Central olfactory connections in the macaque monkey.

The connections between the olfactory bulb, primary olfactory cortex, and olfactory related areas of the orbital cortex were defined in macaque monkeys with a combination of anterograde and retrograde axonal tracers and electrophysiological recording. Anterograde tracers placed into the olfactory bulb labeled axons in eight primary olfactory cortical areas: the anterior olfactory nucleus, piriform cortex, ventral tenia tecta, olfactory tubercle, anterior cortical nucleus of the amygdala, periamygdaloid cortex, and olfactory division of the entorhinal cortex. The bulbar axons terminate in the outer part of layer I throughout these areas and are most dense in areas that are close to the lateral olfactory tract. Labeled axons also were found in the superficial part of nucleus of the horizontal diagonal band. Retrograde tracers injected into the olfactory bulb labeled cells in the nucleus of the diagonal band and in all of the primary olfactory cortical areas except the olfactory tubercle. Electrical stimulation of the olfactory bulb evoked short-latency unit responses and a characteristic field wave in the primary olfactory cortex. Multiunit activity in layer II tended to be of shorter latency than that in layer III and the endopiriform nucleus. Associational connections within the primary olfactory cortex were demonstrated with anterograde tracer injections into the piriform cortex and the entorhinal cortex. Injections into the piriform cortex near the lateral olfactory tract labeled axons in the deep part of layer I of many primary olfactory areas, but especially in areas near the tract. An injection into the rostral entorhinal cortex, distant to the lateral olfactory tract, labeled a complementary distribution of axons in deep layer I of olfactory areas medial and caudoventral to the tract. This organization resembles that reported in the primary olfactory cortex of the rat [Luskin and Price (1983) J. Comp. Neurol. 216:264-291]. The anterograde tracer injections into the piriform cortex and retrograde tracer injections into the orbital and medial prefrontal cortex and rostral insula label connections from the primary olfactory cortex to nine areas in the caudal orbital cortex, including the agranular insula areas Iam, Iai, Ial, Iapm, and Iapl and areas 14c, 25, 13a, and 13m. The piriform cortex projects most heavily to layer I of these areas. Only Iam, Iapm, and 13a receive a substantial projection to the deeper layers. Areas Iam, Iapm, and 13a were also the only areas that responded with multiunit action potentials to olfactory bulb stimulation in anesthetized animals.(ABSTRACT TRUNCATED AT 400 WORDS)

Somatosensory and auditory convergence in the lateral nucleus of the amygdala.

Previous studies have shown that the lateral nucleus of the amygdala (AL) is essential in auditory fear conditioning and that neurons in the AL respond to auditory stimuli. The goals of the present study were to determine whether neurons in the AL are also responsive to somatosensory stimuli and, if so, whether single neurons in the AL respond to both auditory and somatosensory stimulation. Single-unit activity was recorded in the AL in anesthetized rats during the presentation of acoustic (clicks) and somatosensory (footshock) stimuli. Neurons in the dorsal subdivision of the AL responded to both somatosensory and auditory stimuli, whereas neurons in the ventrolateral AL responded only to somatosensory stimuli and neurons in the ventromedial AL did not respond to either stimuli. These findings indicate that the dorsal AL is a site of auditory and somatosensory convergence and may therefore be a locus of convergence of conditioned and unconditioned stimuli in auditory fear conditioning.

Synaptic plasticity in fear conditioning circuits: induction of LTP in the lateral nucleus of the amygdala by stimulation of the medial geniculate body.

Electrical stimulation of the medial geniculate body in the anesthetized rat produces an evoked potential in the lateral nucleus of the amygdala. The potential varies in amplitude with stimulus intensity and reaches peak amplitude in 8.5 msec on the average. High-frequency stimulation of the pathway produces long-lasting increases in the amplitude and slope of the potential. These robust and enduring experience-dependent modifications in neural transmission occur in a pathway known to be involved in the formation of emotional memories and may offer a means for examining the cellular mechanisms of emotional learning, as well as a new approach to questions concerning the relevance of long-term potentiation to normal mnemonic processes.

[Demonstration of olfactory projections and responses within the entorhinal cortex in rats]. / Mise en évidence de projections et de réponses olfactives au sein du cortex entorhinal du rat.

An electrophysiological study was performed in rat entorhinal cortex. The results confirmed anatomical data on its connections with olfactory structures. Unit analysis has shown that neurons respond to odours. This area thus appears as an important structure for olfactory projections, possibly relaying these informations to the hippocampus.

Unit responses evoked in the amygdala and striatum by electrical stimulation of the medial geniculate body.

Unit activity was recorded from cells and cell clusters in the amygdala and striatum in response to electrical stimulation of the medial geniculate body (MGB) in rats anesthetized with chloral hydrate. Responses were mostly excitatory and were evoked against a relatively silent background (i.e., the units seldom fired between stimuli). The shortest latency responses were recorded in the caudate putamen (CPU), lateral amygdaloid nucleus (AL), and amygdalostriatal transition area (AST). Longer latency responses were obtained from neurons in the basolateral (ABL), basomedial (ABM), and central (ACE) nuclei of the amygdala. Moreover, while responses were evoked in AL, AST, and CPU with 300-500 microA stimuli delivered once every 10 sec, more intense and higher-frequency stimuli were required to obtain responses in ABL, ABM, and ACE. These findings are consistent with anatomical tracing studies showing that AL, AST, and CPU receive direct projections from the MGB and related acoustic processing areas of the thalamus but that ACE, ABL, and ABM do not.