Activity of neurons in the region of the substantia nigra during feeding in the monkey

The activity of single neurons in the region of the substantia nigra of the monkey was recorded during feeding to investigate their function in ingestive behavior. It was observed that some neurons in the substantia nigra and the adjacent tegmentum altered their activity during feeding, in relation to mouth movements. The activity of these neurons was related to mouth movements in that the responses of the units were similar when the monkeys drank fluids with different tastes as long as the same movements were made, in that equally good responses could be obtained when the monkey moved his mouth to non-food objects, and in that in some units opposite responses were obtained when ipsilateral as compared with contralateral mouth movements were made. It was also shown that the responses of these units associated with mouth movements were similar when the monkeys were hungry and when they were satiated. These findings suggest that the activity of some neurons in the region of the substantia nigra is related to the execution of movements which may be involved in feeding, and that the activity of these neurons is not related to the initiation of feeding. Self-stimulation through the recording microelectrodes could be obtained just dorsal to the substantia nigra, but the neural basis of this self-stimulation is not known.

Activity of neurones in the inferotemporal cortex of the alert monkey

The activity of neurones in the inferotemporal cortex of the alert rhesus monkey was recorded while the monkey was shown visual stimuli, which included both food and non-food objects for comparison with the activity of neurones in the lateral hypothalamus and substantia innominata. In the anteroventral part of the inferotemporal cortex, neurones were found with visual responses which were sustained while the animal looked at the appropriate visual stimuli. The latency of the responses was 100 msec or more. The majority (96/142 or 68%) of these neurones responded more strongly to some stimuli than to others. These units usually had different responses when objects were shown from different views, and physical factors such as shape, size, orientation, colour and texture appeared to account for the responses of some of these units. Association of visual stimuli with a food reward (glucose solution) or an aversive taste (5% saline solution) did not affect the magnitude of the responses of the neurones to the stimuli either during the learning or after the period of learning. Nor did feeding the monkey to satiety affect the responses of the neurones to their effective stimuli.

Prefrontal cortex and neostriatum self-stimulation in the rat: differential effects produced by apomorphine

In a dose-response experiment, the effects of intraperitoneal injections of the dopamine receptor agonist, apomorphine (0.075, 0.15, 0.3, 0.6 and 1.2 mg/kg) were studied on self-stimulation elicited from electrodes implanted in the medial and sulcal prefrontal cortex and caudate-putamen in the rat. From the medial and sulcal prefrontal cortex electrodes, apomorphine produced a dose-related decrease of self-stimulation rate which was consistent across animals. From the caudate-putamen electrodes, on the contrary, apomorphine produced a facilitatory effect in the majority of the animals at one or more doses, however, at other doses a decreased self-stimulation rate was observed. The clear and consistent effects of apomorphine on self-stimulation of the prefrontal cortex, together with other experimental evidence in the same line, suggest that dopamine is mediating self-stimulation of this cortical area.

Hypothalamic neuronal responses associated with the sight of food

Neurones in the lateral hypothalamic region are described which alter their firing rates when a monkey looks at food. The units responded when the monkey looked at different types of food, but not at non-food objects or simple visual stimuli. The units did not respond in relation to motor movements, intense arousal, nor when a salient aversive stimulus was shown, nor in relation to eye movements, and were thus shown to be different from units in the globus pallidus which did respond in some of these control tests. The neurones did not respond to olfactory stimuli and did not respond if the animal ate in the dark. Because of these findings it is suggested that the activity of these hypothalamic neurones is associated with the sight of food. It is of interest that these neurones which respond when food is shown to a hungry animal are found in a region thought to be involved in the control of feeding.

Is the release of noradrenaline necessary for self-stimulation of the brain?

The hypothesis that a quantity of noradrenaline released contingently on every response made to obtain brain stimulation mediates the reward produced by the stimulation was tested. An alternative hypothesis is that reward is mediated by a different system, but that a steady activation of post-synaptic receptors by noradrenaline is necessary for normal behavior. The synthesis of noradrenaline was inhibited by disulfiran, and when lateral hypothalamic self-stimulation in the rat had ceased, alpha-adrenergic stimulants were injected intraventricularly (IC) or intraperitoneally (IP). The directly acting receptor stimulants oxymetazoline (0.9-250 mug IC), naphazoline (20-250 mug IC), and clonidine (0.75-3 mug IC, 0.037-3 mg/kg IP) did not restore self-stimulation, but the indirectly acting stimulants amphetamine (2 mg/kg IP), methylphenidate (3 mg/kg IP) and phenylephrine (15 mug IC) did not restore self-stimulation. In Experiments 2 and 3, in which either the functional noradrenaline pool was depleted with disulfiram and amphetamine, or the reserve noradrenaline pool was depleted with reserpine, the action of phenylephrine in restoring self-stimulation was shown to be indirect, probably by mobilizing a reserve pool of noradrenaline. Because only indirectly acting noradrenergic stimulants which facilitate the release of noradrenaline restores self-stimulation, it is concluded that noradrenaline must be released contingent on every response for self-stimulation to occur. Whether this released noradrenaline mediates the reward or has some other function associated with bar-pressing behavior remains to be shown.

Effects of dopamine-receptor blockade on self-stimulation in the monkey

In a dose-response experiment it was shown that intraperitoneal injections of 0.062 mg/kg, and 0.1 mg/kg of the dopamine-receptor blocking agent and neuroleptic spiroperidol severely attenuate self-stimulation in the orbitofrontal cortex, hypothalamus, and in the region of the locus coeruleus, in the rhesus monkey and in the squirrel monkey. In the rhesus monkey intracranial injections of 6 mug of spiroperidol bilaterally into the nucleus accumbens or the hypothalamus attenuated self-stimulation of the amygdala, and injections into the orbitofrontal cortex attenuated self-stimulation of the amygdala and lateral hypothalamus. Self-stimulation at other sites tested (including the region of the locus coeruleus) was much less affected by the injections, and injections into the region of the locus coeruleus were ineffective. These results together with other control experiments suggest that spiroperidol can attenuate self-stimulation in the monkey independently of any motor impairment or sedation produced, and that dopamine receptors in particular brain regions are involved in self-stimulation of particular brain sites.