Sandra E. Loughlin

Monoamine innervation of the forebrain: Collateralization

The forebrain is characterized by a dense, localized dopamine (DA) innervation pattern, a diffuse, widespread norepinephrine (NE) innervation pattern, and a serotonin (5-HT) innervation intermediate between the DA and NE patterns. These innervation patterns have implied that basic differences exist in the way DA, NE and 5-HT axons collateralize to different brain structures; that is, DA axons are thought to be poorly collateralized and NE and 5-HT axons are presumed to be more highly collateralized. In the present study, we used injections of retrograde labeling fluorescent dyes into various forebrain regions in order to determine axonal branching patterns from nuclei that contain DA, NE and 5-HT neurons, namely the substantia nigra-ventral tegmental area (SN-VTA), locus coeruleus (LC) and raphe nuclei (DR-MR). The results suggest that at least two subpopulations of neurons can be defined in each monoamine nucleus with respect to the way their axons collateralize. Each area contains a centrally located nuclear area with highly collateralized neurons, and more peripherally situated areas with less highly collateralized neurons. Thus, previous suppositions of the branching of monoamine axons must be revised to account for the existence of cells exhibiting totally different collateralization patterns within each monoamine nucleus.

Monoamine Innervation of Cerebral Cortex and a Theory of the Role of Monoamines in Cerebral Cortex and Basal Ganglia

The historical development of interest in the monoamine innervation of cerebral cortex is highlighted by several key discoveries that have changed our perspectives on cortical function specifically, and brain function in general. The dogma until the early 1960s was that the cortex receives input directly from the thalamus but not from the brain stem. Thus, ascending sensory and nonspecific “activating” neural activity generated in the brain stem was thought to have an obligatory synaptic relay in the thalamus before being sent on to cerebral cortex (Moruzzi and Magoun, 1949; Lorente de No, 1949, Scheibel and Scheibel, 1958; Nauta and Kuypers, 1958). In the early 1960s the fluorescence technique for the visualization of monoamines (Falck et al. 1962) led to the discovery that monoamine cell bodies are present in the brain-stem reticular formation and monoamine fibers are present in cerebral cortex (Dahlstrom and Fuxe, 1964; Fuxe, 1965). Anden et al. (1965), therefore, postulated that these brain-stem norepinephrine-and serotonin-containing neurons in the reticular formation give rise to a direct innervation of cerebral cortex. Moore and Heller (1967) were later able to show biochemically that lesions of the medial forebrain bundle, which contains the ascending monoamine fibers, led to a decrease of serotonin and norepinephrine in cortex. However, because such lesions failed to produce visibly degenerating axons in cortex with silver degeneration methods, it was not clear if the decrease of cortical monoamines was merely a result of a transsynaptic effect induced by the lesions.

Locus coeruleus projections to cortex: Topography, morphology and collateralization

The relationship between individual cells of origin within the nucleus locus coeruleus (LC) and the geometry and distribution of terminal fields in cortex was examined in the albino rat. Computer-assisted 3-dimensional reconstructions of the Nissl-stained LC allowed the characterization of the spatial distribution of LC cells. Similar reconstructions of the distributions of labelled cells following cortical injections of horseradish peroxidase were created. Comparisons of such reconstructions revealed that LC cells projecting to cortex were distributed throughout the compact dorsal LC. These cells were predominantly medium sized multipolar cells. Significant labelling of other morphological sub-populations of LC did not occur following cortical injections. Simultaneous injections of multiple fluorescent retrograde tracers into different cortical regions allowed the characterization of LC axon collateralization in cortex. Individual LC cells innervate functionally and cytoarchitectonically distinct cortical regions simultaneously. LC cells arborize more extensively in the anterior-to-posterior axis of cortex and exhibit relatively minimal medial-to-lateral collateralization. Individual LC cells were also shown to innervate both superficial and deep layers of a cortical region.

Smoking to Self-Medicate Attentional and Emotional Dysfunctions

Individuals with attentional and emotional dysfunctions are most at risk for smoking initiation and subsequent nicotine addiction. This article presents converging findings from human behavioral research, brain imaging, and basic neuroscience on smoking as self-medication for attentional and emotional dysfunctions. Nicotine and other tobacco constituents have significant effects on neural circuitry underlying the regulation of attention and affect. Age, sex, early environment, and exposure to other drugs have been identified as important factors that moderate both the effects of nicotine on brain circuitry and behavior and the risk for smoking initiation. Findings also suggest that the effects of smoking differ depending on whether smoking is used to regulate attention or affect. Individual differences in the reinforcement processes underlying tobacco use have implications for the development of tailored smoking cessation programs and prevention strategies that include early treatment of attentional and emotional dysfunctions.

The origin of cholecystokinin terminals in the basal forebrain of the rat: Evidence from immunofluorencence and retrograde tracing

The origins of cholecystokinin (CCK) fibers in the olfactory tubercle, nucleus accumbens and amygdala of the basal forebrain of the albino rat were studied with combined immunofluorescence and fluorescent retrograde tracing techniques. In each case, the majority of the CCK innervation arises topographically from subpopulations of neurons in the substantia nigra-ventral tegmental area of the midbrain. This ascending CCK input to the forebrain appears to exceed the amount of descending CCK input from the cortex. In this regard, the CCK innervation of limbic structures is quite different from that of the neostriatum. It has been reported that the CCK innervation of the neostriatum is derived primarily from piriform cortex as a descending corticostriatal projection. Limbic structures, on the other hand, are primarily innervated by ascending CCK, as well as local circuit, projections.

Noradrenergic changes and memory loss in aged mice

The present paper addresses the question of whether a decline of central noradrenergic activity is associated with aging and memory loss in mice. Receptor binding techniques were utilized to compare alpha 2-adrenoceptor density in the brains of aged and young mice. Using [3H]rauwolscine, a selective alpha 2-adrenoceptor antagonist, two membrane binding sites were identified which were differentially affected by age. Whereas the density of high-affinity binding sites was unchanged in aged brain as compared to young controls, there was a significant decrease in the number of low-affinity sites. In a separate study, animals were tested for performance on a step-through inhibitory avoidance task, prior to sacrifice and morphological analysis of the brainstem noradrenergic nucleus locus coeruleus (LC). Aged mice exhibited a significant decrease in task retention as compared to young controls; a small, though non-significant, decline was also observed in the numbers of cells within LC. While young mice exhibited low within-group variance, individual aged animals differed greatly in both LC cell number and behavioral performance. Within the aged population, there was a highly significant correlation between the extent of LC cell loss and the degree of memory impairment. These results provide further evidence for an age-related decline in central noradrenergic function which may contribute to an associated memory loss.

Effects of morphine on DNA synthesis in neonatal rat brain.

Several observations have led to the hypothesis that endogenous opioids may modulate the growth and development of the brain. In the present study, we have examined the effect of morphine on the incorporation of [3H]thymidine into the DNA of neonatal rat brains in vivo and in vitro. We have found that morphine, when administered to one-day-old rats, inhibited [3H]thymidine incorporation into brain DNA in a long-lasting, naloxone-reversible manner. Morphine inhibited DNA synthesis in animals one and 4 days of age but not in older animals. This effect was tissue-specific, and did not appear to be due simply to respiratory depression or decreased availability of precursor to the brain. Naloxone, when administered acutely, or naltrexone, chronically, had no effect on [3H]thymidine incorporation, indicating that endogenous opioids do not tonically depress DNA synthesis. When neonatal brain tissue was incubated with morphine in vitro. [3H]thymidine incorporation values were not different from controls. These data indicate that the effect of morphine on DNA synthesis in vivo may be an indirect one, rather than a direct action on proliferating cells.

Adolescent Development of Forebrain Stimulant Responsiveness: Insights from Animal Studies

Abstract: Although initiation of drug abuse occurs primarily during adolescence, little is known about the central effects of nicotine and other abused drugs during this developmental period. Here evidence, derived from studies in rodents, is presented that suggests that tobacco use initiation during early adolescence results from a higher reward value of nicotine. The developmental profiles of the rewarding effects of other abused drugs, such as cocaine, differ from that of nicotine. Using in situ hybridization to quantify mRNA levels of the immediate early gene, cfos, the neuronal activating effects of nicotine in limbic and sensory cortices at different developmental stages are evaluated. Significant age changes in basal levels of cfos mRNA expression in cortical regions are observed, with a peak of responding of limbic cortices during early adolescence. A changing pattern of nicotine‐induced neuronal activation is seen across the developmental spectrum, with unique differences in both limbic and sensory cortex responding during adolescence. An attentional set‐shifting task was also used to evaluate whether the observed differences during adolescence reflect early functional immaturity of prefrontal cortices that regulate motivated behavior and psychostimulant responding. The finding of significantly better responding during adolescence suggests apparent functional maturity of prefrontal circuits and greater cognitive flexibility at younger ages. These findings in rodent models suggest that adolescence is a period of altered sensitivity to environmental stimuli, including abused drugs. Further efforts are required to overcome technical challenges in order to evaluate drug effects systematically in this age group.

Dopaminergic and non-dopaminergic projections to amygdala from substantia nigra and ventral tegmental area

The projections from the substantia nigra (SN) and ventral tegmental area (VTA) to the amygdala of the rat were examined by simultaneous visualization of catecholamine (CA) histofluorescence and retrograde tracer. CA-containing cells in lateral VTA, medial SN and the dorsal edge of SN pars compacta were labeled by injections of propidium iodide (PI) into the amygdala. While CA-containing cells were present in SN pars lateralis (SNl), those cells which were labeled by injections into the amygdala did not contain CA. There is, thus, a significant non-DA projection from SNl to the amygdala.

Mesostriatal projections from ventral tegmentum and dorsal raphe: Cells project ipsilaterally or contralaterally but not bilaterally ☆

The cells of origin of the brainstem projection to the caudate-putamen (CP) were examined in the albino rat by the use of two fluorescent retrograde tracers. Nuclear yellow was injected into the CP of one hemisphere and granular blue was injected into the contralateral CP. Retrogradely labeled cells were studied in the substantia nigra (SN), ventral tegmental area (VTA) and dorsal raphe (DR). The results confirm previous findings indicating the existence of contralateral projections from SN and VTA to the CP. Cells on both sides of the midline of the DR also project to either CP. Contralaterally projecting SN/VTA cells were located in areas of these nuclei which corresponded to the center of the ipsilaterally projecting cell region; the exact locations depended on the placement of the injection in the CP. In DR, contralaterally projecting cells were most often observed near the midline, although labeled cells were occasionally observed in the lateral wings of the DR. Cells labeled by contralateral injections were interdigitated with, but not identical to, cells labelled by ipsilateral injections. That is, contralaterally projecting cells and ipsilaterally projecting cells constituted separate populations within the SN, VTA and DR.