視丘的連結核（nucleus reuniens, Re）與菱形核（rhomboid nucleus, Rh） （合稱ReRh）與海馬迴（hippocampus, HPC）及內前額葉皮質（medial prefrontal cortex, mPFC）有直接且雙向的神經投射關係。過去研究指出連結核與菱形核會參與在許多認知行為實驗中，然而目前尚未有關於這兩個核區在帕弗洛夫痕跡恐懼制約（trace fear conditioning）中的研究。
在痕跡制約中，制約刺激（conditioned stimulus, CS）及非制約刺激（unconditioned stimulus, US）中間有一段時間的間隔，這使得動物較難習得兩個刺激間的關係。根據先前的研究，我們已知海馬迴及內前額葉皮質會參與在痕跡恐懼制約中，而不參與延宕恐懼制約（delay fear conditioning），由於連結核與菱形核可以調控海馬迴及內前核葉皮質的神經迴路，我們因此推測連結核與菱形核也同樣會參與在痕跡恐懼制約，而不參與延宕恐懼制約。在此研究中，我們首先利用c-Fos蛋白表現來確認在公的Long-Evans大鼠中連結核會參與在恐懼記憶的編碼（encoding）階段，而不會參與在記憶的提取（retrieval）階段。接著，我們以行為藥理學的實驗發現抑制連結核與菱形核僅會影響痕跡恐懼記憶的習得（acquisition）階段，不會影響記憶的鞏固（consolidation）以及提取階段。另一方面，在延宕恐懼制約中，儘管從先前的實驗中得知連結核的神經元在記憶編碼階段有活化的現象，在藥理學的實驗中不管是在哪個階段進行抑制都不影響動物的學習。另外，我們也發現，在痕跡恐懼制約中，抑制連結核與菱形核有著狀態依賴的作用（state-dependent effect）。總結來說，我們的實驗結果發現連結核與菱形核在痕跡恐懼制約中扮演了重要的角色。

The reuniens (Re) and rhomboid (Rh) nuclei (ReRh) of the midline thalamus interconnects the hippocampus (HPC) and the medial prefrontal cortex (mPFC). Several studies have suggested that the ReRh participates in various cognitive tasks. However, little is known about the contribution of the ReRh in Pavlovian trace fear conditioning, a procedure with a temporal gap between the conditioned stimulus (CS) and the unconditioned stimulus (US), and therefore making it harder for the animals to acquire. Because the HPC and mPFC are involved in trace, but not delay, fear conditioning and given the role of the ReRh in mediating this neurocircuitry, we hypothesized that ReRh inactivation leads to a learning deficit only in trace conditioning. In a series of experiments, we first examined the c-Fos expression in male Long-Evans rats and established that the ReRh was recruited in the encoding, but not the retrieval phase, of fear memory. Next, we performed behavioral pharmacology experiments, and found that ReRh inactivation impaired only the acquisition, but not the consolidation or retrieval, of trace fear. However, although the ReRh was recruited during the encoding of delay fear demonstrated by c-Fos results, ReRh inactivation in any phases did not interfere with delay conditioning. Finally, we found that trace fear acquired under ReRh inactivation reprised when the ReRh was brought off-line during retrieval. Together, our data revealed the essential role of the ReRh in a learning task with temporally discontinuous stimuli.

Abel T, Lattal KM (2001) Molecular mechanisms of memory acquisition, consolidation and retrieval. Curr Opin Neurobiol 11:180-187.
Agster KL, Burwell RD (2009) Cortical efferents of the perirhinal, postrhinal, and entorhinal cortices of the rat. Hippocampus 19:1159-1186.
Arai R, Jacobowitz DM, Deura S (1994) Distribution of calretinin, calbindin-D28k, and parvalbumin in the rat thalamus. Brain Res Bull 33:595-614.
Bangasser DA, Waxler DE, Santollo J, Shors TJ (2006) Trace conditioning and the hippocampus: the importance of contiguity. J Neurosci 26:8702-8706.
Barker GRI, Warburton EC (2018) A Critical Role for the Nucleus Reuniens in Long-Term, But Not Short-Term Associative Recognition Memory Formation. J Neurosci 38:3208-3217.
Baydin S, Gungor A, Baran O, Tanriover N, Rhoton AL (2016) The double massa intermedia. Surg Neurol Int 7:30.
Beylin AV, Gandhi CC, Wood GE, Talk AC, Matzel LD, Shors TJ (2001) The role of the hippocampus in trace conditioning: temporal discontinuity or task difficulty? Neurobiol Learn Mem 76:447-461.
Blum S, Hebert AE, Dash PK (2006) A role for the prefrontal cortex in recall of recent and remote memories. Neuroreport 17:341-344.
Bokor H, Csaki A, Kocsis K, Kiss J (2002) Cellular architecture of the nucleus reuniens thalami and its putative aspartatergic/glutamatergic projection to the hippocampus and medial septum in the rat. Eur J Neurosci 16:1227-1239.
Bouton ME (2004) Context and behavioral processes in extinction. Learn Mem 11:485-494.
Braak H, Braak E (1991) Alzheimer's disease affects limbic nuclei of the thalamus. Acta Neuropathol 81:261-268.
Braak H, Braak E (1998) Evolution of neuronal changes in the course of Alzheimer's disease. J Neural Transm Suppl 53:127-140.
Brzozka MM, Rossner MJ (2013) Deficits in trace fear memory in a mouse model of the schizophrenia risk gene TCF4. Behav Brain Res 237:348-356.
Bullitt E (1990) Expression of c-fos-like protein as a marker for neuronal activity following noxious stimulation in the rat. J Comp Neurol 296:517-530.
Burman MA, Simmons CA, Hughes M, Lei L (2014) Developing and validating trace fear conditioning protocols in C57BL/6 mice. J Neurosci Methods 222:111-117.
Burwell RD, Amaral DG (1998) Cortical afferents of the perirhinal, postrhinal, and entorhinal cortices of the rat. J Comp Neurol 398:179-205.
Cahill EN, Milton AL (2019) Neurochemical and molecular mechanisms underlying the retrieval-extinction effect. Psychopharmacology (Berl) 236:111-132.
Cassel JC, Pereira de Vasconcelos A, Loureiro M, Cholvin T, Dalrymple-Alford JC, Vertes RP (2013) The reuniens and rhomboid nuclei: neuroanatomy, electrophysiological characteristics and behavioral implications. Prog Neurobiol 111:34-52.
Cholvin T, Hok V, Giorgi L, Chaillan FA, Poucet B (2018) Ventral Midline Thalamus Is Necessary for Hippocampal Place Field Stability and Cell Firing Modulation. J Neurosci 38:158-172.
Delatour B, Witter MP (2002) Projections from the parahippocampal region to the prefrontal cortex in the rat: evidence of multiple pathways. Eur J Neurosci 15:1400-1407.
Dolleman-Van Der Weel MJ, Witter MP (1996) Projections from the nucleus reuniens thalami to the entorhinal cortex, hippocampal field CA1, and the subiculum in the rat arise from different populations of neurons. J Comp Neurol 364:637-650.
Dolleman-van der Weel MJ, Griffin AL, Ito HT, Shapiro ML, Witter MP, Vertes RP, Allen TA (2019) The nucleus reuniens of the thalamus sits at the nexus of a hippocampus and medial prefrontal cortex circuit enabling memory and behavior. Learn Mem 26:191-205.
Drexel M, Preidt AP, Kirchmair E, Sperk G (2011) Parvalbumin interneurons and calretinin fibers arising from the thalamic nucleus reuniens degenerate in the subiculum after kainic acid-induced seizures. Neuroscience 189:316-329.
Dudai Y (2004) The neurobiology of consolidations, or, how stable is the engram? Annu Rev Psychol 55:51-86.
Esclassan F, Coutureau E, Di Scala G, Marchand AR (2009) Differential contribution of dorsal and ventral hippocampus to trace and delay fear conditioning. Hippocampus 19:33-44.
Fritz CO, Morris PE, Richler JJ (2012) Effect size estimates: current use, calculations, and interpretation. J Exp Psychol Gen 141:2-18.
Gilmartin MR, Helmstetter FJ (2010) Trace and contextual fear conditioning require neural activity and NMDA receptor-dependent transmission in the medial prefrontal cortex. Learn Mem 17:289-296.
Gilmartin MR, Kwapis JL, Helmstetter FJ (2012) Trace and contextual fear conditioning are impaired following unilateral microinjection of muscimol in the ventral hippocampus or amygdala, but not the medial prefrontal cortex. Neurobiol Learn Mem 97:452-464.
Gilmartin MR, Miyawaki H, Helmstetter FJ, Diba K (2013) Prefrontal activity links nonoverlapping events in memory. J Neurosci 33:10910-10914.
Gormezano I, Kehoe E, S. Marshall B (1982) Twenty years of classical conditioning with the rabbit. Progress in psychobiology and physiological psychology 10.
Hallock HL, Wang A, Griffin AL (2016) Ventral Midline Thalamus Is Critical for Hippocampal-Prefrontal Synchrony and Spatial Working Memory. J Neurosci 36:8372-8389.
Hallock HL, Wang A, Shaw CL, Griffin AL (2013) Transient inactivation of the thalamic nucleus reuniens and rhomboid nucleus produces deficits of a working-memory dependent tactile-visual conditional discrimination task. Behav Neurosci 127:860-866.
Hoover WB, Vertes RP (2012) Collateral projections from nucleus reuniens of thalamus to hippocampus and medial prefrontal cortex in the rat: a single and double retrograde fluorescent labeling study. Brain Struct Funct 217:191-209.
Ito HT, Moser EI, Moser MB (2018) Supramammillary Nucleus Modulates Spike-Time Coordination in the Prefrontal-Thalamo-Hippocampal Circuit during Navigation. Neuron 99:576-587 e575.
Ito HT, Zhang SJ, Witter MP, Moser EI, Moser MB (2015) A prefrontal-thalamo-hippocampal circuit for goal-directed spatial navigation. Nature 522:50-55.
Jayachandran M, Linley SB, Schlecht M, Mahler SV, Vertes RP, Allen TA (2019) Prefrontal Pathways Provide Top-Down Control of Memory for Sequences of Events. Cell Rep 28:640-654 e646.
Kaczorowski CC, Sametsky E, Shah S, Vassar R, Disterhoft JF (2011) Mechanisms underlying basal and learning-related intrinsic excitability in a mouse model of Alzheimer's disease. Neurobiol Aging 32:1452-1465.
Kafetzopoulos V, Kokras N, Sotiropoulos I, Oliveira JF, Leite-Almeida H, Vasalou A, Sardinha VM, Papadopoulou-Daifoti Z, Almeida OFX, Antoniou K, Sousa N, Dalla C (2018) The nucleus reuniens: a key node in the neurocircuitry of stress and depression. Mol Psychiatry 23:579-586.
Kalat JW, Rozin P (1973) "Learned safety" as a mechanism in long-delay taste-aversion learning in rats. J Comp Physiol Psychol 83:198-207.
Kasicki S, Jelen P, Olszewski M, Slawinska U (2009) Electrical hippocampal activity during danger and safety signals in classical conditioning in the rat. Acta Neurobiol Exp (Wars) 69:119-128.
Klein MM, Cholvin T, Cosquer B, Salvadori A, Le Mero J, Kourouma L, Boutillier AL, Pereira de Vasconcelos A, Cassel JC (2019) Ventral midline thalamus lesion prevents persistence of new (learning-triggered) hippocampal spines, delayed neocortical spinogenesis, and spatial memory durability. Brain Struct Funct 224:1659-1676.
Knight DC, Nguyen HT, Bandettini PA (2006) The role of awareness in delay and trace fear conditioning in humans. Cogn Affect Behav Neurosci 6:157-162.
Kochli DE, Thompson EC, Fricke EA, Postle AF, Quinn JJ (2015) The amygdala is critical for trace, delay, and contextual fear conditioning. Learn Mem 22:92-100.
Lakens D (2013) Calculating and reporting effect sizes to facilitate cumulative science: a practical primer for t-tests and ANOVAs. Front Psychol 4:863.
Laroche S, Davis S, Jay TM (2000) Plasticity at hippocampal to prefrontal cortex synapses: dual roles in working memory and consolidation. Hippocampus 10:438-446.
Layfield DM, Patel M, Hallock H, Griffin AL (2015) Inactivation of the nucleus reuniens/rhomboid causes a delay-dependent impairment of spatial working memory. Neurobiol Learn Mem 125:163-167.
LeDoux JE (1996) The emotional brain: The mysterious underpinnings of emotional life. New York, NY, US: Simon & Schuster.
Likhtik E, Stujenske JM, Topiwala MA, Harris AZ, Gordon JA (2014) Prefrontal entrainment of amygdala activity signals safety in learned fear and innate anxiety. Nat Neurosci 17:106-113.
Lin CH, Yeh SH, Lu HY, Gean PW (2003) The similarities and diversities of signal pathways leading to consolidation of conditioning and consolidation of extinction of fear memory. J Neurosci 23:8310-8317.
Loureiro M, Cholvin T, Lopez J, Merienne N, Latreche A, Cosquer B, Geiger K, Kelche C, Cassel JC, Pereira de Vasconcelos A (2012) The ventral midline thalamus (reuniens and rhomboid nuclei) contributes to the persistence of spatial memory in rats. J Neurosci 32:9947-9959.
Maisson DJ, Gemzik ZM, Griffin AL (2018) Optogenetic suppression of the nucleus reuniens selectively impairs encoding during spatial working memory. Neurobiol Learn Mem 155:78-85.
Marchand A, Faugere A, Coutureau E, Wolff M (2014) A role for anterior thalamic nuclei in contextual fear memory. Brain Struct Funct 219:1575-1586.
Maren S, Phan KL, Liberzon I (2013) The contextual brain: implications for fear conditioning, extinction and psychopathology. Nat Rev Neurosci 14:417-428.
Martin JH (1991) Autoradiographic estimation of the extent of reversible inactivation produced by microinjection of lidocaine and muscimol in the rat. Neurosci Lett 127:160-164.
McGaugh JL (1966) Time-dependent processes in memory storage. Science 153:1351-1358.
McGaugh JL (2000) Memory--a century of consolidation. Science 287:248-251.
McKenna JT, Vertes RP (2004) Afferent projections to nucleus reuniens of the thalamus. J Comp Neurol 480:115-142.
Mei H, Logothetis NK, Eschenko O (2018) The activity of thalamic nucleus reuniens is critical for memory retrieval, but not essential for the early phase of "off-line" consolidation. Learn Mem 25:129-137.
Merlo E, Milton AL, Goozee ZY, Theobald DE, Everitt BJ (2014) Reconsolidation and extinction are dissociable and mutually exclusive processes: behavioral and molecular evidence. J Neurosci 34:2422-2431.
Milad MR, Rauch SL, Pitman RK, Quirk GJ (2006) Fear extinction in rats: implications for human brain imaging and anxiety disorders. Biol Psychol 73:61-71.
Moscovitch A, LoLordo VM (1968) Role of safety in the Pavlovian backward fear conditioning procedure. J Comp Physiol Psychol 66:673-678.
Ogundele OM, Lee CC, Francis J (2017) Thalamic dopaminergic neurons projects to the paraventricular nucleus-rostral ventrolateral medulla/C1 neural circuit. Anat Rec (Hoboken) 300:1307-1314.
Ottersen OP, Storm-Mathisen J (1984) GABA-containing neurons in the thalamus and pretectum of the rodent. An immunocytochemical study. Anat Embryol (Berl) 170:197-207.
Papini MR, Bitterman ME (1990) The role of contingency in classical conditioning. Psychol Rev 97:396-403.
Pavlov IP (1927) Conditioned reflexes: an investigation of the physiological activity of the cerebral cortex. Oxford, England: Oxford Univ. Press.
Phillips RG, LeDoux JE (1994) Lesions of the dorsal hippocampal formation interfere with background but not foreground contextual fear conditioning.
Quet E, Majchrzak M, Cosquer B, Morvan T, Wolff M, Cassel JC, Pereira de Vasconcelos A, Stephan A (2020) The reuniens and rhomboid nuclei are necessary for contextual fear memory persistence in rats. Brain Struct Funct.
Quirk GJ, Mueller D (2008) Neural mechanisms of extinction learning and retrieval. Neuropsychopharmacology 33:56-72.
Ramanathan KR, Ressler RL, Jin J, Maren S (2018a) Nucleus Reuniens Is Required for Encoding and Retrieving Precise, Hippocampal-Dependent Contextual Fear Memories in Rats. J Neurosci 38:9925-9933.
Ramanathan KR, Jin J, Giustino TF, Payne MR, Maren S (2018b) Prefrontal projections to the thalamic nucleus reuniens mediate fear extinction. Nat Commun 9:4527.
Raybuck JD, Lattal KM (2014) Bridging the interval: theory and neurobiology of trace conditioning. Behav Processes 101:103-111.
Richardson JT (2011) Eta squared and partial eta squared as measures of effect size in educational research. Educational Research Review 6:135-147.
Rigoli F, Pezzulo G, Dolan RJ (2016) Prospective and Pavlovian mechanisms in aversive behaviour. Cognition 146:415-425.
Santini E, Ge H, Ren K, Pena de Ortiz S, Quirk GJ (2004) Consolidation of fear extinction requires protein synthesis in the medial prefrontal cortex. J Neurosci 24:5704-5710.
Schafe GE, Atkins CM, Swank MW, Bauer EP, Sweatt JD, LeDoux JE (2000) Activation of ERK/MAP kinase in the amygdala is required for memory consolidation of pavlovian fear conditioning. J Neurosci 20:8177-8187.
Shors TJ, Beylin AV, Wood GE, Gould E (2000) The modulation of Pavlovian memory. Behav Brain Res 110:39-52.
Sierra RO, Pedraza LK, Zanona QK, Santana F, Boos FZ, Crestani AP, Haubrich J, de Oliveira Alvares L, Calcagnotto ME, Quillfeldt JA (2017) Reconsolidation-induced rescue of a remote fear memory blocked by an early cortical inhibition: Involvement of the anterior cingulate cortex and the mediation by the thalamic nucleus reuniens. Hippocampus 27:596-607.
Song C, Ehlers VL, Moyer JR, Jr. (2015) Trace Fear Conditioning Differentially Modulates Intrinsic Excitability of Medial Prefrontal Cortex-Basolateral Complex of Amygdala Projection Neurons in Infralimbic and Prelimbic Cortices. J Neurosci 35:13511-13524.
Song C, Detert JA, Sehgal M, Moyer JR, Jr. (2012) Trace fear conditioning enhances synaptic and intrinsic plasticity in rat hippocampus. J Neurophysiol 107:3397-3408.
Van der Werf YD, Witter MP, Groenewegen HJ (2002) The intralaminar and midline nuclei of the thalamus. Anatomical and functional evidence for participation in processes of arousal and awareness. Brain Res Brain Res Rev 39:107-140.
van Duuren E, van der Plasse G, van der Blom R, Joosten RN, Mulder AB, Pennartz CM, Feenstra MG (2007) Pharmacological manipulation of neuronal ensemble activity by reverse microdialysis in freely moving rats: a comparative study of the effects of tetrodotoxin, lidocaine, and muscimol. J Pharmacol Exp Ther 323:61-69.
Vertes RP (2004) Differential projections of the infralimbic and prelimbic cortex in the rat. Synapse 51:32-58.
Vertes RP, Hoover WB, Do Valle AC, Sherman A, Rodriguez JJ (2006) Efferent projections of reuniens and rhomboid nuclei of the thalamus in the rat. J Comp Neurol 499:768-796.
Viena TD, Linley SB, Vertes RP (2018) Inactivation of nucleus reuniens impairs spatial working memory and behavioral flexibility in the rat. Hippocampus 28:297-311.
Witter MP, Wouterlood FG, Naber PA, Van Haeften T (2000) Anatomical organization of the parahippocampal-hippocampal network. Ann N Y Acad Sci 911:1-24.
Xu W, Sudhof TC (2013) A neural circuit for memory specificity and generalization. Science 339:1290-1295.