Development of isolation-induced vocal behavior in normal-hearing and deafened guinea pig infants.

Infants in many different animal species require auditory information from conspecifics to learn appropriate responses to important environmental and social cues. Isolation calls are emitted by infant guinea pigs in contexts of social separation from their mothers. The aim of the present study was to examine the ontogeny of the isolation calls in normal-hearing and deafened infant guinea pigs, from 2 to 40 days postpartum and to determine the role of hearing maternal vocalization in infant guinea pig vocal responses in contexts of social proximity and isolation. Female newborn pigmented guinea pigs (Cavia porcellus) were housed with their birth mother and siblings. Water and dry food were available ad libitum. One day postpartum, the cochlea of infants in the experimental group was destroyed. The control group consisted of normal-hearing female siblings. Vocalizations from infants in the experimental and control groups were recorded for 6 minutes when with the mother before isolation, 6 minutes when alone, and then 6 minutes when with the mother after reunion. Recordings were made 5 days per week from 2 to 40 days after birth. The duration of calling was calculated for each 6-minute period of recording. Results demonstrated that deaf infants vocalize more than normal-hearing infants during social isolation from their mothers. Vocal activity of isolated deaf and normal-hearing infants decreased substantially over development, almost disappearing by the end of the study period. These results indicate that maternal vocal behavior modulates the vocal responses of guinea pigs early in infant development and supports other evidence that the guinea pig offers a viable model for investigating audition in deaf and normal-hearing human infants.

Gastric distension modulates hypothalamic neurons via a sympathetic afferent path through the mesencephalic periaqueductal gray.

The effects of gastric distension on extracellularly recorded single neuronal activity in the lateral hypothalamuslateral preoptic area-medial forebrain bundle (LPA-LH-MFB), other areas of the hypothalamus, mesencephalic periaqueductal gray (PAG), and other areas associated with the mesencephalic reticular formation were determined in the anesthetized rat. Gastric distension was produced by filling a gastric balloon with water using a calibrated infusion pump. Experimental conditions were based on previous studies that simulated gastric distension during fluid consumption in the rat. The effects of stomach distension using water at body temperature and room temperature were compared. Neurons in both the hypothalamus and mesencephalon were modulated by gastric distension. Hypothalamic neurons exhibited responses associated with gastric distension and exhibited interactions between distension and temperature stimulation of the stomach. Neurons in the mesencephalic periaqueductal gray (PAG) and associated reticular formation also were modulated by these gastric stimuli. When the PAG was electrically stimulated, similar responses to gastric distension and PAG stimulation were observed for hypothalamic neurons. The effects of gastric distension on hypothalamic neurons were reduced or eliminated when the PAG stimulating electrode site was destroyed by electrocoagulation. In addition, the microiontophoretic application of horseradish peroxidase at hypothalamic neuronal recording sites where gastric distension effects were observed resulted in the retrograde labeling of neurons in the PAG. These gastric stimulation-induced effects on hypothalamic and mesencephalic neuronal activity were attenuated but were not permanently eliminated by bilateral cervical vagotomy. However, these effects were significantly reduced or eliminated by bilateral transection of the cervical sympathetic chain or spinal transection at the first cervical level. Because the filling of balloons placed into the abdominal cavity close to the stomach had no similar effects on neural activity, these results can be attributed primarily to the activation of gastric mechano-and temperature-sensitive receptors. These results indicate that the effects of gastric temperature/distension stimulation under these conditions are mediated to a large degree by sympathetic afferents. The PAG is clearly involved as one of the mesencephalic relays for gastric afferent input to the hypothalamus.

Effects of neurotransmitters and vagus nerve stimulation on diencephalic and mesencephalic neuronal activity.

The effects of microiontophoretically applied neurotransmitters and cervical vagus nerve stimulation on neuronal discharge frequency was determined for cells located in the lateral hypothalamus-lateral preoptic area (LH-LPA), medial hypothalamus, thalamus-zona incerta area, and midbrain of anesthetized rats. Seven barrel electrodes were utilized to record simultaneously from and apply gamma aminobutyric acid (GABA), dopamine, glutamate, acetylcholine and norepinephrine to single neurons. Microiontophoretically applied GABA resulted in similar decreases in neuronal discharge frequency in all central areas tested. Thalamus-zona incerta cells were the most sensitive to GABA and required the lowest ejection current for threshold responses. Dopamine application resulted in increases and decreases in hypothalamic and mesencephalic neuronal discharge frequency. Thalamus-zona incerta neurons were the most sensitive to dopamine and exhibited only decreased activity during its application. Glutamate application resulted in a non-specific excitation of neurons. Acetylcholine induced increases and decreases in neuronal discharge frequency. Thalamus-zona incerta cells were the most sensitive to acetylcholine application. Hypothalamic neurons were the most sensitive to norepinephrine induced decreases in neuronal activity and effects in this area persisted for a prolonged period after norepinephrine was applied. Neuronal discharge frequency was significantly increased during vagus nerve stimulation in all central areas studied. Only hypothalamic neurons also exhibited decreased activity during vagus nerve stimulation. Results are discussed in terms of previous neurochemical and neurophysiological data and in terms of the importance of vagal afferents in the control of central neuronal activity.

Mesencephalic reticular formation stimulation effects on hypothalamic neuronal activity.

The effects of mesencephalic reticular formation (RF) single pulse, 0.5 msec and 0-500 microA, stimulation on lateral preoptic-lateral hypothalamic (LPA-LH) neuronal activity were determined in anesthetized rats. In addition, the effects of LH stimulation on neural activity in the RF and periaqueductal gray (PAG) were evaluated. Recordings from 117 neurons indicate reciprocal connections between the LPA-LH and the mesencephalon. Stimulation of the RF affected 70% of the LPA-LH neurons tested. Short latency decreases in activity predominated indicating an inhibitory synaptic input from the RF to the LPA-LH. Short latency increases in discharge frequency were observed infrequently. Stimulation of the LH affected only 32% of the mesencephalic neurons tested. Short latency decreases in activity were usually observed indicating reciprocal inhibitory synaptic connections between the LPA-LH and the RF and periaqueductal gray. Antidromic responses verified these interconnections and revealed relatively slow conduction velocities of approximately 1.0 m/sec. Results are discussed in terms of the involvement of the LPA-LH and RF in sensorimotor functions, spinal motor excitability, and ingestive behavior.

Effects of ventral tegmental area stimulation and microiontophoretic application of dopamine and norepinephrine on hypothalamic neurons.

The effects of ventral tegmental area of Tsai (VTA) stimulation on lateral hypothalamic (LH), lateral preoptic area (LPA). and medial hypothalamic neuronal activity were determined in anesthetized rats. Recordings from 81 hypothalamic neurons indicate that stimulation produces predominantly decreases in hypothalamic neuron activity. Increase in activity due to VTA stimulation occurred less frequently. Following single rectangular pulse stimulation, 0.5 msec. 0-500 microA, short latency decreases in activity occurred. Longer latency increases in discharge frequency were also observed. Dose response relations were established for 56% of the LH neurons, 78% of the LPA neurons, and for 82% of the medial hypothalamic neurons following VTA stimulation. Decreases and in a few cases increases in activity seemed to involve only one or two synapses. Antidromic responses verified interconnections between the VTA and the hypothalamus and revealed relatively slow conduction velocities of 0.45 and 0.81 m/sec. The changes in discharge frequency which occurred following VTA stimulation were similar in direction to the effects of the direct microiontophoretic application of dopamine (DA) or norepinephrine (NE). Since DA increased or decreased while NE decreased discharge frequency, these microiontophoretic tests indicated that the shorter latency VTA stimulation induced increases in decreases in neural activity were associated with VTA dopaminergic neuron stimulation and that in some cases short and long latency decreases in neuronal activity were due to activation of VTA ventral bundle NE fibers of passage or to indirect polysynaptic mechanisms. Results demonstrate the interconnections between various regions of the hypothalamus and the VTA along the extent of the medial forebrain bundle (MFB). The cross-validation of neuroanatomical and various electrophysiological methods in establishing the nature of hypothalamic connections was discussed.

Effects of periaqueductal gray stimulation of diencephalic neural activity.

The effects of ipsilateral mesencephalic periaqueductal gray (PAG) stimulation on lateral hypothalamic (LH), lateral preoptic area (LPA), and ventral and subthalamic activity were determined in anesthetized rats. Recordings from 119 diencephalic neurons indicate that the PAG provides a predominantly inhibitory input to diencephalic neurons. Excitatory input occurred infrequently in the hypothalamus and was not observed in the thalamus. Following single rectangular pulse stimulation, 0.5 msec, 0-500 micro A, short latency decreases in activity occurred. Longer latency increases in discharge frequency were also observed. Dose response relations were established for 74% of the LH neurons, 68% of the LPA neurons, and for 72% of the ventral and subthalamic neurons following VTA stimulation. Decreases and, in a few hypothalamic neurons, increases in activity seemed to involve only one or two synapses. The effects of contralateral PAG stimulation on LPA-LH neuronal activity were alos determined. Dose response relations were established for 66% of the LPA-LH neurons following contralateral stimulation. However, results were different in that many more cells were increased with a shorter latency and at a lower threshold following contralateral stimulation. Antidromic responses verified PAG and diencephalic interconnections and revealed relatively slow conduction velocities, less than 1.0 m/sec. Results were discussed in terms of the anatomy of known PAG pathways, PAG neuronal activation vs. PAG fibers of passage, and the functions of midbrain-hypothalamic interconnections in the integration of somatic, visceral and nociceptive sensory inputs.