Contributions of different afferent pathways to the catecholamine and 5-hydroxytryptamine-innervation of the amygdala: a neurochemical and histochemical study

From chronic stress and anxiety to neurodegeneration: Focus on neuromodulation of the axon initial segment

To adapt to the sustained demands of chronic stress, discrete brain circuits undergo structural and functional changes often resulting in anxiety disorders. In some individuals, anxiety disorders precede the development of motor symptoms of Parkinson's disease (PD) caused by degeneration of neurons in the substantia nigra (SN). Here, we present a circuit framework for probing a causal link between chronic stress, anxiety, and PD, which postulates a central role of abnormal neuromodulation of the SN's axon initial segment by brainstem inputs. It is grounded in findings demonstrating that the earliest PD pathologies occur in the stress-responsive, emotion regulation network of the brainstem, which provides the SN with dense aminergic and cholinergic innervation. SN's axon initial segment (AIS) has unique features that support the sustained and bidirectional propagation of activity in response to synaptic inputs. It is therefore, especially sensitive to circuit-mediated stress-induced imbalance of neuromodulation, and thus a plausible initiating site of neurodegeneration. This could explain why, although secondary to pathophysiologies in other brainstem nuclei, SN degeneration is the most extensive. Consequently, the cardinal symptom of PD, severe motor deficits, arise from degeneration of the nigrostriatal pathway rather than other brainstem nuclei. Understanding when and how circuit dysfunctions underlying anxiety can progress to neurodegeneration, raises the prospect of timed interventions for reversing, or at least impeding, the early pathophysiologies that lead to PD and possibly other neurodegenerative disorders.

Conventional and Novel Pharmacological Approaches to Treat Dopamine-Related Disorders: Focus on Parkinson's Disease and Schizophrenia

The dopaminergic system integrated by cell groups distributed in several brain regions exerts a modulatory role in brain. Particularly important for this task are the mesencephalic dopamine neurons, which from the substantia nigra and ventral tegmental area project to the dorsal striatum and the cortical/subcortical limbic systems, respectively. Dopamine released from these neurons operates mainly via the short distance extrasynaptic volume transmission and activates five different dopaminergic receptor subtypes modulating synaptic GABA and glutamate transmission. To accomplish this task dopaminergic neurons keep mutual modulating interactions with neurons of other neurotransmitter systems, including allosteric receptor-receptor interactions in heteroreceptor complexes. As a result of its modulatory role dopaminergic mechanisms are involved in either the etiology or physiopathology of many brain diseases such as Parkinsońs disease and schizophrenia. The aim of this work is to review some novel and conventional approaches that either have been used or are currently employed to treat these diseases. Particular attention is paid to the approaches derived from the knowledge recently acquired in the realm of receptor-receptor interactions taking place through multiple dopamine heteroreceptor complexes in the plasma membrane. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

The distribution of serotonergic fibers in the macaque monkey amygdala: An immunohistochemical study using antisera to 5-hydroxytryptamine

Though both the amygdala and the serotonin system appear to play critical roles in regulating fear and anxiety, little is known regarding the organization of serotonergic inputs to the primate amygdala. The present study employed immunohistochemistry to determine the distribution of serotonin fibers in the macaque amygdala. The brains of three adult male Macaca fascicularis monkeys were prepared for histological analysis using a polyclonal antibody to serotonin. The macaque amygdala is densely innervated by serotonergic fibers and demonstrates a distinctive pattern of fiber distribution and density among the 13 nuclei and cortical areas. The highest density of 5-hydroxytryptamine immunoreactive fibers is observed in the central nucleus, the nucleus of the lateral olfactory tract, the paralaminar nucleus, the anterior amygdaloid area and a small region of the amygdalohippocampal area. Moderate fiber densities are found in portions of the basal, lateral, and intercalated nuclei. The lowest fiber densities are observed in the accessory basal, posterior cortical, anterior cortical and medial nuclei, and in subregions of the periamygdaloid cortex. The present study provides evidence that the serotonergic system can have substantial influence on the ongoing activity of the amygdaloid complex.

The dopamine D1 receptor-rich main and paracapsular intercalated nerve cell groups of the rat amygdala: relationship to the dopamine innervation

The intercalated cell masses are GABAergic neurons interposed between the major input and output structures of the amygdala. Dopaminergic projections to the main and paracapsular intercalated islands were examined by determining the relationship of the dopamine nerve-terminal networks to the D1-receptor immunoreactive staining of cells within the intercalated islands, using double-fluorescence immunolabelling procedures in combination with confocal laser microscopy. The relationship of terminals positive for both tyrosine hydroxylase and dopamine beta-hydroxylase (noradrenaline and/or adrenaline) to terminals positive for tyrosine hydroxylase but negative for dopamine beta-hydroxylase (dopamine terminals) was studied in relation to the D1-receptor immunoreactivity in adjacent sections at various rostrocaudal levels. The microscopy and image analysis revealed that there was only a minor dopaminergic innervation of the D1 receptor-immunoreactive cells in the rostromedial and caudal component of the main intercalated island, suggesting volume transmission as the main communication mode for dopamine in these regions. In contrast, the D1 receptor-immunoreactive areas in the rostrolateral part of the main island and also the paracapsular intercalated islands showed a high degree of dopaminergic innervation, indicating that synaptic and perisynaptic dopamine transmission plays a dominant role in these regions. It is known that amygdala neurons are involved in the elicitation and learning of fear-related behaviors. We suggest that slow dopaminergic volume transmission in the rostromedial and caudal parts of the main intercalated island may have a role in tonic excitatory modulation in these parts of the main island, allowing GABAergic activity to develop in the central amygdaloid nucleus and thereby contributing to inhibition of fear-related behavioral and autonomic responses. In contrast, a faster synaptic and perisynaptic dopaminergic transmission in the rostrolateral part of the main intercalated island and in the paracapsular intercalated islands may have a role in allowing a more rapid elicitation of fear-related behaviors.

Pharmacological studies on the monoaminergic influence on the synthesis and expression of neuropeptide Y and corticotropin releasing factor in rat brain amygdala

Our earlier findings concerning the 6-OHDA lesion suggested dopaminergic regulation of neuropeptide Y (NPY) and corticotropin releasing factor (CRF) synthesis and expression in amygdala neurons. On the other hand, some other studies indicated that not only dopamine, but also other monoamines may modulate peptidergic neurons. Therefore the present study examined the effect of pharmacological deprivation of monoaminergic influences on NPY and CRF neurons in rat brain amygdala by means of in situ hybridization and immunohistochemical methods. It was found that NPY mRNA expression in the amygdala decreased after 24h blockade of dopaminergic D1 and D2 receptors, by haloperidol or SCH23390. At the same time the NPY-peptide expression measured immunohistochemically was not significantly changed. A prolonged, 14-day, blockade of dopaminergic receptors by haloperidol induced an opposite effect, an increase in NPY mRNA expression. Impairment of the serotonergic transmission by blockade of 5-HT synthesis using p-chlorophenylalanine, as well as attenuation of the noradrenergic transmission by NA depletion from terminals by DSP4, did not significantly change NPY mRNA expression or the mean number of NPY-immunoreactive neurons in the amygdala. Only a decrease in the staining intensity observed as a decreased number of darkly stained neurons was found after both compounds. Neither the dopamine receptor blockade nor the impairment of serotonergic or noradrenergic transmission changed CRF mRNA or the peptide expression in the amygdala. The obtained results indicate that in rat brain amygdala, of all the monoamines, dopamine seems to be the most important modulator of NPY biosynthesis and expression. The effect of blockade of dopaminergic receptors is biphasic: first it induces a decrease and then - after prolonged treatment an increase in NPY mRNA. Serotonergic and noradrenergic systems in the amygdala seem to be connected with regulation of NPY release rather than the biosynthesis.

Amygdala lesions block the effect of cocaine on memory in mice

Post-training cocaine intraperitoneal administration enhanced memory consolidation in unoperated and sham-lesioned mice tested in a one-trial inhibitory avoidance task. Bilateral electrolytical amygdala lesions blocked this effect. The results are interpreted in terms of involvement of neurotransmitter, and in particular dopaminergic, systems in the effects of cocaine on memory.

Effects of cocaine and the cocaine analog CFT on glutamatergic neurons

The effects of cocaine and the cocaine analog methyl-3-beta-(p-fluorophenyl)-1 alpha H, 5 alpha H-tropane-2b-carboxylate (CFT) on glutamate turnover rate were studied in the nucleus accumbens, striatum, frontal cortex, and parietal-cingulate cortex of the rat, using neurotransmitter turnover rate as an estimate of the activity of the glutamatergic neurons. Both cocaine [15 or 30 mg/kg, intraperitoneally (IP)] and CFT (2.2 mg/kg, IP) increased glutamate turnover in the nucleus accumbens, although the time course of their actions differed. These effects on glutamate turnover appeared at times after maximal motor activation of the animals had occurred. On the other hand, neither cocaine nor CFT affected glutamate turnover in the frontal cortex, parietal-cingulate cortex, or striatum. Neither cocaine nor CFT affected the content of glutamate or glucose in any brain region studied. Thus, although cocaine and CFT affect glutamatergic neurons in the CNS, these actions are not generalized across the CNS, but are restricted to a specific brain region.

Thalamoamygdaloid projections in the rat: a test of the amygdala's role in sensory processing

We used the autoradiographic tract-tracing method to define the amygdaloid projection fields after injecting 3H-amino acids into individual thalamic nuclei in the rat. The parvicellular division of the ventroposterior nucleus, the thalamic taste relay, projected lightly to the central and lateral amygdaloid nuclei. The central medial, interanteromedial, and paraventricular thalamic nuclei, viscerosensory relays of the thorax and abdomen, projected heavily to the amygdala. All projected to the basolateral amygdaloid nucleus, the paraventricular nucleus in addition having terminations in the central nucleus, the amygdaloid portion of the nucleus of the stria terminalis, and the amygdalohippocampal transition area. The magnocellular division of the medial geniculate, a thalamic auditory (and, to a moderate degree, a spinothalamic) relay, sent heavy projections to the central, accessory basal, lateral, and anterior cortical nuclei, and to the anterior amygdaloid area and the nucleus of the accessory olfactory tract. Other thalamic nuclei projecting to the amygdala, for which functions could not be associated, were the paratenial and subparafascicular nuclei. The former projected to the lateral, basal, and posterolateral cortical nuclei; the latter projected very lightly to the central, medial, and basal accessory nuclei. These results show that, like the cortical amygdaloid nuclei, which are sensory (olfactory) in nature, the subcortical amygdaloid nuclei must have major sensory functions. These thalamic afferents, when correlated with cortical and brainstem data from the literature, suggested that the amygdala is in receipt of sensory information from many modalities. To uncover the manner by which such information is processed by the amygdala and relayed to effector areas of the brain, six hypothetical mechanisms relating to modality specificity and convergence were posited. By charting sensory-related afferents to all subdivisions of the amygdala, each nucleus was characterized as to its mechanism of information processing. Four proposed amygdaloid systems emerged from this analysis. A unimodal corticomedial amygdaloid system relays pheromonal information from the accessory olfactory bulb to medial basal forebrain and hypothalamic areas. A second system--the lateral-basomedial--collects and combines input from a number of sensory modalities and distributes it to the same basal forebrain and hypothalamic areas as the corticomedial. The central system appears to concentrate the effect of viscerosensory information arriving from multiple brainstem, thalamic, cortical, and amygdaloid sources; this information is combined with significant auditory and spinothalamic inputs from the thalamus and cortex. The central system projects to lateral nuclei in the basal forebrain, hypothalamus, and brainstem.(ABSTRACT TRUNCATED AT 400 WORDS)

Serotonin-, substance P- or leucine-enkephalin-containing neurons in the midbrain periaqueductal gray and nucleus raphe dorsalis send projection fibers to the central amygdaloid nucleus in the rat

By a double-labeling method combining the retrograde tracing of horseradish peroxidase and the immunocytochemical technique, serotonin-, substance P- or leucine-enkephalin-like immunoreactive neurons in the midbrain periaqueductal gray (PAG) and the nucleus raphe dorsalis (DR) of the rat were found to send projection fibers to the central amygdaloid nucleus bilaterally with an ipsilateral dominance. These PAG neurons were chiefly distributed in the ventrolateral PAG subdivision and the ventral parts of medial PAG subdivision at the middle and caudal levels of PAG.

Involvement of amygdala pathways in the influence of post-training intra-amygdala norepinephrine and peripheral epinephrine on memory storage

These experiments examined the role of two major amygdala afferent-efferent pathways--the stria terminalis (ST) and the ventral amygdalofugal pathway (VAF)--in mediating the effects, on memory storage, of post-training intra-amygdala injections of norepinephrine (NE) and subcutaneous (s.c.) injections of epinephrine (E). Rats with either ST lesions or VAF transections and sham-operated rats were trained on a one-trial step-through inhibitory avoidance task and immediately after training received intra-amygdala injections of NE or a buffer solution. Other groups of VAF-transected animals received post-training s.c. injections of E or saline. ST lesions blocked the memory-enhancing effect of intra-amygdala injections of a low dose of NE (0.2 microgram) as well as the amnestic effect of a high dose of NE (5.0 microgram). In contrast, VAF transections did not block the memory-enhancing effect of NE (0.2 microgram). However, VAF transections attenuated the memory-enhancing effect of s.c. injections of E: the effective dose of E was shifted from 0.1 to 0.5 mg/kg. These findings, considered together with previous evidence that ST lesions block the memory-enhancing effect of peripheral E injections, suggest that the VAF is involved in mediating the central influence of peripheral E on amygdala functioning, while the ST is involved in mediating amygdala influences on memory storage elsewhere in the brain.

Differential mechanisms involved in the anticonflict action of benzodiazepines injected into the central amygdala and mammillary body

The present study was designed to clarify the mechanism of action of benzodiazepines (BDZ) injected into the central amygdala (ACE) and mammillary body (MB). When gamma-aminobutyric acid (GABA) at doses of 30 and 70 micrograms, muscimol (0.01 and 0.03 microgram), valproate (200 micrograms), atropine (20 micrograms) and cyproheptadine (3 micrograms) were bilaterally injected into ACE, a significant and marked increase in the punished responses of conflict schedule was observed. These drugs injected into MB failed to increase the punished responses. In MB, only noradrenaline (NA, 20 micrograms) showed the anticonflict action. NA 20 micrograms also produced the anticonflict action in ACE. These results suggest that the mechanism of anticonflict action of BDZ is different in brain areas. The GABA-ergic, cholinergic, serotonergic and NA-ergic systems seem to be involved in the mechanism of anticonflict action of BDZ in ACE. While the NA-ergic system appears to be operative in MB.

Release of [Met]enkephalin in the central nucleus of the amygdala is increased by application of potassium in the substantia nigra

Release of [Met]enkephalin immunoreactivity (Met-IR) in the central nucleus of the amygdala (ACE) was investigated in vivo in anesthetized rats implanted with push-pull cannulae. A stable spontaneous release of this peptide (1.3 fmol/15 min fraction) could be measured in the superfusates using a highly sensitive radioimmunoassay. The addition to the superfusion medium of cocktail of peptidase inhibitors increased three times the spontaneous release of the peptide. Superfusion with 30 mM potassium increased ten times the release of the peptide. Chemical stimulation of the substantia nigra with K+ enhanced four times the Met-IR release in the ipsilateral ACE. The dopaminergic component of the nigro-amygdaloid pathway appeared not to be directly implicated in this effect, since: d(+)amphetamine application in the ACE, which enhanced the local release of DA, remained without effect on Met-IR release and haloperidol-induced blockade of dopaminergic receptors in the ACE similarly did not affect Met-IR release.

Distribution and efferent projections of corticotropin-releasing factor-like immunoreactivity in the rat amygdaloid complex

Using cobalt-enhanced immunohistochemistry, the tracing of retrograde transport of horseradish peroxidase (HRP) and experimental manipulations, a widespread localization of corticotropin-releasing factor-like immunoreactive (CRFI) structures in the rat amygdaloid complex, and CRFI-containing pathways from the amygdala to the lower brainstem, bed nucleus of the stria terminalis (bst) and ventromedial nucleus of the hypothalamus (VMH) have been demonstrated. By means of cobalt-enhanced immunohistochemistry, CRFI cells were detected in almost all the regions of the amygdala, including the central amygdaloid nucleus (Ce), basolateral amygdaloid nucleus (B1), intra-amygdaloid bed nucleus of the stria terminalis (Abst), medial amygdaloid nucleus (Me), amygdalohippocampal area (Ahi), posterior cortical amygdaloid nucleus (Aco), lateral amygdaloid nucleus (La), anterior amygdaloid area (AAA) and basomedial amygdaloid nucleus (Bm). Neural processes with CRFI were found in all of the above areas. The greatest density of CRFI fibres was observed in the Ce, the Me and Ahi. Unilateral lesions located in the Ce and adjacent areas caused an ipsilateral decrease in CRFI fibre number in the lateral hypothalamic area (LH), mesencephalic reticular formation (RF), dorsal (Dpb) and ventral (Vpb) parabrachial nuclei, mesencephalic nucleus of the trigeminal nerve (MeV) and in the lateral division of the bst (bstl). In addition, ipsilateral CRFI fibres decreased in number in the core and shell of the VMH after unilateral lesions of the corticomedial amygdala (CoM) and ventral subiculum (S). These findings suggest that the CRFI cells in the Ce and adjacent areas innervate the Dpb, Vpb and MeV through the LH and RF; the CRFI fibres in the bstl are supplied by the Ce and adjacent areas; and the CoM and S give rise to the CRFI fibres to the VMH. The distribution of retrogradely transported HRP has confirmed these projections. Furthermore, combined HRP and immunohistochemical staining has demonstrated double labeled cells in the Ce following HRP injection into the Dpb, Vpb, MeV and bstl. This provides direct evidence for the amygdalofugal CRF-containing projections to the lower brainstem and bstl. Double-labeled cells were not seen in the CoM and S after HRP injection into the VMH.

Distribution of neuropeptide Y-like immunoreactivity in the rat central nervous system--II. Immunohistochemical analysis

The distribution of neuropeptide Y-like immunoreactivity in the rat brain and spinal cord was investigated by means of the peroxidase-antiperoxidase procedure of Sternberger using a rabbit anti-neuropeptide Y serum. A widespread distribution of immunostained cells and fibres was detected with moderate to large numbers of cells in the following regions: olfactory bulb, anterior olfactory nucleus, olfactory tubercle, striatum, nucleus accumbens, all parts of the neocortex and the corpus callosum, septum including the anterior hippocampal rudiment, ventral pallidum, horizontal limb of the diagonal band, amygdaloid complex. Ammon's horn, dentate gyrus, subiculum, pre- and parasubiculum, lateral thalamic nucleus (intergeniculate leaflet), bed nucleus of the stria terminalis, medial preoptic area, lateral hypothalamus, mediobasal hypothalamus, supramammillary nucleus, pericentral and external nuclei of the inferior colliculus, interpeduncular nucleus, periaqueductal central gray, locus coeruleus, dorsal tegmental nucleus of Gudden, lateral superior olive, lateral reticular nucleus, medial longitudinal fasciculus, prepositus hypoglossal nucleus, nucleus of the solitary tract and spinal nucleus of the trigeminal nerve. In the spinal cord cells were found in the substantia gelatinosa at all levels, the dorsolateral funiculus and dorsal gray commissure in lumbosacral cord. The pattern of staining was found to be similar to that observed with antisera to avian and bovine pancreatic polypeptide, but to differ in some respects from that observed with antisera to molluscan cardioexcitatory peptide. The presence of neuropeptide Y immunoreactive fibres in tracts such as the corpus callosum, anterior commissure, lateral olfactory tract, fimbria, medial corticohypothalamic tract, medial forebrain bundle, stria terminalis, dorsal periventricular bundle and other periventricular areas, indicated that in addition to the localisation of neuropeptide Y-like peptide(s) in interneurons in the forebrain, neuropeptide Y may be found in long neuronal pathways throughout the brain.

Lesions of the stria terminalis attenuate the amnestic effect of amygdaloid stimulation on avoidance responses

The present study investigated the involvement of two amygdala pathways, the stria terminalis (ST) and the ventral amygdalofugal pathway (VAF), in the effect of post-training electrical stimulation of the amygdala on retention. Rats with implanted amygdaloid electrodes and ST lesions, VAF transections or sham pathway operations, were trained on an inhibitory avoidance task and an active avoidance task. Electrical stimulation of the amygdala was given immediately after training and retention was tested 24 h later. In rats with sham ST lesions, post-training amygdaloid stimulation impaired retention in both tasks. Lesions of the ST did not significantly affect retention in the unstimulated rats. However, the ST lesions attenuated the amnestic effect of amygdaloid stimulation. In rats with sham VAF transections, stimulation of the amygdala impaired retention in the inhibitory avoidance task but enhanced retention in the active avoidance task. Transecting the VAF impaired retention performance of the unstimulated rats in the inhibitory avoidance task. However, the VAF transections did not alter the effect of amygdaloid stimulation: in both tasks, the retention performance of stimulated rats with VAF transections did not differ from that of stimulated rats with sham transections. These findings suggest that the ST may be involved in mediating the influences of the stimulated amygdala in modulating memory storage processing in the brain.

5,7-Dihydroxytryptamine lesions of the amygdala reduce amphetamine- and apomorphine-induced stereotyped behaviour in the rat

5,7-Dihydroxytryptamine lesions directed towards the central nucleus of the amygdala produced an apparently selective and localised destruction of 5HT terminals within the amygdaloid complex, without damage to dopamine neurons. Such lesions attenuated the biting responses produced by both amphetamine and apomorphine, suggesting that 5HT within this area may play an important role in the mediation of these behavioural effects.