Noradrenergic modulation of basolateral amygdala neuronal activity: opposing influences of alpha-2 and beta receptor activation

Abstract

Substantial data exists demonstrating the importance of the amygdala and the locus ceruleus (LC) in responding to stress, aversive memory formation, and the development of stress-related disorders; however, little is known about the effects of norepinephrine (NE) on amygdala neuronal activity in vivo. The basolateral nucleus of the amygdala (BLA) receives dense NE projections from the LC, NE increases in the BLA in response to stress, and the BLA can also modulate the LC via reciprocal projections. These experiments examined the effects of noradrenergic agents on spontaneous and evoked responses of BLA neurons. NE iontophoresis inhibited spontaneous firing and decreased the responsiveness of BLA neurons to electrical stimulation of entorhinal cortex and sensory association cortex (Te3). Confirmed BLA projection neurons exhibited exclusively inhibitory responses to NE. Systemic administration of propranolol, a beta-receptor antagonist, decreased the spontaneous firing rate and potentiated the NE-evoked inhibition of BLA neurons. In addition, iontophoresis of the alpha-2 agonist clonidine, footshock administration, and LC stimulation mimicked the effects of NE iontophoresis on spontaneous activity. Furthermore, the effects of LC stimulation were partially blocked by systemic administration of alpha 2 and beta receptor antagonists. This is the first study to demonstrate the actions of directly applied and stimulus-evoked NE in the BLA in vivo, and provides a mechanism by which beta receptors can mediate the important behavioral consequences of NE within the BLA. The interaction between these two structures is particularly relevant with regard to their known involvement in stress responses and stress-related disorders.

Figure 1.

Diagram demonstrating sites of recording and stimulating electrodes. A, Neurons recorded from were located within the basolateral complex of the amygdala (lateral, basolateral, and basomedial nuclei). B, Stimulating electrode sites localized to the LC. C, Stimulating electrode sites localized to the EC. D, Stimulating electrode sites localized to the Te3.

Figure 2.

Norepinephrine inhibits spontaneous activity of BLA neurons. A, Firing rate histogram showing dose-dependent inhibition of spontaneous activity during microiontophoretic application of NE. B, The majority of neurons displayed an inhibition of spontaneous activity during NE application (p = 0.02). C, In some cases, NE microiontophoresis resulted in a dose-dependent excitation of neuronal activity. D, A small number of neurons showed an increase in spontaneous activity with NE application (p = 0.02). E, Neurons confirmed as projection cells via antidromic activation only displayed inhibition to NE application (p = 0.002). F, Neurons in all nuclei of the BLA displayed excitatory and inhibitory responses.

Figure 3.

Norepinephrine inhibits evoked activity of BLA neurons. A, Electrophysiological recording trace of a BLA neuron exhibiting orthodromic activation after EC stimulation. B, Iontophoretic application of NE suppresses responses to EC stimulation (p = 0.005). C, BLA neurons are also activated by Te3 stimulation. D, Iontophoretic application of NE suppresses responses to Te3 stimulation (p = 0.002). Light lines in B and D illustrate individual neuronal responses; the dark line represents the average response.

Figure 4.

Inhibitory effects of NE are mediated via α-2 receptors. A, Firing rate histogram showing that iontophoretic application of 10 nA clonidine causes an inhibition of spontaneous activity of a BLA neuron. This same neuron exhibited inhibition of spontaneous activity in response to 40 nA NE application. B, All neurons that display inhibitory responses to clonidine iontophoresis (p = 0.02) also display inhibitory responses to NE iontophoresis (p = 0.03).

Figure 5.

Inhibitory effects of NE are potentiated under β receptor blockade. Propranolol administration potentiates the dose-dependent inhibitory effects of NE iontophoresis.

Figure 6.

LC stimulation and NE iontophoresis cause similar effects on BLA neurons. A, Firing rate histogram of a neuron showing inhibition in response to LC stimulation. B, This same neuron shows inhibition in response to NE iontophoresis. C, All neurons that display inhibitory responses to LC stimulation (p = 0.03) also display inhibitory responses to NE iontophoresis (p = 0.004). D, Example of a neuron showing excitation in response to LC stimulation. E, This same neuron shows excitation in response to NE iontophoresis. F, The majority of neurons that display excitatory responses to LC stimulation (p = 0.01) also display excitatory responses to NE iontophoresis (p = 0.01).

Figure 7.

Footshock stimulation and NE iontophoresis cause similar effects in BLA neurons. A, Firing rate histogram of a neuron showing inhibition in response to footshock. B, This same neuron shows inhibition in response to NE iontophoresis. C, All neurons that display inhibitory responses to footshock (p = 0.003) also display inhibitory responses to NE iontophoresis (p = 0.004). D, Example of a neuron showing excitation in response to footshock. E, This same neuron shows excitation in response to NE iontophoresis. F, The majority of neurons that display excitatory responses to footshock administration (p = 0.003) also display excitatory responses to NE iontophoresis (p = 0.002).