Impaired patterned responding in rats with electrolytic median raphe lesions

Rats with electrolytic lesions of the median nucleus of the raphe were severely impaired in the acquisition of a straight alley, single alternation response compared to sham operated controls. This deficit was due to the failure of lesioned animals to reduce their running speeds on non-reinforced trials. As similar results have been obtained following damage to several limbic structures, this study provides behavioral evidence for the concept of a limbic midbrain circuit.     

Ingestive behaviors of rats with electrolytic median raphe lesions maintained on a food deprivation schedule

The current study examines the effects of electrolytic median raphe lesions on the feeding and drinking behaviors of rats maintained on a 23-hr food deprivation schedule. Measurement of food intake during the feeding period indicated no difference between the two groups but lesioned animals showed significantly more spillage than controls. Furthermore, measurement of water intakes indicated that median raphe lesioned rats consume significantly less water than controls in the absence of food and come to ingest virtually all of their daily fluid intake during the feeding period. Additionally, rats with lesions of this nucleus take significantly more drinks during the feeding period, often punctuating their meal with short draughts. Since similar behaviors are seen following the destruction of certain hypothalamic and limbic nuclei anatomically related to the median raphe, it is possible that these syndromes share an anatomical basis.     

Distribution of dopaminergic cell bodies in the median raphe nucleus of the rat brain

An increasing amount of data suggests that a dysfunction in dopamine (DA) neurotransmission is involved in the pathophysiology of various neurological and psychiatric disorders. With this in mind, the distribution and connectivity of the dopaminergic system in the rat brain has been studied extensively. So far, little is known about the distribution of DA containing neurons in the median raphe nucleus (MnR). This nucleus is mainly defined by a large population of serotonin containing neurons. Using quantitative immunohistochemistry, we observed the presence of a small number of DA containing neurons in the rat MnR, which was in contrast to a previous report.     

Potential noradrenergic regulation of serotonergic neurons in the median raphe nucleus

Summary Pharmacological and morphological evidence suggests that the functional activity of serotonergic neurons may be regulated by catecholamines. We have attempted to reveal a potential pathway by which this interaction might occur. Rats received bilateral knife cut lesions of the ventral noradrenergic bundle which severed the A-1 and A-2 cell body contributions to this projection. Controls received a sham lesion into the cerebellum. Two weeks later all animals were sacrificed, and norepinephrine and serotonin levels were measured in discrete nuclei of the brain. Lesion placement was confirmed histofluorometrically. Serotonin levels in the median raphe nucleus were significantly reduced by 40%, but levels of serotonin were unaffected in the dorsal raphe nucleus and 8 serotonergic terminal regions. The lesions did not affect levels of norepinephrine in the locus coeruleus, cingulate cortex, or habenula. This study suggests that a noradrenergic projection to the median raphe nucleus from the A-1 and A-2 cell body groups may modulate serotonergic neuronal function.This work was presented, in part, at the Society for Neuroscience Annual Meeting, Anaheim, California, 1977     

Evidence for regulation of water intake by median raphe serotonergic neurons

Radiofrequency lesions of the median raphe (MR) nucleus did not alter daily water intake in the rat, whereas electrolytic lesions produced a transient polydipsia only during the first day following lesioning. In contrast, microinjection of the specific serotonin neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) (8 μg/l μl) directly into the MR nucleus produced a small but non-significant drop in water consumption during the first postoperative day, followed by the development of a marked polydipsia peaking at Day 6 and declining again to approach control levels by Days 8–10. The results suggest that the transient polydipsia which occurred during the first 24-hours following electrolytic MR lesioning can be attributed to a different mechanism than the neurotoxin-induced polydipsia, perhaps to stimulation of cholinergic neurons known to lie in this area. The more slowly developing polydipsia which occurred following 5,7-DHT lesions of the MR nucleus can be attributed to destruction of serotonergic neurons. These results strongly implicate MR serotonergic neurons as inhibitory regulators of water intake mechanisms.     

Behavioral inhibition induced by electrical stimulation of the median raphe nucleus of the rat.

Earlier studies using brain lesions or drugs that impair 5-hydroxytryptamine (5-HT) neurotransmission suggest that the ascending serotonergic system is involved in behavioral inhibition. Confirming this hypothesis, the present study demonstrates that electrical stimulation of the median raphe nucleus (NMR) of the rat induces behavioral inhibition. Rats implanted with brain electrodes in the NMR were trained to lever press under a variable-interval 1 min schedule of water presentation. At variable time intervals averaging 7 min, periods of 90 sec of brain stimulation were superimposed on VI responding without any exteroceptive stimulus change. Electrical stimulation of the NMR caused suppression of ongoing lever-pressing behavior, proportional to the stimulus intensity. In addition, NMR stimulation elicited defecation, also directly related to the stimulus intensity, and a cluster of somatic and autonomic changes, such as crouching, micturition, piloerection and teeth chattering, that are characteristic of the rat's emotional behavior in stressful situations. In four out of six rats, the response-suppressant effect of NMR stimulation was counteracted by the 5-HT synthesis inhibitor, para-chlorophenylalanine (PCPA), three to five days following the administration of a single dose of 500 mg/kg of the drug. The effect of PCPA tended to disappear after fifteen days and in one rat, was reversed by 5-hydroxytryptophan, given after benserazide, on the third day from PCPA injection. These results support the suggestion that the mesolimbic serotonergic pathway originating in the NMR and projecting to the septal area and the hippocampus is a substrate of behavioral inhibition, in the rat.     

5-HT mechanisms of median raphe nucleus in the conditioned freezing caused by light/foot-shock association

We have shown that 5-HT mechanisms of the median raphe nucleus (MRN) are involved in contextual fear-conditioning processes as electrolytic or neurotoxic lesions with N-methyl-D-aspartate (NMDA) or injections of 8-hydroxy-2-(di-n-propilamino)-tetralin (8-OH-DPAT) into this structure inhibit freezing behavior in a contextual fear paradigm. In this work, we extend these studies by analyzing the behavioral responses in a classical fear-conditioning paradigm (light or tone/foot-shock association) in rats with either neurochemical lesion with NMDA or injected with 8-OH-DPAT into the MRN. The animals received NMDA or 8-OH-DPAT or saline microinjections into the MRN and were submitted to conditioning trials in an experimental chamber, where they received 10 foot-shocks (0.6 mA, 1 s, variable interval between 10 and 50 s) paired with tone or light (CS). On the next day, they were tested in a different experimental chamber, with or without CS presentation, where the duration of freezing and the number of rearing episodes were recorded. Light or tone alone caused a significant amount of freezing. NMDA lesions or 8-OH-DPAT injections into the MRN clearly inhibited freezing behavior in rats conditioned to light/foot-shock association, but not in the conditioning sessions with tones. Besides the proposed role in contextual fear conditioning, these results clearly show that MRN is involved in the fear conditioning with light as conditioned stimuli. Distinct neural substrates seem to subserve conditioning fear with acoustic stimuli.     

Involvement of median raphe nucleus 5-HT1A receptors in the regulation of generalized anxiety-related defensive behaviours in rats

Changes in 5-HT_1A receptor-mediated neurotransmission at the level of the median raphe nucleus (MRN) are reported to affect the expression of defensive responses that are associated with generalized anxiety disorder (e.g. inhibitory avoidance) but not with panic (e.g. escape). The objective of this study was to further explore the involvement of MRN 5-HT_1A receptors in the regulation of generalized anxiety-related behaviours. Results of experiment 1 showed that intra-MRN injection of the 5-HT_1A/7 receptor agonist 8-OH-DPAT (0.6 nmol) in male Wistar rats impaired the acquisition of inhibitory avoidance, without interfering with the performance of escape in the elevated T-maze test of anxiety. Pre-treatment with the 5-HT_1A receptor antagonist WAY-100635 (0.18 nmol) fully blocked this anxiolytic-like effect. As revealed by experiment 2, intra-MRN injection of 8-OH-DPAT (0.6, 3 or 15 nmol) also caused anxiolytic effect in rats submitted to the light–dark transition test, another animal model that has been associated with generalized anxiety. In the same test, intra-MRN injection of WAY-100635 (0.18, 0.37 or 0.74 nmol) caused the opposite effect. Overall, the current findings support the view that MRN 5-HT neurons, through the regulation of 5-HT_1A somatodendritic autoreceptors, are implicated in the regulation of generalized anxiety-associated behaviours.     

The adrenaline microinjection into the median raphe nucleus induced hypophagic effect in rats submitted to food restriction regimen

The effect of the equimolar doses (6, 20 and 60 nmol) of either adrenaline (AD) or noradrenaline (NA) microinjected into the median raphe nucleus (MR) on feeding behavior of food-restricted rats (15 g/day/rat) was investigated. The data indicated that 20 nmol AD microinjection, but not NA, into the MR decreased the animal food intake. This hypophagic effect induced by AD may be ascribed to a feeding bout conclusion (satiation process) and not to any changes in non-ingestive behaviors induced by drug microinjection. Since equimolar doses of NA failed to change the animal feeding behavior, it is possible to say that AD-induced hypophagia may be due to either changes in tonic stimulatory control exerted by endogenous noradrenaline on MR or to AD-β₂ receptor activation in the MR. We claim that such activation may be much more importantly exerted by adrenaline-containing afferents to MR neurons involved with ingestive behavior than by noradrenergic inputs.     

A generalized learning deficit in albino rats with early median raphe or pontine reticular formation lesions

Recent studies suggest that lesions of the median raphe or pontine reticular formation in adult rats are associated with a nonspecific (generalized) learning impairment. The present study showed that lesions in the region of the median raphe or pontine reticular formation in 21 day old rats likewise produced a nonspecific learning impairment, as evidence by significant deficits on a visual discrimination, nonvisual incline plane discrimination, 3-cul maze, and three simple spatial problems. The finding that relatively large lateral pontomesencephalic lesions did not lead to deficient learning of any of these tasks indicates that lesion locus rather than lesion size is responsible for the production of the learning deficits observed in this study.     

Evidence that serotonergic projections to the substantia nigra in the rat arise in the dorsal, but not the median, raphe nucleus

Following microinjections of the fluorescent retrograde tracer Fast blue into the substantia nigra, large numbers of retrogradely labeled cells were observed in the dorsal raphe nucleus. Using immunocytochemical techniques it could be demonstrated that the majority of these cells contained serotonin-like-immunoreactivity. In contrast, careful examination of the region of the median raphe nucleus revealed no suggestion of a significant projection from the median raphe to the substantia nigra.     

Preoptic implants of estradiol increase wheel running but not the open field activity of female rats

In order to localize the neural substrate for estrogen induction of increases in activity, bilateral implants of dilute estradiol (in concentrations of 1:10, 1:100, and 1:250) were made into the medial preoptic area (n = 5), the anterior hypothalamus (n = 8), the ventromedial hypothalamus (n = 6), and the posterior hypothalamus (n = 4) of ovariectomized female rats housed in running wheels. The same animals were also tested for open field activity and lordosis. The preoptic area was the most effective site tested for estrogenic stimulation of running wheel activity, with 3/5 animals responding consistently to estradiol application. 3/8 animals with cannulae in the anterior hypothalamus (AHA) responded with increases in activity measured in the running wheel on at least two out of three estradiol treatments. No animals with cannula placements posterior to the AHA exhibited consistent running wheel responses to estradiol. No estrogen-induced increase in open field activity was found, regardless of cannula location or concentration of estradiol; however, there was a tendency for the number of squares entered during the later tests to decrease in the ventromedial hypothalamic nucleus group and in the subgroup of animals not showing estrogen-sensitive wheel running in the anterior hypothalamic area group. Only the ventromedial hypothalamic estradiol treatments stimulated lordosis behavior. These results support earlier reports suggesting that the medial preoptic area (and possibly the adjacent anterior hypothalamic area) is the critical brain site for estrogenic stimulation of running wheel activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence for a supportive role of classical transient receptor potential 6 (TRPC6) in the exploration behavior of mice

Non-selective classical transient receptor potential (TRPC) cation channels share important roles in processes of neuronal development and function. To test the influence of TRPC6 activity on behavior, we developed a TRPC6-deficient (TRPC6^−/−) mouse model in a BALB/c genetic background. Both, TRPC6^−/− and wild-type (WT) mice were analyzed first for their general health and reflex status (modified SHIRPA protocol) and then in three different behavioral tests (marble-burying test, square open field and elevated star maze). No abnormalities were detected in the SHIRPA protocol. Most interestingly, TRPC6^−/− mice showed no significant differences in anxiety in a marble-burying test, but demonstrated reduced exploration in the square open field and the elevated star maze. Therefore, TRPC6 channel activity may play a yet unknown role for exploration behavior.