Improving Database Design for a Notification System in MySQL - mysql

Question

I'm in the process of building a common notification system for a webapp. The main technologies I'm using are Java, Spring MVC and Hibernate. I've been looking at several posts here and there, trying to come up with the solution that suits me best, taking into account recommended practices.
I've already coded my database tables and would like to receive some feedback in order to
improve my design to avoid big changes while I'm implementing my Java classes. My goal is to make it as complete, scalable and optimal as possible mantaining the complexity to the minimum possible.
Here's my code:
NOTIFICATION SAMPLE:
The #user added a new comment.
The user [USER_ID] [USER_ACTION] a new [OBJECT].
>>> Updates >>>
[05/02/14]
Fields id_recipient and seen removed from notification table. (#Kombajn zbożowy)
New table notification_user created. (#Kombajn zbożowy)
Lowercase identifiers. (#wildplasser)
notification
CREATE TABLE notification (
id_notification BIGINT(20) NOT NULL AUTO_INCREMENT,
id_notification_type BIGINT(20) NOT NULL,
id_action_type BIGINT(20) NOT NULL,
id_sender BIGINT(20) NOT NULL,
created_date TIMESTAMP NOT NULL,
url VARCHAR(300) NULL,
PRIMARY KEY (id_notification),
FOREIGN KEY (id_notification_type) REFERENCES notification _type (id_notification_type),
FOREIGN KEY (id_action_type) REFERENCES action_type (id_action_type),
FOREIGN KEY (id_sender) REFERENCES user (id_user)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
notification_user: so that one notification can be sent to many recipients (users).
CREATE TABLE notification_user (
id_notification BIGINT(20) NOT NULL,
id_recipient BIGINT(20) NOT NULL,
seen TINYINT(1) DEFAULT 0,
PRIMARY KEY (id_notification , id_recipient),
FOREIGN KEY (id_notification) REFERENCES notification (id_notification),
FOREIGN KEY (id_recipient) REFERENCES user (id_user)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
notification_type: refers to the type of object that was modified by the actions of a certain user. Example: comment, post, etc.
CREATE TABLE notification_type (
id_notification_type BIGINT(20) NOT NULL AUTO_INCREMENT,
notification_name VARCHAR(100) NOT NULL,
description VARCHAR(300) NULL,
PRIMARY KEY (id_notification_type)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
action_type: actions executed by the users which trigger the notifications. Typically: update, add, remove, etc.
CREATE TABLE action_type (
id_action_type BIGINT(20) NOT NULL AUTO_INCREMENT,
action_name VARCHAR(100) NOT NULL,
PRIMARY KEY (id_action_type)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;