Na+,K(+)-ATPase activity is reduced in hippocampus of rats submitted to an experimental model of depression: effect of chronic lithium treatment and possible involvement in learning deficits

This study was undertaken to verify the effects of chronic stress and lithium treatments on the hippocampal Na+,K(+)-ATPase activity of rats, as well as to investigate the effects of stress interruption and post-stress lithium treatment on this enzyme activity and on spatial memory. Two experiments were carried out; in the first experiment, adult male Wistar rats were divided into two groups: control and submitted to a chronic variate stress paradigm, and subdivided into treated or not with LiCl. After 40 days of treatment, rats were killed, and Na+,K(+)-ATPase activity was determined. In the second experiment, rats were stressed during 40 days, and their performance was evaluated in the Water Maze task. The stressed group was then subdivided into four groups, with continued or interrupted stress treatment and treated or not with lithium for 30 additional days. After a second evaluation of performance in the Water Maze, rats were killed and Na+,K(+)-ATPase activity was also measured. Results showed an impairment in Na+,K(+)-ATPase activity and in Water Maze performance of chronically stressed rats, which were prevented by lithium treatment and reversed by lithium treatment and by stress interruption. These results suggest that the modulation of Na+,K(+)-ATPase activity may be one of the mechanisms of action of lithium in the treatment of mood disorders.     

Towards a mathematical model of within-session operant responding

Operant response rate changes within the course of a typical free-operant experimental session. These changes are orderly, and reliably demonstrated with subjects from different species, responding under different experimental conditions. Killeen (1995) postulated that the response rate changes are a function of the interplay between arousal and satiation and offered a mathematical model for this hypothesis. Here we analyze Killeen's model, demonstrating that, although solid in its principles, it presents some flaws in its implementation. Then, based on the same principles, we build and test a new model of within-session motivation dynamics. We also demonstrate that, by representing arousal as a variable that ranges between 0 and 1, we can obtain a surprisingly simple model of free-operant response rate.     

Interference processes in monkey auditory list memory

A rhesus monkey’s memory was tested for single items and four-item lists of natural and environmental sounds. Memory items were presented from a center speaker, followed by a retention delay and then a choice response to a test sound presented simultaneously from two side speakers. Recognition of the last item of four-item lists was much poorer than that of single items at 0-, 1-, and 2-sec delays, despite there being the same temporal relations between study and test. This result showed that the first three items proactively interfered with memory of the last list item. Proactive interference dissipated after 2 sec, revealing a recency effect that eventually equaled single-item performance. Recognition of the first item of four-item lists was much poorer than single items at 20- and 30-sec delays, showing that the last three items retroactively interfered with memory of the first list item. The results point to the critical nature of interference processes in the understanding of serial position functions.     

The role of nucleus accumbens and dorsolateral striatal D2 receptors in active avoidance conditioning

The role of dopamine (DA) in rewarding motivated actions is well established but its role in learning how to avoid aversive events is still controversial. Here we tested the role of D2-like DA receptors in the nucleus accumbens (NAc) and the dorsolateral striatum (DLS) of rats in the learning and performance of conditioned avoidance responses (CAR). Adult male Wistar rats received systemic, intra-NAc or intra-DLS (pre- or post-training) administration of a D2-like receptor agonist (quinpirole) or antagonist ((−)sulpiride) and were given two sessions in the two-way active avoidance task. The main effects observed were: (i) sulpiride and lower (likely pre-synaptic) doses of quinpirole decreased the number of CARs and increased the number of escape failures; (ii) higher doses of quinpirole (likely post-synaptic) increased inter-trial crossings and failures; (iii) pre-training administration of sulpiride decreased the number of CARs in both training and test sessions when infused into the NAc, but this effect was observed only in the test session when it was infused into the DLS; (iv) post-training administration of sulpiride decreased CARs in the test session when infused into the NAc but not DLS. These findings suggest that activation of D2 receptors in the NAc is critical for fast adaptation to responding to unconditioned and conditioned aversive stimuli while activation of these receptors in the DLS is needed for a slower learning of how to respond to the same stimuli based on previous experiences.     

Stimulus generalization in two associative learning processes

Recent studies involving nonlinear discrimination problems suggest that stimuli in human associative learning are represented configurally with narrow generalization, such that presentation of stimuli that are even slightly dissimilar to stored configurations weakly activate these configurations. The authors note that another well-known set of findings in human associative learning, cue-interaction phenomena, suggest relatively broad generalization. Three experiments show that current models of human associative learning, which try to model both nonlinear discrimination and cue interaction as the result of 1 process, fail because they cannot simultaneously account for narrow and broad generalization. Results suggest that human associative learning involves (a) an exemplar-based process with configural stimulus representation and narrow generalization and (b) an adaptive learning process characterized by broad generalization and cue interaction.     

Antagonism of lateral amygdala alpha1-adrenergic receptors facilitates fear conditioning and long-term potentiation

Norepinephrine receptors have been studied in emotion, memory, and attention. However, the role of alpha1-adrenergic receptors in fear conditioning, a major model of emotional learning, is poorly understood. We examined the effect of terazosin, an alpha1-adrenergic receptor antagonist, on cued fear conditioning. Systemic or intra-lateral amygdala terazosin delivered before conditioning enhanced short- and long-term memory. Terazosin delivered after conditioning did not affect consolidation. In vitro, terazosin impaired lateral amygdala inhibitory postsynaptic currents leading to facilitation of excitatory postsynaptic currents and long-term potentiation. Since alpha1 blockers are prescribed for hypertension and post-traumatic stress disorder, these results may have important clinical implications.Although norepinephrine (NE) has been widely studied as an important modulator of memory and emotion, comparatively little is known about the role of NE in amygdala-dependent Pavlovian fear conditioning, a major model for understanding the neural basis of fear learning and memory. In fear conditioning, an emotionally neutral conditioned stimulus (CS; i.e., tone) is temporally paired with an aversive unconditioned stimulus (US; i.e., footshock). After very few pairings, a lasting, robust CS–US association is acquired, and the CS elicits stereotypical defensive responses, including behavioral freezing (Blanchard and Blanchard 1969; Bolles and Fanselow 1980).The lateral nucleus of the amygdala (LA) is a key structure underlying fear conditioning (LeDoux 2000). Convergence of CS and US information in LA is believed to play an important role in initiating synaptic plasticity. Long-term potentiation (LTP)-like changes in LA CS processing are critical for fear memory storage (LeDoux 2000; Blair et al. 2001; Maren 2001; Walker and Davis 2002). LA receives auditory CS inputs from the thalamus and cortex and connects directly and indirectly with the central nucleus of the amygdala to control expression of Pavlovian fear responses.Of the noradrenergic receptor subtypes, alpha1 receptors have received the least attention in fear conditioning. LA receives NE-containing inputs from the locus coeruleus that fire tonically and phasically in response to aversive stimuli like footshock (Pitkänen 2000; Tanaka et al. 2000; Aston-Jones and Cohen 2005). Alpha1-adrenergic receptors are expressed in LA, most likely on both excitatory and inhibitory neurons (Jones et al. 1985; Domyancic and Morilak 1997). Alpha1 receptor activation stimulates GABA-mediated miniature inhibitory postsynaptic currents in LA (Braga et al. 2004), suggesting that alpha1 receptors contribute to inhibition in fear conditioning pathways. Several elegant experiments recently demonstrated that LA inhibition gates synaptic plasticity necessary for fear conditioning, and this inhibitory gate can be influenced by neuromodulators including NE (Stutzmann and LeDoux 1999; Shumyatsky et al. 2002; Bissière et al. 2003; Shaban et al. 2006; Shin et al. 2006; Tully et al. 2007). However, the role of alpha1 receptor activity in gating amygdala LTP and fear learning has never been examined.We hypothesized that alpha1 blockers would facilitate fear learning, possibly by impairing LA inhibition and facilitating LA LTP. To test this hypothesis, we injected rats with terazosin, a selective alpha1-adrenergic receptor antagonist, systemically or directly into LA before or after fear conditioning. We examined in vitro the effect of terazosin on LA pyramidal cell inhibitory postsynaptic currents (IPSCs) and excitatory postsynaptic currents (EPSCs) in response to fiber stimulation of the thalamic CS input pathway to LA, as well as the effect of terazosin on LA LTP in this same pathway. We found that intra-LA terazosin facilitated fear conditioning when injected before but not after training. We also found that terazosin impaired IPSCs in LA pyramidal cells, leading to facilitated EPSCs and LTP.Behavioral experiments were conducted on adult, male Sprague–Dawley rats (Hilltop Laboratory Animals) weighing approximately 300 g upon arrival. Rats were individually housed, maintained on a 12/12 h light/dark schedule, and allowed free access to food and water. Testing was conducted during the light phase. All procedures and experiments were approved by NYU''s Animal Care and Use Committee.For systemic injections, terazosin (20 mg/kg; Sigma) was dissolved in saline and injected intraperitoneally (i.p.) 30 min prior to conditioning in 1 mL/kg volume. For bilateral infusions, terazosin (125 ng/0.25 µL) was dissolved in aCSF and infused into the LA at 0.1 µL/min 30 min prior to or immediately after fear conditioning. Bilateral guide cannulae (22 gauge; Plastics One) aimed at LA (−3.3 mm anterior, 5.2 mm lateral, −7 mm dorsal to bregma) were surgically implanted as previously described (Sotres-Bayon et al. 2009). Rats were given at least 7 d to recover from surgery before testing. For infusions, dummy cannulae were removed, and infusion cannulae (28 gauge, +1 mm beyond guides) were inserted into guides. Infusion cannulae were connected to a 1.0 μL Hamilton syringe via polyethylene tubing. Infusion rate was controlled by a pump (PHD22/2000; Harvard Apparatus), and infusion cannulae were left in place for an additional 60 sec to allow diffusion of the solution away from the cannula tip, then dummy cannulae were replaced. Upon completion of the experiment, rats were euthanized, brains removed, and cannulae placements verified histologically as previously described (Sotres-Bayon et al. 2009).Two contexts (A and B) were used for all testing as previously described (Schiller et al. 2008). The grid floors in Context B were covered with black Plexiglas inserts to reduce generalization. No odors were used and chambers were cleaned between sessions. CSs were 30 sec, 5 kHz, 80 dB tones, and USs were 1 sec, 0.8 mA scrambled electric footshocks. Experiments consisted of two phases separated by 48 h: (1) fear conditioning in Context A and (2) long-term memory (LTM) test in Context B. On Day 1, rats were placed in Context A, allowed 5 min to acclimate, and then received three CS–US pairings separated by variable 5 min ITIs. On Day 3, rats were placed in Context B and allowed 5 min to acclimate before receiving one CS-alone presentation.The index of fear in behavioral experiments was “freezing,” the absence of all non-respiratory movement (Blanchard and Blanchard 1971; Fanselow 1980). Following testing, freezing was manually scored from DVDs by a scorer blind to group specification. Graphs represent group means ± SEM. Statistical analysis was conducted with GraphPad Prism.Whole-cell electrophysiological recordings were obtained from LA pyramidal cells using in vitro coronal slices from rats aged P21–P30 d as described in Cunha et al. (2010). Terazosin was bath-applied for 10 min to achieve stable responses before testing. The cells were voltage-clamped using an Axopatch 200B amplifier at −35 mV for recording EPSCs and IPSCs. Synaptic responses were evoked with sharpened tungsten bipolar stimulating electrodes. Internal capsule fibers containing thalamic afferents were stimulated for paired-pulse facilitation (PPF) (ISI = 50 msec; 0.1 Hz) using a photoelectric stimulus isolation unit with a constant current output. Cells were rejected if access resistance (8–26 MΩ) changed more than 15%. Signals were filtered at 2 kHz and digitized (Digidata 1440 A; Axon Instruments), and peak amplitude, 10%–90% rise time, and IPSC decay time constants were analyzed offline using pCLAMP10.2 software (Axon Instruments).Brain slices for LTP experiments were prepared from rats aged 3–5 wk as in Johnson et al. (2008) and maintained on an interface chamber at 31°C. Glass recording electrodes (filled with aCSF, 5 MΩ resistance) were guided to LA neurons. Bipolar stainless steel stimulating electrodes (75 kΩ) were positioned medial to LA in internal capsule fibers. Orthodromic synaptic potentials were evoked via an isolated current generator (Digitimer; 100 μsec pulses of 0.3–0.7 mA). Evoked field potentials were recorded with an Axoclamp 2B amplifier and Axon WCP software (Axon Instruments). Data were analyzed offline using WCP PeakFit (Axon Instruments). LTP was measured as a change in evoked field potential amplitude.Baseline responses were monitored at 0.05 Hz for 30 min with a stimulus intensity of 40%–50% of maximum fEPSP before LTP induction. Terazosin (10 µM) was superfused for 15 min, and then LTP was elicited by three tetanus trains (100 Hz × 1 sec, 3 min ITI) with the same intensity and pulse duration as the baseline stimuli. In one experiment, picrotoxin (PTX; 75 µM) was present in the perfusion solution to block fast GABAergic signaling.