Amygdalofugal axon terminals immunoreactive for L-aspartate or L-glutamate in the nucleus accumbens of rats and domestic chickens: a comparative electron microscopic immunocytochemical study combined with anterograde pathway tracing

Early cellular and synaptic changes in dopaminoceptive forebrain regions of juvenile mice following gestational exposure to valproate

Gestational exposure of mice to valproic acid (VPA) is one currently used experimental model for the investigation of typical failure symptoms associated with autism spectrum disorder (ASD). In the present study we hypothesized that the reduction of dopaminergic source neurons of the VTA, followed by perturbed growth of the mesotelencephalic dopamine pathway (MT), should also modify pattern formation in the dopaminoceptive target regions (particularly its mesoaccumbens/mesolimbic portion). Here, we investigated VPA-evoked cellular morphological (apoptosis-frequency detected by Caspase-3, abundance of Ca-binding proteins, CaBP), as well as synaptic proteomic (western blotting) changes, in selected dopaminoceptive subpallial, as compared to pallial, regions of mice, born to mothers treated with 500 mg/kg VPA on day 13.5 of pregnancy. We observed a surge of apoptosis on VPA treatment in nearly all investigated subpallial and pallial regions; with a non-significant trend of similar increase the nucleus accumbens (NAc) at P7, the age at which the MT pathway reduction has been reported (also supplemented by current findings). Of the CaBPs, calretinin (CR) expression was decreased in pallial regions, most prominently in retrosplenial cortex, but not in the subpallium of P7 mice. Calbindin-D 28K (CB) was selectively reduced in the caudate-putamen (CPu) of VPA exposed animals at P7 but no longer at P60, pointing to a potency of repairment. The VPA-associated overall increase in apoptosis at P7 did not correlate with the abundance and distribution of CaBPs, except in CPu, in which the marked drop of CB was negatively correlated with increased apoptosis. Abundance of parvalbumin (PV) at P60 showed no significant response to VPA treatment in any of the observed regions we did not find colocalization of apoptotic (Casp3+) cells with CaBP-immunoreactive neurons. The proteomic findings suggest reduction of tyrosine hydroxylase in the crude synaptosome fraction of NAc, but not in the CPu, without simultaneous decrease of the synaptic protein, synaptophysin, indicating selective impairment of dopaminergic synapses. The morpho-functional changes found in forebrain regions of VPA-exposed mice may signify dendritic and synaptic reorganization in dopaminergic target regions, with potential translational value to similar impairments in the pathogenesis of human ASD.

Neurochemical fingerprinting of amygdalostriatal and intra-amygdaloid projections: a tracing-immunofluorescence study in the rat

Amygdalostriatal and intra-amygdaloid fiber connectivity was studied in rats via injections of one of the tracers Phaseolus vulgaris leucoagglutinin (PHA-L) or biotinylated dextran amine (BDA) into various amygdaloid nuclei. To determine the neurotransmitter identity of labeled fibers we combined tracer detection with immunofluorescence staining, using antibodies against vesicular transporters (VTs) associated with glutamatergic (VGluT1, VGluT2) or GABAergic (VGAT) neurotransmission. High-magnification confocal laser scanning images were screened for overlap: occurrence inside tracer labeled fibers or axon terminals of immunofluorescence signal associated with one of the VTs. Labeled amygdalostriatal fibers were seen when tracer had been injected into the magnocellular and parvicellular portions of the basal amygdaloid nucleus and the lateral amygdaloid nucleus (nuclei belonging to 'cortical type' amygdaloid nuclei). Intra-amygdaloidal projection fibers were mostly found after tracer injections in the central and medial amygdaloid nuclei ('striatal type' amygdaloid nuclei). Terminals of tracer-labeled amygdalostriatal fibers contained immunofluorescence signal associated mostly with VGluT1 and to a lesser degree with VGluT2 or VGAT. Intra-amygdaloid labeled fibers showed colocalization mostly of VGluT1, followed by VGAT. VGluT2 co-occurred in a minority of intra-amygdaloid tracer-containing fiber terminals. We conclude from our observations that both amygdalostriatal and intra-amygdaloid projections, arising from, respectively, 'cortical type' and 'striatal type' amygdaloid nuclei contain strong glutamatergic and modest GABAergic components. The glutamatergic fibers express either VGluT1 or VGluT2. The absence in large numbers of tracer labeled fibers of expression of one of the selected VTs leads us to suspect that amygdalostriatal projection fibers may contain hitherto neglected neurotransmitters in these connections, e.g., aspartate.

Age-related and function-dependent regional alterations of free L- and D-aspartate in postembryonic chick brain

D-aspartate (D-Asp) modulates adult neural plasticity and embryonic brain development by promoting cell proliferation, survival and differentiation. Here, developmental changes of the excitatory amino acids (EAAs) L-Glu, L-Asp and D-Asp were determined during the first postembryonic days, a time window for early learning, in selected brain regions of domestic chickens after chiral separation and capillary electrophoresis. Extracellular concentration (ECC) of EAAs was measured in microdialysis samples from freely moving chicks. ECC of D-Asp (but not L-EAAs) decreased during the first week of age, with no considerable regional or learning-related variation. ECC of L-Asp and L-Glu (but not of D-Asp) were elevated in the mSt/Ac in response to a rewarding stimulus, suggesting importance of Asp-Glu co-release in synaptic plasticity of basal ganglia. Potassium-evoked release of D-Asp, with a protracted transient, was also demonstrated. D-Asp constitutes greater percentage of total aspartate in the extracellular space than in whole tissue extracts, thus the bulk of D-Asp detected in tissue appears in the extracellular space. Conversely, only a fraction of tissue L-EAAs can be detected in extracellular space. The lack of changes in tissue D-Asp following avoidance learning indicates a tonic, rather than phasic, mechanism in the neuromodulatory action of this amino acid.

Update on forebrain evolution: From neurogenesis to thermogenesis

Comparative developmental studies provide growing understanding of vertebrate forebrain evolution. This short review directs the spotlight to some newly emerging aspects, including the evolutionary origin of the proliferative region known as the subventricular zone (SVZ) and of intermediate progenitor cells (IPCs) that populate the SVZ, neural circuits that originated within homologous regions across all amniotes, and the role of thermogenesis in the acquisition of an increased brain size. These data were presented at the 8th European Conference on Comparative Neurobiology.

Multiple amygdaloid divisions of arcopallium send convergent projections to the nucleus accumbens and neighboring subpallial amygdala regions in the domestic chicken: a selective pathway tracing and reconstruction study

Retrograde tracing with choleratoxin B, injected into the nucleus accumbens (Ac) and bed nucleus of stria terminalis, lateral part (BSTL), yielded labeled perikarya in a ring-shaped area of arcopallium, including dorsal and hilar subdivisions, with a wedge-shaped node of dense accumulation in the amygdalopiriform area (APir). Also, the position of source neurons for this arcopallio-subpallial pathway was verified by anterograde tracing. Three subregions of arcopallium (amygdalopiriform, dorsal, hilar) were injected with dextran (10 kDa), and fibers and terminal fields were detected in Ac, BSTL and extended amygdala (EA). Most abundant projections to Ac arose from APir. The study enabled precise description of the main output fiber streams: the dorsal stream follows the dorsal border of arcopallium and, continuing in the ventral amygdalofugal tract, it traverses the EA and the BSTL before reaching the Ac. The ventral stream of fibers enters the EA along the ventral subpallial border and terminates in the basal nucleus and ventral pallidum. The course of the pathway was reconstructed in 3D. Retrogradely labeled arcopallial neurons were devoid of DARPP-32. DARPP-32 was present in the Ac but not the BSTL. No colocalization between the calcium binding proteins calbindin, parvalbumin and calretinin, and retrogradely labeled neurons was detected, despite a considerable territorial overlap. This finding further supports the excitatory nature of the arcopallial-accumbens pathway. Conjoint and convergent amygdalar input to EA, including BSTL, as well as to Ac subregions likely transmits fear and aggression related signals to both viscerolimbic (EA) and learned reward- and motivation-related (Ac) ventrobasal forebrain regions.