Faculty Bibliography

The goal of the present study was to examine the contribution of thalamo-amygdala and thalamo-cortico-amygdala projections to fear conditioning. Lesions were used to destroy either the thalamo-cortico-amygdala projection, the thalamo-amygdala projection, or both projections, and the effects of such lesions on the acquisition of conditioned fear responses (changes in arterial pressure and freezing behavior) to a tone paired with footshock were measured. In each group of animals examined, a large lesion of the acoustic thalamus, including all nuclei of the medial geniculate body and adjacent portions of the posterior thalamus, was made on one side of the brain to block auditory transmission to the forebrain at the level of the thalamus on that side. In this way, experimental lesions could be made on the contralateral side of the brain. Thus, animals with thalamo-amygdala pathway lesions received a large lesion of the acoustic thalamus on one side. Contralaterally, only the nuclei that project to the amygdala (the medial division of the medial geniculate body, the posterior intralaminar nucleus, and the suprageniculate nucleus) were selectively destroyed, leaving much of the thalamo-cortico-amygdala projection intact. For thalamo-cortico-amygdala pathway lesions, the acoustic thalamus was destroyed on one side and temporal and perirhinal cortices were ablated contralaterally. In these animals, thalamo-amygdala projections were intact on the side of the cortical lesion. Destruction of either pathway alone had no effect on auditory fear conditioning. However, combined lesions of the two sensory pathways disrupted conditioning.(ABSTRACT TRUNCATED AT 250 WORDS)

The ultrastructure and synaptic associations of terminals immunoreactive for L-glutamate (Glu) were examined in the lateral nucleus of the amygdala (AL). All results reported here involved tissue fixed only with paraformaldehyde. The specificity of the antiserum with paraformaldehyde fixation conditions was assessed and confirmed by immuno-dot blot analysis: the reactivity of anti-Glu to glutamic acid was at least 1,000 times greater than the reactivity to other amino acids. At the light microscopic level, Glu-immunoreactive punctate processes and somata were present in AL. At the electron microscopic level, many Glu-immunoreactive terminals were identified. Data analysis was performed on 365 of these labeled terminals. Glu-immunoreactive terminals were 0.3-1.5 microns in diameter and contained numerous small, clear vesicles as well as mitochondria. Many (77%) of the terminals analyzed had morphologically identifiable synaptic specializations. Most (90%) of the Glu-immunoreactive terminals with synaptic specializations formed asymmetric synapses on spines or small dendrites; synaptic specializations on soma or proximal dendrites were rarely seen (< 1%). Glu-immunoreactive terminals were qualitatively compared to terminals in AL labeled with two other antisera: anti-glutaminase, a marker for the enzyme that catalyzes the conversion of glutamine to the releasable or transmitter form of Glu, and anti-gamma-aminobutyric acid (anti-GABA), a marker for the major inhibitory amino acid transmitter in the brain. Terminals immunoreactive for glutaminase, like those immunoreactive for Glu, formed mostly asymmetric synaptic specializations on spines or small dendrites. In contrast, GABA-immunoreactive terminals usually formed symmetric synapses on soma or proximal dendrites and were never observed to form asymmetric axo-spinous contacts. Although Glu is a metabolic precursor to GABA, these data indicate that the majority of Glu-immunoreactive terminals reflect the site of synthesis and release of Glu and not of GABA. In addition, these results provide morphological evidence that Glu plays a role in excitatory neurotransmission at synapses in AL and support the growing body of data implicating excitatory amino acid-mediated synaptic plasticity in-emotional learning and memory processes in AL.

A recent study, carried out in the monkey brain demonstrated a hitherto undescribed projection from the lateral to the basal nucleus of the amygdaloid complex. In the present study, we used light and electron microscopic techniques to determine whether a similar connection exists in the rat brain and to define what type(s) of synaptic contacts are produced by fibers of this projection. Injections of the lectin tracer Phaseolus vulgaris leucoagglutinin (PHA-L) were placed into several levels of the lateral nucleus and the distribution of fibers in the basal (basolateral) nucleus was evaluated. All lateral nucleus injections resulted in labeled fibers in the basal nucleus, though the density and distribution of labeled fibers depended on the position of the injection site within the lateral nucleus. In general, the heaviest labeling of the basal nucleus was observed after injections at midrostrocaudal levels of the lateral nucleus, especially when the injection was located ventrally. Fibers originating from cells labeled by these injections were observed throughout much of the rostrocaudal extent of the basal nucleus. Rostrally situated injections resulted in substantially lower levels of labeled fibers in the basal nucleus. Injections placed caudally in the lateral nucleus resulted in light to medium levels of labeled fibers in the basal nucleus; the terminal field in these cases did not extend as far rostrally as after the rostral and midlevel injections. Electron microscopic analysis of PHA-L labeled fibers revealed that they contributed synapses to the basal nucleus. The majority of PHA-L labeled terminals formed asymmetric contacts on dendritic spines or shafts; a smaller number of PHA-L labeled terminals formed symmetrical synapses

The present study examined whether complete bilateral destruction of auditory cortex would interfere with auditory fear conditioning in rats. Complete destruction of auditory cortex required lesions of temporal neocortical and perirhinal periallocortical areas. Fear conditioning was assessed by measuring freezing and arterial pressure responses elicited by an acoustic stimulus after pairing with footshock. Animals with complete bilateral lesions of auditory cortex showed conditioned arterial pressure and freezing responses comparable to those of unoperated controls. In contrast, bilateral destruction of the acoustic thalamus interfered with the conditioning of both responses. These results demonstrate that the auditory cortex is not required for the conditioning of fear responses to simple acoustic stimuli and add to the growing body of evidence that fear conditioning can be mediated by subcortical (amygdaloid) projections of the acoustic thalamus

The lateral amygdaloid nucleus (AL) is anatomically connected with sensory processing structures in the thalamus and cortex and is believed to be critically involved in emotional processing by virtue of these connections. In order to understand further how auditory projections to AL contribute to emotional processing, acoustic response properties of single AL neurons were characterized in rats. Recordings were also made in the posterior striatum dorsal to AL. Many cells in AL and the striatum could be driven by broad-band auditory stimulation with white noise or clicks. Initial onset latencies were typically between 12 and 25 msec. Most cells also had later responses (60-150 msec), and a few only had late responses. In frequency receptive field tests, different classes of cells were identified. One group had relatively clear frequency preferences. Thresholds for these relatively tuned cells tended to be somewhat higher in AL than in the striatum. Frequency preferences for AL cells were always above 10 kHz. Although most striatal cells had preferences for frequencies above 10 kHz, some cells were found with frequencies below 10 kHz as well. A second group of acoustically responsive neurons, much more common in AL than in the striatum, showed no frequency specificity (untuned cells). These responded to a wide range of frequencies, even at intensities near threshold. A third group, found mainly in AL (approximately 60% of the total population of cells examined in AL), exhibited rapid habituation to auditory stimuli. These tended to have high thresholds (80-100 dB). Because these cells habituated so quickly, frequency specificity could not be determined. Responses in AL and the striatum were compared with responses in the "specific" auditory relay nucleus of the thalamus, the ventral division of the medial geniculate body, where cells had shorter onset latencies, narrower tuning functions, and lower-intensity thresholds than cells in AL and striatal areas. These findings show that cells in AL exhibit a wide range of auditory tuning properties and suggest that information processing in the amygdala might be fruitfully studied as a direct extension of processing in sensory afferent structures.

The amygdala appears to play an essential role in many aspects of emotional information processing and behavior. Studies over the past year have begun to clarify the anatomical organization of the amygdala and the contribution of its individual subregions to emotional functions, especially emotional learning and memory. Researchers can now point to plausible circuits involved in the transmission of sensory inputs into the amygdala, between amygdaloid subregions, and to efferent targets in cortical and subcortical regions, for specific emotional learning and memory processes

The contribution of the amygdala and hippocampus to the acquisition of conditioned fear responses to a cue (a tone paired with footshock) and to context (background stimuli continuously present in the apparatus in which tone-shock pairings occurred) was examined in rats. In unoperated controls, responses to the cue conditioned faster and were more resistant to extinction than were responses to contextual stimuli. Lesions of the amygdala interfered with the conditioning of fear responses to both the cue and the context, whereas lesions of the hippocampus interfered with conditioning to the context but not to the cue. The amygdala is thus involved in the conditioning of fear responses to simple, modality-specific conditioned stimuli as well as to complex, polymodal stimuli, whereas the hippocampus is only involved in fear conditioning situations involving complex, polymodal events. These findings suggest an associative role for the amygdala and a sensory relay role for the hippocampus in fear conditioning