Responses of morphologically identified cortical neurons to intracellularly injected cyclic GMP

cGMP: a second messenger for acetylcholine in the brain?

Already 30 years ago, it became apparent that there exists a relationship between acetylcholine and cGMP in the brain. Acetylcholine plays a role in a great number of processes in the brain, however, the role of cGMP in these processes is not known. A review of the data shows that, although the connection between NO-mediated cGMP synthesis and acetylcholine is firmly established, the complexities of the heterosynaptic pathways and the oligosynaptic structures involved preclude a clear definition of the role of cGMP in the functioning of acetylcholine presently.

Increasing the activity of the neostriatum cholinergic system alters an established type of motor behavior in animals

Chronic experiments on four dogs were performed to study the effects of bilateral microinjection of the choline receptor agonist carbacholine and the blocking agent scopolamine into the dorsolateral part of the head of the caudate nucleus on the performance of an operant defensive reflex involving maintenance of a specified amount of flexion and on the differentiation of meaningful signals. Bilateral microinjection of carbacholine (0.1-0.4 micrograms in 1.5 microliters of solvent) reduced the phasic component and amplified the tonic component of the operant responses, inhibited intersignal leg lifts, normalized posture, and calmed the animals, and also led to a sharp improvement in the differentiation of meaningful signals. These changes were expressed as increases of three-fold or more in the latent period of movement initiation when the differentiation signal was used, as compared with the baseline latent period before the injections. Microinjection of the choline receptor blocker scopolamine into the striatum had the opposite effects. Unilateral microinjection of these substances produced changes mainly only on the day of dosage and had no effect on subsequent behavior, while bilateral microinjection altered the established motor behavior for a longer period of time. This affected both the motor and the sensory components of the operant response.

Changes in membrane currents during Pavlovian conditioning of single cortical neurons

Single electrode voltage clamp recordings were made during Pavlovian conditioning of single units of the motor cortex of cats. Units that developed a conditioned spike discharge in response to a click conditioned stimulus (CS) after pairing the click with glabella tap and local ionophoretic application of glutamate showed increases in input resistance and reductions of an early outward current induced by depolarizing commands and by return to holding potentials after hyperpolarizing commands. Changes in later currents were also found in some cells. Units that failed to develop a conditioned response did not show these changes. The decreases in membrane currents could contribute to an increased spike discharge in response to the CS as could the increased input resistance observed after conditioning. Conductance changes of this type may serve as engrams by which some forms of memory and learning are expressed across both vertebrate and invertebrate species.

The role of cGMP in the extinction of the reactions of identified neurons of the edible snail in response to acetylcholine

The possible role of cGMP in the regulation of the extinction of the reactions of the RPa4, RPa3, and LPa3 neurons of the edible snail in response to acetylcholine (ACh), applied rhythmically to the soma of the neuron by means of microiontophoresis, has been investigated. It was demonstrated that activators of guanylate cyclase which increased the level of cGMP in the cell, namely, sodium nitroprusside and sodium azide (5.10(-4)-10(-3) mole/liter), when applied intracellularly, intensify the extinction of inward transmembrane current and of depolarization of the membrane in response to ACh. The hypothesis of the participation of cGMP-dependent phosphorylation of membrane proteins in the regulation of the rate of development, depth, and duration of short-lived plasticity of the cholinoreceptors of the neuron is proposed.

In vivo properties of neurons of the precruciate cortex of cats

The electrical properties of 409 cells of the precruciate cortex of cats were measured intracellularly, in vivo. Resting potentials (RP) averaged -54 +/- 11 mV (SD), and action potentials (AP) of up to 80 mV were found. The magnitude of RP was correlated with the size of AP recorded. Input resistance averaged 8.4 +/- 8.0 megohms (n = 180 cells) and was uncorrelated with AP or RP. There were no significant differences in the above electrical properties between HRP-identified layer V pyramidal cells (n = 56) and unidentified cells (n = 353). However, within layer V pyramidal cells, the size of the soma was relatable to input resistance. Comparisons of present in vivo data with in vitro data obtained by other investigators from cells of the same region, type and species indicate that resting potentials are more positive in vivo than in vitro, but that critical firing thresholds are the same. Injections of ramp depolarizing currents in 118 unidentified cells disclosed 82% simple (no or minimal accommodation) responses. 18% ceiling (small accommodation) responses, and no minimal gradient (large accommodation or injury) responses. This finding was similar to that found in layer V pyramidal cells in vitro.

Intracellular injection of cGMP-dependent protein kinase results in increased input resistance in neurons of the mammalian motor cortex

Purified, cyclic GMP-dependent protein kinase (cGPK) was pressure-injected into neurons of the precruciate cortex of awake cats. Input resistances increased within seconds after injection and remained elevated for 2 min or longer. The increases were larger when cGPK was injected in a mixture with 10 microM cGMP than when injected alone. Injections of heat-inactivated cGPK, with or without 10 microM cGMP, failed to produce increases in input resistance. The present results indicate that injection of activated cGPK into neurons of the mammalian motor cortex can mimic actions of extracellularly applied acetylcholine and intracellularly applied cGMP, the latter in 100-fold higher concentrations than those used here, in neurons of the same cortical areas.

Responses of morphologically identified cortical neurons to intracellularly injected cyclic AMP

Intracellular injections of cyclic adenosine monophosphate (cAMP) and horseradish peroxidase (HRP) were made in neurons of the motor cortex of awake cats. Eighty-six percent of injected cells responded to cAMP and HRP with a rapid decrease in input resistance. The decreases in input resistance occurred immediately after injection and began to return toward baseline 2 to 3 min later. The decreases were significantly greater than the small decreases in input resistance normally seen in uninjected cells held for 2 min or more after penetration and exceeded comparably small decreases in input resistance seen after control injections of 5'-AMP plus HRP. Pyramidal cells of layer V were identified as responding to cAMP with a decreased input resistance. A spiny stellate cell of layer III and a pyramidal cell of layer VI were also identified that showed similar responses. Increased rates of discharge were also observed after penetration with electrodes containing cAMP, but significant changes in input resistance were not found in association with the increased rates of discharge. After pressure injection of cAMP, the rates of discharge decreased toward more normative values. Our findings indicate that cAMP has an effect on cortical neurons similar to that found in some types of invertebrate (molluscan) neurons and dissimilar to the effect of cyclic guanosine monophosphate.