Parabrachial inputs to Fos-immunoreactive neurons in the lateral central nucleus of amygdala activated by hypotension: a light and electron microscopic study in the rat

Neurochemical properties of the synapses between the parabrachial nucleus-derived CGRP-positive axonal terminals and the GABAergic neurons in the lateral capsular division of central nucleus of amygdala

The lateral capsular division of central nucleus of amygdala (CeC) contains neurons using γ-amino butyric acid (GABA) as the predominant neurotransmitter and expresses abundant calcitonin gene-related peptide (CGRP)-positive terminals. However, the relationship between them has not been revealed yet. Using GAD67-green fluorescent protein (GFP) knock-in mouse, we investigated the neurochemical features of synapses between CGRP-positive terminals and GABAergic neurons within CeC and the potential involvement of CGRP1 receptor by combining fluorescent in situ hybridization for CGRP1 receptor mRNA with immunofluorescent histochemistry for GFP and CGRP. The ultrastructures of these synapses were investigated with pre-embedding electron microscopy for GFP and CGRP. We found that some GABAergic neurons in the CeC received parabrachial nucleus (PBN) derived CGRP innervations and some of these GABAergic neurons can be activated by subcutaneous injection of formalin. Moreover, more than 90 % GABAergic neurons innervated by CGRP-positive terminal also express CGRP1 receptor mRNA. The CGRP-positive fibers made symmetric synapses onto the GABAergic somata, and asymmetric synapses onto the GABA-LI dendritic shafts and spines. This study provides direct ultrastructural evidences for the synaptic contacts between CGRP-positive terminals and GABAergic neurons within the CeC, which may underlie the pain-related neural pathway from PBN to CeC and be involved in the chronic pain modulation.

Differential postsynaptic compartments in the laterocapsular division of the central nucleus of amygdala for afferents from the parabrachial nucleus and the basolateral nucleus in the rat

Neurons in the laterocapsular division of the central nucleus of the amygdala (CeC), which is known as the "nociceptive amygdala," receive glutamatergic inputs from the parabrachial nucleus (PB) and the basolateral nucleus of amygdala (BLA), which convey nociceptive information from the dorsal horn of the spinal cord and polymodal information from the thalamus and cortex, respectively. Here, we examined the ultrastructural properties of PB- and BLA-CeC synapses identified with EGFP-expressing lentivirus in rats. In addition, the density of synaptic AMPA receptors (AMPARs) on CeC neurons was studied by using highly sensitive SDS-digested freeze-fracture replica labeling (SDS-FRL). Afferents from the PB made asymmetrical synapses mainly on dendritic shafts (88%), whereas those from the BLA were on dendritic spines (81%). PB-CeC synapses in dendritic shafts were significantly larger (median 0.072 μm(2)) than BLA-CeC synapses in spines (median 0.058 μm(2); P = 0.02). The dendritic shafts that made synapses with PB fibers were also significantly larger than those that made synapses with BLA fibers, indicating that the PB fibers make synapses on more proximal parts of dendrites than the BLA fibers. SDS-FRL revealed that almost all excitatory postsynaptic sites have AMPARs in the CeC. The density of AMPAR-specific gold particles in individual synapses was significantly higher in spine synapses (median 510 particles/μm(2)) than in shaft synapses (median 427 particles/μm(2); P = 0.01). These results suggest that distinct synaptic impacts from PB- and BLA-CeC pathways contribute to the integration of nociceptive and polymodal information in the CeC.

Expression and colocalization of NADPH-diaphorase and Fos in the subnuclei of the parabrachial nucleus in rats following visceral noxious stimulation

To investigate whether neural nitric oxide synthase (nNOS) in the parabrachial nucleus (PB) is involved in processing visceral noxious stimulation, we mapped the distribution of histochemical staining for nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d), a marker for nNOS, and immunohistochemical staining for Fos, a neuronal activity marker, in the subnuclei of the PB following 2% formalin injection into the stomach of rats. NADPH-d and noxious-stimuli induced Fos staining were also examined in tissue containing PB cells labeled by the retrograde transport of fluogold (FG) injected into the central nucleus of the amygdala (CeA). We found that the number of Fos immunoreactive (Fos-IR) neurons was significantly increased in the dorsal lateral (dl), external lateral (el) and Kölliker-Fuse (KF) subnuclei of the PB. We observed that intensely labeled (type 1) NADPH-d positive neurons were mainly located in the rostral part of the PB; they extended long processes adjacent Fos-IR neurons, but no Fos/type 1 NADPH-d double-labeled neurons were seen. In contrast, lightly labeled (type 2) NADPH-d positive neurons were principally localized in the dl of the PB, in which a few Fos/type 2 NADPH-d double-labeled neurons were detected. Additionally, a large number of FG/Fos double-labeled neurons were observed to be surrounded closely by the intensive NADPH-d staining in the el of the PB. These results suggest that neurons in the el of the PB that project to the CeA are activated by visceral noxious stimulation and could be indirectly influenced by nitric oxide in the PB.

Aldosterone-sensitive neurons in the nucleus of the solitary tract: bidirectional connections with the central nucleus of the amygdala

The HSD2 (11-beta-hydroxysteroid dehydrogenase type 2-expressing) neurons in the nucleus of the solitary tract (NTS) of the rat are aldosterone-sensitive and have been implicated in sodium appetite. The central nucleus of the amygdala (CeA) has been shown to modulate salt intake in response to aldosterone, so we investigated the connections between these two sites. A prior retrograde tracing study revealed only a minor projection from the HSD2 neurons directly to the CeA, but these experiments suggested that a more substantial projection may be relayed through the parabrachial nucleus. Small injections of cholera toxin beta subunit (CTb) into the external lateral parabrachial subnucleus (PBel) produced both retrograde cell body labeling in the HSD2 neurons and anterograde axonal labeling in the lateral subdivision of the CeA. Also, injections of either CTb or Phaseolus vulgaris leucoagglutinin into the medial subdivision of the CeA labeled a descending projection from the amygdala to the medial NTS. Axons from the medial CeA formed numerous varicosities and terminals enveloping the HSD2 neurons. Complementary CTb injections, centered in the HSD2 subregion of the NTS, retrogradely labeled neurons in the medial CeA. These bidirectional projections could form a functional circuit between the HSD2 neurons and the CeA. The HSD2 neurons may represent one of the functional inputs to the lateral CeA, and their activity may be modulated by a return projection from the medial CeA. This circuit could provide a neuroanatomical basis for the modulation of salt intake by the CeA.

Cardiovascular dysautonomia as a cause of falls in Parkinson's disease

Parkinson's disease (PD) patients have a ninefold increased risk of recurring falls compared to healthy controls. The risk of falling due to cardiovascular dysautonomia (CVD) is not quantifiable. But, CVD is an integral part of the disease and at least 20% of PD patients suffer from orthostatic hypotension, an expression of CVD. One way to reduce falls due to CVD in PD patients could be to give adequate information on the relationship between falling risks and cardiovascular dysautonomia to patients and their caregivers. Moreover, drugs given for PD might contribute to OH and we propose that education and non-pharmacological strategies for its treatment might be preferable, especially because of the low efficacy of drugs available for the treatment of OH and the frailty of elderly PD patients.