Jonathan C. Gewirtz

Second-order fear conditioning prevented by blocking NMDA receptors in amygdala

Antagonists of NMDA (N-methyl-D-aspartate)-type glutamate receptors disrupt several forms of learning. Although this might indicate that NMDA-receptor-mediated processes are critical for synaptic plasticity, there may be other mechanisms by which NMDA-receptor antagonism could interfere with learning,. For instance, fear conditioning would be blocked by microinfusion of the NMDA-receptor antagonist AP5 (D,L-2-amino-5-phosphonovalerate) into the basolateral amygdala, if AP5 inhibited routine synaptic transmission, thereby reducing the ability of stimuli to activate amygdala neurons. In second-order fear conditioning, the reinforcer is a fear-eliciting conditioned stimulus rather than an unconditioned stimulus. Expression of conditioned fear is amygdala-dependent and so provides a behavioural assessment of the ability of the reinforcer to activate amygdala neurons in the presence of AP5. We report here that intra-amygdala AP5 actually enhances expression of conditioned fear to the conditioned stimulus that provides the reinforcement signal for second-order conditioning. Nevertheless, acquisition of second-order fear conditioning is completely blocked. Our findings strongly support the view that NMDA receptors are critically involved in synaptic plasticity.

Behavioral Evidence for Interactions between a Hallucinogenic Drug and Group II Metabotropic Glutamate Receptors

Recent electrophysiological studies in our laboratory have demonstrated a physiological interaction between 5-HT2A and metabotropic glutamate2/3 (mGlu2/3) receptors in the medial prefrontal cortex. Several behavioral studies have found that phenethylamine hallucinogens with partial agonist activity at 5-HT2A receptors induce head shakes when directly administered into the medial prefrontal cortex. The purpose of the present experiments was to examine whether an interaction occurs between mGlu2/3 and 5-HT2A receptors on a behavioral level using head shakes induced by phenethylamine hallucinogens as a model of 5-HT2A receptor activation. Administration of the mGlu2/3 agonist LY354740 (0.3–10 mg/kg, ip) suppressed head shakes induced by the phenethylamine hallucinogen 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI). Conversely, administration of the mGlu2/3 antagonist LY341495 (1 mg/kg, ip) enhanced the frequency of DOI-induced head shakes. Taken together, these results raise the possibility that the psychomimetic properties of hallucinogenic drugs may be mediated in part, via increased glutamate release following activation of 5-HT2A receptors.

A MAJOR ROLE FOR THALAMOCORTICAL AFFERENTS IN SEROTONERGIC HALLUCINOGEN RECEPTOR FUNCTION IN THE RAT NEOCORTEX

Activation of 5-hydroxytryptamine(2A) (5-HT(2A)) receptors by hallucinogenic drugs is thought to mediate many psychotomimetic effects including changes in affect, cognition and perception. Conversely, blockade of 5-HT(2A) receptors may mediate therapeutic effects of many atypical antidepressant and antipsychotic drugs. The purpose of the present study was to determine the source of subcortical glutamatergic afferents, which would project widely throughout the anterior-posterior axis of the rat brain to the apical dendrites of layer V pyramidal cells of the medial prefrontal cortex, from which serotonin induces transmitter release via activation of 5-HT(2A) receptors. Fiber-sparing chemical lesions of the medial thalamus selectively decreased the frequency of serotonin-induced excitatory postsynaptic currents recorded from layer V pyramidal cells in the prelimbic region of the medial prefrontal cortex by 60%. In contrast, large bilateral lesions of the amygdala did not alter the serotonin response. These thalamic lesions significantly decreased the amount of binding to either mu-opioid or metabotropic glutamate 2/3 receptors in the prelimbic region of the medial prefrontal cortex as expected from previous evidence that these agonists for these receptors suppress serotonin-induced excitatory postsynaptic currents by a presynaptic mechanism. Surprisingly, the amount of specific binding to cortical 5-HT(2A) receptors was significantly increased by the medial thalamic lesions. Thus, these experiments demonstrate that activation of cortical 5-HT(2A) receptors modulates transmitter release from thalamocortical terminals. Unexpectedly, lesioning the thalamocortical terminals also alters 5-HT(2A) receptor binding in the prefrontal cortex. These findings are of interest with respect to understanding therapeutic effects of antidepressant/antipsychotic drugs and the known behavioral effects of thalamic lesions in humans.

Stress differentially regulates synaptophysin and synaptotagmin expression in hippocampus

BACKGROUND In view of the effects of stress on synaptic plasticity, the regulation of synaptophysin and synaptotagmin expression by immobilization was analyzed by in situ hybridization. METHODS Rats were exposed to immobilization stress, which induced typical behavioral alterations, such as reduced locomotor activity after stress exposure. Determination of mRNA levels of the integral synaptic vesicle proteins was performed immediately after acute or chronic immobilization. RESULTS The results demonstrate that stress exposure leads to reduced expression of synaptophysin but increased expression of synaptotagmin in the hippocampus. CONCLUSIONS This rapid and differential regulation of synaptic vesicle proteins could be responsible for some of the morphological, biochemical, and behavioral changes observed after stress exposure. These changes may be relevant to such clinical disorders as psychoses, depression, and posttraumatic stress disorder that are sensitive to stress and involve changes in neural and synaptic plasticity.

Neural Systems Involved in Fear Inhibition: Extinction and Conditioned Inhibition

“I can’t get the memories out of my mind! The images come flooding back in vivid detail, triggered by the most inconsequential things, like a door slamming or the smell of stir-fried pork. Last night, I went to bed, was having a good sleep for a change. Then in the early morning a storm-front passed through and there was a bolt of crackling thunder. I awoke instantly, frozen in fear. I am right back in Viet Nam, in the middle of the monsoon season at my guard post. I am sure I’ll get hit in the next volley and convinced I will die. My hands are freezing, yet sweat pours from my entire body. I feel each hair on the back of my neck standing on end. I can’t catch my breath and my heart is pounding. I smell a damp sulfur smell. Suddenly I see what’s left of my buddy Troy, his head on a bamboo platter, sent back to our camp by the Viet Cong. Propaganda messages are stuffed between his clenched teeth. The next bolt of lightning and clap of thunder makes me jump so much that I fall to the floor..... ” (Paraphrased from a war veteran’s conversations with Dr. R. L. Gelman, Dept. of Psychiatry, Yale University School of Medicine).

Modulation of DOI-induced increases in cortical BDNF expression by group II mGlu receptors

Previous studies have shown that 5-hydroxytryptamine(2A) (5-HT(2A)) receptor activation induces changes in the pattern of brain-derived neurotrophic factor (BDNF) mRNA expression in the neocortex and hippocampus, and that 5-HT(2A) receptor blockade interferes with the induction of BDNF mRNA by stress. Recent studies have also shown that activation of metabotropic glutamate group II (mGlu2/3) receptors suppresses 5-HT(2A) receptor-stimulated excitatory postsynaptic potentials/currents (EPSP/Cs) in pyramidal neurons in medial prefrontal cortex. Conversely, blockade of mGlu2/3 receptors enhances 5-HT-induced EPSCs. The current study examined the effects of the highly selective mGlu2/3 agonist (1S,2S,5R,6S)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylate monohydrate (LY354740) and the mGlu2/3 antagonist 2S-2-amino-2-(1S,2S-2-carboxycycloprop-1-yl)-3(xanthy-9-yl)propanoic acid (LY341495) on BDNF mRNA expression in medial prefrontal cortex induced by the hallucinogen and 5-HT(2A/2B/2C) agonist 1-(2,5-dimethoxy-4-iodophenethyl)-2-aminopropane (DOI). LY354740 (0.1-10 mg/kg) dose-dependently suppressed DOI-induced BDNF mRNA levels in medial prefrontal cortex. In contrast, the mGlu2/3 antagonist LY341495 (1 mg/kg) enhanced DOI-induced BDNF mRNA levels. BDNF mRNA expression was not altered by administration of the mGlu agonist or the antagonist alone. These results are discussed with respect to a potential role for group II mGlu agonists in the treatment of depression and schizophrenia.

The nucleus accumbens is not critically involved in mediating the effects of a safety signal on behavior.

Although considerable progress has been made towards understanding the neural systems mediating conditioned fear, little is known about the neural mechanisms underlying conditioned inhibitors of fear (or safety signals). The present series of experiments examined the involvement of the nucleus accumbens (NAC) in mediating the effects of safety signals on behavior using a conditioned inhibition of fear-potentiated startle paradigm. Neither increasing dopaminergic nor decreasing glutamatergic function in the NAC altered the magnitude of conditioned fear or conditioned inhibition of fear in rats. Furthermore, large pre- or post-training electrolytic lesions of the NAC did not affect acquisition or expression of fear-potentiated startle or conditioned inhibition of fear-potentiated startle. Taken together, these data suggest that the NAC is not critically involved in the acquisition or expression of fear-potentiated startle or conditioned inhibition of fear-potentiated startle. Previous research has implicated the NAC in ‘reward-attenuated startle’ in which presentation of a stimulus paired with food decreased startle responding. The present results, therefore, indicate important neural dissociations between the processing of appetitive and safety signals, even though behavioral studies and learning theories have suggested that these two forms of learning share some commonalities.

Potentiated startle and hyperalgesia during withdrawal from acute morphine: effects of multiple opiate exposures

RationaleAdministration of an opiate antagonist following acute morphine exposure elevates the startle response in rodents, a phenomenon that may reflect the anxiogenic effects of withdrawal. Previous acute dependence studies have demonstrated escalated withdrawal severity following multiple withdrawal episodes.ObjectivesTo examine the effects of prior opiate exposure on the magnitude of withdrawal-potentiated startle and an additional measure of acute dependence, withdrawal-induced hyperalgesia.MethodsThe effects of repeated naloxone-precipitated morphine withdrawals on acoustic startle responding were evaluated in experiments that varied either the dose of the opiate antagonist (8-day, repeated measures procedure) or agonist (3-day procedure). Additional experiments examined withdrawal-induced hyperalgesia utilizing either a single-day dependence paradigm or the same 3-day procedure as in the startle experiment.ResultsRepeated naloxone-precipitated withdrawals from acute morphine exacerbated withdrawal severity in both startle procedures, although this effect varied biphasically (inverted-U function) with morphine dose in the 3-day dependence paradigm. Withdrawal from a single morphine exposure also induced hyperalgesia, and this effect was intensified by prior withdrawal episodes.ConclusionsThese data demonstrate that repeated withdrawals from acute morphine exacerbate the severity of potentiated startle and hyperalgesia. These paradigms may be useful in examining the neural plasticity underlying the development of opiate dependence.

Disruption of contextual freezing, but not contextual blocking of fear-potentiated startle, after lesions of the dorsal hippocampus.

The role of the dorsal hippocampus in contextual fear conditioning was investigated with a contextual blocking paradigm. In Experiment 1, rats were given pairings of a light conditioned stimulus (CS) and footshock after preexposure either to footshock or to the context alone. The group preexposed to footshock showed poorer fear conditioning to the light CS, as measured by the fear-potentiated startle reflex. In Experiment 2, a group preexposed to footshock in the same context showed poorer fear conditioning to the light CS than did a group preexposed to footshock in a different context, indicating contextual blocking of fear-potentiated startle. In Experiment 3, lesions of the dorsal hippocampus had no effect on contextual blocking, even though contextual freezing was disrupted. The sparing of contextual blocking indicated that contextual memory was intact following hippocampal lesions, despite the disruption of contextual freezing.

Habituation of prepulse inhibition of the startle reflex using an auditory prepulse close to background noise

This study examined whether prepulse inhibition habituates with repeated presentation of the prepulse alone. Prepulse inhibition was determined by measuring the decrement in the startle response when the acoustic startle-eliciting stimulus was preceded by an auditory prepulse. Rats received repetitive exposures of the same auditory prepulse alone (experimental condition) and of a visual prepulse alone (control condition). To reduce habituation of startle itself and the possible dishabituating influence the startle stimulus might have on habituation of prepulse inhibition, startle stimulus presentations were infrequently interspersed among a much larger number of prepulse-alone presentations. Stimulus-specific habituation of prepulse inhibition occurred using an auditory prepulse 2.5 dB, but not 13 dB, above background noise. Implications are discussed for the role of prepulse inhibition in sensory gating.