Neurotransmitters in the cerebral cortex

This article surveys the conventional neurotransmitters and modulatory neuropeptides that are found in the cerebral cortex and attempts to place them into the perspective of both intracortical circuitry and cortical disease. The distribution of these substances is related, where possible, to particular types of cortical neuron or to afferent or efferent fibers. Their physiological actions, where known, on cortical neurons are surveyed, and their potential roles in disease states such as the dementias, epilepsy, and stroke are assessed. Conventional transmitters that occur in afferent fibers to the cortex from brain-stem and basal forebrain sites are: serotonin, noradrenaline, dopamine, and acetylcholine. All of these except dopamine are distributed to all cortical areas: dopamine is distributed to frontal and cingulate areas only. The transmitter in thalamic afferent systems is unknown. Gamma aminobutyric acid (GABA) is the transmitter used by the majority of cortical interneurons and has a profound effect upon the shaping of receptive field properties. The vast majority of the known cortical peptides are found in GABAergic neurons, and the possibility exists that they may act as trophic substances for other neurons. Levels of certain neuropeptides decline in cases of dementia of cortical origin. Acetylcholine is the only other known transmitter of cortical neurons. It, too, is contained in neurons that also contain a neuropeptide. The transmitter(s) used by excitatory cortical interneurons and by the efferent pyramidal cells is unknown, but it may be glutamate or aspartate. It is possible that excitotoxins released in anoxic disease of the cortex may produce damage by acting on receptors for these or related transmitter agents.