Brain Mechanisms

Neurotransmitter organization of aggressive behavior. Adams rightly tries to reduce the possible aggressive behavioral forms to an acceptable minimum by dividing them into just two distinct groups: offense and defense (submission being the extreme manifestation of the latter). Such a simplified approach has likewise been chosen in the present commentary, in which an attempt is made to elucidate the neurotransmitter basis of various aggressive (emotional) behavioral reactions of the cat. Only two neurotransmitter systems (the cholinergic and the catecholaminergic) will be touched upon; the roles of other putative transmitters. such as serotonin, GABA, and so forth, lie beyond the scope of the present discussion.

Let us begin with one of Darwin's principal laws: self-preservation. Most probably, this drive is of utmost importance in determining the actual behavior of any individual. When consummatory behavior is not taken into account, the phenomenon of self-preservation can be reduced to mere defense. When looked at from an ethological point of view, and disregarding, again, consummatory or sexual drives, offense seems to be just a special defense strategy, one that is only manifest when the animal (or human?) is in a somewhat "desperate" situation.

Affective defense behavior. This is the term used by Adams, and it seems to be roughly equivalent to the "affective defense reaction" (ADA) described by Hess and Brugger (1943) and by Macphail and Miller (1968), or to the behavior called "rage reaction' in some of our own earlier papers (Varszegi and Decsi 1967; Decsi, Varszegi, and Mehes 1969; Decsi 1974: Nagy and Decsi 1973). Eglin (cited by Maclean, 1969) was the first to describe direct cholinergic stimulation of the cat's hypothalamus as resulting in a characteristic emotional reaction. The response to cholinergic stimulation is very similar to, but not quite identical with, the response to electrical stimulation of the same locus (cf. Baxter 1967; Decsi, Varszegi; and Mehes 1969). Hunsperger and Bucher (1967) summarized the behavioral pattern seen alter electrical stimulation as'' characterized by lowering of the head, laying back ot the ears, hunching the back, accompanied by growling and hissing and signs of sympathetic discharge, such as pupillary dilation and pilo-erection".

This is practically the same picture as that seen after chemical (muscarinergic) stimulation (Varszegi and Decsi 1967: Decsi, Varszegi, and Mehes 1969). Thus, the neurotransmitter background of this form of behavior should be cholinergic. This assumption is corroborated by several findings: e.g. topical carbachol (CCh) pretreatment decreases the threshold of the electrically-evoked rage reaction (hissing response), while atropine increases it (Decsi, Varszegi, and Mehes 1969); systemic pretreatment with atropine or t-hyosciamine fully counteracts the effect of intrahypothalamic CCh (Varszegi and Decsi 1967) Since this reaction pattern - i.e. the ADR - plays a crucial role in the behavior of the animal, investigations have begun in this laboratory to analyse it in detail. First, it has turned out that the reaction is not specific to the hypothalamus but can also be evoked from several other brain regions (Decsi 1974). This recognition led to the conception that a complex subcortical functional circuit should be responsible for the behavioral manifestation in question. Among others, this circuit comprises the hypothalamus, thalamus, and central gray matter (Nagy and Decsi 1977). The role of the limbic system in modifying this reaction pattern has been described in previous years (Nagy and Decsi 1973; 1974; Decsi and Nagy 1974; 1977b). The reaction can be started by cholinergic stimulation of any of the relay stations (but not from other parts of the brain Decsi 1974) and is inhibited by local atropine blockade (or electrocoagulation) of any of these stations. For instance, the ADR ("rage reaction') evoked from the hypothalamus can be prevented by topical application of atropine in the thalamus, central gray matter, red nucleus, or septal region, but not by that in the amygdala.

Adams says: "A defense motivational mechanism is hypothesized to be located in the midbrain central gray." Not only there. Using the method of electrical stimulation, Hess and Brugger (1943), Hunsperger (1956), and many others had demonstrated long ago that this mechanism could be triggered from the hypothalamus and also from some other parts of the brain. Our own experiments with chemical stimulation have also demonstrated that this mechanism has a rather extended subcortical organization, and that the "mechanism" involves a complex cholinergic circuit (circuitry, in Adams's terminology).

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