Efferent projections of the anterior and posterodorsal regions of the medial nucleus of the amygdala in the mouse.

The efferent projections of the anterior and posterodorsal part of the medial nucleus (MePD) in the mouse were studied by means of anterograde axonal tracing using biotinylated dextran amine. The MePD axons ran mainly via the stria terminalis and to a lesser extent via the ventral amygdalofugal pathway. The projections to the forebrain were broadly distributed and varied from very strong to scant. The most significant connections were destined to the bed nucleus of the stria terminalis in which all parts of the medial division were innervated by MePD neurons. Moderate projections reached the limbic striatum (nucleus accumbens), olfactory tubercle and the lateral septal nucleus. The substantia innominata was also innervated by the MePD, and especially the projection to its ventral portion was substantial. The profuse innervation of the medial preoptic nucleus and medial preoptic area indicated significant involvement of the MePD in sexual behavior. Many hypothalamic nuclei were innervated but to a different extent. The very strong innervation of the ventral premammillary nucleus further indicated the involvement of the MePD in the neuronal circuitry for sexual behavior. Substantial projections also reached the anterior hypothalamus and tuber cinereum, while the connections to the lateral hypothalamus were widespread but showed moderate density. MePD strongly innervated the ventrolateral part of the ventromedial hypothalamic nucleus and moderately its remaining parts. The neurosecretory hypothalamic nuclei and the arcuate nucleus contained only a few MePD terminals. The thalamic innervation was very scant and reached the lateral habenular nucleus and the nuclei of the midline. The mesencephalic connections were moderate to sparse and projected to the mesolimbic dopaminergic groups in the ventral tegmental area, the pars lateralis and the dorsal tier of the substantia nigra pars compacta, the periaqueductal gray and the dorsal raphe nucleus. The present results principally resembled data known in other rodent species; however, the efferents of the MePD often differed in extent and/or topical distribution.

Efferent connections of the parabigeminal nucleus to the amygdala and the superior colliculus in the rat: a double-labeling fluorescent retrograde tracing study.

The parabigeminal nucleus (Pbg) is a subcortical visual center that besides reciprocal connections with the superior colliculus (SC), also projects to the amygdala (Am). The Pbg-Am connection is part of a multineuronal pathway that conveys extrageniculostriate inputs of the retina to the Am, and it rapidly responds to the sources of threat before conscious detection. The present study demonstrates that Pbg projects bilaterally to Am and SC. The ipsilateral projections arise from separate cell populations, whilst the contralaterally projecting Pbg neurons emit branching axons that simultaneously innervate Am and SC.

Glutamate stimulation of acetylcholine release from myenteric plexus is mediated by endogenous nitric oxide.

Glutamate was found to be an excitatory neurotransmitter in the enteric nervous system. Although several lines of evidence indicate a role of glutamate in the regulation of gut motility and secretion the physiological significance of glutamatergic transmission is not clear yet. We studied the effect of glutamate on [3H]acetylcholine release and nicotinamide adenine dinucleotide phosphate-diaphorase staining in longitudinal muscle strips with attached myenteric plexus of guinea pig ileum. L-glutamate (100 microM) significantly enhanced both the evoked [3H]acetylcholine release and the optical density of nicotinamide adenine dinucleotide phosphate-diaphorase positive neurones, i.e. the intensity of staining. The non-competitive N-methyl-D-aspartate receptor antagonist MK-801 (3 microM) abolished the stimulatory effect of L-glutamate on acetylcholine efflux. Similarly, the nitric oxide synthase inhibitor N(omega)-nitro-L-arginine (100 microM) significantly reduced the effect of L-glutamate on [3H]acetylcholine release and nicotinamide adenine dinucleotide phosphate-diaphorase staining. Our data suggest that endogenous nitric oxide seems to mediate the stimulatory effect of glutamate on acetylcholine release from guinea pig myenteric neurons.

Orexinergic innervation of the extended amygdala and basal ganglia in the rat.

The orexinergic system interacts with several functional states of emotions, stress, hunger, wakefulness and behavioral arousal through four pathways originating in the lateral hypothalamus (LH). Hundreds of orexinergic efferents have been described by tracing studies and direct immunohistochemistry of orexin in the forebrain, olfactory regions, hippocampus, amygdala, septum, basal ganglia, thalamus, hypothalamus, brain stem and spinal cord. Most of these tracing studies investigated the whole orexinergic projection to all regions of the intracranial part of the CNS. To identify the orexinergic efferents at the subnuclear level of resolution, we focussed on the orexinergic target in the amygdala, which is substantially involved in the LH output and contributes mostly to the functional outcome of the orexinergic system and the basal ganglia. Immunohistochemical identification of axonal orexin A and orexin B in male adult rats has been performed on serial sections. In the extended amygdala many new orexinergic targets were found in the anterior amygdaloid area (dense), anterior cortical nucleus (moderate), amygdalostriatal transition region (moderate), basolateral regions (moderate), basomedial nucleus (moderate), several bed nucleus of the stria terminals regions (few to dense), central amygdaloid subdivisions (dense), posteromedial cortical nucleus (moderate) and medial amygdaloid subnuclei (dense). Furthermore, the entopeduncular nucleus has been newly identified as another target for orexinergic fibers with a high density. These results suggest that subdivisions and subnuclei of the extended amygdala are specific targets of the orexinergic system.

Amygdalotrigeminal projection in the rat: An anterograde tracing study.

Previous neurophysiological studies have demonstrated that the amygdala has a direct influence upon trigeminal motoneuron activity. The existence of a direct amygdalotrigeminal pathway in rats was proved by anterograde tracing with the neuroanatomical tracer, biotinylated dextran amine (BDA). After ipsilateral BDA application to the central nucleus of the amygdala (AmCe), widespread ipsilateral projections emerging from its medial subnucleus were traced to the trigeminal brainstem nuclear complex, including the principal sensory (Pr5) and mesencephalic trigeminal nucleus (Me5), and their premotoneurons and interneurons, located in the supratrigeminal, intertrigeminal and peritrigeminal nuclei. Sparse BDA-labeled axons and their terminals were also distributed in the contralateral Pr5, interpolar and caudal subnuclei of the spinal trigeminal nucleus. The central lateral amygdaloid nucleus gives rise to a light ipsilateral projection to the pontine part of the Me5. The present data indicate that AmCe sends massive efferents to the trigeminal nuclei in the brainstem, wherein its medial subnucleus sends the major input to them. The medial amygdaloid nucleus sparsely innervates Me5 neurons, specifically those located in its mesencephalic portion, while basomedial and basolateral efferents do not target the trigeminal nuclear complex. These results suggest that the amygdaloid input may modulate the activity of trigeminal sensory and motor neurons and, thus, the amygdala is possibly involved in the control of masticatory behavior.