Conditioning of single units in visual association cortex: cell-specific behavior within a small population

Validating TDP1 as an Inhibition Target for the Development of Chemosensitizers for Camptothecin-Based Chemotherapy Drugs

Cancer chemotherapy sensitizers hold the key to maximizing the potential of standard anticancer treatments. We have a long-standing interest in developing and validating inhibitors of the DNA repair enzyme tyrosyl-DNA phosphodiesterase 1 (TDP1) as chemosensitizers for topoisomerase I poisons such as topotecan. Herein, by using thieno[2,3-b]pyridines, a class of TDP1 inhibitors, we showed that the inhibition of TDP1 can restore sensitivity to topotecan, results that are supported by TDP1 knockout cell experiments using CRISPR/Cas9. However, we also found that the restored sensitivity towards topoisomerase I inhibitors is likely regulated by multiple complementary DNA repair pathways. Our results showed that one of these pathways is likely modulated by PARP1, although it is also possible that other redundant and partially overlapping pathways may be involved in the DNA repair process. Our work thus raises the prospect of targeting multiple DNA repair pathways to increase the sensitivity to topoisomerase I inhibitors.

The Sensory Neocortex and Associative Memory

Most behaviors in mammals are directly or indirectly guided by prior experience and therefore depend on the ability of our brains to form memories. The ability to form an association between an initially possibly neutral sensory stimulus and its behavioral relevance is essential for our ability to navigate in a changing environment. The formation of a memory is a complex process involving many areas of the brain. In this chapter we review classic and recent work that has shed light on the specific contribution of sensory cortical areas to the formation of associative memories. We discuss synaptic and circuit mechanisms that mediate plastic adaptations of functional properties in individual neurons as well as larger neuronal populations forming topographically organized representations. Furthermore, we describe commonly used behavioral paradigms that are used to study the mechanisms of memory formation. We focus on the auditory modality that is receiving increasing attention for the study of associative memory in rodent model systems. We argue that sensory cortical areas may play an important role for the memory-dependent categorical recognition of previously encountered sensory stimuli.

Pharmacological induction of use-dependent receptive field modifications in the visual cortex

Lasting modifications of the receptive fields of neurons in the visual cortex can be induced by pairing visual stimuli with iontophoretic application of the neuromodulators acetylcholine and noradrenaline or the excitatory amino acids N-methyl-D-aspartate (NMDA) and L-glutamate. The modifications are obtained in less than 1 hour and persist for more than 40 minutes. Thus, acetylcholine and norepinephrine have a permissive role in use-dependent neuronal plasticity. These results support the notion of a postsynaptic threshold for neuronal malleability that differs from that of sodium-dependent action potentials.

"Learned" changes in the responses of the rat barrel field neurons

The effect of pairing two vibrissa stimulations on unit responses of the barrel field of the somatosensory cortex were studied in partially restrained but awake and undrugged rats. Before pairing, one of the stimulations (S2) evoked a stable, short-latency and excitatory response from the recorded unit. Depending on the neuron, the other stimulation (S1), preceding S2 by 500 ms, did or did not have an effect before pairing. In a number of cases, the S1-S2 association produced significant changes in the unit responses: (1) the appearance of an excitatory response to S1 when that stimulus was ineffective before pairing; (2) the modification of pre-existing responses to S1 and/or S2. In all instances these modifications consisted in the decrease or disappearance of the "afferent inhibition" and/or the appearance of long-latency excitatory components. These effects appeared after some 30-100 trials and persisted in some cases up to 20 min after interruption of pairing. Our observations provide the first physiological data on the plasticity of the vibrissa projections in the chronic adult rodent. Though the underlying plastic neural elements and mechanisms remain to be specified, these phenomena suggest that "learned" changes in unit activity may occur in sensory systems and not only in "non-specific" ones.

Model of cortical organization embodying a basis for a theory of information processing and memory recall

Motivated by V. B. Mountcastle's organizational principle for neocortical function, and by M. E. Fisher's model of physical spin systems, we introduce a cooperative model of the cortical column incorporating an idealized substructure, the trion, which represents a localized group of neurons. Computer studies reveal that typical networks composed of a small number of trions (with symmetric interactions) exhibit striking behavior--e.g., hundreds to thousands of quasistable, periodic firing patterns, any of which can be selected out and enhanced with only small changes in interaction strengths by using a Hebb-type algorithm.

Two neuronal systems are involved in a classical conditioning in the rat

Unit activity of the hippocampus, the centrum-medianum-parafascicularis and medialis dorsalis nuclei of the thalamus, was recorded in chronic rats during a classical conditioning; neocortical electroencephalographic and somatic responses were also recorded. Conditioned unit responses of the different groups of neurons were compared according to the precocity of their appearance, their stability, their latency and the differentiation between the positive (reinforced) and the negative (non-reinforced) conditioned stimuli. Conditioned unit responses of type I hippocampal neurons (probably pyramids) and of neurons located in the centrum-medianum-parafascicularis nucleus did not differentiate between the positive and negative conditioned stimuli; they progressed rapidly, then declined and disappeared. They were contemporary with an initial conditioning stage which was characterized by undifferentiated arousal responses (desynchrony of electroencephalographic activity) to both conditioned stimuli. Conditioned unit responses of type II hippocampal neurons (probably granule cells or interneurons of the fascia dentata) and of neurons located in the medialis dorsalis nucleus progressed slowly and differentiated between the conditioned stimuli during a late conditioning stage which was characterized by the regression of arousal responses and the differentiation of somatic responses. These data strongly suggest that two neuronal systems, each having a different role, are involved in classical conditioning in the rat.

Dynamic interactions of evoked potentials in a polysensory cortex of the cat

Interactions of acoustic and somatosensory evoked potentials were studied in the anterior suprasylvian gyrus of the cat. Two kinds of interaction could be observed: occlusion or facilitation. In most cases occlusion was observed. The interactions showed dynamic changes and were susceptible to different kinds of influences. After having determined the control values of interaction over a period of several minutes, reversible enhancement of occlusion was observed after synchronous activation of the acoustic and somatosensory inputs with 2 Hz frequency. The same effect could be observed during stimulation of the mesencephalic reticular formation with 200 Hz frequency. The interactions could also be influenced by amphetamine and gamma-glutamyltaurine, known as drugs capable of influencing the arousal level of the brain. During treatment with amphetamine the interaction was shifted in the facilitatory direction. The antagonists of amphetamine (haloperidol and reserpine) prevented this effect. The authors suggest that the interactions of acoustic and somatosensory responses are mediated by interneurons (inhibitory and/or excitatory) and particular stimulus situations and drugs are able to modify the equilibrium between excitatory and inhibitory subsystems.