Alterations in size, number, and morphology of gustatory papillae and taste buds in BDNF null mutant mice demonstrate neural dependence of developing taste organs

Sensory ganglia that innervate taste buds and gustatory papillae (geniculate and petrosal) are reduced in volume by about 40% in mice with a targeted deletion of the gene for brain-derived neurotrophic factor (BDNF). In contrast, the trigeminal ganglion, which innervates papillae but not taste buds on the anterior tongue, is reduced by only about 18%. These specific alterations in ganglia that innervate taste organs make possible a test for roles of lingual innervation in the development of appropriate number, morphology, and spatial pattern of fungiform and circumvallate papillae and associated taste buds. We studied tongues of BDNF null mutant and wild-type littermates and made quantitative analyses of all fungiform papillae on the anterior tongue, the single circumvallate papilla on the posterior tongue, and all taste buds in both papilla types. Fungiform papillae and taste buds were reduced in number by about 60% and were substantially smaller in diameter in mutant mice 15–25 days postnatal. Remaining fungiform papillae were selectively concentrated in the tongue tip region. The circumvallate papilla was reduced in diameter and length by about 40%, and papilla morphology was disrupted. Taste bud number in the circumvallate was reduced by about 70% in mutant tongues, and the remaining taste buds were smaller than those on wild-type tongues. Our results demonstrate a selective dependence of taste organs on a full complement of appropriate innervation for normal growth and morphogenesis. Effects on papillae are not random but are more pronounced in specific lingual regions. Although the geniculate and petrosal ganglia sustain at least half of their normal complement of cell number in BDNF −/− mice, remaining ganglion cells do not substitute for lost neurons to rescue taste organs at control numbers. Whereas gustatory ganglia and the taste papillae initially form independently, our results suggest interdependence in later development because ganglia derive BDNF support from target organs and papillae require sensory innervation for morphogenesis. J. Comp. Neurol. 409:13–24, 1999.  © 1999 Wiley-Liss, Inc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/34453/1/2_ftp.pd

Gene Expression Changes in the Motor Cortex Mediating Motor Skill Learning

The primary motor cortex (M1) supports motor skill learning, yet little is known about the genes that contribute to motor cortical plasticity. Such knowledge could identify candidate molecules whose targeting might enable a new understanding of motor cortical functions, and provide new drug targets for the treatment of diseases which impair motor function, such as ischemic stroke. Here, we assess changes in the motor-cortical transcriptome across different stages of motor skill acquisition. Adult rats were trained on a gradually acquired appetitive reach and grasp task that required different strategies for successful pellet retrieval, or a sham version of the task in which the rats received pellet reward without needing to develop the reach and grasp skill. Tissue was harvested from the forelimb motor-cortical area either before training commenced, prior to the initial rise in task performance, or at peak performance. Differential classes of gene expression were observed at the time point immediately preceding motor task improvement. Functional clustering revealed that gene expression changes were related to the synapse, development, intracellular signaling, and the fibroblast growth factor (FGF) family, with many modulated genes known to regulate synaptic plasticity, synaptogenesis, and cytoskeletal dynamics. The modulated expression of synaptic genes likely reflects ongoing network reorganization from commencement of training till the point of task improvement, suggesting that motor performance improves only after sufficient modifications in the cortical circuitry have accumulated. The regulated FGF-related genes may together contribute to M1 remodeling through their roles in synaptic growth and maturation.McGovern Institute for Brain Research at MITNational Institutes of Health (U.S.) ((NIH grant 1-RC1-NS068103-01)National Institutes of Health (U.S.) (NIH grant R01-MH084966)Roberto Rocca Education Program (Fellowship)Massachusetts Institute of Technology. Undergraduate Research Opportunities Program (Fellowship)Italy. Ministero dell'istruzione, dell'università e della ricerca (MIUR grant RBIN04H5AS)Italy. Ministero dell'istruzione, dell'università e della ricerca (MIUR grant RBLA03FLJC)Italy. Ministero dell'istruzione, dell'università e della ricerca (FIRB n. RBAP10L8TY

Changes in Brain MicroRNAs Contribute to Cholinergic Stress Reactions

Mental stress modifies both cholinergic neurotransmission and alternative splicing in the brain, via incompletely understood mechanisms. Here, we report that stress changes brain microRNA (miR) expression and that some of these stress-regulated miRs regulate alternative splicing. Acute and chronic immobilization stress differentially altered the expression of numerous miRs in two stress-responsive regions of the rat brain, the hippocampal CA1 region and the central nucleus of the amygdala. miR-134 and miR-183 levels both increased in the amygdala following acute stress, compared to unstressed controls. Chronic stress decreased miR-134 levels, whereas miR-183 remained unchanged in both the amygdala and CA1. Importantly, miR-134 and miR-183 share a common predicted mRNA target, encoding the splicing factor SC35. Stress was previously shown to upregulate SC35, which promotes the alternative splicing of acetylcholinesterase (AChE) from the synapse-associated isoform AChE-S to the, normally rare, soluble AChE-R protein. Knockdown of miR-183 expression increased SC35 protein levels in vitro, whereas overexpression of miR-183 reduced SC35 protein levels, suggesting a physiological role for miR-183 regulation under stress. We show stress-induced changes in miR-183 and miR-134 and suggest that, by regulating splicing factors and their targets, these changes modify both alternative splicing and cholinergic neurotransmission in the stressed brain

Contextual and auditory fear conditioning are mediated by the lateral, basal, and central amygdaloid nuclei in rats

A large body of literature implicates the amygdala in Pavlovian fear conditioning. In this study, we examined the contribution of individual amygdaloid nuclei to contextual and auditory fear conditioning in rats. Prior to fear conditioning, rats received a large electrolytic lesion of the amygdala in one hemisphere, and a nucleus-specific neurotoxic lesion in the contralateral hemisphere. Neurotoxic lesions targeted either the lateral nucleus (LA), basolateral and basomedial nuclei (basal nuclei), or central nucleus (CE) of the amygdala. LA and CE lesions attenuated freezing to both contextual and auditory conditional stimuli (CSs). Lesions of the basal nuclei produced deficits in contextual and auditory fear conditioning only when the damage extended into the anterior divisions of the basal nuclei; damage limited to the posterior divisions of the basal nuclei did not significantly impair conditioning to either auditory or contextual CS. These effects were typically not lateralized, although neurotoxic lesions of the posterior divisions of the basal nuclei had greater effects on contextual fear conditioning when the contralateral electrolytic lesion was placed in the right hemisphere. These results indicate that there is significant overlap within the amygdala in the neural pathways mediating fear conditioning to contextual and acoustic CS, and that these forms of learning are not anatomically dissociable at the level of amygdaloid nuclei.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/61940/1/goosensLM01.pd

Contextual and Auditory Fear Conditioning are Mediated by the Lateral, Basal, and Central Amygdaloid Nuclei in Rats

A large body of literature implicates the amygdala in Pavlovian fear conditioning. In this study, we examined the contribution of individual amygdaloid nuclei to contextual and auditory fear conditioning in rats. Prior to fear conditioning, rats received a large electrolytic lesion of the amygdala in one hemisphere, and a nucleus-specific neurotoxic lesion in the contralateral hemisphere. Neurotoxic lesions targeted either the lateral nucleus (LA), basolateral and basomedial nuclei (basal nuclei), or central nucleus (CE) of the amygdala. LA and CE lesions attenuated freezing to both contextual and auditory conditional stimuli (CSs). Lesions of the basal nuclei produced deficits in contextual and auditory fear conditioning only when the damage extended into the anterior divisions of the basal nuclei; damage limited to the posterior divisions of the basal nuclei did not significantly impair conditioning to either auditory or contextual CS. These effects were typically not lateralized, although neurotoxic lesions of the posterior divisions of the basal nuclei had greater effects on contextual fear conditioning when the contralateral electrolytic lesion was placed in the right hemisphere. These results indicate that there is significant overlap within the amygdala in the neural pathways mediating fear conditioning to contextual and acoustic CS, and that these forms of learning are not anatomically dissociable at the level of amygdaloid nuclei

Pretraining NMDA receptor blockade in the basolateral complex, but not the central nucleus, of the amygdala prevents savings of conditional fear

The acquisition of conditional freezing is abolished by N-methyl-D-aspartate (NMDA) receptor antagonism in the basolateral complex of the amygdala (BLA) during fear conditioning, suggesting that memory formation is prevented. The present study examined whether there is residual memory, or "savings," for fear conditioning in rats trained under amygdaloid NMDA receptor blockade. Rats infused with D,L-2-amino-5-phosphonovalerate (APV) into the BLA or central nucleus of the amygdala (CEA) during fear conditioning did not acquire either auditory or contextual fear conditioning. However, savings of conditional fear was exhibited by rats infused with APV into the CEA but not the BLA. These results suggest that both the BLA and CEA play a critical role in the acquisition of conditional fear but that the BLA is able to process and retain some aspects of aversive memories in the absence of the CEA.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/61952/1/goosensBN03.pd

Context-dependent neuronal activity in the lateral amygdala represents fear memories after extinction

The context in which fear memories are extinguished has important implications for treating human fear and anxiety disorders. Extinction of Pavlovian fear conditioning is context specific; after extinction, fear responses are reduced only in the extinction context and remain elevated in every other context. Contextual modulation of spike firing in the amygdala is a putative mechanism for the context-specific expression of extinguished fear. To test this possibility, we conditioned rats to fear two auditory conditional stimuli (CSs) and then extinguished each CS in separate and distinct contexts. Thereafter, single-unit activity in the lateral nucleus of the amygdala (LA) and freezing behavior were recorded during tests in which each CS was presented in each extinction context. Hence, each CS was tested in its own extinction context and in the context of the other CS. Conditional freezing was context dependent; fear to an extinguished CS was low in its own extinction context and high in the other test context. Similarly, the majority of LA neurons exhibited context-dependent spike firing; short-latency spike firing was greater to both CSs when they were presented outside of their own extinction context. In contrast, behavioral and neuronal responses to either non-extinguished CSs or habituated auditory stimuli were not contextually modulated. Context-dependent neuronal activity in the LA may be an important mechanism for disambiguating the meaning of fear signals, thereby enabling appropriate behavioral responses to such stimuli.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/61951/1/hobinJN03.pd

Auditory-evoked spike firing in the lateral amygdala and Pavlovian fear conditioning: mnemonic code or fear bias?

Amygdala neuroplasticity has emerged as a candidate substrate for Pavlovian fear memory. By this view, conditional stimulus (CS)-evoked activity represents a mnemonic code that initiates the expression of fear behaviors. However, a fear state may nonassociatively enhance sensory processing, biasing CS-evoked activity in amygdala neurons. Here we describe experiments that dissociate auditory CS-evoked spike firing in the lateral amygdala (LA) and both conditional fear behavior and LA excitability in rats. We found that the expression of conditional freezing and increased LA excitability was neither necessary nor sufficient for the expression of conditional increases in CS-evoked spike firing. Rather, conditioning-related changes in CS-evoked spike firing were solely determined by the associative history of the CS. Thus, our data support a model in which associative activity in the LA encodes fear memory and contributes to the expression of learned fear behaviors.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/61948/1/goosensNEURON03.pd

The amygdala is essential for the development of neuronal plasticity in the medial geniculate nucleus during auditory fear conditioning in rats

The medial geniculate nucleus of the thalamus (MGN) and the basolateral complex of the amygdala (BLA) are critical components of the neural circuit that mediates auditory fear conditioning. Several studies indicate that neurons in both the MGN and BLA exhibit associative plasticity of spike firing during auditory fear conditioning. In the present study, we examined whether the development of plasticity in the MGN requires the BLA. Single units were recorded from chronic multichannel electrodes implanted in the medial division of the MGN of conscious and freely moving rats. Rats received auditory fear conditioning trials, which consisted of a white-noise conditional stimulus (CS) and a co-terminating footshock unconditional stimulus (US). Unpaired (sensitization) controls received the same number of trials as paired animals, but the CS and US were explicitly unpaired. Before fear conditioning, rats received either an intra-amygdala infusion of muscimol, a GABA(A) receptor agonist, to inactivate BLA neurons or an infusion of the saline vehicle. Auditory fear conditioning produced a substantial increase in both CS-elicited spike firing in the MGN and conditional freezing behavior in vehicle-treated rats receiving paired training. Muscimol inactivation of the BLA severely attenuated the development of both conditioning-related increases in CS-elicited spike firing in the MGN and conditional freezing to the auditory CS. Unpaired training did not yield increases in either CS-elicited spike firing or freezing to the tone CS. These results reveal that the BLA is essential to the development of plasticity in the auditory thalamus during fear conditioning.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/61941/1/marenJN01.pd

A role for amygdaloid PKA and PKC in the acquisition of long-term conditional fear memories in rats

Although there is great interest in the cellular mechanisms underlying Pavlovian conditioning, few studies have directly examined the contribution of intracellular signaling pathways in the amygdala to the acquisition and expression of conditional fear memories. In the present study, we examined this issue by infusing 1-(5'-isoquinolinesulfonyl)-2-methylpiperazine (H7), a potent inhibitor of both protein kinase C (PKC) and cAMP-dependent protein kinase (PKA), directly into the amygdala prior to fear conditioning or retention testing. We found that infusion of H7 prior to training attenuated long-term conditional fear in a dose-dependent manner (Experiment 1), but short-term fear memories were spared. The contribution of protein kinases to conditional fear was region-specific within the amygdala: infusion of H7 into the basolateral amygdala (BLA) but not the central nucleus of the amygdala (CEA) resulted in attenuated freezing (Experiment 2). Moreover, the deficits in fear conditioning produced by PKA/PKC inhibition were not modality-specific, insofar as intra-BLA H7 reduced both contextual and auditory fear. The effects of H7 on conditional freezing were not attributable to either state-dependency or performance deficits (Experiment 3). Together, these experiments suggest that amygdaloid PKA and PKC play an important role in the acquisition of fear memories.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/61935/1/goosensBBR00.pd