Responses of morphologically identified cortical neurons to intracellularly injected cyclic AMP

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.

Responses of morphologically identified cortical neurons to intracellularly injected cyclic GMP

Cyclic nucleotides are thought to act as second messengers of neurotransmission inside central neurons, and cyclic guanosine monophosphate (cGMP) has been postulated to act as a messenger for muscarinic, cholinergic transmission. Nonetheless, the action of cGMP has not yet been established in identified cortical neurons. We injected cGMP and horseradish peroxidase (HRP) intracellularly in neurons of the motor cortex of awake cats. Fifty-four percent of injected cells responded to cGMP and HRP with an increase in input resistance within 30 s after injection. None of a control group of cells injected with HRP without cGMP so responded. In cells receiving intracellular depolarizing current sufficient to produce repeated spike discharge at the time of injection, the increase in input resistance after cGMP persisted for as long as the cells could be held. There was no significant increase in firing rate after injection of cGMP. Cells responding to cGMP with an increased input resistance were identified as pyramidal cells of layer V. One inverted pyramidal cell of layer VI also showed an increase in input resistance in response to cGMP. Previous studies have suggested that muscarinic cholinergic agents produce an increased input resistance (thought to reflect a decreased potassium conductance) underlying an increased rate of discharge in neocortical neurons. Our results favor a dual action of muscarinic cholinergic transmission in mammalian cortical neurons--the increase in input resistance in layer V pyramidal cells mediated by cGMP, and the increase in rate of discharge mediated by other means.