Hypothalamic self-stimulation: the role of dopamine and possible relations to neocortical slow wave activity

Hippocampal conjunctive and complementary CA1 populations relate sensory events to movement

Hippocampal CA1 neurons respond to sensory stimuli during enforced immobility, movement, and their transitions in a new conveyor belt task. Head-fixed mice were exposed to light flashes or air streams while at rest, spontaneously moving, or running a fixed distance. Two-photon calcium imaging of CA1 neurons revealed that 62% of 3341 imaged cells were active during one or more of 20 sensorimotor events. Of these active cells, 17% were active for any given sensorimotor event, with a higher proportion during locomotion. The study found two types of cells: Conjunctive cells that were active across multiple events, and complementary cells that were active only during individual events, encoding novel sensorimotor events or their delayed repetitions. The configuration of these cells across changing sensorimotor events may signify the role of hippocampus in functional networks integrating sensory information with ongoing movement making it suitable for movement guidance.

Cortical State Fluctuations during Sensory Decision Making

In many behavioral tasks, cortex enters a desynchronized state where low-frequency fluctuations in population activity are suppressed. The precise behavioral correlates of desynchronization and its global organization are unclear. One hypothesis holds that desynchronization enhances stimulus coding in the relevant sensory cortex. Another hypothesis holds that desynchronization reflects global arousal, such as task engagement. Here, we trained mice on tasks where task engagement could be distinguished from sensory accuracy. Using widefield calcium imaging, we found that performance-related desynchronization was global and correlated better with engagement than with accuracy. Consistent with this link between desynchronization and engagement, rewards had a long-lasting desynchronizing effect. To determine whether engagement-related state changes depended on the relevant sensory modality, we trained mice on visual and auditory tasks and found that in both cases desynchronization was global, including regions such as somatomotor cortex. We conclude that variations in low-frequency fluctuations are predominately global and related to task engagement.

Synergistic anticholinergic and antiserotonergic effects in humans

Animal research suggests an important interactive role for ascending cholinergic and serotonergic systems in modulation of cerebral function. Employing a randomized, double-blind, crossover design, 11 healthy young adults were tested in each of four conditions: (1) placebo, (2) fenfluramine (a serotonin depleting agent), (3) scopolamine (a muscarinic antagonist), and (4) fenfluramine and scopolamine. P3 latency was slowed by the dual drug treatment to an extent greater than the sum of individual drug effects. EEG mean frequency was decreased by behavioral activation, and this decrease was reversed by the combined drug treatment but not by single drugs. In contrast, verbal memory, EEG alpha power, and P3 amplitude were significantly affected only by scopolamine. No drug effects were found for the N1 and P2 potentials. The results provide the first demonstration of combined anticholinergic and antiserotonergic effects in humans, and offer partial support to the concept of an interactive role of cholinergic and serotonergic systems in cerebral mechanisms.

Differential effects of atropine, procaine and dopamine in the rat ventral tegmentum on lateral hypothalamic rewarding brain stimulation

Microinjections of the muscarinic antagonist, atropine, of dopamine, or of the local anesthetic, procaine, in the ventral tegmentum elevated frequency thresholds for lateral hypothalamic self-stimulation. The largest and most robust effects were observed following atropine (30 or 60 micrograms) microinjections. The most sensitive sites for the atropine effect were near dopamine cells. In order to determine if the effects of atropine can be reversed by pretreatment with a cholinergic agonist, carbachol (1-3 micrograms) was microinjected 15 min prior to atropine. Carbachol pretreatment attenuated the frequency threshold elevation of atropine by 47-95%. Since atropine is a local anesthetic, the effects of procaine on self-stimulation thresholds were tested as well. Procaine (100 or 250 micrograms) in ventral tegmentum elevated frequency thresholds by much less than atropine. Therefore, while atropine attenuates reward primarily through blockade of muscarinic receptors, the local anesthetic effect of atropine may enhance the threshold elevation. Dopamine (1-10 micrograms) also elevated frequency thresholds, but when dopamine injections were repeated daily, the threshold elevations were attenuated. This attenuation contrasted with the robust effects of atropine, and may reflect the development of autoreceptor subsensitivity. Hence, both dopaminergic and muscarinic receptors in ventral tegmentum are involved in lateral hypothalamic brain stimulation reward.

EEG spectral analysis in delirium

Spectral analysis of EEG was conducted for 51 elderly delirious patients meeting the Diagnostic and Statistical Manual of Mental Disorders III (DSM-III) criteria and for 19 controls. As a whole group, and also when subdivided according to the type of delirium, severity of cognitive decline or the type of central nervous system disease, delirious patients showed significant reductions of alpha percentage, increased theta and delta activity and slowing of the peak and mean frequencies and these changes were also obvious in individual recordings. The alpha percentage and various ratio parameters correlated significantly with Mini Mental State score, and delta percentage and mean frequency with the lengths of delirium and hospitalisation. The results indicate an association between spectral EEG changes and severity of cognitive deterioration in delirium.

Cholinergic antagonists in ventral tegmentum elevate thresholds for lateral hypothalamic and brainstem self-stimulation

Frequency thresholds for lateral hypothalamic self-stimulation are elevated following microinjections of atropine into ventral tegmentum (73). Many self-stimulation sites in brainstem are situated near cholinergic cell groups and axons, and ventral tegmentum receives cholinergic afferents terminals. To test the hypothesis that ventral tegmental muscarinic receptors are involved in lateral hypothalamic and brainstem self-stimulation, stimulating electrodes were placed in lateral hypothalamus and dorsal tegmentum near the midbrain-pons border, and cannulae were implanted in ventral tegmentum. Microgram injections of muscarinic antagonists, atropine or scopolamine, or a choline uptake blocker, hemicholinium-3, elevated frequency thresholds for both self-stimulation sites in a dose-dependent and time-dependent fashion. In addition, summation and collision between the two self-stimulation sites was tested using paired-pulse methods (53). Summation ranged from 31 to 87% (i.e., 24 to 47% reductions in frequency threshold were observed at long intrapair intervals), but no collision-like effects were observed at short intrapair intervals. The ventral tegmentum is a likely site for the convergence of dorsal tegmental and lateral hypothalamic self-stimulation pathways.

Suppression of serotonin-dependent cerebral activation: a possible mechanism of action of some psychotomimetic drugs

Rats treated with centrally acting anti-muscarinic (atropinic) drugs display large amplitude irregular slow waves in both the neocortex and hippocampus during behavioral immobility and some stereotyped automatic behaviors (Type 2 behavior). However, rhythmical slow activity (RSA) in the hippocampus and low voltage fast activity (LVFA) in the neocortex occur in close correlation with spontaneous changes in posture, head movement, walking, rearing, swimming or struggling when held (Type 1 behavior). Previous research has indicated that such atropine-resistant RSA and LVFA is dependent on brain serotonin. In the experiments reported here, atropinized rats were given a test drug or a control injection while hippocampal and neocortical activity and behavior were recorded. Several psychotomimetic drugs (phencyclidine; (d,l)-N-allyl-N-normetazocine (SKF-10,047); d,l-cyclazocine; and N-ethyl-1-phenyl-cyclohexylamine) strongly suppressed atropine-resistant RSA and LVFA in doses that were compatible with active behavior. Ketamine had a weak effect but a variety of other drugs were inactive in this test. It is suggested that the psychotomimetic effect of phencyclidine and the psychotomimetic opioids is due, at least in part, to suppression of serotonin-dependent activation of the cerebral cortex.

Near-total loss of 'learning' and 'memory' as a result of combined cholinergic and serotonergic blockade in the rat

Previous work has indicated that activation of the cerebral cortex (i.e. elicitation of low-voltage fast activity in the neocortex and rhythmical slow activity in the hippocampus) is dependent on corticipetal cholinergic and serotonergic projections. Treatment with a combination of p-chlorophenylalanine (an inhibitor of the synthesis of serotonin) plus atropine or scopolamine (muscarinic cholinergic antagonists) can suppress all cerebral activation. In this paper, the behavioral effects of single or combined blockade of cholinergic and serotonergic neurotransmission were studied using a shock avoidance test, an open field test, a swim-to-platform test, a hypothalamic self-stimulation test and a test of grooming behavior. The results show that blockade of cerebral activation produces a condition analogous to global dementia but does not produce sleep or coma. The hypothesis that cholinergic and serotonergic neurotransmission provides a basis for learning and memory is discussed critically.

Evidence that serotonin mediates non-cholinergic neocortical low voltage fast activity, non-cholinergic hippocampal rhythmical slow activity and contributes to intelligent behavior

Previous research has shown that low voltage fast activity (LVFA) in the neocortex and rhythmical slow activity (RSA) in the hippocampus can result from activity in either of two ascending pathways. Activity in neurons in the basal forebrain may produce atropine-sensitive (presumably cholinergic) LVFA and RSA during both Type 1 behavior (e.g., head movement, walking) and Type 2 behavior (e.g., waking immobility, face-washing, tremor). Activity in an aminergic pathway may produce atropine-resistant LVFA and RSA during Type 1 behavior only. The role of 5-hydroxytryptamine (5-HT) in this pathway was studied in rats treated with p-chlorophenylalanine (PCPA; 500 mg/kg/day X 3, i.p.). Amine levels were measured by high pressure liquid chromatography with electrochemical detection. Brain slow wave and multi-unit activity was assessed by inspection and by a procedure of filtering and integration. PCPA treatment alone had little effect on LVFA or RSA, but following PCPA and atropine (50 mg/kg) together, both LVFA and RSA were attenuated or eliminated. Thus, atropine-resistant LVFA and RSA may be dependent on 5-HT transmission. A combination of PCPA and atropine produced a very severe deficit in performance in a simple water maze. Rats treated with this drug combination may provide an animal model of human global dementia.