**1. Introduction**

Throughout the 55-year history of the G.B. Elyakov Pacific Institute of Bioorganic Chemistry (PIBOC), Far Eastern Branch, Russian Academy of Science, a vast variety of new marine-derived natural compounds including different glycosides, polar steroids, alkaloids, lipids, and other metabolites have been isolated from echinoderms and sponges, ascidians, algae, fungi, and marine bacteria and thoroughly studied. Many of the discovered new natural compounds proved to have a protective action and were repeatedly mentioned in well-known series of review articles published in Natural Products Reports [1]. To protect themselves against predators and pathogens, marine organisms use a variety of structural groups of toxins. The latter are synthesized by lower plants, including microalgae [2–4], and then accumulated by some invertebrates that transfer them up to higher trophic levels. Marine excitatory amino acids (MEAAs) that belong to this category and play an important role in marine communities have found application in pharmacology. To date, there are only three known groups of marine-derived excitatory amino acids: (**1**) kainic acid and related compounds, (2) domoic acid and some its derivatives, (3) sponge-derived dysiherbaine and neodysiherbaine A, which specifically bind and activate the kainate-type of glutamate receptors (GluRs).

MEAAs, including the main compounds shown in Figure 1, are a chemical group of particular research interest to us, although many of them were discovered long ago. This is explained by our studies on highly polar marine metabolites from northwestern Pacific invertebrates, including sponges, and marine bacteria and search for new excitatory compounds or their analogs. Another explanation is the long-term interest of one of the coauthors of the present review (S.I.) [5] in domoic acid (DA), and in DA-producers from the northern Pacific Ocean.

In general, these compounds are known as bioactive agents, particularly some of them as biotoxicants, contaminating marine waters. The application of these compounds in experimental pharmacology has expanded the knowledge about transmission of nerve impulses in higher animals. In fact, excitatory amino acids have a structure similarity with that of the principal excitatory neurotransmitter glutamic acid (**1**) and act as central nervous system (CNS) excitants. In the past decade, MEAAs, sometimes called kainoids, have become an important research field, with several discoveries made.

**Figure 1.** Chemical structures of glutamic acid (**1**) and marine excitatory acids (**2**–**5**).

Despite the successes of the most recent studies on biosynthesis of these natural products, a number of questions about their appearance and disappearance in producers and consumers, transfer via food chains, action on own producers, and origin of some of them still remain unresolved.

The last review [6] considering excitatory amino acids was published more than 20 years ago. Herein, we discuss the recent results of studies on marine-derived natural products belonging to this group, including their discovery, structure, biosynthesis, recent approaches to syntheses, origin, biological action in producers and recipients, and environmental roles.
