PP1 inhibitors depolarize Hermissenda photoreceptors and reduce K+ currents

In vitro extinction learning in Hermissenda: involvement of conditioned inhibition molecules

Extinction of a conditioned association is typically viewed as the establishment of new learning rather than the erasure of the original memory. However, recent research in the nudibranch, Hermissenda crassicornis (H.c.) demonstrated that extinction training (using repeated light-alone presentations) given 15 min, but not 23 h, after memory acquisition reversed both the cellular correlates of learning (enhanced Type B cell excitability) and the behavioral changes (reduced phototaxis) produced by associative conditioning (pairings of light, CS, and rotation, US). Here, we investigated the putative molecular signaling pathways that underlie this extinction in H.c. by using a novel in vitro protocol combined with pharmacological manipulations. After intact H.c. received either light-rotation pairings (Paired), random presentations of light and rotation (Random), or no stimulation (Untrained), B cells from isolated CNSs were recorded from during exposure to extinction training consisting of two series of 15 consecutive light-steps (LSs). When in vitro extinction was administered shortly (2 h, but not 24 h) after paired training, B cells from Paired animals showed progressive and robust declines in spike frequency by the 30th LS, while control cells (Random and Untrained) did not. We found that several molecules implicated in H.c. conditioned inhibitory (CI) learning, protein phosphatase 1 (PP1) and arachidonic acid (AA)/12-lipoxygenase (12-LOX) metabolites, also contributed to the spike frequency decreases produced by in vitro extinction. Protein phosphatase 2B (PP2B) also appeared to play a role. Calyculin A (PP1 inhibitor), cyclosporin A (PP2B inhibitor), and baicalein (a 12-LOX inhibitor) all blocked the spike frequency declines in Paired B cells produced by 30 LSs. Conversely, injection of catalytically-active PP1 (caPP1) or PP2B (caPP2B) into Untrained B cells partially mimicked the spike frequency declines observed in Paired cells, as did bath-applied AA, and occluded additional LS-produced reductions in spiking in Paired cells.

Protein tyrosine kinase involvement in learning-produced changes in Hermissenda type B photoreceptors

Learning-correlated changes in the excitability and photoresponses of Hermissenda's ocular type B photoreceptors are mediated by reductions in two distinct K(+) currents, I(A) and I(K-Ca). The suppression of these K(+) currents has been linked to conditioning-produced activation of protein kinase C (PKC). The question of whether PKC accounts completely for the changes in excitability and K(+) currents or whether other kinase(s) are involved has received little attention. In the present experiments, we asked whether protein tyrosine kinases (PTKs) might also contribute to conditioning-produced alterations in B cells. We found that the PTK inhibitors genistein and lavendustin A greatly reduced cumulative depolarization of type B cells, a short-term correlate of associative learning. This disruption occurred even when PKC activation had been either occluded by preexposure of type B cells to a phorbol ester or otherwise prevented by the pseudosubstrate inhibitor peptide PKC[19-31]. PTK inhibitors also increased the amplitude of the transient (I(A)) and delayed (I(Delayed)) components of voltage-dependent K(+) current that have previously been shown to be selectively reduced by conditioning and to contribute to cumulative depolarization. Genistein partially prevented the reduction of I(A) and I(Delayed) due to in vitro conditioning and blocked the changes in their voltage dependencies. Ionophoresis of pervanadate ion, a potent inhibitor of protein tyrosine phosphatases, depolarized type B photoreceptors and occluded conditioning-produced cumulative depolarization. Pervanadate also suppressed I(A) and I(Delayed), reduced their voltage dependence, and altered inactivation kinetics for I(A), mimicking conditioning. Western blot analysis using a phosphotyrosine antibody indicated that conditioning increased the phosphotyrosine content of many proteins within the Hermissenda CNS. Collectively, our results suggest that in addition to PKC, one or more PTKs play an important role in conditioning-produced changes in type B cell excitability. PTKs and PKCs converge to effect reductions in B cell K(+) currents during conditioning, apparently through distinct biophysical mechanisms.

Transient mitogen-activated protein kinase activation is confined to a narrow temporal window required for the induction of two-trial long-term memory in Aplysia

Although it is commonly appreciated that spaced training is superior to massed training in memory formation, the molecular mechanisms underlying this feature of memory are largely unknown. We previously described the selective benefit of multiple spaced (vs massed) training trials in the induction of long-term memory (LTM) for sensitization in Aplysia californica. We now report that LTM can be induced with only two spaced training trials [tail shocks (TSs)] when the second TS is administered 45 min after the first. In contrast, spacing intervals of 15 and 60 min are ineffective. This surprisingly narrow permissive training window for two-trial LTM is accompanied by an equally narrow window of transient mitogen-activated protein kinase (MAPK) activation, a necessary signaling molecule for LTM induction, at 45 min after a single TS. Thus, the transient recruitment of MAPK following a single TS may provide a narrow molecular window for two-trial LTM formation.

Mouse brain localization of the protein kinase C-enhanced phosphatase 1 inhibitor KEPI (Kinase C-Enhanced PP1 Inhibitor)

We recently identified the protein kinase C-enhanced protein phosphatase 1 (PP1) inhibitor KEPI based on its morphine-induced upregulation in striatum. Regulation of protein serine/threonine dephosphorylation by PP1 can modulate important brain signaling pathways. To improve understanding of KEPI's role in the brain, we have developed anti-KEPI sera in rabbits immunized with a hemocyanin conjugate of KEPI residues 66-80, characterized the specificity that this serum provides, mapped the distribution of immunoreactive KEPI (iKEPI) in mouse brain, rat dorsal root ganglia and striatal cultures and documented KEPI binding to PP1 in vitro. Staining is found in apparently neuronal processes and, often less intensely, in neuronal perikarya in primary cultures and in neurons and neuronal elements from a number of brain regions. iKEPI fiber/terminal patterns are relatively densely distributed in striatum, nucleus accumbens, septum, bed nucleus of the stria terminalis, hippocampus, paraventricular thalamus, ventromedial hypothalamus, interpeduncular nucleus, raphe nuclei, nucleus caudalis of the spinal tract of the trigeminal and dorsal horn of the spinal cord. iKEPI-positive cell bodies lie in the nucleus accumbens, striatum, lateral septal nucleus, granular layer of dentate gyrus, interpeduncular nucleus, dorsal root ganglia and cerebellar vermis. These expression patterns point to possible roles for KEPI in regulating protein dephosphorylation by inhibiting PP1 activities in a number of brain pathways likely to use several different neurotransmitters and to participate in a number of brain functions. Dense KEPI immunoreactivity in nucleus accumbens perikarya, combined with evidence for its regulation by opiates, supports possible roles for KEPI in molecular signal transduction pathways important for drug reward and addiction.

The effect of intensity and duration on the light-induced sodium and potassium currents in the Hermissenda type B photoreceptor

Light duration and intensity influence classical conditioning in Hermissenda through their effects on the light-induced currents. Furthermore, the contribution of voltage-dependent potassium currents to the long-lasting depolarization in type B photoreceptors depends on light-induced currents active at resting potentials. Thus, the present study measures the effect of holding potential, duration, and intensity on the light-induced currents in discontinuous single-electrode voltage clamp mode. Three distinct current components are distinguished by their temporal and voltage characteristics and sensitivity to pharmacological agents. One current component is a transient sodium current, I(Nalgt); another is a plateau sodium current, I(plateau), which persists for the duration of the light stimulus. Substitution of trimethylammonium chloride for sodium reduces both currents equally, suggesting that I(plateau) represents partial inactivation of I(Nalgt). The third current component is a prolonged reduction in potassium currents, I(Klgt); it is accompanied by an increase in input resistance, and it appears at potentials close to rest. An increase in light duration or intensity causes an increase in the peak conductance of both I(Nalgt) and I(Klgt). Latency of I(Nalgt) is decreased by intensity, whereas rise time is increased by duration. An increase in light duration or intensity causes an increase in the time-to-peak and duration of I(Klgt). Characteristics of these currents suggest that I(Klgt) is responsible for the long-lasting depolarization seen after light termination, and thus plays a role in classical conditioning.

Learning in simple systems

Cellular processes that mediate learning and memory show a remarkable level of conservation between vertebrates and invertebrates. Recent studies have shown that learning and memory formation in invertebrates, so-called 'simple systems', involves a highly complex arrangement of cellular pathways. Some pathways contribute to a single stage of memory formation, whereas others impact on multiple stages of memory development. Distinct cellular pathways may also act in series or in parallel during various stages of memory formation.