Anette Hoffmann

Central chemoreceptor drive to breathing in unanesthetized toads, Bufo paracnemis

Central chemoreceptor drive to breathing was studied in unanesthetized toads, equipped with face masks to measure pulmonary ventilation and arterial catheters to analyze blood gases. Two series of experiments were performed. Expt. 1: The fourth cerebral ventricle was perfused with solutions of mock CSF, adjusted to stepwise decreasing pH values. Concomitant perfusion-induced increases of pulmonary ventilation, pHa and PaO2 were measured. Expt. 2: Inspiration of hypercapnic gas mixtures was applied to stimulate both central and peripheral chemoreceptors. Subsequently, only peripheral chemoreceptors were stimulated. This was accomplished by repeating the hypercapnic conditions while the fourth ventricle was perfused with mock CSF at pH 7.7. This procedure reduced the slope of the ventilatory response curve by about 80%. Taken together, the experiments suggest a highly dominant role of central chemoreceptors in the ventilatory acid-base regulation of the toad.

Temperature and central chemoreceptor drive to ventilation in toad (Bufo paracnemis)

The central chemoreceptor drive to ventilation was assessed in unanesthetized toads, Bufo paracnemis, exposed to three different temperatures: 15, 25 and 35 degrees C. The acid-base status of the fourth ventricle was manipulated by mock CSF perfusion. In additional experiments, arterial pH was varied by inspiration of hypercapnic gas mixtures. Ventilation was measured directly by pneumotachography and arterial blood samples were analyzed using electrodes for pH and PO2. Regardless of temperature, the ventilatory control of acid-base status was predominantly central. Moreover, an increase in temperature was accompanied by a proportional increase in the ventilatory response to chemoreceptor stimulation by either lowered mock CSF pH or hypercapnia. The alphastat hypothesis could not adequately account for the temperature effects on the ventilatory responses to hypercapnia or on air convection requirements in the toad.

Involvement of the Cholinergic System and Periaqueductal Gray Matter in the Modulation of Tonic Immobility in the Guinea Pig

Unilateral microinjection of carbachol (CCh, 1.0 microg/0.2 microl) into the ventrolateral periaqueductal gray matter (vPAG) increased the duration of tonic immobility (TI) episodes induced by postural inversion and by movement restriction maneuvers in adult male guinea pig (Cavia porcellus), while stimulation with the same drug at the same concentration into the dorsolateral and dorsomedial periaqueductal gray matter (dl/dmPAG) decreased the duration of TI. Pretreatment with atropine (7.6 microg/0.4 microl) showed that the action of CCh is mediated by muscarinic receptors in the ventrolateral PAG but not in the dorsomedial and dorsolateral regions. These data suggest that the PAG and the cholinergic system are involved in the modulation of TI episodes and that different regions of the guinea pig PAG play distinct roles in the organization of this behavior.

The parabrachial region as a possible region modulating simultaneously pain and tonic immobility.

Unilateral microinjection of carbachol (1 microgram) into the dorsal parabrachial region (PBR) of conscious guinea pigs produced a 100% increase in the duration of restraint-induced tonic immobility (TI) episodes. In another group of animals with a subcutaneous electrode introduced into the thigh region, microinjections of equivalent doses of the same drug in similar sites also significantly reduced the motor defense and vocalization responses elicited by the application of a noxious electrical stimulus to the skin. Both effects were blocked by pretreatment with atropine. The possible simultaneous activation of mechanisms modulating TI and the response to a noxious stimulus may be of adaptive importance since analgesia may reinforce immobility to permit the use of other defense mechanisms in a situation of prey/predator confrontation.

Influence of some limbic structures upon somatic and autonomic manifestations of pain

Abstract In guinea pigs under pentobarbital anesthesia a painful stimulation was applied through a monopolar electrode introduced in the dental pulp of one incisor and the effects on motor movements, respiration and vocalization were registered. It was observed that association of painful and septal stimuli resulted either in abolition or diminution of painful manifestations. Sometimes, according to the electrode position, septal stimulation elicited painful expression. Changes in frequency of stimulation without changing the electrode placement resulted either in analgesic-like effect or algesic-like effect. The stimulation of amygdala, stria terminalis and hippocampus elicited also both kinds of manifestations, sedative and painful. The results here reported support the existence of a limbic modulation of painful messages.

Behavioral characterization of the alarm reaction and anxiolytic-like effect of acute treatment with fluoxetine in piauçu fish

In Ostariophysan fish, the detection of the alarm substance liberated into the water as a consequence of an attack by a predator elicits an alarm reaction or anti-predatory behavior. In this study, experiments were performed to: (i) describe and quantitatively characterize the behavioral and ventilatory responses in piauçu fish (Leporinus macrocephalus), individually and as part of a school, to conspecific alarm substance (CAS) and; (ii) test the effect of acute fluoxetine treatment on alarm reaction. Histological analysis revealed the presence of club cells in the intermediate and superficial layers of the epidermis. The predominant behavioral response to CAS was freezing for fish held individually, characterized by the cessation of the swimming activity as the animal settles to a bottom corner of the aquarium. Fish exposed to CAS showed decrease in the mean ventilatory frequency (approximately 13%) relative to control. In schools, CAS elicited a biphasic response that was characterized by erratic movements followed by increased school cohesion and immobility, reflected as an increased school cohesion (65.5% vs. -5.8% for controls) and in the number of animals near the bottom of the aquarium (42.0% vs. 6.5% for controls). Animals treated with single i.p. injections of fluoxetine (10 μg/g b.w.) did not exhibit alarm behavior following CAS stimulation. These results show that an alarm pheromone system is present in piauçu fish, evidenced by the presence of epidermal club cells and an alarm reaction induced by CAS and consequently of a chemosensory system to transmit the appropriate information to neural structures responsible for initiating anti-predator behavioral responses. In addition, fluoxetine treatment caused an anxiolytic-like effect following CAS exposure. Thus, the alarm reaction in piauçu can be a useful model for neuroethological and pharmacological studies of anxiety-related states.

The 'club' cell and behavioural and physiological responses to chemical alarm cues in the Nile tilapia

The alarm response to skin extract has been well documented in fish. In response to skin extract, there is a decline in both locomotion activity and aggressive interactions. Our observation herein of these responses in the cichlid Nile tilapia, Oreochromis niloticus, confirmed the existence of the alarm response in this species. However, so far there has been a paucity of information on the autonomic correlates of this response. In this study, the ventilatory change in response to the chemical alarm cue was evaluated. This parameter was measured 4 min before and 4 min after exposure to 1 mL of either conspecific skin extract or distilled water (extract vehicle). Skin extract induced an increase in the ventilation rate, which suggested an anticipatory adjustment to potentially harmful stimuli. The chemical cue (alarm substance) also interfered with the prioritisation of responses to different environmental stimuli (stimuli filtering); this was suggested by the observation that the Nile tilapia declined to fight after exposure to a cue that indicates a risk of predation. Furthermore, histological analysis of the Nile tilapia skin revealed the presence of putative alarm substance-producing (club) cells.

Afferent and Efferent Connections of the Dorsocentral Telencephalon in an Electrosensory Teleost, Gymnotus carapo

Biotinylated dextran amine was injected unilaterally into dorsal regions of the telencephalon of the weakly electric fish Gymnotus carapo in order to study the afferent and efferent connections of specific dorsal regions with ventral regions of the telencephalon and with other regions of the central nervous system. Efferent pathways from the dorsolateral area of the telencephalon project ipsilaterally to the anterior hypothalamic nucleus, the ventral thalamus and magnocellular tegmental nucleus, whose axons reach the spinal cord. Anterograde labeling showed that the central division of the dorsal telencephalon sends efferent projections through the lateral forebrain bundle towards the ipsilateral lateral and medial preglomerular nucleus, the pretectal nucleus, the optic tectum and the dorsal torus semicircularis, regions that are all involved in the processing of electrosensory and/or multisensory information. In addition, when biotinylated dextran amine was injected into the dorsal torus semicircularis, retrogradely labeled neurons were observed in the dorsocentral area of the telencephalon. The dorsocentral area is also a target of the extra-telencephalic afferents originating from rostral, lateral and medial regions of preglomerular complex. Within the telencephalon, neurons of many ventral subdivisions project ipsilaterally to the dorsocentral area. The dorsocentral, dorsolateral and dorsomedial areas are connected ipsilaterally and reciprocally. The dorsocentral area is reciprocally connected with its contralateral homologue through the anterior commissure.

The lateral hypothalamus in the modulation of tonic immobility in guinea pigs.

THE lateral hypothalamus has been reported to be involved in the organization of aggression and predatory attack but not in behavioral inhibition responses such as tonic immobility (TI). TI may be defined as an inborn behavioral inhibition characterized by profound physical inactivity and relative lack of responsiveness to the environment, triggered by an intense sensation of fear generated during prey—predator confrontation. Our study indicates that cholinergic stimulation of anterior regions of the lateral hypothalamus of guinea pigs potentiates the duration of TI episodes, while stimulation of medial and posterior regions of this structure promotes a decrease in TI duration, suggesting that the lateral hypothalamus modulates the duration of TI episodes in a differentiated manner.

Cardiovascular reflexes in conscious toads.

Methods used for implanting sensors and catheters in temporarily ether-anesthetized toads (Bufo paracnemis) are described. Following recovery it was found that distension of the pulmocutaneous arterial trunk and high frequency electrical stimulation of the laryngeal nerve of conscious toads induce an abrupt fall in arterial pressure accompanied or not by bradycardia or cardiac arrest. A brief suppression of throat movements may occur but this is not a constant finding. The response is blocked by atropine or methyl-homatropine and persists in animals with high spinal sectioning, thus indicating its cholinergic parasympathetic nature. However a certain amount of sympathetic inhibition is not ruled out. Perfusion of the artery with lobeline and electrical stimulation of the laryngeal nerve at low frequency (1/s) induces a rise in arterial pressure which is blocked by phentolamine. The hypertension is followed by enhancing of both throat oscillations and electromyographic discharges. The occurrence of chemoreceptors in the pulmocutaneous arterial wall in these animals is discussed. Blockage of the laryngeal nerve with lidocaine or perfusion of the pulmocutaneous arterial trunk with the same solution elicited a blood pressure rise, tachycardia and enhanced ventilatory movements. This was attributed to suppression of the baroreceptor tonus.