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2001 Third Web Report
The objective of our class and the neuroscientific community is to understand the complex neural pathways of the brain. The neural mechanisms of anxiety have long puzzled researchers. Thus far, the belief is that two structures of the limbic system known as the lateral septum and the amygdala regulate anxiety. The present paper will examine the role of the lateral septum and the amygdala in propagating anxiety (supporting the brain = behavior paradigm), discuss the neural connection that exist between the Am and LS, consider the effects that benzodiazepine anxiolytics have on this neural connection, as well as introduce findings from my current research that supports the "lesser wrong" model of anxiety.
The amygdala (hereafter cited as Am) is a known anxiety producing or anxiogenic structure (1). Anxiety is assessed through a number of animal studies that examine fear and stress responses in the presence of aversive stimuli such as a shock. Fear and stress responses in animals are good measures of anxiety because they correlate well with the symptoms observed in people with generalized anxiety disorder. For instance, studies that assess anxiety in animal paradigms report increased stress induced ulceration, increased heart beat, and increased galvanic skin conductance. Similarly, individuals diagnosed with generalized anxiety disorder show upset stomach, increased hear rate, and increased sweating. Hence, anxiety, fear, and stress responses are closely related.
Fear responses are behavioral changes that occur in the presence of aversive stimuli. The most common behavioral response (in rats) to aversive stimuli is behavioral arrest also referred to as "freezing". Behavioral arrest occurs when the animal "freezes following a cue for the aversive stimulus. Davis (1992) describes the freezing behavior as the animal's apprehensive expectation that something bad is about to happen.
Stress responses are physiological changes that occur in response to aversive stimuli. Stress responses in animal and human studies include pupil dilation, increased heart beat, decreased salivation (or dry mouth), increase galvanic skin conductance (or increased sweating) and frequent defecation (or diarrhea). Studies have shown that during the presentation of a tone that forewarns the animal of a shock, the animal show a multitude of stress responses including the ones listed above.
Similarly, in a fMRI scan study, human subjects showed Am activation in instructed fear or anticipatory anxiety task. In this task, subjects are manipulated into believing that they will receive a shock when a threatening stimulus is presented. Three types of stimuli were presented: a blue square, a yellow square, and the word "rest". Subjects were told that they would receive no more than three shocks and no less than one shock when one of the colored squares, threatening stimulus was presented. Results show that there was a significant increase in Am activity in the subjects who believed that they were going to receive a shock when compared to the control group who was not told they would receive a shock (Phelps et al., 2001). These findings support observations from animal studies that the Am is critical in producing fear, stress, and anxiety behaviors/responses. This also supports the brain = behavior paradigm in that Am = anxiogenic behaviors.
Conversely, lesioning the Am results in anxiolytic (anxiety reducing) effects during aversive situations (2). Freezing behaviors, decreased salivation, and changes in the galvanic skin response, are not observed in anxiety studies following Am ablation. For instnce, "in rabbits, a cue paired with a shock leads to bradycardia [decrease heart beat], that can be blocked by either chemical or mechanical lesions of the central nucleus of the Am (Aggleton, 2000)," thereby suggesting that absene of the Am results in absence of anxiogenic responses.
The lateral septum (hereafter cited as LS) is believed to produce anxiolytic anxiety reducing physiological and behavioral responses in the presence of aversive stimuli. Autonomic anxiolytic effects in response to aversive stimuli are opposite from anxiogenic changes seen with amygdaloid activation (3). Anxiolytic stress responses include reduced heart rate, galvanic skin conductance, pupil dilation, stress induced ulceration etc. in the presence of aversive stimuli. For example, in the cold immobilization task rats are placed on cold surfaces and tightly restrained to prevent movement. The cold immobilization task is a stressful event that produces a number of physiological changes such as stress induced ulceration. However, it has been shown that septal stimulation reduces physiological fear responses such as stress induced ulceration in cold immobilization tasks if the LS is stimulated (Yadin et al., 1993). Studies also show that behavioral arrest or freezing is reduced in aversive situations when the LS is simultaneously stimulated.
These observations suggest that the LS is critical in producing anxiolytic effects, possibly by inhibiting the amygdala and its anxiogenic responses.
Based on a number of observations from stimulation and single unit recording activity, researchers believe that the LS and the Amygdala communicate via pathways that indirectly connect both structures. For instance, Melia et al. (1992) state that the lateral septum may communicate with the Am via projections to the mammillary, lateral and paraventricular nuclei of the hypothalamus and that the Am communicates with the LS via projections to the bed nucleus of the stria terminalis and lateral hypothalamus. King and Meyer (1958) also proposed that a neural connection exist between the Am and LS. They observed that Am lesions abolished the hyperemotional states characterized by fearfulness and distress that followed LS lesioning. This suggest that septal ablation disrupts the balance between the Am and LS such that the activity of the Am is not properly inhibited.
Recent observations have surmised the existence of a direct neural connection between the LS and Am, thereby introducing a "less wrong" hypothesis to the neural mechanisms that in mediate anxiety. Evidence suggests that the LS inhibit the amygdala via a GABA-GABA system. GABA is a major inhibitory neurotransmitter and is widely distributed within the LS and Am. The premise is that GABA-A, located in the septum when stimulated, inhibits a second GABA neuron (hereafter cited as GABA-B) that is also located in the septum but junctions to the Am. In the Am, GABA-B connects to a third GABA that is located in the Am (hereafter cited as GABA-C). GABA-B cannot inhibit GABA-C because its effects are being blocked by GABA-A. As a result, GABA-C, no longer being inhibited by GABA-B, suppresses the Am and its anxiogenic responses. This inhibitory process is known as disinhibition. The septal disinhibition activity is believed to work via the surmised septo-amygdaloid pathway (4).
Animal studies assessing anxiety show that benzodiazepines generate anxiolytic stress and fear responses by enhancing the inhibitory effect that the LS has on the Am (5). For instance, in the water lick conflict task (classical fear conditioning task) thirsty rats are given the opportunity to drink from a spout. However, with each lick the animal is given an aversive stimulus-a shock. The animal's water lick behavior decreases once it associate the water lick with the shock. However animals given benzodiazepine anxiolytics continue to lick during the aversive stimulus. Moreover, benzodiazepine medicated rats show no stress responses such as increased heart rate and decreased salivation that one typically finds in an animal that is about to be shocked. These observations suggest that benzodiazepine anxiolytics reduce the anxiety, fear, and stress responses to aversive stimuli.
Past research has found that the GABA neurons widely distributed in the LS and Am have benzodiazepine receptors (6). Past observations have shown that benzodiazepine anxiolytics work to enhance the inhibitory effects of the LS by producing more of the inhibitory neurotransmitter-GABA. First benzodiazepine anxiolytics bind to the GABA-A receptors in the LS and hyperpolarize the cell (increase the frequency of chloride channel openings). GABA-A hyperpolarization increases the neuron's production of the GABA neurotransmitter. The increase in GABA neurotransmitter inhibits GABA-B. As a result, GABA-B can no longer inhibit GABA C (located in the amygdala). Therefore, GABA-C continues to inhibit the amygdala, thereby suppressing anxiogenic effects. If administered to the Am the benzodiazepine bind to the GABA-C receptor to increase GABA neurotransmitter production that inhibits the cell. Hence, benzodiazepines have an anxiolytic effect if infused into the LS or the Am.
The "less wrong" model of anxiety suggest that benzodiazepines inhibitory GABA system works via the direct pathway between the LS and the Am. To validate this notion my study microinfused (directly administered via cannula) the benzodiazepine, chlordiazepoxide (hereafter cited as CDP) into the LS and recorded the firing activity in the central nucleus of the Am. The premise is that if there is a connection between the LS and Am, then CDP infused into the LS should produce anxiolytic responses in the Am.
Results indicated a decrease in Am cell firing when CDP was infused into the LS, thereby supporting that the "less wrong" model of anxiety that the LS inhibits the Am via the septo-amygdaloid pathway to promote anxiolytic responses.
Understanding the neural mechanisms of anxiety or becoming less wrong is an essential feature to treating post-traumatic stress disorders, panic disorders, and phobias. Thousands of people suffer from anxiety disorders that may have debilitating effects on their personal as well as professional lives. Understanding the neural workings of fear, stress, and anxiety allows for better treatment.
The "lesser wrong" model of anxiety also supports the brain = behavior paradigm in that stimulating the Am = anxiogenic responses whereas lesioning the Am produce an absence in anxiogenic behaviors. Conversely, stimulating the LS = anxiolytic responses and lesioning the LS shows an absence in anxiolytic behaviors.
Aggleton, J.P. The Amygdala. New York: Oxford Press, 2000 pp. 213-287.
Davis, Michael (1992). The Role of the amygdala in fear and anxiety. Annual Review of Neuroscience, Vol. 15:353-375.
King, F.A and Meyer, P.M. (1958). Effects of amygdaloid lesions upon septal hyperemotionality in the rat. Science, Vol. 128:655-656.
Melia, K.R., Sananes, C.B, and Davis, M. (1992). Lesions of the central nucleus of the amygdala block the excitatory effects of septal ablation on the acoustic startle reflex. Physiology and Behavior, Vol. 51(1): 175-180.
Phelps, E.A., O'Conner, K.J., Gatenby, J.C., Gore, J.C, Grillon, C., and Davis, M (2001). Activation of the left amygdala to a cognitive representation of fear. Nature Neuroscience, Vol. 4(4): 437-441.
Thomas, E. and Evans, G.J. (1983). Septal inhibition of aversive emotional states. Physiology and Behavior, Vol. 31(5): 673-678.
1) Emotion, Memory, and the Brain
2) Human Fear Responses
3) Ongoing Research Efforts
4) Benzodiazepine drugs and Anxiety
6) Benzodiazepine drugs and Anxiety