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Chronic bipolar electrodes were implanted into various forebrain structures in 45 ovariectomized hormone-treated female rats. A significant decrease in copulatory behavior as measured by the lordosis-to-mount ratio and an increase in the female's avoidance of sexual contact was observed following electrical stimulation of the olfactory bulbs, habenula region, caudate-putamen region and preoptic area. This decrease could not be attributed to motor effects nor aversive side effects of stimulation. On the other hand, anterior hypothalamic, premamillary and medial forebrain bundle stimulation resulted in no apparent change in mating behavior. Tests for self-stimulation in all forebrain sites revealed that only implants in the medial forebrain elicited positive responses.

Lesions made via self-stimulation electrodes in the medial forebrain bundle (MFB) of rats always resulted in monoamine accumulation, as studied with the histochemical fluorescence method. Build-up was often in the monoamine systems of the MFB itself, those systems being the ventral norepinephrine bundle, the nigro-neostriate bundle, and the mesolimbic fibers. In other cases, build-up was restricted to a bundle of fibers dorsomedial to the MFB, probably originating in the locus coeruleus or the subcoeruleus area. In still other instances, build-up was seen in both this dorsal bundle of fibers and the catecholamine systems of the MFB. Electrodes in rats with sites of implantation in the region of the A1, A2 and A5 cell groups of origin of the ventral norepinephrine system, as identified with combined Nissl and histochemical examination, were never positive with respect to self-stimulation; nor were placements in this ventral system caudal to the origin of the dorsal bundle positive with respect to that behavior. These results argued against the association of the ventral system with self-stimulation. By implication, the catecholamine fibers of the MFB which are capable of supporting self-stimulation might be the two ascending dopamine systems of the ventral mesencephalic tegmentum. Lesions made via positive electrodes in the vicinity of the brachium conjuctivum always resulted in monoamine build-up in the dorsal norepinephrine fiber system. In sum, our results implicate the A9 and A10 dopamine ventral midbrain systems and the A6 norepinephrine dorsal bundle in intracranial self-stimulation.

Eighty-six intracranial cannula placements in 51 rats were tested with unilateral, 2-mul injections of a 0.60-osmol/kg solution, sucrose dissolved in isotonic sodium chloride. To assess antidiuretic hormone (ADH) release, a water diuresis was induced and spontaneous urinations were collected and analyzed for sodium by flame photometry. On alternate test days the 0.60-osmol/kg solution was injected into sleeping rats, and latencies to drink and volume drunk were recorded. Injections at 42 placements elicited neither drinking nor adtidiuresis on two separate test days each; at 15, both antidiuresis and drinking on at least two of three tests each; at 19, drinking but not antidiuresis; at 10, antidiuresis but not drinking. Positive drinking and ADH placements were not distinctly separated. They clustered in the bed nucleus of the stria terminalis, the preoptic areas, and the anterior portions of the hypothalamus. Placements in the medial forebrain bundle and dorsal to the anterior hypothalamic area elicited thirst but not ADH release for the most part. Placements nearest the supraoptic nucleus were weak or negative for ADH release. Central nervous system osmo-receptors exist and seem not to be the neurosecretorycells. Thirst osmoreceptors and antidiuretic osmoreceptors seem to be contiguous, but distinct.

The projections of neurons in the nucleus locus coeruleus (LC) of the rat were traced by radioautography following stereotaxic injections of 3H-proline. An ascending bundle consisting of a dense cluster of labeled axons accompanied the medial forebrain bundle (MFB) into the lateral hypothalamus. Rostral to the anterior commissure, the labeled fibers contributed to other major pathways: the stria terminalis to the amygdala, and the cingulum bundle and the supracallosal stria to the cingulate cortex, subiculum, and hippocampus. The rest of the ascending fibers terminated diffusely in the pyriform cortex and frontal neocortex. In addition, a lateral group of labeled fibers entered the cerebellum through the superior cerebellar peduncle and terminated around the Purkinje cell somata and in the molecular layer. Descending fibers which entered the medulla either terminated around brainstem nuclei or passed into the ventral portion of the spinal cord. In all areas except the brainstem, the labeling was unilateral with respect to the injection site. This distribution of label from LC provides direct confirmation for connections which have been difficult to visualize in their entirety by any other histological or cytochemical method.

The regenerative growth of central monoamine neurons in the adult rat brain, into an autologous iris transplant placed in the caudal diencephalon, was studied. One injection of 2 μl of anti-NGF serum, given intracerebrally close to the ascending noradrenergic axons from the locus coeruleus, caused a marked reduction in the regrowth of new axonal sprouts into the transplant, as observed 2 weeks after operation. The inhibitory effect was pronounced when the injection was given at the time of transplantation (which was also the time of axonal damage), and it was considerably less effective when given 4 days after the transplantation,i.e. at a time when the sprouting from the lesioned axons had already started. A similar inhibition of the regrowth of the lesioned noradrenergic locus coeruleus axons was also obtained after preincubation of the iris transplant in the anti-NGF serum. In contrast, the regrowth from the lesioned dopamine and indolamine fibers in the medial forebrain bundle system seemed unaffected. None of the inhibitory effects was seen in the control specimens treated with different types of control sera. This indicates that the observed impairment of the formation and regrowth of new axon sprouts from lesioned central noradrenergic neurons was the result of a specific action of the NGF-antibodies by an interference with NGF (or an NGF-like substance) in the brain tissue and/or in the transplanted iris tissue. These observations point to a possible role of endogenous NGF, or an NGF-like substance, in adult central noradrenergic neurons during regeneration.

Prolonged changes during 10 sec poststimulus periods in unit firing rate following stimuli (0.2 sec, 100 Hz, 0.5 msec cathodal pulses) at reward sites in medial forebrain bundle and aversive midbrain reticular sites have been observed in unanesthetized postcollicular cerveau isole rats. However, longlasting medial forebrain bundle-elicited inhibition and reticular-elicited excitation converged on single cells only in intralaminar thalamus. Since these opposite effects had a comparatively long duration, the present study postulated that a related brain region might also show opposite medial forebrain bundle and reticular responses, but the inverse of those seen in intralaminar thalamus. Using similar methods, convergence on single units of prolonged excitation by medial forebrain bundle and by reticular stimuli was typical in internal capsule and to a lesser extent amygdala. Ventral anterior-ventral lateral nuclei of thalamus were characterized by converging inhibition. Contrasting actions of the stimuli which might involve presynaptic interactions were observed in lateral pallidum. However, direct opposite effects, medial forebrain bundle-elicited excitation and reticular-elicited inhibition, were demonstrated and confined anatomically to a medial pallidal region. The opposite responses summated or cancelled when the medial forebrain bundle and reticular stimuli were given simultaneously. A medial pallidal-intralaminar thalamus system may be central to the neural coding of certain affective states, integrating these with motor control and sensory information.

Electrolytic lesions of the hypothalamic medial forebrain bundle (MFB) produced substantial deficits in the copulatory behavior of male rats, but no changes in testicular, seminal vesicle, ventral prostate or penis weights. Radioimmunoassay of serum from lesioned and control animals revealed a small but statistically insignificant reduction in testosterone and LH levels, and no change in FSH levels of lesioned males. In a second experiment, androgen levels were maintained in castrated males by subcutaneous implants of silastic capsules containing crystalline testosterone; this procedure failed to prevent the copulatory deficit which accompanied subsequent MFB lesions. We have concluded that the behavioral deficit produced by such lesions is not a direct result of pituitary-gonadal dysfunction.

The afferent and efferent connections of the septum with the hippocampus (fornix) of with the hypothalamus and lower brain stem (medial forebrain bundle) were transected by means of an encephalotome near the point where these pathways enter or leave the septal area. A transection of the fornix that produced minimal direct damage to cellular components of the septum of hippocampus reproduced the effects of large septal lesions on responding in several temporally defined paradigms that involve periods of response suppression (differential reinforcement of low rates, discriminated Sidman avoidance, fixed interval). Transection of the medial forebrain bundle fibers that interconnect the septum with the hypothalamus and lower brain stem did not affect behavior in any of these paradigms. These observations should be veiwed in the context of the results of earlier investigations which demonstrated that transection of the medial forebrain bundle reproduces several other components of the septal lesion syndrome.

Publisher Summary This chapter elaborates the histochemical demonstration of monoamine neuron systems in the human fetal brain. In chemical investigations on human fetal brain, significant but low concentrations of dopamine, noradrenaline and serotonin have been reported. The monoamine axon bundles had a notably high fluorescence in the fetal brain and could thus be traced for long distances through the lower brain stem, and the hypothalamus up to the basal ganglia and the septal region. One major system of CA fibers could be followed from the region of the lower medulla oblongata through pons and mesencephalon, and at the level of the red nucleus; it joined the medial forebrain bundle. Within the MFB, one system of CA fibers could be further traced up to the septal region. A second large system of ascending CA fibers was observed to originate in the fluorescent cell bodies in the substantia nigra and through the tegmental fields of Forel, the lateral hypothalamus and the zona incerta, ran into the internal capsule toward the basal ganglia.

The efferent connections of the suprachiasmatic nucleus of the hypothalamus have been studied in the rat by the injection of 3H-proline into the nucleus and the surrounding regions of the rostral hypothalamus, and by the injection of the enzyme marker, horseradish peroxidase, into the region of the ventromedial hypothalamic nucleus. After an injection of 3H-proline confined to the ventral portion of the suprachiasmatic nucleus, transported label can be followed, in the autoradiographs, dorsally and caudally in the periventricular area as far as the caudal end of the ventromedial nucleus, into the triangular area between this nucleus and the arcuate nucleus, and along the ventral aspect of the tuberal region, just lateral to the ventromedial nucleus. A small number of silver grains are also seen over the internal lamina of the median eminence. No label can be followed rostrally or immediately lateral to the nucleus. Comparable injections into adjoining regions of the hypothalamus (especially the anterior hypothalamic area, the medial preoptic area, and the retrochiasmatic region) show transported label over the same regions, but with a somewhat different pattern of grain distribution; in addition, the anterior hypothalamic area shows an extensive projection through the medial forebrain bundle to the mammillary and supramammillary nuclei, the midbrain tegmentum, and certain of the midline thalamic nuclei. Although it is difficult in our autoradiographs to distinguish between the course of the efferent fibers from the suprachiasmatic nucleus and the zones in which they terminate, our evidence favors a termination among the cells of the periventricular area, and upon dendrites of the cells in the ventromedial, dorsomedial and arcuate nuclei, which extend beyond the limits of the nuclei into the periventricular area and to the area beneath the ventromedial nucleus.

Summary After injections of 6-hydroxydopamine (6-OHDA) along the medial forebrain bundle in the lateral hypothalamus, rats failed to acquire a one-way active avoidance response or failed to perform a previously acquired active avoidance response. Such rats, however, acquired a passive avoidance response and a conditioned taste aversion normally. Thus the effect of lateral hypothalamic injections of 6-OHDA was not a total loss of the capacity to acquire or perform conditioned responses, but was a failure to initiate forward movement in the presence of a conditional stimulus. Most of these rats could initiate a similar forward movement to escape from the unconditioned stimulus (foot shock). Failure to acquire or perform the active avoidance responses was correlated with the loss of hypothalamic, striatal and forebrain catecholamines produced by lateral hypothalamic 6-OHDA injections. Identical injections of 6-OHDA placed along the medial hypothalamus produced a similar loss of regional catecholamines, but medial 6-OHDA injections did not affect active avoidance responding. We interpret this dissociation between loss of catecholamines and the capacity for active avoidance responding to mean that medial 6-OHDA injections did not damage the same catecholaminergic terminal fields as lateral 6-OHDA injections and that the integrity of the terminal fields damaged by lateral 6-OHDA injections is necessary for active avoidance responding.

The effect of norepinephrine (NE), dopamine and serotonin were studied on self-stimulation behavior and on the unit activity in the medial forebrain bundle (MFB) correlated with this behavior. Cumulative doses of 10, 20, 30 and 40 μg of each substance were tested. The drugs were dissolved in artificial cerebrospinal fluid and were injected into the lateral ventricle of the brain. The duration of the injection was 15 min for 10 μg of the substance, and an additional 15 min for each increment of 10 μg until a total of 40 μg had been injected. Effects of these 4 doses on behavior and on neuronal activity were determined after each 10 μg. It was found that all 4 substances facilitated self-stimulation behavior, but to different degrees and at different doses. L -NE was the most effective of the substances tested because it facilitated behavioral responding in a majority of the animals at each dose; D -NE was the least effective. The effects of dopamine and serotonin were facilitatory in some animals but not in others. Excitatory neuronal activity occurring during self-stimulation behavior in the MFB was reduced by L -NE, whereas inhibitory neuronal activity in the same pathway was potentiated. The significance of these results is discussed in terms of the view that NE could be one of the transmitters in the MFB and that its synaptic action there is likely to be inhibitory.

—A specific and sensitive, radioisotopic microassay for tryptophan hydroxylase (EC 1.14.36) is described, which is capable of determining enzymatic activity in as little as 5 μg of crude brainstem homogenate. 5-Hydroxytryptophan, the immediate product of hydroxylation of tryptophan is enzymatically converted to N-acetylserotonin. A radioisotopic label is then introduced by the enzymatic methylation of N-acetylserotonin in the presence of [3H]methyl-S-adenosyl-methionine. The [3H]-melatonin thus formed is isolated by extraction and counted. With this assay, the activity in individual hypothalamic nuclei (arcuate nucleus, median eminence, suprachiasmatic nucleus, and medial forebrain bundle) has been measured.

The effect of electrode implantation and unilateral and bilateral intracerebral lesions on the gastric secretory response to insulin-induced hypoglycemia was examined in six unanesthetized, unrestrained cats. Lesions located in the supraoptic nucleus, preoptic area, mamillothalamic tract region of the dorsal hypothalamus, and the supraoptic, medial forebrain bundle and mamillothalamic tract regions of the lateral hypothalamus significantly reduced or completely abolished the increase in gastric secretory volume, titratable acidity, chloride and pepsin output evoked by insulin-induced hypoglycemia. Bilateral lesions located in the superior border of the anterior portion of the caudate nucleus and the lateral region of the corpus callosum did not alter the gastric secretory response of one animal to insulin-induced hypoglycemia. The results suggest that fibers originating in or passing through the supraoptic-preoptic region, mamillothalamic tract region of the dorsal hypothalamus, and various regions of the lateral hypothalamus project directly or indirectly to the dorsal motor nuclei of the vagi. The results also suggest the possibility that the increase in vagal activity observed during insulin-induced hypoglycemia may represent an integrated response involving a number of stimulatory, and possibly inhibitory, pathways in the hypothalamus.

Rats immobilized in a restraining cage were trained to move their tails back and forth using lateral hypothalamic stimulation as reinforcement. The same rats were also trained to bar press in an ordinary cage for the same reinforcement. Following lesions in the medial forebrain bundle anterior to the stimulation site the stimulation no longer appeared to be positively reinforcing when the animals were tested on the tail movement task. When these subjects were tested on bar pressing, the stimulation still appeared to be reinforcing. Various interpretations of the data, including the possibilities that the lesions changed the rats' emotional responsiveness, and that a different set of fibers within the medial forebrain bundle mediates the reinforcement of each of the two tasks were considered.

In three experiments, electrodes were chronically implanted into the medial hypothalamic area of male rats. Escape latencies, motivated by electrical stimulation of the brain and footshock presented both separately and together in a shuttle box, were measured. In all cases where cessation of medial hypothalamic stimulation was contingent upon escape, the combination of footshock and intracranial stimulation decreased escape latencies to a level below that motivated by footshock alone. Increases in the current stimulating the medial hypothalamus generally caused more rapid escape, both when the intracranial stimulation was presented alone and when combined with footshock. In contrast, stimulation of sites within the medial forebrain bundle region, which supported self-stimulation, systematically increased latencies to escape from footshock. Facilitation of escape behavior was no longer observed when the presentation of medial hypothalamic stimulation was nonresponse contingent. These effects are interpreted as consistent with a predominantly aversive character attributable to electrical stimulation of the medial hypothalamus, but not with the view that this part of the brain mediates general behavioral suppression.