Olfactory tubercle

The olfactory tubercle (OT), also known as the tuberculum olfactorium, is a multi-sensory processing center that is contained within the olfactory cortex and ventral striatum and plays a role in reward cognition. OT has also been shown to play a role in locomotor and attentional behaviors, particularly in relation to social and sensory responsiveness, and it may be necessary for behavioral flexibility. The OT is interconnected with numerous brain regions, especially the sensory, arousal, and reward centers, thus making it a potentially critical interface between processing of sensory information and the subsequent behavioral responses. The olfactory tubercle (OT), also known as the tuberculum olfactorium, is a multi-sensory processing center that is contained within the olfactory cortex and ventral striatum and plays a role in reward cognition. OT has also been shown to play a role in locomotor and attentional behaviors, particularly in relation to social and sensory responsiveness, and it may be necessary for behavioral flexibility. The OT is interconnected with numerous brain regions, especially the sensory, arousal, and reward centers, thus making it a potentially critical interface between processing of sensory information and the subsequent behavioral responses. The OT is a composite structure that receives direct input from the olfactory bulb and contains the morphological and histochemical characteristics of the ventral pallidum and the striatum of the forebrain. The dopaminergic neurons of the mesolimbic pathway project onto the GABAergic medium spiny neurons of the nucleus accumbens and olfactory tubercle (receptor D3 is abundant in this two areas ). In addition, the OT contains tightly packed cell clusters known as the islands of Calleja, which consist of small granule cells. Even though it is part of the olfactory cortex and receives direct input from the olfactory bulb, it has not been shown to play a role in processing of odors. The olfactory tubercle differs in location and relative size between humans, non-human primates, rodents, birds, and other animals. In most cases, the olfactory tubercle is identified as a round bulge along the basal forebrain anterior to the optic chiasm and posterior to the olfactory peduncle. In humans and non-human primates, visual identification of the olfactory tubercle is not easy because the basal forebrain bulge is small in these animals. With regard to functional anatomy, the olfactory tubercle can be considered to be a part of three larger networks. First, it is considered to be part of the basal forebrain, the nucleus accumbens, and the amygdaloid nuclei because of its location along the rostral ventral region of the brain, that is, the front-bottom part. Second, it is considered to be part of the olfactory cortex because it receives direct input from the olfactory bulb. Third, it is also considered to be part of the ventral striatum based on anatomy, neurochemical, and embryology data. One of the most striking features of the olfactory tubercle is the closely packed crescent-shape cell clusters, which are located mostly in layer III and sometimes in layer II. These cells clusters, called the islands of calleja, are innervated by dopaminergic projections from the nucleus accumbens and the substantia nigra, suggesting the role that the olfactory tubercle plays in the reward system. The olfactory tubercle is a multi-sensory processing center due to the number of innervations going to and from other brain regions such as the amygdala, thalamus, hypothalamus, hippocampus, brain stem, auditory and visual sensory fibers, and a number of structures in the reward–arousal system, as well as the olfactory cortex. Due to its many innervations from other brain regions, the olfactory tubercle is involved in merging information across the senses, such as olfactory—audition and olfactory—visual integrations, possibly in a behaviorally relevant manner. Thus, damage to the olfactory tubercle is likely to affect the functionality of all these areas of the brain. Examples of such disruption include changes in normal odor-guided behavior, and impairments in modulating state and motivational behavior, which are common in psychiatric disorders such as schizophrenia, dementia and depression. The olfactory tubercle has been shown to play a large role in behavior. Unilateral lesions in the olfactory tubercle have been shown to alter attention, social and sensory responsiveness, and even locomotor behavior. Bilateral lesions have been shown to reduce copulatory behavior in male rats. The olfactory tubercle has also been shown to be especially involved in reward and addictive behaviors. Rats have been shown to administer cocaine into the olfactory tubercle more than the nucleus accumbens and ventral pallidum, other reward centers in the brain. In fact, they will administer cocaine into the olfactory tubercle at about 200 times per hour and even till death. Functional contributions of the olfactory tubercle to olfaction are currently unclear; however, there is evidence of a perceptual role that it may play. Work from Zelano, et al. suggest that the olfactory tubercle may be crucial in sorting out the sources of olfactory information. This suggests that it may also play a role in odor guided behavior. Thus, it may link perception of odor with action through its connections with attention, reward, and motivation systems of the basal forebrain. Functional imaging data from this same group also shows that the olfactory tubercle is highly activated during tasks that engage attention, thus playing a large role in arousal-related systems. Because the olfactory tubercle is a component of the ventral striatum, it is heavily interconnected with several affective-, reward-, and motivation-related centers of the brain. It also sits at the interface between the olfactory sensory input and state-dependent behavioral modulatory circuits, that is the area that modulates behavior during certain physiological and mental states. Thus, the olfactory tubercle may also play an important role in the mediation of odor approach and odor avoidance behavior, probably in a state-dependent manner. In general, the olfactory tubercle is located at the basal forebrain of the animal within the medial temporal lobe. Specifically, parts of the tubercle are included in the olfactory cortex and nested between the optic chiasm and olfactory tract and ventral to the nucleus accumbens. The olfactory tubercle consists of three layers, a molecular layer (layer I), the dense cell layer (layer II), and the multiform layer (layer III). Other than the islands of Calleja, which are characteristic of the tubercle, it is also noted for the being innervated by dopaminergic neurons from the ventral tegmental area. The olfactory tubercle also consists of heterogeneous elements, such as medial forebrain bundle, and has a ventral extension of the striatal complex. During the 1970s, the tubercle was found to contain a striatal component which is composed of GABAergic medium spiny neurons. The GABAergic neurons project to the ventral pallidum and receive glutamatergic inputs from cortical regions and dopaminergic inputs from the ventral tegmental area.