Chemical circuitry and central cardiovascular regulation

In attempting to sort out the functional components responsible for central cardiovascular regulation, circuits which can be defined by their neurotransmitter provide a focus for an initial simplified scheme. Three classes of transmitter-specific circuits can be developed from this perspective. The most prominent class, amino acid-mediated circuits, are illustrated by the presumptive excitatory glutamatergic excitatory efferent projections from baroreceptors to the nucleus tractus solitarius (NTS) and by presumptive inhibitory interneurons intrinsic to reticular system which use gamma-aminobutyrate as a transmitter. Secondly, monoamine-mediated intrinsic and extrinsic projections to the NTS are generally subsumed neuroanatomically within the so-called "non-specific" projection systems but recent cytochemical studies indicate that some organizational principles exist within NTS regions. A third class of chemically defined transmission consists of those circuits attributed to peptidergic neurons, such as endorphins, enkephalins, Substance P, or vasoactive-intestinal peptide (VIP). Although no classes of transmitters or transmitter mechanisms wholly unique to cardiovascular regulation have yet been identified, these 3 classes of transmitter can each mediate a broad range of integrative actions, through unique molecular mechanisms.