Enhancement of learning four weeks after stimulation of the nucleus locus coeruleus in the rat: differential effects of dorsal noradrenergic bundle lesion and lesion of the locus coeruleus proper

Facilitation of learning consecutive to electrical stimulation of the locus coeruleus: cognitive alteration or stress-reduction?

This chapter summarizes behavioral and neurochemical data on the delayed effect of locus coeruleus stimulation on learning capabilities in the rat. The initial observation showed that electrical stimulation of the locus coeruleus of a 15-day-old-rat improved the early stages of acquisition and extinction of a food-reinforced task performed 4 weeks later. Neurochemical lesion of the dorsal noradrenergic bundle performed 10 days before the stimulation did not attenuate the behavioral effect, whereas the lesion of the locus coeruleus proper suppressed the subsequent behavioral improvement. More recently we showed that the increase of adrenocorticotrophin release consecutive to a moderate stressful situation was significantly lower in previously stimulated rats than in implanted non-stimulated animals. Furthermore, we demonstrated that the neurochemical lesion of the locus coeruleus increased neophobia in the open-field as well as in a specific exploration task. Taken together these data strongly suggest that the long-term improvement in acquisition and extinction of locus coeruleus-stimulated rats results mainly from an attenuated stress reaction when these animals are confronted with a new environment (beginning of acquisition) or a new situation (beginning of extinction). Finally, we were interested in investigating the possibility of some long-term neurochemical modifications that could be related to the observed behavioral effects. The most significant modification observed concerned certain subpopulations of adrenoceptors in specific brain regions. By using specific ligands of the beta-, alpha 1- and alpha 2-adrenoceptors, we studied the long-term effect (4 weeks) of the locus coeruleus stimulation on the kinetic characteristics of these three sub-types of receptors in four brain areas (the cortex, hippocampus, hypothalamus and brainstem). No significant alteration in the density of beta binding sites was observed in any of the four structures analyzed; likewise locus coeruleus stimulation did not modify the density or affinity of the beta-, alpha 1- and alpha 2-receptors in the brainstem. The density of alpha 1- and alpha 2-receptors was significantly increased in the cortex whereas in the hippocampus only the density of the alpha 2-receptors was increased. Finally, a very large increase of the density of alpha 2-adrenoceptors was observed in the hypothalamus (113%). In each case the increase in receptor density was also associated with a decreased affinity. A behavioral counterpart of these changes in the kinetic properties of the alpha 2-receptors has been observed by using a pharmacological approach.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of sex steroids on the development of the locus coeruleus in the rat

The effects of postnatal (on day 1 (D1) after birth) male orchidectomy and female androgenization on the locus coeruleus (LC) are studied. Normal adult female rats show greater LC volume and cell number than males. Testosterone treatment of females on D1 eliminates these differences. However, D1 male orchidectomy does not affect either the volume or the number of cells.

Delayed up-regulation of alpha-adrenoceptor populations in particular regions of the rat brain after stimulation of the nucleus locus coeruleus

We previously showed that electrical stimulation of the nucleus locus coeruleus resulted 4 weeks later in greatly improved performance in the acquisition and extinction of a food-reinforced operant task. To ascertain whether adrenergic receptors of particular brain regions were involved in this long term behavioral modification, we studied the characteristics of alpha 1, and alpha 2 and beta-binding sites after stimulation of the locus coeruleus. In the first experiment these characteristics were studied, 4 weeks after treatment, in cortex, hippocampus, hypothalamus and the brainstem. Neither the number, nor the affinity of beta-receptors ([125I]iodocyanopindolol binding sites) was modified in any brain region. A significant increase in the number of alpha 1-receptors ([3H]prazosin binding sites) was observed in the cortex (62%). The number of alpha 2-receptors, ([3H]yohimbine binding sites), was significantly increased in cortex (99%), hippocampus (33%) and hypothalamus (113%). No significant alteration of the alpha 1, alpha 2 and beta-adrenoceptors was observed in the brainstem. To investigate the time course of these adrenoceptor changes, the characteristics of alpha 1, alpha 2 and beta-adrenoceptors were studied 2 weeks after stimulation using the same ligands and in the same brain regions. The only significant modifications observed were an increase of the alpha 2-adrenoceptors in the cortex (19.4%) and in the hypothalamus (54%). Furthermore, in both experiments, the increase in the number of alpha 1 and alpha 2-receptors was associated with a significant decrease in affinity. These results are discussed in relation to our previous behavioral and pharmacological findings.

The role of noradrenaline in tuning and dopamine in switching between signals in the CNS

Neuronal catecholaminergic activity modulates central nervous function. Specifically noradrenaline can exert a tuning or biassing function whereby the signal to noise ratio is altered. Dopamine activity may promote switching between inputs and outputs of information to specific brain regions. It has been ten years since evidence for a tuning function was advanced for noradrenaline and in the last 5 years the switching hypothesis for dopamine has been tentatively put forward. Recent studies are reviewed to show that while catecholamine activity contributes to neural interactions in separate brain regions that give rise to the organization of different functions, their working principles may be common between species and independent of the nucleus of origin. Behavioral examples are discussed and an attempt is made to integrate this with evidence from intracellular recording studies. It is suggested that the tuning principle in noradrenergic systems is particularly important for the formation of associations and neural plasticity (interference control) and that the switching principle of dopaminergic systems modulates the timing, time-sharing and initiation of responses (program-control).

Neurochemical lesion of the locus coeruleus of the rat does not suppress the sedative effect of clonidine

The locus coeruleus of male rats was destroyed bilaterally by injection of 6-hydroxydopamine. Rats injected with the vehicle and normal rats served as controls. Starting 20 days after the lesion, the locomotor activity of all rats was measured for 5 min every day. For the first 6 days, the lesioned rats were significantly less active than control rats; from the 7th to the 15th day, on the other hand, the locomotor activity of the two groups of rats was the same. From the 16th day onwards, the sedative effect of small doses of clonidine (2.5-100 micrograms/kg) was measured in lesioned and control animals. In spite of an almost total loss of noradrenaline in the cerebral cortex and hippocampus and a 33% loss of noradrenaline in the brain-stem of the lesioned rats, the sedative effect of clonidine was the same as in the control rats. This result suggests that the sedation produced by clonidine is not dependent on presynaptically located alpha 2-adrenoceptors.