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Chapter one


Literature review

Chapter two

Materials and method

Obtaining the tissue

Processing of the brain tissue

Chapter three


Medial subdivision

Lateral subdivision

Lateral capsular subdivision

Intermediate subdivision


Chapter four

Discussion and conclusion







The central amygdaloid nucleus was studied in the guinea pig to determine its cytoarchitecture, from stained sections of the nucleus I prepared (stained sections of the nucleus I prepared (sextions were stained with cresyl echt villet) it was seen that the nuclear consist of four subdivisions viz (1) the medial and  (2) intermediate subdivisions which were found to be cytoarchitecturally similar and show affiliations with the adjacent regions of the force brain and (3) the lateral and (4) lateral capsular subdivisions which were found to be cytoarchitecturally similar and show affiliation with the astriatum.

The medial subdivision consist of cells that are slightly larger than what obtains in the lateral and lateral capsular subdivisions. The intermediate subdivision is located lateral to the medical subdivision and consistent of tightly packed avoid neurons. The lateral subdivision is located at the caudal two-thirds of the nucleus and dis round in cornonal section, while the lateral capsula subdivision is the lateral portion of the capsule which surrounds the lateral subdivision and has a lower cell density than that in the lateral subdivision.




The amygdaloid nucleus is so named because it resembles an  almond in shape. It consist of an aggregation of neuronal masses and associated nerve fibres. It is located in the dorsonedial part of the temporal pole of the cerebrum and forms the ventral, superior and medial walls of the ventral tip of the inferior horn of the lateral ventricle. It is fused with the tip of the tail of the caudate nucleus and the stria terminalis merges from its posterior aspect. The amygdalid nucleus constitutes one of the four nuclei comprising the basal ganglia (other components of the basal ganglis are

  1. The caudate nucleus
  2. Putamen and
  • The globus pallidus)

But is functionally a part of the limbic syst4em. The amygdalid nucleur complex is covered by a rudimentary cortex and is continuous caudally with the uncus of the parahippocampal gyrus (Fig. 1)

The amygdalid nuclear complex is divided into two main groups of nucleaur viz;

  1. A corticomedial amygdalid complex which consists of the following nuclei:
  • The central nucleus
  • The medial nucleus
  • The cortical nucleus (which may be regarded as the rudimentary cortex, with irregular groups of pyramidal and granule cells and which occupies the surface elevation of the gyrus semilunaris).

The corticomedial complex is continuous through transitional zones with the anterior perforated substance, the diagonal band of Broca. The substantia innominate, and with the putamen, caudate nucleus, and surrounding cortical areas of the uncus and parahippocampal cortical areas of the uncus and parahippocampal gyrus. It is relatively small in  the human brain.

(2) A basilateral amygdaloid complex which is well differentiated in the human brain and consists of the following nuclei:

(a) The lateral nucleus

(b) The basal nucleus

© The accessory basal amygdaloid nucleus.

It is continuous with the calausrum, and, through a corticoamygdaloid transitional zone, with the cortex of the parahippocampal gyrus.

The aim of this study is to elucidate the cytoarchitecture of the central nucleus of the Guinea pig amygdala. Although, a lot of work has been done by other investigators on this nuycleus in the Rat (Mc Donald, 1982), Rabbit (Hopkins, 1975; Hopkins and Holstege, 1978; Schwaber et al, 1980) Cat (Lescault, 1971; Hakk, 1972 a, b; Bdchstead, 1979), not much work and been done on this nucleus in the Guinea pig. Very little information was got by the few investigators who worked on this particular experimental animal due to the difficulty of correlating the different subdivisions of the nucleus in this animal with that in the rat, cat or rabbit. This diffuc8lty has however been surmounted due to a more precise information about the nucleus that have been acquired over the years.

The central mygdalid nucleus (CN) which is located in the dorsal portion of the amygdala adjacent to the corpus striatum is unique among amygdaloid nuclei in that it has extensive connections with brain stem areas involved in visceral functions (e.g. Hopkins, 1975; Norgren, 1976; Ricardo and Koh, 1978; Ottersen and Ben-Ari, 1978). Stimulation and ablation studies suggest that the central nucleus (CN) participates in numerous adaptive behaviours, including arousal, feeding, flight, avoidance and the defence that accompany such behaviours including changes in heart rate, blood pressure, respiration, and gastrointestinal function. It has been shown that the central nucleus contains high concentration of dopamine (Ungerstedt, 1971) and other neurotransmitters and appear closely related to the neostriatum.

Recent evidence has implicated the central nucleus of the amygdala in somatomotor response acquisition deficits observed during a variety of adverse conditioning paradigms. Over the past years, research has been focused on the contribution of the central nucleus of the amygdala to autonomic response acquisition examining vagus nerve-mediated bradycardia responses to a conditioned stimulus during a pavlovian fear conditioning procedure in the rabbit. The results have demonstrated that small lesions within the central nucleus attenuate the magnitude of the conditioned braduycardia response and that this response can be similarly attenuation by local administration of either B-adrenergic antagonists or opiate agonists into the central nucleus. In addition, recent horse radish peroxidase and autoradiographic analyses in the rabbit suggest that the central nucleus project monosynaptically to the nucleus tractus solitaries (NTS) and the dorsal motor nucleus of the vagus (DMV), the latter a site of origin of cardioinhibitory neurons which may mediate the conditioned bradycardia response. Electrical stimulation of the central nucleus elicits short latency vagus nerve mediated bradycardia responses in both the awake and anaesthetized rabbit. This suggest that, at least under some conditions, central nucleus activity should be related to the expression of heart rate decelerative responses. Specifically it was hypothesized that changes in central nucleus neuronal activity should develop to a conditioned stimulus (CS) during the course of an aversive pavlovian conditioning procedure and that these changes should correlate with the conditioned bradycardia response which emerge under these conditions.

Based on anatomical and electrophysiological included as part of a forebrain module for cardioscular regulation. The selective activation to neuronal activity to the CS were observed at some central nucleus placements only under conditions where it is given motivational significance via pairing with an aversive unconditioned stimulus (UCS). This suggests that the central nucleus contribution to the expression of caridiovascular responses to conditioned fear stimuli, suggest that the central nucleus contribution to cardioregulation may be confined to threatening situations which evoke a constellation of autonomic and somatomotor emotional responses from the organism.

Removal of the amygdala has been shown to result in the following:-

  • Increased aggressiveness (Bard and Rioch, 1937).
  • Postoperative rage and seizures which play a role in savage behaviour and this is though to be due to the loss of inhibitory influences.

Bilateral lesions of the amygdala complex results in the following:

  • Decreased aggressive and assaultive behaviour (Green et al, 1951; pool, 1954; Scoville, 1954).
  • Reduction in emotional excitability (Narabayashi et al, 1963).
  • Conversion of wild animals into docile tamed beasts.
  • ‘’psychic objects visually, tactually and orally.
  • Hypersexualty characterized by pairing with both male and female partners
  • Hyperphagia or hypophagia due to loss of control of food and water in take. (fonberg, 1968) while stimulating of this part p[reduces an arrest of feeding behaviour (Fonberg and Delgado, 1961). The corticomedial part of the amygdala of which the central nucleus as a part has been implicated in this.

I strongly believe that a sound knowledge of the cytoarchitecture of the central amygdaloid nucleus will provide a better platform for future investigators to correlate the components of the different subdivisions of the amygdala with their function.



The central amygdalid nucleus which is located in the dorsal portion of the amygdala adjacent to the corpus striatum, is unique among amygdaloid nuclei in that it has extensive connections with brainsteim areas involved in visceral function. Stimulation and ablation studies suggest that the central nucleus participates in numerous adaptive behaviours, including arousal, feeding, flight, avoidance, and the defence reaction, and produces the visceral responses that accompany such behaviours including changes in heart rate, blood pressure, respiration, and gastrointestinal function (Kaada 1972).

Electrical stimulation and ablation of the nucleus in animals have produced a wide variety of behavioural, visceral, somatic and endcrine changes (Maclean and Delgado, 1953; Shealy and peele, 1957; Gloor, 1960; Kaada, 1951, 1972). Pronounced behavioural changes are elicited by stimulation of the nucleus in unacesthetized animals. The most common response under such conditions is an ‘’arrest’’ reaction in which all spontaneous ongoing activities cease as the animal assumes an attitude of aroused attention. This response is indistinguishable from the arousal reaction obtained from brain stem reticular actibvation and is associated with cortical desynchronization. The ‘’arrest’’ reacto nappears as the initial phase of flight or defence, reactions obtained by amygdaloid stimulation. Flight (fear) and defensive (rage and aggression) reactions, termed agonistic behaviour, have been elicited from the central amygdaloid nucleus (Ursin and Kaada, 1960). The intensity of the electrical current determines the intensity of the response, but unlike similar hypothalamic stimulation; the response builds gradually and outs two the period of stimulation (Zbrozyna, 1972). The intense reactions of fear and rage are associated with pupillary dilatation; piloerection, growling, hissing, unmistakable signs of emotional involvement and participation of the autonomic nervous system. Stimulation no the amygdaloid region in man procedures feelings of ear, confusional states, distursbances of awareness and amnesis for events taking place during the stimulation (Feindel and penfiled, 1954; Mullan and Penfied, 1959; Gloor, 1972). Although, rage is the most common behavioural response in animals, is the most common is associated with temporal lobe animals, it rarely is associated with temporal lobe seizures or deep stimulation of the temporals lobe in man (Gloor, 1972).

Visceral and autonomic responses: these include alterations of respiratory rate, rhythm, and amplitude, as well as inhibition of respiration. The most common response of stimulation in anaesthetized animals is an acceleration of the respiratory rate associated with a reduction in amplitude (Kaada, 1972).  Cardiovascular responses involve both increases and decreases in arterial blood pressure and alterations of heart rate. Pressor responses appear to predonimate following amygdaloid stimulation in the anaesthetized animal (Reis and Oliphant, 1964). Gastrointestinal motility and secretion and micturition may be induced. Piloerection, salivation; pullary changes, and altercations of body sympathetic and parasympathetic in nature.

Somatic Responses:- on stimulation of the ucleus somatic responses include turning of the head and eyes to the opposite side, and complex hythmic movements related to chewing, licking and wallowing. The varied somatic and autonomic effects of stimulation constitute an insignificant part of the syndrome produced by lesion produced in this nucleus.

Endocrine response: Stimulaiton of the central amygdaloid nucleus results in endocrine responses which include:

  • Release of Adrenocorticotrophic hormone (ACTH) and gonadotrophic hormone, and lactogenic responses (Kaada, 1972; Zolovick, 1972).
  • Stimulation of the central nucleus and other parts of the certicomedial division of the amygdala may induce ovulation, but this response is abolished by transection of the stria terminalis (Shealy and Peele, 1957; Everette, 1959; velasco and Taleisnik, 1969). The nucleus is also concerned with the lutecinizing (LII) and follicular-stimulating (FSH) hormones (Kaada, 1972).
  • It is quite clear that the central amygdala participates with the hypothalamus in the control and regulation of hypo physical secretions.

Recent interest in the central amygdaloid nucleus has been stimulated by the finding that it contains numeruous putative neurotransmitters and neuromodulators including several neuropeptides. Fluorescence histochemcal and immonocy to chemical studies reveal that the central amygdaloid nucleus receives a substantial projection from brainstem monoaminergic systems. (e.g. Ungerstedt, 1971; Lindval and Bjorklund, 1974; Swanson and Hartman, 1975). There is biochemical evidence that the central amygdaloid nucleurs contains GABAergic neurons that projects to the bed nucleus of the stria terminalsis (le Gal La Salle et al, 1978). It also contains opiate receptors (Atweh and Kuhar, 1977) and has high levels of encephalin which immunocy to chemical studies have demonstrated in cell bodies and fibres (e.g. Elde et al, 1976; Gros et al, 1978; Wamsley et al, 1980; Finley et al, 1981). Cell bodies and nerve terminates in the central nucleus contain substance P (Cuello and Kanagawa, 1978; Ljungdahl et al 1978; Emson et al, 1978; Leranth et al 1981), and somatostatin (Epelbaum et al, 1979; Benneth-clark et al, 1980). Fibres containing vasopressin, oxytocin, vasoactive intestinal polypeptide, and cholecystokinin are also seen in the cental amydgaloid nucoleus. (e.g. Buijs, 1978; Loren et al, 1979; Robertys et al, 1980; Sims et al, 1980; vanderhacghen et al, 1980).

Recent neurochemical (Fallon et al, 1978; Hokfelt et alk, 1982) and hodological investigation (e.g. Hopkins, 1979; Hopkins and Holstege, 1978; saper et al, 1979, Hopkins and Holstege, 1978; saper ert al, 1976; Krieger et al, 1979; Bechstead, 1979; Schwabert et al, 1980; Ottersen, 1980) have shown that distinct transmitters and projections are associated with discrete portions of the central nucleus. These findings call for a detailed study of the cyto architecture of the central amygdaloid nucleus’s information acquired from these neurobiological studies must ultimately be correlated with structural subdivision of central amygdaloid nucleus in order to understand the organization of this nucleus.

The cytoarchitecture of the amygdala, including the central nucleus has been studied in numerous specie with NISSL techniques (Hall, 1972b). In the Cat (Fox, 1940), Rat (brodal, 1947; Uchida, 1950a, b; Uchida, 1950a, b; Koikegami, 1963) medial and lateral subdivisions have  been recognised. Golgi studies of the Cat amygdala have shown that the medial subdivision of the central nucleus contains large neurons with a moderate number of dendritic spinesc whkle the lateral subdivision consist of smaller cells with a dense covering of dendritic spines (Hall, 1972a, b; Tombol and Szarfranska-kasmal, 1972; Kamal and Tombol, 1975). The spiny neurons of the laterial subdivision in the cat closely resemble medium-seize spiny neurons of the adjacent putamen and the exact border separating the central nucleus from the putamen is indistinct in both NISSL and golgi preparations (Fox, 1940; Hall, 1972 a, b; wakefield and Hall, 1974). In the guinea pig, Hall and Geneser-Jensen (1971) noted a region (area x) at the junction of the central nucleus and the putamen which was acetylcholinesterase-positive like the putamen, but which could be distinguished from both central nucleus and the putamen based on differences in monoamine oxidase histochemistry. It has not been determined whether area x represents an extreme lateral portion of the cenetal nucleus or a medial part of the putamen (Hall, 1972 b).

It has been shown-latancy bradycardia and depressor responses can be elicited from the amygdala central nucleus in the anesthetized rabbit. Furthermore, the largest response magnitudes were elicited from sites located within the medial component of the nucleus, particularly at its larger, more anterior extent. This is consistent with recent autoradiographic studies in cat and rabbit which demonstrate that a prominent projection originates within the central nucleus and descends to directly innervate the vagal dorsal motor nucleus and the nucleus of the solitary tract. This projection hs been demonstrated with the use of Horse Roddish peroxidase (HRP) technique to originate primarily form the medial component of the central nucleus in the rabbit. Furthermore, within this medial component the major portion of the projection originates at an anterior level where the medial component approaches its largest size. Given the similarities between the location of the most sensitive sites for the production of the bradycardia response in this study and the site of origin of the direct projection to the vagal dorsal motor nucleus and nucleus solitaries, it is not reasonable to assume that the profound bradycardia responses may be due to stimulation of the cells of origin of this projection system. Such stimulation could either be directly or indirectly (eg.g via the nucleus of solitary tract) activate cardioinhibitory neurons located within the vagal dorsal motor nucleus in the rabbit. It has also been observed that in cases in which atropine methilnitrate abolishes the stimulation – induced bradycardia, the depressor responses in these cases, although attenuated, were not totally abolished. This finding demonstrates a partial dissociation between the bradycardia and depressor responses and is consistent with the observation that depressor responses can be elicited from the amygdala central nucleus of rats in the absence of significant changes of heart rate. These observations, as well as those demonstrating that

  • Manipulations of the central nucleus decrease the magnitude of the conditioned bradyucardia response in the rabbit during pavlovian fear conditioning.
  • Direct projection exists in the rabbit from the central nucleus to the site of origin of cardioinhibitory neurons, and
  • Prominent, short-latency bradycardia and depressor responses can be elicited from the origin of this pathway, are consistent with the notion that the central nucleus, and its medial component in particular, may play a direct role in the expression of cardiovascular and perhaps other autonomic and somatomotor responses to fear-arousing stimuli.

Observations that stimulation of the central nucleus induces fear like behaviour in the cat and that lesion of the central nucleus reduce fear-like behaviour in the rat are consistent with this interpretation.

Recent demonstrations (Peter et al, 1982) have shown that administration of Neurotensin (NT) into the central nucleus prolongs hot plate response latency in the rat, and the existence of immunoreactive neurotensin and NT. Receptors in the central nucleus supports the possible physiological significance of this findings. The increase in nociceptive threshold following intra-cental nucleus administration of NT. Is dependent upon an intact stria terminalis. This is consistent with the view that NT. Acts in the central nucleus either to modulate neuronal perikarya having projections through the stria terminalis, or alter the response of central neurons to afferent input from the stria terminalis.

Gurdjain (1928) described the central nucleus of the rat as a homogenous mass that was surrounded, especially on its lateral aspect, by a distinct capsule of fibres. Later investigators (Brodal, 1947; Uchida, 1950 a; Yu, 1969) recognized medial and lateral subdivisions of the rat central nucleus but there were discrepancies as to the exact delineation of each. Subdivision. Valverde (1965) and Yu (1969) studied the rat amygdala with the Glogi technique but did not comment on differences in neuronal morphology in different subdivision of the central nucleus.

It is clear that there is confusion regarding the manner in which the central nucleus should be subdivided. This lack of knowledge hinders efforts to correlate discrete portions of the nucleus with particular projections, transmitters and functions.

The present investigation attempts to define subdivisions of the central nucleus of the Guinea pig amygdala by correlating NISSL – and Golgi – stained brains. The Golgi techniques reveals details of dendritic and axonal morphology which permits a more accurate determination of the boundaries and extent of nucleus subdivisions than that afforded by NISSL techniques, as well as characterization of distinct cell types within particular subdivisions.



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