with its two receptor types, its binding protein and its closely related peptide urocortin forms a network of neuronal pathways capable of interacting with other circuitries controlling food intake and sympathetically-mediated thermogenesis. A defect in the synthesis and release of corticotropin-releasing hormone has been implicated in the development of obesity in laboratory animals. This condition is alleviated by exogenous corticotropin-releasing hormone treatment. The relationship between the neuropeptide Y system and the hypothalamic-pituitary-adrenal axis is complex and seems to include positive feedback between neuropeptide Y and corticosteroids and negative feedback between corticotropin-releasing hormone and neuropeptide Y. Leptin is involved in the regulation of energy balance by interacting with the hypothalamic-pituitary-adrenal axis. In the past, we have shown by cross-correlation analysis, that under physiological conditions cortisol and plasma leptin levels are related to each other in a time-related negative and positive fashion over 24 h.

Topographic distribution of direct and hippocampus- mediated entorhinal cortex activity evoked by olfactory tract stimulation.

Olfactory information is central for memory-related functions, such as recognition and spatial orientation. To understand the role of olfaction in learning and memory, the distribution and propagation of olfactory tract-driven activity in the parahippocampal region needs to be characterized. We recently demonstrated that repetitive stimulation of the olfactory tract in the isolated guinea pig brain preparation induces an early direct activation of the rostrolateral entorhinal region followed by a delayed response in the medial entorhinal cortex (EC), preceded by the interposed activation of the hippocampus. In the present study we performed a detailed topographic analysis of both the early and the delayed entorhinal responses induced by patterned stimulation of the lateral olfactory tract in the isolated guinea pig brain. Bi-dimensional maps of EC activity recorded at 128 recording sites with 4 x 4 matrix electrodes (410 microm interlead separation) sequentially placed in eight different positions, showed (i) an early (onset at 16.09 +/- 1.2 ms) low amplitude potential mediated by the monosynaptic LOT input, followed by (ii) an associative potential in the rostral EC which originates from the piriform cortex (onset at 33.2 +/- 2.3 ms), and (iii) a delayed potential dependent on the previous activation of the hippocampus. The sharp component of the delayed response had an onset latency between 52 and 63 ms and was followed by a slow wave. Laminar profile analysis demonstrated that in the caudomedial EC the delayed response was associated with two distinct current sinks located in deep and in superficial layers, whereas in the rostrolateral EC a small-amplitude sink could be detected in the superficial layers exclusively. The present report demonstrates that the output generated by the hippocampal activation is unevenly distributed across different EC subregions and indicates that exclusively the medial and caudal divisions receive a deep-layer input from the hippocampus. In the rostrolateral EC, specific network interactions may be generated by the convergence of the direct olfactory input and the olfaction-driven hippocampal output.

Projections from the amygdaloid complex to the piriform cortex: A PHA-L study in the rat.

Projections from the amygdala to the piriform cortex are proposed to provide a pathway via which the emotional system can modulate the processing of olfactory information as well as mediate the spread of seizure activity in epilepsy. To understand the details of the distribution and topography of these projections, we injected the anterograde tracer Phaseolus vulgaris-leucoagglutinin into different nuclear divisions of the amygdaloid complex in 101 rats and analyzed the distribution and density of projections in immunohistochemically processed preparations. The heaviest projections from the amygdala to the piriform cortex originated in the medial division of the lateral nucleus, the periamygdaloid and sulcal subfields of the periamygdaloid cortex, and the posterior cortical nucleus. The heaviest terminal labeling was observed in layers Ib and III of the medial aspect of the posterior piriform cortex. Lighter projections to the posterior piriform cortex originated in the dorsolateral division of the lateral nucleus, the magnocellular and parvicellular divisions of the basal and accessory basal nuclei, and the anterior cortical nucleus. The projections to the anterior piriform cortex were light and originated in the dorsolateral and medial divisions of the lateral nucleus, the magnocellular division of the basal and accessory basal nuclei, the anterior and posterior cortical nuclei, and the periamygdaloid subfield of the periamygdaloid cortex. The results indicate that only selective amygdaloid nuclei or their subdivisions project to the piriform cortex. In addition, substantial projections from several amygdaloid nuclei converge in the medial aspect of the posterior piriform cortex. Via these projections, the amygdaloid complex can modulate the processing of olfactory information in the piriform cortex. In pathologic conditions such as epilepsy, these connections might provide pathways for the spread of seizure activity from the amygdala to extra-amygdaloid regions.

Cytoarchitectonic and connectional organization of the ventral lateral geniculate nucleus in the cat.

The ventral lateral geniculate nucleus is a small extrageniculate visual structure that has a complex cytoarchitecture and diverse connections. In addition to small-celled medial and lateral divisions, we cytoarchitectonically defined a small-celled dorsal division. A large-celled intermediate division intercalated between the three small-celled divisions, which we divided into medial and lateral intermediate subdivisions. In WGA-HRP injection experiments, the different cytoarchitectonic divisions were shown to have connections with different nuclei. The medial division was reciprocally connected to the pretectum and projected to the superficial layers of the superior colliculus and the intralaminar nuclei. The medial intermediate division received projections from the intermediate layer of the superior colliculus and the lateral and interpositus posterior cerebellar nuclei, and projected to the intermediate layer of the superior colliculus, the periaqueductal gray of midbrain, and the intralaminar nuclei. The lateral intermediate divisions received projections from the pretectum, the intermediate layer of the superior colliculus, and the lateral and interpositus posterior cerebellar nuclei, and projected to the pretectum, superficial layers of the superior colliculus, and the pulvinar. The lateral division received projections from superficial layers of the superior colliculus and had reciprocal connections with the pretectum. The dorsal division received projections from the pretectum and had reciprocal connections with the periaqueductal gray of midbrain. The different cytoarchitectonic divisions of the ventral lateral geniculate nucleus are thus suggested to play different functional roles related to vision, eye and head movements, attention, and defensive reactions.

Regional brainstem expression of Fos associated with sexual behavior in male rats

This study utilized Fos expression to map the distribution of activated cells in brainstem areas following masculine sexual behavior. Males displaying both appetitive and consumatory sexual behaviors (Cop) were compared to animals prevented from copulation (NC) and to socially isolated (SI) animals. Following copulation, Fos was preferentially augmented in the caudal ventral medulla (CVM), a region mediating descending inhibition of penile reflexes, and which may be regulated by a forebrain circuit that includes the medial preoptic area (MPOA). Copulation-induced Fos was observed in the medial divisions of both the dorsal cochlear nucleus (DC) and trapezoid bodies (Tz), areas which are part of a circuit processing auditory information. In addition, the medullary linear nucleus (Li) displayed comparable amounts of Fos in Cop and NC as compared to the SI animals. Other regions of the pontomedullary reticular system, which may mediate sleep and arousal, did not exhibit Fos expression associated with consumatory sexual behavior. We suggest that Fos is associated with the inhibition of sexual behavior following ejaculation in the CVM, and that auditory information arising from the DC and Tz is combined with copulation-related sensory information in the subparafasicular nucleus and projected to the hypothalamus. In addition, equal amounts of Fos expression observed in the Li in both the Cop and NC animals suggests that this region is involved in sexual arousal. Overall, the data suggest that processing by brainstem nuclei directly contributes to the regulation of mating behavior in male rats.

Ionic mechanism of long-lasting discharges of action potentials triggered by membrane hyperpolarization in the medial lateral habenula.

The activation of inhibitory synapses typically suppresses the generation of action potentials by hyperpolarizing the membrane of postsynaptic cells. In contrast to such conventional action of inhibitory synapses, we report here the ionic mechanism through which hyperpolarizing synapses trigger long-lasting discharges of action potentials that persist up to several tens of seconds. By using extracellular and intracellular recordings in slice preparations, we demonstrate that the activation of synaptic input from the limbic forebrain generates transient hyperpolarizing postsynaptic potentials in neurons of the medial part of the lateral habenular nucleus of the epithalamus. The synaptic hyperpolarization then sets off the coordinated activation of a distinct set of membrane ion channels and intracellular Ca2+ mobilization by internal stores. The activation of these cellular events in distinct temporal order drives a persistent depolarization of habenular cells and promotes long-lasting discharges of tonic action potentials. The cells in the medial division of the lateral habenula project to dopamine and serotonin cells in the midbrain. We suggest that these habenular cells, by generating persistent action potentials in response to a transient increase in the activity of the limbic forebrain, may contribute to the regulation of the serotonergic and dopaminergic activity in the brain.

The dual pattern of corticothalamic projection of the primary auditory cortex in macaque monkey

The distribution and terminal morphology of the corticothalamic projection originating from the primary auditory cortex (A1) were established in a macaque monkey, using the anterograde (and retrograde) tracer biotinylated dextran amine. A dense corticothalamic projection from A1 was found in the ventral (vMGB) and dorsal (dMGB) divisions of the medial geniculate body and, to a lesser extent, in the medial division (mMGB), the posterior thalamic nucleus (PO) and the suprageniculate nucleus. Most terminal boutons were small (<1 μm), except some large boutons (2–6 μm) located in PO and vMGB. The data demonstrate that the corticothalamic projection from A1 in primate consists of two types of terminals (small and giant endings) in line with previous observations in rat and cat. Retrogradely labeled thalamocortical neurons formed clusters generally overlapping the corticothalamic terminal fields.

Chronic Low Back Pain of Facet (Zygapophysial) Joint Origin: Is There a Difference Based on Involvement of Single or Multiple Spinal Regions?

Facet (zygapophysial) joint pain can be diagnosed by anesthetization of the medial branch divisions of the dorsal rami. In accordance with the criteria established by the International Association for the Study of Pain, lumbar facet (zygapophysial) joints have been implicated as the source of chronic pain in 15% to 45% of the patients with chronic low back pain. The reasons for the wide variations have not been systematically evaluated. This study was designed to determine the prevalence of facet (zygapophysial) joint pain in patients suffering with only low back pain and compare this prevalence to the prevalence in patients with painful involvement of multiple regions of the spine. A total of 300 patients in an interventional pain management setting, presenting and undergoing diagnostic interventional procedures consecutively, either with involvement of a single region (low back only) or multiple spinal regions (low back pain and neck pain or thoracic pain) were evaluated. There were 150 patients in each group. All patients were treated with diagnostic medial branch blocks with 1% lidocaine to test the presence of facet joint pain. Lidocaine-positive patients underwent a subsequent confirmatory block with 0.25% bupivacaine. Medial branches were blocked at two levels to block a single joint. Prevalence of lumbar facet joint pain in patients with low back only was 21%, compared to 41% of the patients with low back pain with involvement of other regions of the spine with controlled comparative local anesthetic blocks. A false-positive rate of 17% in patients with low back pain only and 21% in patients with involvement of multiple regions of the spine was demonstrated with single blocks. This study demonstrated a lower incidence of facet joint pain in patients with spinal pain of a single region in the low back compared to the patients with multiple region involvement of the spine (21% vs 41%), in an interventional pain management setting. These results may not be extrapolated to the general population or chronic low back pain population at large.

Blink-related sensorimotor anatomy in the rat.

Protection of the eye and maintenance of the precorneal tear film depend on sensory innervation of the cornea and eyelids and motor innervation of muscles involved in closing and opening the eyes. Using a variety of fluorescent and transganglionic tracers, the sensorimotor innervation of blink-related orbital and periorbital structures was studied in Sprague-Dawley rats. The orbicularis oculi muscle surrounded the entire palpebral fissure and was innervated by motoneurons located along the dorsal cap of the ipsilateral facial motor nucleus. Upper and lower eyelid orbicularis oculi motoneurons were strictly ipsilateral and co-extensive, but upper eyelid orbicularis oculi motoneurons were, on average, slightly rostral and lateral to lower eyelid orbicularis oculi motoneurons. Facial motoneurons supplying the frontoscutularis, a muscle that helps to elevate the upper eyelid, were located in the medial division of the ipsilateral facial motor nucleus. Presumptive type Abeta afferents from the cornea terminated most prominently at the junction of the first cervical segment and the spinal trigeminal nucleus, pars caudalis. There was a second concentration of corneal terminations at the junction of pars caudalis and pars interpolaris of the spinal trigeminal nucleus. Sparse projections to the spinal trigeminal nucleus, pars oralis and the principal trigeminal nucleus were also detected. Presumptive type Abeta afferents from the eyelids terminated throughout the rostrocaudal extent of the spinal trigeminal nucleus with a heavy concentration within laminae III and IV of the first cervical segment. Presumptive types Adelta and C terminals from the eyelids were virtually limited to laminae I and II of the first cervical segment. Central terminations from the frontal nerve were present in the principal trigeminal nucleus and throughout the spinal trigeminal nucleus, but were most prominent within the dorsal horn of the first cervical segment. Our comprehensive description of blink-related sensorimotor anatomy in rats will provide a foundation for future physiological studies of blinking.

Sex-specific patterns of galanin, cholecystokinin, and substance P expression in neurons of the principal bed nucleus of the stria terminalis are differentially reflected within three efferent preoptic pathways in the juvenile rat.

Neurons in the principal bed nucleus of the stria terminalis (BSTp) integrate hormonal and sensory information associated with reproduction and transmit this information to hypothalamic nuclei that regulate neuroendocrine and behavioral functions. The neuropeptides galanin (GAL), cholecystokinin (CCK), and substance P (SP) are highly expressed in BSTp neurons and are differentially regulated by sex steroids. The current experiments investigated whether developmental or peripubertal hormone-mediated changes in GAL, CCK, and SP expression are reflected within efferent pathways to the preoptic structures that regulate gonadotropin secretion and sexual behavior. Anterograde labeling of projections from the BSTp of male and female juvenile rats combined with immunohistochemical labeling of GAL-, CCK-, and SP-containing fibers in the anteroventral periventricular preoptic nucleus (AVPV) and the central and medial divisions of the medial preoptic nucleus (MPNc, MPNm, respectively) revealed unique sex differences in each region. In the AVPV, Phaseolus vulgaris leucoagglutinin-labeled fibers were seen at a greater density in males than in females, and higher percentages of these fibers contained GAL in males than in females. In contrast, fibers projecting from the BSTp to the MPNc were more likely to contain SP in females than in males. Treatment of gonadectomized, peripubertal males and females with exogenous testosterone and estradiol did not alter the densities of GAL-, CCK-, or SP-containing fibers in any of the three brain areas examined. Collectively, these results suggest that patterns of neuropeptide expression in BSTp projections are established during development, resulting in a distinct, stable, and sex-specific chemoarchitectural profile for each projection pathway.

Differential neuronal activation in the hypothalamic paraventricular nucleus and autonomic/neuroendocrine responses to I.C.V. endotoxin

The paraventricular nucleus (PVN) of the hypothalamus is a key site for regulating neuroendocrine and autonomic activities. To study the role of the PVN activation in brain inflammation-induced autonomic/endocrine responses, lipopolysaccharide (LPS; 0.5 or 5 μg) was administered i.c.v. and rats were killed 1, 3 or 6 h after the injection. I.c.v. LPS-0.5 μg did not cause changes in mean arterial pressure (MAP) over 6 h, whereas LPS-5 μg induced a temporary decrease in MAP approximately 30 min after the injection. LPS at either dose increased heart rate. Whereas induction of Fos-like immunoreactivity was confined to the dorsal medial parvocellular division (mpd) of the PVN with the lower dose, labeling was found throughout the PVN with the higher dose. At 3 h, LPS-5 μg also stimulated increases in arginine vasopressin (AVP) heteronuclear RNA levels in the posterior magnocellular and dorsal parvocellular divisions of the PVN at 3 h, and activation of catecholaminergic neurons in the hypothalamus and brainstem. Increases in tyrosine hydroxylase (TH) mRNA levels were found in the locus coeruleus at 6 h. LPS at both doses elevated plasma ACTH levels and corticotropin-releasing factor gene expression in the mpd of the PVN. I.c.v. LPS induced IL-1β mRNA in the meninges and ventricular ependymal lining at 1 h, and in the periventricular PVN at 3 h. Induction of IL-1β mRNA was found in the lung at 1 h, and a significant increase in plasma LPS binding protein occurred at 3 h. These findings suggest that PVN activation induced by the lower dose of LPS is related primarily to increases in activity of the HPA axis, whereas the higher dose of LPS more widely activates autonomic regulatory centers including the PVN and also stimulates changes in sympathetic output and hypothalamic AVP synthesis. Activation of the PVN by i.c.v. LPS likely occurs through both central and systemic routes. Differential neuronal activation in the PVN is functionally related to autonomic/endocrine responses elicited by brain inflammation.

Discrimination of striatopallidum and extended amygdala in the rat: a role for parvalbumin immunoreactive neurons?

Synaptic effects of parvalbumin-immunoreactive (-ir) interneurons (PVs) upon medium spiny neurons may be essential to neural processing in the striatum and, in effect, may serve as an additional feature distinguishing striatum from extended amygdala. The present immunohistochemical study in the rat was done to evaluate the distributions of PVs in the striatum and extended amygdala. Numerous PVs occupy all structures currently regarded as having a striatal composition, including the caudate-putamen, nucleus accumbens, and olfactory tubercle, as well as structures that receive outputs from these, including the globus pallidus, ventral pallidum, entopeduncular nucleus and substantia nigra reticulata. The morphologies of striatal PVs and their distribution were similar to what has been previously reported. In addition, we found that the density of larger neostriatal PVs with extensive and densely immunoreactive dendritic and local axonal arbors is greatest laterally, particularly in striatal districts with slight calbindin-ir, including the striatal patch compartment. In contrast to the situation in striatum, few PVs were observed in the central and medial divisions of the extended amygdala, including the bed nucleus of stria terminalis, interstitial nucleus of the posterior limb of the anterior commissure and central and medial nuclei of the amygdala, or in mesopontine, peribrachial and medullary structures that receive extended amygdala output. The paucity of PVs may be a characteristic feature distinguishing extended amygdala and its projection areas from striatopallidum, as well as the general character of neural processing that occurs in each.

Extended amygdala and basal forebrain.

The basal forebrain is a confluence of systems that are crucial to understanding some of the most important functions of the brain, including reward and punishment, learning and cognition, and feeding and reproduction. Basic to understanding this broad spectrum of behavior is untangling the interwoven functional systems in basal forebrain. This has been grounded by the appreciation that the major nearby structures, that is, amygdala and basal ganglia, provide a context for interpreting basal forebrain areas that are best viewed as extensions of either of these larger regions. The components of basal forebrain, the ventral striatopallidal system and the medial and central divisions of extended amygdala, are subcortical relays for information garnered from brain stem, thalamus, and cortical areas. With respect to the classically defined amygdala of the temporal lobe, the lateral-basolateral complex, and the superficial amygdaloid nuclei may tentatively be viewed as specialized cortical regions. Their output targets both the striatopallidal complex and the extended amygdala, with some of the most massive basal forebrain efferents originating in the basolateral amygdaloid complex. The subcortical projections of the basolateral nucleus, at least in the rat, appear to be dichotomous, with anterior (or magnocellular) portions of the nucleus preferentially targeting striatum and ventral striatum (including the core of the nucleus accumbens), while the posterior (small-celled) portions of the basolateral nucleus target the extended amygdala as well as the shell of the nucleus accumbens. This divergence represents a particular opportunity for behavioral neuroscientists analyzing basal forebrain functions. Studies exploiting the dual subcortical projection of basolateral amygdala indicate distinct functional roles for striatum versus extended amygdala. These reinforce the identification of extended amygdala as a functional-anatomical entity distinct from the striatopallidal system.

Descending pathways from the paraventricular nucleus contribute to the recruitment of brainstem nuclei following a systemic immune challenge

Hypothalamic nuclei, particularly the paraventricular nuclei (PVN), are important brain sites responsible for central nervous system responses during an immune challenge. The brainstem catecholamine cells of the nucleus tractus solitarius (NTS) and ventrolateral medulla (VLM) have been shown to play critical roles in relaying systemic immune signals to the PVN. However, whilst it is well recognised that PVN divisions also innervate the NTS and VLM, it is not known whether descending PVN pathways can modulate the recruitment of brainstem cells during an immune challenge. Using systemic administration of the proinflammatory cytokine interleukin-1β, in combination with Fos immunolabelling, we firstly investigated the effect of PVN lesions on NTS and VLM catecholamine and non-catecholamine cell responses. We found that ibotenic acid lesions of the PVN significantly reduced numbers of Fos-positive non-catecholamine, noradrenergic and adrenergic cells observable in the VLM and NTS after interleukin-1β administration. We then investigated the origins of descending inputs to the VLM and NTS, activated by systemic interleukin-1β, by mapping the distribution of Fos-positive retrogradely-labelled cells in divisions of the PVN after iontophoretically depositing choleratoxin-b subunit into the NTS or VLM one week prior to interleukin-1β administration. We found that, after either NTS or VLM deposits, the majority of retrogradely-labelled Fos-positive cells activated by interleukin-1β were localised in the medial and lateral parvocellular PVN divisions. Retrogradely-labelled Fos-positive cells were also observed in the NTS after VLM deposits, and in the VLM after NTS tracer deposits, suggesting reciprocal communication between these two nuclei after systemic interleukin-1β. Thus the present study shows that the PVN has the capacity to modulate NTS and VLM responses after an immune challenge and that these may result from descending projections arising in the medial and lateral PVN divisions. These findings suggest that central nervous system responses to an immune challenge are likely to involve complex reciprocal connections between the PVN and the brainstem as well as between brainstem nuclei themselves.

Auditory thalamic nuclei projections to the temporal cortex in the rat

Thalamocortical projections from the auditory thalamic nuclei were examined systematically in the rat, including those from the dorsal division (MGD) of the medial geniculate body (MG), which were less clearly determined in previous studies. Injections of biocytin confined in each thalamic nucleus revealed characteristic features of projections in terms of cortical areas and layers of termination. In contrast to exclusively selective projections to cortical area Te1 from the ventral division (MGV) of the MG, diffuse and selective terminations were observed in the projections from the dorsal (MGD) and medial divisions (MGM) of the MG and the suprageniculate nucleus (SG). Diffuse termination was continuous in layer I or VI of the temporal cortex, while selective termination was in layers III and IV of discrete cortical areas. In addition to diffuse termination in the upper half of layer I of cortical areas Te1, Te2d and Te3v, the MGD and SG projections formed plexuses of axons selectively in lower layer III and layer IV of Te2d and Te3v. The SG projections targeted further the dorsal bank of the perirhinal cortex (PRh), while the MGD projections targeted in part the ventral fringe of Te1. The MGM projections terminated diffusely in layer VI of Te1 and Te3v, and selectively in lower layer III and layer IV of the rostral part of Te3v. Diffuse projections to layers I and VI from the SG and MGM extended in cortical regions over the dorsal fringe of Te1. Selective dense projections to middle cortical layers of Te2d and Te3v (especially its rostral part) indicate the existence of auditory areas, which could be involved in cross-modal interaction with visual and somatosensory system, respectively. Diffuse projections are supposed to bind information processings in these areas and the primary auditory area (Te1).

Contributions of Thalamic Nuclei to Declarative Memory Functioning

In spite of the acknowledged role that the thalamus plays in declarative memory, details about the precise memory processes it is involved in and which are the structures of the thalamus that contribute to these processes remain unknown. An overview is presented of human clinical and animal experimental findings showing the involvement of the thalamus, at the level of white matter tracts and separate nuclei, in aspects of memory functioning. The region in the thalamus that contributes to declarative memory is the anterior and medial division, containing the anterior nuclei, the medial dorsal nucleus and the intralaminar and midline nuclei. A lesion to the anterior nuclei or their afferent white matter tract, the mammillothalamic tract, results in deficits of encoding of new stimuli. Lesions to the medial dorsal nucleus affect executive processes pertaining to declarative memory, such as the use of memory strategies for retrieval; damage to the intralaminar and midline nuclei results in decreased arousal and thus affects the declarative memory process. Based on anatomical and functional data, a theory is proposed of how the thalamus might play a role at different levels of declarative memory functioning. Firstly, the anterior and mediodorsal nucleus are involved in processing the contents of the stimuli for storage and recall. The anterior nuclei influence the selection of material to be stored and remembered, whereas the mediodorsal nucleus is involved in the coordination and selection of the strategies used to retrieve material. Secondly, the intralaminar and midline nuclei and specifically the lateral and ventral components, maintain a necessary state of the cortical regions involved in the ongoing memory processes. The two types of function subserved by these groups of thalamic nuclei, focussing on contents vs. state, need to work in parallel to mediate and allow memory functioning, respectively.

Projections from the periamygdaloid cortex to the amygdaloid complex, the hippocampal formation, and the parahippocampal region: a PHA-L study in the rat.

The periamygdaloid cortex, an amygdaloid region that processes olfactory information, projects to the hippocampal formation and parahippocampal region. To elucidate the topographic details of these projections, pathways were anterogradely traced using Phaseolus vulgaris leukoagglutinin (PHA-L) in 14 rats. First, we investigated the intradivisional, interdivisional, and intra-amygdaloid connections of various subfields [periamygdaloid subfield (PAC), medial subfield (PACm), sulcal subfield (PACs)] of the periamygdaloid cortex. Thereafter, we focused on projections to the hippocampal formation (dentate gyrus, hippocampus proper, subiculum) and to the parahippocampal region (presubiculum, parasubiculum, entorhinal, and perirhinal and postrhinal cortices). The PACm had the heaviest intradivisional projections and it also originated light interdivisional projections to other periamygdaloid subfields. Projections from the other subfields converged in the PACs. All subfields provided substantial intra-amygdaloid projections to the medial and posterior cortical nuclei. In addition, the PAC subfield projected to the ventrolateral and medial divisions of the lateral nucleus. The heaviest periamygdalohippocampal projections originated in the PACm and PACs, which projected moderately to the temporal end of the stratum lacunosum moleculare of the CA1 subfield and to the molecular layer of the ventral subiculum. The PACm also projected moderately to the temporal CA3 subfield. The heaviest projections to the entorhinal cortex originated in the PACs and terminated in the amygdalo-entorhinal, ventral intermediate, and medial subfields. Area 35 of the perirhinal cortex was lightly innervated by the PAC subfield. Thus, these connections might allow for olfactory information entering the amygdala to become associated with signals from other sensory modalities that enter the amygdala via other nuclei. Further, the periamygdalohippocampal pathways might form one route by which the amygdala modulates memory formation and retrieval in the medial temporal lobe memory system. These pathways can also facilitate the spread of seizure activity from the amygdala to the hippocampal and parahippocampal regions in temporal lobe epilepsy.

The Mesencephalic Trigeminal Nucleus of the Duck: Development and Apoptosis

The normal development of the mesencephalic trigeminal nucleus (MesV) of the white Peking duck (Anas platyrhynchos) was studied from the 9th day of incubation until hatching and during adulthood. In the early days of embryonic development, neurons are present in the posterior commissure and in the mesenchymal tissue outside the leptomeninges in addition to those in the tectal commissure (TC) and in the optic tectum. Following the internucleosomal cleavage of DNA, a massive loss of neurons in the MesV starts in the 11-day embryo and continues until the 15th day of incubation. On the 16th day, the nucleus consists of a numerically larger medial division located in the TC and a smaller lateral division within the stratum griseum periventriculare as is found in the adult animal. The programmed cell death occurring in the MesV is discussed herein and correlated with the analogous apoptotic phenomena observed in the trigeminal motor nucleus.

Stereotaxic localization of corticosterone to the amygdala enhances hypothalamo-pituitary–adrenal responses to behavioral stress

The amygdala is involved in behavioral, autonomic, and neuroendocrine responses to stressful stimuli. The goal of the current study was to determine the effect of directly elevating glucocorticoids in the amygdala on hypothalamo-pituitary–adrenocortical (HPA) responses to the elevated plus maze, a behavioral stressor known to activate the amygdala. Micropellets (30 μg) of crystalline corticosterone or cholesterol (control) were implanted bilaterally at the dorsal margin of the CeA in male Wistar rats; vascular catheters were also placed at this time. Five days post-surgery, blood samples were drawn at 07:00 and 19:00 h to assess diurnal rhythm of plasma corticosterone. At 7 days post-implantation, rats were subjected to behavioral stress using an elevated plus maze and blood was collected 15 min prior to stress, and at 15, 45, and 90 min after the initiation of the stressor. Corticotropin releasing factor (CRF) and arginine vasopressin (AVP) mRNA levels were analyzed by in situ hybridization in the medial parvocellular division of the hypothalamic paraventricular nucleus (mpPVN) in corticosterone- and cholesterol-implanted rats either not exposed to the elevated plus maze (control) or 4 h post-behavioral stress. Localization of corticosterone to the amygdala had no effect on diurnal rhythm of corticosterone secretion. Behavioral stress significantly increased peak plasma corticosterone levels in both groups to a similar level. However, in the corticosterone implanted rats, plasma corticosterone concentrations at 45 and 90 min post-stress were significantly greater compared to control rats indicating a prolonged corticosterone response to behavioral stress. In non-stressed rats, corticosterone delivery to the amygdala elevated basal CRF mRNA in the mpPVN to levels similar to those observed post-stress in control animals; no further increase was observed in CRF mRNA following stress. Behavioral stress resulted in a significant elevation in CRF mRNA in cholesterol controls. Basal AVP mRNA levels were unaffected by corticosterone implants. AVP mRNA did not increase in cholesterol implanted rats in response to behavioral stress. However, AVP mRNA levels were higher in corticosterone implanted rats post stress compared to cholesterol treated controls. In conclusion, direct administration of corticosterone to the amygdala increases plasma corticosterone in response to a behavioral stressor without altering the diurnal rhythm in plasma corticosterone. Elevated basal levels of mpPVN CRF mRNA, and the induction of a mpPVN AVP mRNA response to the behavioral stressor implicate enhanced ACTH secretagogue expression in the increased HPA response to corticosterone modulation of amygdala function.