The cell types present in the amygdala were, as in most otherbrain regions, initially described using Golgi techniques.More recently, single-cell recordings have been made in bothin vivo and in vitro preparations, the cells filled with dyes,and their morphology reconstructed after physiological recording.These studies have allowed a correlation of morphological andphysiological properties of neurons in several nuclei. AlthoughGolgi studies have been carried out in most regions of the amygdala,investigations of the electrophysiological characteristicsof neurons have centered mainly on the basolateral complexin vitro in the rat (56, 61, 219, 283) and in vivo in thecat (107–109, 196, 201).
Initially, two main types of neuron were described based onGolgi studies. The first type comprises 70% of the cell populationand has been described as pyramidal (73, 174, 283), spiny,or class I cells (153, 150). Many have pyramidal-like somatawith three to seven dendrites emanating from the soma. Thesecondary and tertiary dendrites of these cells are spiny. Oneof the dendrites is usually more prominent than the othersand thus has been likened to the apical dendrite of corticalneurons (56, 73). Some neurons appear to have two apicaldendrites and are more like the spiny stellate cells of thecortex (155). Unlike pyramidal neurons in the cortex or hippocampus,these cells are not arranged with parallel apical dendritesbut are randomly organized, particularly close to the nuclearborders (56, 155, 201, 219, 283). Thus, while this celltype has been described as pyramidal, these neurons differfrom cortical pyramidal neurons in several ways. The primarydendrite of the apical and basal dendrites is of equivalentlength, the dendrites taper rapidly, the distal dendrites donot have an elaborate terminal ramification, and as mentionedabove, there is no rigid orientation of the pyramids in oneplane (56, 112). Because of these clear differences, it maybe more appropriate to call these cells pyramidal-like or projectionneurons (56, 201). The axons of these cells originate eitherfrom the soma or from the initial portion of the primary dendrite(56, 150). They give off several collaterals within the vicinityof the cell before projecting into the efferent bundles ofthe amygdala, showing that they are projection neurons (150,260). For neurons within the basolateral complex, cells describedas pyramidal comprise a morphological continuum ranging frompyramidal to semi-pyramidal to stellate (56, 156, 201, 219,283). However, it should be noted that when reconstructedin coronal sections, cells can sometimes appear stellate becausethey have a largely rostrocaudal orientation (56, 174, 204,283). In general, neurons in the B are somewhat larger thanin the LA with an average soma diameter of 15–20 µmcompared with 10–15 µm in the LA (150, 174). Noclear morphological distinctions have been found between neuronsin the different subdivisions of the lateral or basal nuclei.As mentioned above, the large dendritic arbors of pyramidal-likeneurons indicate that the dendritic trees of these cells wouldcover the boundary between subdivisions (56, 200). Theseconsiderations call into question the functional parcellationof neurons in the basolateral complex into different subdivisions.
The second main group of cells found within the basolateralcomplex has slightly smaller somata (10–15 µm)and resembles nonspiny stellate cells of the cortex. Thesewere termed "S," for spiny cells by Hall (73) and "stellate"or "class II" cells by Millhouse and De Olmos (174). Thesecells have two to six primary dendrites that lack spines andform a relatively spherical dendritic field (109, 150). Thereis no apparent apical dendrite and, as with the pyramidal likeneurons, they form a heterogeneous population that has beensubdivided into multipolar, bitufted, and bipolar cells accordingto their dendritic trees by McDonald (150). These neurons areclearly GABAergic (160) and are local circuit interneurons.Their axons originate from the soma or from the proximal portionof a primary dendrite (150). Consistent with local circuitinterneurons, the axons branch several times and thus have a"cloud of axonal collaterals and terminals" near the cell body(174). Some of these interneurons form a pericellular basketor axonal cartridge around the perikarya and initial segmentof pyramidal cells, respectively, allowing a tight inhibitorycontrol over the output of the cell (28, 109, 161, 261).
Like interneurons in other cortical areas, these cells expressseveral calcium binding proteins (98, 163). About one-halfof the cells express parvalbumin, whereas the other half expresscalbindin and/or calretinin in their cytosol (98, 158), suggestingthat there are different classes of interneurons in the basolateralcomplex. However, there is significant overlap between thesethree markers. While the calretinin and parvalbumin positiveneurons form separate populations, a large proportion of theparvalbumin positive cells also express calbindin (98, 163).The functional relevance, if any, of these different populationsof interneurons is currently not known.
In addition, although uncommon, several other types of cellshave also been described in the basolateral complex on thebasis of distinctive axonal or dendritic patterns. These havebeen termed extended neurons, cone cells, chandelier cells,and neurogliaform cells (61, 95, 153, 150, 174). Extendedcells are large cells with long thick dendrites with few branchesand few spines and are found in the rostral parts of the basalnucleus. Cone cells, which have only been described in therat, have large cell bodies (20–30 µm) and cone-shapeddendritic trees that are nonspiny and are found in the dorsalangle of the lateral nucleus (174). Chandelier cells resemblecortical chandelier cells and have clustered axon varicositiesthat form synapses with the initial segment of pyramidal likeneurons (150). Finally, neurogliaform cells are another typeof small nonspiny stellate neuron found in the basolateralcomplex (95, 153, 150). These cells are small (10 µm)with a restricted spherical dendritic tree and branching axonsthat travel little further than the confines of their dendritictrees. They form numerous synaptic connections along the dendritesof pyramidal-like neurons and therefore probably representinhibitory local circuit neurons (150).
Electrophysiological studies of neurons in the basolateral complexhave been made in vivo from cats and in vitro in acute brainslices, largely from the rat. These neurons have been dividedaccording to whether they are located in the lateral or basalnucleus. However, no attempt has been made to separate neuronslocated in different subdivisions. In our experience, this islargely because internuclear boundaries that can be delineatedin Nissl-stained sections are not readily apparent in acutelyprepared coronal brain slices when viewed under the light microscope.However, the lateral and basal nucleus can be readily distinguished(Fig. 5).
Recordings both in vivo and in vitro from neurons in the LAshow extremely low levels of spontaneous activity (135, 200,201). Based on their firing properties in response to currentinjections, neurons in the LA have been broadly divided intotwo types (Fig. 6) (135, 201). The first type, comprising95% of total cells, fires broad action potentials (half-width1.2 ms measured at 28–30°C) and shows varying degreesof spike frequency adaptation in response to a prolonged depolarizingcurrent injection. Action potential trains are followed bya prolonged (1–5s) afterhyperpolarization (AHP), whichis largely responsible for the spike frequency adaptation (57).The second population fires short-duration action potentials(half-width 0.7 ms) and shows little spike frequency adaptationin response to a prolonged depolarizing current injection (109, 135, 201) (Fig. 6). Due to the similarities with corticaland hippocampal neuron firing properties (109, 135), the firsttype was classified as pyramidal or projection neurons andthe second as interneurons. This electrophysiological distinctionbetween projection neurons and local circuit interneurons issimilar to that seen in other brain regions (37, 148). A detailedanalysis of repetitive firing patterns of pyramidal neuronsin the lateral nucleus has recently been carried out usingwhole cell patch-clamp recordings from coronal rat brain sections(56). These characteristics were then correlated with morphologicalproperties by filling cells with neurobiotin. In this study,cells were classified according to the degree of spike frequencyadaptation that they displayed in response to a prolonged currentinjection. It was found that pyramidal-like neurons formed acontinuum of firing properties (Fig. 7). At one end of thespectrum cells fire two to three spikes only and show markedspike frequency accommodation, whereas at the other end of thespectrum cells fire repetitively throughout the current injectionwith little accommodation (Fig. 7A) (56, 65). In betweenwere cells that fire several times but show clear spike frequencyadaptation. The majority of the cells lay at the end of thespectrum that fired fewer spikes and showed marked accommodation.These neurons did not show any difference in resting membraneproperties. Quantitative analysis of the morphological propertiesof neurons at each end of the electrophysiological spectrumrevealed no significant differences between cells (56, 65).Thus it was concluded that these neurons have differentialdistributions of voltage-gated and calcium-activated potassiumchannels that determine their repetitive firing properties(56, 57). In accordance with this, cells that show spike frequencyadaptation were shown to have larger AHPs than those that firerepetitively (Fig. 7B) (56). This wide distribution of firingproperties is consistent with the distribution of morphologicalfeatures that have been described for projection neurons inthe basolateral complex (see above). However, no correlationwas found between the cells' firing properties and their morphology(56).
Finally, one other cell type, termed a single firer, that standsout from the above classification has also been described inthe LA and comprises 3% of recorded cells (33, 56, 61, 297).In this cell type only a single action potential is evokedin response to a prolonged current injection; the excitabilitycould not be enhanced by giving larger current injections orby depolarizing the cell. Despite the marked accommodation thatit showed, no prolonged AHP followed the action potential (243). These cells appear to express a dendrotoxin-sensitivevoltage-gated potassium current that is responsible for theirmarked spike frequency adaptation (E. Faber and P. Sah, unpublishedobservations). Thus this cell was considered to be in a discreteclass from the above neurons. Faulkner and Brown (61) recoveredone of these cells for morphological analysis and found thatit was pyramidal-like but with few spines. In contrast, Yajeyaet al. (296) who recovered two of these neurons reported themto have a round soma from which four or five spiny dendritesemanated in a spherical fashion. Yajeya et al. (297) have proposedthe single firing neurons to represent the neurogliaform (typeIII) cells described by McDonald (150).
Intracellular recordings from LA neurons made in vitro in thecat and guinea pig and in vivo in cats have found a large proportionof LA projection neurons to display intrinsic voltage-dependentoscillations. These increased in frequency in a voltage-dependentmanner until repetitive spiking was evoked with larger depolarizingcurrent injections (196, 201). As first reported in entorhinalcortex (6), these oscillations have been suggested to be dueto the activity of subthreshold tetrodotoxin-sensitive sodiumchannels (196). Upon depolarization, these cells fire a burstof action potentials followed by a slow rhythmic firing ofsingle spikes, but do not fully accommodate. Action potentialsare followed by an AHP. A small number of nonoscillating burstingneurons were also described that fired a burst of two to threeaction potentials before firing in a sustained fashion andshowing no accommodation. As with the results described invitro (56, 65), no morphological differences were found betweenneurons with different firing properties (201).
Thus, although there are some similarities, there are also cleardifferences in the description of pyramidal neurons recordedin vivo in cat and those recorded in vitro in the rat. First,the in vitro recordings from neurons in rat slices using wholecell patch-clamp techniques have not shown the membrane oscillationsdescribed in vivo. Second, while the firing patterns were notdescribed in detail for recordings in vivo, these studies didnot describe any fully accommodating projection neurons inthe LA, the major cell type described in vitro. The reasonfor this disparity is not clear. However, the recordings invivo were made with sharp intracellular microelectrodes, whichleads to a lower input resistance due to the membrane leak aroundthe electrode. This would have attenuated the impact of theAHP in the neurons, and thus all cells would appear as repetitivelyfiring neurons. Another possibility is wash out of chemicalmediators of the oscillations in whole cell recordings. Infact, in the few cells that showed full accommodation in thecat and guinea pig LA, the oscillations were absent, suggestingthat adaptation may reflect an inactivation of the oscillationswhen recorded in the whole cell mode (196). Finally, thesedifferences may be species dependent, since microelectrode recordings in the LA in vivo have revealed that accommodatingneurons can also be found in rats (35).
Two studies have examined the electrophysiological propertiesof neurons in the basal nucleus using intracellular recordingsin rat brain slices and correlated them with their morphologicalproperties (219, 283). As in the LA, these cells have beendivided into pyramidal or projection neurons, comprising 95%of the neuronal cell mass, and local circuit interneurons, whichcomprise the remaining 5%. Pyramidal neurons have been furtherclassified into two electrophysiological groups based on theirfiring patterns, burst firing, and repetitive firing. Burstfiring cells fired one or two spikes before ceasing firing,whereas repetitively firing cells fired throughout the currentinjection but showed little accommodation (219, 283, 297).A number of intermediate neurons have also been described (219, 283). Thus as with LA neurons, basal neurons form acontinuum of firing patterns. The repetitive firing neuronsdescribed by Washburn and Moises (283) differ from those inthe LA because they show a delay in firing when depolarizedfrom more negative membrane potentials and have therefore beentermed "late firing" neurons (Fig. 7C). This effect has beenshown to be due to the presence of a low-threshold, slowlyinactivating potassium current (ID) in these neurons (283).Similar to LA projection neurons, action potentials in basalprojection neurons are broad, and accommodating neurons havea significantly larger AHP than repetitively firing neurons.The difference in AHP is most likely the basis of the differencein two extremes of firing pattern (56, 243). The electrophysiologicalproperties of pyramidal neurons in the LA and B are subjectto modulation by ascending and local transmitter systems. Acetylcholine,norepinephrine, glutamate, serotonin, and opioids all modulatevoltage- or calcium-dependent potassium currents leading tochanges in spike frequency adaptation (57, 285) (E. Faberand P. Sah, unpublished observations).
Neurons in the basal nucleus have been examined in vivo in thecat (200, 201). In contrast to LA neurons, which are virtuallysilent, basal nucleus neurons were reported to fire in burstsat rest. The bursts of spikes were followed by a nonadaptingtrain of spikes, due to activation of a slow afterdepolarization.These constituted 80% of basal nucleus neurons recorded from.The remaining 20% of neurons in the basal nucleus were nonburstingcells that accommodated and showed oscillations similar tothose recorded in the LA. After reconstruction of these cells,they were all described as modified pyramids, and no consistentdifferences in morphology were noted. In recordings made inin vitro brain slices, Rainnie et al. (219) described thebursting cells to be spiny stellate, whereas Washburn and Moises(283) reported them to be spiny pyramidal. In contrast, athird study (297) reported the repetitively firing neuronsto be stellate. The simplest explanation for these discrepanciesis likely to be the large variation in the orientation of theapical dendrite which makes clear classification of cell morphologydifficult (201). In summary, in contrast to the discrepanciesin recordings from LA pyramidal cells, there is more consensusin the properties of neurons in the basal nucleus.
Recordings from interneurons in the basolateral complex havebeen made both in vitro and in vivo in both the lateral andbasal nuclei. These neurons show a similar pattern of physiologicalproperties. In all cases, they generate narrow action potentials(half width 0.7 ms) and in response to a depolarizing currentinjection fire nonadapting trains of action potentials (109,135, 201, 284). In contrast to pyramidal neurons, interneuronsfire spontaneously in vivo at high frequencies (10–15Hz) (109, 201). As described above, interneurons can be dividedinto at least two classes based on their content of calciumbinding proteins. However, no differences in physiological properties between interneurons have been reported.
C. Synaptic Properties
Pyramidal-like neurons in the basolateral complex show highlevels of immunoreactivity for glutamate and aspartate (260)but not glutamic acid decarboxylase (28). Thus these cellsare presumed to be glutamatergic and form the output cells ofthis structure. These neurons receive both cortical and thalamicinputs which form asymmetrical synapses (58). Consistent withtheir morphology, these inputs are glutamatergic and form synapsescontaining both -amino-3-hydroxy-5-methyl-4-isoxazolepropionicacid (AMPA) and N-methyl-D-aspartate (NMDA) receptors (59,60). Three types of ionotropic glutamate receptors, AMPA,NMDA, and kainate receptors, are recognized in the mammaliancentral nervous system (83). The presence of AMPA and NMDAreceptors at excitatory synapses within the central nervoushas been known for many years. Electrophysiological studies(Figs. 7D and 8) have confirmed that cortical and thalamicafferents to pyramidal neurons form dual-component glutamatergicsynapses (32, 136, 220, 290). Analysis of spontaneous,miniature synaptic currents has shown that AMPA and NMDA receptorsare present at individual synapses in these neurons (136).The properties of these two inputs are similar with regardto the type of AMPA receptors that they express. However, ithas been suggested that NMDA receptors present at thalamicinputs might be different from those present at cortical inputs(291). At most synapses, NMDA receptors are not active atresting membrane potentials due to their voltage-dependentblock by extracellular Mg2+ (146, 191). The suggestion isthat NMDA receptors present at thalamic inputs have lower levelsof Mg2+ block such that they are active at resting membranepotentials (127, 291, but see Ref. 136). This finding hasnot been further studied but has major implications for theinterpretation of experiments in which NMDA receptors are blockedby specific antagonists (see below).
NMDA receptors are hetero-oligomers assembled from two typesof subunits, NR1 and NR2. The NR1 subunit is a single geneproduct, whereas the NR2 subunit is encoded by four differentgenes: NR2A–NR2D (147). Native NMDA receptors are thoughtto be heteromultimers containing four or five subunits consistingof two NR1 subunits and two or three NR2 subunits (38). Atmost synapses throughout the central nervous system, NMDA receptorsare composed of NR1 subunits in combination with either NR2Aor NR2B subunits. NR2A and NR2B subunits are ubiquitously distributedthrough the central nervous system and have been shown to undergoa developmental switch in hippocampal and cortical neurons(179). At birth NMDA receptors are composed of NR1/NR2B subunits,and there is a switch from NR2B to NR2A subunits around P7.However, in the LA, a recent study has shown that applicationof the NR2B-selective antagonist ifenprodil blocks the inductionof fear conditioning, suggesting that receptors containingNR2B subunits are present at synapses in the adult lateralamygdala where they are involved in initiating synaptic plasticity(227). While both NR2A and NR2B subunits are present in thelateral amygdala, the subunit composition of these receptorsat synapses in the amygdala has not been determined.
Recently, the presence of kainate receptors at synapses hasalso been demonstrated (31, 280). It has been suggested thatkainate receptors are also present at some glutamatergic inputsto pyramidal neurons in the basal nucleus, where they are proposedto be involved in basal synaptic transmission (126). All threetypes of ionotropic glutamate receptor have been suggestedto underlie different forms of synaptic plasticity in the amygdala(see below).
Glutamate also activates metabotropic receptors that are coupledvia G proteins to phospholipase C or adenylyl cyclase (207).These receptors are found both presynaptically and postsynapticallyin many regions of the central nervous system. However, onlya few effects resulting from synaptically released glutamatehave been described (206). Activation of metabotropic receptorsby application of exogenous agonists in basal amygdala neuronshas both presynaptic and postsynaptic actions (221, 222).However, effects of metabotropic glutamate receptors by synapticallyreleased glutamate have only been described during the inductionof synaptic plasticity (see below).
Neurons in the LA have also been suggested to have a fast excitatoryinputs mediated by 5-hydroxytryptamine (5-HT) receptors (264).However, since this initial report, subsequent experimentshave been unable to reproduce these results as all inputs tothese neurons can be blocked with a combination of glutamatergicand GABAergic antagonists (136, 259, 290). Instead, 5-HT3receptors in this nucleus have been proposed to be present presynaptically on interneuron terminals (103, 104).
Interestingly, although heterogeneity in firing properties hasbeen described in pyramidal neurons, there have been no reportsof differences in synaptic properties between cells, suggestingthat the properties of exogenous inputs to all pyramidal neuronsare similar. As discussed above, the axons of pyramidal neuronshave substantial local collaterals (150, 260). Many of thelocal targets of these collateral are interneurons (262),but they are also likely to contact nearby pyramidal neurons(150). The properties of any of these local connections arenot known.
Interneurons in the basolateral complex receive excitatory inputsfrom local, cortical, and thalamic sources (109, 135, 270).In addition, these neurons are connected in local networkssuch that interneurons have synaptic connections with eachother (109). In contrast to pyramidal-like neurons, glutamatergicinputs to interneurons activate synapses that express few orno NMDA receptors in the postsynaptic membrane (135). Furthermore,the AMPA receptors present at these inputs show marked inwardrectification and appear to be calcium permeable (135). AMPAreceptors are heteromultimers assembled from four genes, GluR1–GluR4.Receptors that lack GluR2 subunits have strong inward rectification(Fig. 8) and a high calcium permeability (93, 286). Consistentwith the marked inward rectification reported at these synapses(135), GABAergic cells in the basolateral complex have beenshown to express low levels of GluR2 subunits (157). As describedabove, interneurons in the basolateral complex are a heterogeneouspopulation of neurons that can be separated on morphologicalgrounds and their content of calcium binding proteins. Physiologicalstudies of these neurons have not thus far reported differencesin synaptic properties between different cells. However, recentresults from our laboratory suggest that some interneurons inthe basolateral complex do express synaptic NMDA receptors(A. Woodruff and P. Sah, unpublished observations).
As in most other parts of the central nervous system, the fastspiking cells are GABAergic and constitute local circuit interneurons(160, 189, 202, 209). Activation of these cells in thebasolateral complex generates inhibitory synaptic potentialsthat have fast and slow components (Fig. 7D) (108, 218,284). As originally described in the hippocampus (52), thefast component is mediated by GABAA receptors while the slowcomponent is mediated by activation of GABAB receptors (135,218, 284). Measurements of spontaneously occurring miniatureinhibitory synaptic currents have suggested that differentinterneurons in the basolateral complex are responsible forgenerating the GABAA and GABAB receptor-mediated componentof inhibitory synaptic current (263). Direct evidence for this proposal, for example, by paired interneuron/pyramidalcell recordings, is currently lacking. However, stimulationof different afferents indicates that these different interneuronscannot be independently stimulated by extrinsic inputs in vivo(107). In contrast to most other cells, the slow componentof the inhibitory synaptic potential in LA pyramidal neuronsis in part generated by a calcium-activated potassium conductancethat is activated by calcium influx via NMDA receptors (39,108). This observation raises the possibility that, as recentlydescribed in the olfactory bulb (85), NMDA receptors in lateralamygdala neurons might be coupled to calcium-activated potassiumchannels.
Interneurons can mediate both feed-forward or feedback inhibition(3). In the basolateral complex, whether interneurons mediatefeed-forward inhibition or feedback inhibition (or both) hasnot been fully determined. Electrophysiological studies inacute slices in the rat have shown that these cells receiveboth cortical and thalamic excitatory inputs, consistent witha role of these cells in feed-forward inhibition (109, 135,270). However, it is possible that the excitatory inputs tointerneurons are due to activation of axon collaterals of pyramidalcells, which indicates a feedback role for interneurons. Tracttracing studies in the cat and monkey found that cortical afferentsform few if any synapses with parvalbumin positive neurons,while local afferents do make synapses onto them (262). Incontrast, a similar study in the rat has described thalamicinputs to interneurons in the LA (295). These findings areconsistent with the proposal that different populations ofinterneurons can have a feed-forward and/or a feedback rolein the basolateral complex.
In summary, pyramidal neurons and local circuit interneuronsin the basolateral nuclei can be separated on electrophysiologicalgrounds. As in the cortex, pyramidal neurons have a range offiring properties. However, unlike in the cortex, these differentrepetitive firing properties are not accompanied by clear morphologicaldifferences. Among the interneurons, several classes of cellcan be identified based on the presence of different calciumbinding proteins. The roles of these cells with different firingproperties are not currently understood. However, it seemslikely that neurons with differing electrophysiological propertieswill involve different local circuits and have distinct afferent/efferentconnections.