Neuroscience. Cajal's rational psychology

Optimization of Somatic Inhibition at Critical Period Onset in Mouse Visual Cortex

Local GABAergic circuits trigger visual cortical plasticity in early postnatal life. How these diverse connections contribute to critical period onset was investigated by nonstationary fluctuation analysis following laser photo-uncaging of GABA onto discrete sites upon individual pyramidal cells in slices of mouse visual cortex. The GABA(A) receptor number decreased on the soma-proximal dendrite (SPD), but not at the axon initial segment, with age and sensory deprivation. Benzodiazepine sensitivity was also higher on the immature SPD. Too many or too few SPD receptors in immature or dark-reared mice, respectively, were adjusted to critical period levels by benzodiazepine treatment in vivo, which engages ocular dominance plasticity in these animal models. Combining GAD65 deletion with dark rearing from birth confirmed that an intermediate number of SPD receptors enable plasticity. Site-specific optimization of perisomatic GABA response may thus trigger experience-dependent development in visual cortex.

Differential processing of excitation by GABAergic gain modulation in canine caudal ventral respiratory group neurons

The discharge frequency (F(n)) patterns of medullary respiratory premotor neurons are subject to potent tonic GABAergic gain modulation. Studies in other neuron types suggest that the synaptic input for tonic inhibition is located on the soma where it can affect total neuronal output. However, our preliminary data suggested that excitatory responses elicited by highly local application of glutamate receptor agonists are not gain modulated. In addition, modulation of the amplitude of spike afterhyperpolarizations can gain modulate neuronal output, and this mechanism is located near the spike initiation zone and/or soma. The purpose of this study was to determine if these two gain-modulating mechanisms have different functional locations on the somatodendritic membrane of bulbospinal inspiratory and expiratory neurons. Four-barrel micropipettes were used for extracellular single-neuron recording and pressure ejection of drugs in decerebrate, paralyzed, ventilated dogs. The net increases in F(n) due to repeated short-duration picoejections of the glutamate receptor agonist, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), was quantified before and during locally induced antagonism of GABA(A) receptors by bicuculline or small-conductance, calcium-activated potassium channels by apamin. The AMPA-induced net increases in F(n) were not significantly altered by BIC, although it produced large increases in the respiratory-related activity. However, the AMPA-induced net responses were amplified in accordance with the gain increase of the respiratory-related activity by apamin. These findings suggest that GABAergic gain modulation may be functionally isolated from the soma/spike initiation zone, e.g., located on a dendritic shaft. This could allow other behavioral signals requiring strong neuronal activation (e.g., coughing, sneezing, vomiting) to utilize the same neuron without being attenuated by the GABAergic modulation.

Adenosine action on interneurons and synaptic transmission onto interneurons in rat hippocampus in vitro

To investigate the action of adenosine on interneurons as well as on excitatory synaptic transmission onto interneurons in the hippocampus, intracellular recordings were made from electrophysiologically identified interneurons in the CA1 region of the hippocampal slice in vitro. The effects of adenosine and the preferential adenosine A1 receptor agonist, chloroadenosine, were examined. Application of 50 microM adenosine and 20 microM chloroadenosine to the bath produced a hyperpolarization of 5.6+/-1.6 (n=5) and 6.1+/-1.4 mV (n=6), respectively, as well as a decrease in membrane input resistance of 18.1+/-3.5% (n=5) and 18.5+/-1.4% (n=6), respectively. Adenosine depressed the postsynaptic potentials (PSPs) elicited in the interneurons by stimulation of Schaffer collateral fibers by 73+/-6.8% (n=5). The amplitude and the duration of the afterhyperpolarization which followed the spike of the action potential were attenuated by 48+/-6.9% and 31+/-8.6%, respectively (n=5). Chloroadenosine depressed the evoked PSPs in these interneurons by 61.2+/-2.7% (n=6) and depressed the duration and the amplitude of the afterhyperpolarization by 85.2+/-4.5% and by 72.6+/-4.8%, respectively (n=6). The data show that adenosine and chloroadenosine directly inhibit hippocampal CA1 interneurons by blocking the synaptic input, by hyperpolarizing the membrane potential and by depressing the afterhyperpolarization following individual action potential spikes. It is proposed that adenosine A1 receptors are present at pre- and/or postsynaptic sites of interneuron synapses in the hippocampal CA1 region. The present findings demonstrate that adenosine A1 receptor activation in CA1 interneurons is able to modulate the excitatory synaptic input to, and excitability of, these neurons. Thus, as adenosine is released during ischemia and epilepsy, adenosine may protect both interneurons and pyramidal cells from glutamate excitotoxicity through activation of adenosine A1 receptors on these neurons in the hippocampus.

How many subtypes of inhibitory cells in the hippocampus?

Hippocampal inhibitory cells are diverse. It is supposed that they fall into functionally distinct subsets defined by a similar morphology and physiology. Switching between functions could be accomplished by activating receptors for modulating transmitters expressed selectively by different subsets of interneurons. We tested this hypothesis by comparing morphology, physiology, and neurotransmitter receptor expression for CA1 hippocampal interneurons. We distinguished 16 distinct morphological phenotypes and 3 different modes of discharge. Subsets of inhibitory cells were excited or inhibited by agonists at receptors for noradrenaline, muscarine, serotonin, and mGluRs. Most cells responded to 2 or 3 agonists, and 25 different response combinations were detected. Subsets defined by morphology, physiology, and receptor expression did not coincide, suggesting that hippocampal interneurons cannot easily be segregated into a few well-defined groups.

Glycine and GABAA receptor-mediated synaptic transmission in rat substantia gelatinosa: inhibition by mu-opioid and GABAB agonists

1. Bicuculline-sensitive and strychnine-sensitive inhibitory postsynaptic currents (IPSCs) could be evoked in neurones of the rat substantia gelatinosa of the spinal trigeminal nucleus pars caudalis. 2. Spontaneous tetrodotoxin (TTX)-insensitive-mediated miniature IPSCs (mIPSCs) blocked by strychnine or bicuculline were also present in many neurones. The decay of the glycine receptor-mediated mIPSCs was fitted by a single exponential, whereas the decay of the GABAA receptor-mediated mIPSCs could in some instances be fitted by a single exponential, but in other instances required two exponentials. 3. An increase in baseline current noise developed during the course of the recording. This noise was abolished by strychnine (1 microM) but was insensitive to bicuculline (10 microM), TTX (0.5 microM), [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO, 1 microM) or baclofen (30 microM). The single-channel conductance underlying the noise was estimated to be 21 pS. 4. The mu-opioid agonist DAMGO (1-10 microM) reduced the amplitude of the evoked glycine receptor-mediated IPSC and the evoked GABAA receptor-mediated IPSC. The mu-opioid antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP, 1 microM) reversed the DAMGO inhibition. 5. The GABAB agonist baclofen (30 microM) reduced the amplitude of the evoked glycine receptor-mediated IPSC and the GABAA receptor-mediated IPSC. The inhibition was reversed by the selective GABAB antagonist 3-N[1-(S)-(3,4-dichlorophenyl)ethyl]amino- 2-(S)-hydroxypropyl-P-benzyl-phosphinic acid (CGP 55845A, 1 microM). 6. Both DAMGO and baclofen reduced the frequency of glycine and GABAA receptor-mediated mIPSCs without affecting average amplitude, and increased the percentage of failures of the evoked glycine and GABAA receptor-mediated IPSCs, suggesting a presynaptic site of action.

Differences between somatic and dendritic inhibition in the hippocampus

Hippocampal synaptic inhibition is mediated by distinct groups of inhibitory cells. Some contact pyramidal cells perisomatically, while others terminate exclusively on their dendrites. We examined perisomatic and dendritic inhibition by recording from CA3 inhibitory and pyramidal cells and injecting biocytin to visualize both cells in light and electron microscopy. Single perisomatic inhibitory cells made 2-6 terminals clustered around the soma and proximal pyramidal cell processes. Dendritic cells established 5-17 terminals, usually on different dendrites of a pyramidal cells. Perisomatic terminals were larger than those facing dendritic membrane. Perisomatic inhibitory cells initiated the majority of simultaneous IPSPs seen in nearby pyramidal cells. Single IPSPs initiated by perisomatic sodium-dependent action potentials. Activation of inhibitory fibers terminating on dendrites could suppress calcium-dependent spikes. Thus, distinct inhibitory cells may differentially control dendritic electrogenesis and axonal output of hippocampal pyramidal cells.

Dynamics control of semantic processes in a hierarchical associative memory

A neural mechanism for control of dynamics and function of associative processes in a hierarchical memory system is demonstrated. For the representation and processing of abstract knowledge, the semantic declarative memory system of the human brain is considered. The dynamics control mechanism is based on the influence of neuronal adaptation on the complexity of neural network dynamics. Different dynamical modes correspond to different levels of the ultrametric structure of the hierarchical memory being invoked during an associative process. The mechanism is deterministic but may also underlie free associative thought processes. The formulation of an abstract neural network model of hierarchical associative memory utilizes a recent approach to incorporate neuronal adaptation. It includes a generalized neuronal activation function recently derived by a Hodgkin-Huxley-type model. It is shown that the extent to which a hierarchically organized memory structure is searched is controlled by the neuronal adaptability, i.e. the strength of coupling between neuronal activity and excitability. In the brain, the concentration of various neuromodulators in turn can regulate the adaptability. An autonomously controlled sequence of bifurcations, from an initial exploratory to a final retrieval phase, of an associative process is shown to result from an activity-dependent release of neuromodulators. The dynamics control mechanism may be important in the context of various disorders of the brain and may also extend the range of applications of artificial neural networks.

Tonic inhibition originates from synapses close to the soma

Central neurons are subject to a tonic barrage of randomly occurring spontaneous inhibitory events (mIP-SCs) resulting from the action potential-independent release of gamma-aminobutyric acid (GABA). Do the terminals making synapses onto somatic versus dendritic sites, which arise from specific populations of interneurons, differ in their ability to generate mIPSCs? We have combined the techniques of whole-cell patch-clamp recording and computational simulation to demonstrate that in granule cells of the dentate gyrus, most of the action potential-independent inhibition taking place as mIPSCs originates from proximal sites. Indeed, removal of the bulk (> 50%) of the dendritic tree did not change the characteristics of mIPSCs. These results are consistent with a functional segregation of GABAergic terminals synapsing at proximal versus distal portions of central neurons. Thus, proximal GABAergic terminals are responsible for tonic inhibition targeted at the soma.