Periaqueductal gray spike trains recorded in frontal or horizontal mesencephalic brain slices from the rat

Fos-like immunoreactivity in the brain associated with freezing or escape induced by inhibition of either glutamic acid decarboxylase or GABAA receptors in the dorsal periaqueductal gray

GABAergic neurons exert tonic control over the neural substrates of aversion in the dorsal periaqueductal gray (dPAG). It has been shown that electrical stimulation of this region at freezing or escape thresholds activates different neural circuits in the brain. Since electrical stimulation activates cell bodies and fibers of passage, it is necessary to use chemical stimulation that activates only post-synaptic receptors. To investigate this issue further, reduction of GABA transmission was performed with local injections of either the GABA-A receptor antagonist bicuculline or the glutamic acid decarboxylase (GAD) inhibitor semicarbazide into the dorsolateral periaqueductal gray (dlPAG). Local infusions of semicarbazide (5.0 microg/0.2 microl) or bicuculline (40 ng/0.2 microl) into this region caused freezing and escape, respectively. The results obtained showed that freezing behavior induced by semicarbazide was associated with an increase in Fos expression in the laterodorsal nucleus of the thalamus (LD) and ventrolateral periaqueductal gray (vlPAG), while bicuculline-induced escape was related to widespread increase in Fos labeling, notably in the columns of the periaqueductal gray, hypothalamus nuclei, the central amygdaloid nucleus (Ce), the LD, the cuneiform nucleus (CnF) and the locus coeruleus (LC). Thus, the present data support the notion that freezing and escape behaviors induced by GABA blockade in the dlPAG are neurally segregated: freezing activates only structures that are mainly involved in sensory processing, and bicuculline-induced escape activates structures involved in both sensory processing and motor output of defensive behavior. Therefore, the freezing elicited by activation of dlPAG appears to be related to the acquisition of aversive information, whereas most brain structures involved in the defense reaction are recruited during escape.

Characterization of synaptic transmission in the ventrolateral periaqueductal gray of rat brain slices

Synaptic transmission evoked by focal stimulation in the ventrolateral periaqueductal gray was characterized using the whole-cell recording technique in rat brain slices. At resting membrane potential (-62+/-1 mV), focal stimulation (0.05-0.1 ms, 0.03 Hz) usually evoked a 6-cyano-7-nitroquinoxaline-2, 3-dione-sensitive fast excitatory postsynaptic potential and a DL-2-amino-5-phosphonopentanoic acid-sensitive slow excitatory postsynaptic potential with a bicuculline-sensitive inhibitory postsynaptic potential in between. In the presence of kynurenic acid, bicuculline-sensitive inhibitory postsynaptic currents recorded in the voltage-clamp mode displayed a reversal potential of -68+/-3 mV, resembling GABA(A) receptor-mediated inhibitory postsynaptic currents. However, no GABA(B) receptor-mediated inhibitory postsynaptic current was evoked, even at stronger stimulating intensity. 6-Cyano-7-nitroquinoxaline-2,3-dione-sensitive fast excitatory postsynaptic currents were isolated by DL-2-amino-5-phosphonopentanoic acid plus bicuculline and DL-2-amino-5-phosphonopentanoic acid-sensitive slow fast excitatory postsynaptic currents by bicuculline plus 6-cyano-7-nitroquinoxaline-2,3-dione. Both types of excitatory postsynaptic current reversed at potentials near 0 mV. The I-V curve of slow fast excitatory postsynaptic currents or N-methyl-D-aspartate currents displayed a negative slope at potentials more negative than -30 mV in an Mg(2+)-sensitive manner. The control postsynaptic currents reversed at potentials between -50 and -35 mV, inclined to the reversal potential of GABA(A), but not glutamate, receptor channels. It is concluded that, in the ventrolateral periaqueductal gray, focal stimulation elicits both inhibitory and excitatory transmission, while the former is dominant. The inhibitory transmission is mediated by GABA(A) but not GABA(B) receptors. The excitatory transmission is mediated by glutamate acting on alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate as well as N-methyl-D-aspartate receptors.

In vitro responses of neurons in the periaqueductal gray to hypoxia and hypercapnia

Hypoxia-sensitive neurons in the caudal hypothalamus (CH) have been shown to project to the periaqueductal gray (PAG) which, in turn, sends descending projections to an area of the ventrolateral medulla (VLM) containing neurons inherently excited by hypoxia. The purpose of this study was to determine if neurons in the PAG are excited by hypoxia or hypercapnia in an in vitro environment. Extracellular responses to hypoxia and hypercapnia of neurons located throughout the PAG were recorded in a rat brain slice (400-500 microm thick) preparation. Hypoxic (10% O(2)/5% CO(2)/85% N(2)) and hypercapnic (7% CO(2)/93% O(2)) stimuli were delivered to the tissue through gas bubbled into the brain slice chamber. A majority (39 of 53) of the neurons tested responded to hypoxia. Of these neurons, 92% responded to hypoxia with an increase in firing rate. Neurons in the dorsolateral/lateral regions increased firing rates to a greater extent than neurons located in ventrolateral regions. All neurons tested (n=6) also responded to hypoxia after perfusion of the tissue with a low Ca(2+)/high Mg(2+) solution to block classic synaptic transmission. Only a small proportion (7/33) of neurons tested responded to hypercapnia. These findings indicate that neurons in the periaqueductal gray region of the brain have an inherent responsiveness to hypoxia and, thus, may contribute to the overall coordination of cardiorespiratory responses to systemic hypoxia.

Modulation of the activity of midbrain central gray substance neurons by calcium channel agonists and antagonists in vitro

Changes in the background impulse activity of midbrain central gray substance neurons have been studied on slice preparations from the rat midbrain upon application of calcium-free solution, an activator of calcium channels, BAY-K 8644 (10 nM), organic (verapamil, 40 microM; D600, 10 microM; nifedipine, 1-10 microM; amiloride, 1 microM) and inorganic (Co2+, 1.5 mM) calcium channel blockers. Besides BAY-K 8644, all the substances inhibited most of the neurons studied. Verapamil, BAY-K 8644 and Co2+ also revealed facilitatory effects. Facilitatory action of BAY-K was most effective in silent neurons and in those previously inhibited by amiloride. Latent period values of inhibition in calcium-free solution and upon application of organic and inorganic blockers have the following sequence: D600 > amiloride > verapamil > Co2+ > nifedipine > calcium-free solution. Maximum rise time had the following order: amiloride > D600 > nifedipine > verapamil > Co2+ > calcium-free solution. Complete suppression of the neuronal activity induced by amiloride lasted twice as long as that induced by calcium-free solution, Co2+ and nifedipine, and six times as long as verapamil-induced suppression. Preliminary application of calcium channel blockers reduced facilitatory and increased inhibitory effects of serotonin and substance P. Data obtained are discussed with the supposition in mind that inhibition of the function of calcium channels in central gray substance neurons could be one of the mechanisms underlying the analgesic effect of a series of neurotropic agents after their introduction into this structure.

What brain structures are active during emotions? Effects of brain stimulation elicited aversion on c-fos immunoreactivity and behavior

Aversive behavior is produced by stimulating some brain structures, such as the dorsal periaqueductal gray and the medial hypothalamus. We have used c-fos immunoreactivity to map brain areas which are influenced by stimulation of these two structures. Stimulation was produced in freely moving rats by electrical stimulation or by microinjections of either excitatory amino acids or GABA blocking drugs. Behavior was monitored to detect emotional changes. The effects on labeling induced by the stimulation of either structure were then compared. Structures labeled include the amygdala, the stria terminalis, the supramamillary area, the hypothalamus, the periaqueductal gray, the superior colliculus, the nucleus cuneiformis, and the locus coeruleus. Regardless whether chemical or electrical stimulation was used or the structure stimulated, there was a large overlap among the brain areas labeled. We then compared our results with data from the literature where other methods of inducing aversion have been used, including pain and stress. There was remarkable similarity in the patterning of labeling irrespective of the type of stimulation (central-peripheral, chemical-electrical). There was, however, one interesting difference produced by central vs. peripheral stimulation. Labeling was unilateral in the former case and bilateral in the latter case. Our results suggest that there is a neural substrate that mediates aversive behavior, no matter how it is produced. Nevertheless, that peripheral stimulation produces mainly bilateral activation of this substrate whereas central stimulation produces mainly unilateral activation suggests that natural peripheral stimuli are also integrated at a higher functional level. Future work could be directed toward explicit comparisons of central versus peripheral stimulation to identify the structures involved in higher level integration of aversive behavior.