The effect of ventral medial frontal cortex (MFC) lesions on respiratory rate (RESP), immobility ("freezing"), and ultrasonic vocalizations (USVs) during conditioned emotional responses (CERs) was investigated. Male Sprague-Dawley rats were divided into two groups: MFC-lesioned rats (N = 11) sustained bilateral lesions of the infralimbic region of the MFC via microinjection of the neurotoxin NMDA; controls received sterile saline. Following a 2 week recovery period, all animals were differentially conditioned to two tones; a 2 mA footshock served as the unconditioned stimulus (US). The CS+tone was consistently paired with the US, while the CSr tone was randomly paired with the US. On the following day, RESP, freezing, and USVs were recorded during CER testing (no US were presented). All animals responded during the CS+ with increased RESP compared to baseline; the increase in MFC rats was significantly larger than in controls. All animals also froze at the onset of the CS+. Following the CS+, controls displayed a prolonged period of freezing (265 +/- 37 sec) and decreased RESP compared to baseline, and 92% emitted USVs. In contrast, MFC rats displayed a significantly shorter period of freezing (86 +/- 25 sec) and little or no USV; RESP remained significantly elevated throughout the remainder of the trial. These behaviors indicate a significantly altered stress response following ventral MFC lesions, implying that the MFC may be necessary for complete expression of various behavioral responses to "stressful" stimuli. The role of the MFC in emotion is also discussed.
Classical conditioning of Hermissenda produces neurophysiological correlates in the primary sensory neurons of the pathway mediating the conditioned stimulus (CS), the type B and type A photoreceptors. Biophysical and biochemical changes intrinsic to the type B photoreceptors have been studied extensively in conditioned animals. A second site for intrinsic modification with learning has been recently identified in type A photoreceptors. We have recorded from identified medial and lateral type A photoreceptors in conditioned animals and animals that received pseudorandom presentations of light (CS) and rotation (unconditioned stimulus). Here we report that conditioning produces differential effects upon CS-elicited spike frequency and intrinsic excitability detected in identified lateral and medial type A photoreceptors. Lateral type A photoreceptors from conditioned animals exhibited significant increases in spike frequency elicited by the CS as compared to pseudorandom controls. In contrast, CS-elicited spike frequency recorded in medial type A photoreceptors was not significantly different from random controls. The amplitude of the peak and plateau phases of the generator potential of medial and lateral type A photoreceptors was decreased in conditioned animals as compared to random controls. In addition, only lateral type A photoreceptors exhibited enhanced cellular excitability as expressed by increased spike discharges produced by the injection of extrinsic depolarizing current pulses. Conditioning also decreased spike frequency accommodation in lateral type A photoreceptors. In normal controls, medial type B photoreceptors produced stronger synaptic inhibition of medial type A photoreceptors than the lateral type B photoreceptors inhibition of lateral type A photoreceptors.(ABSTRACT TRUNCATED AT 250 WORDS)
Intrinsic changes have been identified in isolated and intact type A and type B photoreceptors following classical conditioning of the nudibranch mollusk Hermissenda. Aspects of various intrinsic, nonsynaptic modifications are expressed by alterations in the excitability of identified photoreceptors in response to the conditioned stimulus. In addition to changes in cellular excitability, changes in synaptic strength between identified neurons have been proposed as a possible mechanism of associative learning in several invertebrate preparations. Here we report that classical conditioning produces differential effects upon the strength of inhibitory monosynaptic connections between identified pairs of type B and type A photoreceptors. The amplitude of IPSPs elicited by an action potential in the medial type B photoreceptor and recorded from medical type A photoreceptors was significantly enhanced in conditioned animals as compared to pseudorandom controls. In contrast, the amplitude of IPSPs elicited by an action potential in the lateral type B photoreceptor and recorded from lateral type A photoreceptors did not show significant synaptic enhancement following conditioning. These results provide additional evidence for differential effects of conditioning upon cellular modifications in identified type A and type B photoreceptors, and further indicate that multiple sites of cellular plasticity exist in the visual system of conditioned Hermissenda.
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