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

Abstract

Several studies have shown that L-aspartate (Asp) is present in synaptic vesicles and released exocytotically from presynaptic terminals, possibly by Ca(2+)-dependent corelease of Asp and L-glutamate (Glu). It has been demonstrated that both excitatory amino acids (EAAs) are released from the rat striatum as part of corticostriatal neurotransmission. The single or colocalized occurrence of Asp and Glu in specific synaptic boutons of the chicken medial striatum/nucl. accumbens has been demonstrated by our group using ultrastructural immunocytochemistry. However, evidence for the presence of EAAs in any specific striatal pathway was only circumstantial. Here, we report on the distribution of Asp and Glu in specific synaptic terminals of the amygdalostriatal pathway, both in rat and chicken brains, combining anterograde tracing with postembedding immunogold labeling of Asp or Glu. Immunoreactivity for Asp and Glu was observed in amygdalofugal terminals with asymmetrical synaptic junctions (morphologically representing excitatory synapses) in both species. The postsynaptic targets were either dendritic spines or small dendrites, whereas axosomatic or axo-axonic connections were not observed. Ultrastructurally, the synaptic terminals immunoreactive for Asp were indistinguishable from those immunoreactive for Glu. The findigs are consistent with an Asp-Glu corelease mechanism, with a distinct synaptic contingent, evolutionarily conserved in the amygdalostriatal pathway.

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

“…The arcopallium harbors subsets of parvalbumin + , calbindin + and calretinin + neurons (Cornez et al 2015; Roberts et al 2002). The observed lack of calcium binding proteins is in harmony with our previous finding that at least a contingent of the source neurons of the amygdalofugal pathway are excitatory based on the presence of glutamate and aspartate in asymmetrical synaptic terminals (of excitatory morphological type), deriving from amygdalofugal axons (Hanics et al 2012). The calcium binding proteins calbindin D28K, calretinin and parvalbumin tend to occur in smooth non-pyramidal interneurons (and some pyramidal neurons) of mammalian cortex (for review: DeFelipe (1997)).…”

“…According to previous observations, the BSTL is primarily involved in contextual fear (Duvarci et al 2009; Phelps and LeDoux 2005; Walker and Davis 2008), whereas the central amygdala is more involved in lasting fear responses, similar to anxiety (Duvarci et al 2009; Walker and Davis 2008; Walker et al 2003, 2009). Based on evidence from the previous (Hanics et al 2012) and present observations, this pathway is excitatory, with potential cotransmission of Glu and Asp. Dopaminergic input to the source neurons of this pathway is unlikely to involve DARPP-32 as the main signal transducer, as evidenced by the present study.…”

“…Earlier results from our laboratory, based on electron microscopic immunocytochemistry, have indicated the presence of l -Asp and l -Glu in excitatory axodendritic boutons in the striatum/accumbens region of rats and chickens (Adam and Csillag 2006; Hanics et al 2012). The source of these terminals proved to be the basolateral amygdala (BLA) in the rat, whereas in chicks the source region was located in the amygdaloid arcopallium (Hanics et al 2012).…”

“…The source of these terminals proved to be the basolateral amygdala (BLA) in the rat, whereas in chicks the source region was located in the amygdaloid arcopallium (Hanics et al 2012). Further, to investigate the specificity of l -Asp containing input pathways in the domestic chicken, it was necessary to carry out a detailed pathway tracing study, combined with immunohistochemistry, relevant to the chemical nature of source neurons and of potential target areas.…”

Selective contribution of the telencephalic arcopallium to the social facilitation of foraging efforts in the domestic chick

“…The present tracing results show that the lateral Arco projects to the NAc and MSt, whereas the intermediate and dorsal Arco project mainly to the MSt (Figs and ). A recent neuroanatomical study also revealed amygdalo‐fugal terminals in the MSt/NAc with the morphological features of excitatory synapses (Hanics et al ., ). Taken together, these data indicate that Arco‐MSt/NAc pathways are involved in determining foraging effort.…”