The effects of stimulation of substantia innominata and sensory receiving areas of the forebrain upon the activity of neurons within the amygdala of the anesthetized cat

Electrical stimulation of the nucleus basalis of meynert: a systematic review of preclinical and clinical data

Deep brain stimulation (DBS) of the nucleus basalis of Meynert (NBM) has been clinically investigated in Alzheimer’s disease (AD) and Lewy body dementia (LBD). However, the clinical effects are highly variable, which questions the suggested basic principles underlying these clinical trials. Therefore, preclinical and clinical data on the design of NBM stimulation experiments and its effects on behavioral and neurophysiological aspects are systematically reviewed here. Animal studies have shown that electrical stimulation of the NBM enhanced cognition, increased the release of acetylcholine, enhanced cerebral blood flow, released several neuroprotective factors, and facilitates plasticity of cortical and subcortical receptive fields. However, the translation of these outcomes to current clinical practice is hampered by the fact that mainly animals with an intact NBM were used, whereas most animals were stimulated unilaterally, with different stimulation paradigms for only restricted timeframes. Future animal research has to refine the NBM stimulation methods, using partially lesioned NBM nuclei, to better resemble the clinical situation in AD, and LBD. More preclinical data on the effect of stimulation of lesioned NBM should be present, before DBS of the NBM in human is explored further.

Revealing the ventral amygdalofugal pathway of the human limbic system using high spatial resolution diffusion tensor tractography

The amygdala is known to have a role in core processes regulated by the limbic system such as motivation, memory, emotion, social behavior, self-awareness as well as certain primitive instincts. Several functional studies have investigated some of these brain tasks of the human limbic system. However, the underlying neuronal fiber connectivity of the amygdalo-diencephalon, as part of the limbic system, has not been delineated separately by prior diffusion-weighted imaging studies. The ability to trace the underlying fiber connections individually will be helpful in understanding the neurophysiology of these tracts in different functions. To date, few diffusion-weighted studies have focused on the amygdala, yet the fine connections of the amygdala, hypothalamus, septum or other adjacent limbic structures have yet to be elucidated by diffusion-weighted tractography studies. We therefore aimed to further investigate these fine neuronal connections using fiber tractography and high spatial resolution diffusion tensor imaging on 3T on 15 healthy right-handed male human subjects (age range 24-37 years). The ventral amygdalofugal pathway, anterior commissure and stria terminalis are the three main efferent pathways of the amygdala. We delineated the detailed trajectories of the ventral amygdalofugal tract, anterior commissure and their connections bilaterally in 15 normal adult human brains. Using a high-resolution diffusion tensor tractography technique, for the first time, we were able to demonstrate the trajectory of amygdalofugal tract and its connections to the hypothalamic and septal nuclei. We further revealed, for the first time, the close relationship of the amygdalofugal tract and anterior commissure with the fornix, stria terminalis and uncinate fasciculus bilaterally in 15 healthy adult human brains.

Cortical pathways to the mammalian amygdala

The amygdaloid nuclear complex is critical for producing appropriate emotional and behavioral responses to biologically relevant sensory stimuli. It constitutes an essential link between sensory and limbic areas of the cerebral cortex and subcortical brain regions, such as the hypothalamus, brainstem, and striatum, that are responsible for eliciting emotional and motivational responses. This review summarizes the anatomy and physiology of the cortical pathways to the amygdala in the rat, cat and monkey. Although the basic anatomy of these systems in the cat and monkey was largely delineated in studies conducted during the 1970s and 1980s, detailed information regarding the cortico-amygdalar pathways in the rat was only obtained in the past several years. The purpose of this review is to describe the results of recent studies in the rat and to compare the organization of cortico-amygdalar projections in this species with that seen in the cat and monkey. In all three species visual, auditory, and somatosensory information is transmitted to the amygdala by a series of modality-specific cortico-cortical pathways ("cascades") that originate in the primary sensory cortices and flow toward higher order association areas. The cortical areas in the more distal portions of these cascades have stronger and more extensive projections to the amygdala than the more proximal areas. In all three species olfactory and gustatory/visceral information has access to the amygdala at an earlier stage of cortical processing than visual, auditory and somatosensory information. There are also important polysensory cortical inputs to the mammalian amygdala from the prefrontal and hippocampal regions. Whereas the overall organization of cortical pathways is basically similar in all mammalian species, there is anatomical evidence which suggests that there are important differences in the extent of convergence of cortical projections in the primate versus the nonprimate amygdala.

Recurrent inhibition in amygdalo-cortical projection neurons of the rat via collaterals of fast-conducting axons

On stimulation of the medial prefrontal cortex (mPFC) of the rat, the amygdala neurons projecting to the mPFC exhibited an IPSP, which caused IS-SD blockage in the antidromic spikes of these neurons. The IPSP was not preceded by EPSP, but was not monosynaptic in nature. Its onset latencies were mostly confined to within 10 ms, which was shorter than the time for orthodromic conduction from the mPFC to the amygdala. The findings indicated that the IPSP was produced via recurrent collaterals of the amygdalo-cortical projection neurons. In view of the wide range of distribution of the time for antidromic conduction in the amygdalo-cortical projection neurons (3.4-27.2 ms), the projection neurons with a fast-conducting axon were considered to be responsible for the mPFC-evoked recurrent IPSP.

Pericapillary and distant axon terminals in the nuclei of the cat amygdala: a morphometric study

According to some ultrastructural studies, the pericapillary axon terminals in the central nervous system (CNS) are functionally connected with the capillary vessel wall. Thus, it may be expected that the population of pericapillary axon terminals will be morphologically distinct from the terminals at a further distance from the capillary walls. To test this hypothesis, morphometrical analysis of 3,048 axon terminals was performed, comparing terminals situated in the close vicinity of the capillary vessel with those at a distance from the vessels in the lateral, basal, medial, central and cortical nuclei of the amygdaloid body of eight cats. The cross-sectional area and circumference of each identified axon terminal profile were measured, and the shape of synaptic vesicles and the presence of synaptic contacts and granular vesicles were recorded. The statistical evaluation of results was performed by means of the Newman-Keuls' test, Wilcoxon's test, Fisher's contingency-table test and the test for two coefficients of structure. The morphometric examination revealed two ultrastructurally distinct groups of axon terminals, pericapillary and distant terminals, in all the nuclei of the amygdaloid body. The differentiating features were the shape of the synaptic vesicles, the number of synaptic contacts, and the size of the axon terminals. These results further support the hypothesis of a functional connection between axon terminals and the capillary vessel wall in the CNS.

Suppression of amygdaloid kindled convulsion following unilateral injection of 2-amino-7-phosphonoheptanoic acid (2-APH) into the substantia innominata of rats

The comparative effect of intracerebral injection of 2-APH, a selective antagonist for N-methyl-D-aspartate (NMDA) receptors, into the substantia innominata (SI) and the amygdala (AM) of AM-kindled rats was examined. The intra-SI injection (ipsilateral to the kindled AM) induced a transient incoordination followed by immobility with loss of the rightening reflex, beginning at about 5 min following the injection and lasting for about 3 h. When the animals were stimulated at the previously established generalized seizure triggering threshold (GST) 45 min after the injection, the kindled seizure regressed to earlier stages although the afterdischarge (AD) duration remained unchanged. At 24 h, kindled seizure was readily activated at the GST. When 2-APH was injected into the kindled AM, no behavioural change occurred but AM stimulation at the GST failed to produce AD 45 min after the injection. Kindled seizure could be elicited, however, when the stimulus intensity was increased. This elevation of the GST lasted for 1-18 days. The findings suggest that NMDA receptors in the AM and SI play a differential role in AM seizure initiation and propagation, respectively. They also provide further support to the role presumed to be played by the SI in transforming the limbic seizure into motor seizure.

Suppression of amygdala-kindled seizure in cats by enhanced GABAergic transmission in the substantia innominata

The chronological effect of intracerebral injection of GABAergic drugs, either muscimol or gabaculine, into the substantia innominata, was examined in amygdala-kindled cats. Results obtained indicate: that the substantia innominata may play a significant role in ictal linkage between a nonmotor system such as the amygdala and the motor mechanism responsible for amygdala-kindled convulsion, and that the GABA terminals in the substantia innominata exert a suppressive action toward convulsive seizure generalization of amygdala origin.