Neuroanatomical and neurochemical mechanisms underlying amygdaloid control of defensive rage behavior in the cat

Abstract

1. It is well established that the hypothalamus and midbrain periaqueductal gray (PAG) play important roles in the expression of defensive rage behavior. While defensive rage is not elicited from the amygdala, this region of the limbic system nevertheless serves an important role in the modulation of defensive rage behavior. The present paper attempts to address the question of how the amygdala modulates defensive rage behavior in the cat. The studies were conducted using brain stimulation, pharmacological, neuroanatomical and immunocytochemical methods to identify the likely neural pathways and their associated neurotransmitters by which different regions of the amygdala modulate defensive rage behavior in the cat. 2. The experimental evidence provided thus far establishes that three regions of the amygdala have been identified as powerful modulators of defensive rage behavior. These include the medial nucleus, basal complex and central nucleus of the amygdala. Experiments involving dual stimulation of an amygdaloid nucleus and sites within the medial hypothalamus or PAG from which defensive rage behavior was elicited demonstrated that two of the regions facilitated defensive rage --the medial nucleus and basal complex--and a third region--the central nucleus--suppressed defensive rage. The mechanisms and substrates underlying modulation for each of these regions are different. Medial amygdaloid facilitation of defensive rage involves a pathway (i.e., the stria terminalis) that projects directly to the medial hypothalamus and utilizes substance P as a neurotransmitter. Basal amygdaloid facilitation of defensive rage behavior makes use of a pathway to the PAG in which excitatory amino acids acting on NMDA receptors are utilized as a neurotransmitter. The central nucleus also projects to the PAG. However, it is strongly inhibitory and utilizes enkephalins that act upon mu receptors within the PAG.