Self-stimulation at the lateral hypothalamus and locus coeruleus after specific unilateral lesions of the dopamine system

A group of rats was trained to press levers for electrical stimulation from bipolar electrodes aimed at the lateral hypothalamus (LH), and another group was trained to self-stimulate from electrodes in the locus coeruleus (LC). All rats in both groups were subjected to unilateral injections of 6-hydroxydopamine into the substantia nigra and midbrain ventral tegmentum. The lesions produced profound depletions of dopamine from the ipsilateral frontal cortex, nucleus accumbens/olfactory tubercle, and corpus striatum. Pretreatment with desmethylimipramine prevented loss of noradrenaline in excess of that caused by electrode implantation. The destruction of the dopamine projections produced a persistent and pronounced deficit in intracranial self-stimulation from ipsilateral electrodes in both the LH and LC groups, but only transient effects on self-stimulation from contralateral electrodes at these sites. These results suggest that an intact dopamine system is required for the expression of self-stimulation behaviour.

Integrated excitatory response to microinjections of L-glutamic acid into the reticular system of the toad

Microinjections of L-glutamic acid into the mesencephalic tegmentum and diencephalon of anesthetized toads evoked an excitatory response consisting of electrocephalographic and electromyographic activation, and a rise in arterial blood pressure. Electromyographic responses were reduced or absent in diencephalically injected toads. Except for this case, all other components of the response always appeared together and presented the same time course. L-glutamic acid was able to activate a group of neurons at different levels of the reticular system involved in an integrated response of ergotropic nature.

Diencephalic sites responsive to prostaglandin E2 facilitation of sexual receptivity in estrogen-primed ovariectomized rats

Crystalline prostaglandin E2 (PGE2) induced high levels of sexual receptivity when stereotaxically implanted into the antirior-bala hypothalamic and preoptic regions of estrogen-primed, ovariectomized rats. Anterior- as well as medial basal hypothalamic implants of PGE2 induced a significant elevation of rectal body temperature, but had no effect on open-field activity scores. The possibility that PGE2 exerts its facilitatory effects upon sexual receptivity via an LHRH mechanism is discussed.

The nucleus accumbens--possible site of antipsychotic action of neuroleptic drugs?

The hypothesis that neuroleptic drugs exert their therapeutic effects by blocking dopaminergic transmission has been investigated by examining the effects of 3 neuroleptic drugs on dopamine turnover in 2 dopaminergically innervated regions of brain--the neostriatum and nucleus accumbens. The drugs chlorpromazine, thioridazine and fluphenazine, known to be therapeutically active in the treatment of schizophrenia, but to have differing incidences of extrapyramidal side effects, were administered to rats in dose ratios approximating to those effective in man. All 3 drugs induced a similar rise in the content of the dopamine metabolite homovanillic acid (HVA) in the nucleus accumbens, whilst the changes in HVA observed in the neostriatum were in the rank order in which these drugs produce extrapyramidal side effects. While the concentrations of dopamine metabolites in the frontal cortex were too low to assess the possibility that neuroleptic drugs have actions at this level, our results are consistent with the hypothesis that these drugs exert their therapeutic effects by dopamine receptor blockade in the nucleus accumbens.

Serotonin, the superior colliculus, and grooming behavior in cats with neocortical lesions

Previous studies of cats with pontile lesions indicate that a serotonergic deficit exists in the superior colliculi and that this deficit is involved in the genesis of an abnormal grooming behavior. Cats with frontal neocortical lesions exhibit the same serotonergic deficit and the same abnormal grooming behavior. The present study established that the serotonergic deficit is involved in the mediation of the abnormal grooming behavior in cats with frontal neocortical lesions. Microinjections of 5-hydroxytryptophan (5-HTP) and 5-hydroxytryptamine (5-HT) into the superior colliculi abolished or signigicantly reduced the abnormal behavior in cats with frontal neocortical lesions, whereas no effects of 5-HTP were observed after injections into the cerebrospinal fluid above the superior colliculi, into the tegmentum beneath the superior colliculi, or into the medial dorsal nucleus rostral to the superior colliculi. Other substances (tryptophan, noradrenaline, and gamma-amino-butyric acid) had no effect on the abnormal behavior when injected into the superior colliculi. Further evidence implicating a serotonergic deficit in the mediation of the abnormal behavior was obtained by systemic injections: The abnormal behavior was abolished with 5-HTP in cats with frontal neocortical lesions and in adrenalectomized cats that were previously treated with p-chlorophenylalanine.. The present study also demonstrated that the abnormal grooming behavior is induced by frontal neocortical lesions and not by more caudal lesions of the neocortex. The anatomical relations between the frontal neocortex and the superior colliculus and the role of these structures in grooming behavior are discussed.

Self-stimulation of the MFB or VTA after microinjection of haloperidol into the prefrontal cortex of the rat

Haloperidol, a dopamine receptor antagonist, was microinjected in doses of 12 or 24 microng into the prefrontal cortex of the rat. Its effects on self-stimulation of the ventral tegmental area (VTA) or the medial forebrain bundle (MFB) were examined. It was found that these injections failed to attenuate self-stimulation at either structure. However, when haloperidol was injected into the caudate-putamen complex, a decrease in self-stimulation occurred within these structures. These results suggest that dopamine in the medial prefrontal cortex is not necessarily a part of the neurochemical substrate underlying self-stimulation of the ventral tegmental area or medial forebrain bundle.

[Respective effects on locomotor activity of injections of 5, 7-DHT in median raphe nucleus and of 6-OHDA in the mesolimbic dopaminergic group area in the rat]

This experiment was performed in order to demonstrate that the locomotor hyperactivity provoked by a radiofrequency lesion of the ventral mesencephalic tegmentum-A10 DA group area was not due to a 5-HT fiber damage. Four groups of Rats were used. First groups II and IV received a 5, 7-DHT injection in the median raphe; groups I and III received the vehicle. Locomotor activity was measured in a circular corridor 10 and 30 days; no hyperactivity was obtained. Then the same groups received a 6-OHDA injection, bilaterally in the A 10 area (groups III and IV) or the vehicle (groups I and II); the activity was measured 10 days later: significant hyperactivity was obtained with groups III and IV, without statistical differences between these two groups. In conclusion (i) 5-HT neurons are not directly involved in the VMT-hyperactivity, (ii) the DA A 10 neurons seem to be a critical anatomical target for this symptom.

The afferent input and functional organization of the hypothalamus in reactions regulating pituitary-adreno-cortical activity

In chronic experiments on rabbits, stimulation of the dorsal tegmentum of the midbrain caused an increase in serum corticosteroid level. In acute experiments, stimulation of the same region resulted in changes in the activity of hypothalamic neurons. Neurons in the anterior (AHA), medial (MHA), including the medial dorsal (MHAd) and medial ventral (MHAv) areas, and lateral (LHA) areas of the hypothalamus were studied. When the midbrain was stimulated, more neurons in LHA, MHAd and MHAv were excited than inhibited; in AHA, more neurons were inhibited. These changes suggest that the activation of the pituitary-adrenocortical system caused by stimulation of the dorsal tegmentum is associated with changes in the influx of afferent impulses to the hypothalamus, which are responsible for AHA inhibition and MHA and LHA excitation.

Raphe neurons and barbiturate induced rhythmic activity in the subthalamic nucleus and ventral tegmental area

A continuous rhythmic discharge in the subthalamic nucleus (STN), lateral habenular nucleus, and ventral tegmental area (VTA) is associated with tremor in cats lightly anesthetized with barbiturate. The pontine raphe nuclei centralis superior and inferior also show barbiturate dependent slow wave and unitary activity. Stimulation of the raphe region produces evoked potentials and synchronization of the oscillatory activity in STN and VTA. The latency of the evoked response is shortest in STN. Extensive raphe lesions abolish the rhythmic activity in STN and VTA, but lesions which do not completely destroy nuclei centralis superior and inferior are not effective. In single cell recordings 81% of 142 cells in the pontine raphe region and medial reticular formation are inhibited following VTA stimulation. Fifty-four percent of 129 cells in the same region were inhibited when tested with STN stimulation. Forty-seven raphe paramedian reticular formation cells showed convergence of inhibitory input from STN, VTA, and contralateral sciatic nerve (Group II and III fibers). Depletion of serotonin with rho-chlorophenylalanine did not affect the appearance of barbiturate rhythmic activity. It is concluded that the pontine raphe region is not essential for maintenance of the oscillatory activity in STN and VTA, and that the raphe region is on the output side of the system.

Degenerative processes after punctate intracerebral administration of 6-hydroxydopamine

Various doses of the neurotoxin 6-hydroxydopamine (6-OHDA) were infused into the substantia nigra-ventromedial tegmental area, caudateputamen nucleus, and red nucleus. A maximum of four successively occurring zones of neuropathology could by detected at all injection sites: (1) A zone of complete absence of neural, glial, and vascular elements due to tissue displacement by the cannula, (2) an area of glial cells, developing over time, which surrounded the cannula tract, (3) a region exhibiting virtually complete loss of neuronal elements, and (4) a zone of selective neuropathological reaction or cellular loss in which some neurons were affected and others were not. All investigators who have histochemically and/or histologically evaluated the effects of intracerebrally administered 6-OHDA agree on the existence and genesis, some non-selective process, of the first three zones. Although some scientists maintain that the fourth zone is where 6-OHDA operates selectively, data are presented in this report that other, well-established processes can account for neuropathology beyond zone 3. Prominent among these are retrograde and anterograde degeneration, ischemia as a function of interruption of blood supply, and nonselective traumatization. In addition, the topography of the injected site (e.g., morphology of affected neurons, degree of myelination) was found to be as important a determinant of the locus of tissue damage as the presumed selectivity of 6-OHDA's action. Furthermore, monoamine oxidase inhibition by nialamide did not appear to increase the neurotoxicity of 6-OHDA. Similarly, 1-(2,5-dihydroxy-4-methylphenyl)-2-aminopropane was not a more potent cytotoxin than 6-OHDA even though this new neurotoxin has a propane side chain which renders it immune to monoamine oxidase. These observations, taken together, suggest that considerable cautioon should be exercised in interpreting data from experiments in which 6-OHDA, or related neurotoxins, are used to uncover catecholaminergic mechanisms of behavior and other functional processes.

Monoaminergic mechanisms in aversive brain stimulation

In these experiments we have examined the role of brain monoamines in the fearlike aversive responses produced by the electrical stimulation of the dorsal midbrain tegmentum (DMT). Chronic bipolar stimulating electrodes were bilaterally implanted into the DMT of 77 rats. Electrical stimulation via 34 of these electrodes produced fearlike, escape seeking responses. These animals were then trained for stable stimulus escape using a decremental bar pressing paradigm. In this paradigm, each bar press reduced the stimulation current by a predetermined fraction (5 percent) of the initial current level. Perceived aversive strength of the initial stimulus current was thereby represented by an increasing function of the number of bar presses to escape. Administration of the catecholamine depleting drug alpha-methyl-para-tyrosine yielded no change in bar pressing relative to saline-injected controls. However, the serotonin depleting drub para-chlorophenylalanine produced a marked increase in decremental bar pressing compared to saline-injected controls. These results suggest that fearlike responses to DMT stimulation may be dependent upon brain serotonin levels and relatively insensitive to levels of brain catecholamines.