Involvement of mesencephalic reticular formation neurons in cat conditioned reflexes

Traditional defensive and operant food reflexes were used to investigate neuronal responses of the mesencephalic reticular formation. It was found that these neurons may be divided into different groups according to function, depending on how they respond to positive conditioning stimuli. Of the two main groups of neurons with sustained tonic reactions one is activated in response to positive acoustic conditioning stimulation; it no longer reacts to the same stimulus after extinction of the reflex, while the other only becomes involved in response to positive stimulation accompanying the initiation of movement. Neurons belonging to the second group begin to respond directly to acoustic stimulation after extinction of the conditioned reflex. Neurons of the mesencephalic reticular formation can thus exercise additional tonic ascending effects both in the production and inner inhibition of the conditioned reflex. The group of neurons with a phasic reaction, i.e., a double response (a direct response to sound and another produced by movement) displayed a drop in spontaneous activity during the shaping of inhibition of differentiation and of extinction in particular. It was found that the initial changes in the spike response of reticular formation neurons during conditioning and pseudo-conditioning are similar. There are thus grounds for stating that neurons of the mesencephalic reticular formation participate in the shaping, production, and inner inhibition of traditional and operant conditioned reflexes in a differentiated capacity rather than as a population reacting identically.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 18, No. 2, pp. 161–171, March–April, 1986.     

Participation of angiotensin II in the establishment of negative emotional reactions

In waking rats and rabbits systemically injected angiotensin II was shown to participate predominantly in the mechanisms of negative emotional reactions. The effects of angiotensin II were observed at the behavioural level as well as at the neuronal one. Depending on the dose and the time of injection of angiotensin II and its specific antagonist saralasin they inhibited or facilitated elaboration and extinction of automatized conditioned active avoidance independently of arterial, pressure changes, the pain threshold being altered. Injection of angiotensin II abolished individual behavioural reactions of the animals in response to stress factors and increased their resistability to emotional stress. The negative emotional reactions were found to induce changes of chemosensitivity of neurones of the parafascicular complex of the medial thalamus and the midbrain reticular formation during microionophoretic application of angiotensin II. A supposition is made about the increase of angiotensin II brain synthesis under conditions of emotional stress.     

Putative nociceptive neurotransmitters in the mesencephalic reticular formation

The neurotransmitter(s) involved in the transmission of nociceptive information in the mesencephalic reticular formation (MRF) of the rat have not been identified. Acetylcholine (ACh), substance P (SP), neurotensin (NT), norepinephrine (NE) and dopamine (DA) have all been implicated as putative neurotransmitters involved in nociception. All of these compounds were microiontophoretically administered in the MRF of rats to determine which, if any, mimicked the effects produced by a nociceptive stimulus (foot pinch). This is only one of several criteria that a substance should meet to be considered a nociceptive neurotransmitter in the MRF. ACh and NE mimicked the effects of the nociceptive stimulus in 61% and 67% respectively of the cells tested; NT, DA and SP mimicked the effects of the nociceptive stimulus less frequently (33%, 30%, 23% respectively). Therefore, the nociceptive neurotransmitters in the MRF appear to be ACh and NE; NT, DA and SP may be neurotransmitters with a less important role in nociception in the MRF.     

Temporal patterns of unit activity of mesencephalic reticular nuclei during sleep and waking

Temporal patterns of unit activity in the mesencephalic reticular nuclei (n. cuneiformis, n. parabrachialis) were studied in unrestrained rats during the sleep-waking cycle; activity was derived by means of movable metallic microelectrodes. Analysis of the data showed that most neurons of these mesencephalic reticular nuclei (76 and 66% respectively) generate activity with the highest frequency during active waking and the emotional stage of paradoxical sleep; they discharge with lower frequency during passive wakefulness and the nonemotional stage of paradoxical sleep, and they exhibit least activity during slow-wave sleep. Comparatively few neurons (24 and 15%) demonstrate the opposite kind of temporal pattern of activity: They discharge more intensively during slow-wave sleep and more slowly during active wakefulness and the emotional stage of paradoxical sleep. Activity of these neurons during quiet wakefulness and the nonemotional stage of paradoxical sleep reaches the level of activity observed during slow-wave sleep. Neurons discharging intensively during active wakefulness were found in n. parabrachialis; their discharge frequency during passive wakefulness and slow-wave sleep and its frequency was least during paradoxical sleep. The similarity and differences of the neurophysiological mechanisms of regulation of the phases and stages of the sleepwaking cycle are discussed.I. S. Beritashvili Institute of Physiology, Academy of Sciences of the Georgian SSR, Tbilisi. Translated from Neirofiziologiya, Vol. 16, No. 5, pp. 678–690, September–October, 1984.     

Electrophysiological properties of mesencephalic ventral tegmental area neurons in awake rats

A statistical analysis was made of spike activity and the shape and duration of individual action potentials in cells from the mesencephalic ventral tegmental area and adjacent regions during experiments on awake rats. Four groups of cells were identified in this test population whith electrophysiological features which may relate to their distinctive neurochemical profile and to their specific afferent connections.Institute of Pharmacology, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 18, No. 6, pp. 729–737, November–December, 1986.     

Interaction of amygdalar neurons of active and passive rabbits in negative emotional situations

Interaction of basal and central nuclear neurons of amygdala was studied by plotting histograms of crosscorrelation in passive and active rabbits exposed to emotionally significant stimuli. The behavior of animals was studied in the open field, light-dark test and during presentation of emotionally significant stimuli. Rabbits of different typological groups applied a certain behavioral strategy in a variety of behavioral tests. Intergroup differences were revealed in the interaction of neighboring cells of amygdala. Passive rabbits (as comparied to active rabbits) demonstrated more excitatory connections and less inhibitory connections with the latency from 50 to 150 ms. Interactions with the delta1-range and theta2-range frequencies in passive animals were more rarely observed. The asymmetry of the interhemispheric neuronal interaction in amygdala with the right dominance was revealed in passive but not active animals. The results testify that amygdala is involved in the choice of behavioral strategy, and the level of its activation is higher in passive animals.     

Comparison of spontaneous activity of mesencephalic reticular neurones in the waking state and during pentobarbital anaesthesia.

In rats immobilized by d-Tubocurarine the spontaneous activity of 100 mesencephalic reticular neurones was recorded extracellularly and statistically evaluated before and after repeated intravenous administration of 15 mg/kg doses of Pentobarbital. Number of spontaneously active neurones decreases quasi-linearly with repeated 15 mg/kg Pentobarbital doses. After a 75 mg/kg cumulative dose practically all neurones ceased firing spontaneously, whereas cortical EEG activity fully disappeared after the 90 mg/kg Pentobarbital dose. The firing rate was characterized by the mean interval with its standard deviation. Mean value for the total sample of spontaneously active neurones was 146.7 +/- 192.3 msec without Pentobarbital and increased to 302.7 +/- 367.5 msec after 15 mg/kg and to 400.6 +/- 452.5 msec after 30 mg/kg cumulative dose of Pentobarbital. The 15 mg/kg dose increased the frequency of firing in 5% of neurones only. The most often encountered type of interval histogram in the mesencephalic reticular formation was the exponential type (59% in unanaesthetized state), which was also most sensitive to Pentobarbital. Synchronized activity in bursts, characterized by periodical peaks and dips frequently occurred in neurones with the exponential-like interspike interval density after Pentobarbital administration. On the contrary, neurones with gamma-like and especially with symmetrical-like types of density were less influenced by Pentobarbital. In many neurones a periodical increase in the firing rate (with intervals of tens of seconds) related to the occurrence of spindles was present in the cortical EEG activity.     

Interactions of morphine with putative neurotransmitters in the mesencephalic reticular formation

Acetylcholine (ACh) and norepinephrine (NE) have been identified previously as putative nociceptive neurotransmitters in the mesencephalic reticular formation (MRF) of the rat because they frequently mimic the change in neuronal firing (usually an increase) evoked by a noxious stimulus (NS). The purpose of this study was to determine if 1.) morphine (M) acts to prevent the increase in firing evoked by a NS by blocking the effects of either of these two neurotransmitters and 2.) if this effect is a specific narcotic effect. Using the technique of microiontophoresis in conjunction with extracellular recording, we located single units in the MRF in which 1.) neuronal firing was accelerated by a NS: 2.) M blocked this response; and 3.) either ACh or NE mimicked the effect of the NS. Neurons meeting these three criteria were studied further to determined if morphine would also block the response to either of the neurotransmitters and if this was a specific narcotic effect. We found that morphine blocked the increase in neuronal firing evoked by the NS and ACh or the NS and NE in over 50% of the cells meeting the above criteria. Some neurons were found in which both ACh and NE mimicked the NS and M blocked all three responses. This blockade of these neurotransmitters was a specific narcotic effect because it could be reversed by the systematic administration of naloxone. These data lead to the tentative hypothesis that M, acting via an opiate receptor, blocks the increase in neuronal firing evoked by a NS by blocking the postsynaptic effects of either ACh or NE. This may be one of the mechanisms by which morphine acts to produce analgesia.